WO2000024093A1 - Planar waveguide laser amplifier - Google Patents

Abstract

The invention concerns a power optical amplifier with planar waveguide optically pumped and a power laser using said amplifier. The amplifier planar waveguide comprises layers (14, 16, 18) forming a multimode guide with large numerical aperture for the pump light (4) and a monomode guide (16) for the light to be amplified (2). The latter is amplified in said guide by the optical pumping.

Description

 PLANAR WAVEGUIDE LASER AMPLIFIER

DESCRIPTION

TECHNICAL AREA

The present invention relates to an optical power amplifier with an optically pumped planar waveguide and to a power laser using this amplifier. The invention finds applications in particular in the following fields: laser marking and micro-marking, long distance telemetry, telecommunications, in particular space telecommunications (between satellites), laser display, scientific instrumentation and biomedical instrumentation.

The invention is advantageously usable in a structure of the “master oscillator power amplifier” type, more simply called MOPA.

This structure uses a low power laser source (master oscillator) capable of emitting a laser beam of high optical quality, as well as an optical power amplifier which is capable of providing the power necessary for the amplification of this beam. , while deteriorating the original qualities as little as possible.

In the optical amplifier, a laser medium, doped with optically active ions, is pumped optically for example by power laser diodes to amplify the incident laser beam. It is a traveling wave amplifier more simply called TWA. Indeed, the amplification is carried out during the crossing, by the laser beam, of the amplifying laser medium.

Such an optical amplifier is therefore not a laser. It would become so by adding a resonant cavity. However, an optical amplifier - in particular that of the invention - must not include one: it is necessary to prevent such an amplifier from becoming a laser in order to have good gain.

PRIOR STATE OF THE ART

Massive optical amplifiers (“bulk”) in the form of a rod (“rod”) or in the form of a plate (“slab”) are known. These optical amplifiers can amplify a high power but with a low gain. In addition, their lengths are of the order of several centimeters or even several tens of centimeters. They are therefore not compact.

Such amplifiers are for example considered in document [4] which, like the other documents cited below, is mentioned at the end of this description.

There are also known optical amplifiers in the form of a thin plate (“thin slab”). However, these amplifiers do not make it possible to guide an incident laser beam in a single mode so that the quality of the output beam is poor. As a result, for example, the focus such an output beam over a very small area is difficult. In particular, such a beam is almost unusable for precision engraving.

Such amplifiers are mentioned in document [8].

Fiber optic amplifiers are also known which have a high gain but a low power. In addition, such amplifiers are not compact because they use a large length of optical fiber.

Such amplifiers are mentioned in document [5].

There are also known optical amplifiers with planar waveguide for telecommunications (at 1.3 μm or 1.55 μm). Such amplifiers are pumped by optical fibers, longitudinally, so that they are ill-suited to applications requiring power laser beams. Such amplifiers are mentioned in documents [3] and [6].

There is also known an optical power amplifier with planar waveguide which is compatible with micro-lasers emitting at 1.064 μm. The layers of this planar guide are formed by liquid phase epitaxy on a substrate.

Reference is made to document [1] where this amplifier is described.

It is specified that it is possible to form the epitaxial layers of the planar guide from the teaching of document [2] to which we will refer. It should be noted that this document [2] discloses the use of epitaxial layers to form compact laser cavities. There is no question of optical amplification in this document [2].

If we consider Figure 2 of document [1] we see that the optical pumping of the amplifier described in this document is carried out longitudinally. This requires a quasi-alignment and even a quasi-superposition of the pump beam and the beam to be amplified. However, as each of these has a size of a few tens of micrometers, such a technique is very difficult to implement and to make reliable.

In addition, a power amplification requires a pumping technique having a good coupling efficiency and making it possible to bring high energies to the amplifying medium. However, the amplifier described in document [1] is not compact and does not make it possible to reduce the optical losses (since this amplifier requires too many optics). In addition, it does not make it possible to obtain an optimal coupling efficiency. Indeed, for this amplifier the optical pumping is obtained thanks to a power laser diode whose emission has an elliptical configuration and which emits a laser beam which is not limited by diffraction.

In addition, the amplifier described in this document [1] does not make it possible, from an incident beam of good optical quality, to provide an amplified beam of high power which retains this good optical quality. STATEMENT OF THE INVENTION

The present invention relates to an optical amplifier with planar waveguide for amplifying a beam emitted by a laser source, for example a micro-laser, by achieving optimal coupling of the pump light, for example emitted by a laser diode or a plurality (for example a strip) of laser diodes, while preserving the optical qualities of the beam coming from the laser source after amplification of this beam.

Specifically, the present invention firstly relates to an optical amplifier intended to amplify a first light, by optical pumping by means of a second light, this amplifier comprising a substrate and, on this substrate, a waveguide planar optic comprising a plurality of layers, this amplifier being characterized in that this plurality of layers forms

a multimode optical waveguide with large digital aperture for the second light, this multimode waveguide being able to accept the major part of this second light and to guide it, and

a single-mode optical waveguide for the first light, this single-mode waveguide being able to guide the first light and to amplify the latter thanks to the optical pumping generated by the second light.

The amplifier which is the subject of the invention is capable of being compact and comprises a waveguide with a plurality of layers, allowing efficient coupling (preferably transverse coupling) of a multimode pump light beam which is not limited by diffraction and which for example comes from one or more arrays of power laser diodes, while allowing a single mode amplification of a pulsed or continuous laser beam which crosses this waveguide.

The planar waveguide used in the invention is compatible with a high pump power and with good quality signals which it is desired to amplify. In addition, a collective manufacturing process can be used to manufacture, in a large number of copies, an amplifier according to the invention.

According to a first particular embodiment of the amplifier which is the subject of the invention, the planar waveguide comprises:

a first layer formed on the substrate and containing activating ions for the amplification of the first light, and a second layer formed on the first layer, the first layer forming the single-mode guide for the first light and all of the first and second layer forming the multimode guide with large digital aperture for the second light, the optical index of the first layer being greater than that of the substrate, the optical index of the second layer being less than that of the first layer and greater than that of the substrate.

According to a second particular embodiment, the planar waveguide comprises:

- a first layer formed on the substrate, a second layer formed on the first layer and containing activating ions for the amplification of the first light, and

a third layer formed on the second layer, the second layer forming the single-mode guide for the first light and all of the first, second and third layers forming the multimode guide with large digital aperture for the second light, the optical index of the second layer being greater than that of the substrate and the optical index of each of the first and third layers being less than that of the second layer and greater than that of the substrate.

In either of these two particular embodiments, the amplifier may further comprise a protective layer formed on said assembly and having an optical index less than or equal to that of the substrate.

Preferably, the planar guide has the shape of a rectangular parallelepiped and has four polished faces.

In this case, according to a preferred embodiment, the lateral face of the planar waveguide, the face which is opposite to that by which the second light penetrates, is covered with a layer which is highly or totally reflective with respect to this second light.

Preferably, the layer which is highly or totally reflective with respect to the second light is also anti-reflection with respect to the first light and the lateral face of the planar waveguide, which face is opposite to the one through which the first light penetrates is covered with an anti-reflection layer vis-à-vis the first light. As a variant, instead of using such layers, it is possible to ensure that the lateral faces of the planar waveguide, faces which are opposite to those through which the first and second lumens penetrate respectively, are approximately 1 °.

According to a particular embodiment of the invention, the planar guide in the shape of a rectangular parallelepiped is further bevelled at the same angle at two opposite lateral edges and thus comprises two additional opposite lateral faces which are respectively intended to receive the first light and providing this first light in amplified form in a multiple pass configuration. According to a preferred embodiment of the invention, the layers of the planar guide are formed by epitaxy in the liquid phase.

The present invention also relates to a power laser comprising: the optical amplifier which is the subject of the invention,

- a laser source intended to supply the first light to this amplifier and

- a pumping light source intended to supply the second light to the amplifier. Preferably, the pumping light source is arranged so as to allow optical pumping in the amplifier.

This pumping light source comprises for example at least one power laser diode. For certain applications of the invention, it may be necessary to use a laser diode capable of emitting in the near infrared. The laser source can, for its part, comprise at least one micro-laser, for example an array of micro-lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which: • Figures 1 and 2 are sectional views schematic cross section of two particular embodiments of the amplifier object of the invention, with the corresponding optical index profiles, • FIG. 3 is a schematic view of a power laser using an amplifier according to the invention,

FIG. 4 is a schematic top view of an amplifier according to the invention, the planar guide of which comprises two bevelled side faces, and

• Figure 5 is a schematic top view of another amplifier according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the case of each of FIGS. 1 to 4, transverse pumping is used for the amplification of a light beam by means of an amplifier according to the invention. The direction of propagation of the or light beams allowing this optical pumping is then perpendicular to the direction of propagation of the beam that we want to amplify.

The present invention can also be used in the case of longitudinal pumping but transverse pumping is preferable because it is simpler to implement.

However, the overlap of the pump beam (s) and of the beam which it is desired to amplify is less favorable in the case of transverse pumping than in the case of longitudinal pumping.

This makes it necessary to use, both for the pump beam and for the beam to be amplified, a planar guide having low optical losses. To form the stack of layers of this planar guide which comprises an amplifier according to the invention, liquid phase epitaxy is used which is a very suitable technique (although other techniques are also suitable) for making a stack adequate. Indeed, by this liquid phase epitaxy technique, layers of good optical quality are obtained, comprising few volume and surface defects and therefore having low optical losses for propagation. The good uniformity allowed by this technique, on a substrate whose diameter can go up to 5 cm or even more, as well as its flexibility of use, as regards the choice of the optical parameters (doping and therefore optical indices) and geometric parameters (growth and cutting), allow an optimized and collective realization of optical amplifiers in accordance with the invention. The amplifier which is schematically represented in cross section in FIG. 1 is intended to amplify a laser beam 2 (which can be pulsed or continuous) by transverse optical pumping by means of another laser beam 4.

FIG. 1 is thus a cross-sectional view perpendicular to the axis of the beam to be amplified 2.

The amplifier of FIG. 1 comprises a monocrystalline substrate 6 and, on this substrate, a planar optical waveguide 8. This planar guide has the shape of a rectangular parallelepiped and includes four polished side faces. Two opposite side faces are visible in FIG. 1. where they have the references 10 and 12.

In addition, this planar guide comprises three monocrystalline layers 14, 16 and 18 epitaxially grown on the substrate, preferably by liquid phase epitaxy.

For the production of this planar guide one can use the teaching of the document [2] already cited.

The layer 14 is formed on the substrate 6. The layer 16 is formed on this layer 14 and contains activating ions for the amplification of the beam 2 and the layer 18 is formed on this layer 16. All three layers 14, 16, 18 forms a multimode optical waveguide with large digital aperture for the pumping beam 4 (which has a given wavelength).

This multimode guide is able to accept the major part of this pumping beam and to guide it.

The second layer 16 forms a single-mode optical waveguide for the beam to be amplified 2 (which has a given wavelength). This single-mode guide is capable of guiding this beam 2 and of amplifying the latter thanks to the optical pumping generated by the pumping beam 4.

The third layer 18, or upper layer, can be left in the open air but, in the case of FIG. 1, it is covered by a protective layer 20.

We also see in this figure 1 the variations of the optical index n as a function of the thickness e which is counted going from the protective layer 20 to the substrate 6.

The optical indices of the substrate 6, of the layer 14, of the layer 16, of the layer 18 and of the layer 20 are respectively denoted ni, n2, n3, n4 and n5.

We see that n3 is greater than ni and that n2 and n4 are both slightly less than n3 but greater than ni, n2 being equal to n4 in the example shown. We also see that n5 is less than or equal to ni.

It can be seen that the stack of monocrystalline layers 14, 16 and 18 forms a multimode guide (for the wavelength of the pump beam 4) over the entire width 1 of this guide. It can also be seen that the single-mode guide (for the wavelength of the beam to be amplified 2), that is to say the layer 16, is a buried single-mode guide.

The two layers 14 and 18 are containment layers. They are not doped by the activating ions but their doping is chosen so that they have an optical index (n2 = n4) much higher than that (nor) of the substrate. The stack of layers 14, 16 and 18 has optical indices which are both close to one another, to allow single-mode guidance of the beam to be amplified 2, and highly contrasted with respect to the substrate and to the external medium (that is ie the air or the protective layer 20), which gives it a large numerical aperture with respect to the pump beam 4.

As a purely indicative and in no way limitative, the beam to be amplified 2 has a wavelength of 1.064 μm and the pumping beam 4 has a wavelength of the order of 807 nm to 808 nm; the substrate 6 is made of white YAG (Y ₃ Al ₅ 0ι ₂ ); the confinement layers 14 and 18 are made of YAG doped with 12% at Ga and 35% at Lu; the active layer 16 is made of YAG doped with 1 to 3% at of Nd, 14% at of Ga and 46% at of Lu; the protective layer 20 may be made of silica (optical index represented by dotted lines, lower than that of the substrate) or of YAG (same index as that of the substrate); the thicknesses of the substrate, of layer 14, of layer 16, of layer 18 and of protective layer 20 are respectively 550 μm, 2.5 μm, 5 μm, 2.5 μm and 10 μm; the thickness of the multimode guide is thus 10 μm; the difference between n3 and n2 (and therefore between n3 and n4) is around 10 ^"3 ; the difference between n2 (or n4) and ni is around 2x10 ^{~ 2} and so is even for the difference between n3 and ni.

The large digital opening thus obtained allows optimal coupling of the pump beam 4 with the active medium (layer 16). Indeed, this digital opening defines the cone of acceptance of the pump beam 4 in the multimode waveguide (all of the layers 14, 16 and 18). The pump harness 4 not being not limited by diffraction, the guide of this pump beam must have the largest possible digital aperture to absorb most of the pump beam 4 while retaining a single-mode guide zone (layer 16) for the beam to amplify 2.

For comparison, a guide with the same numerical aperture of 0.26 (remember that the numerical aperture is the square root of the difference between the square of the optical index of the guide doped with active ions and the square of the optical index of the substrate) and having only a guiding layer of 10 μm would not be monomode and would therefore have a lower beam quality. It is important that the amplifier has a thick multimode guide. This makes it easier to focus the pump beam 4 on the edge of the stack of layers 14, 16 and 18 and to make the assembly more compact. The amplifier in Figure 2 differs from that in Figure 1 in that the layer 14 is removed. The layer 16, referenced 16a in FIG. 2, rests directly on the substrate 6. In addition, the layer 18, referenced 18a in FIG. 2, has a thickness greater than the thickness that it had in the case of FIG. 1 In the case of FIG. 2, the single-mode guide is formed by the layer 16a while the multimode guide is formed by all of the layers 16a and 18a. In the example shown, the indices ni, n3, n4 and n5 retain the values they had in the case of FIG. 1, the thickness of the layer 18a is twice the thickness of the layer 18 of the figure 1 and a thickness of 10 μm is also obtained for the multimode guide.

Figure 3 is a schematic perspective view of a power laser according to the invention. This power laser comprises an amplifier 22 in accordance with the invention (the amplifier of FIG. 2 in the example shown) as well as a laser source 24 (a micro-laser in the example shown) intended to supply the beam 2 which we want to amplify and a source 26 of pumping light (a strip of power laser diodes in the example shown) which provides several pumping laser beams which have the collective reference 28. It is also a question of a transverse pumping and the array of power laser diodes is arranged so that the pumping beams coming from the different diodes of this array are perpendicular to the axis of the beam to be amplified 2. The power laser also comprises a cylindrical lens 30 provided to collimate the pumping beams and a cylindrical coupling lens 32 which receives these collimated beams and focuses them on the edge of the multimode guide (layers 16a and 18a).

We also see the laser beam 34 which has been amplified by optical pumping and which exits through the edge of the single-mode guide (layer 16a), through a lateral face 36 of the planar guide, adjacent to the face 10 which receives the pumping beams.

As a purely indicative and in no way limitative, a strip of laser diodes is used. power of 20 W, emitting at a wavelength of 807 to 808 nm.

The lateral face 12 of the planar guide (face opposite to that through which the pumping beams penetrate) is advantageously covered with a highly or totally reflective layer 38 vis-à-vis these pumping beams.

Preferably, this layer 38 is also anti-reflection with respect to the beam to be amplified 2 and another anti-reflection layer 40 with respect to the beam to be amplified 2, also covers the lateral face 36 of the planar guide ( opposite side to the lateral entry side of the beam to be amplified).

These layers 38 and 40 are multi-electric deposits which allow:

- optimize the transverse dimensions of the planar guide while ensuring good uniformity of pumping in the amplifier,

- avoid parasitic oscillations which contribute to degrading the performance of the amplifier and

- Eliminate the injection losses in the guiding structure and the light returns in the pump diodes or in the micro-laser 24 (master oscillator), which would disturb the functioning of these.

Instead of using such layers, it is possible to bevel, as is schematically illustrated by FIG. 4 in top view, the lateral faces 12 and 36 of the planar guide. We see that the face 12 is bevelled with an angle OC and that the face 36 is bevelled with an angle β. These angles α and β are of the order of 0.5 ° to 1 °. These bevels therefore replace the dielectric treatments intended to combat parasitic oscillations (undesirable laser effect).

If there is no layer 38, it is possible to provide, opposite the lateral face 12, another optical pumping assembly for sending pumping beams 28a into the edge of the multimode guide, this other assembly successively comprising a strip 26a of power laser diodes, a collimating lens 30a (homologous to lens 30) and a focusing lens 32a (homologous to lens 32).

The planar structure of an amplifier according to the invention facilitates: - temperature regulation as well as minimization of thermo-optical distortion effects

(thermo-induced birefringence), which allow high gains with high output powers

the passage from an amplification of a single beam to an amplification of several beams, by means of the pumping being a transverse pumping carried out over the entire width 1 (FIG. 3) of the planar guide and

- the use of optical and mechanical microtechnology to produce power laser sources, which can be integrated into microsystems.

The amplifier object of the invention is capable of operating at multiple wavelengths and with different crystalline materials (for example YAG, LMA, YSO, YAP, YAB, YLF and COB), which can be deposited by epitaxy , as well as with different activating ions (for example Nd, Yb, Er, Pr and Cr) and various co-dopants (for example Ga, Lu, Se and Bi). The concentration of doping and co-doping ions in the different layers makes it possible to:

- master the respective optical indices of the materials, - master the mesh disagreements between the adjacent layers (this is important for the growth of multilayers and the reduction of losses by diffusion of volume or interface) and

- fix the spectroscopic quantities such as the fluorescence wavelength, the cross section

(“Cross section”) of absorption and the effective section of stimulated emission, which make it possible to size the amplifying structures.

The amplifier object of the invention therefore allows, in a compact form, the power amplification of a pulsed or continuous laser source

(typically a micro-laser) while allowing it to retain the original spatial qualities of its beam which are essential for its integration.

Other advantages of the amplification which is the subject of the invention are indicated below.

1) The configuration of this amplifier facilitates heat dissipation while reducing the impact of thermal effects on the distortion of the beam to be amplified. This is important when it comes to a power amplifier.

2) The design of the stack of dielectric layers is such that one or more optical pump beams (for example guided transversely), which can be easily introduced (with simple optics), or several power laser diodes, in the active area of the amplifier. This active area must have a large opening to optimize the absorption of the radiation from this or these diodes, radiation which is not limited by diffraction. 3) The stack, as designed, allows single-mode guidance of the beam to be amplified in the amplifier guide, which ensures good quality of the amplified beam at the output of the amplifier. The invention therefore makes it possible to generate optical power while maintaining good beam quality comparable to that of the beam before it is amplified.

This provides a compact system with modular pump power (for example with laser diodes of 10, 20, 30 and 40 W without changing the structure and therefore having access to different power ranges).

It should be noted that the amplifier which is the subject of the invention is particularly suitable for transverse pumping by laser diodes since the elliptical configuration of the output beams of the power laser diodes (FIG. 3) allows easy coupling by means of simple optics.

This amplifier can be used with one or more laser beams (pulsed or continuous) to be amplified: for example, in the case of FIG. 3, the microlaser 24 (emitting a single beam) can be replaced by a strip of microlasers 24a emitting several parallel beams 2a, which are sent simultaneously into the single-mode guide, by the edge thereof, to obtain several amplified beams 34a. The invention solves for the first time the problem of coupling the power of a multimode pump beam in a power waveguide amplifier while retaining good optical quality for the amplified beam.

In addition, given the technique used, it is easy to produce multiple passage structures, compact and efficient, comparable to those mentioned in the document [7]. Figure 5 is a schematic top view of an amplifier according to the invention having a multiple pass configuration. In the case of this FIG. 5, the planar waveguide 8 of parallelepipedal shape of FIG. 1 or of FIG. 2 is still used, but, in the case of FIG. 5, this planar guide is bevelled, at an angle γ equal for example to 45 °, at two opposite lateral edges and thus comprises two additional opposite lateral faces 42 and 44 which are parallel and respectively intended to receive the laser beam to be amplified 2 and to provide an amplified beam 34.

FIG. 5 also shows the array of power laser diodes supplying the pump beams 28 as well as the collimating lens 30 and the focusing lens 32.

We see the many passages made, in the single-mode guide, by the laser beam that we want to amplify. We also see a dielectric deposit 38a highly or totally reflecting in the light of the pumping beams, formed on the lateral face 12 of the planar guide, opposite to where the pumping beams arrive.

The documents cited in this description are as follows:

[1] DP Sheperd; CTA Brown, TJ Warburton, DC Hanna, AC Tropper and B. Ferrand, A diode-pumped, high gain, planar waveguide, Nd: Y ₃ Al ₅ 0ι ₂ amplifier, Applied Physics Letters 71 (7), August 18, 1997

[2] Invention of B. Chambaz, I. Chartier, B. Ferrand and D. Pelenc: FR 2685135 A - see also EP 0547956 A and US5,309,471 A

[3] RN Ghosh, J. Shmulovich, CF. Kane, MRX De Barros, G. Nykolak, AJ Bruce and PCBecker, 8-mW Threshold Er ³⁺ -Doped Planar Waveguide Amplifier, IEEE Photonics Technology Letters, vol. 8, n ° 4, April 1996

[4] C. Kennedy, High Power Diode Pumped Laser Puise Amplifier, Lasers & Optronics, October 1997

[5] JD Minelly, WL Barnes, RI Laming, PR Morkel, JE Townsend, SG Grubb and DN Payne, Diode Array Pumping of Er ³⁺ / Yb ³⁺ Co-Doped Fiber Lasers and Amplifiers, IEEE Photonics Technology Letters, vol. 5, n ° 3, March 1993

[6] Y.C. Yan, A.J. Faber, H. De Waal, P.G. Kik and A. Polman, Appl. Phys. Lett., 71, 2922 (1997)

[7] JJ Degnan, Optimal Design of Passively Q-Switched Microlaser Transmitters for Satellite Laser Ranging, presented at "Laser Technology Developement Session" from "Tenth International Workshop on Laser Ranging Instrumentation", Shanghai, China, November 11 to 15, 1996

[8] JE Bernard, E. McCullough and AJ Alcock, High-gain, diode-laser-pumped Nd: YV0 ₄ slab amplifier, Cleo 1994, CTUK54, p.116, 1994

[9] D.C. Hanna et al., Optics Communications, vol.91, n ° 3, 1992, p.229 to 235

[10] EP 0 320 990 A (POLAROID CORP.).

The device described in document [9] is a planar laser pumped transversely by a low power laser diode (500 mW), the beam of this diode being guided in the structure described. This device does not provide single-mode guidance of the laser beam and it is not an optical amplifier but a laser. However, we saw above that an optical amplifier must not become a laser. The device described in document [10] is a structure of the “cladding pumping” type conventionally used in fiber optic devices to improve the coupling of the pump light. In this device, there is no question of a pump light propagating transversely to the light to be amplified. In addition, this device cannot be extended to a multibeam configuration by the very structure of the optical fiber which can only propagate light in its cross section. In addition, the planar configuration of the invention is much more favorable to good control of thermal effects than the circular configuration described in document [10]. Furthermore, only a planar structure such as that of the invention makes it possible, by guiding the pump light transversely to the light to be amplified, to amplify in a compact and monomode fashion a plurality of beams, for example from pulsed microlaser (s) or continuous.

Claims

1. An optical amplifier intended to amplify a first light (2), by optical pumping by means of a second light (4), this amplifier comprising a substrate (6) and, on this substrate, a planar optical waveguide ( 8) comprising a plurality of layers (14, 16, 18; 16a, 18a), this amplifier being characterized in that this plurality of layers forms - a multimode optical waveguide and of large digital aperture for the second light, this multimode waveguide being able to accept most of this second light and to guide it, and - a single-mode optical waveguide for the first light, this single-mode waveguide being able to guide the first light and to amplify it thanks to the optical pumping generated by the second light. 2. Amplifier according to claim 1, in which the planar waveguide comprises: - a first layer (16a) formed on the substrate (6) and containing activating ions for the amplification of the first light, and - a second layer (18a) formed on the first layer, the first layer forming the single-mode guide for the first light and all of the first and second layers forming the multimode guide of large digital aperture for the second light, the optical index of the first layer being greater than that of the substrate, the optical index of the second layer being lower to that of the first layer and higher than that of the substrate. 3. Amplifier according to claim 1, in which the planar waveguide comprises: - a first layer (14) formed on the substrate, - a second layer (16) formed on the first layer and containing activating ions for the amplification of the first light, and - A third layer (18) formed on the second layer, the second layer forming the single-mode guide for the first light and all of the first, second and third layers forming the multimode guide with large digital aperture for the second light, the the optical index of the second layer being greater than that of the substrate and the optical index of each of the first and third layers being less than that of the second layer and greater than that of the substrate. 4. Amplifier according to any one of claims 2 and 3, further comprising a protective layer (20) formed on said assembly and having an optical index less than or equal to that of the substrate. 5. Amplifier according to any one of claims 1 to 4, wherein the planar guide has the shape of a rectangular parallelepiped and has four polished side faces. 6. Amplifier according to claim 5, in which the lateral face of the planar waveguide, the face which is opposite to that by which the second light enters (4), is covered with a layer (38) of highly or totally reflecting screws. opposite this second light. 7. Amplifier according to claim 6, in which the layer (38) which is highly or totally reflective with respect to the second light (4) is also anti-reflection with respect to the first light (2) and the lateral face of the planar waveguide, the face which is opposite to that through which the first light penetrates, is covered with a layer (40) anti-reflection vis-à-vis the first light. 8. Amplifier according to claim 5, in which the lateral faces (12, 36) of the planar waveguide, faces which are opposite to those through which the first and second lumens penetrate respectively, are bevelled at approximately 1 °. 9. An optical amplifier according to claim 5, in which the planar guide is also bevelled at the same angle at two opposite lateral edges and thus comprises two additional opposite lateral faces (42, 44) which are respectively intended to receive the first light (2) and providing this first light in amplified form (34) in a multiple pass configuration. 10. Amplifier according to any one of claims 1 to 9, in which the layers (14, 16, 18; 16a, 18a) of the planar guide are formed by liquid phase epitaxy. 11. Power laser comprising: - the optical amplifier (22) according to any one of claims 1 to 10, - a laser source (24) intended to supply the first light (2) to this amplifier and - a pump light source (26) intended to supply the second light (4) to the amplifier. 12. The laser of claim 11, wherein the pumping light source (26) is arranged so as to allow transverse optical pumping in the amplifier (22). 13. The laser as claimed in claim 11, in which the pumping light source comprises at least one power laser diode (26, 26a). 14. Laser according to any one of claims 11 to 13, wherein the laser source comprises at least one microlaser (24).