Laser phasing device with a multiplane converter

Abstract

The invention relates to a laser beam phasing device (10) comprising: a plurality of N semi-open cavities, a multiplane 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, a partially reflecting output mirror (OM) configured to form, with the N cavity back mirrors, a common laser cavity for said plurality of gain media and enabling the coherent combination of said elementary beams, the N phase-collimated elementary beams being configured to form an input spatial mode of the chosen multiplane conversion device such that the multiplane conversion device generates the principal mode (PM), associated with a principal beam (PB), the output mirror being configured to partially reflect said principal beam. Figure 8

Description

Title of the invention: Laser phasing device with a multiplane converter FIELD OF INVENTION

[0001] The present invention relates to the coherent combination of lasers and more particularly to coherent combination using a spatial light multiplexer of the multiplane light conversion (MPLC) type. PRIORITY OF THE TECHNOLOGY

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

[0003] Indeed, for example, optronic countermeasures applications require the generation of sufficiently powerful infrared mid-range signals to dazzle or even destroy the thermal sensors of missiles. This requires power levels exceeding 10 W. The development of such lasers is a significant technological challenge.

[0004] These power levels can be obtained by parametric conversion of near-infrared sources, but at the cost of high 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 approximately 2 W in continuous operation. Thus, to obtain the necessary power, the coherent combination of several 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 the coherent combination of lasers can be applied to different laser technologies, for different applications such as fiber lasers for laser weapons, or free-space telecommunications.

[0006] Numerous coherent combination methods have been proposed, some more complex to implement than others, and with varying combination efficiencies. One of the main challenges is to phase the different lasers to form a single beam of minimum diffraction at the system output.

[0007] Active and passive phasing are distinguished. 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. Furthermore, 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 implementation complexity because it does not require the addition of phase modulators to be controlled (passive phasing).

[0009] This has been implemented in various configurations to form a common cavity, including the Dammann grating (diffractive optical element or DOE). The passive coherent combination of QCLs with a diffractive component, called a Dammann grating, is described, for example, 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 [Fig. 1]. The principle consists of generating and amplifying the laser beams in individual gain media located within a common resonator. This common resonator generates feedback on all the lasers and selects an output supermode.Practically a set of QCLs (QCL1 to QCL5), each QCL being semi-open cavity (a single mirror present at the cavity back) and collimated by a CL lens, are incident on the Dammann DG grating at angles associated with the different diffraction orders of the grating. The grating is calculated so that all the energy is theoretically concentrated in the 0th order. A partially reflecting mirror OC closes the laser cavity common to all the QCLs.

[0010] In this configuration, the resonator, common to all QCLs, must "find" a longitudinal mode common to all channels, within the QCL gain band. To do this, an optical path difference must be introduced between the different channels, with the expectation that statistically a longitudinal mode common to all channels exists.

[0011] In addition to a Damman network, passive phasing based on the selection of a supermode 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 the need to introduce an optical path difference between all channels, which limits the maximum output power. Thus, this configuration has limited efficiency. Furthermore, implementing a high-performance Damman network is not easy.

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

[0014] The present invention relates to a laser beam phasing device comprising: • a plurality of N semi-open cavities, each semi-open cavity comprising a gain medium and an associated mirror called the cavity back mirror, and configured to generate a collimated elementary beam, • a multiplane 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 primary mode, and Nl other output modes called secondary modes, • a partially reflective output mirror configured to form, with the N cavity back mirrors, a common laser cavity for said plurality of gain media in which the multiplane conversion device is disposed and which allows the coherent combination of said elementary beams, • the N collimated in-phase elementary beams being configured to form a spatial input mode of the chosen multiplane conversion device such that the multiplane conversion device generates the principal mode, associated with a principal 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 Nl secondary modes.

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

[0017] According to one embodiment, the exit mirror has a dimension adapted to reflect 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 back mirror is a fiber Bragg mirror.

[0020] According to yet another embodiment each gain medium is a laser crystal, the cavity back mirror being made up of a face treated to reflect a laser wavelength serving as a cavity back mirror or of 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 spatial input mode corresponds to a substantially equal amplitude of the N elementary beams.

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

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

[0025] According to another embodiment 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 servo 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 embodiments of the device of the invention: these examples are not limiting to the scope of the invention. These embodiments present both the essential features of the invention and additional features related to the embodiments considered.

[0027] The invention will be better understood and other features, objectives and advantages thereof will become apparent from the following detailed description and with reference to the accompanying drawings given by way of 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 prior art.

[0029] Figure [Fig. 2] illustrates the schematic diagram of a known MPLC-type device the state of the art.

[0030] Figure 3 illustrates a known prior art MPLC-type device in which a succession of reflections on phase masks occurs, separated by free-space propagations and reflections on a mirror.

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

[0032] Figure 5 illustrates a second example of input / output of a multiplexer with a MPLC, called "non-mode selective". The input basis consists of four combinations of four beams lit simultaneously, whose amplitude is substantially equal and whose phase relationship is controlled (the constant amplitude and the phase relationships defining the input basis), and the output basis is identical to that of the example in [Fig.4].

[0033] Figure 6 illustrates another example of input / output of a multiplexer with an MPLC of the "non-mode-selective" type. The input basis consists of four combinations of four beams simultaneously lit and whose amplitude is substantially equal and whose phase relationship is controlled as in [Fig.5], and the output basis is identical to the input basis of the example in [Fig.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 the realization of the device according to the invention, in which the MPLC is configured so that all secondary modes are located at the same position different from the position of the main beam.

[0037] Fig. 10 illustrates the overall beam exiting the MPLC, before (A) and after (B) the closure of the cavity by the exit mirror, for an exit basis composed of spatially separated Gaussian beams.

[0038] Fig. 11 illustrates an embodiment in which the exit mirror has a concave surface and is arranged so that the main beam is located at the center of the exit mirror.

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

[0040] Fig. 13 illustrates an embodiment in which each gain medium is an optically pumped active optical fiber and the associated cavity back 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 to optimize the output beam power. According to a first embodiment illustrated in (a), the detector is configured to detect a fraction of the output beam, and the SCE electronics provide feedback on the supply currents so as to maximize the power of the detected beam and thus 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 bar 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 the 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 into a cavity common to the different lasers to be phased. This component is known from the prior art, and its operating principle is described below. Its manufacture is mastered, for example, by Cailabs in France.

[0045] In this application, light radiation is defined as radiation composed of at least one mode of the electromagnetic field, each mode forming a spatiotemporal distribution of the amplitude, phase, and polarization of the field. The modification or transformation of the phase of the light radiation refers to the spatiotemporal modification or transformation of each of the radiation modes.

[0046] The term "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 [Fig.2], implements means capable of modifying the form of an incident light beam. This modification can be precisely described in modal form, that is, 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 input and output mode families form orthonormal bases. Thus, a multiplane MPLC device is designed to transform a basis of predefined input modes into a basis of equally predefined output modes. The choice of the input and output bases determines the design of the MPLC. The input mode base implements N spatially separated elementary input beams FLI, 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 [Fig.2] via the FLS output beam.

[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 an output mode called the target.

[0049] The MPLC 1 device, for example described in documents WO2019 / 162637 and WO 2020 / 161126, performs a unitary modal transformation.

[0050] In device 1, the incident light 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. An example in which a succession of reflections occurs is illustrated [Fig. 3]. Typically, at least some of the optical components 5 on which the reflections (and / or transmissions) occur and which guide the propagation of the radiation, have a microstructured surface that modifies the light incident on said component. Such an optical component forms a phase mask introducing a local phase shift within the cross-section of the radiation reflected therein or which is 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 so as to transform the incident light radiation, which has a specific shape, into output radiation with a different shape.

[0052] In [Fig. 3], the DC conversion device consists of two reflective optical pieces 2a, 2b arranged opposite each other. Semi-structured phase masks 5 are carried by one of these pieces. A multi-pass cavity is thus formed in which the incident light is reflected and phase-shifted a plurality of times at each mask 5 to form the transformed radiation, which in turn forms the output radiation. In this example, the phase masks 5 are all carried by a phase plate 2a having semi-structured areas forming the masks 5, the second optical piece 2b being a simple mirror.

[0053] The optical input stage EE is, for example, formed of a bundle of optical fibers allowing the input light beams FLI to be positioned opposite the rest of the optical device 1. The output stage may include optical elements for shaping the output beam and / or complementary output beams. However, the input and / or output plane may also be formed by 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 along a single direction, in a linear array. They can also be distributed in this PE plane along two directions, in a matrix or in a hexagonal pattern. Any number of such beams can be provided, for example from 2 to 10, more than 10, or even several dozen.

[0055] Device 1 thus transforms a basis of input modes associated with the N elementary incident beams FLI into a basis of output modes. The basis change performed by the MPLC is a spatial unitary transformation of the input electromagnetic field. As is known, the transfer matrix T describing the basis change performed by the MPLC is a Hadamard or Fourier matrix. The principle of encoding 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 No. 2 (2021). The Hadamard unitary matrix comprises only 1s and -1s, while the Fourier matrix comprises complex coefficients of amplitude 1. Once the Given a defined Hadamard or Fourier matrix, the structures of the phase masks are calculated by various 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 correspondence is shown [Fig. 4], with N=4. In MS operation, the input basis consists of four spatial input modes (EM1, EM2, EM3, EM4) corresponding to four combinations of four spatially separated beams, only one of which is illuminated at a time. The output basis consists of four Hermite-Gauss modes (OMI, OM2, OM3, OM4) located at the same position. All output modes are spatially superimposed, and each output mode originates from a single input light beam (FLI).

[0057] A second example of multiplexing with an MPLC, called "non-mode selective" (NMS), is illustrated in Figures 5 and 6. The input basis consists of four combinations of four beams simultaneously illuminated, with substantially equal amplitudes and controlled phase (0 or jt). The constant amplitude and phase values ​​define the input basis. In NMS operation, each output mode results from the superposition of all the input light beams (FLI).

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

[0059] In [Fig. 6] the output base is identical to the input base of [Fig. 4], that is to say Let us say a set of four spatially separated beams. As illustrated [Fig. 7], the OLB output beam then comprises four spatially separated FLS output elementary beams. For example, an FLS output beam is an HGoo mode. When the phase of the FLI input beams is controlled as illustrated [Fig. 6], only one FLS output beam is lit at a time. MPLC theoretically allows the full power of the N FLI input beams to be combined into the "perfect" HGoo mode.

[0060] A preferred output mode is designated as the target or main mode OMp, corresponding to the FLSp beam of [Fig. 7], and the three (Nl) other output modes as secondary modes OMs, corresponding to the FLSs beams. The input mode base consists of a super mode EMs transformed by the MPLC into the main output mode, and 3 (Nl) 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 offset of an output beam HGoo as a function of the phases applied to the input beams FLI. This "pixel-like" NMS application illustrates the ability of an MPLC to perform output beam offsetting, that is, to shift the position of the output beam according to the phase differences applied to the N input beams. This application of interest is described, for example, in the publication of 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 in the prior art to use an MPLC component to achieve active phasing of several laser beams (QCLs) from the same master source and individually amplified, in conjunction with phase modulators (the 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 spatially combines them. This solution, although efficient and capable of operating with a large number of channels, is complex to implement, particularly with QCL-type amplifying media.

[0063] The phase-locking device 10 according to the invention is illustrated [Fig. 8]. It comprises a plurality of N semi-open cavities, each semi-open cavity comprising a gain medium GM and an associated mirror MC referred to as the cavity back mirror. Furthermore, each semi-open cavity is configured to generate an elementary beam EB (on the side opposite the mirror MC) which is collimated, typically with a dedicated microlens CL.

[0064] The device 10 also includes a multiplane 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 includes a desired output spatial mode, called the main mode PM, and Nl other output modes called secondary modes SM.

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

[0066] The N collimated in-phase elementary beams are configured to form an input spatial mode of the chosen MPLC multiplane conversion device such that the multiplane conversion device generates the desired output spatial mode or principal mode PM, associated with a principal beam PB. The plurality of elementary beams The input signal thus enters an MPLC component designed to perform coherent beam combination. The N elementary beams form the supermode as described above: a particular output mode, the so-called main mode, corresponds to a given phase set 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 in the same position (example in [Fig. 5]), the cavity is capable of laser in one or the 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 MPLC conversion device is further configured so that the main mode is spatially separated from the 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 allows, via the mirror OM, the main mode to be reliably selected as the mode in which the resonator formed by the common cavity will laser.

[0068] An example of an input / output of an MPLC suitable for the implementation of 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 an input / output of an MPLC according to the invention suitable for implementing the device 10 according to the invention is illustrated [Fig. 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 PI of the beam PB. In this example, the output basis is a Hermite-Gauss basis.

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

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

[0072] Preferably, the chosen spatial input 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 identical intensity. However, the practical implementation of the MPLC leads to different losses on each of the channels, and according to one embodiment, these losses are compensated for. these differentiated losses on the input power of the N elementary beams.

[0073] Thus, the set of gain media constitutes the free-space input of an MPLC, enabling the coherent combination of beams, i.e., an output mode corresponds to a phase distribution of the input modes, for substantially equal power across 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 principal mode is a Gaussian beam, the beam at the output of the device 10 according to the invention is then a laser beam of minimum diffraction (Gaussian mode).

[0074] Figure 10 illustrates the overall beam output of the MPLC, OLB, before (A) and after (B) the closure of the cavity by the mirror OM, for an output basis composed of spatially separated Gaussian beams. At A, the elementary input beams have an arbitrary phase relationship to each other, the OLB beam is a random superposition of all the output modes, and the incident energy is distributed among all the output beams. At B, the cavity is closed and focused 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 in the MPLC component.

[0075] Using an MPLC as a common resonator for passive phasing overcomes the limitations of known state-of-the-art techniques, with simpler practical implementation. Indeed, the production of MPLCs is controlled by a supplier (Cailabs) who precisely controls their alignment, unlike solutions based on diffractive components (DOE, for example, Dammann grating). While the alignment of the MPLC component is certainly critical, similar to that of a DOE, it is controlled by the supplier who mass-produces it. Thus, the proposed MPLC-based solution according to the invention benefits from an existing value chain, facilitating its implementation.

[0076] The MPLC component theoretically allows for 100% concentration of the energy from the input elementary beams into the main output beam. An MPLC has the advantage of low losses, thus limiting resonator losses and optimizing laser power. Furthermore, this component is robust, its alignment is well controlled, and its size 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 several optical channels, making it possible to reach tens of watts of power Optical output is required for optronic countermeasures 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 at the center of the output mirror, as illustrated [Fig. 11]. The radius of curvature of the output coupler naturally selects a single spot.

[0079] According to an illustrated embodiment [Fig. 12], the exit mirror has dimensions adapted to reflect only the main beam. This mirror is, for example, flat or concave with the main beam positioned at the center of the mirror. In the latter case, the alignment of the resonator is 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-infrared (including the 3-12 pm band). Preferably, the cavity back mirror MC is a reflective deposit placed 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 microlens array, or individually placed lenses, achieves beam collimation.

[0081] According to a second embodiment, each gain medium is an optically pumped AF active optical fiber, typically pumped by a DL laser diode, and the associated cavity back mirror is a fiber-reinforced BM Bragg mirror, as illustrated [Fig. 13]. The Bragg mirror is either external and connected to the fiber or directly embedded in the active fiber. The active optical fiber is typically doped with Yb, Er, Tm, Ho, Nd... This embodiment is suitable for emitting wavelengths around 1pm, 1.5pm, and 2pm.

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

[0083] According to one embodiment, the output power of the device 10 is fed back to the pump current of the different laser channels, so as to optimize the total output power by properly balancing the channels. To implement this embodiment, each gain medium must 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 optical fibers. Active lasers are optically pumped by laser diodes, which are themselves electrically pumped via a supply current. With feedback, the power of each semi-open cavity is optimized to minimize losses for a given cavity and thus maximize output power. Feedback also allows for the identification of 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 is operated and a servo electronics SCE configured to provide feedback on the different AC supply currents.

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

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

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

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

Claims

Demands

1. A laser beam phasing device (10) 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 back mirror, and configured to generate a collimated elementary beam (EB), • a multiplane 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 primary mode (PM), and Nl other output modes called secondary modes (SM), • a partially reflective output mirror (OM) configured to form, with the N cavity back mirrors,a laser cavity common to said plurality of gain media in which the multiplane conversion device is disposed and allowing the coherent combination of said elementary beams, • the N phase-collimated elementary beams being configured to form a spatial input mode of the chosen multiplane conversion device such that the multiplane conversion device generates the principal mode (PM), associated with a principal beam (PB), • the output mirror being configured to partially reflect said principal beam.

2. A phasing device according to the preceding claim in which the multiplane conversion device is further configured so that the main mode is spatially separated from the secondary modes (SM).

3. A phasing device according to any 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 mirror. output mirror.

4. A phasing device according to any one of claims 2 or 3 wherein the output mirror has a dimension adapted to reflect only the main beam.

5. A phasing device according to any one of the preceding claims wherein each semi-open cavity is a semi-open cavity of a quantum cascade laser.

6. A phasing device according to any one of claims 1 to 4 wherein each gain medium is an optically pumped active optical fiber and the associated cavity back mirror is a fiber Bragg mirror.

7. A phasing device according to any one of claims 1 to 4 in which each gain medium is a laser crystal, the cavity back mirror being made up of a face treated to reflect a laser wavelength serving as a cavity back mirror or a separate mirror.

8. A phasing device according to any one of the preceding claims wherein the multiplane conversion device is configured so that the main beam is Gaussian.

9. A phasing device according to any 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.

10. A phasing device according to any one of the preceding claims, wherein the multiplane conversion device is configured so that all secondary modes are located in the same position.

11. A phasing device according to any one of the preceding claims wherein 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 cavity outlet (PBout) and a servo electronics (SCE) configured to provide feedback on the supply currents so as to maximize a detected beam power.

12. A phasing device according to any one of claims 1 to 10, wherein 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 a control electronics (SCE) configured to provide feedback on the supply currents so as to minimize a power of at least one secondary beam detected.

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