WO2001048882A1

WO2001048882A1 - Laser source wavelength multiplexer

Info

Publication number: WO2001048882A1
Application number: PCT/FR2000/003644
Authority: WO
Inventors: Christian Larat; Jean-Pierre Huignard
Original assignee: Thales
Priority date: 1999-12-23
Filing date: 2000-12-21
Publication date: 2001-07-05
Also published as: FR2803116A1; AU2857001A; EP1155484A1; FR2803116B1

Abstract

The invention concerns a wavelength multiplexer comprising: a plurality of laser sources (S1 to Sn, 1.1 to 1.n) emitting beams at different wavelengths; a first diffraction grating (3) forming a cavity mirror for the laser diodes. Said grating comprises an assembly of juxtaposed sub-gratings with different pitches. Each sub-grating receives one or several laser beams, retroreflects part of the beam(s) and transmits the other part of the beam in an output direction (U), said direction being the same for all the sub-beams; a focusing optics which is located in the output direction (U) (lens 5), and which focuses into a focal point the beams emitted by the laser sources; a second diffraction grating (6) which is located at said focal point, receives said emitted beams. The angles of incidence of said beams on the diffraction grating are such that the latter superimposes the beams after diffraction.

Description

WAVELENGTH MULTIPLEXER OF LASER SOURCES

The invention relates to a device for wavelength multiplexing of a plurality of laser sources and in particular of beams emitted by an array of laser diodes.

The power delivered by a laser diode being limited (typically less than 10 W), it is conventional to use monolithic assemblies of diodes (called arrays of laser diodes) making it possible to deliver powers up to typically 100 W These arrays have the following characteristics: disadvantage of delivering an optical beam of very poor quality, the elementary diodes emitting side by side Generally 1 cm wide in the direction parallel to the plane of the junction (D _// ), the emissive surface of the bars is about 1 μm high in the perpendicular direction (Dj _. ) A bar is thus a highly asymmetrical source, being approximately 10 000 times wider than high Similarly, the divergence of the radiation is non-symmetrical greater than 25 ° (30 ° to 50 °) according to D _± , it is approximately 10 ° according to D // The combination of these two characteristics leads to a geometric extent approximately 2000 times greater according to D _// than selo n D _± This high asymmetry is completely detrimental to the effective use of these components for many applications. In fact, in addition to the difficulty of handling, it is generally advantageous to have a beam of geometric extent close to the symmetry of revolution, for example, for injection into an optical fiber or for the longitudinal optical pumping of solid lasers

Superimposing different laser beams is possible by wavelength multiplexing, each laser emits at a different wavelength from the others and the beams are superimposed using dichroic mirrors or dispersive elements (networks, prisms) [CC Cook and TY Fan, in Advanced Solid State Laser 99, OSA Technical Digest, paper PD7, MC Famés et al, Electron Lett, vol 27, pp 1498-1499 (15 08 91), IH White et al, Electron Lett, vol 26, pp 832-834 (21/6/90)] The invention therefore relates to a wavelength multiplexer of laser sources, characterized in that it comprises

A plurality of laser sources emitting beams at different wavelengths, A first diffraction grating forming a cavity mirror for the laser diodes, this grating comprising a set of juxtaposed sub-grids of different pitches, each sub-grating receiving one or more of said laser beams, retroreflecting part of the beam or beams and transmitting the other part of the beam in an exit direction, this direction being the same for all the sub-beams;

• a focusing optic located on the output direction, focusing at a focal point the beams emitted by the laser sources;

• a second diffraction grating located at said focal point and receiving said emitted beams, the angles of incidence of these beams on the diffraction grating being such that the latter superimposes the beams after diffraction.

The various objects and characteristics of the invention will appear more clearly in the description which follows, given by way of example of embodiment, as well as in the appended figures which represent: FIGS. 1a to 1c, an example of a detailed embodiment of the multiplexer of the invention; Figures 2a and 2b, an alternative embodiment of the invention. Figures 1 a to 1 c show an embodiment of the multiplexer according to the invention.

Figures 1a and 1b show a device for the emission of n laser beams of different wavelengths.

Figure 1a shows n laser diodes 1.1 to 1.n. Preferably, these n diodes are in fact a strip of laser diodes 1. The beams emitted by these diodes are collimated by a first cylindrical lens 21, then focused by a second cylindrical lens 22 on a diffraction grating 3 located substantially at the focusing plane. of the lens 22. The axes of the cylindrical lenses 21, 22 are orthogonal to the long length of the diode array and form an afocal system.

The diffraction grating 3 is in reality a juxtaposition of diffraction sub-grids 3.1 to 3.n having different pitches so as to be effective at different wavelengths. The wavelength of each of the n laser diodes 1.1 to 1.n of the strip 1 is imposed by making an external cavity closed by a network 3 placed in the Littrow position, the plane of incidence of which is perpendicular to the plane. of the junction of the laser diodes. This orientation of the network is chosen to ensure good selection in wavelength, which would not necessarily be the case for an orientation parallel to the plane of the junction due to the non-monomode spatial emission in this direction of the power diodes. . Order 0 of the network corresponds to the output beams 4.1 to 4.n of the cavity and order -1 to the beam reinjected into the cavity. To avoid the phenomenon of sub-cavity which could compromise the efficiency of the wavelength selection, it is preferable to anti-reflective treat the output face of the laser diodes. The angle of incidence of the beams on the grating and the pitch of the grating fix the value of the emission wavelength of each laser diode. It is necessary for the beam to be collimated in the plane of incidence of the grating, for example by putting a third cylindrical lens 23 between the strip 1 and the grating 3. This third cylindrical lens 23 is orthogonal to the lenses 21, 22. This is visible in Figure 1b which shows a bottom view of the system of Figure 1a. Finally, each laser diode should light only one sub-array. This is achieved by afocal mounting of the two cylindrical lenses 21 -22 which image the laser diodes on the array 3. Thus, at the output of the array 3, the n beams coming from the diodes each have a different wavelength and are parallel between them. They are spaced by a distance dx which corresponds to the distance between diodes in the strip multiplied by the magnification of the lenses 21-22.

A preferential example consists in having the beams 4.1 to 4.n parallel to each other and spatially distributed as a function of their wavelengths. The distance dx between two beams, 4.1 and 4.2 for example, is such that: dx = f.dλ.D

• f being the focal distance of the lens 5;

• dλ, the difference in wavelengths of beams 4.1 and 4.2;

• D, the diffraction power of the diffraction grating 6.

The n beams 4.1 to 4.n are then superimposed locally by a lens 5 on a diffraction grating 6. The focal point of the lens 5 is placed on network 3. Network 6 is placed in the Fourrier plane (image focus). The lens 5 can be cylindrical or spherical. The network 6 is a network jaded to have a preferential direction at the output 7. For the beams 4n to be spatially superimposed after the network 6, their wavelength λ _{n must} be chosen judiciously.

By this method, the beams of the N laser diodes making up a strip are therefore spatially superimposed.

As a first approximation, the wavelength spacing dλ _n between two elementary beams is independent of n (this is verified as long as the incidences on the lens 5 remain low). We therefore have: dλ _n = λ _n - λ _n -ι = dλ which is constant. This value is chosen to be greater than the spectral width of the laser diodes operating in an external cavity. Typically, we will take dλ = 0.5 at 1 nm. Under these conditions, for network 3, the variation (da) of the step (a) of a sub-network to the adjacent sub-network is given by:

da _ dλ a ^" λ

For example with λ = 800 nm, dλ = 1 nm and a = 0.8 μm (network with 1/1200 lines / mm) we obtain da = 1 nm. To produce n gratings whose pitch varies from close to close to 0.5 nm, the invention proposes, for example, to use a photosensitive polymer to produce the gratings 3 in the form of a thick hologram, either by using a length d 'fixed writing wave and by varying the incidences of the beams according to the location on the network 3; either by varying the write wavelength for each sub-network.

The second network 6 has a fixed number of lines, which corresponds to a dispersing power D (which also depends on the working angle chosen). In the case of the optical assembly presented in FIG. 1 c, the relationship between the spatial spacing dx and spectral dλ between sub-beams is easily obtained: dx = f.dλ.D where f is the focal length of the lens 5. For example , with dx = 0.2 mm, dλ = 1 nm and D = 1200 mm ^' Vcos 45 °, we obtain f = 120 mm. Without departing from the scope of the invention, it is also provided, for example, that:

• Other combinations of lenses and other network effects are used. • In-plane imaging of the diode junction on the network

3 is not compulsory, it is sufficient that the beams are separated at the level of the network 3. This is carried out over a distance of the order of a millimeter at the outlet of the bar (spacing of 0.2 mm, divergence of 10 °). We can thus remove the lenses 21 and 22 as we can see in Figures 2a and 2b.

• At the output of network 6, the beam can be used directly or focused in an optical fiber.

• We can take advantage of the external cavity to not only fix the wavelength of each laser diode but also operate a spatial filtering of the lateral mode to reduce the number of transverse modes emitted by each laser diode.

• Diffraction grids 3 and 6 can operate in transmission or in reflection.

Claims

1. Multiplexer in wavelengths of laser sources, characterized in that it comprises:

• a plurality of laser sources (S1 to Sn, 1.1 to 1.n) emitting beams at different wavelengths; • a first diffraction grating (3) forming a cavity mirror for the laser diodes, this grating comprising a set of juxtaposed sub-grids of different pitches, each sub-grating receiving one or more of said laser beams, retroreflecting part of the beams and transmitting the other part of the beam in an exit direction (U), this direction being the same for all the sub-beams;

• a focusing optic located on the output direction (U) (lens 5), focusing at a focal point the beams emitted by the laser sources;

• a second diffraction grating (6) located at said focal point and receiving said emitted beams, the angles of incidence of these beams on the diffraction grating being such that the latter superimposes the beams after diffraction.

2. Multiplexer according to claim 1, characterized in that before focusing by the focusing optics (5), the beams are parallel and the distances between the axes of the beams are calculated and distributed to obtain the superposition of the beams after diffraction by the diffraction grating.

3. A multiplexer according to claim 2, characterized in that the distance dx between two beams is given by the formula: dx = f.dλ.D in which:

• f being the focal length of the focusing optics (5); • dλ, the difference in wavelengths of the beams;

• D is the dispersing power of the second diffraction grating (6).

4. Multiplexer according to claim 1, characterized in that the two diffraction gratings (3, 6) operate in reflection or in transmission.

5. Multiplexer according to claim 2, characterized in that it comprises, between the laser diodes and the first diffraction grating (3), a first cylindrical lens (21) collimating the laser beams and a second cylindrical lens (22) located between the first cylindrical lens and the first diffraction grating, and focusing the collimated beams on the plane of the first diffraction grating and imaging the emissive faces of the laser diodes on this first grating, the axes of the cylindrical lenses (21, 22) being orthogonal to the line of laser diodes.

6. Multiplexer according to claim 5, characterized in that it comprises a third cylindrical lens (23), orthogonal to the first and to the second lenses, located between the laser diodes and the second diffraction grating (3) and collimating the beams emitted by laser diodes.