FR2826192A1 - Frequency stabilized solid state laser includes window of transparent and thermally conductive material to cool optically pumped material - Google Patents

Abstract

The solid state laser (10) comprises a cavity (102) which is defined by an entry mirror (103) and an exit mirror (104). A window (105) lies is in contact with the entry mirror, and is made from a material with a thermal conductivity which is greater than that of the active element (101). This facilitates dissipation of the thermal energy produced in the active material during pumping. The solid state laser (10) comprises a cavity (102) which is defined by an entry mirror (103) and an exit mirror (104). Inside the cavity is an element of active material (101) for optical pumping. The entry mirror (103) is arranged on one of the faces (101a) of the active material and forms a cavity between two planes, in conjunction with the exit mirror (104). A system is provided for cooling the solid state laser, in order to ensure that the cavity remains stable. This is provided by a window (105) which is in contact with the entry mirror, made from a material which is transparent to the wavelength of the pump signal, with a thermal conductivity which is greater than that of the active element (101). This facilitates dissipation of the thermal energy produced in the active material during pumping. The window may be made of sapphire or AIN.

Description

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SOLID LASER WITH CAVITY PLAN / PLAN AND LASER SOURCE
FREQUENCY STABILIZED IMPLEMENTING SAID LASER
The present invention relates to a new solid laser structure.

The solid laser of the invention finds in particular, but not exclusively, its application to the production of a laser source stabilized in frequency, and usable in particular as a standard in the field of telecommunications.

In the present text, the term “solid laser” designates any laser intended to be optically pumped and comprising a pumping cavity inside which is positioned an element made of active material, crystalline or amorphous, doped, and allowing the emission of 'laser radiation, when subjected to a pump beam. The pump cavity is delimited by two mirrors: a first mirror designated in the remainder of this text input mirror (and also commonly called pump mirror), through which the element made of active material is pumped; a second mirror designated in the remainder of this text as output mirror, and which allows the transmission of the laser beam produced in the cavity by the element made of active material. For further explanations on the structure and operation of solid lasers known to date, one can in particular refer to chapter 7.3. 1 titled solid state lasers from the book titled Lasers Theory and Practice, by the authors John
Hawkes and Lan Latimer, and edited by Prentice Hall International Ltd.

 The main objective of the present invention is to provide a new solid laser which allows the emission of radiation in transverse single mode and longitudinal single mode mode, with very good transverse quality, and which has a small footprint.

 The solid laser of the invention is known in the prior art in that it comprises a cavity which is delimited by an input mirror and

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 an outlet mirror, and inside which is positioned an element made of active material intended to be pumped optically.

 Typically according to the invention, the input mirror is deposited on one of the faces of the element made of active material and forms with the output mirror a plane / plane cavity; the solid laser comprises means for cooling the input mirror / active material element assembly, which cooling means make it possible to make the plane / plane cavity stable.

 According to the invention, the input mirror being deposited directly on the element made of active material, the size of the solid laser is advantageously reduced. Also, the use of a plane / plane cavity (plane input mirror and plane output mirror) makes it possible to improve the transverse quality of the radiation at the output of the solid laser, compared to a hemispherical type cavity generally used. works in this type of laser. Finally, it should be emphasized that in operation of the laser, the pumping of the active material element results in a strong creation of heat in said element, which, in the absence of any cooling means, results in particular by deformation and expansion of the input mirror / active material element assembly, which becomes comparable to a lens (thermal lens effect). Knowing that in the solid laser of the invention, the output mirror is plane, it is not possible, in the absence of any cooling means, to design a laser whose plane / plane cavity is stable and weak dimension. In the solid laser of the invention, the cooling means of the input mirror / active material element assembly thus have the main function of making the plane / plane cavity of the laser stable.

 The invention also relates to a frequency stabilized laser source. This laser source comprises a solid laser which conforms to the invention and which also comprises means for

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 adjustment of the length (L) of its cavity; the source also comprises means for controlling (9) the emission frequency (f1) of the solid laser which controls said adjustment means.

 Other characteristics and advantages of the invention will appear more clearly on reading the description below of two alternative embodiments of the invention, which description is given by way of non-limiting example, and with reference to the accompanying drawings. in which: - Figure 1 is a schematic representation of a first alternative embodiment of a laser source of the invention, - Figure 2 is a schematic presentation of a second alternative embodiment of a laser source of the invention, - figure 3 represents the energy levels of the two isotopes (85) and (87) of Rubidium (Rb), - figure 4 schematically represents a theoretical curve of the intensity of the beam having crossed the cell absorption after a single pass (simple absorption), on the purely theoretical assumption where there would be no widening of the absorption peak by Doppler effect.

 - Figure 5 schematically represents the actual curve of the intensity of the beam having crossed the absorption cell after a single pass (single absorption), with widening of the absorption peak by Doppler effect, - Figure 6 schematically represents the curve real intensity of the beam crossing the absorption cell, after two successive passages and with saturation of the atomic transition used for the servo-control.

 With reference to FIG. 1, and in accordance with a first alternative embodiment of the invention, the laser source comprises: an assembly 1 which is formed by a polarized miniature solid laser 10, optically pumped by a pump diode 11 via a

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short optical fiber 12, and which outputs a first laser beam 2 of frequency (f1); a generator 3 of N @ th harmonic, which makes it possible to generate, from said first laser beam 2, a light beam 4 composed of a second laser beam 4a of the same frequency (f1) and of a light beam 4b, called N "th harmonic, whose frequency (f2) is equal to (N) times the frequency (f1) of the laser beam 2 (N being an integer greater than or equal to 2); an absorption cell 7 containing a chemical element of absorption, which in the particular example described below is a gas (or a vapor) of Rubidium (Rb); - a dichroic mirror 5 (or any other known equivalent optical means fulfilling the same function) which generally has the function of separating the aforementioned beams 4a and 4b, by allowing the second laser beam 4a of frequency (f1) to pass through to form the output laser beam, and by deflecting the light beam 4b (N'e harmonic) to direct it towards the absorption cell 7 so to pass it through the el ment chemical absorption of that cell 7; means 6 for polarizing the laser beam, interposed between the absorption cell 7 and the dichroic mirror 5; a focusing lens (D interposed between the dichroic mirror
5 and the polarization means 6; this lens (D makes it possible to reduce the size of the beam 4b and thereby increase its power density; the use of this lens is not necessary when the beam 4b has sufficient power; - a mirror 8 ( or any other equivalent reflective optical means fulfilling the same function) which has the function of returning (deflection to 180) the light beam 4 "b coming from the absorption cell 7 in the direction of said absorption cell, so

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 to make it pass through the absorption cell, a second time and along the same optical path, which allows operation in saturated absorption; - And control means 9 which are designed to control precisely the frequency (f1) of the first beam 2 on a transition of the chemical absorption element (Rubidium).

 According to an essential characteristic of the invention, the solid laser 10 and the generator 3 are designed such that the power of the harmonic output at the output of the generator 3 is sufficient to obtain saturation of said transition of the chemical element absorption, that is to say a saturation of the absorption of Rubidium in the case of the particular example which will now be detailed.

Set 1: Solid laser 10 / optical fiber 11 / pump diode 12
The solid laser 10 comprises an element made of active material 101 intended to be optically pumped, and more particularly in the form of a glass rod of very small thickness (approximately 1 mm / dimension depending on the length of the cavity), which is co-doped with Ytterbium and Erbium. The Erbium ion inserted in a vitreous matrix allows, by optical pumping at a pump wavelength (λ) of the order of 975nm, a laser emission by the glass rod 101 at a wavelength () included in the spectral band 1, 5dd. m and 1.6m.

 The invention is not however limited to the use of a Ytterbium and Erbium co-doped glass rod, the solid laser being more generally able to be produced from an element made of doped active material. It could in particular be a crystal (instead of a vitreous matrix), which can more particularly be semiconductor.

 The bar 101 is positioned in a laser cavity 102 forming a Fabry-Perrot interferometer. This bar 101 has two opposite flat faces 101a and 101b, which are parallel and oriented

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 perpendicular to the longitudinal axis of the cavity 101. The cavity 102 is delimited by two mirrors 103,104. According to a preferred characteristic, the input mirror is a dichroic mirror. In addition to reduce the bulk, this input mirror 103 is deposited directly on the face 101a of the bar 101, and forms a plane mirror.

In contrast to the input mirror 103, the output mirror 104 is a plane mirror, so that the cavity 102 is a plane / plane type cavity. The length L of this cavity 102, which corresponds to the distance between the two mirrors 103 and 104 along the longitudinal optical axis of the cavity, is adjustable in a fine manner, so as to adjust the length of the cavity, and thereby even allowing fine adjustment of the emission frequency (f1) of the solid laser 1. This fine adjustment is obtained by means of a piezoelectric shim 108 on which the output mirror 104 is mounted, preferably (but not necessarily) by means of a coupling element 107 (for example glass substrate), and by acting on the control voltage the piezoelectric shim 108. This control voltage is controlled automatically, by a feedback loop which is described later , and which in general allows a control of the emission wavelength λ 1 (λ 1 = c / fl, c being the speed of light) of the solid laser 10, on a transition (designated below) generally after transition (T)) of the absorption chemical element of cell 7, which chemical element is chosen so that this emission wavelength λ 1 is stabilized around
1.56m.

The use of a cavity 102 of plan / plane type and of short length advantageously makes it possible to obtain at the output of the solid laser
10 radiation in single transverse and longitudinal modes, of very good quality.

 The inlet mirror 103 / bar 101 assembly forms the inlet face of the cavity 102, and is mounted for cooling, on

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 a first face 105a of a sapphire window 105. The sapphire window 105, which is in contact with the input mirror 103, has the function of dissipating the thermal energy accumulated by the glass rod 101 during the operation of the solid laser 10. This sapphire window could be replaced in another variant by any element made of a material, which is transparent to the pump beam of the pump diode 11, and which has a coefficient of thermal conductivity greater than that of the rod 101 so as to dissipate the thermal energy generated in this bar 101 during pumping. The higher the coefficient of thermal conductivity of the window 105, the better the cooling of the bar 101. For example, in another variant using a glass bar 101, and a pump wavelength of around 975nm, we could replace the sapphire window with an AIN window.

106b 106Q, qui permet le passage du faisceau de pompe jusqu'à l'ensemble fenêtre en saphir 105/miroir 103/barreau 101, la seconde face 105b de la fenêtre 105 venant fermer cette ouverture 106. Q. Ce radiateur 106, qui est en contact avec la fenêtre 105, a pour fonction d'évacuer vers l'extérieur l'énergie thermique qui est absorbée par la fenêtre en saphir 105 en cours de pompage, et présente à cet effet une surface de dissipation importante (comparativement à la surface de la fenêtre 105). Ce radiateur 106 est par exemple en cuivre ou en aluminium. Il pourrait également s'agir d'un élément Peltier permettant d'absorber activement l'énergie thermique de la fenêtre 105. The sapphire window is mounted on one 106a of the faces of a radiator 106, having in its center a through opening

106b 106Q, which allows the passage of the pump beam to the sapphire window 105 / mirror 103 / bar 101 assembly, the second face 105b of the window 105 closing this opening 106. Q. This radiator 106, which is in contact with the window 105, has the function of evacuating to the outside the thermal energy which is absorbed by the sapphire window 105 during pumping, and has for this purpose a significant dissipation surface (compared to the surface from window 105). This radiator 106 is for example made of copper or aluminum. It could also be a Peltier element making it possible to actively absorb the thermal energy from window 105.

 The use in combination of the radiator 106 and the window 105 advantageously makes it possible to make the plane / plane cavity 102 stable, by effective cooling of the input mirror 103 / glass rod 101 assembly. Indeed, in l '' absence of cooling of the entire input mirror 103 / bar 101, or

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 if the cooling of this assembly is not effective enough, during operation of the laser, under the action of the pump beam, there is a deformation and expansion of the input mirror 103 / bar 101 assembly, which together becomes comparable to a lens (thermal lens effect). This lens associated with a plane output mirror 104 in this case forms an unstable cavity, which makes the laser inoperative. In addition, inefficient cooling can lead to destruction of the bar 101.

 Upstream of the radiator 106 / sapphire window 105 / entrance mirror 103 / glass rod 101 assembly is mounted a focusing lens 112, which is centered on the optical axis of the cavity.

 The solid laser 1 also usually comprises a rotary standard 109 (thin blade), which is positioned in the cavity 102, being centered on the optical axis of this cavity, and which allows a first coarse adjustment of the frequency of emission of the solid laser 10, by adjusting the initial angular position of the standard 109.

 To stabilize the polarization of the radiation at the output of the solid laser 10, when the active material (bar 101) is isotropic, such as co-doped glass Ytterbium / Erbium, the standard 109 is made of a material with polarizing power, such as for example the material sold under the registered trademark POLARCOR. On the other hand, if the active material is anisotropic, the standard 109 can be produced in an isotropic material, such as for example glass.

 The pump diode 11 allows optical pumping of the solid laser through the input mirror 103. In a particular embodiment, a multimode laser diode of power less than 1W is used. More particularly, for pumping co-doped Ytterbium / Erbium 101 glass, a GatnAsP semiconductor diode is used, for example, which makes it possible to obtain a pump wavelength (cap) of the order of 975nm.

 The use of an active medium produced by means of a glass (or

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 crystal) co-doped with Ytterbium and Erbium, combined with cooling of this active medium by the input face (dichroic mirror 103 associated with the window 105 and with the dissipation radiator 106) and with the use of a cavity 102 of the plane / plane type advantageously makes it possible to obtain, at low cost, a solid laser 10 which has a small footprint and which is capable of outputting a laser beam 2 of wavelength λ 1 (λ 1 = c / f1) regulated at 1.56 gm, and having sufficient power in single transverse and longitudinal single mode regimes.

Harmonic generator (3) The generator 3 comprises two focusing lenses 31 and 32 between which is placed a non-linear crystal 30 based on crystals (PP: KTiOPO,) making it possible to obtain a quasi-phase agreement. In general, this crystal (PP: KTiOP04) could be replaced by any known equivalent crystal fulfilling the same function, and for example by a KNbO-based crystal. The generator 3 makes it possible, from the first beam 2 at the input, to generate at the output a second harmonic (N = 2) whose frequency (f2) is twice the frequency of the first beam 2, which second harmonic is deflected by the dichroic mirror 5 in the direction of the absorption cell 7 (beam 412.). The use of a crystal 30 of the (PP: KTP) type advantageously makes it possible to obtain a higher second harmonic power compared, for example, to the use of a single crystal of the Knob03 type.

Polarization means 6 and Absorption cell 7
The polarization means 6 successively comprise and aligned along the same optical axis: a half-wave plate 62, a polarizing cube 61, and a quarter-wave plate 63. In operation, the incoming beam 4b of frequency (f2) corresponding at the second harmonic generated by crystal 30 is sent to the polarizing cube
61. The half-wave blade 62 makes it possible to direct a very small part of

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l'opposée de la première photodiode 9a. the power of this incoming beam 4b towards a first photodiode 9a. Most of the power of this incoming beam 4b passes through the polarizing cube 61 for the first time, then passes through the quarter-wave plate 63. This blade makes the polarization of the beam coming from the polarizing cube 61 circular (right or left). The beam 4′b circularly polarized passes through the absorption cell 7, is reflected at 1800 by the mirror 8, then crosses back, in the opposite direction, successively the absorption cell 7 and the quarter-wave plate 63. from its circular polarization, this return beam is reflected by the polarizing cube 61 towards a second photodiode 9b, positioned at

the opposite of the first photodiode 9a.

Control by saturated absorption
FIG. 3 represents the energy levels of Rubidium (Rb) 85 and 87. To achieve the control of the frequency (f1) of the laser beam (2), for example, the transition D (SS1) is used as transition (T) // F = 2 5PF = 3) Rubidium (Rb) at 0.7801) m. This transition D2 couples the levels 5S1, (fundamental level) and 5 P3 / 2 (excited level, lifetime 0, 15rus) which advantageously have a fine structure. Other atomic transitions of Rubidium could also be used to carry out the control.

When the pulsation (Wl) of the beam 4b crossing the cell 7 of Rubidium is close to the pulsation (w.) Of the chosen atomic transition (T), the atom absorbs part of the intensity of the beam 4b (see Figure 4 ). This energy absorption is important when the frequency of the beam 4b is not distant from the excited level more than once. The displacement of the atoms of the gas
Rubidium, however, causes the Doppler effect to widen the width of the transition (T). Thus in the event of simple absorption (beam 4b passing through the absorption cell 7 only once), in reality an absorption curve is obtained of the type of that of FIG. 5. Thus, in the event of simple absorption the peak absorption (Figure 5)

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 is in practice too wide to allow a control which is sufficiently precise to within a few MHz.

 Saturated absorption overcomes the aforementioned problem of widening the absorption peak by Doppler effect. Indeed, the return beam, which is returned by the mirror 8 in the example of FIG. 1, by crossing again through the absorption cell 7, makes it possible to carry out a population inversion, which makes the medium (Rb gas) transparent for atoms of zero velocity. In this case, if the intensity of the beam 4b is sufficient to saturate the transition (T), the electrical signal, as a function of the frequency of the laser, returned by the photodiode 9b has the appearance given in FIG. 6. We understand as well as the slaving can thus advantageously be much more precise than in the case of simple absorption.

 It is understood in the light of the explanation above, that in order to obtain an operation in saturated absorption, it is essential that the power of the Nth harmonic (second harmonic / beam 4b in the example of FIG. 1) which is used for the servo is sufficient to saturate the chosen transition and obtain on the photodetector 9b a signal in accordance with FIG. 6.

Control means 9 / modulation of the atomic transition
In the particular example of FIG. 1, the control means 9 make it possible to control the frequency of the laser beam 2 by synchronous detection, thanks to a modulation of the atomic transition (T).

 These control means 9 comprise a solenoid 90 which surrounds the absorption cell 7, and a feedback loop (I).

 This feedback loop (I) essentially comprises a sinusoidal voltage generator 91, a synchronous detection module 92, and a voltage amplifier 93.

 The sinusoidal voltage generator 91 makes it possible to supply the solenoid 90 with a sinusoidal voltage of frequency f given by

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 for example the order of a few hundred KHz. In operation, the absorption cell is thus subjected to an amplitude modulated magnetic field created by the solenoid 90. This modulated magnetic field makes it possible to move the Zeeman sub-levels of the transition (T) (for example the atomic transition D2 in the case of Rubidium). This displacement is to a large extent proportional in particular to the value of the magnetic field created. As a result, the absorption signal S1 (delivered by the photodetector 9a) is continuously frequency modulated over a very small range (typically to move the energy levels over a few MHz).

 The absorption signal S1 is processed by the synchronous detection module 92. This module 92 also receives as input the sinusoidal voltage delivered by the generator 91, and usually makes it possible to extract from the absorption signal 51 a signal error (e), in the form of a DC voltage, the value of which is proportional to the frequency offset between the frequency of the beam 4b passing through the absorption cell 7 and the frequency of the transition (T) chosen. The error signal (E) is amplified by a voltage amplifier 93, which delivers to the piezoelectric shim 108 a control signal 94 (direct voltage). This control signal 94 makes it possible to continuously and finely adjust the length of the cavity of the solid laser (distance between the input 103 and output 104 mirrors) and thereby the emission frequency f1 of the laser solid 10.

 The laser source of the invention, a particular variant of which has been described with reference to FIG. 1, advantageously makes it possible to produce a frequency standard which has the following characteristics and advantages: - emission of transverse single-mode radiation (beam 4a) , which facilitates the coupling of the electromagnetic wave with optical fibers and planar waveguides, - emission of a longitudinal single-mode radiation (beam 4a),

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 which makes it possible to obtain good spectral fineness, - very good stability in frequency and in power of the output beam 4a, - emission frequency (f1) known in an absolute manner, which makes it possible to use the beam 4a in active multiplexing / demultiplexing techniques, - emission wavelength (At 1) stabilized around 1.56 gm, which allows its use as a frequency standard in the telecommunications field, compact and compact laser source, this which allows it to be easily inserted into embedded systems and into processing chains in the telecommunications field.

As an indication, in an embodiment in accordance with the example of FIG. 1 which has just been described, the solid laser 10 was designed so that the laser beam 2 generated by this solid laser was stabilized around 1.56 μm with an accuracy of +/- 10-8, and had an output power (beam power 4a) greater than or equal to
50mW in transverse single-mode and longitudinal single-mode regimes, this power being able to reach 100mW. The length L of the cavity was about 5mm. The length, width and height of the solid laser 10 were worth 50mm, 30mm and 30mm respectively. The laser beam 2 at the output of the solid laser 10 exhibited very good transverse quality, and was therefore easy to couple with an optical fiber. In particular, the parameter M2 which defines the transverse quality of this beam was less than 1.1. The second harmonic (beam 4b) generated by generator 3 from this laser beam 2 had a power greater than 1 W, which was sufficient to saturate the transition (S5, dF = 2 -7 SP3dF = 3) of Rubidium from the absorption cell 7.

Variant of embodiment of Figure 2
The variant embodiment of FIG. 2 differs from

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 in general of the variant of FIG. 1 by the implementation of a synchronous detection based no longer on a modulation of the transition (T), but on a frequency modulation of the beam 4b. For the sake of simplification, in this FIG. 2, the elements which have been taken from the variant of FIG. 1 have been referenced in the same manner as in FIG. 1.

9a , lequel signal est traité de la même manière par le module de détection synchrone pour extraire le signal d'erreur (s). In the variant of FIG. 2, to carry out the frequency modulation of the beam 4h, the control means comprise an acousto-optical element 95 which is interposed between the mirror 5 and the polarization means 6; In the example illustrated, this acousto-optical element 95 is more particularly interposed between the focusing lens (D and the half-wave plate 62. In another variant, this acousto-optical element could be interposed between the mirror 5 and the focusing lens (D. This acousto-optical element, as a function of the alternating frequency signal (f) which applied to it as an input, allows frequency modulation of the laser beam 4b. In a similar manner to the variant of FIG. 1, we thus achieves a modulation of the absorption signal (ground) delivered by the photodetector

9a, which signal is processed in the same way by the synchronous detection module to extract the error signal (s).

 In another alternative embodiment, the acoustooptic element could be replaced by an electro-optic carrying out a sinusoidal phase modulation of the beam 4b.

 The invention is not limited to the two variant embodiments which have been described by way of examples with reference to FIGS. 1 and 2.

In particular, and in a non-exhaustive manner, as a replacement for Rubidium, all the known alkalis could also be used, and in particular Potassium, Cesium, Hydrogen, Sodium, Lithium, ... More generally, any chemical element or compound can be used having differentiated absorption lines, and suitable for implementing a control system according to the variant of FIG. 1 (modulation

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 of the atomic transition), or of the variant of FIG. 2 (modulation of the 4Q beam). The invention can more generally be applied to the production of a laser source stabilized around any predetermined wavelength (λ 1), which may be different from the value 1.56 gm, which is specific day in the telecommunications field. In the context of the invention, the laser source may be produced by means of an optically pumped solid state laser which may be different from the particular solid laser 10 of FIG. 1, that is to say d 'more generally by means of a laser implementing in a cavity, an element of crystalline or amorphous active material. Finally, the solid laser 10 described with reference to FIG. 1, by way of a preferred variant of embodiment being new in itself, it can advantageously be used to produce any type of laser source, and its use is not necessarily limited to the production of a frequency stabilized laser source according to the invention.

Claims (17)

 CLAIMS 1. Solid laser (10) comprising a cavity (102) which is delimited by an entry mirror (103) and an exit mirror (104), and inside which is positioned an element made of active material ( 101) intended to be optically pumped, characterized in that the inlet mirror (103) is deposited on one (101a) of the faces of the element made of active material and forms with the outlet mirror (104) a plane / plane cavity (102), and in that it includes means for cooling the input mirror / active material element assembly, which cooling means make it possible to make the plane / plane cavity stable. 2. Solid laser according to claim 1 characterized in that the cooling means comprise a window (105) which is in contact with the input mirror (103), and which is made of a material transparent to the wavelength of pump, and having a thermal conductivity greater than the thermal conductivity of the active element (101), so as to allow dissipation of the thermal energy generated in the active material element (101) during pumping. 3. Solid laser according to claim 2 characterized in that the window (105) is made of sapphire or AIN. 4. Solid laser according to claim 2 or 3 characterized in that the cooling means comprise a radiator (106), which is in contact with the window (105) and which makes it possible to evacuate towards the outside the absorbed thermal energy through this window (105) during pumping.  5. Solid laser according to one of claims 1 to 4 characterized in that the element made of active material (101) is a glass or crystal co-doped with Ytterbium / Erbium ions.  6. Solid laser according to any one of claims 1 to 5 characterized in that it is designed to emit radiation <Desc / Clms Page number 17>  in transverse single-mode and longitudinal single-mode regimes, with a power greater than or equal to 50mW. 7. Solid laser according to one of claims 1 to 6 characterized in that it comprises means (108) for adjusting the length (L) of the cavity (102). 8. Solid laser according to claim 7 characterized in that the means (108) for adjusting the length (L) of the cavity (102) comprise a piezoelectric shim, the output mirror (104) being mounted on one of the faces of said piezoelectric shim. 9. Frequency stabilized laser source characterized in that it comprises a solid laser (10) referred to in one of claims 7 or 8, and means for controlling (9) the emission frequency (f1) of the solid laser which controls the means (108) for adjusting the length (L) of the cavity (102). 10. Laser source according to claim 9 characterized in that it further comprises an absorption cell (7) containing a chemical absorption element, and in that the control means (9) making it possible to control on a transition (T) of the chemical absorption element, the frequency (f1) of the beam emitted by the solid laser (10).  11. Laser source according to claim 10 characterized in that the chemical absorption element is chosen from the group: Rubidium, Potassium, Cesium, Hydrogen, Sodium, Lithium.  12. Laser source according to claim 10 or 11 characterized in that it further comprises a generator (3) which makes it possible to generate, from the laser beam (2) emitted by the solid laser (10), a light beam ( 4b), called N 'harmonic, the frequency of which is equal to (N) times the frequency (f1) of the laser beam (N integer greater than or equal to 2), and which is intended to pass through the absorption cell (7) .  13. Laser source according to claim 12 characterized in that it <Desc / Clms Page number 18>  further comprises optical deflection means (8) which have the function of passing the N th harmonic at least twice in succession through the absorption cell (7), so as to allow operation in saturated absorption, and in that the solid laser (10) and the generator of NI, e harmonic are designed such that the power of the N @ th harmonic at the output of the generator (3) is sufficient to obtain saturation of said transition (T) of the chemical element of absorption. 14. Laser source according to claim 13 characterized in that the harmonic generator (3) comprises a non-linear crystal (30) based on crystals (PP: KTP). 15. Laser source according to one of claims 9 to 14 characterized in that the servo means (9) are designed to deliver a control signal (94) for adjusting the emission frequency (f1) of the laser solid (10), from a synchronous detection based on a modulation of the transition (T). 16. Laser source according to one of claims 12 to 14 characterized in that the servo means (9) are designed to deliver a control signal (94) for adjusting the emission frequency (f1) of the laser solid (10), from a synchronous detection based on a modulation of the Nth harmonic (4b). 17. Laser source according to one of claims 1 to 16 characterized in that the wavelength (λ 1) of the laser beam (2) emitted by the solid laser (10) is stabilized around 1.56 gm.