Abstract:
The invention concerns a system for measuring and controlling the wave-front of a coherent light beam ( 32 ), comprising: a device for generating a reference light beam ( 36 ) that is coherent with said coherent light beam ( 32 ); a partially reflective monolithic device ( 38 ) comprising an array of elementary cells, each elementary cell comprising at least one element ( 40 ) for measuring a portion of the incident wave interfering between the coherent light beam and the reference light beam and a phase modulation element ( 42 ) for modulating the phase of the reflected beam; and a control device ( 46 ) for controlling said phase modulation element making use of the information from the associated measuring element.

Description:
FIELD OF THE INVENTION 
     The present application is a National Stage of PCT International Application Serial Number PCT/FR2013/052936, filed Dec. 4, 2013, which claims priority under 35 U.S.C. §119 of French Patent Application Serial Number 12/61594, filed Dec. 4, 2012, the disclosures of which are incorporated by reference herein.  
     1. Background 
     The present invention relates to a device and a method for measuring and controlling the wavefront of a coherent light beam, for example, a laser beam. More specifically, the present invention relates to such a device enabling to measure and modulate the phase or the amplitude of such a wavefront. 
     2. Discussion of Related Art 
     Currently, coherent light, and in particular laser light, is used for many applications, for example for medical applications, in microscopy, or also for data processing with optical fibers. The wavefront of a laser beam depends on the type of medium that the laser beam has crossed. It may be advantageous to study the wavefront of a laser beam to determine the nature and the shape of the medium that it has crossed. It may also be provided, after finding out the shape of the wavefront, to modulate it so that the modulated wavefront is adapted to a given application. 
     U.S. Pat. No. 5,994,687 describes a system for controlling the wavefront of a laser beam. 
     SUMMARY 
     An object of an embodiment of the present invention is to provide a system overcoming all or part of the disadvantages of prior art. 
     To achieve this, a system according to an embodiment comprises an array of pixels which each provide, independently, the detection of a portion of the wavefront and the modulation of a corresponding portion of a reflected beam. 
     Thus, an embodiment provides a system for measuring and controlling the wavefront of a coherent light beam, comprising: a device for generating a reference light beam coherent with the coherent light beam; a partially reflective monolithic device comprising an array of elementary cells, each elementary cell comprising at least one element for measuring a portion of the incident wave interfering between the coherent light beam and the reference light beam and an element for modulating the phase of the reflected beam; and a phase modulation element control device making use of the information from the associated measurement element. 
     According to an embodiment, each elementary cell comprises at least one photodetection element topped with at least a first electrode, a first liquid crystal layer, and a second electrode, the second electrode being transparent, the first electrode being provided to let through a portion only of the incident interfering beam towards the at least one photodetection element, the other portion being reflected. 
     According to an embodiment, the control device controls the voltage applied between the first and the second electrode. 
     According to an embodiment, the first electrode is metallic and comprises at least one opening opposite the at least one photodetection element. 
     According to an embodiment, the first electrode is made of indium-tin oxide (ITO). 
     According to an embodiment, the second electrode is common to all the elementary cells of the partially-reflective device. 
     According to an embodiment, the second electrode is made of indium-tin oxide. 
     According to an embodiment, the system is adapted to a phase modulation of the beam reflected by the partially reflective device, the system further comprising a second liquid crystal layer extending over the first liquid crystal layer, the crystals of the first and of the second liquid crystal layer being parallel nematic crystals, the orientation of the crystals in the second liquid crystal layer being perpendicular to the orientation of the crystals in the first liquid crystal layer. 
     According to an embodiment, the system is adapted to a phase modulation of the beam reflected by the partially reflective device, the first liquid crystal layer being nematic with a parallel orientation. 
     According to an embodiment, the system is adapted to an amplitude modulation of the beam reflected by the partially-reflective device, the liquid crystals of the first liquid crystal layer being twisted nematic crystals, a polarizer being placed between the photodetection element and the first electrode, the orientation of the polarizer being parallel to the orientation of the liquid crystal molecules on the polarizer side. 
     According to an embodiment, the system is adapted to an amplitude modulation of the beam reflected by the partially-reflective device, the liquid crystals of the first liquid crystal layer being nematic with a parallel orientation, a polarizer being placed between the photodetection element and the first electrode, the orientation of the polarizer being at a 45° angle relative to the director of the liquid crystals of the first liquid crystal layer. 
     According to an embodiment, the system further comprises at least one polarizer placed opposite the partially-reflective device. 
     According to an embodiment, the control device outputs a temporally-continuous or discrete control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1  illustrates a device enabling to control the wavefront of a coherent light beam; 
         FIG. 2  schematically illustrates the principle of a system for controlling the wavefront of a coherent light beam according to an embodiment; 
         FIGS. 3, 4, and 5  illustrate two alternative embodiments of pixels of a coherent light beam control device according to an embodiment; 
         FIGS. 6A and 6B  illustrate the alternative embodiments of counter electrodes according to an embodiment; 
         FIG. 7  illustrates another alternative embodiment of pixels of a coherent light beam control system according to an embodiment; and 
         FIG. 8  illustrates an application of a device according to an embodiment. 
     
    
    
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of optical structures and of integrated systems, the various drawings are not to scale. 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example of a coherent light beam control system. 
     This system comprises a coherent light source  10  which delivers a beam  11 . As a non-limiting example, coherent light source  10  may be a laser source. Beam  11  crosses a correction device  12  controlled by a control device  14 . The beam originat-ing from correction device  12  is directed towards a beam splitter  16  which deviates a portion towards a wavefront detection device  18  and which transmits the rest of the beam for a subsequent use (on the right-hand side of the drawing). Detection device  18  comprises a liquid crystal display  20  coupled to a CCD camera  22 . The data originating from detection device  18  are transmitted to control device  14 . 
     In operation, the wavefront of the coherent light beam is analyzed, by means of the beam deviated by beam splitter  16 , by detection device  18 . The data relative to the shape of the wavefront are delivered to control device  14 , which determines the correction to be brought to coherent light beam  11  so that it exhibits, at the device output (to the right of the beam splitter), a desired wavefront. 
     A disadvantage of a system such as that in  FIG. 1  is that it dissociates the detection, control, and modulation functions in separate blocks (modulation block  12 , detection block  18 , and control block  14 ). Such a device thus necessarily implies a significant latency due to signal transmissions between the different blocks. Further, for the modulation applied to coherent light beam  11  to be efficient, an iterative correction is necessary. Thus, the system of  FIG. 1  does not enable to rapidly modulate the wavefront of the coherent light beam, and is not adapted to the modulation of a coherent light beam exhibiting a time-variable wavefront. 
     Another disadvantage of the device of  FIG. 1  is its manufacturing complexity. Indeed, for the wavefront modulation applied to coherent light beam  11  by block  12  to be efficient, the pixels of modulator  12  should be perfectly aligned with those of detector  18 , which implies a fine alignment step which is particularly difficult to implement. 
     Thus, there is a need for a system enabling to combine a detection and a modulation of the wavefront of a coherent light beam, which is easy to manufacture and more efficient than existing devices. 
     To satisfy this and other needs, the inventors have provided a system comprising an array of pixels which each ensure, independently, the detection of a portion of the wavefront of a coherent light beam and the modulation of a corresponding portion of a reflected beam. 
       FIG. 2  schematically illustrates the general operation of a system according to an embodiment. 
     The system receives a coherent light beam  32  originating from a source  30 , for example, a laser source. It is here desired to determine the shape of the wavefront of coherent light beam  32  and to modulate this beam. The system further comprises a second source  34  of a reference light beam  36 , coherent with light beam  32 . In practice, light beams  32  and  36  may originate from a same source, coherent light beam  32  having been submitted to transformations before reaching the device, for example, through a diffusing medium. Beams  32  and  36  are directed so as to interfere. 
     The system further comprises a single device  38 , partly reflective, simultaneously performing a function  40  (DETECT) of detection of the wavefront of the interference beam (between beams  32  and  36 ) and a function  42  (MODULATE) of modulation of the beam reflected on the device. The structure of the pixels of device  38  will be described in further detail hereafter in relation with  FIGS. 3 to 7 . Thus, light beam  42  reflected by device  38  exhibits a controlled wavefront. To modulate the reflected light beam, device  38  comprises at least one liquid crystal layer sandwiched between two electrodes, the orientation of the liquid crystals being controlled by the voltage applied between the two electrodes, which enables to perform a phase or amplitude modulation of the reflected beam, as will be seen hereafter. 
     The system further comprises a processing and calculation device  44  (PROCESS) which receives the data from detection device  40  and which defines, according to the wavefront data detected by the detection device, the phase modulation that modulation device  42  should apply. It should be noted that, unlike what is shown in  FIG. 2 , detection device  40  and modulation device  42  are integrated in a same single device  38 , comprising a pixel array, each pixel being defined to simultaneously detect and modulate the portion of the wavefront that it receives. Advantageously, the device provided herein operates whatever the light source used, provided for the wavelength of this source to be compatible with the characteristics of the detector and of the modulator. 
     Advantageously, the above-described device enables to perform many processings based on the spatial modulation of the wavefront. Such processings comprise, without this being a limitation, adaptive optical techniques, parallel optical communication channel modulation techniques, or also interferometry techniques. This last application will be described in further detail hereafter. 
     The use of source  34  of a reference beam  36  coherent with object beam  32  enables to do interferometry between the two beams, and thus to directly access the wavefront of object beam  32 . The phase of the object beam received by the device can thus be measured pixel by pixel, and the phase or the amplitude of the wave reflected by the device can be controlled even in response mode. 
     The phase measurement is performed by the pixels of detector  40 , which are sensitive to the intensity of the light field. The superposing, at any point of the detector, of the object wave to be analyzed/processed due to beam  32  of value U 0 =A 0 ·exp(iΦ 0 ) and of the reference wave due to beam  36  of value U R =A R ·exp(iΦ R ) provides, at the level of each pixel of detector  40 , an intensity proportional to the cosine of phase Φ 0  of object beam  32 , according to the following equation (Φ R  being the phase of the reference field, which is constant):
 
 I=|U   R   +U   0 | 2   =A   0   2   +A   R   2 +2 A   0   A   R  cos(Φ R −Φ 0 )
 
     Thus, the phenomenon of interference between object and reference beams  32  and  36  enables to directly measure at the level of each pixel the phase shift of incident light beam  32 , and thus of the wavefront of this beam. 
     To perform the modulation, an at least partially reflective layer, defined by pixels, topped with a liquid crystal layer and with a main transparent electrode, is placed in device  40 . For each pixel, the orientation of the liquid crystals may be controlled for each pixel, that is, at an elementary level. The incident beam is thus partly reflected by the reflective layer and crosses the liquid crystal layer twice, which enables to modulate it. 
     Such a basic structure of device  38  enables to modulate the phase of the light beam. It will also be possible, as will be seen hereafter in relation with embodiments, to perform an amplitude modulation by adding at least one polarizer to the system. 
       FIGS. 3 to 7  illustrate several structures of pixels of a detection and modulation device  38 . Such structures provide a fast detection of the wavefront for a feedback action adapted to the desired modulation. 
       FIGS. 3 and 4  respectively illustrate a perspective view and a cross-section view of a pixel forming a device  38  according to a first embodiment. 
     The pixel comprises a semiconductor substrate  50  having a photodetection device  52  defined at its surface. The photodetection device may be a photodiode, a fully depleted diode, an avalanche diode, or also a photogate having its photogenerated charge collection area located at the surface of substrate  50 . 
     Substrate  50  is topped with a stack of metallization layers comprising conductive tracks  54  separated by an insulating material  56 . For clarity, the conductive tracks of the first metallization levels are not shown in the perspective view of  FIG. 3 . 
     An at least partly reflective conductive track  54 ′, which at least partly covers a portion of the pixel, is defined in an upper level of the interconnection stack. In the shown example, region  54 ′ is made of an opaque material, for example, of the same material as the conductive tracks of the lower interconnection levels (generally a metal such as aluminum or copper). An opening  58  is defined in region  54 ′ opposite photodetection area  52 . Thus, incident light beams which arrive at the level of opening  58  reach the surface of photodetection area  52 , while incident light beams which reach region  54 ′ are reflected by said region. The relative surface areas of region  54 ′ and of opening  58  are selected to reflect a desired proportion of the incident beam, for example, from 40 to 50% of the incident light flow of beam  32 . 
     At the surface of the interconnection stack is formed a stack of a first alignment layer  60  (liquid crystal bonding and alignment layer), of a liquid crystal layer  62 , of a second alignment layer  64 , and of a transparent conductive layer  66 . The stack further comprises as an example an upper glass plate  68 . Transparent conductive layer  66  forms a first electrode for controlling the liquid crystals of layer  62 , while region  54 ′ forms the second control electrode (counter electrode). Trans-parent conductive layer  66  is made of a conductive material transparent to the considered wavelengths. As an example, trans-parent conductive layer  66  may be made of indium-tin oxide (ITO). The materials of the various layers and regions  56 ,  60 ,  62 ,  64 ,  66 , and  68  and their interfaces are selected to avoid parasitic reflections. 
     The elements of application of control voltages to electrodes  54 ′ and  66  will not be detailed, the forming of tracks and/or vias of access to electrodes such as electrodes  54 ′ and  66  being well known in integrated circuit techniques. 
     The application of a voltage between electrodes  54 ′ and  66  enables to modify the structure of the liquid crystals of layer  62 , and thus the phase of the reflected light beam. It should be noted that such an adjustment is performed pixel by pixel, a counter electrode  54 ′ being independently defined in each of the pixels. 
     To perform a phase modulation of the reflected beam, in the structure of  FIGS. 3 and 4 , a layer of liquid crystals capable of imposing a phase shift to the incident wave is provided, such a phase shift depending on the voltage applied between electrodes  54 ′ and  66 . This voltage is defined from the amplitude detected by the detection device. 
     In an embodiment, nematic crystals having their directors parallel in a plane parallel to the surface of layers  60  and  64  are selected. Alignment layers  60  and  64  are defined to obtain such a liquid crystal distribution. In practice, this means that, if alignment layers  60  and  64  are made of a polymer material such as polyimide, parallel ridges are formed at the surface of these layers, on the side of the liquid crystals, to force their alignment. 
     A layer of nematic liquid crystals of parallel orientation enables to perform a pure phase modulation of an incident polarized light, the modulation depending on the electric field applied between two electrodes formed on either side of the liquid crystal layer. If the polarization of the incident light is parallel to the direction of the directors of the liquid crystals, the applied voltage is a direct image of the phase shift of the light. Maximum delay ΔΦ, generated on an incident light of wavelength λ by a liquid crystal layer of thickness d and where the difference between ordinary refraction index no and extraordinary refraction index ne can be written Δn (Δn=n o −n e ), can be expressed as: 
     
       
         
           
             ΔΦ 
             = 
             
               
                 
                   2 
                   ⁢ 
                   π 
                 
                 λ 
               
               ⁢ 
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 n 
                 · 
                 d 
               
             
           
         
       
     
     Δn depending on the applied voltage. Thus, for the provided system which acts on the reflected light beam, after a return travel in the liquid crystal layer, the generated phase shift will thus be equal to 2ΔΦ. 
     It should be noted that the difference between ordinary and extraordinary indexes Δn of the crystal and their variation according to the applied voltage is a characteristic of the liquid crystal molecules used, and typically varies from 0.1 to 0.2. The selection of the molecules thus provides freedom as to the thickness of the liquid crystal layer for a phase modulation of the reflected beam. 
     It should be noted that, for this first embodiment, the incident light (beam  32 ) should be polarized in alignment with the orientation of the liquid crystals. Indeed, to apply a phase modulation, it is necessary for the polarization of the incident light to be parallel to the direction of the directors of the liquid crystal molecules. 
     To do away with such a constraint, a second alternative embodiment illustrated in  FIG. 5  may be used. 
     In the device of  FIG. 5 , each pixel comprises a semiconductor substrate  50  having a photodetection device  52  defined at its surface. Substrate  50  is topped with a stack of metallization layers comprising conductive tracks  54  separated by an insulating material  56 . In an upper level of the interconnection stack is defined an at least partly reflective conductive region  54 ′, which covers a portion of the pixel. In the example of  FIG. 5 , region  54 ′ is made of an opaque material, for example, metallic, and an opening  58  is defined in region  54 ′ opposite photodetection area  52 . 
     At the surface of the interconnection stack is formed a stack comprising:
         a first alignment layer  60  for a first liquid crystal layer,   a first liquid crystal layer  62 ,   a second alignment layer  64  for first liquid crystal layer  62 ,   a first alignment layer  70  for a second liquid crystal layer,   a second liquid crystal layer  72 ,   a second alignment layer  74  for second liquid crystal layer  72 ,   a transparent conductive layer  66 , and   a protective glass plate  68 .       

     Transparent conductive layer  66  forms a first electrode for controlling the positioning of the liquid crystals of layers  62  and  72 , while region  54 ′ forms the second control electrode. As in the case of the structure of  FIGS. 3 and 4 , the element of application of a control voltage between electrodes  54 ′ and  66  will not be described in detail. 
     The device of  FIG. 5  enables to perform a phase modulation of the wave reflected by the pixel (on layer  54 ′), without requiring for the incident waves to have a specific polarization. To achieve this, the liquid crystals of layers  62  and  72  should be oriented in directions parallel to the surface of the layers, and in perpendicular directions (the liquid crystals of layer  62  have their directors, in top view, perpen-dicular to the directors of the liquid crystals of layer  72 ). It should be noted that, as a variation, alignment layers  64  and  70  may be gathered in a single layer, for example, made of a polymer. 
     According to the voltage applied to electrodes  54 ′ and  66 , the liquid crystals of layers  62  and  72  orient, which enables to modulate the phase of the light reflected by electrode  54 ′. Publication “Polarization-independent liquid crystal phase modulator using a thin polymer-separated double-layered structure”, OPTICS EXPRESS, 31 Oct. 2005, Vol. 13, No. 22 (8746), specifies values of the voltages to be applied to a stack of two liquid crystal layers to obtain a desired phase modulation of the light beam. 
     In this second embodiment, the phase modulation is performed independently from the polarization of the incident light. Indeed, liquid crystal layers  62  and  72  enable to modulate all the field components in the same way. 
     It can also be shown that the phase delay introduced by the intermediate layers ( 64 ,  70 ) between the two liquid crystal layers compensates, which enables to keep the same phase-shift dynamics. 
     The above-described systems enable to provide a phase modulation at the level of each pixel. To provide an amplitude modulation, it may be provided to add to the system one or a plurality of polarizers delivering different amplitudes for waves having different phase shifts. 
     For example, in a device comprising pixels similar to that of  FIGS. 3 and 4 , a polarizer may be interposed between interconnection stack  56  and alignment layer  60 . The crystals of liquid crystal layer are provided (due to alignment layers  60  and  64 ) to be in twisted nematic phase. A polarizer is placed in front of the device, in the interference area between beams  32  and  36  (or integrated at the surface of the device). When a potential is applied on either side of the liquid crystal layer, the helix of the liquid crystals rotates more or less, which, with the association of the polarizer, modifies the amplitude of the reflected beam. In this example, the polarizer interposed between layers  56  and  60  is oriented perpendicularly to the polarizer placed in front of the device, and the interposed polarizer is oriented in a direction parallel to the direction of the liquid crystals at the interface with layer  60 . 
     A device similar to that of  FIGS. 3 and 4 , where a polarizer is interposed between interconnection stack  56  and alignment layer  60 , may also be considered. The crystals of the liquid crystal layer are provided in this variation to be in parallel alignment nematic phase (due to alignment layers  60  and  64 ). A polarizer is placed in front of the device, in the interference area between beams  32  and  36  (or integrated at the surface of the device). The polarizers are oriented with a 45° angle relative to the orientation of the liquid crystals and are placed at a 90° angle relative to each other. 
     In this last variation, the intensity reflected by the device can be written as: 
     
       
         
           
             
               I 
               = 
               
                 
                   
                     sin 
                     2 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         d 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         n 
                       
                       λ 
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   I 
                   0 
                 
               
             
             , 
           
         
       
     
     I 0  being the intensity incident on device, λ the wavelength of the light beam, d the thickness of layer  62 , and Δn the difference between ordinary refraction index n o  and extraordinary refraction index n e  of the liquid crystals. The amplitude of the reflected beam thus is a direct function of the voltage applied on either side of liquid crystal layer  62 . 
     One or a plurality of polarizers may similarly be associated with the structure of  FIG. 5  to obtain an amplitude modulation. 
       FIGS. 6A and 6B  illustrate, in top view, two alter-native embodiments of a lower electrode  54 ′ formed above a pixel of a coherent light beam detection and modulation device  38 . 
       FIG. 6A  shows electrode structure  54 ′ of  FIGS. 3, 4, and 5 , that is, for each pixel of the detector, electrode  54 ′ comprises a peripheral reflective region having an opening  58  formed opposite a photodetection area  52  defined in its middle. 
     In  FIG. 6B , structure  54 ′ differs from that of  FIG. 6A  in that a plurality of openings  58 ′ are defined in the reflective region. In practice, each opening  58 ′ may be associated with an independent photodetector formed in substrate  50 . Thus, each pixel, having its surface area defined by the size of electrode  54 ′, comprises a plurality of photodetectors. This structure may be advantageous, for example, in the case where a plurality of measurements per pixel are necessary. The different measurements can thus be performed simultaneously. Other alternative embodiments of counter electrode  54 ′ may also be provided. Particularly, the number and the shape of openings  58 / 58 ′ may be different from those provided herein. 
       FIG. 7  illustrates another embodiment of a pixel of a device  38 . 
     The pixel illustrated in  FIG. 7  is similar to that of  FIGS. 3 and 4 , except as concerns lower electrode  54 ′, which is absent from  FIG. 7 . In this embodiment, this electrode is replaced with a transparent conductive layer  76  which is placed between interconnection stack  56  and alignment layer  60 . 
     The material of transparent conductive layer  76  is selected with adapted reflection and transmission coefficients so that a portion of the incident light beam reaches one or a plurality of underlying photodetection areas and that another portion is reflected and modulated as desired. As an example, transparent conductive electrode  76  may be made of indium-tin oxide (ITO) and have a thickness in the range from 50 to 100 nm. Indeed, this material is well adapted to transmitting part of the incident light rays towards the underlying reflection and photodetection areas. In this case, as previously, the reflective layer may be a dielectric or a metal. 
     The variation of  FIG. 7  can allow an amplitude or phase modulation, if the liquid crystal layer is provided as described in relation with  FIGS. 3 and 4 . It may also be provided to combine the variations of  FIGS. 5 and 7 , by forming a second layer of liquid crystals properly oriented at the surface of layer  62 , to form a device enabling to modulate the phase of the beam reflected by the device. The operation of such devices (the orientation of the liquid crystals in the liquid crystal layer(s) being defined in adapted fashion) being similar to the above operations, it will not be described in detail again herein. It should also be noted that, in the variation of  FIG. 7 , the processing and calculation device should be adapted to take into account the fact that the signal received by the photodetection areas is influenced by a passage through liquid crystal layer  62  submitted to a control voltage. 
     Advantageously, the system provided herein ensures a greater detection/modulation reactivity than known devices, since the two functions are integrated in a same device, and this, in compact fashion. Further, the integration of these two functions directly within the detection pixels eliminates the alignment issues of previously-provided devices. 
       FIG. 8  illustrates a possible application of a device according to an embodiment to a coherent light beam focusing system. 
     In this example of application, a coherent light beam  80 , for example, a laser beam, reaches a diffusing medium  82 . Diffusing medium here means an inhomogeneous medium disturbing the propagation of a wave, for example, a rough interface such as ground glass, a turbid medium such as milk or a biological medium, or also a strongly-diffusing medium such as white paint. As it comes out of the diffusing medium (in reflection or transmission), obtained beam  84  is strongly deformed and is transmitted to an analysis and modulation device  38  such as described hereabove. A reference light beam  86 , coherent with beam  80  (for example, originating from the same source as beam  80 ), is provided to interfere with beam  84  at the level of the detector of device  38 . Device  38  is associated with a processing and calculation device, not shown. As described hereabove, beam  94  reflected by device  38  may be phase and/or amplitude modulated. To achieve this, a polarizer  88  may be placed opposite device  38 , as previously described. 
     Advantageously, the detection of the beam by detection device  40  of device  38  enables to know the effect of diffusing medium  82  on the light beam. Once this effect is known, a phase and/or modulation algorithm may be applied so that beam  94  reflected by device  38  is adapted to this medium. It may in particular be provided to apply an algorithm enabling to focus the reflected light beam onto a predetermined point through or inside of the diffusing medium. 
     Such a solution may be particularly advantageous in the medical field, and more particularly for the treatment of patients implementing the focusing of coherent light beams. Indeed, it may be provided to simultaneously determine the nature of a diffusing medium such as a portion of the human body, and to focus the reflected beam on a portion of this diffusing medium, for example, to destroy a diseased cell or area. 
     It may also be provided for device  38  to ensure a phase conjugate function  94 . Such a function enables to focus the reflected beam on the source of the incident light beam. In this case, the phase of the object field, φ OBJET , is determined, after which a phase equal to −φ OBJET  is imposed on the modulator. A phase shift enables, as a variation, to displace the focusing point of the reflected beam. 
     As an example, in the case where the phase of a frequency-modulated signal is desired to be measured, the phase of the object field, φ OBJET , may be measured by a method of phase-shift interferometry based on the recording of a plurality of measurements called holograms. This measurement may be performed with 2, 3, or 4 holograms. In the case of a two-hologram measurement, two holograms having their reference phase shifted by π are recorded. 
     In this case, the intensity for each hologram on the pixels is, with the same notations as previously:
 
 I   0   =|U   0 | 2   =A   0   2   +A   R   2 +2 A   0 A R  cos(Φ R (0)−Φ 0 )
 
 I   π   =|I   π | 2   =A   0   2   +A   R   2 +2 A   0 A R  cos(Φ R (π)−Φ 0 )
 
     Phase Φ 0  can thus be obtained by writing difference I 0 −I π : 
     I 0 −I π =4A 0 A R  cos(Φ 0 ), and thus: 
     
       
         
           
             
               Φ 
               0 
             
             = 
             
               
                 arccos 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         I 
                         0 
                       
                       - 
                       
                         I 
                         π 
                       
                     
                     
                       4 
                       ⁢ 
                       
                         
                           
                             A 
                             0 
                           
                           ⁢ 
                           
                             A 
                             R 
                           
                         
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     Value −Φ 0  to be applied for the modulation of the reflected beam is thus determined. It should be noted that, for a method with four holograms, the reference phase will be shifted between each hologram by π/2 and, for a method with three holograms, the reference phase will be shifted between each hologram by 2π/3. 
     In the case where the signal is not frequency-modulated, the phase of the object field, φ OBJET , simply corresponds to the measured intensity. 
     It should be noted that this interferometry method is particularly adapted to the forming of liquid crystal control electrodes such as that in  FIG. 6B . Indeed, it may be provided for the different photodetection areas to simultaneously perform the above acquisitions. 
       FIG. 8  shows a beam splitter cube  96  which is positioned between beam  80  and diffusing medium  82 . The splitter cube is provided to divert part of the reflected beam coming out of diffusing medium  82  towards a second detector  98  which enables, if desired, to perform a measurement in another plane that that of detection and modulation device  38 . 
     Another alternative application of a device such as provided herein comprises placing two or more elementary bricks of devices  38 , associated with reference sources. Particularly, it may be provided to place two devices  38  opposite each other, a beam being brought onto a first one of these devices, for example, by a beam splitter. The beam originating from the first device is modulated, by means of a detection of interferometry with a reference source, and is sent back towards a second device  38 . The latter sends back a wave modulated as desired towards first device  38 , and so on. Particularly, by placing a diffusing medium between the two devices  38 , such a device enables to converge towards a maximum transmission mode of the diffusing medium. 
     The modulation devices provided hereabove may also be used for a wide range of applications, to perform a wavefront detection in parallel with a modulation of this wavefront (possibly with an intermediate calculation step). Such devices may particularly be used to qualify transfers in optical fibers, and to verify that a matching of the optical fibers actually corresponds to a desired aim. 
     In all the above applications, the modulation device may have any state, known or not, during the measurement phases. 
     Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. Although the use of liquid crystals, and as an example, of nematic liquid crystals, have been provided, any other structure capable of giving a light wave a variable phase shift as a response to a control signal may be used. Further, various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step.