US20120114003A1

US20120114003A1 - Laser device, a light signal generation device, and an optical resonator and a method for producing light

Info

Publication number: US20120114003A1
Application number: US13/234,256
Authority: US
Inventors: John F. Donegan; Wei-Hua Guo; Qiao-Yin Lu; Diarmuid Byrne; Brian Corbett; Paul Martin Lambkin; Brendan John Roycroft; Jan-Peter Engelstaedter; Frank Peters
Original assignee: University College Cork; College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin; UNDIVIDED TRINITY OF QUEEN ELIZABETH NEAR DUBLIN
Current assignee: University College Cork; College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin; UNDIVIDED TRINITY OF QUEEN ELIZABETH NEAR DUBLIN
Priority date: 2006-09-20
Filing date: 2011-09-16
Publication date: 2012-05-10
Also published as: EP2064786A2; EP2064785A1; US20100290495A1; WO2008035321A2; US20100046562A1; WO2008035321A3; WO2008035320A1; EP2064785B1; US8238388B2; EP2064785B9

Abstract

A laser device includes a ridge waveguide having an active layer between upper and lower cladding layers. A ridge formed in the upper cladding layer defines the width of a light guiding region in the active layer, and is formed so that a portion of the light guiding region extends into the ridge. A plurality of reflecting slots extend across and into the ridge to a depth sufficient to extend into the extending portion in order that the reflectivity of each slot is on the order of 2%. The slots intersect more than 20% of the total mode energy in the light guiding region, and this in combination with the gain of the active layer facilitates lasing within the light guiding region independently of the reflectivity of end facets of the waveguide. The laser device is particularly suitable for integrally forming with other optical components on a single semiconductor chip.

Description

This is a continuation of U.S. Ser. No. 12/442,337 filed Jun. 23, 2009, which is a 371 of PCT/IE2007/000086 filed Sep. 20, 2007 and published as WO 2008/035321, which claims priority from Ireland Application No. S2006/0692 filed Sep. 20, 2006, the disclosures of which are incorporated in their entirety.
The present invention relates to a laser device, and in particular, though not limited to a laser device for producing light, and the invention also relates to a light signal generating device. The invention also relates to a method for producing light from a semiconductor waveguide, and to an optical resonator.
In this specification and claims unless specifically stated otherwise, the term “reflectivity” means the amplitude of the reflectivity, and the term “transmission” means the amplitude of the transmission.
Semiconductor laser devices such as optical resonators, light generators, light detectors, waveguides, amplifiers, splitters, interferometers, modulators, multiplexers and the like are commonly used in telecommunications in the transmission and reception of data by optical signals. However, due to the nature of such components, in general, it is necessary to produce all such components as discrete components, which must be subsequently assembled together. The assembly of such components is a complex and time consuming task, due to the relatively small size of the components, and furthermore, the assembly of such components requires that they be assembled with a considerable degree of precision, due to the requirement that the light guiding region of each component must be accurately aligned with the light guiding region of its next adjacent component. Misalignment of a light guiding region of one component with that of its adjacent component or components results in loss of some or all of the light signal.
It would therefore be desirable if all such components required to produce, for example, an optical signal transmitter could be integrally formed on a single semiconductor chip. However, known optical resonators and light generators such as laser devices do not lend themselves to integration with other components on a single semiconductor chip.
Optical resonators and laser light generators such as ridge waveguides comprise a light guiding region, which in the case of a light generator comprises an active region which is provided by one or more quantum wells or quantum dots. The active region is located between an upper cladding layer and a lower cladding layer. A longitudinally extending ridge which is formed in the upper cladding layer defines the lateral width of the light guiding region in the active layer. The upper cladding layer typically is doped to be p-type, and the lower cladding layer is doped to be n-type. An electrical current pumped through the active region from the upper p-type cladding layer to the lower n-type cladding layer produces light in the active region. Lasing requires optical feedback in the light guiding region. In known laser devices, for example, in Fabry-Perot cavities, optical feedback is achieved by cleaving the ends of the waveguide to form two reflective facets at the respective opposite longitudinally spaced apart ends of the light guiding region, which reflect the light back into the light guiding region. Due to this requirement to cleave the ends of an optical resonator, optical resonators must therefore be formed as discrete components.
There is therefore a need to provide a laser device, for example, an optical resonator, a light signal generating device and the like which would lend itself to being integrally formed with other optical components on a semiconductor chip.
The present invention is directed towards providing such a laser device, and the invention is also directed towards providing a light signal generating device, an optical resonator and a method for generating a light signal in a laser device.
According to the invention there is provided a laser device comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
In one embodiment of the invention at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of any of the facets in which the light guiding region terminates.
In another embodiment of the invention the reflecting means are located intermediate the opposite longitudinally spaced apart ends of the light guiding region.
Preferably, the reflecting means are located in the waveguide.
In one embodiment of the invention the amplitude of reflectivity of each reflecting means lies in the range of 2% to 20%. Preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 5% to 15%. Advantageously, the amplitude of the reflectivity of each reflecting means is approximately 10%.
In another embodiment of the invention each reflecting means extends into the light guiding region.
In a further embodiment of the invention the light guiding region comprises an active region, and each reflecting means extends into the light guiding region to a location spaced apart from the active region.
In another embodiment of the invention at least four reflecting means are provided.
Preferably, at least six reflecting means are provided.
In another embodiment of the invention the outer two of the reflecting means define a volume in the light guiding region in which lasing occurs.
In a further embodiment of the invention each reflecting means comprises a refractive index altering means for altering the refractive index of the light guiding region adjacent the location of the refractive index altering means.
In a still further embodiment of the invention each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot.
Preferably, each reflecting slot extends substantially laterally of the light guiding region.
In another embodiment of the invention at least some of the reflecting slots are at least partially filled with a reflecting medium.
In a further embodiment of the invention the reflecting medium is a metal material.
In another embodiment of the invention the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region.
In another embodiment of the invention each reflecting means is located in the ridge.
In a further embodiment of the invention each reflecting means extends into the ridge to a depth substantially similar to the depth of the ridge.
In a still further embodiment of the invention the light guiding region extends into the ridge.
In another embodiment of the invention the waveguide comprises an active layer located between an upper cladding layer and a lower cladding layer, the active layer forming the active region.
In a further embodiment of the invention the ridge is formed in the upper cladding layer and defines the lateral width of the light guiding region in the active layer.
In another embodiment of the invention each reflecting means extends into a portion of the light guiding region defined in the upper cladding layer, and terminates at a location therein spaced apart from the active layer.
In a still further embodiment of the invention the waveguide is in the form of an optical resonator.
In another embodiment of the invention the laser device is a tuneable laser device, and the light guiding region defines a first light guiding region and a second light guiding region communicating with the first light guiding region, at least two of the reflecting means being at locations spaced apart longitudinally relative to the first light guiding region to produce a first mirror loss spectrum associated with the first light guiding region with minimum peak values at respective first wavelength values, at least two of the reflecting means at locations spaced apart longitudinally relative to the second light guiding region to produce a second mirror loss spectrum associated with the second light guiding region with minimum peak values at respective second wavelength values, and a refractive index varying means for selectively varying the refractive index of at least the first light guiding region for in turn varying the first mirror loss spectrum until one of the first wavelength values is similar to one of the second wavelength values to produce light of a selected wavelength.
In a further embodiment of the invention the refractive index varying means comprises a means for injecting a first electrical current into the first light guiding region for altering the refractive index thereof. In a still further embodiment of the invention a means is provided for varying the first current for in turn varying the refractive index of the first light guiding region.
In one embodiment of the invention the refractive index varying means comprises a means for injecting a second electrical current into the second light guiding region for varying the refractive index thereof.
In a still further embodiment of the invention a means is provided for varying the second current for in turn varying the refractive index of the second light guiding region.
In a further embodiment of the invention the means for injecting the first and second currents are operable independently of each other for independently varying the refractive indices of the respective first and second light guiding regions.
Preferably, an electrical isolating means is provided for electrically isolating the first and second light guiding regions from each other.
In another embodiment of the invention the light guiding region defines a third light guiding region intermediate the first and second light guiding regions and communicating therewith, the active region extending in the third light guiding region, and the third light guiding region being adapted to be pumped with an electrical current for generating light therein.
Preferably, respective electrical isolating means are provided for substantially electrically isolating the third light guiding region from the respective first and second light guiding regions.
In another embodiment of the invention the first and second light guiding regions are passive regions.
In another embodiment of the invention the first and second light guiding regions are active regions.
In a further embodiment of the invention the active region extends into the first and second light guiding regions, and the first and second light guiding regions are adapted to be pumped with an electrical current.
In another embodiment of the invention the waveguide is a semiconductor laser.
In another embodiment of the invention the waveguide comprises a laser diode for producing light.
In a further embodiment of the invention the waveguide is adapted for receiving a pumping current.
The invention also provides a light signal generating device comprising a waveguide defining a laser device according to the invention for producing light, and an optical component integrally formed with the laser device, a light guiding region being defined in the waveguide, which forms the light guiding region of the laser device and a light guiding region of the optical component.
In one embodiment of the invention the waveguide comprises an active region located between an upper cladding layer and a lower cladding layer, the active region forming the light guiding region of the laser device and the optical component, and a longitudinally extending ridge being formed in the upper cladding layer defining the lateral width of the light guiding region of at least the laser device.
In another embodiment of the invention the ridge defines the lateral width of the light guiding region of the optical component.
In a further embodiment of the invention an electrical isolating means is provided for substantially electrically isolating the laser device from the optical component.
In a further embodiment of the invention the electrical isolating means is formed by an isolating slot extending into the ridge intermediate the laser device and the optical component.
In a still further embodiment of the invention the laser device and the optical component are integrally formed in a single piece of material.
In another embodiment of the invention the laser device and the optical component are integrally formed in a single piece of semiconductor material.
In another embodiment of the invention the optical component is selected from one or more of the following:

- an optical modulator,
- an optical sensor,
- an optical detector,
- an optical amplifier,
- an optical splitter,
- an optical interferometer and
- an optical multiplexer.

The invention also provides a light signal generating device comprising an elongated waveguide formed on a single piece of semiconductor material, an elongated longitudinally extending light guiding region being defined in the waveguide, the light guiding region defining a first light guiding region of a light generating device and a second light guiding region of an optical component, the second light guiding region communication with the first light guiding region for receiving light generated therein, at least two reflecting means at locations spaced apart longitudinally relative to the first light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light in the first light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
In one embodiment of the invention at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing in the first light guiding region is independent of the reflectivity of any of the facets in which the light guiding region may terminate.
Preferably, the respective reflecting means are located in the waveguide adjacent the first light guiding region.
In one embodiment of the invention each reflecting means extends into the first light guiding region. Preferably, the light guiding region comprises an active region, and each reflecting means extends into the first light guiding region to a location spaced apart from the active region.
In one embodiment of the invention the optical component is an optical modulator, and the waveguide adjacent the second light guiding region is adapted for receiving a control voltage signal for modulating light.
In another embodiment of the invention the optical component is an optical detector, and the waveguide adjacent the second light guiding region is adapted for producing an emf across the second light guiding region in response to detecting light.
In a further embodiment of the invention an electrical isolating means for substantially electrically isolating the first light guiding region and the second light guiding region from each other is provided.
In another embodiment of the invention the waveguide adjacent the first light guiding region defines a laser diode for producing the light.
In another embodiment of the invention the first and second light guiding regions defined in the waveguide are integrally formed on a semiconductor chip.
The invention also provides an optical resonator comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.
The invention further provides a method for producing light in a waveguide of the type having a longitudinally extending light guiding region defined therein, the method comprising providing at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region, and at least one of the reflecting means is located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facet in which the light guiding region may terminate.
In one embodiment of the invention the laser device is of a buried waveguide structure.
The advantages of the invention are many. By virtue of the fact that lasing is independent of at least one of any of the facets at the respective opposite longitudinally spaced apart ends of the light guiding region, the laser device according to the invention is particularly suitable for being integrally formed with at least one other optical component and in many cases two or more optical components on a semiconductor chip. Where the laser device is adapted to lase independently of both facets at the respective opposite longitudinally spaced apart ends of the light guiding region, the laser device is particularly suitable for integrally forming with two or more components on a semiconductor chip, and furthermore, components can be integrally formed with the laser device at the respective opposite ends thereof.
Additionally, by virtue of the fact that lasing is achieved independently of the end facets, cleaving of the waveguide to produce reflective end facets is no longer required. Accordingly, the waveguide does not have to be formed along the natural crystalline direction of the semiconductor material, which is essential in laser devices known heretofore, since cleaving of a semiconductor material in order to produce suitably reflective end facets can only be achieved in a direction transversely of the crystalline direction of the semiconductor material.
A further advantage relates to the yield of lasers from a semiconductor wafer. The cleaving process naturally produces a distribution of lengths for the laser structure, thus varying the optical power and wavelength of such devices. Since this step is no longer required, the overall yield from a wafer increases.
A still further advantage of the laser device according to the invention is that since lasing is independent of the reflectivity of the end facets, the distance from an end facet and the one of the reflecting means, be it a slot or otherwise, which is closest to the end facet is not critical, since the phase of reflections from the end facets do not have an effect on the lasing modes.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a laser device according to the invention,

FIG. 2 is a side elevational view of the laser device of FIG. 1,

FIG. 3 is a top plan view of the laser device of FIG. 1,

FIG. 4 is an end elevational view of the laser device of FIG. 1,

FIG. 5 is a side elevational view of a light generating device also according to the invention,

FIG. 6 is a top plan view of the light generating device of FIG. 5,

FIG. 7 is an end elevational view of the light generating device of FIG. 5,

FIG. 8 is a side elevational view of a light signal generating device according to another embodiment of the invention,

FIG. 9 is a top plan view of the light signal generating device of FIG. 8,

FIG. 10 is an end elevational view of the light signal generating device of FIG. 8,

FIG. 11 is a side elevational view of a light signal generating device according to a further embodiment of the invention,

FIG. 12 is a top plan view of the light signal generating device of FIG. 11,

FIG. 13 is an end elevational view of the light signal generating device of FIG. 11,

FIG. 14 illustrates a mirror loss spectrum produced by the light signal generating device of FIG. 11,

FIG. 15 illustrates another mirror loss spectrum produced by the laser device of FIG. 11,

FIG. 16 illustrates a plot of power reflectivity against wavelength of light produced by the laser device of FIG. 11,

FIG. 17 is a side elevational view of a light signal generating device according to a further embodiment of the invention,

FIG. 18 is a top plan view of the light signal generating device of FIG. 17,

FIGS. 19 to 23 illustrate waveforms produced by computer simulations and experimentation carried out on laser devices according to the invention,

FIG. 24 is a top plan view of a laser device also according to the invention,

FIGS. 25 to 27 illustrate waveforms of various aspects of light produced by the laser device of FIG. 24,

FIG. 28 is a side elevational view of a tuneable laser device according to another embodiment of the invention,

FIG. 29 is a top plan view of the tuneable laser device of FIG. 28,

FIGS. 30 to 34 illustrate waveforms of various aspects of light produced by the tuneable laser device of FIG. 28,

FIG. 35 is a side elevational view of a tuneable laser device according to another embodiment of the invention,

FIG. 36 is a top plan view of the tuneable laser device of FIG. 35,

FIG. 37 is a top plan view of a tuneable laser device according to a further embodiment of the invention,

FIG. 38 is a top plan view of a tuneable laser device according to a still further embodiment of the invention,

FIG. 39 is a top plan view of a laser device according to another embodiment of the invention,

FIG. 40 is a top plan view of a laser device according to a still further embodiment of the invention,

FIG. 41 is a top plan view of a laser device according to another embodiment of the invention,

FIG. 42 is a top plan view of a laser device according to another embodiment of the invention, and

FIGS. 43 and 44 illustrate waveforms for varying depths of slots in a laser device according to the invention.

Referring to the drawings and initially to FIGS. 1 to 4, there is illustrated a laser device according to the invention, indicated generally by the reference numeral 1, for generating light and in which lasing of the light is independent of the reflectivity or otherwise of end facets of the device 1. In this case the laser device is provided as an optical resonator in the form of a ridge waveguide 2. The ridge waveguide 2 comprises an active region 4 located between an upper cladding layer 5 which is doped to be p-type, and a lower cladding layer 6 which is doped to be n-type. The active region 4 is provided by an active layer 7 which comprises a gain medium of relatively high refractive index, which may be provided in the form of one or more quantum wells or quantum dots.
An elongated ridge 8 is formed in the upper cladding layer 5 and extends longitudinally along the waveguide 2 and defines a longitudinally extending light guiding region 9 within which light is generated and guided. The width W of the ridge 8 is selected to define the lateral width w_lof the light guiding region 9 in the active layer 7. In this embodiment of the invention the lateral width w_lof the light guiding region 9 is selected to restrict the guided modes of the light to one mode. The mode is guided in the transverse direction, namely, in the Y-direction by the layered construction of the waveguide 2 and the relatively high refractive index of the active layer 7. Additionally, the ridge 8 is dimensioned to allow upper and lower portions 13 and 14 of the light guiding region 9 to extend above and below the active layer 7 into the upper and lower cladding layers 5 and 6 with the width w₁of the light guiding region 9 in the lateral X-direction being greater than its height h₁in the transverse Y-direction. In this case the light guiding region extends into the ridge 8.
An upper electrically conductive layer 10 is formed on the ridge 8, and a lower electrically conductive layer 12 is formed on the lower cladding layer 6 for facilitating pumping current into the active layer 7 through the upper and lower cladding layers 5 and 6.
A plurality of reflecting means, which in this embodiment of the invention are formed by respective lateral reflection causing slots 15 formed in and extending laterally of the ridge 8. The lateral slots 15 alter the refractive index of the light guiding region 9 for partially reflecting the guided light mode in the light guiding region 9, and are equi-spaced apart a distance d₁, centre-to-centre. In this embodiment of the invention the lateral slots 15 are of length l in the longitudinal Z-direction parallel to the ridge 8 and of width Win the lateral X-direction similar to the width W of the transverse ridge 8. The lateral slots 15 extend downwardly into the ridge 8 to a depth d₂, which in this embodiment of the invention is similar to the depth D of the ridge 8, and extend into the portion of the wave guiding region 9 which extends into the ridge 8. However, the lateral slots 15 do not extend into the active layer 7, and are spaced apart therefrom by a depth d₃of the upper cladding layer 5 between the active layer 7 and the ridge 8.
In this embodiment of the invention the lateral slots 15 extend into the light guiding region 9 such that there is a significant overlap of the fundamental mode intensity with each lateral slot 15 sufficient to cause a relatively large perturbation of the mode at the lateral slot 15, such that a relatively large reflectivity, 2% or greater is obtained at each lateral slot 15. Preferably, each lateral slot 15 produces a reflectivity in the range of 2% and 20%, and ideally a reflectivity of 10% approximately. The number of lateral slots 15 and their respective reflectivities is selected so that the combined reflectivity of the lateral slots is sufficient for lasing independently without the necessity for reflectivity from any other feedback elements, such as the end facets 18 and 19 at the respective opposite ends of the light guiding region 9.
By producing the laser device 1 according to the invention to lase independently of the reflectivity of the ends 18 and 19 of the waveguide 2 irrespective of whether they are reflective end facets or not, the laser device 1 according to the invention lends itself readily to integration with other optical components on a single integrated semiconductor chip.
Accordingly, referring now to FIGS. 5 to 7, there is illustrated a light signal generating device which is also according to the invention and which is indicated generally by the reference numeral 30 for producing light signals. The light signal generating device 30 is suitable for integration on a single semiconductor chip. The light signal generating device 30 comprises a single elongated ridge waveguide 31 which is formed in one piece of semiconductor material on a single semiconductor chip (not shown). The waveguide 31 comprises an upper cladding layer 33 of p-type material, a lower cladding layer 34 of n-type material, and an active layer 35 located between the upper and lower cladding layers 33 and 34. The upper and lower cladding layers 33 and 34, and the active layer 35 are similar to the upper and lower cladding layers 5 and 6 and the active layer 7 of the laser device 1 described with reference to FIGS. 1 to 4. A longitudinally extending ridge 36 formed in the upper cladding layer 33 extends the length of the waveguide 31 and defines a light guiding region 39 which is similar to the light guiding region 9 of the laser device 1. Electrically conductive layers 37 and 38 are provided on the ridge 36 and on the lower cladding layer 34, respectively, for facilitating pumping of the device 30 as will be described below.
In this embodiment of the invention an optical resonator 40 within which light is generated is formed in the waveguide 31, and an optical modulator 41 is also formed in the waveguide 31 which communicates with the optical resonator 40 and modulates the light produced by the optical resonator 40 for producing light signals for use, for example, in the transmission of data by a telecommunications system. For convenience the optical modulator 41 is illustrated in block representation only. A first portion of the light guiding region 39 forms a first light guiding region 44 of the optical resonator 40, and a second portion of the light guiding region 39 forms a second light guiding region 45 of the optical modulator 41. A portion 51 of the waveguide 31 between the optical resonator 40 and the optical modulator 41 forms a light guiding region 52 for guiding light from the optical resonator 40 to the optical modulator 41. In order to minimise absorption of light in the optical modulator 41 and in the portion 51 of the waveguide 31, the portions of the laser device 30 forming the optical modulator 41 and the portion 51 are treated to increase the bandgap of the optical modulator 41 and the portion 51. Any suitable treatment to increase the bandgap may be used, for example, intermixing, resulting from subjecting the portions of the waveguide forming the optical modulator 41 and the portion 51 to a relatively high temperature for a relatively short duration. Such treatments will be well known to those skilled in the art.
The ridge 36 is dimensioned for defining the light guiding region 39 so that the light guiding region 39 is substantially similar to the light guiding region 9 of the laser device 1 with upper and lower portions 47 and 48 of the light guiding region 39 extending above and below the active layer 35 into the upper cladding layer 33 and the lower cladding layer 34, respectively. The upper portion 47 of the light guiding region 39 which extends into the upper cladding layer 33 also extends into the ridge 36.
A plurality of equi-spaced apart lateral reflecting slots 49 of length l extend into and laterally of the ridge 36 to a depth d₂substantially similar to the depth D of the ridge 36, in similar fashion as the lateral slots 15 extend into the ridge 8 of the laser device 1. The number of lateral slots 49, their combined reflectivity and the gain of the active layer 35 are selected as described with reference to the laser device 1 so that pumping of the optical resonator 40 with a suitable pumping current results in lasing in the optical resonator 40 between the outermost lateral slots 49 a and 49 b. Light lasing in the first light guiding region 44 is guided through the light guiding region 52 into the second light guiding region 45 of the modulator 41, where the light is modulated and transmitted through an end 50 of the light guiding region 39.
The electrically conductive layer 37 on the ridge 36 is severed at 53 in order that the optical resonator 40 can be pumped and the optical modulator 41 can be operated independently of each other. The optical resonator 40 is pumped by applying a current through the electrically conductive layers 37 and 38 adjacent the resonator 40, and the light is modulated by applying the appropriate electrical signals to the optical modulator 41 as will be well known to those skilled in the art.
In this embodiment of the invention the outermost lateral slots 49 a and 49 b define the optical resonator 40 and the length of the first light guiding region 44 within which lasing occurs to produce the light which is communicated into the second light guiding region 45 of the modulator 41.
Referring now to FIGS. 8 to 10, there is illustrated a light signal generating device according to another embodiment of the invention, indicated generally by the reference numeral 60, which is also integrated on a single semiconductor chip (not shown) as a single integral unit. In this embodiment of the invention the light signal generating device 60 is substantially similar to the light signal generating device 30, and similar components are identified by the same reference numerals. However, in this embodiment of the invention the light guiding region 39 as well as defining the first and second light guiding regions 44 and 45 which form the optical resonator 40 and the optical modulator 41, respectively, and the light guiding region 52 of the portion 51, also forms a third light guiding region 61 which forms part of a light detector 62, for detecting light produced by the optical resonator 40. In this embodiment of the invention the three light guiding regions 44, 45, 52 and 61 are integrally formed by the light guiding region 39, and light produced by the optical resonator 40 is communicated into the second and third light guiding regions 45 and 61 from the first light guiding region 44. An electrical isolating slot 63 is formed in the ridge 36 between the light detector 62 and the optical resonator 40, so that the optical resonator 40 can be pumped, and electrical signals read from the light detector 62 independently of each other.
Referring now to FIGS. 11 to 13, there is illustrated a light signal generating device according to a further embodiment of the invention, indicated generally by the reference numeral 70, for producing light signals of selectable wavelength. The light signal generating device 70 is substantially similar to the light signal generating device 30, and similar components are identified by the same reference numerals. However, in this embodiment of the invention instead of providing an optical resonator 40 in the waveguide 31, a tuneable laser device 72 is provided for producing light of selectable wavelength. In this case the first light guiding region 44 forms the light guiding region of the tuneable laser device 72. Additionally, a light detector 73, which is similar to the light detector 62 of the light signal generating device 60, is also formed in the waveguide 31. In this embodiment of the invention the ridge 36 defines the light guiding region 39, which forms the first light guiding region 44 for the tuneable laser device 72, the second light guiding region 45 of the optical modulator 41, the light guiding region 52 of the portion 51, and a third light guiding region 74 of the light detector 73.
The tuneable laser device 72 is a three-section tuneable laser device 72 comprising a central gain section 75 located between left and right feedback sections 76 and 77. The first light guiding region 44 forms the light guiding region for the gain section 75 and the first and second feedback sections 76 and 77. A plurality of first and second lateral slots 78 and 79 are formed in the first and second feedback sections 76 and 77, respectively. The first and second lateral slots are substantially similar to the lateral slots 15 of the laser device 1 and extend the width and almost the depth of the ridge 36 to form respective first and second optical resonators 80 and 81 within which lasing occurs when the first and second optical resonators 80 and 81 and the gain section 75 are pumped, or when the gain section 75 is pumped. The reflectivity of each of the first slots 78 is sufficient such that lasing occurs in the first feedback section 76 independently without the necessity for reflectivity from any other feedback element such as the opposite ends 82 and 83 of the light guiding region 39. Similarly the reflectivity of each of the second slots 78 is sufficient such that lasing occurs in the second feedback section 77 independently without the necessity for reflectivity from any other feedback element such as the opposite ends 82 and 83 of the light guiding region 39.
In this embodiment of the invention the first lateral slots 78 are equi-spaced apart from each other a distance d_l, centre to centre, and the second lateral slots 79 are equi-spaced apart from each other a distance d_rcentre to centre. However, the spacing between the first lateral slots 78 is different to the spacing between the second lateral slots 79 so that the respective mirror loss spectra produced by the first and second optical resonators 80 and 81 are different. The first mirror loss spectrum which is produced by the first optical resonator 80 and the second mirror loss spectrum which is produced by the second optical resonator 81 are such that only one of the minimum peak values of each first and second mirror loss spectrum occurs at the same wavelength value.
Varying the refractive index of the portion of the first light guiding region 44 which forms the first optical resonator 80 relative to the refractive index of the portion of the first light guiding region 44 which forms the second optical resonator 81, results in variation of the wavelength at which the two minimum peak values of the respective first and second mirror loss spectra coincide, thereby facilitating tuning of the laser device 72 to produce light of the selectable wavelengths.
In this embodiment of the invention the first and second feedback sections 76 and 77 may be active or passive. If the feedback sections 76 and 77 are active, then all three sections, namely, the gain section 75 and the two feedback sections 76 and 77 are pumped independently of each other with respective pumping currents. The pumping currents for pumping the first and second feedback sections 76 and 77 are variable independently of each other. By varying the currents with which the first and second feedback sections 76 and 77 are being pumped, the refractive indices of the light guiding regions of the respective first and second feedback sections 76 and 77 are varied, thereby varying the minimum peak values of the respective first and second mirror loss spectra produced by the first and second resonators 80 and 81, for in turn varying the wavelength at which two minimum peak values, one from each of the first and second mirror loss spectra occur for in turn varying the wavelength of the light produced by the tuneable laser device 70. This is described in more detail below.
Where the first and second feedback sections 76 and 77 are passive sections, only the gain section 75 is pumped with a pumping current, and respective tuning currents, which are variable independently of each other are injected into the first and second feedback sections 76 and 77 for varying the refractive indices of the light guiding regions of the first and second feedback sections 76 and 77 for in turn varying the wavelength at which the minimum peak values of the first and second mirror loss spectra produced by the first and second resonators 80 and 81 occur, for in turn varying the wavelength of light produced by the tuneable laser device 72.
Accordingly, tuning of the tuneable laser device 72 is carried out either by independently varying the pumping currents with which the first and second feedback sections 76 and 77 are pumped when the first and second feedback sections 76 and 77 are active sections, or by independently varying the tuning currents injected into the first and second feedback sections 76 and 77 when the first and second feedback sections 76 and 77 are passive sections. This type of tuning is effectively a Vernier type tuning.
In all cases the pumping currents with which the gain section 75 and the first and second feedback sections 76 and 77 are pumped are independent of each other, and the pumping currents with which the first and second feedback sections 76 and 77 are pumped are variable independently of each other. Similarly, the pumping current with which the gain section 75 is pumped and the tuning currents with which the first and second feedback sections 76 and 77 are pumped are independent of each other, and the tuning currents with which the first and second feedback sections 76 and 77 are injected are variable independently of each other.
Whether the first and second feedback sections are operated as active or passive sections is determined based on optimising between sensitivity with which the wavelength of light can be produced, and the losses at each first and second slot. Each first and second slot produces a loss, and by pumping the first and second feedback sections 76 and 77, the losses at the first and second slots 78 and 79 can be compensated for. However, by only pumping the gain section 75, and using only tuneable currents in the first and second feedback sections 76 and 77, tuning of the laser device 70 is more sensitive than when all three sections 75, 76 and 77 are pumped.
Tuning of the tuneable laser device 72 may be discontinuous whereby the wavelength of the light produced by the tuneable laser device 72 is varied in steps or hops, or continuous whereby the wavelength of the light produced by the tuneable laser is progressively varied from the lowest wavelength of the range over which the tuneable laser device 72 is tuneable to the highest of the wavelengths of the tuneable range, or vice versa. By holding the pumping current or the tuning current, as the case may be, of one of the first and second feedback sections 76 and 77 constant, while varying the pumping current or the tuning current, as the case may be, of the other of the first and second feedback sections 76 and 77, tuning is discontinuous. By simultaneously and appropriately varying the pumping currents or the tuning currents, as the case may be, of the first and second feedback sections 76 and 77, tuning of the tuneable laser 72 is continuous.
Electrical isolating means provided by laterally extending left and right isolating slots 84 and 85 are provided in the ridge 36 between the gain section 75 and the first and second feedback sections 76 and 77, respectively, so that the gain section can be pumped independently of the first and second feedback sections 76 and 77, and so that the pumping currents or the tuning currents can be applied to the first and second feedback sections 76 and 77 independently of each other and independently of the gain section 75.
By virtue of the fact that the light of the selectable wavelength is produced by the laser device 72 independently of reflection from end facets, the tuneable laser device 72 can be formed with the optical modulator 41 and the light detector 73 or with any other optical components as one single integral unit on a single semiconductor ship.
FIGS. 14 and 15 illustrate typical first and second mirror loss spectra produced by the tuneable laser device 72 which produces light within a selectable wavelength range from approximately 1,500 nm to 1,600 nm with a centre wavelength of 1,550 nm approximately. In FIGS. 14 and 15 mirror loss in cm⁻¹is plotted on the Y-axis against wavelength which is plotted on the X-axis in microns. The waveform A of FIG. 14 represents the first mirror loss spectrum produced by the first feedback section 76, while the waveform B of FIG. 15 represents the second mirror loss spectrum produced by the second feedback section 77, when one of the minimum peak values of each first and second mirror loss spectrum coincide at the centre wavelength of 1,550 nm. FIG. 16 illustrates a plot of power reflectivity against wavelength produced by the first and second feedback sections 76 and 77 of the tuneable laser device 72 superimposed one upon the other, where it can be seen that the peak values of the power reflectivities of the first and second feedback sections 76 and 77 coincide at the centre wavelength of 1,550 nm.
By varying the current with which the first and second feedback sections 76 and 77 are being pumped, the wavelength at which the two peak values of the power reflectivities of the respective first and second feedback sections 76 and 77 occur is varied. By holding the pumping current of one of the first and second feedback sections 76 and 77 constant while varying the current with which the other of the first and second feedback sections 76 and 77 are being pumped results in the wavelength of the light being varied in hops or steps. However, by appropriately and simultaneously varying the pumping currents with which the first and second feedback sections 76 and 77 are being pumped can be made to result in progressive variation of the wavelength of the light produced by the laser device 72 from the lowest selectable wavelength to the highest selectable wavelength, or vice versa, as discussed above.
Referring now to FIGS. 17 and 18, there is illustrated a light signal generating device 90 according to another embodiment of the invention for producing light signals suitable for use in the transmission of data signals in telecommunications. The light signal generating device 90 is substantially similar to the light signal generating device 70, and similar components are identified by the same reference numerals. The main difference between the light signal generating device 90 and the light signal generating device 70 is that the tuneable laser device 72 is provided as a two-section laser device without the central gain section 75. In this embodiment of the invention the tuneable laser device 72 comprises the first and second optical resonators 80 and 81, which are formed by the first and second feedback sections 76 and 77, and which in this case are active sections and which produce the respective first and second mirror loss spectra similar to those produced by the first and second optical resonators 80 and 81 of the light signal generating device 70, and which are variable by varying the pumping current with which the first and second optical resonators 80 and 81 are pumped. In this embodiment of the invention a laterally extending electrical isolating slot 91 is provided in the ridge 36 between the first and second optical resonators 80 and 81 in order to facilitate independent pumping of the first and second optical resonators 80 and 81. By virtue of the fact that the tuneable laser device 72 according to this embodiment of the invention comprises only the two feedback sections 76 and 77 without the central gain section 75 results in a more easily controlled device, since only two pumping currents are required.
In what follows the terms “first” and “second” are used interchangeably with the terms “left” and “right”, the terms “first” and “left” corresponding, and the terms “second” and “right” corresponding.
The reflectivity and the transmitivity of each lateral reflection causing slot of the laser devices according to the invention is relatively complex. For example, in a laser device according to the invention which is similar to the tuneable laser device 72 of the light signal generating device 70 of FIGS. 11 to 13, where the first (left) and second (right) lateral slots 78 and 79 are provided at spaced apart intervals along the waveguide to provide an overall reflectivity which is independent of any reflectivity of the ends 82 and 83 of the waveguide 31, and thus, obviate the need to include the reflectivity of the ends 82 and 83, whether they are cleaved or otherwise, the reflectivity of the series of the lateral slots, in order to obtain lasing, can be obtained by an optical matrix formalism or equivalent by repeated applications of the Fabry-Perot reflectivity expression, namely:
$r_{r} = r_{LHS} + \frac{t_{LHS}^{} r_{RHS} \exp (-  2 φ)}{1 - r_{LHS} r_{RHS} \exp (-  2 φ)}$
accounting for the reflectivity of each lateral slot in turn.

Where

- r_rrepresents overall reflectivity from a combination of two slots,
- r_LHSrepresents the slot reflectivity as incident from the left-hand side of the slot,
- t_LHSrepresents the slot transmission incident from the left-hand side,
- r_RHSrepresents the slot reflectivity as incident from the right-hand side,
- i represents the imaginary unit, and
- φ represents the complex optical path length between the two slots.

However, it has been noted that if there is a gain in the first and second feedback sections 76 and 77, the feedback reflectivities can have moduli greater than unity.
The following examples demonstrate the efficacy of a number of ridge waveguides which have been formed according to the invention, and compared with ridge waveguides not according to the invention.

EXAMPLE 1

In this first example it is demonstrated that by increasing the number of lateral slots a single electrical contact ridge waveguide laser structure internal reflectance resulting from the lateral slots reduces the role of the reflectivity of the end facets. The epitaxial layer structure used in this example was of similar construction to that of the laser device 1 described with reference to FIGS. 1 to 4, and was of an edge emitting laser design, having an active layer containing five InGaAlAs quantum wells emitting light at a wavelength around 1540 nm in a 540 nm thick waveguide. The ridge was 3.0 microns wide, and the laser was cleaved to a length of 607 microns. Five adjacent lasers on the same bar had in sequence, no slots, two slots, four slots, six slots and eight slots. The positions of the lateral slots were positioned with a spacing of 37 microns at 0.39, 0.45, 0.51, 0.58, 0.64, 0.70, 0.76 and 0.82 of the cavity length. The etch depth of the lateral slots was significantly greater than prior art slotted ridge waveguide lasers which use the slots to provide a perturbation of the Fabry-Perot modes of the cavity formed between the two cleaved mirror ends. In this case each lateral slot penetrated just into the top of the light guiding region, but not into the quantum wells.
The lasers can be considered as three section devices—the long cavity to the bottom (which is 0.51 of the cavity length for the six slotted device, for example), short cavity at the top (1−0.82=0.18 of the cavity length), and the region containing the slots.
The general principles of the invention can be shown using data from the zero, two and six slot devices. As all the laser devices were formed from the same bar, they have the same physical length and the same waveguide effective refractive index, n_g. The wavelength spacing, in nanometres for the Fabry Perot (FP) modes in a cavity of length L is given by
$Δ λ = \frac{λ^{2}}{2 n_{g} L} .$
For the zero slot laser device this spacing was measured to be 1.6 nm, which gives a waveguide effective refractive index of 3.50. The emission spectra below threshold from each facet for the series of lasers was measured from a wavelength of 1470 nm to 1670 nm. FIG. 19 illustrates the emission spectra, which for clarity are displaced vertically, for the four laser devices. In FIG. 19 power in decibels is plotted on the Y-axis, and wavelength in nanometres is plotted on the X-axis. The waveform A represents the emission spectrum for the zero slot laser device. The waveform B represents the emission spectrum for the two slot laser device. The waveform C represents the emission spectrum of the six slot laser device (long end of the laser). The waveform D represents the six slot laser device (short end of the laser device). The emission spectrum A of the zero slot laser device was taken at a pumping current I of 10 mA, the emission spectrum B of the two and six slot laser devices were taken at a pumping current I of 15 mA. The threshold currents for lasing in the three devices are for the zero slot device, a threshold current of 17 mA, for the two slot laser device a threshold current of 21 mA, and for the six slot device, a threshold current of 26 mA.
For devices with zero slots and two slots, the emission spectra were qualitatively the same from each of the two facets of the laser. However, for the four or more slot devices, the emission spectra from each end facet were quite different, as can be seen from the waveforms C and D of FIG. 19. This is an indication that the asymmetries in the laser layout are significant enough to create quite different optical field strengths at each end of the laser device, and each of the main cavities start to act semi-independently. In order to assess the perturbations of the slots on the emission spectra of the waveforms A to D, a Fourier transform (FT) was taken of the respective spectra of the waveforms A, B and C of FIG. 19. In FIG. 20 Fourier amplitude is plotted on the Y-axis against normalised cavity length on the X-axis. The waveform E of FIG. 20 represents the Fourier transform of the emission spectrum A of the zero slot device. The waveform F of FIG. 20 represents the Fourier transform of the emission spectrum B of the two slot device. The waveform G of FIG. 20 represents the emission spectrum of waveform C of the six slot device (long cavity). For a ridge laser with no intra-cavity slots, only the end facet reflectivities define the modal structure, and a series of peaks at the cavity length in the Fourier transform are calculated. In preparing the Fourier transforms of FIG. 20, the spectra-vs-wavelength data was first converted to wavelength-vs-frequency, which was then Fourier transformed to amplitude-vs-time, and the time axis was converted to distance using the waveguide effective refractive index, and normalised to the cavity length. Thus, the Fourier transform spectrum for the zero slot device, namely, the waveform E of FIG. 20 has a peak at “1” corresponding to the facet reflectivity, while the waveform F of FIG. 20 for the two slot device has peaks at 0.76 and 0.82, corresponding to the slots at these physical positions within the cavity. In order to obtain clean peaks at the slots positions, it was necessary to convert the data versus wavelength spectrum to an equally spaced energy axis, in order to have the Fabry-Perot peaks spaced equally apart. For the Fourier transform E of the zero slot laser, a slight change in the Fabry-Perot peak spacing with frequency which was due to the dispersion in refractive index with wavelength was found between 3.49 and 3.51.
For the Fourier transform spectrum of the two slot laser, there is a replica of the peaks at 0.76 and 0.82 to be found at 0.18 and 0.24. This is easily explained, as the short end of the laser also will produce Fabry-Perot oscillations at frequencies corresponding to (1-L) where L is the cavity length. However, for the Fourier transform G of the six slot laser this is found not to be the case. For the six slot laser, peaks in the Fourier transform G occur at 0.51, 0.58, 0.64, 0.70, 0.76 and 0.82, and there are no corresponding 1-L peaks. This is a clear indication that the cavities at each end of the laser are not well coupled, and are semi-independent. This further indicates that there is a mirror separating the end sections. This is confirmed by considering the emission spectra from the each end of the six slot laser, namely, the spectra represented by the waveforms C and D of FIG. 19. These spectra are clearly different and are dominated by the distances between the respective end facets and the slotted region.
Accordingly, from the above it will be clearly appreciated that the mode spacing of the multi-slot devices is not determined by the spacing of the cleaved facets but rather is determined by the slots and their separation and the separation between the slots and the facets. It has thus been clearly shown that the introduction of these slots does not perturb the modes of the Fabry-Perot cavity formed by the cleaved facets but rather form their own cavities.
FIG. 21 illustrates computer simulations of the sub-threshold laser emission spectra of the zero slot, two slot and six slot device. In FIG. 21 power in decibels is plotted on the Y-axis against wavelength, which is plotted in microns on the X-axis. In FIG. 21 the waveform A represents the sub-threshold emission spectrum of the zero slot device, the waveform B represents the sub-threshold laser emission spectrum of the two slot device, the waveform C represents the sub-threshold laser emission spectrum of the six slot device (long end of the laser), and the waveform D represents the sub-threshold laser emission spectrum of the six slot device (short end of laser). The formalism which assumes that the spontaneous emission sources are continuously distributed within the laser cavity of the respective devices was used in the preparation of the spectra of the waveforms A to D of FIG. 21. The feedback effects of the slots are introduced by associating with each ridge-to-slot interface, a scattering matrix for the fundamental mode. This is valid since whilst the slot induces waveguide discontinuities, it does not produce waveguide terminations. Here the scattering matrix elements are treated as fitting parameters although, in principle, a rigorous mode-matching analysis can be used to estimate the proportion of reflected and transmitted power that remains in the fundamental mode and that proportion lost from it. Such an analysis would reveal that whilst losses occur in equal measure both entering and leaving the slot, a significant reflection only occurs on entering the slot. As such, the slot does not form a cavity within itself.
FIG. 22 illustrates Fourier transforms of the emission spectra of waveforms A, B and C of FIG. 21. In FIG. 22 Fourier amplitude is plotted on the Y-axis against normalised cavity length, which is plotted on the X-axis. The waveforms E, F and G are the Fourier transforms which correspond to the emission spectra A, B and C of FIG. 21.
The relationship between the reflectivity and the slot etch-depth is exponential as the slot approaches and enters the light guiding region without actually penetrating the active layer of quantum wells, thereby increasing feedback and maintaining threshold gain values at reasonable levels. Although a comprehensive parameter extraction has not been performed, broad agreement between experiments and simulation can be obtained when a Fresnel reflectivity of r=0.1 is assumed on entering the slot, accompanied by a 40% loss in power scattered to radiation modes.
With these parameters the model captures the essential features of the experimental spectra, namely, highly asymmetric emission with respect to each facet for higher slot number, and a tendency for either end of the laser to behave independently as the number of slots is increased.
When lasing, the four, six and eight slot lasers are single mode with a side-mode-suppression-ratio greater than 30 dB for currents 1.3 I_th<I<3 I_th, where I_this the laser threshold current. The lasing spectrum for each end of the six slotted device is shown in FIG. 23, in which power in dB is plotted on the Y-axis against wavelength, which is plotted on the X-axis in nanometres. The waveform A of FIG. 23 represents the lasing spectrum of the long cavity end, and the waveform B represents the lasing spectrum of the short cavity end. The reason for the single moded behaviour however, is not that of asymmetric cavity lengths, but is predominantly due to the specific effect of the slotted region. The slots have such reflectivity that they form slot-slot cavities, and the regular spacing causes a 10 nm periodicity in the gain spectrum. The mode matching between the long cavity and the inter-slot cavities is as important as the mode matching between the long and short cavity.

EXAMPLE 2

This example demonstrates a laser device formed by a series of etched slots at one end of the laser device with one cleaved facet at the other end. Additionally, this example shows how an integrated power sensor can be easily and efficiently implemented in the device. A ridge type laser device 100 with the configuration indicated in FIG. 24 is discussed. The total length of the device is 1600 μm and is divided into two electrical sections, namely, a front section 110 and a rear section 111 of approximately equal length. The sections 110 and 111 are electrically isolated by a gap 106 in an electrical contact of the two sections 110 and 111. A plurality of etched slots 108 is located near the centre of the device. Light is emitted through a front end facet 109. In order to avoid any influence from a rear end facet 107, the rear end facet 107 is etched to an angle of 45° by focused ion beam etching disabling laser operation in the rear section. This creates a 800 μm long device with a cleaved front facet 109 and a slotted section. Due to the specific device configuration, the slotted section can be pumped independently of the front section 110.
Two modes of operation are distinguished. The front section 110 is pumped and back section 111 is absorbing (mode 1) and the back section 111 pumped with front section 110 absorbing (mode 2). FIG. 25 shows the light-current curve before and after the etching of the 45° spoiling rear end facet 107, the waveform A before etching and the waveform B after etching. The graph clearly indicates that there is virtually no change in the characteristics and that the reflectivity of the slots is sufficient to enable lasing operation, the threshold current being 25 mA.
When the device 100 is drive in mode 2 no lasing occurs verifying that the reflectivity of the rear facet 107 is indeed negligible. If its depth is sufficient, it has been found that even a single slot provides the necessary reflectivity. FIG. 26 illustrates the power-current characteristic and power spectrum for such a single slot device. The amplitude reflectivity of a single slot can be estimated to be γ=0.15 for this particular device.
If the two section device 100 of FIG. 24 is operated with current injected only in the front section 110 so that the front section lases, the rear section 111 can be used as an integrated power sensor. The light emitted internally from the front section 110 through the slot reflector into the rear section 111 causes a photocurrent in the rear section. The waveform A in FIG. 27 shows the detected rear section current as a function of the front-section drive current. No bias voltage is applied to the rear section and so the slot section is not electrically pumped, but it is optically pumped from the front section. The rear section 111 which forms the detector section can also be reverse biased to increase the collected current.
The detected current also depends on whether the slots are in the front section 110 or the rear section 111 of the device. If the slot is placed in the front section 110 of the device 100, but current is collected from the rear section as was done previously, the waveform B in FIG. 27 is obtained. This has a six times lower detected current, which shows that to get greatest sensitivity the slot should be placed in the unpumped section.

EXAMPLE 3

In this example another laser device according to the invention in which the reflecting means is provided by slots in a Fabry-Perot optical cavity is described.
Referring to FIGS. 28 and 29, the laser device according to this embodiment of the invention is illustrated and indicated generally by the reference numeral 120. The device 120 is a three section laser device having a central gain section 128 and left and right feedback sections 127 and 129, respectively. Nine spaced apart lateral reflecting left slots 130 are provided in the left section 127, and nine spaced apart lateral reflecting right slots 131 are provided in the right section 129, to form the respective mirror sections of the left and right sections 127 and 129. End facets 134 and 135 of the left section 127 and the right section 129, respectively, which are cleaved are treated to minimise the reflectivity from the two facets by use of a dielectric anti-reflecting coating. The laser device 120 comprises a lower cladding layer 136, an upper cladding layer 137 and an active layer 138 located between the lower and upper cladding layers 136 and 137. A ridge 140 is formed in and extends longitudinally along the upper cladding layer 137 to define the lateral width of the light guiding region in the active layer 138, and the slots 130 and 131 are formed in the ridge 140. A lower electrically conductive layer 144 is provided on the lower cladding layer 136 and an upper electrically conductive layer 145 is provided on the ridge 140. The laser device 120 according to this embodiment of the invention is substantially similar to the tuneable laser device 72 of the light signal generating device 70 described with reference to FIGS. 11 and 12.
In this case the spacing d₁between the left slots 130 is 97 microns, and the spacing d_rbetween the right slots 131 is 108 microns. In this embodiment of the invention the effective refractive index of the mode is taken as Ng of 3.5. The free spectral range (FSR) for the left mirror is 3.5 nm and the free spectral range for the right mirror is 3.2 nm. The slot length l of the respective left and right slots 130 and 131 is 1 micron, and the amplitude reflection and transmission of a single slot is assumed as 0.1 and 0.8. The left and right slots 130 and 131 are deep etched into the ridge 140 to a depth of 1.3 microns, and extend into the light guiding region. All three sections of the laser device 120, namely, the central gain section 128, and the left and right sections 127 and 129 are active, in other words, all three sections are pumped. Additionally, the three sections, namely, the central gain section 128 and the left and right sections 127 and 129 can be pumped independently of each other. Suitable electrical isolating means are provided between the three sections 127, 128 and 129 by providing suitable gaps (not shown) in the electrically conductive layer on the ridge 140.
The total length of the cavity, in other words, the total length L of the light guiding region which extends between the left and right end facets 134 and 135 is 2,345 microns. FIG. 30 illustrates a plot of output power in decibels on the Y-axis against wavelength in nanometres plotted on the X-axis for the laser device 120. From the waveform of FIG. 30 the longitudinal mode spacing Δλ is determined to be 0.25 nm, giving an effective optical cavity length of 1,250 μm from the equation:
$L_{e} = \frac{λ^{2}}{2 n_{g} Δ λ},$
where λ is the wavelength of the longitudinal mode. This implies that the left and right end facets 134 and 135 do not contribute to the laser operation.
Referring now to FIG. 31, modelled results of the mirror reflection spectra for both the left and right sections 127 and 129 of the laser device 120 are illustrated. In FIG. 31 power reflection is plotted on the Y-axis against wavelength in nanometres, which is plotted on the X-axis. The waveform A in full lines represents the mirror reflection spectrum of the left section 127, while the waveform B in broken lines represents the mirror reflection spectrum of the right section 129. The maximum peak values of the mirror reflection spectra A and B coincide at the wavelength 1,550 nm, thereby causing lasing at this wavelength. By tuning one or both of the left and right sections 127 and 129, by altering one or both of the pumping currents of the left and right sections 127 and 129, the maximum peak values of the respective mirror reflection spectra A and B coincide at different wavelengths, thereby causing lasing to occur at different wavelengths. Thus, by tuning the left and right sections 127 and 129, light of selectable wavelengths is produced by the laser device 120. By holding the pumping current of one of the left and right sections 127 and 129 constant and varying the pumping current of the other of the left and right sections 127 and 129, the wavelengths of the light produced by the laser device 120 is varied in steps or hops. The wavelength of the light produced by the laser device 120 can be progressively altered from the lowest to the highest wavelengths of the tuneable range or vice versa by simultaneously and appropriately altering the pumping current of the left and right sections 127 and 129.
FIG. 32 illustrates a plot of the power reflection on the Y-axis against wavelength in nanometres on the X-axis of the lasing mode and the adjacent modes of the laser device 120. From this it can be seen that there is a large difference in the power of the laser between the lasing mode and the adjacent modes, which provides a large side mode suppression ratio.
FIGS. 33 and 34 show some experimental results which have been obtained from the laser device 120. FIG. 33 illustrates a plot of the side mode suppression ratio plotted on the Y-axis against wavelength in nanometres plotted on the X-axis for the laser device 120, while FIG. 33 illustrates a plot of the output power in dbm plotted on the Y-axis against wavelengths in nanometres plotted on the X-axis of the laser device 120. The change in the wavelength in FIGS. 33 and 34 relative to the wavelengths of FIGS. 30, 31 and 32 is due to the intermixing effect between the quantum wells and the barrier layers in the active layer of the light guiding region. This intermixing results in an increase in the bandgap. FIG. 33 shows that there are eleven accessible modes with side mode suppression ratios of over 30 dbm with longitudinal mode spacings of approximately 3 nm (400 GHz). FIG. 34 shows some of these lasing modes, and as can be seen, good single mode operation can be observed.
Referring now to FIGS. 35 and 36, there is illustrated a laser device 150 according to another embodiment of the invention, which is substantially similar to the laser device 120, and similar components are identified by the same reference numerals. The main difference between the laser device 150 and the laser device 120 is that the reflective effect of the left and right end facets 134 and 135 is cancelled by angling the end facets 134 and 135, instead of by coating the end facets as in the case of the laser device 120. Otherwise, the laser device 150 is similar to the laser device 120.
Referring now to FIG. 37, there is illustrated a laser device 155 according to another embodiment of the invention, which is somewhat similar to the laser device 120, and similar components are identified by the same reference numerals. The main difference between the laser device 155 and the laser device 120 is that the central gain section 128 has been omitted. The left and right sections 127 and 129 are active sections, and are pumped independently of each other. In this case the left and right end facets 134 and 135 are coated with a suitable anti-reflection coating. The operation of the laser device 155 is substantially similar to that of the laser device 120, and tuning of the laser device 155 is likewise substantially similar to that of the laser device 120. The pumping currents with which the left and right sections 127 and 129 are pumped are varied for tuning the laser device 155. Tuning of the laser device 155 may be continuous, whereby the wavelength of the light produced by the laser device 155 is progressive from the lowest to the highest tuneable wavelengths, or vice versa, by simultaneously and appropriately varying the pumping currents of the left and right sections 127 and 129, or may be discontinuous, in other words, the wavelength of the light produced by the laser device 155 may be varied in steps by holding the pumping current of one of the left and right sections 127 constant while varying the pumping current of the other of the left and right sections 127 and 129.
Referring now to FIG. 38, there is illustrated a laser device 160, which is somewhat similar to the laser device 120, and is substantially similar to the laser device 155, and similar components in this case are identified by the same reference numerals as the laser device 120. In this case the central gain section 128 has been omitted, and the laser device 160 comprises left and right sections 127 and 129 only, both of which are active sections. In this case the left and right end facets 134 and 135 are rendered to be non-reflective by angling the respective left and right end facets 134 and 135. Otherwise, operation of the laser device 160 is similar to that of the laser devices 120 and 155, and its tuning is likewise similar.
Two other laser devices according to the invention, indicated generally by the reference numerals 165 and 170, are illustrated in FIGS. 39 and 40. The laser devices 165 and 170 are single section devices, and are suitable for producing light of a single wavelength or multiple wavelengths with specified spacing. In both cases the laser devices 165 and 170 are provided with left and right end facets 134 and 135, which are rendered to be non-reflective, and thus, the laser devices 165 and 170 according to these embodiments of the invention lase independently of the end facets. The left and right end facets 134 and 135 of the laser device 165 are rendered to be non-reflective by a suitable non-reflective coating being provided on the left and right end facets 134 and 135, while in the laser device 170 the left and right end facets 134 and 135 are rendered to be non-reflective by angling the respective left and right end facets 134 and 135.
Referring now to FIGS. 41 and 42, laser devices 175 and 180 according to further embodiments of the invention are illustrated. The laser devices 175 and 180 in these cases are planar buried heterostructural (PBH) devices, and comprise respective buried optical waveguides 176 and 181. In this case the reflecting means instead of being formed by lateral slots in a ridge, the reflecting means are formed by laterally extending slots 177 a and/or 177 b, and 182 a and/or 182 b, which extend laterally of the buried optical waveguides 176 and 181, however, they are located on respective opposite sides of the buried optical waveguide 176 and 181 in the case of the slots 177 b and 182 a and 182 b, and in the case of the slots 177 a, the slots are located above or below the buried optical waveguide 176.
Since the principle of the invention can be applied to the laser devices of buried waveguide construction of FIGS. 40 and 41, the principle of the invention can also be applied to regrown structures such as planar buried heterostructure laser (PBH) devices similar to the laser devices 175 and 180. The width of the high index region in a typical single later mode PBH semiconductor laser is much smaller than for a ridge waveguide structure. This is necessary to restrict the modes, since the optical lateral optical confinement is much less for a PBH laser compared to a ridge waveguide laser. A PBH laser formed with a grating and therefore requires at least two regrowths: one for the grating or lateral slots 177, and another to bury the optical waveguide 176. Using the principle of the invention as described herein, while a regrowth is still required to bury the optical waveguide 176, a regrowth is not required to make a single mode PBH laser, the lasing within which is independent of its respective end facets.
Slotted reflectors 177 can be made in the PBH laser 175 using etched slots that are the width of the buried optical waveguide 176, or greater than the width of the optical waveguide 176 in order to increase the reflectance. Alternatively, referring to FIG. 41, since the optical mode exists substantially outside of the high index region in a PBH laser 180, the slots 182 can also be etched to the side of the high index portion of the optical waveguide 181. In this case it is possible to etch the slots 182 much deeper, since the lateral extent of the active region 181 is confined to the width of the high index region of the optical waveguide 181.
Additionally, where it is desired to control the free spectral range (FSR) of an optical cavity, this can be achieved by using the principle of the invention. Using the principle of the invention it is possible to create optical resonators (which may be lasers), which have an FSR corresponding to a small optical cavity. For example, a 20 micron Fabry-Perot will have a very large FSR. It is not realistic to make 20 micron long semiconductor diode lasers. However, using the principle of the invention, much longer semiconductor lasers can be made which due to the design of the slots have a FSR that is equivalent to that of a 20 micron long cavity.
This can be used to create a cavity whose FSR corresponds to wavelengths or frequencies along the International Telecommunication Union (ITU) grid. This could be used to reduce the amplified spontaneous emission (ASE) from a semiconductor optical amplifier (SOA), while ensuring that the semiconductor optical amplifier operates at the required frequencies.
Referring now to FIG. 43, a plot of reflection amplitude |r| and transmission amplitude |t| against slot depth is illustrated. As can be seen from the waveform A of FIG. 43, which is a plot of reflection amplitude against slot depth, the reflection amplitude increases with slot depth. However, from the waveform B of FIG. 43, which is a plot of transmission amplitude against slot depth, the light transmission amplitude decreases with slot depth, and thus, a trade-off must be made reflection amplitude and transmission amplitude when selecting slot depth of the first and second slots. Additionally, in order to minimise internal losses in the laser devices it is necessary to keep the number of first and second slots to a minimum. This requires a slot pattern which provides just enough optical feedback to ensure that the laser operates in a single longitudinal mode over the wide tuning range of interest.
It is also important that the length of the first and second slots, in other words, the length of the first and second slots in the direction of light propagation is relatively small, typically, less than 3 μm. This is required in order to ensure that the internal loss in the respective first and second light guiding regions is minimised, since the internal losses in the light guiding regions is substantially higher under the first and second slots than elsewhere in the first and second light guiding regions, and also as a result of the fact that the dopant concentration in semiconductor material at the bottom of a slot may be less than one-tenth of the level adjacent the dopant level adjacent the top of the ridge, and thus, it would be difficult if not impossible to create a low resistance metal contact on the ridge adjacent the bottom of the first and second slots. Thus, if the length of the slots in the direction of light propagation is increased arbitrarily, a portion of the waveguide beneath the slot will remain unpumped.
FIG. 44 illustrates a plot of reflection amplitude r against slot length in the direction of light propagation.
While the laser device according to the invention and the embodiments thereof which have been described with reference to FIGS. 1 to 40 of the drawings have been laser devices of ridge type construction, it will be readily apparent to those skilled in the art that laser devices according to the invention can also be produced other than as ridge waveguide type laser devices. For example, curved mirror devices, intermixing, and off-axis laser devices can also be produced in accordance with the invention, as well as the buried waveguide devices of FIGS. 40 and 41.
While the laser devices and laser signal generating devices described with reference to the drawings have been described as being suitable for use in the optical transmission of data signals in telecommunications applications, it will be readily apparent to those skilled in the art that the laser devices and light signal generating devices will be suitable for many other purposes, for example, in the analysis of gases and other substances, such as gas chromatography and the like. Needless to say, the laser devices and the light signal generating devices may be used for many other purposes, which effectively are unlimited.
While the laser devices described with reference to the drawings have been described in general as comprising at least two reflecting means formed by at least two reflecting slots, it is envisaged in certain cases that a single reflecting means, namely, a single slot may be used. In which case, the single reflecting slot would co-operate with one of the end facets of the waveguide, however, lasing would be independent of the other end facet.

Claims

1. A laser device comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.

2. A laser device as claimed in claim 1 in which at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of any of the facets in which the light guiding region terminates, and preferably, the reflecting means are located intermediate the opposite longitudinally spaced apart ends of the light guiding region, and preferably, the reflecting means are located in the waveguide, and advantageously, the amplitude of reflectivity of each reflecting means lies in the range of 2% to 20%, and preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 5% to 15%, and advantageously, the amplitude of the reflectivity of each reflecting means is approximately 10%, and preferably, each reflecting means extends into the light guiding region, and advantageously, the light guiding region comprises an active region, and each reflecting means extends into the light guiding region to a location spaced apart from the active region, and preferably, at least four reflecting means are provided, and preferably, at least six reflecting means are provided, and advantageously, the outer two of the reflecting means define a volume in the light guiding region in which lasing occurs.

3. A laser device as claimed in claim 1 in which each reflecting means comprises a refractive index altering means for altering the refractive index of the light guiding region adjacent the location of the refractive index altering means, and preferably, each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot, and advantageously, each reflecting slot extends substantially laterally of the light guiding region, and preferably, at least some of the reflecting slots are at least partially filled with a reflecting medium, and preferably, the reflecting medium is a metal material, and advantageously, the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region, and preferably, each reflecting means is located in the ridge, and advantageously, each reflecting means extends into the ridge to a depth substantially similar to the depth of the ridge, and preferably, the light guiding region extends into the ridge.

4. A laser device as claimed in claim 1 in which the waveguide comprises an active layer located between an upper cladding layer and a lower cladding layer, the active layer forming the active region, and preferably, the ridge is formed in the upper cladding layer and defines the lateral width of the light guiding region in the active layer, and advantageously, each reflecting means extends into a portion of the light guiding region defined in the upper cladding layer, and terminates at a location therein spaced apart from the active layer, and preferably, the waveguide is in the form of an optical resonator.

5. A laser device as claimed in claim 1 in which the laser device is a tuneable laser device, and the light guiding region defines a first light guiding region and a second light guiding region communicating with the first light guiding region, at least two of the reflecting means being at locations spaced apart longitudinally relative to the first light guiding region to produce a first mirror loss spectrum associated with the first light guiding region with minimum peak values at respective first wavelength values, at least two of the reflecting means at locations spaced apart longitudinally relative to the second light guiding region to produce a second mirror loss spectrum associated with the second light guiding region with minimum peak values at respective second wavelength values, and a refractive index varying means for selectively varying the refractive index of at least the first light guiding region for in turn varying the first mirror loss spectrum until one of the first wavelength values is similar to one of the second wavelength values to produce light of a selected wavelength.

6. A laser device as claimed in claim 5 in which the refractive index varying means comprises a means for injecting a first electrical current into the first light guiding region for altering the refractive index thereof, and preferably, a means is provided for varying the first current for in turn varying the refractive index of the first light guiding region, and advantageously, the refractive index varying means comprises a means for injecting a second electrical current into the second light guiding region for varying the refractive index thereof, and preferably, a means is provided for varying the second current for in turn varying the refractive index of the second light guiding region.

7. A laser device as claimed in claim 5 in which the means for injecting the first and second currents are operable independently of each other for independently varying the refractive indices of the respective first and second light guiding regions, and preferably, an electrical isolating means is provided for electrically isolating the first and second light guiding regions from each other, and advantageously, the light guiding region defines a third light guiding region intermediate the first and second light guiding regions and communicating therewith, the active region extending in the third light guiding region, and the third light guiding region being adapted to be pumped with an electrical current for generating light therein, and preferably, respective electrical isolating means are provided for substantially electrically isolating the third light guiding region from the respective first and second light guiding regions, and advantageously, the active region extends into the first and second light guiding regions, and the first and second light guiding regions are adapted to be pumped with an electrical current, and preferably, the waveguide is a semiconductor laser, and preferably, the waveguide comprises a laser diode for producing light, and advantageously, the waveguide is adapted for receiving a pumping current.

8. A light signal generating device comprising a waveguide defining a laser device as claimed in claim 1 for producing light, and an optical component integrally formed with the laser device, a light guiding region being defined in the waveguide, which forms the light guiding region of the laser device and a light guiding region of the optical component, and preferably, the waveguide comprises an active region located between an upper cladding layer and a lower cladding layer, the active region forming the light guiding region of the laser device and the optical component, and a longitudinally extending ridge being formed in the upper cladding layer defining the lateral width of the light guiding region of at least the laser device, and preferably, the ridge defines the lateral width of the light guiding region of the optical component, and advantageously, an electrical isolating means is provided for substantially electrically isolating the laser device from the optical component, and preferably, the electrical isolating means is formed by an isolating slot extending into the ridge intermediate the laser device and the optical component, and advantageously, the laser device and the optical component are integrally formed in a single piece of material, and preferably, the laser device and the optical component are integrally formed in a single piece of semiconductor material, and advantageously, the optical component is selected from one or more of the following:

an optical modulator,

an optical sensor,

an optical detector,

an optical amplifier,

an optical splitter,

an optical interferometer and

an optical multiplexer.

9. A light signal generating device comprising an elongated waveguide formed on a single piece of semiconductor material, an elongated longitudinally extending light guiding region being defined in the waveguide, the light guiding region defining a first light guiding region of a light generating device and a second light guiding region of an optical component, the second light guiding region communication with the first light guiding region for receiving light generated therein, at least two reflecting means at locations spaced apart longitudinally relative to the first light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light in the first light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.

10. A light signal generating device as claimed in claim 9 in which at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing in the first light guiding region is independent of the reflectivity of any of the facets in which the light guiding region may terminate, and preferably, the respective reflecting means are located in the waveguide adjacent the first light guiding region, and preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 2% to 20%, and advantageously, the amplitude of the reflectivity of each reflecting means lies in the range of 5% to 15%, and preferably, the amplitude of the reflectivity of each reflecting means is approximately 10%, and advantageously, each reflecting means extends into the first light guiding region, and preferably, the light guiding region comprises an active region, and each reflecting means extends into the first light guiding region to a location spaced apart from the active region, and advantageously, at least four reflecting means are provided, and preferably, the outer two of the reflecting means define a volume in the first light guiding region in which lasing occurs.

11. A light signal generating device as claimed in claim 9 in which each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot, and preferably, each reflecting slot extends substantially laterally of the light guiding region, and advantageously, at least some of the reflecting slots are at least partially filled with a reflecting medium, and preferably, the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region, and preferably, each reflection causing means is located in the ridge, and advantageously, the waveguide comprises an active layer located between an upper cladding layer and a lower cladding layer, the active layer forming the active region, and preferably, the ridge is formed in the upper cladding layer and defines the lateral width of the wave guiding region in the active layer, and each reflecting means extends into a portion of the light guiding region defined in the upper cladding layer, and terminates at a location therein spaced apart from the active layer, and advantageously, the optical component is selected from one or more of the following:

an optical modulator,

an optical sensor,

an optical detector,

an optical amplifier,

an optical splitter,

an optical interferometer and

an optical multiplexer.

12. A light signal generating device as claimed in claim 9 in which the optical component is an optical modulator, and the waveguide adjacent the second light guiding region is adapted for receiving a control voltage signal for modulating light, and preferably, the optical component is an optical detector, and the waveguide adjacent the second light guiding region is adapted for producing an emf across the second light guiding region in response to detecting light, and advantageously, an electrical isolating means for substantially electrically isolating the first light guiding region and the second light guiding region from each other is provided, and preferably, the waveguide adjacent the first light guiding region defines a laser diode for producing the light, and preferably, the first and second light guiding regions defined in the waveguide are integrally formed on a semiconductor chip.

13. An optical resonator comprising a waveguide, a longitudinally extending light guiding region defined within the waveguide, at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region and at least one of the reflecting means being located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facets in which the light guiding region may terminate.

14. An optical resonator as claimed in claim 13 in which at least two reflecting means are provided intermediate the longitudinally spaced apart end of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of any of the facets in which the light guiding region terminates, and preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 2% to 20%, and preferably, the amplitude of the reflectivity of each reflecting means lies in the range of 5% to 15%, and advantageously, the amplitude of the reflectivity of each reflecting means is approximately 10%, and preferably, each reflecting means extends into the light guiding region, and preferably, the light guiding region comprises an active region, and each reflecting means extends into the light guiding region to a location spaced apart from the active region, and advantageously, the outer two of the reflecting means define a volume in the light guiding region in which lasing occurs.

15. An optical resonator as claimed in claim 13 in which each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot, and preferably, each reflecting slot extends substantially laterally of the light guiding region, and advantageously, at least some of the reflecting slots are at least partially filled with a reflecting medium, and preferably, the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region, and each reflection causing means is located in the ridge.

16. A method for producing light in a waveguide of the type having a longitudinally extending light guiding region defined therein, the method comprising providing at least two reflecting means at locations spaced apart longitudinally relative to the light guiding region, and at least one of the reflecting means is located intermediate longitudinally spaced apart ends of the waveguide for partially reflecting light being guided in the light guiding region, the amplitude of the reflectivity of each reflecting means being at least 2%, and the amplitude of the combined reflectivity of the respective reflecting means being such that lasing is independent of the reflectivity of at least one of any facet in which the light guiding region may terminate.

17. A method as claimed in claim 16 in which at least two reflecting means are provided intermediate the longitudinally spaced apart ends of the waveguide, and the amplitude of the combined reflectivity of the respective reflecting means is such that lasing is independent of the reflectivity of any of the facets in which the light guiding region terminates.

18. A method as claimed in claim 16 in which the amplitude of the reflectivity of each reflecting means lies in the range 2% to 20%, and preferably, the amplitude of the reflectivity of each reflecting means lies in the range 5% to 15%, and advantageously, the amplitude of the reflectivity of each reflecting means is approximately 10%, and preferably, each reflecting means extends into the light guiding region, and advantageously, the light guiding region comprises an active region, and each reflecting means extends into the light guiding region to a location spaced apart from the active region, and advantageously, the outer two of the reflecting means define a volume in the light guiding region in which lasing occurs, and preferably, each reflecting means is formed by a slot located in the waveguide for providing the partial reflection of the light in the light guiding region adjacent the slot, and advantageously, each reflecting slot extends substantially laterally of the light guiding region, and preferably, at least some of the reflecting slots are at least partially filled with a reflecting medium, and advantageously, the waveguide is provided by a ridge waveguide having an elongated ridge extending longitudinally along the waveguide, the ridge defining the light guiding region, and each reflection causing means is located in the ridge.

19. A method as claimed in claim 16 in which the waveguide is of a semiconductor laser, and preferably, the waveguide is adapted for receiving a pumping current.

20. A laser device as claimed in claim 7 in which the first and second light guiding regions are passive regions, and alternatively, the first and second light guiding regions are active regions, and preferably, the laser device is of a buried waveguide structure.