FR2885267A1 - Active element for laser source, has different crystals presenting low doping at upstream face of elongated core, and absorption unit arranged in core periphery for absorbing any radiation presenting wavelength of laser radiation - Google Patents

Abstract

The active element has an elongated core (2) with a doped matrix to absorb an incoming pumping beam (3) at an upstream face (11), to amplify a laser radiation (4). The element is formed of materials including an yttrium aluminum garnet/neodymium crystal (13) and an yttrium vanadate/neodymium crystal (14). The crystals present a longitudinal doping variation with a low doping, which is limited to a preset value, at the level of the face. An absorption unit (12A) is arranged at the periphery of the core and is formed so as to absorb any radiation presenting a wave length of the laser radiation.

Description

The present invention relates to an active element for laser source, as well as

  a laser source comprising such an active element.

  More specifically, said laser source is of the type comprising: an active element comprising an elongated bar, of generally circular cross-section, but not exclusively, comprising a doped matrix capable of absorbing a pumping beam to amplify propagating laser radiation longitudinally with or without rebound; a pumping system comprising pumping (laser) diodes capable of emitting a pumping beam; an optical transport system for directing the pumping beam emitted by said pumping system into said active element so as to obtain longitudinal pumping; and an optical cavity for extracting said laser radiation.

  It is known that to be effective, the pump beam must be spectrally tuned to the absorption spectrum of the active element so that said pump beam is absorbed and transfers its energy to the rare earth ion doping said active element. .

  It is also known that the pump (laser) diodes have an emission spectrum, generally a few nanometers wide, which shifts from 0.25 to 0.3 nanometer per degree, when the temperature of said diodes is varied. pumping.

  To ensure a satisfactory conformity of the wavelength of the pumping beam (resulting from said pumping diodes) with the absorption spectrum of the active medium, it is known to mount said diodes on Peltier modules, the function of which is to stabilize their temperature better than 0.5 C so that a wavelength centering is ensured at least 0.2 nm.

  However, particularly in the context of military applications, the parameters of compactness, consumption and speed of implementation are of particular importance. Also, the use of Peltier modules, which induces a high consumption and which requires a stabilization time of the order of one minute, is a brake on the use of laser sources pumped by diode (s) in compact systems. The same is true for other active stabilization systems of the temperature of the diodes. Also, the technology still used today, for example for terrestrial laser designators, is a flash pumping technology, which is inefficient and cumbersome.

  To try to remedy this problem, it is necessary either to increase the tolerance of the active medium to drift wavelength, which is proposed for example by the patent FR-2 803 697, for which the pump beam is guided to pass several times through the active medium; either to implement a passive stabilization of the emission of the wavelength of the pump diodes, as proposed for example in the patent application US-2005/0018743 which describes the use of a system including one or more Volume Bragg gratings (RBV or VBG) for conditioning one or more emission characteristics of the laser.

  However, the above solutions only make it possible to obtain insensitivity over 3 to 10 nanometers corresponding to a diode temperature drift of 15 to 40 C. Such a range of thermal insensitivity is largely insufficient to use the pumping system, for example in a terrestrial laser designator, between -40 C and +70 C.

  The object of the present invention is to provide an active element and a laser source making it possible to obtain a thermal insensitivity of the laser emission over more than 15 nanometers.

  It is known that the proportion of pumping energy absorbed by the active element depends, on the one hand, on the absorption coefficient a (2) of said active element and, on the other hand, on the length of material L, crossed by the pumping beam. This proportion of absorbed energy Abs satisfies the relation Abs = 1-Exp [-a (?) L]. Also, to optimize said Abs proportion, it is necessary to maximize, on the one hand, said absorption coefficient a for all wavelengths of interest and, on the other hand, said length L traversed by the pumping beam.

  It is known, moreover, that it is difficult to extract properly the energy of a large volume of active medium (active element), in which the pumping energy would be dispersed. Also, the proposed configuration is a longitudinal pumping configuration, for which the absorption length of the pump beam can be long, provided that it is collinear (or almost) to the axis of the laser source.

  The main difficulty of longitudinal pumping at high power levels (greater than 500 W) lies in the production of parasitic ef-fects such as spontaneous emission amplification (ASE amplification below) or parasitic emission (MEP modes below). The ASE amplification comes from a spontaneous radiation, naturally emitted by the ions excited by the pumping beam and amplified by the gain resulting from the presence of these excited ions. MEP modes, meanwhile, come from the combination of: reflections present at the edges of the active element and / or any other reflector; and laser gain from the excited ions.

  The combination of these two factors generates a parasitic laser emission along one or more axes which are usually distinct from the main laser axis.

  ASE amplification is a parameter governed mainly by the gain and the maximum gain length possible in the active element. The only way to reduce its effect is to limit the gain length or the gain value.

  In addition, the MEP modes are governed by the gain and the presence of parasitic reflections that return photons to the laser and thus allow a gain cycle of these photons.

  The present invention aims to overcome these disadvantages. It relates to an active element for laser source, to obtain a high thermal insensitivity, while limiting the generation of spurious effects of the aforementioned type (ASE amplification and MEP modes).

  For this purpose, according to the invention, said active element of the type comprising an elongated bar which comprises a doped matrix capable of absorbing at least one pump beam entering an upstream face, for amplifying at least one longitudinally propagating laser radiation, is remarkable in that said active element is formed of at least one material having a longitudinal variation of doping with the lowest doping, which is limited to a predetermined value, for example 0.15%, at said upstream face and in that said active element further comprises an absorption means which is arranged at the periphery of said bar and which is shaped to absorb any radiation having the wavelength of said laser radiation.

  Preferably, the peripheral portion of said upstream face not useful, because external said pumping beam, is also provided with such absorption means.

  Thus, thanks to the invention, one acts: on the one hand, on the gain, by limiting the doping at the beginning of the bar (at the level of said upstream face) to limit the absorption in this zone and thus limit the transverse gain which makes it possible to reduce both the appearance of ASE amplification and the appearance of MEP modes; and secondly, on the reflectivity of the environment, surrounding the periphery of the active element of an absorption means for the (or) wavelengths of the laser radiation.

  The drop in doping at the beginning of the bar decreases the absorption efficiency. It is therefore important to provide, above the first millimeters of absorption, a higher level of doping.

  To maximize the absorption of the pumping beam, said active element advantageously comprises, at least partially, as a material, a YVO4 / Nd crystal.

  In a first embodiment, said material of the active element has a longitudinal variation of continuous doping, while, in a second embodiment, it has a longitudinal variation of doping step.

  In the first embodiment, said material preferably comprises a doping gradient ceramic and, in the second embodiment, it comprises several crystals having different dopings. These two modes can be combined.

  In addition, in a particular embodiment variant, said material comprises both a YAG / Nd crystal and a YVO4 / Nd crystal. As the absorption bands of these two crystals are different, the range of insensitivity is thus extended. In addition, said YAG / Nd crystal is preferably arranged upstream of said YVO4 / Nd crystal.

  Advantageously, the doping of the active element is carried out in such a way as to obtain a predetermined compromise between pa-rasite phenomena (ASE amplification, MEP modes) and a maximum thermal (insensitivity) band.

  So that the spontaneous emission ASE is reflected, internally to said bar, on said upstream face, it is advantageous for said active element to comprise an undoped portion, made of the same material as said upstream face, which it covers and of which it is united.

  The present invention also relates to a laser source of the type comprising: an active element for a laser source; 1 o a pumping system provided with laser diodes (pumping) which are capable of emitting at least one pump beam; an optical transport system for directing the pump beam emitted by said laser diodes into said active element so as to obtain longitudinal pumping; and an optical cavity for extracting at least one laser radiation.

  According to the invention, said laser source is remarkable in that said active element is of the aforementioned type.

  Advantageously, said laser source further comprises means for evacuating a heat flow from said pumping system.

  In a particular embodiment, said pumping system comprises modules (or stacks) of diodes formed of semiconductors from different disks (waffers). The sum of the spectral emissions of the different semiconductors thus generates a spectrum wider than that of a single diode. In addition, advantageously, each diode module includes an individual cooling means, which makes it possible to obtain an equally spread spectrum operation.

  Advantageously, said laser source is formed so as to use the active element oscillator and amplifier.

  Furthermore, advantageously, said laser source further comprises: means for generating at least a double passage of the pump beam in the active element; and / or a mirror transparent to laser radiation and reflecting the pump beam, which is arranged downstream of said active element. This mirror thus allows the part of the unabsorbed pump beam to be returned to the active element while allowing the laser radiation to pass, which makes it possible to increase the efficiency of the pumping.

  The figures of the appended drawing will make it clear how the invention can be realized. In these figures, identical references designate similar elements.

  Figure 1 is a block diagram of a laser source according to the invention.

  FIG. 2 schematically shows an example of an elongated bar of an active element according to the invention.

  The active element 1 according to the invention comprises an al-longed bar 2 which comprises a doped matrix capable of absorbing a pump beam 3, for amplifying at least one laser radiation 4 propagating longitudinally along an axis X-X.

  This active element 1 can be integrated in a laser source 5 as represented by way of example in FIG.

  Said laser source 5 comprises, in the usual way, in addition to said active element 1: a conventional pumping system 6 which comprises pump diodes 6A of the laser type and which is capable of emitting at least one pumping beam 3; a conventional optical transport system 7 for directing the pumping beam 3 emitted by said pumping system 6 into said active element 1 so as to obtain longitudinal pumping; and a conventional optical cavity 8, of axis X-X, including in particular a reflective mirror 9 and a mirror 10 partially transparent, which are placed opposite one another. This optical cavity 8 gives the laser radiation 4 obtained by the laser amplification and emitted through said mirror 10 along the X-X axis, its directivity and geometry characteristics.

  According to the invention, the bar 2 of said active element 1 is formed of at least one material having a longitudinal doping variation, along the axis XX, with the weakest doping which is limited to a predetermined value, for example 0 , 15%, at the upstream face 11 (or inlet of the pump beam 3) of said active element 1. In addition, said active element 1 comprises an absorption means 12 which is formed in such a way as to absorb any radiation having a wavelength substantially equal to that of said laser radiation 4, this absorption means 12 having a portion 12A which is arranged at the periphery of the bar 2 and a portion 12B covering the peripheral portion of said upstream face 11 not useful for the pump beam 3, because surrounding it externally.

  Thus, thanks to the preceding features of the invention, it acts.

  on the one hand, on the gain, by limiting the doping at the beginning of the bar 2 (at the level of said upstream face 11) to limit the absorption in this zone and thus limit the transverse gain, which makes it possible to reduce the appearance both ASE amplification and MEP modes; and secondly, on the reflectivity of the environment, surrounding the periphery of the bar 2 with an absorption means 12 associated with the (or) wavelengths of the laser radiation 4.

  The drop in doping at the beginning of the bar 2 decreases the absorption efficiency. It is therefore important to provide, beyond a predetermined distance, for example a few millimeters downstream of the upstream face Il, a higher doping level.

  In addition, in order to maximize the absorption of the pump beam 3, the active element 1 preferably comprises as a material, at least in part, a YVO4 / Nd crystal. Neodymium Nd has a high absorption cross-section in this material, which enables the bar 2 to effectively absorb the pump beam 3 over short distances over a spectral range of the order of 20 nm. This material is orthotropic and is therefore sensitive to absorption and emission. The absorption coefficient is higher when the polarization 7E is used.

  In a first embodiment, said material of the active element 1 has a longitudinal variation of doping which is continuous, while, in a second embodiment, it has a longitudinal variation of doping step.

  In the first embodiment, said material is preferably a doping gradient material. Such materials can be made by ceramic process.

  It is also possible to use several progressive doping crystals to reach at the input of each of them the maximum starting gain of the ASE amplification. In this case, the doping of the crystals is adapted so that at the temperature corresponding to the peak of absorption of the pump beam 3, the first crystal, then the following, has maximum doping without the parasitic effects (MEP modes and amplification ASE) are too important. However, since it is at this point (for maximum absorption) that the yield is the highest, it is conceivable to accept that the spurious effects still have an influence for this temperature.

  One possible improvement is to combine a YAG / Nd crystal and a YVO4 / Nd crystal. As the absorption bands of these two crystals are different, the range of insensitivity is thus extended. In particular, the absorption of the YVO4 / Nd crystal is stronger around 808-815 nm, whereas the YAG / Nd crystal has an absorption band at 792-797 nm which the YVO4 / Nd crystal does not possess. Such a combination is possible since the two crystals each emit at 1064 nm.

  In this case, if the YAG / Nd crystal is further placed upstream of said YVO4 / Nd crystal, an additional advantage is obtained. Indeed, the crystal YAG / Nd will be subjected to the strongest pumping power and it has an effective stimulated emission sequence less important than the crystal YVO4 / Nd. It will therefore convert the absorption into a lower gain than if the YVO4 / Nd crystal was placed upstream, which makes it possible to push back the threshold of appearance of parasitic effects. In this case, it is also advantageous to partially retain the YVO4 / Nd crystal in order to ensure sufficient longitudinal gain for the laser effect to be effective.

  Consequently, according to the invention, the doping of the active element 1 is performed so as to obtain a predetermined compromise between parasitic phenomena (ASE amplification, MEP modes) and a maximum thermal insensitivity band.

  It will be noted that the present invention does not use regulation of the temperature of the laser diodes 6A of the pumping system 6, leaving it to drift several tens of degrees. However, the laser source 5 may comprise, in a preferred embodiment, a means 15 for evacuating the heat flow from said pumping system 6. This evacuation of the heat flow can be carried out in particular: by solid conduction; by convective exchange with a fluid; by changing the liquid / gas phase of a fluid; or by solid / liquid phase change of a body thermally connected to said laser diodes 6A of the pumping system 6.

  In addition, said laser source 5 can be formed so as to use the active element 1 both as an oscillator and as an amplifier; and so that the diameter of the pump beam 3 remains approximately constant throughout the active element 1 so as to obtain a form of approximately constant deposition integral, regardless of the wavelength of the laser diodes 6A of the pumping system 6 and therefore the absorption coefficient of the active element 1.

  Moreover, said laser source 5 furthermore comprises a mirror 16 which is transparent to the laser radiation 4 and which is reflective for the pump beam 3, and which is arranged downstream from said active element 1. This mirror 16 thus makes it possible to send back to the active element 1 the part of the pump beam 3 unabsorbed, while allowing the laser radiation 4, which increases the efficiency of pumping. Preferably, this recycling mirror 16 is a meniscus, whose concave face is directed towards the active element 1, in order to recollimate the pump beam 3 in this active element 1.

  In a preferred embodiment, the bar 2 thus comprises a composite crystal composed of two different crystals 13 and 14: a crystal 13 of YAG / Nd (doped at x1% of length 21) followed by a crystal 14 of YVO4 / Nd (doped at x2% length 22), which are pumped longitudinally, as shown in FIG.

  The total length L = f1 + 2 of said composite crystal (bar 2) is determined by the length of the focusing horn of the pumping system 6.

  In the case of longitudinal pumping, the ASE amplification is essentially gou-vernée by the maximum value of the transverse gain input of the two crystals 13 and 14, where the absorption is the strongest. The transverse gain is represented by the product go.X (where X is the diameter of the pump beam 3 at the inlet of the two crystals 13 and 14).

  In a general way, the gain (longitudinal or transversal) can be expressed by the following formula: go.e (lo) = ae.tifluo.rio.riq 1o.Pp. (1 T (1o)) 1 expHP 1 (1) h.c 7Ldp uo wherein: Io is the central wavelength of the pump laser diodes 6A (which fluctuates with temperature); - ae is the stimulated emission cross section. The typical value is: ae = 2.5 × 10 19 cm 2 for YAG / Nd and this = 1, 2.10-18 cm 2 for YVO 4 / Nd; Tfluo is the fluorescence time (typically: Tfluo = 230 ps for YAG / Nd and Tfluo = 100 ps for YVO4 / Nd); no is the optical efficiency of the pumping system 6; riq is the fluorescence quantum yield; tip is the pumping time; rp is the radius of the pump beam 3; and Pp is the pumping power.

  T (a, o) is the transmission of the composite crystal (bar 2) crossed, in the case where all the composite crystal is crossed and verifies the expression: f Ixo (1). exp [aYAG (X) .e1- aYVO4 (1) .e2]. T (Xo) = f IXo (X) .dX in which: aYAG (X) and aYVO4 (X) are respectively the YAG and YVO4 absorption spectra; and IXo (X) is the emission spectrum of a laser diode 6A.

  A possible method for determining the parameters 1, 2, x1 and x2 of the bar 2 is as follows: A / x1 is determined directly so that the maximum value of the transverse gain goYAG (Xo) .X does not exceed the value of maximum transverse gain of start of ASE amplification. If, for example, this value is goYAG.Xmax = 3.5, considering that the emission spectrum of the laser diodes 6A is a Gaussian (centered on Io), taking the following values Pp = 4000 W (pumping power QCW during a time tp of 200, us) and rp = 3 mm, and taking the other parameters relating to the standard YAGINd and YVO4 / Nd crystals, a high doping x1 of the order of 1.3 (expressed as a percentage) is obtained; B / the length 1 of the crystal YAGINd is determined by iteration: One fixes a length 1 which gives directly 2 (L being fixed) and one determines x2 so that the maximum value of the transverse gain goYVO4 (X) .X does not exceed the value maximum transverse gain of start of ASE amplification (same method as in step A / previous). We note the values of longitudinal gains go (Xo) .L and absorption 1-T (Xo) on the whole bar 2; and C / we set a new value of 1 and repeat step B / previous.

  We choose the value of 1 (which then directly determines 2 and x2) (2) which gives the highest longitudinal gain go (Xo) .L and absorption values over the desired spectral range.

  The previous method often gives a crystal YAG / Nd (in input) of length 1 much weaker than the crystal YVO4 / Nd. For example, for a composite crystal (bar 2) 15 mm long, the crystal YAG / Nd typically has a length 1 of between 3 and 6 mm.

  Similarly, this configuration makes it possible to access YVO4 / Nd crystal do-page rates which are much greater than if the protective YAG / Nd crystal were removed.

  For example, if a maximum starting transverse gain value of the ASE amplification goYVO4.Xmax = 3.5 is chosen, a single YVO4 / Nd crystal subjected to the previous 4000 W pumping power should have a Approximate doping 0.13% to avoid ASE amplification.

  In the case of the composite crystal, the doping rate x1 is much higher, which allows for much better gains and absorptions.

  As shown in FIG. 2, the most upstream portion of the active element 1 may comprise an undoped portion 17 made of the same material as said crystal 13 and secured to said upstream face 11. Such undoped portion 17 only can serve as a heat sink, but also prevents the ASE radiation reflected on said upstream face 1 January.

Claims (19)

  An active element for laser source, said active element (1) comprising an elongated bar (2) which comprises a doped matrix capable of absorbing at least one pump beam (3) entering an upstream face (II), to amplify at least one laser radiation (4) propagating longitudinally, characterized in that said active element (1) is formed of at least one material (13, 14) having a longitudinal variation of doping with the lowest doping, which is limited to a predetermined value, at said upstream face (II), and in that said active element (1) further comprises an absorption means (12A) which is arranged at the periphery of said bar (2) and which is shaped to absorb any radiation pre-sensing the wavelength of said laser radiation (4).   2. Active element according to claim 1, characterized in that said active element (1) further comprises an absorption means (12B) which is arranged at the peripheral portion of said upstream face (II) outside said pump beam ( 3).   3. Active element according to one of claims 1 or 2, characterized in that said active element (1) comprises at least partially as material a crystal YVO4 / Nd.   4. Active element according to one of claims 1 to 3, characterized in that said material has a longitudinal variation of doping which is continuous.   5. Active element according to one of claims 1 to 3, characterized in that said material has a longitudinal variation of doping step.   6. Active element according to claim 4, characterized in that said material comprises a doping gradient ceramic.   7. Active element according to claim 5, characterized in that said material comprises several crystals (13, 14) having different dopings.   8. Active element according to claim 7, characterized in that said material comprises a YAG / Nd crystal and a YVO4 / Nd crystal.   9. Active element according to claim 8, characterized in that said crystal YAG / Nd is arranged upstream of said crystal YVO4 / Nd.   10. Active element according to any one of the preceding claims, characterized in that the doping of the active element (1) is performed so as to obtain a predetermined compromise between parasitic phenomena and a maximum thermal band.   11. Active element according to any one of the preceding claims, characterized in that it comprises an undoped portion (17), made of the same material as said upstream face (Il) that it covers and of which it is integral.   Laser source comprising: an active element (1) for a laser source; a pumping system (6) provided with laser diodes (6A) which are capable of emitting at least one pumping beam (3); an optical transport system (7) for directing the pump beam (3) emitted by said laser diodes (6A) into said active element (1) so as to obtain longitudinal pumping; and an optical cavity (8) for extracting at least one laser radiation (4), characterized in that said active element (1) is of the type specified in any one of claims 1 to 1 1.   13. Laser source according to claim 12, characterized in that it further comprises means (15) for evacuating a heat flow from said pumping system (6).   14. Laser source according to one of claims 12 or 13, characterized in that said pumping system (6) is formed so as to generate a pumping beam (3) which is approximately cons-along the entire length of the bar (2).   15. Laser source according to one of claims 12 to 14, characterized in that said pumping system (6) comprises semiconductor diode modules formed from different disks.   16. Laser source according to claim 15, characterized in that each diode module comprises a cooling means.   17. Laser source according to one of claims 12 to 16, characterized in that it is formed to use the active element (1) oscillator and amplifier.   18. Laser source according to one of claims 12 to 17 characterized in that it further comprises means for generating at least a double passage of the pump beam (3) in the active element (1).   19. Laser source according to one of claims 12 to 18, characterized in that it further comprises a mirror (16) transparent to laser radiation (4) and reflecting the pump beam (3), which is arranged downstream of said active element (1).