Abstract:
A device for phase distortion compensation across an optical beam is provided. The device is a part of an optical receiver, which can be used in free space optical communications, remote sensing, optical imaging and others. 2 M  inputs of the combiner interfere with each other via a system of tunable coupled waveguides. The phases in interleaved waveguides of the combiner are adjusted to maximize the resulting output signal. The combiner may be used for coherent communication in combination with a balanced 90° hybrid. Integrated solutions for the proposed device are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of provisional Application Ser. No. 61/090,404 filed Aug. 20, 2008. It is also a continuation-in-part of U.S. patent application Ser. No. 10/669,130 filed Sep. 22, 2003 now U.S. Pat. No. 7,327,913, No. 11/610,964 filed Dec. 14, 2006 now U.S. Pat. No. 7,397,979; No. 11/672,372 filed Feb. 7, 2007 now U.S. Pat. No. 7,483,600; No. 11/695,920 filed Apr. 3, 2007 now U.S. Pat. No. 7,715,720; No. 12/045,765 filed Mar. 11, 2008; No. 12/137,352 filed Jun. 11, 2008, all of which applications are fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to laser systems and methods of receiving at least a portion of the laser beam after its transmission through turbulent media. The system includes means for compensation of the atmospheric perturbations in the received beam. The applications comprise free space optical communications, remote sensing, optical imaging and others. 
     BACKGROUND OF THE INVENTION 
     Laser radars, remote sensing equipment, free space, non-line of sight, satellite communication systems are affected by optical turbulence. In this invention we disclose the atmospheric turbulence mitigation approach in view of communication systems, however this approach is applicable for variety of other arrangements. 
     The atmospheric turbulence effect, noticeable as beam drift and scintillation, is the main source of errors in the free-space optical communications. It leads to the decreased link capacity, BER deterioration and sometimes unavailability of the transmission. Adaptive optics schemes are widely used for the phase correction. Adaptive optical systems require direct measurement of the wave-front phase using wave-front sensors such as a Shack-Hartmann sensor or shearing interferometer, followed by some type of wavefront reconstruction and conjugation. In the presence of the strong phase and intensity fluctuations characteristic of near-earth propagation paths, these types of systems perform poorly. Besides, such systems cannot compensate fast phase change; their operation speed is limited by the data processing time. 
     There is a need in an efficient solution of the turbulence effects mitigation in optical systems with laser beam propagation through the atmosphere. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an electro-optically adjustable optical device that provides compensating of phase distortion across the optical beam caused by the turbulence. An optical device has 2M input waveguides (M is integer≧2), each receiving a portion of the incoming optical beam. The waveguides are connected by (2M−1) couplers forming a tree-like structure, each coupler is formed by two waveguides, coming in and out of the coupler. In each coupler one output waveguides is used in control means for changing an input phase of the optical beam portion in the same waveguide before its coupling. Another output waveguide forms an input waveguide for a consequent coupler from (2M−1) couplers. A final output waveguide from the last coupler is an output beam of the device. The control means change the phases of the beams propagating in the waveguides before their coupling. This change aims to maximize the output beam energy. The control means may include a photodetector receiving a beam in the output waveguide, producing an electrical signal being used to change the input phase of the optical beam portion in the same waveguide before its coupling. The input phase is changed in a phase modulator connected to the same waveguide before coupling. Optionally, the control means may also control the coupling rate of all couplers to maximize the output beam energy. 
     The output signal from the device can be detected and used for further processing, information recovery and display. The device may also include an optical receiver to detect the output beam. In the preferred embodiment this signal is received by a coherent optical receiver. In the coherent receiver it is combined with a local oscillator beam. In one embodiment the receiving beam and the local oscillator beam interfere in a 90-degrees optical hybrid and the output signals are processed by a set of balanced detectors. 
     It is another object of the present invention to provide a device for optical beam combining, which has at least one beam combining unit, however it may contain any number of those units, being connected between them. The unit includes a first, a second, a third, a fourth waveguides to receive a first, a second, a third, a fourth portions of the optical beam respectively; a first coupler combining the first and the second waveguides and outputting a fifth and a sixth waveguides; a second coupler combining the third and the fourth waveguides and outputting a seventh and a eighth waveguides; a third coupler combining the fifth and the seventh waveguides and outputting a ninth and a tenth waveguides; the sixth, the eighth and the tenth waveguides being connected to a first, a second and a third photodetectors respectively; output signals from the first, the second and the third photodetectors controlling a phase of the second, the fourth and the seventh portions of the optical beam via a first, a second phase and a third modulators respectively; wherein the control includes minimizing the output signals from the first, the second and the third photodetectors; the ninth portion of the optical beam providing an output signal from the device. 
     Another object of the present invention is to provide an integrated single monolithic adjustable device to perform this operation. However the description is provided of any kind of device: integrated device, a free-space optical link device, and a fiber optics device. In the preferred embodiment the integrated device is a chip made of LiNbO 3  material. 
     Yet another object of the present invention is to provide a system for information recovery, which can find applications in optical communications, remote sensing, optical imaging and other fields. The receiving unit includes an optical beam combiner with a set of input waveguides, each receiving a portion of incoming optical beam. 2 M  inputs of the combiner interfere with each other via a system of tunable coupled waveguides. The phases in interleaved waveguides of the combiner are adjusted to maximize the resulting output signal. The combiner may be used for coherent communication in combination with a balanced 90° hybrid. The receiving unit may be located as far as 2000 meters from the transmitter. The transmitter may include a light source that generates multiple wavelengths in the UV, optical or infrared ranges. In one embodiment the light source generates a pulsed optical signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  One embodiment of the combiner of the present invention. 
         FIG. 2  One example of a random power distribution, which was used in the modeling. 
         FIG. 3  ( a )-( d ) The results of modeling. 
         FIG. 4  One embodiment of the receiver of the present invention. 
         FIG. 5  One of the embodiments of the coherent receiver. 
         FIG. 6  Integrated solution for the combiner of  FIG. 1 . 
         FIG. 7  The preferred embodiment of the integrated receiver. 
         FIG. 8  Optical systems that benefit from using the combiner of the present invention: (a) line-of-sight and (b) non-line-of-sight optical communications, (c) systems with reflected optical beam, such as imaging systems, remote sensing, etc. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which the preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Optical signal transmission in free space is susceptible to atmospheric-induced attenuation and scattering. At the receiver side the beam must be collected and focused on the detector. However spatial distribution of phase and intensity is not uniform across the beam. This invention provides a solution for the atmospheric effects compensation and mitigation of the beam non-uniformity. An optical beam combiner is proposed which allows compensating phase distortions across the optical beam wavefront. 
     A schematic diagram of the optical beam combiner of the present invention is shown in  FIG. 1 ; it is a part of a receiver. Besides the receiver also includes an optical system to focus the incoming beam on the inputs  1 ,  2 ,  3 ,  4 −2 M , M is an integer≧2, a set of detectors and amplifiers (not shown in  FIG. 1 ). For example, the optical system can be a set of lenses, each lens focusing the part of the beam on a corresponding input  1 ,  2 ,  3 ,  4 −2 M  of the combiner. Alternatively, the incoming beam can be collected by an array of single-mode optical fibers, each fiber being connected to one of the 2 M  inputs of the combiner  10 . All signal entering the inputs  1 ,  2 ,  3 ,  4 −2 M  have different power. The combiner has (M−1) stages of directional couplers with two output branches each, where one output branch serves for control. The operation of each stage is the following. The waveguides  1  and  2  are coupled together by a coupler  11 . The signal from the output  12  is detected by a first detector  13 . The power detected by the detector  13  is minimized by detecting the signal and applying it via electronic unit  15  to the phase modulator  16  that changes the phase of the signal in the input waveguide  2  until it is shifted by exactly 90 degrees from the signal in the upper waveguide  1 . When the phase shift is equal to 90 degrees, a constructive interference occurs in the upper output branch  17  and a destructive in the lower one  12 . When the constructive interference is achieved, the output of the coupler (m,n) becomes 
     
       
         
           
               
             
               
                 
                   
                     
                       P 
                       out 
                       
                         ( 
                         
                           m 
                           , 
                           n 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           1 
                           2 
                         
                         ⁢ 
                         
                           P 
                           
                             in 
                             ⁢ 
                             .1 
                           
                           
                             ( 
                             
                               m 
                               , 
                               n 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           1 
                           2 
                         
                         ⁢ 
                         
                           P 
                           
                             in 
                             ⁢ 
                             .2 
                           
                           
                             ( 
                             
                               m 
                               , 
                               n 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           
                             P 
                             
                               in 
                               , 
                               1 
                             
                             
                               ( 
                               
                                 m 
                                 , 
                                 n 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             P 
                             
                               in 
                               , 
                               2 
                             
                             
                               ( 
                               
                                 m 
                                 , 
                                 n 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   
                     = 
                     
                       
                         P 
                         
                           in 
                           , 
                           1 
                         
                         
                           ( 
                           
                             m 
                             , 
                             n 
                           
                           ) 
                         
                       
                       + 
                       
                         P 
                         
                           in 
                           , 
                           2 
                         
                         
                           ( 
                           
                             m 
                             , 
                             n 
                           
                           ) 
                         
                       
                       - 
                       
                         
                           1 
                           2 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 
                                   P 
                                   
                                     in 
                                     , 
                                     1 
                                   
                                   
                                     ( 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                     ) 
                                   
                                 
                               
                               - 
                               
                                 
                                   P 
                                   
                                     in 
                                     , 
                                     2 
                                   
                                   
                                     ( 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     When the signal in the lower branch is equal to zero, it does not control the phase modulator any longer. 
     The output signal  18  serves for the further processing, for example, for recovery of the information encoded in the beam. 
     The combiner consists of multiple interconnected modules as shown in  FIG. 1 . The smallest module  20  has four inputs, three photodetectors  21 ,  22 ,  23  that control phase in the lower waveguides  24 ,  25 , and  26 . The output  19  is used for the information recovery. The controls procedure minimizes the signals received by the detectors  21 ,  22 ,  23  thus maximizing the output signal  19 . These smallest modules, similar to  20 , may be combined together in a one-dimensional or two-dimensional matrix. Four inputs of the module can form a line or a square or a rectangular. In the preferred embodiment the input waveguides collect all light across the received optical beam, which typically has round shape. However the geometrical solution of the combiner may or may not follow the shape of the beam. a three-dimensional system of receiving waveguides may be connected to an integrated combiner device with two-dimensional arrangement. 
     The integrated solution for the combiner will be described in the following sections. 
     A computer modeling for up 32 input waveguides has been performed, where the system contains up to 5 cascades (M is from 1 to 5) in order to estimate the achievable output power. The number of inputs is related to the number of cascades as 2 M . 
     It is assumed that the input powers in are distributed randomly. One example of a random power distribution, which was used in the modeling, is shown in  FIG. 2 . 
     The results of the device operation modeling are shown in the histograms of  FIGS. 3  ( a )-( d ). The total power combining efficiency is 
             η   =       P   out     /       ∑   n     ⁢       P   n     .               
The analytical expression of the mean of the power combining efficiency is:
 
                 〈   η   〉     =       8   9     +     1     9   ⁢           ⁢   N           ,         
where N=2 M . One can see that typically nearly 90% of the input power is achievable at the output.
 
     Besides phase modulators, the system parameters can be adjusted by controlling the coupling rate of the couplers. The modeling shows that adjustment of the coupling rate of the combiner couplers allows achieving almost 100% of output energy. 
       FIG. 4  presents a version of the combiner module  20  of  FIG. 1 , which is suitable for an integrated solution. Phase shifters  31 - 36  are controlled by the signal  37  from the detectors  38 ,  38   a  and  38   b  after its processing in a digital signal processing (DSP) unit  39 . The DSP unit may also controls the coupling rate of the couplers  40 - 42 . The output signals  27 ,  27   a  and  27   b  detected by the detector  38 ,  38   a  and  38   b  are amplified in TIAs  43 ,  43   a  and  43   b , then converted into digital signals  44  in A/D converters  45 ,  45   a ,  45   b . In the preferred embodiment the control signal  44  is chosen to minimize the signal  37  from by the detectors  38 ,  38   a ,  38   b  similar to described above procedure. A digital signal processing (DSP) unit  46  changes the control signal to achieve minimization of the signal  37 . 
     In the preferred embodiment the combiner is a part of a coherent optical receiver with 90-degrees optical hybrid such as described in US Patent Application Pub. No. 20070274733 by the same inventive entity. 
     An optical receiver  30  of  FIG. 5  consists of three main blocks: the combiner  20 , an optical hybrid  30  and a detector array  50 . It serves for the signal receiving and coherent detection. The combiner  20  allows energy concentrating into the output  19  as described above. Note, that output branches  27   a  and  27   b  are not shown in this  FIG. 5  for simplicity, however in the preferred embodiment the combiner is the device shown in  FIG. 4 . The output signal  19  is mixed with a local oscillator signal  51  from a light source  52  in a 90-degrees optical hybrid  30  followed by a set of detectors  53 - 56 . (The waveguide  27  is a part of the combiner  20 .) In the preferred embodiment balanced detectors are used. After the signal amplification in TIAs  57  and  58 , they are converted into digital signals  59  and  60  in A/D converters  61  and  62 . The signals  59  and  60  can be used for further processing, for the data recovery and display. The signal  37  controls all phase modulators and coupling rates of the couplers. In the preferred embodiment the control signal are chosen to minimize the signal  37  from by the detector  38 , similar to described above procedure. 
     It is beneficial to have phase modulators in both upper and lower branches of the combiner, for example,  31  and  32 ,  33  and  34 , etc. In this case push-pull modulators can be used. However, in principle, the same phase shift can be achieved by implementing only one modulator in one of the branches. 
     In the preferred embodiment the interface optical unit  63  includes focusing lenses and a bundle of optical fibers. In one embodiment, the focusing optics can be cylindrical. 
       FIG. 6  shows an integrated layout of the combiner  10  of  FIG. 1 . Phase modulators  70 - 76  are controlled by the signals  77 - 83  from the set of photodetectors  84 - 90 . In the preferred embodiment, the control of the modulators  70 - 76  includes minimizing the output signal from the detectors  84 - 90  as described above. In another embodiment the signals  77 - 83  also control the coupling rate of the couplers  91 - 97 , which allows achieving better compensation of the beam non-uniformity. 
       FIG. 7  represents a coherent optical receiver with the beams combiner as shown in  FIG. 6 . The disclosed combiner allows phase adjusting of the corrupted light beam, and thus the beam insertion in a single mode fiber or waveguide. 
     The disclosed beam combiner can be implemented in a variety of optical systems ( FIGS. 8  ( a - c )). In one embodiment it is a part of a line-of-sight free space communication system as shown in  FIG. 8  ( a ). In  FIG. 8  ( a ) a transmitter  100  sends an encoded laser beam  101  towards a receiver  104 . In the preferred embodiment the light source of the transmitter is a pulsed light source. The beam  101  is scattered on multiple inhomogeneities along the optical path forming a cone of light  103 . A receiver  104  includes a beam combiner of the present invention, an optical hybrid and a series of photodetectors and amplifiers to control the phase modulators of the combiner followed by DSP unit to recover the transmitted data. The transmitter and receiver may be from 1 to 2000 meters apart. 
     Another embodiment of the communication system is shown in  FIG. 7  ( b ), where the combiner of the present invention is used in case of non-line of sight transmission. 
     In yet another embodiment the combiner is used in a system with light reflected from the surface  105  as shown in  FIG. 7  ( c ). Such systems are used, for example, for optical imaging, for remote sensing and other applications. 
     A free-space optical communications system with the beam combiner as shown in  FIGS. 1 ,  4 ,  6  is another object of the present invention. In the preferred embodiment the data is transmitted using a phase-shift-keying modulation, preferably QPSK. In yet another embodiment the transmission is performed using orthogonal frequency division multiplexed communications as disclosed in co-pending patent application of the same inventive entity application Ser. No. 12/045,765 filed Mar. 11, 2008 and 12/137,352 filed Jun. 11, 2008. 
     Data transmission in such system can be performed using a light source generating radiation in multiple wavelengths in UV, visible or infrared range. In the preferred embodiment UV laser radiation is used. 
     The description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.