Laser device comprising means for phasing a large number of coherent sources

0001The field of the invention is that of laser power sources using a network of fiber lasers and capable of providing a significant gain in the energy extracted. The increase in power and especially in energy of fiber lasers constitutes a major challenge for many industrial, scientific applications of metrology, lidar, ... or optronics type.

0002One of the limiting factors at the level of the power generated at the fiber outlet is constituted by the small core diameter of the fiber, since the latter is monomode and leads to the appearance of non-linear effects and / or of the threshold. amplifier damage.

0003Two research avenues are currently being explored in laboratories and aim to parallel a linear or matrix network of N or N² amplifying fibers through the following two techniques.

0004This involves wavelength multiplexing and coherent phasing of the fibers then constituting the equivalent of a phased network, carried out with individual unitary phase modulators coupled to each of the fibers.

0005However for a very large number of fibers (typically greater than 100) the wavelength recombination solution has limits and in particular in terms of spectral control.

0006In this context, the present invention provides a laser power source using a coherent and collective phasing technique, compatible with a large number of fibers constituting a fiber network.

0007More specifically, the subject of the present invention is a laser device comprising a set of amplifying fibers delivering a set of optical waves, powered by an oscillator delivering a signal wave <b>characterized in that</b>'it comprises :<ul id="ul0001" list-style="dash"><li>a coherent source emitting a coherent wave substantially at the same wavelength as the signal wave and the direction of propagation of which is inclined relative to the direction of propagation of the optical waves from the amplifying fibers;</li><li>means for making the coherent wave and the optical waves from the amplifying fibers interfere and generating an interferogram consisting of a network of fringes;</li><li>means for detecting the interferogram, the relative positioning of the fringes transcribing a phase law between fibers;</li><li>a spatial modulator;</li><li>means for processing and displaying the phase law detected on the spatial modulator, said spatial modulator being positioned so as to be able to be read by the signal wave and thus generate a signal wave modulated in phase selectively at the level of each of the fibers compensating for the phase shifts induced by said fibers.</li></ul>

0008Thus, the coherent addition makes it possible to realize a phased network with one dimension or two dimensions of N or N² fibers ensuring both a power output equal to N or N² times the elementary power of a fiber, and a source brightness corresponding to the total dimension of the emissive pupil constituting the network.

0009According to a variant of the invention, the detection means comprise a detection matrix of the charge transfer device type commonly referred to as CCD “Charge Couple Device” or of the field effect transistor detector type commonly referred to as CMOS “Complemantary Metal Oxide Semiconductor "

0010According to a variant of the invention, the detection matrix comprises a number of pixels of the order of a few 10⁶ to some 10⁷ pixels, compatible with a high spatial resolution image.

0011According to a variant of the invention, the spatial phase modulator is of the LCD liquid crystal screen type.

0012According to a variant of the invention, the spatial phase modulator has a response time of the order of ten milliseconds compatible with an image rate greater than or equal to about 100 Hz.

0013According to a variant of the invention, the means for making said coherent source and the optical beams interfere comprise a mirror.

0014According to a variant of the invention, the fibers are provided with collimating lenses of the type: networks of individual microlenses or ordered in a 1-dimensional or two-dimensional 2D network.

0015According to a variant of the invention, the amplifying fibers are polarization maintaining.

0016According to a variant of the invention, the laser device also comprises conventional optical components for shaping the beam, separating blades having a sampling rate adapted to the desired function, fiber connectors.

0017According to a variant of the invention, the device further comprises reference means whose optical axis is inclined relative to the direction of propagation of the waves from the amplifying fibers, making it possible to generate from the signal wave, a signal wave correctly oriented and comprising a direction of propagation inclined with respect to those of the waves from the amplifying fibers.

0018According to a variant of the invention, the angle between the direction of propagation of the waves from the amplifying fibers and that of the signal plane wave which interferes is of the order of a few degrees to detect each spatial period Λ of the law of phase with a sufficient number of pixels.

0019According to a variant of the invention, the wavelength of the signal wave being of the order of 1 micron to 1.5 microns, each spatial period is detected by about ten pixels and can be of the order from 50 to 100µm.

0020According to a variant of the invention, the means for displaying and processing the phase law further comprise frequency filtering means so as to retain only the spatial frequencies corresponding to the interference fringes.

0021The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which:<ul id="ul0002" list-style="dash"><li>the <figref idref="f0001">figure 1</figref> illustrates a first example of a laser device of the invention;</li><li>the <figref idref="f0001">figure 2</figref> illustrates the phenomenon of interference of the optical beams from the amplifying fibers and the inclined optical beam;</li><li>the <figref idref="f0002">figure 3</figref> illustrates the diffraction phenomenon at the level of the spatial modulator;</li><li>the <figref idref="f0002">figure 4</figref> illustrates a variant of a compact laser device according to the invention.</li></ul>

0022The general principle of the present invention lies in the integration of means for phasing coherent sources from amplifying fibers. This principle is illustrated in<figref idref="f0001">figure 1</figref> which shows a first example of a laser device according to the invention as well as the relative positioning of the basic components of the optical architecture. These components are mainly:<ul id="ul0003" list-style="dash"><li>amplifying fibers 3 provided with collimating lenses of the type: individual or ordered microlens arrays in a one-dimensional or two-dimensional network. The fibers are preferably polarization maintaining;</li><li>a one-dimensional or two-dimensional spatial modulator 2 suitable for controlling the phase of each elementary amplifying fiber. The response time of this spatial modulator of the LCD screen type must be compatible with an image rate greater than or equal to 100 Hz typically;</li><li>a matrix of detectors 7 of the CCD or CMOS type having a number of pixels compatible with a high spatial resolution image typically of some 10⁶ to some 10⁷ pixels or more;</li></ul><ul id="ul0004" list-style="dash"><li>conventional optical components for shaping the beam, not shown, separating blades having a sampling rate adapted to the desired function, fiber connectors, etc .;</li><li>display and signal processing means 6 for controlling the spatial modulator.</li></ul>

0023The signal wave 1 coming from an oscillator not shown feeds through a selectively transparent mirror the amplifier fibers 3, the amplified waves coming from said fibers are reflected by the mirror 5.

0024A coherent annex 4 source is transmitted through the mirror 5. It has a propagation direction making an angle θ with the axis of propagation of the amplified waves coming from the amplifying fibers so as to be able to interfere with them, part of said coherent source being taken to probe all the fibers.

0025The phase of the fiber network can thus be probed by the very low power annex 4 coherent source, of wavelength substantially equal to that of oscillator 1 which generates a coherent wave which is used for the injection of the fiber network. amplifiers.

0026The relative phase between fibers is detected by interference from a plane reference wave coming from the coherent source annex 4 and the plane waves coming from the amplifying fibers.

0027The result of these interferences is a network of fringes of the same pitch and whose relative phase is equal to ϕ<i>_i</i>(<i>x</i>), collected by a detector 7 and then sent to a spatial modulator 2, one-dimensional or two-dimensional.

0028According to this principle detailed on the <figref idref="f0001">figure 2</figref>is the relative positioning of the fringes f₁, ..., f_i on the detector which translates the phase shift between fibers F₁, ..., F_i. If the phases change, taking into account the "slow" effects of temperature, constraints, acoustic noise, etc., the interference is detected in real time at a rate greater than or equal to 100 Hz.

0029The angle between the direction of propagation of the waves from the amplifying fibers and that of the signal plane wave which interfere is chosen to be small (typically less than 1 degree) to detect each spatial period Λ (typically of the order of 50 μm) with a sufficient number of pixels, for example less than 10 pixels, corresponding to a size of the elementary CCD pixel of 5 μm for a value of Λ = 50 μm. or 100µm typically.

0030The interferogram detected according to the principles shown on the <figref idref="f0001">Figures 1 and 2</figref> is registered on the two-dimensional phase modulator.

0031The wave coming from oscillator 1 is then spatially modulated and it diffracts, under the conditions of reading of the <figref idref="f0002">figure 3</figref>, a wave whose phase at the level of each fiber is worth -ϕ<i>_i</i>(<i>x</i>). This phase law therefore compensates at all times for the phase shifts induced by the fibers of the two-dimensional network of amplifiers, thus performing the phase-shifting function at the level of the resulting wave 8.

0032More precisely, the spatial phase distribution detected after CCD reading is worth: <maths id="math0001"><math display="block"><mi mathvariant="normal">Φ</mi><mfenced><mi>x</mi></mfenced><mo>=</mo><msub><mi>ϕ</mi><mi>o</mi></msub><mo>+</mo><msub><mi>δϕ</mi><mi>i</mi></msub><mo>⁢</mo><mi>cos</mi><mfenced open="[" close="]"><mi mathvariant="italic">Kx</mi><mo>+</mo><msub><mi>ϕ</mi><mi>i</mi></msub><mfenced><mi>t</mi></mfenced></mfenced></math><img file="EP2136239B1_D0001.tif" /></maths>

0033The phase law written on the LCD according to the reading diagram of the <figref idref="f0001">Figures 1 and 2</figref> is worth: <maths id="math0002"><math display="block"><mi mathvariant="normal">Φ</mi><mfenced><mi>x</mi></mfenced><mo>=</mo><msub><mi>ϕ</mi><mi>o</mi></msub><mo>+</mo><msub><mi>δϕ</mi><mi>i</mi></msub><mo>⁢</mo><mi>cos</mi><mfenced open="[" close="]"><mi mathvariant="italic">Kx</mi><mo>-</mo><msub><mi>ϕ</mi><mi>i</mi></msub><mfenced><mi>t</mi></mfenced></mfenced></math><img file="EP2136239B1_D0002.tif" /></maths>

0034After diffraction it generates a wave whose phase is -ϕ<i>_i</i>(<i>x</i>).

0035Oscillator 1 is coupled by diffraction by the liquid crystal spatial modulator over all of the amplifying fibers. This function is performed by a spatial modulator screen having a resolution equivalent to that of the detection matrix and having to perform this function at least a few 10⁶ to some 10⁷ pixels.

0036These orders of magnitude are perfectly compatible with the LCD technology displaying images called "high definition television" commonly known as HDTV at a rate of 100 Hz. This screen can be considered as a dynamic diffractive element within the framework of the invention.

0037The general idea of ​​the phasing proposed in the present invention thus consists in carrying out in parallel and at a given time a measurement of the relative phase between the different fibers on the CCD detector in order to then compensate in real time for this relative phase shift effect. between fibers of the network by diffraction from the LCD phase screen, small and compatible with the pixel size. The detected interferogram is displayed on the spatial modulator according to the diagram of the<figref idref="f0001">figure 2</figref>. Under these conditions, the spatial phase modulator diffracts a wave ensuring the phasing of the entire fiber network.

0038According to a variant of the invention, it is also possible to probe at the wavelength λ the phase of each amplifier by a very weak retroreflection of the wave at the end of each fiber (double passage of the probe beam). The detector receives an interference network of the form:<maths id="math0003"><math display="block"><msub><mi mathvariant="normal">I</mi><mi>o</mi></msub><mo>⁢</mo><mfenced><mn>1</mn><mo>+</mo><mi>cos</mi><mfenced open="[" close="]"><mi mathvariant="italic">Kx</mi><mo>+</mo><mn>2</mn><mo>⁢</mo><msub><mi>ϕ</mi><mn>2</mn></msub><mfenced><mi>t</mi></mfenced></mfenced></mfenced><mn>.</mn></math><img file="EP2136239B1_D0003.tif" /></maths>

0039After processing the signal from the CCD or CMOS type detection matrix, a diffraction law is displayed on the LCD spatial modulator: <maths id="math0004"><math display="block"><mi mathvariant="normal">Φ</mi><mfenced><mi>x</mi></mfenced><mo>=</mo><msub><mi>ϕ</mi><mi>o</mi></msub><mo>+</mo><msub><mi>δϕ</mi><mi>i</mi></msub><mo>⁢</mo><mi>cos</mi><mfenced open="[" close="]"><mi mathvariant="italic">Kx</mi><mo>-</mo><msub><mi>ϕ</mi><mi>i</mi></msub><mfenced><mi>t</mi></mfenced></mfenced><mn>.</mn></math><img file="EP2136239B1_D0004.tif" /></maths>

0040This phase law diffracts under the same conditions as those illustrated in <figref idref="f0001">figure 2</figref> and compensates for the single phase shift simple<i>_i</i>(<i>x</i>) due to each amplifier.

0041The invention is particularly well suited in the context of the use of single-mode fibers with polarization maintenance.

0042In the case where the fibers are depolarizing, the principles used and described above are compatible with the simultaneous processing of the two polarization components from a polarization splitter.

0043According to a variant of the invention, it is advantageously possible to integrate a component of the polarization controller type allowing for each amplifying fiber to re-establish a state of linear polarization and adapted to diffraction by the LCD matrix. It can be as described in the patent application filed by the applicant and published under the number<patcit id="pcit0001" dnum="FR2907547"><text>FR 2 907 547</text></patcit> concerning a polarimetric imaging system with a matrix of programmable material-based wave blades with an isotropic electro-optical tensor, of a component corresponding to a means for programmable analysis of an incident polarization distribution

Examples of embodiment of a laser device according to the invention:

0044The device includes a signal wave source of wavelength typically 1 μm or 1.5 μm, the angle θ between the two beams interfering on the detection matrix is ​​of the order of 10 mradian leading to a spatial period of l 'around 100 µm.

0045The two-dimensional network of single-mode amplifying fibers can typically be a network of 10 × 10, collimated at each end, by microlenses, the beam diameters being of the order of 1 mm to 2 mm. Each elementary beam is collected by approximately 200 pixels of CCD matrix and LCD matrix.

0046The device can also comprise an “LCOS” type matrix corresponding to a liquid crystal matrix on silicon substrate and operating in reflection, typically comprising 4x10⁶ pixels.

0047According to a variant of the invention, the phase modulator can also be of the “MOEMS” type of English terminology: Micro-Opto-Electro-Mechanical Systems, simultaneously offering mechanical, electrical and optical functions. Under the effect of an applied electric field and by electrostatic force, certain unitary elements can be brought closer to the substrate thereby creating a network structure. Typically these elements can have lengths of around a few tens of microns for widths of around a few microns. Arrays of micro-mirrors are thus produced capable of reflecting or not reflecting the light beam, electrically controllable and thereby enabling binary or analog phase control.

0048A diffraction efficiency of the maximum phase modulator is obtained of approximately 34% and typically greater than 20% taking into account the near IR operation.

0049The diffraction orders are filtered by the core of each fiber. Control of the transmission direction is made possible by introducing a linear phase law superimposed on the compensation law.

0050According to a variant of the invention illustrated in <figref idref="f0002">figure 4</figref>, the emission pupil may be incomplete, thanks to the presence of fibers distributed in a more or less dense manner in space with the absence of local fibers and a more or less distributed pupil. It is thus possible from a surface of about 1 cm² at the fiber outlet to create a very large source of about 1 m² without necessarily needing a large quantity of fibers constituting the fiber network.

0051The embodiment is compatible with operation in pulse mode of the source.

0052It should be noted that the wavelength of the probe beam which is used for the phase measurement of each fiber can be slightly different from the emission wavelength of the source, it is possible to display on the modulation modulation matrix phase, a calculated phase law taking this wavelength difference into account. Typically if the coherent source emits at a wavelength λ 'close to λ corresponding to the wavelength of the oscillator, the phase to be displayed corresponds to the measured phase multiplied by a parameter λ' / λ.

0053In this method, signal processing is simple and rapid, even nonexistent, since the interferogram detected by the matrix, for example of the CCD type, is used, which is displayed directly on the phase modulator to compensate for the phase deviations of the various pupils.

0054However, the detected signal is tainted by low frequency components, linked to the lack of uniformity of the beams, as well as other artifacts, frequently encountered when detecting interference patterns.

0055It may therefore be useful to apply to the detected signal, filtering making it possible to retain only the spatial frequencies corresponding to the interference fringes. The low frequency components of the spectrum of the image are deleted, and they are replaced by an average component chosen so as to make best use of the dynamics of the spatial modulator.

0056In the previous device, we are content to apply to the beam, a phase shift opposite to that which we have measured. It is therefore a priori compensation commonly called "feed-forward". This takes into account neither the measurement errors (non-flatness of the reference, non-linearity of the detector), nor the faults of the spatial modulator.

0057It is nevertheless possible to locally correct the phase of the interferogram detected before applying it to the modulator for compensation. This is done very simply in the Fourier space, by multiplying the Fourier Transform of the signal detected by a complex matrix representing the phase correction to be applied for each sub-pupil. This operation can advantageously be combined with the filtering operation presented above, since the two treatments simply consist in a multiplication in space of the Fourier transforms.

0058The phase correction matrix can be developed during a calibration operation of the multi-pupil source.

0059It can also be changed in real time (to adapt for example to mechanical deformations, the effects of temperature, etc., which would not be taken into account in the direct control described above).

0060In this case, it is advantageous to provide a device for observing residual errors of the wave front of the multi-pupil source, in order to carry out a feedback feedback by modifying the phase correction matrix. . The bandwidth of this external loop is chosen as a function of the time constant of the residual errors.