US20080212635A1

US20080212635A1 - High efficiency distributed feedback (dfb) laser with low-duty cycle grating

Info

Publication number: US20080212635A1
Application number: US12/036,809
Authority: US
Inventors: Manoj Kanskar; Yiping He; Steve H. Macomber
Original assignee: Alfalight Inc
Current assignee: Alfalight Inc
Priority date: 2007-02-23
Filing date: 2008-02-25
Publication date: 2008-09-04
Also published as: WO2008103188A1; US20080205476A1; US7586970B2

Abstract

The invention provides a grating for a distributed feedback laser having decreased diffraction loss with reduced +/−1 order diffraction and scattering loss resulting from the reduced imperfections in the grating fabrication. In various embodiments, the grating has a low duty cycle wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5. Further, in some preferred embodiments, the invention includes a laser, the laser comprising a distributed feedback laser wherein the laser includes a grating having less diffraction and less scattering loss. In various exemplary embodiments, the grating is further a partial grating, thereby providing increased efficiency resulting from a decrease in first-order diffraction loss due to the grating being separated from the front and rear facets and in some exemplary embodiments being situated at the area of lowest electric filed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application seeks priority from U.S. provisional application 60/891,396 filed on Feb. 23, 2007, which is incorporated herein by reference in its entirety, for all purposes.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor diode lasers and particularly to edge-emitting distributed feedback (DFB) semiconductor lasers.

BACKGROUND OF THE INVENTION

Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. Semiconductor lasers may be constructed to be either edge-emitting or surface-emitting. In edge-emitting lasers, the layers adjacent to the active layer typically have a higher bandgap to confine the electrons. Then a lower index of refraction material (cladding layer) is required to sandwich the active layer and the adjacent layers to form waveguide to confine the optical field.
In an edge-emitting Fabry-Perot type semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge-emitting or surface-emitting, utilizes distributed feedback structures rather than conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index (grating) or the gain or both of the semiconductor laser structure.
High power diode lasers have been extensively used for pumping high power solid-state lasers such as the thin disk, slab, rod, micro-chip and fiber lasers that are useful for industrial, printing, medical applications and scientific instrumentation. There are also emerging alkali-vapor gas lasers that are pumped by semiconductor lasers. Multimode 975 nm diode lasers are of particular interest for pumping the upper transition states of rare-earth doped (such as Yb, Er and Yb/Er co-doped) solid state lasers, fiber lasers and amplifiers. At this pump wavelength, the quantum defect is minimal and the absorption cross-section is much higher (2.5 dB/m) relative to the 920 nm transition states (0.7 dB/m). Hence, shorter gain fibers may be used to mitigate deleterious nonlinear effects such as the Stimulated Raman Scattering (SRS) and the Stimulated Brillouin Scattering (SBS) that can occur in high average or peak power application. However, the absorption bandwidth at 975 nm is quite narrow (<9 nm full width half maximum (FWHM)). Similarly, the absorption peak is narrow for other solid-state host materials with Yb, Er or Yb/Er co-doping. Nd-doped solid-state gain media such as Nd:YAG also has narrow absorption cross-section near 808 nm and 885 nm absorption bands. As a result, either expensive thermal stabilization measures or very sensitive external wavelength-locking methods such as use of diffraction gratings, Fiber Bragg Gratings (FBG) or Volume Bragg Gratings (VBG) in an external cavity configuration have to be employed making diode lasers less attractive as pump sources for these applications to pump at the narrow gain band region. A monolithic integration of Bragg grating inside the semiconductor laser cavity is a simpler and a more cost-effective means of achieving both the wavelength stabilization as well as emission linewidth narrowing making multimode DFB laser an attractive pump source for the aforementioned precision-pumping applications.
Semiconductor lasers having continuous wave (CW) output power in the several waft-range and narrow bandwidth, e.g., less than 3 Å. FWHM, would be desirable for a variety of applications. Conventional, FP broad stripe (≧25 μm) semiconductor lasers used for obtaining high powers typically have a spectral width of about 20 Å FWHM or more at high drive levels and broaden further under quasi-CW operation. Since the lasing wavelength in FP diode laser is determined by the peak wavelength of the gain spectrum, the center of the lasing wavelength shifts as a function of temperature. This temperature tuning rate is approximately 0.32 nm per Centigrade. Significant improvements in spectral width and temperature tuning rate can be obtained using distributed feedback (DFB) gratings as previously reported (M. Kanskar, et al, Electron. Lett. Vol. 42, p. 1455, 2006), or distributed Bragg reflectors (DBR) rather than FP mirror facets for optical feedback. A CW power of about 278 mW with about 1 Å of wavelength variation, resulting from mode hopping, has been reported for narrow-stripe DBR lasers. (J. S. Major, et al., Electron. Lett. Vol. 29, No. 24, p. 2121, 1993). Using DFB phase-locked laser arrays, narrow bandwidth operation has been obtained from large apertures at relatively long wavelengths (λ=1.3 μm to 1.5 μm). Pulsed operation at a power level of 120 mW has been reported from a 45 μm aperture device (λ=1.3 μm), (Y. Twu, et al., Electron. Lett. Vol. 24, No. 12, p. 1144, 1988), and 85 mW CW from a 72 μm aperture device (λ=1.55 μm), (K. Y. Liou, et al., Tech. Dig. 13th IEEE Int. Semicond. Laser Conf., Paper D7, 1992). For applications where (lateral) spatial coherence is not necessary, a broad-stripe laser with a DFB grating is apparently well suited for achieving high CW powers with narrow spectral linewidth and more robust temperature tuning characteristics.
For diode lasers operating in the near infrared spectral region, it is simple and cost-effective to fabricate a second-order grating since the grating pitch is typically submicron in length. However, when a second-order distributed feedback laser is fabricated there is an additional optical power loss incurred compared to a FP or the first-order DFB laser. This problem arises because the second-order grating has first-order diffractions (e.g., there are +/−1 diffraction orders) that scatters light out in directions that are normal to the propagation direction of the fundamental mode. As a result, the differential quantum efficiency (DQE) is usually lower than that for the FP or the first-order DFB laser making the power-conversion efficiency of a second-order DFB laser poorer.
It is known that the magnitude of the first-order diffraction loss in DFB lasers can also be minimized by reducing the index contrast of the grating and/or by placing the grating far away from the peak of the transverse optical intensity, as discussed in U.S. Pat. No. 6,455,341 by Macomber. As disclosed by Macomber, the first-order diffraction loss is minimized by introducing low-index contrast which leads to lower scattering strength for the grating thereby reducing scattering loss. Additionally, by locating the grating where the transverse optical intensity is lower, the fraction of the diffracted light is reduced. This technique pertains to minimization of the transverse optical field only and does not address the issue of continual diffraction loss which occurs during propagation along the longitudinal direction as the laser light oscillates back and forth numerous times inside the DFB laser cavity.
Currently, the most straightforward method to overcome diffraction loss from the laser cavity is to introduce a first-order grating in the laser cavity. (See, for example, (e.g. H.-P Gauggel, et al., Electron. Lett. Vol. 31, No. 5, p. 367, 1995). As a result, there are no possible diffraction orders that could lead to radiation loss from the cavity modes. However, making a first-order grating can be impractical and expensive, especially for short wavelength radiation. In order to overcome this practical problem, a second-order grating that is distributed over the entire gain volume (e.g. laser cavity) is used to both stabilize and narrow the emission bandwidth of a laser. While this method stabilizes the wavelength and narrows the emission bandwidth, the second-order grating distributed across the entire laser cavity leads to continuous first-order diffraction loss of radiation out of the cavity as the laser modes oscillate back and forth inside the resonator (or cavity). Monolithic distributed Bragg reflector (DBR) lasers have also been used in the past to stabilize and lock the wavelength, such as those available from, for example, TOPTICA PHOTONICS (Westfield, Mass.). This technique has an additional disadvantage. The DBR section is not electrically pumped; hence, the gain section underneath the DBR acts as a saturable absorber, reducing the overall efficiency of the laser. Additionally, the use of a saturable absorber can also lead to a deleterious effect known as self-pulsation.
In DFB lasers, periodic variations of the refractive index are fabricated in a way that is overlapped with the confined optical field of the laser cavity, e.g., a grating is fabricated inside a laser cavity. It is usually a two-step epitaxial growth process. The first growth comprises epitaxy of the bottom cladding layer, active layer, its adjacent layers, and a thin grating layer. Grating patterns can be generated using holographically exposed photoresist patterning or other methods and subsequent transfer of these grating patterns onto the underlying grating layer using an etching technique, for example wet-chemical or dry reactive-ion etching techniques. The remaining top cladding layer is epitaxially grown on the fabricated grating layer, a process called regrowth. There are usually additional optical power losses incurred compared to an FP laser. The grating is usually designed to utilize second-order diffraction as feedback. As discussed above, when second-order DFB laser is fabricated, there are one and minus-one (+/−1) diffraction orders that diffract light out in directions that are normal to the propagation direction of the fundamental mode. The +/−1 diffractions represent another loss in second-order DFB laser. As a result, the differential quantum efficiency (DQE) is usually lower than for the FP or the first-order DFB laser making the power-conversion efficiency (PCE) of a second-order DFB laser poorer. Additionally, factors including: the shape of the grating, i.e. a relative length of the low-index portion of the grating layer with respect to the pitch of the grating; the quality of regrown layer; and imperfections in the grating fabrication process, all lead to additional scattering loss, which is determined by the spatial frequencies introduced by the size of the imperfections. These diffused scattering losses apply not only to the second-order grating but to any order grating. The ability to provide a laser and, in particular, a second-order DFB laser, that suffers less from these shortcomings would allow for the advantages of a second-order DFB, and provide the laser with a high PCE.
It would be desirable therefore, to provide a grating, and in particular, a second-order grating that reduced first-order diffraction losses. In addition, reduction in the scattering loss due to imperfections inherent in the grating fabrication process would further improve the efficiency of the grating. Incorporation of these benefits in a distributed feedback laser, and in particular a second-order distributed feedback laser would provide would provide greater lasing efficiency than heretofore available.

SUMMARY OF THE INVENTION

Disclosed is a grating for a semiconductor diode laser wherein the relative length of the low-index portion grating part with the pitch of the grating having a low duty factor results in a laser in which the diffraction loss is suppressed and the imperfections in the grating leads to less scattering loss of the light waves. The laser includes a low duty cycle grating housed in the laser structure that is overlapped with confined optical field. The low duty cycle grating reduced the one and minus-one diffraction losses and maximizes the useful feedback from second-order diffraction. In addition, the inventors have found that by utilizing a low duty cycle grating, high quality materials can be regrown on the grating layer, thus, the scattering loss at the grating is significantly reduced while providing sufficient feedback to lock the wavelength and narrow the emission spectral bandwidth.
Therefore, in one exemplary embodiment, the invention comprises an edge-emitting semiconductor laser including a diffraction grating having ±1 diffraction orders, a relative length of the low refractive index portion of the grating with the pitch of the grating having a low duty factor. In various exemplary embodiments, the invention comprises an edge-emitting semiconductor laser including second-order grating. The grating has a relative length of the low-index portion to the pitch of the grating comprising a low duty factor. In this embodiment, the standing wave amplitude is approximately aligned to the grating via longitudinal mode hopping resulting in lower diffraction loss and a consequent increase in energy efficiency. While the grating, according to the invention, may be any order, in various exemplary embodiments, the grating is a second-order diffraction grating.
In various other exemplary embodiments, the invention provides a grating for a laser wherein the ratio of the length of the low-index portion (‘a’, see, FIG. 1B) to the length of the pitch of the grating (‘b’, see, FIG. 1B) is less than 0.5. In some exemplary embodiments, the fabrication of the low duty cycle grating is accomplished by regrowth of high-index material on a low-index grating. It should be appreciated that the grating can be fabricated using any grating material known in the art such as, for example, GaAs, AlGaAs, InGaP, InGaAsP, AlInGaP, InP, GaSb, InGaSb, GaN or InGaN. However, any material usable for a grating according to the invention is contemplated, depending on the emission wavelength of the laser.
Further, in various other exemplary embodiments, the invention comprises an edge emitting distributed feedback semiconductor laser having a front facet and a rear facet which together define a laser cavity and having therein a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5. In various exemplary embodiments according to the invention, the laser is a distributed feedback laser. In various other exemplary embodiments the invention includes a second-order distributed feedback laser. In some exemplary embodiments according to the invention, the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.49. In still other exemplary embodiments, the ratio of the low-index portion to the pitch of the grating is between about 0.15 to about 0.45.
In still other exemplary embodiments, the invention comprises a semiconductor laser having a front facet and a rear facet which together define the laser cavity and a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5 and wherein the grating is a partial grating. The partial grating is separated from the front facet and the rear facet. In some exemplary embodiments, the grating has a ratio of the low-index portion ‘a’ to the pitch of the grating of from about 0.15 to about 0.45. In other exemplary embodiments, the partial grating has a length that is less than about 75% of the length of the laser cavity. In various exemplary embodiments the laser according to the invention has a grating having a ratio of the low-index portion ‘a’ to the pitch of the grating is from about 0.15 to about 0.45, 0.25 to about 0.40 and a partial grating that is 75% or less than 50% the length of the laser cavity. In some exemplary embodiments, the grating is positioned within the laser cavity such that it is separated from the front and the back facet. In various exemplary embodiments, the grating is situated in the laser cavity at the area of lowest electrical intensity.
In yet another exemplary embodiment of the invention, the invention includes a method for fabricating a semiconductor laser having a laser cavity defined by a front facet and a rear facet. The method includes fabricating a grating within the laser cavity wherein the ratio of the length of the low-index portion ‘a’ to the pitch of the grating ‘b’ is less than 0.5. In some exemplary embodiments, the method includes fabricating the grating wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.5. In various embodiments, the method includes fabrication completed by regrowth of high-index material on low-index grating. In various embodiments of the invention, the method further includes fabricating a grating having a ratio of the length of the low-index portion ‘a’ to the pitch of the grating ‘b’ is less than 0.5 and wherein the grating is a partial grating. In some exemplary embodiments the partial grating is situated in the laser cavity such that it is separated from the front facet and the rear facet. In various other exemplary embodiments, the grating is situated in the laser cavity at the area of the lowest electric field.
In yet other exemplary embodiments, the invention comprises a composite laser having more than one semiconductor laser, wherein at least one semiconductor laser includes a front facet, a rear facet and a grating having a ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5. In various exemplary embodiments according to the invention, the composite laser may further include at least one semiconductor laser having a partial grating. In various embodiments, the partial grating is positioned in the laser cavity such that it is separated from both the front facet and the back facet. In various other embodiments, the grating is situated in the cavity at about the area of the lowest electric field.
Other objects, features and advantages of the present invention will become apparent after review of the specification, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing of a conventional distributed feedback (DFB) semiconductor laser with second-order grating, located at a distance, ‘d’ from the peak of the optical intensity in a vertical direction. The arrows indicate the directions of the fundamental mode propagation, second-order feedback propagation and the first-order diffraction losses. FIG. 1B is an exploded view of the grating shown in FIG. 1A, illustrating the definition of grating parameters used: ‘a’ illustrates the low-index portion; ‘b’ illustrates the pitch; ‘n₁’ is the refractive index of the low-index grating layer; and ‘n₂’ is the refractive index of the high-index regrown layer. As shown, the ratio of the low-index portion to the pitch (period) of the grating is generally from 0.1 to 0.5.

FIG. 2 is a scanning electron micrograph showing the fabricated low duty cycle grating layer with lower index of refraction before the regrowth is done.

FIG. 3 is a scanning electron micrograph image showing the completed grating structure after the high-index of refraction portion and the cladding layers have been regrown on the grating layer.

FIG. 4 is a scanning electron micrograph image showing the fabricated high duty cycle grating after the top cladding layer had been regrown.

FIG. 5 is a graph showing plots comparing the output CW power (L-I), voltage (V-I), and power conversion efficiency (PCE) with respect to injected electrical current of DFB lasers with low-duty cycle grating (solid lines) and high-duty cycle grating (dotted lines). Arrows indicate the appropriate axes.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

I. In General

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. As well, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. It is also to be noted that the terms “comprising”, “including”, “characterized by” and “having” can be used interchangeably.
Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications and patents specifically mentioned herein are incorporated by reference in their entirety for all purposes including describing and disclosing the chemicals, instruments, statistical analyses and methodologies which are reported in the publications which might be used in connection with the invention. All references cited in this specification are to be taken as indicative of the level of skill in the art. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention.
As used herein, the terms ‘low-index portion’ portion refers to section of the grating where refractive index of length ‘a’ has a lower refractive index relative to the rest of the grating length (b-a) as illustrated in FIG. 1B. As used herein, the terms ‘period’ and ‘pitch’ are used interchangeably and refer to the distance from beginning of the low-index portion to the next beginning of the low-index portion, illustrated as ‘b’ in FIG. 1B. As used herein, “duty cycle” refers to the ratio of the length of the low-index portion ‘a’ of the grating to the pitch (or period) of the grating ‘b’ e.g., a/b.
As used herein, the terms ‘partial’ and ‘abbreviated’ are used interchangeably and mean shortened; reduced in length; abridged. For example, an abbreviated or partial grating means a grating that is shortened to a length such that placement in the laser cavity does not fill the entire cavity but rather results in the ability to place the grating more toward the front or the back of the cavity. The term “proximate” means near to, or nearer to one area than an opposing area. Thus, the term “proximate to the back” means closer to the back than the front.
FIGS. 1A and 1B illustrate some concepts of a conventional distributed feedback laser 40. As shown and used herein, the term laser “cavity” 12 refers to the space between the high reflection coating (HR) 14 and the anti-reflection coating (AR) 16 of the laser. As used herein, the term “laser facet” refers to the facet holding the HR and AR coating. Thus, the back, or rear facet 18 is defined by the HR coating and the front facet 20 is defined by the AR coating. A grating 42 is a dielectric layer with a periodic perturbation of the refractive index so that sufficient reflectivity may be reached at a wavelength due to Bragg scattering which provides optical feedback for the lasing to be established. Gratings are constructed to reflect only a narrow band of wavelengths and thus, produce a narrow linewidth of laser output.
The invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well known components and processing techniques are omitted so as not to unnecessarily obscure the invention in detail but such descriptions are, nonetheless, included in this disclosure by incorporation by reference of all the citations included herein. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this detailed description.

II. The Invention

The invention provides a grating for a distributed feedback laser having decreased diffraction loss with reduced +/−1 order diffraction. In various embodiments according to the invention, the grating has a low duty cycle wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5. However, in various other exemplary embodiments, the ratio of the length of the low-index portion to the pitch of the grating is from about 0.15 to about 0.45. Further, in still other exemplary embodiments, the invention includes a laser, the laser comprising a distributed feedback laser wherein the laser includes a low duty cycle grating having less first-order diffractions and scattering losses.
In various exemplary embodiments, the invention provides a grating for a laser wherein the ratio of the length of the low-index portion (a, as in FIG. 1B) to the length of the pitch of the grating (b, as in FIG. 1B) is less than 0.5. In some exemplary embodiments, the fabrication of the low duty cycle grating is accomplished by regrowth of high-index material on low-index grating or low-index material on high-index grating. It should be appreciated that the grating can be fabricated using any grating material known in the art such as, for example, GaAs, AlGaAs, InGaP, InGaAsP, AlInGaP, InP, GaSb, InGaSb, GaN or InGaN. However, any material usable for a grating according to the invention is contemplated.
Further, in various other exemplary embodiments, the invention comprises an edge emitting distributed feedback semiconductor laser having a front facet and a rear facet which together define a laser cavity and having therein a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less then 0.5. In still other exemplary embodiments according to the invention, the laser is a distributed feedback laser. In various other exemplary embodiments the invention includes a second order distributed feedback laser. In some exemplary embodiments the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.49. In still other exemplary embodiments, the ratio of the low-index portion to the pitch of the grating is between about 0.15 to about 0.45.
In various other exemplary embodiments, the invention comprises a semiconductor laser having a front facet and a rear facet which together defines the laser cavity and a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less then 0.5 and wherein the grating is a partial grating and separated from the front facet and the rear facet. In some exemplary embodiments, the grating has a ratio of the low-index portion ‘a’ to the pitch of the grating is from about 0.15 to about 0.45. In other exemplary embodiments, the partial grating has a length that is less than about 75% of the length of the laser cavity. In various other exemplary embodiments the laser according to the invention has a grating having a ratio of the low-index portion ‘a’ to the pitch of the grating is from about 0.15 to about 0.45, 0.25 to about 0.40 and a partial grating that is 75% or less than 50% the length of the laser cavity. In still other exemplary embodiments, the grating is positioned within the laser cavity such that it is separated from the front and the back facet. In various exemplary embodiments, the grating is situated in the laser cavity at the area of lowest electrical intensity.
In yet another exemplary embodiment of the invention, the invention includes a method for fabricating a semiconductor laser having a laser cavity defined by a front facet and a rear facet. The method includes fabricating a grating within the laser cavity wherein the ratio of the length of the low-index portion ‘a’ to the pitch of the grating ‘b’ is less than 0.5. In some exemplary versions, the method includes fabricating the grating wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.45. In various embodiments, the method includes fabrication completed by regrowth of high-index material on low-index grating. In various embodiments of the invention, the method further includes fabricating a grating having a ration of the length of the low-index portion ‘a’ to the pitch of the grating ‘b’ is less than 0.5 and wherein the grating is a partial grating. In various exemplary embodiments the partial grating is situated in the laser cavity such that it is separated from the front facet and the rear facet. In various other exemplary embodiments, the grating is situated in the laser cavity at the area of the lowest electric field.
In yet other exemplary embodiments, the invention comprises a composite laser. The composite laser having more than one semiconductor laser, wherein at least one semiconductor laser includes a front facet, a rear facet and a grating having a ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ less than 0.5. In various exemplary embodiments according to the invention, the composite laser may further include at least one semiconductor laser having a partial grating. In various other exemplary embodiments the partial grating is positioned in the laser cavity such that it is separated from both the front facet and the back facet. In various embodiments, the grating is situated in the cavity at about the area of the lowest electric field.
Grating Fabrication
Generally it should be appreciated that conventional gratings for lasers are fabricated by first growing a crystal on a wafer substrate, such as, for example a GaAs, substrate. These grating layers comprise low-index and high-index sections in each pitch (period) of the grating. Such conventional gratings typically have a pitch of
$b = \frac{q λ}{2 n_{eff}},$
where λ wavelength, q is the order the grating (e.g. q=2 for second-order grating, for some exemplary embodiments of the invention) and n_effis the effective refractive index of the waveguide. The ratio of the low-index section of the grating to the pitch of the grating can be anywhere between ≧0 to ≦1.
FIGS. 1A and 1B illustrate some features of a conventional distributed feedback second-order grating with the exception that the grating 42 has a low duty cycle. Low duty cycle, as defined herein, means that the ratio of the low-index portion ‘a’ to the pitch ‘b’ of the grating is less than 0.5. FIG. 1B illustrates such a low duty-cycle grating. FIG. 1A shows a second-order diffraction grating 42 that has first-order diffraction orders (upward and downward arrows indicate the light propagation directions) that lead to continuous radiation loss while the fundamental mode (0^thorder) and the feedback (2^ndorder) beam propagate forward and backward (illustrated by the arrows). This loss occurs along the entire length of the grating 42. The inventors have identified that radiation loss occurs along the entire length of the grating, thus, the inventors hypothesized that one method of limiting this diffraction loss is to limit the length of the grating (e.g. provide a partial grating). See priority document, U.S. provisional application 60/891,396 and U.S. patent application Ser. No. 11/777,913 each of which is incorporated herein by reference in its entirety. The inventors have further hypothesized that one method of limiting this diffraction loss is to fabricate gratings with a duty cycle factor away from 0.5. Thus, according to one exemplary embodiment, the invention includes a laser having a low duty cycle grating housed in the laser cavity that is overlapped with the confined optical field. The low duty cycle grating reduces the first-order diffraction losses and maximizes the useful feedback from second-order diffraction. FIG. 1B, particularly points out the low-index ‘a’ and high-index ‘b-a’ portions of the grating.
FIG. 2 is a scanning electron micrograph showing the fabricated low duty cycle grating layer before the regrowth is done. FIG. 3 is a scanning electron micrograph image showing the completed grating structure after the top cladding layer has been regrown on grating etched on a semiconductor layer.
While conventional gratings have generally been fabricated as described above, the inventors have found that the low duty cycle grating acts to suppress the first-order diffraction losses and improve the regrown material quality thereby reducing the scattering of the light which results from imperfections in the grating fabrication process. By reducing the diffraction and scattering losses, more light is extracted on the output facet thereby increasing the efficiency of the laser.
In contrast, FIG. 4 shows a scanning electron micrograph image of a fabricated high duty cycle grating after the rest of the cladding layer had been regrown. In the high duty cycle grating, the ratio of the low-index portion to the pitch of the grating is greater than 0.5. This grating was fabricated by the same approach as for FIG. 3 with the exception of fabricating the grating with a high duty cycle.

Example 1

Comparison of Power Conversion Efficiency of DFB Lasers with Low Duty Cycle and High Duty Cycle Gratings

FIG. 5 is a graph showing plots comparing the output CW power (L-I), voltage (V-I), and power conversion efficiency (PCE) with respect to injected electrical current of DFB lasers with low-duty cycle gratings (solid lines) and high-duty cycle gratings (dotted lines). (L-I: light power vs. electrical current curve; V-I: voltage vs. electrical current; PCE: PCE vs. electrical current curves). As shown, in each instance for the low duty cycle grating, the power conversion efficiency is significantly greater than for the high duty cycle, the result of less diffraction and scattering losses in the laser cavity. Similarly, the constant wave (CW) power output is significantly higher for the low duty cycle DFB laser compared to that of the high duty cycle DFB laser.
The inventors initially fabricated a grating with a duty cycle away from 0.5 to test the hypothesis that the +/−1 diffractions loss would be limited. During the process of fabricating of the low-duty cycle grating, the inventors surprisingly found that the quality of the regrown material (low-index portion) is greater with a low duty cycle grating. Thus, because the quality is higher there are less imperfections in the grating and the scattering is also less.
Thus, by using a grating having a low duty cycle the overall power conversion efficiency is increased and the CW power is higher at all current levels and continues to increase up to the maximum current, 7 Amperes, tested.
Further, as discussed above, and more fully described in U.S. patent application Ser. No. 11/777,913 hereby incorporated by reference in its entirety, the inventors have previously identified that by using a partial grating in a distributed feedback laser, the amount of diffraction loss along the grating is limited. Therefore, it should be appreciated that by providing a laser having both a partial grating and a low duty cycle the overall efficiency of the grating would be substantially increased.
Therefore, the inventors have shown that by fabricating the grating with low duty cycle factor, increased electrical-to-optical power conversion efficiency, wavelength stabilization and narrowed linewidth emission is achieved. The optical power of the first-order diffractions are proportional to the Fourier transformation of the grating shape. Unlike high duty cycle gratings, low duty cycle gratings help improve the quality of regrown material, leading to less scattering loss. Therefore, as shown, a higher output power and a higher electrical-to-optical power conversion efficiency in the laser was achieved. Thus, in various embodiments according to the invention, the invention provides a grating and/or a laser housing a grating in which the grating has a low duty cycle and thus, a higher output power and a higher electrical-to optical power conversion efficiency than heretofore achieved in distributed feedback lasers.
In addition, as discussed above, the inventors have also shown that abbreviating the grating allows for greater ability of the light waves to be in sympathy with the peaks (low-index) and valleys (high-index) of the grating. Therefore, the DFB laser should not be very sensitive to the random facet-grating phase problem (e.g., conventional lasers are fabricated with a random cleave of the grating with respect to the facet) which leads to variable spectral output. Therefore, in various other exemplary embodiments according to the invention, the invention comprises a grating and/or a laser housing the grating wherein the grating has a low duty factor as well as being abbreviated or partial. In these various embodiments, the diffraction loss due to the grating having a low duty cycle is reduced providing less scattering loss and therefore a higher output power and a higher electrical-to-optical power conversion efficiency in the laser as well as increased efficiency due to synchronization of the standing waves over the partial grating due to longitudinal mode hopping.
While this invention has been described in conjunction with the various exemplary embodiments outlined above, various alternatives, modifications, variations, improvements and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the exemplary embodiments according to this invention as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention. Therefore, the invention is intended to embrace all known or later developed alternatives, modifications, variations improvements, and/or substantial equivalents of these exemplary embodiments.

Claims

1. A grating for a laser wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5.

2. The grating of claim 1, wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.49.

3. The grating of claim 1, wherein the fabrication is completed by regrowth of high-index material on low-index grating layer.

4. The grating of claim 1, wherein the fabrication is completed by regrowth of low-index material on a high-index grating or high index material on a low index grating.

5. The grating of claim 1, wherein the grating has a high-index layer of GaAs, AlGaAs, InGaP, InGaAsP, AlInGaP, InP, GaSb, InGaSb, GaN or InGaN.

6. An edge emitting distributed feedback semiconductor laser comprising:

a front facet,

a rear facet, which defines, together with the front facet, a laser cavity;

a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less then 0.5.

7. The laser of claim 6, wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.49.

8. The laser of claim 7, wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is between from about 0.15 to about 0.45.

9. The laser of claim 6, wherein the grating has a low-index layer of InGaP and a regrown layer of AlGaAs.

10. A semiconductor laser comprising:

a front facet,

a rear facet, which defines, together with the front facet, a laser cavity;

a grating positioned within the laser cavity, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is less then 0.5; and

wherein the grating is a partial grating and separated from the front facet and the rear facet.

11. The laser of claim 8, wherein the grating has a ratio of the low-index portion of the grating ‘a’ to the pitch of the grating is from about 0.15 to about 0.45.

12. The laser of claim 10, wherein the partial grating has a length that is less than about 75% of the length of the laser cavity.

13. The laser of claim 10, wherein the partial grating is positioned in the laser cavity separated from both the front facet and the back facet.

14. The laser of claim 10, wherein the partial grating is situated proximate to the back facet.

15. The laser of claim 10, wherein the grating is placed in the laser cavity at the area of the lowest electric field.

16. The laser of claim 10, wherein the edge emitting semiconductor laser is a distributed feedback laser.

17. A method for fabricating a semiconductor laser having a laser cavity defined by a front facet and a rear facet, the laser comprising:

fabricating a grating within the laser cavity wherein the ratio of the length of the low-index portion of the grating ‘a’ to the pitch of the grating ‘b’ is less than 0.5.

18. The method of claim 17, wherein the ratio of the length of the low-index portion of the grating ‘a’ to the length of the pitch of the grating ‘b’ is from about 0.1 to about 0.5.

19. The method of claim 18, wherein the fabrication is completed by regrowth of high-index material on a low-index grating.

20. The method of claim 18, wherein the fabrication is completed by regrowth of low-index material on a high-index grating.

21. The method of claim 18 wherein fabrication of the grating further comprises fabricating a partial grating that is less then the length of the laser cavity.

22. A composite laser having more than one semiconductor laser, wherein at least one semiconductor laser comprises:

a front facet,

a rear facet, which defines, together with the front facet, a laser cavity;

a grating wherein the ratio of the length of the low-index portion ‘a’ to the length of the pitch of the grating ‘b’ is less than 0.5.

23. The composite laser of claim 22, wherein at least one semiconductor laser further has a grating that is a partial grating.

24. The composite laser of claim 23, wherein the partial grating is positioned within the laser cavity separated from the front facet and the back facet.

25. The composite laser of claim 24, wherein the partial grating is positioned within the cavity proximate to the back facet.