Laser phasing device with a multi-plane converter

Abstract

The invention relates to a laser beam phasing device (10), comprising: − a plurality of N semi-open cavities; − a multi-plane conversion device (MPLC) configured to convert a set of N input spatial modes forming an input base into a set of N output spatial modes forming an output base; − a partially reflective output mirror (OM) configured to form, with the N end mirrors of the cavity, a laser cavity common to the plurality of gain media and allowing the coherent combination of the elementary beams; − the N phase-collimated elementary beams being configured to form a input spatial mode of the multi-plane conversion device chosen so that the multi-plane conversion device generates the principal mode (PM), associated with a principal beam (PB); − the output mirror being configured to partially reflect the principal beam.

Description

DESCRIPTION TITLE: Laser phasing device with a multiplane converter FIELD OF THE INVENTION

[0001] The present invention relates to the coherent combination of lasers and more particularly to the coherent combination using a spatial light multiplexer of the multiplane converter device type or MPLC, for MultiPlane Light Conversion. STATE OF THE ART

[0002] Coherent laser combining is an effective solution for pooling the power of N lower power lasers and thus increasing output power while maintaining beam quality close to the diffraction limit.

[0003] Indeed, for example, optronic countermeasure applications require the generation of mid-infrared signals powerful enough to dazzle or even destroy missile thermal sensors. This requires powers greater than 10 W. The development of such lasers is a significant technological challenge.

[0004] These powers can be obtained by parametric conversion of near-infrared sources but at the cost of great complexity and are limited to pulsed regimes. On the other hand, quantum cascade lasers (QCLs) are good candidates for these applications but are limited to about 2 W in continuous mode. Thus, to obtain the necessary power, the coherent combination of a few QCLs is an option that can solve this problem.

[0005] Besides quantum cascade lasers (QCLs) emitting in the mid-infrared for optronic countermeasures, more generally coherent laser combining can be applied to different laser technologies, for different applications such as fiber lasers for laser weapons, or free space telecommunications.

[0006] A great many coherent combination methods have been proposed, more or less complex to implement, and with efficiencies of variable combination. One of the main challenges is to phase the different lasers to form a single beam with minimal diffraction at the output of the system.

[0007] A distinction is made between active and passive phasing. Active phasing involves measuring the phase of each laser and using a phase modulator associated with each phase-locked laser. This solution is complex and expensive to implement. In addition, there are no efficient phase modulators with low losses for all wavelengths of interest, for example, for the mid-IR range 3-12 pm.

[0008] For applications requiring a limited number of channels to be phased, the formation of a single laser cavity containing all the channels is a suitable solution to reduce the complexity of implementation because it does not require the addition of phase modulators to be controlled (passive phasing).

[0009] This has been implemented in different configurations to form a common cavity, among which we can cite the Dammann grating (diffractive optical element or DOE). The passive coherent combination of QCLs with a diffractive component, called a Dammann grating, is for example described in the publication by G. Bloom et al "Passive coherent beam combining of quantum-cascade lasers with a Dammann grating," Opt. Lett. 36, 3810-3812 (2011) and illustrated in Figure 1. The principle consists of generating and amplifying the laser beams in individual gain media located inside a common resonator. This common resonator generates the feedback on all lasers and selects an output supermode.Practically a set of QCLs (QCL1 to QCL5), each QCL being semi-open cavity (a single mirror present on the cavity bottom side) and collimated by a lens CL, are incident on the Damman grating DG according to angles associated with the different diffraction orders of the grating. The grating is calculated so that all the energy is theoretically concentrated in order 0. A partially reflecting mirror OC closes the laser cavity common to all QCLs.

[0010] In this configuration the resonator, common to all QCLs, must “find” a longitudinal mode common to all channels, in the bandwidth of gain of the QCLs. To do this, it is necessary to introduce a difference in optical path between the different channels, hoping that statistically a longitudinal mode common to all channels exists.

[0011] Besides a Damman grating, passive phasing based on supermode selection can be achieved with another spatial filter such as a Michelson cavity or by Talbot effect.

[0012] However, these configurations are complex to implement, particularly in terms of alignment tolerance, stability and because of the need to introduce an optical path difference between all channels, which limits the maximum output power. Thus, this configuration has limited efficiency. Moreover, the realization of a high-performance Damman network is not easy.

[0013] An aim of the present invention is to remedy the aforementioned drawbacks by proposing a laser phasing device according to a configuration based on the production of a common cavity and using an easily available component which does not present alignment problems. DESCRIPTION OF THE INVENTION

[0014] The present invention relates to a device for phasing laser beams comprising: - a plurality of N semi-open cavities, each semi-open cavity comprising a gain medium and an associated mirror called a cavity bottom mirror, and being configured to generate a collimated elementary beam, - a multi-plane conversion device configured to convert a set of N input spatial modes forming an input basis into a set of N output spatial modes forming an output basis, the output basis comprising a desired output spatial mode, called the main mode, and N-1 other output modes called secondary modes, - a partially reflective output mirror configured to form, with the N cavity bottom mirrors, a laser cavity common to said plurality of gain media in which the multiplane conversion device is arranged and allowing the coherent combination of said elementary beams, - the N elementary beams collimated in phase being configured to form an input spatial mode of the multiplane conversion device chosen so that the multiplane conversion device generates the main mode, associated with a main beam, - the output mirror being configured to partially reflect said main beam.

[0015] According to one embodiment, the multiplane conversion device is further configured so that the main mode is spatially separated from the N-1 secondary modes.

[0016] According to one embodiment, the output mirror has a concave surface and is arranged so that the main beam is located in the center of said output mirror.

[0017] According to one embodiment, the output mirror has a dimension suitable for reflecting only the main beam.

[0018] According to one embodiment, each semi-open cavity is a semi-open cavity of a quantum cascade laser.

[0019] According to another embodiment, each gain medium is an optically pumped active optical fiber and the associated cavity bottom mirror is a fiber Bragg mirror.

[0020] According to yet another embodiment, each gain medium is a laser crystal, the cavity bottom mirror consisting of a face treated to reflect a laser wavelength serving as a cavity bottom mirror or a separate mirror.

[0021] According to one embodiment, the multiplane conversion device is configured so that the main beam is Gaussian.

[0022] According to one embodiment, the multiplane conversion device is configured so that the chosen input spatial mode corresponds to a substantially equal amplitude of the N elementary beams.

[0023] According to one embodiment, the multiplane conversion device is configured so that all the secondary modes are located at the same position.

[0024] According to a variant, each gain medium is pumped via a supply current, further comprising a detector configured to detect a fraction of the main beam at the exit of the cavity and servo electronics configured to feedback on the supply currents so as to maximize a power of the detected beam.

[0025] According to another variant, each gain medium is pumped via a supply current, further comprising a detector configured to detect at least a fraction of at least one secondary beam associated with a secondary mode and control electronics configured to feedback on the supply currents so as to minimize a power of the at least one detected secondary beam.

[0026] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0027] The invention will be better understood and other characteristics, aims and advantages thereof will appear during the detailed description which follows and with regard to the appended drawings given as non-limiting examples and in which:

[0028] Figure 1 already cited illustrates the phasing of lasers with a diffractive component arranged in a common cavity of the state of the art.

[0029] Figure 2 illustrates the block diagram of an MPLC type device known from the state of the art.

[0030] Figure 3 illustrates a state-of-the-art MPLC-type device in which a succession of reflections on phase masks separated by propagations in free space and reflections on a mirror takes place.

[0031] Figure 4 illustrates a first example of input / output of a multiplexing operated with an MPLC type device, called “mode-selective”.

[0032] Figure 5 illustrates a second example of input / output of a multiplexing with an MPLC, called "non-mode selective". The input base is formed by four combinations of four simultaneously switched beams, whose amplitude is substantially equal and whose phase relationship is controlled (the constant amplitude and the phase relationships defining the input base), and the output base is identical to that of the example of Figure 4.

[0033] Figure 6 illustrates another example of input / output of a multiplexing with a "non-mode-selective" type MPLC. The input base is formed by four combinations of four beams simultaneously switched on and whose amplitude is substantially equal and the phase relationship controlled as in Figure 5, and the output base is identical to the input base of the example in Figure 4.

[0034] Figure 7 illustrates an MPLC type device in which the output beam comprises four spatially separated FLS output elementary beams.

[0035] Figure 8 illustrates a phasing device according to the invention.

[0036] Figure 9 illustrates another example of input / output of an MPLC according to the invention suitable for producing the device according to the invention, in which the MPLC is configured so that all the secondary modes are located at the same position different from the position of the main beam.

[0037] Figure 10 illustrates the overall beam at the output of the MPLC, before (A) and after (B) the closing of the cavity by the output mirror, for an output basis composed of spatially separated Gaussian beams.

[0038] Figure 11 illustrates an embodiment in which the output mirror has a concave surface and is arranged such that the main beam is located in the center of the output mirror.

[0039] Figure 12 illustrates another embodiment in which the output mirror has a dimension adapted to reflect only the main beam.

[0040] Figure 13 illustrates an embodiment in which each gain medium is an optically pumped active optical fiber and the associated cavity bottom mirror is a fiber Bragg mirror.

[0041] Figure 14 illustrates a variant of the device according to the invention in which a detected power is used for optimizing the power of the output beam. According to a first embodiment illustrated in a) the detector is configured to detect a fraction of the output beam and the SCE electronics feed back on the supply currents so as to maximize the power of the detected beam and therefore of the output beam. According to a second embodiment illustrated in b) the detector is configured to detect at least a fraction of at least one secondary beam associated with a secondary mode.

[0042] Figure 15 illustrates a device according to the invention with a strip of QCLs as a gain medium and feedback according to the second embodiment of the variant.

[0043] Figure 16 illustrates a device according to the invention with active optical fibers as gain medium and feedback according to the first embodiment of the variant. DETAILED DESCRIPTION OF THE INVENTION

[0044] The phasing device according to the invention uses a spatial light multiplexer called a multiplane conversion device or MPLC inserted in a cavity common to the different lasers to be phased. This component is known from the state of the art and its operating principle is recalled below. Its production is mastered for example by the Cailabs Company in France.

[0045] In this application, light radiation is defined as radiation formed from at least one mode of the electromagnetic field, each mode forming a spatio-temporal distribution of the amplitude, phase and polarization of the field. The modification or transformation of the phase of the light radiation designates the spatio-temporal modification or transformation of each of the modes of the radiation.

[0046] The "shape" of a light radiation / beam refers to the transverse distribution of the amplitude and phase of the mode or the combination of the transverse distributions of amplitude and phase of the modes composing this radiation / beam.

[0047] The MPLC device, illustrated in Figure 2, implements means capable of modifying the shape of an incident light beam. This modification can be precisely described in modal form, that is to say by defining how input modes forming a family of modes defined in a transverse input plane PE, are transformed into output modes of a family of output modes defined in a transverse output plane PS. The families of input and output modes form orthonormal bases. Thus, a multiplane MPLC device is designed to transform a base of predefined input modes into a base of equally predefined output modes. The choice of the input base and the output base determines the design of the MPLC. The base of input modes implements N elementary input beams FLI spatially separated, collectively forming, in an input plane transverse to said beams PE, an incident light beam ILB.The MLPC component generates in an output plane PS an output light beam OLB materialized in figure 2 by the output beam FLS.

[0048] During the modal conversion implemented by the device 1, a maximum of energy (theoretically 100%) of the incident radiation present in the input modes is transported into a so-called target output mode.

[0049] The MPLC device 1, for example described in documents WO201 9 / 162637 and WO 2020 / 161126, performs a unitary modal transformation.

[0050] In the device 1, the incident light radiation undergoes, in the DC conversion device arranged between the input plane PE and the output plane PS, a succession of reflections and / or transmissions through phase masks, each reflection and / or transmission being followed by propagation of the radiation in free space. Thus, a multi-plane conversion device comprises a plurality of phase masks separated by propagation in free space, and provides a succession of reflections and / or transmissions through these phase masks. An example in which a succession of reflections is illustrated Typically at least some of the optical parts 5 on which the reflections (and / or transmissions) take place and which guide the propagation of the radiation, have a microstructured surface which modifies the light radiation incident on said part. Such an optical part forms a phase mask introducing a local phase shift within the transverse section of the radiation which is reflected there or transmitted there.

[0051] Thus, light radiation propagating within an MPLC device undergoes a succession of local phase shifts separated by propagations. The succession of these elementary transformations (for example 4, 8, 10, 12, 14 or even 20 elementary transformations) establishes a global transformation of the spatial profile of the incident light radiation. The microstructured surfaces are therefore configured in such a way as to transform the incident light radiation, which has a specific shape, into an output radiation whose shape is different.

[0052] In Figure 3 the DC conversion device consists of two reflective optical parts 2a, 2b arranged opposite each other. Microstructured phase masks 5 are carried by one of these parts. A multi-passage cavity is thus formed in which the incident light radiation is reflected and phase-shifted a plurality of times at each mask 5 to form the transformed radiation, making it possible to form the output radiation. In this example the phase masks 5 are all carried by a phase plate 2a having microstructured zones forming the masks 5, the second optical part 2b being a simple mirror.

[0053] The optical input stage EE is for example formed from a bundle of optical fibers making it possible to position the input light beams FLI opposite the rest of the optical device 1. The output stage may comprise optical elements for shaping the output beam and / or complementary output beams. But the input and / or output plane may also be formed from a fictitious plane transverse to the input or output beams, which in this case propagate freely.

[0054] The FLI input light beams can be distributed, in the PE plane, in a single direction, in a bar. They can also be distributed in this PE plane in two directions, in a matrix or in a hexagonal distribution. Any number of such beams can be provided, for example 2 to 10, more than 10, or even several dozen.

[0055] The device 1 thus transforms a base of input modes associated with the N elementary incident beams FLI into a base of output modes. The base change operated by the MPLC is a spatial unitary transformation of the input electromagnetic field. In a known manner, the transfer matrix T describing the base change operated by the MPLC is a Hadamard or Fourier matrix. The principle of coding an MPLC by a Hadamard or Fourier matrix is ​​described for example in the document by Wen et al “Scalable non-mode selective Hermite-Gaussian mode multiplexer based on multi-plane light conversion” Photonic Research Vol. 9 n°2 (2021). The unitary Hadamard matrix only comprises 1s and -1s, the Fourier matrix comprises complex coefficients of amplitude 1. Once the Hadamard or Fourier matrix is ​​defined, the structures of the phase masks are calculated by different algorithms also known to those skilled in the art.

[0056] A first example of multiplexing with an MPLC is called "mode-selective" MS, and an illustration of an input / output mapping is shown in Figure 4, with N=4. In MS usage the input basis is formed by four spatial input modes (EM1, EM2, EM3, EM4) corresponding to four combinations of four spatially separated beams, only one of which is on at a time. The output basis consists of four Hermite-Gauss modes (OM1, OM2, OM3, OM4) located at the same position. All output modes are spatially superimposed, and each output mode comes from a single FLI input light beam.

[0057] A second example of multiplexing with an MPLC, called "non-mode selective" NMS, is illustrated in Figure 6. The input base is formed by four combinations of four beams simultaneously lit and whose amplitude is approximately equal and whose phase is controlled (0 or K). The constant amplitude and phase values ​​define the input base. In NMS use each output mode is derived from the superposition of all FLI input light beams.

[0058] In Figure 5 the output base is identical to the example in Figure 4 (HG modes).

[0059] In Figure 6 the output base is identical to the input base of Figure 4, i.e. a set of four spatially separated beams. As illustrated in Figure 7 the OLB output beam then comprises four spatially separated FLS elementary output beams. For example an FLS output beam is an HGoo mode. When the phase of the FLI input beams is controlled as illustrated in Figure 6, only one FLS output beam is on at a time. The MPLC theoretically makes it possible to combine all the power of the N FLI input beams into the "perfect" HG mode. 00 .

[0060] A preferred output mode is designated as the target or main mode OMp corresponding to the beam FLSp in Figure 7, and the three (N-1) other output modes as secondary modes OMs, corresponding to the beams FLSs. The input mode base consists of a super mode EMs transformed by the MPLC into the main output mode, and 3 (N-1) other input modes EMe.

[0061] In this example, the EMs supermode is transformed into a Gaussian beam at the desired position (A) and the other EMe input modes are converted into an FLSs beam at a spatially separated position (B, C and D). This results in a discrete shift of an HGoo output beam as a function of the phases applied to the FLI input beams. This pixel-like NMS use illustrates the ability of an MPLC to perform output beam depointing, i.e. to move the position of the output beam as a function of the phase differences applied to the N input beams. This application of interest is for example described in the publication by Billaud et al “Optimal coherent beam combining based on Multi-Plane Light Conversion for high throughput optical feeder links”, International Conference on Space Optical Systems and Applications (ICSOS) IEEE, 2019.

[0062] It is also known from the state of the art to use an MPLC component to carry out active phasing of several laser beams. (QCLs) from the same master source and amplified individually, in association with phase modulators (so-called amplifier configuration) as described in the aforementioned document WO 2020 / 161126. The beams from the different amplifiers are phase-controlled using the modulators before entering the MPLC component which combines them spatially. This solution, although effective and able to operate with a large number of channels, is complex to implement, in particular with QCL-type amplifying media.

[0063] The phasing device 10 according to the invention is illustrated It comprises a plurality of N semi-open cavities, each semi-open cavity comprising a gain medium GM and an associated mirror MC called cavity back mirror. In addition, each semi-open cavity is configured to generate an elementary beam EB (on the side opposite that of the mirror MC) which is collimated, typically with a dedicated microlens CL.

[0064] The device 10 also comprises a multi-plane MPLC conversion device configured to convert a set of N input spatial modes forming an input basis into a set of N output spatial modes forming an output basis, the principle of which has been described above. The output basis comprises a desired output spatial mode, called the main mode PM, and N-1 other output modes called secondary modes SM.

[0065] The device further comprises a partially reflecting OM output mirror configured to form, with the N cavity bottom mirrors, a laser cavity common to the plurality of gain media, in which the MPLC device is arranged. With the presence of the OM mirror which closes the cavity, this cavity common to all gain media allows the coherent combination of the elementary beams EB. Indeed, the use of a cavity common to all the "lasers" (semi-open cavities + OM mirror), where the optical feedback is the same for all the lasers, naturally selects a laser mode (longitudinal and transverse) common to all the lasers. This performs the coherent combination of all the channels. The main beam after its exit from the cavity (fraction of PB transmitted by the OM mirror) is called PBout.

[0066] The N elementary beams collimated in phase are configured to form an input spatial mode of the multiplane conversion device MPLC chosen so that the multiplane conversion device generates the desired output spatial mode or main mode PM, associated with a main beam PB. The plurality of elementary beams thus enters an MPLC component designed to perform the coherent combination of the beams. The N elementary beams form the supermode as described above: a particular output mode, the so-called main mode, corresponds to a given phase play of the input channels. The reflecting mirror is configured to partially reflect the main beam FB.

[0067] Secondary beams SB are associated with the secondary output modes. If all the output beams, i.e. the main beam (main mode) and the secondary beams (secondary modes), are located at the same position (example in Figure 5), the cavity is capable of lasing on one or other of these modes, or even of jumping from one mode to another, and the selection of the main mode is not easy. Thus, preferably, the multiplane conversion device MPLC is further configured so that the main mode is spatially separated from the N-1 other output modes called secondary modes SM, and the main beam is partially reflected by the output mirror. The spatial separation of the main beam PB from the secondary beams SB makes it possible, via the mirror OM, to select with certainty the main mode as the mode on which the resonator formed by the common cavity will lase.

[0068] An example of input / output of an MPLC suitable for producing the device 10 according to the invention is illustrated in FIG. 6. In this example the multiplane conversion device is configured so that the secondary modes, i.e. the secondary beams SB, are located at a different position B, C, D, themselves different from the position A of the main beam FB.

[0069] Another example of input / output of an MPLC according to the invention suitable for producing the device 10 according to the invention is illustrated in figure 9. In this example the multiplane conversion device is configured so that all the secondary modes, i.e. all the secondary beams SB, are located at the same position P2, different from the position P1 of the beam PB. In this example the output base is a Hermite-Gauss base.

[0070] The output basis is for example chosen from: a Hermite-Gauss basis (see figure 9), a Laguerre-Gauss basis, a basis of spatially separated beams (see figure 6), any other basis.

[0071] Preferably for the quality of the output beam (minimal diffraction), the multiplane conversion device according to the invention is configured so that the main beam is Gaussian, i.e. according to the HGoo- mode.

[0072] Preferably, the chosen input spatial mode corresponds to a substantially equal amplitude of the N elementary beams. In theory, the MPLC is preferably designed so that the elementary beams of the input supermode have an identical intensity. However, the practical implementation of the MPLC leads to different losses on each of the channels, and according to one embodiment, these differentiated losses are compensated for on the input power of the N elementary beams.

[0073] Thus the set of gain media constitutes the free space input of an MPLC allowing the coherent combination of beams to be carried out, i.e. an output mode corresponds to a phase distribution of the input modes, for a power substantially equal on all channels. At the output of the MPLC, a laser output coupler (partially reflecting mirror) closes the resonator common to all channels. For an MPLC designed so that the main mode is a Gaussian beam, the beam at the output of the device 10 according to the invention is then a laser beam of minimal diffraction (Gaussian mode).

[0074] Figure 10 illustrates the overall beam at the output of the MPLC, OLB, before (A) and after (B) the closure of the cavity by the OM mirror, for an output basis composed of spatially separated Gaussian beams. In A the elementary input beams have any phase between them, the OLB beam is a random superposition of all the output modes and the incident energy is distributed in all the output beams. In B the cavity is closed and lases on the main mode, the elementary input beams have a phase relationship corresponding to the input supermode associated with the main mode. In theory all the energy is concentrated in the main beam FP due to the closure of the cavity whose resonance naturally selects the main beam FP. In practice a small fraction of the energy is found in the secondary beams due to manufacturing imperfections of the MPLC component.

[0075] The use of an MPLC as a common resonator for passive phasing allows to overcome the constraints of known state-of-the-art techniques, with a simpler practical implementation. Indeed, the production of MPLCs is controlled by a supplier (Cailabs company) who finely controls the alignment, unlike solutions based on diffractive components (DOE, for example Dammann grating). The alignment of the MPLC component is certainly critical, similar to that of a DOE, but is controlled by the supplier who produces it in series. Thus, the proposed MPLC-based solution according to the invention benefits from an already existing value chain, facilitating its implementation.

[0076] The MPLC component theoretically allows for 100% concentration of the energy of the elementary input beams into the main output beam. An MPLC has the advantage of having low losses and therefore limiting resonator losses and optimizing laser power. In addition, this component is robust, its alignment is well controlled and the component's footprint is limited. The resulting solution according to the invention is therefore compact and suitable for military environments.

[0077] The device according to the invention is particularly well suited to the pooling of a few optical channels, making it possible to achieve the ten watts of optical output power required for optronic countermeasure applications. The resulting system is compact, robust and easy to implement. It does not require the addition of phase modulators.

[0078] According to one embodiment, the output mirror OM has a concave surface and is arranged so that the main beam is located in the center of the output mirror, as illustrated in Figure 11. The radius of curvature of the output coupler naturally selects a single spot.

[0079] According to an embodiment illustrated in Figure 12, the output mirror has a dimension adapted to reflect only the main beam. This mirror is for example flat or concave with the main beam arranged in the center of the mirror. The alignment of the resonator is in the latter case greatly simplified.

[0080] According to a first embodiment, each semi-open cavity is a semi-open cavity of a quantum cascade laser or QCL. Typically, QCL lasers emit in the mid to far IR (including the 3-12 pm band). Preferably, the MC cavity bottom mirror is a reflective deposit deposited on one face of the gain medium. Preferably, the QCLs are arranged on a strip, or are placed side by side on the same chip, so that they are close enough to each other to constitute the input of the MPLC component. Coupling the beams at the output of the gain medium on the side opposite the MC mirrors with a matrix of micro-lenses, or individually placed lenses, achieves the collimation of the beams.

[0081] According to a second embodiment, each gain medium is an active optical fiber AF optically pumped, typically by a laser diode DL, and the associated cavity bottom mirror is a fibered Bragg mirror BM, as illustrated The Bragg mirror is either external and connected to the fiber or directly inscribed in the active fiber. The active optical fiber is typically doped with Yb, Er, Tm, Ho, Nd, etc. This embodiment is suitable for the emission of wavelengths around 1 pm, 1.5 pm and 2 pm.

[0082] According to a third embodiment, each gain medium is a laser crystal, the cavity bottom mirror being made up of either a face treated to reflect a laser wavelength serving as a cavity bottom mirror, or a separate mirror. The associated wavelengths are diverse, depending on the crystal used, for example 1064 nm for Nd, 1030 nm for Yb, and around 1550 nm for Er.

[0083] According to a variant, a feedback of the output power of the device 10 is carried out on the pump current of the different laser channels, so as to optimize the total output power by correctly balancing the channels. To implement this variant, it is appropriate for each gain medium to be pumped via an AC supply current. This pumping is either direct, as for example for QCLs which are electrically pumped, or indirect as for example for active optical fibers which are optically pumped by laser diodes, themselves electrically pumped via a supply current. With the feedback, the power of each semi-open cavity is optimized to minimize losses for a given cavity and therefore optimize output power. Feedback also helps find a common longitudinal laser mode.

[0084] According to this variant, the device according to the invention further comprises a detector Det configured to detect the power on which the feedback operates and a control electronics SCE configured to feedback on the different AC supply currents.

[0085] According to a first embodiment illustrated in figure 14 a) the detector Det is configured to detect a fraction of the output beam PBout and the electronics SCE feedbacks on the supply currents so as to maximize the power of the fraction of PBout detected, and therefore the power of the output beam Pout. For this, a sampling blade BS is positioned on the path of PBout, which has the disadvantage of sampling a fraction of PBout which is therefore not available for the application.

[0086] According to a second embodiment illustrated in Figure 14 b) the detector Det is configured to detect at least a fraction of at least one secondary beam SB associated with a secondary mode, and to feedback on the supply currents so as to minimize the power of the at least one secondary beam detected. Preferably the detector is configured to detect all the secondary beams, either using focusing optics or because the MPLC has been designed to locate all the secondary modes at the same position (see for example Figure 9). This configuration has the advantage of not impacting the power of the output beam PBout.

[0087] A device 10 according to the invention with a strip of QCLs as gain medium and feedback according to the second embodiment of the variant is illustrated in figure 15.

[0088] A device 10 according to the invention with active optical fibers as gain medium and feedback according to the first embodiment of the variant is illustrated in Figure 16.

Claims

CLAIMS 1. Device (10) for phasing laser beams comprising: - a plurality of N semi-open cavities, each semi-open cavity comprising a gain medium (GM) and an associated mirror (MC) called a cavity bottom mirror, and being configured to generate a collimated elementary beam (EB), - a multi-plane conversion device (MPLC) configured to convert a set of N input spatial modes forming an input basis into a set of N output spatial modes forming an output basis, the output basis comprising a desired output spatial mode, called the main mode (PM), and N-1 other output modes called secondary modes (SM), - a partially reflecting output mirror (OM) configured to form, with the N cavity bottom mirrors, a laser cavity common to said plurality of gain media in which the multiplane conversion device is arranged and allowing the coherent combination of said elementary beams, - the N elementary beams collimated in phase being configured to form an input spatial mode of the multiplane conversion device chosen so that the multiplane conversion device generates the main mode (PM), associated with a main beam (PB), - the output mirror being configured to partially reflect said main beam.

2. Phasing device according to the preceding claim wherein the multiplane conversion device is further configured so that the main mode is spatially separated from the N-1 secondary modes (SM).

3. Phasing device according to one of the preceding claims wherein the output mirror has a concave surface and is arranged so that the main beam is located at the center of said output mirror.

4. Phasing device according to one of claims 2 or 3 in which the output mirror has a dimension adapted to reflect only the main beam.

5. Phasing device according to one of the preceding claims in which each semi-open cavity is a semi-open cavity of a quantum cascade laser.

6. Phasing device according to one of claims 1 to 4 in which each gain medium is an optically pumped active optical fiber and the associated cavity bottom mirror is a fiber Bragg mirror.

7. Phasing device according to one of claims 1 to 4 in which each gain medium is a laser crystal, the cavity bottom mirror consisting of a face treated to reflect a laser wavelength serving as a cavity bottom mirror or a separate mirror.

8. Phasing device according to one of the preceding claims in which the multi-plane conversion device is configured so that the main beam is Gaussian.

9. Phasing device according to one of the preceding claims in which the multiplane conversion device is configured so that the chosen input spatial mode corresponds to a substantially equal amplitude of the N elementary beams.

0. Phasing device according to one of the preceding claims wherein the multi-plane conversion device is configured so that all the secondary modes are located at the same position.

11. Phasing device according to one of the preceding claims in which each gain medium is pumped via a supply current (AC), further comprising a detector (Det) configured to detect a fraction of the main beam at the output of the cavity (PBout) and servo electronics (SCE) configured to feedback on the supply currents so as to maximize a power of the detected beam.

12. Phasing device according to one of claims 1 to 10 in which each gain medium is pumped via a supply current, further comprising a detector (Det) configured to detect at least a fraction of at least one secondary beam associated with a secondary mode and servo electronics (SCE) configured to feedback on the supply currents so as to minimize a power of the at least one detected secondary beam.

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