Abstract:
An adaptive free-space laser communication system and a method of communication between laser communication transceivers of the system. Communication channel condition information communicated via laser beams transmitted between a first laser communication transceiver and a second laser communication transceiver is monitored and the information is shared between the transceivers. A wave-front phase of the laser beams is shaped with wave-front correctors of the first and second laser communication transceivers.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used and/or licensed by or for the United States government. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to laser communication systems and methods and more particularly to duplex free-space laser communication systems and methods. 
   2. Description of the Related Art 
   Random variations in the refractive index, commonly referred to as atmospheric or optical turbulence, significantly limit the performance of free-space laser communication systems. Optical turbulence-induced random phase and intensity fluctuations (scintillations) across the received wave-front lead to intensity fading at the receiver, with the result being increased system bit error rates. Attempts have been made at applying some type of adaptive optics correction scheme to reduce the effects of optical turbulence on free-space laser communication systems. Adaptive optical systems that have previously been used for free-space laser communication 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 wave-front reconstruction and conjugation. Disadvantageously however, in the presence of the strong phase and intensity fluctuations characteristic of near-earth propagation paths, these types of systems perform poorly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a full duplex free-space laser communication system with exemplary laser communication transceivers. 
   

   DETAILED DESCRIPTION 
   The following describes a novel approach to monitoring the atmospheric channel during communication, and to sharing this channel condition information between two free-space laser communication transceivers operating in duplex (two-way) mode for adaptive compensation of atmospheric turbulence-induced distortions. As will be shown below, improvements in the ability to point (direct) and focus (lock) a laser beam in a duplex free-space laser communication system are made. The resulting beam position stabilization and improved beam focusing reduces system bit error rates due to optical turbulence, offering more robust and reliable free-space laser communication system performance and the potential for increased communication distances. 
   Referring to  FIG. 1 , a schematic diagram of an adaptive duplex free-space laser communication system  100  comprising two laser communication transceivers  102 ,  202  is shown. In one example, the two transceiver modules  102 ,  202  are identical. In the exemplary embodiment shown in  FIG. 1 , first laser communication transceiver  102  comprises: a laser transmitter  104 ; photo-receiver  106 ; transmitter optics (collimating telescope)  108 ; receiver telescope (focusing lens)  110 ; wave-front corrector (deformable mirror or liquid crystal spatial light modulator)  112 ; encoder  114 ; decoder  116 ; beam quality image (BQM)  118 ; and control unit  120 . In the example of  FIG. 1 , the second laser communication transceiver  202  comprises: laser transmitter  204 ; photo-receiver  206 ; transmitter telescope (collimating lens)  208 ; receiver telescope (focusing lens)  210 ; wave-front corrector (deformable mirror or liquid crystal spatial light modulator)  212 ; encoder  214 ; decoder  216 ; beam quality imager (BQM)  218 ; and control unit  220 . 
   The following description of laser communication system  100  operation applies regardless of whether transmission is from first laser communication transceiver  102  to second laser communication transceiver  202 , or vice versa. The following description refers to an example embodiment where information is sent through optical communication channel  122  from transceiver  102  to transceiver  202 . In another example, such as in actual practice both processes of communication can and would occur simultaneously: information being sent from transceiver  102  to transceiver  202 , and vice versa. 
   An improved ability to direct and focus the information-carrying laser beam  124  transmitted from transceiver  102  is achieved through continuous shaping of the wave-front phase of the outgoing beam  124  by the wave-front corrector  112 . In this example, the wave front corrector is positioned immediately in front of laser transmitter telescope  108 . The wave-front phase of the transmitted laser beam  124  from laser communication transceiver  102  is continuously shaped based upon the measured value of the beam quality metric J 1 , determined at beam quality imager  218  of the second transceiver  202 . This beam quality metric contains information about phase and/or intensity distortions along the optical communication channel  122  between transceiver  102  and transceiver  202 . A signal describing the instantaneous value of the beam quality metric J 1  is measured at transceiver  202  and is then returned as part of the overall laser communication link signal to the control unit  120  of the other transceiver, in this case transceiver  102 . This is accomplished by modulating the laser beam  224  transmitted from transceiver  202  with an encoded signal that is then sent, and subsequently received, by both the photo-receiver  106  and the beam quality metric imager  118  in transceiver  102 . In this example, photo-receiver  106  and beam quality metric imager  118  are located at the same or conjugate planes with respect to the receiver telescope  110  of transceiver  102 . The beam quality metric imager  118  measures information about distortions (phase and/or intensity) along the optical communication channel between transceiver  202  and transceiver  102 . This information, contained in beam quality metric J 2 , is then returned as part of the overall laser communication link signal to be received by transceiver  202  and subsequently control wave-front corrector  212  in front of the beam  224  transmitted by transceiver  202 . The beam quality metric information received from one transceiver (such as transceiver  202 ) is used to control the wave-front corrector of the partner transceiver (such as wave-front corrector  112  of transceiver  102 ) using a model-free (blind) optimization algorithm. In general, the purpose of the beam quality metric imagers  118 ,  218  is to produce the signals J 2 , J 1  that characterize the concentration of laser beam energy onto the respective photo-receivers  106 ,  206 . 
   The concentration of laser energy onto the photo-receivers  106 ,  206  is affected by turbulence-induced phase fluctuations along the optical communication channel: strong fluctuations reduce the concentration of energy onto the photo-receivers  106 ,  206  due to focusing and pointing errors, and signal fading due to scintillation. The notation J 1  (i=1,2) is used to designate the signals that contain information about the value of the beam quality metric received from the other transceiver. For example, the beam quality metric J 1  shown in transceivers  102  and  202  characterizes the intensity distribution of the laser beam  124  emitted by laser transmitter  104 , after propagation through the optical communication channel, as measured by beam quality imager  218  at transceiver  202  and then encoded by encoder  214  into the signal sent by transceiver  202  to be subsequently received by the decoder  116  in transceiver  102 . 
   Criterion dependent on the intensity distribution I(x,y) registered in the focal plane of the receiver (x,y) may selectively be used as the beam quality metric J. For purposes of illustration provided as an example: 
           J   =       ∫   D     ⁢         I   n     ⁡     (     x   ,   y     )       ⁢           ⁢     ⅆ   x     ⁢     ⅆ   y               
where n=1,2, . . . is an integer and D is the aperture of the beam quality metric imager, defined here by either  118  or  218 . Implementations for measuring the beam quality metric J in the receiver plane may include (1) photocurrent of a photo-receiver (avalanche photo-detector, pin-diode etc.) or a function dependent on photo-receiver output signal (in this case both the beam splitter and beam quality imager in  FIG. 1  are not required); (2) incident light power transmitted through an optical fiber placed in the plane of the photo-receiver; (3) energy incident onto a photo-receiver after being passed through a pinhole (energy inside pinhole); or (4) any technique for measuring intensity onto a receiver and computing the beam quality metric dependent on this intensity.
 
   Information about the current state of the beam quality metric J 1  as registered by the beam quality imager  218  or directly by a photo-sensor (located in a plane equivalent to that of the photo-receiver  206 ) is employed to shape the wave-front phase of the beam before it is transmitted from transceiver  102 . The beam quality metric J 1  measured in transceiver  202  is accordingly transmitted back to transceiver  102  where it can be used to control wave-front corrector  112  with the goal of improving subsequent reception at photo receiver  206 . 
   For example, shaping of the wave-front phase may be accomplished as follows. As previously explained, information concerning the value of the beam quality metric J 1  dependent on the laser intensity emitted from laser transmitter  104  that is incident on photo-receiver  206  is encoded by the encoder  214 , of transceiver  202 , into the signal transmitted from transceiver  202  to transceiver  102 . The decoder  116  in transceiver  102  extracts the information of the beam quality metric J 1 . This information is delivered to the control unit  120  of transceiver  102  which utilizes a model-free optimization technique (for example, gradient descent/ascent, stochastic parallel perturbation gradient descent optimization, decoupled stochastic gradient descent, genetic optimization, etc.) to compute the control signals to be applied to the wave-front corrector  112  of transceiver  102 . These output control signals applied to the wave-front corrector  112  modulate (shape) the wave-front phase of the beam  124  before it is transmitted through the optical communication channel to transceiver  202 . A successful wave-front phase spatial shaping should to the maximum extent possible attempt to negate the effects of any phase distortions due to inhomogeneities in the refractive index along the optical communication channel. That is, the wave-front phase modulation applied by the wave-front corrector ( 112 ,  212 ) should be a two-dimensional spatial phase map computed to compensate the negative influence of phase distortions along the propagation path even in the possible presence of intensity scintillations typical of laser beam propagation through an optically non-homogeneous media. As the value for the beam quality metric J 1  is continuously updated, the control unit  120  continuously operates to optimize this value by updating the output signals to the wave-front corrector  112  in transmitter  102 . This in turn continuously shapes the phase of the transmitted beam  124  to counter the effect of phase distortions along the optical channel. Reducing wave-front phase distortions reduces errors in pointing, focusing and signal fading due to scintillation, resulting in increased energy onto the photo-receiver  206  at transceiver  202  and reduced bit error rates. 
   Feedback circuits implementing a model-free optimization technique are used to continuously optimize the beam quality metric value measured by beam quality imagers ( 118 ,  218 ). This is accomplished by sending signals to adjust either a wave-front phase modulator or an adaptive mirror (deformable or pixelated) placed directly in front of the transmitter ( 104 ,  204 ). The modulated beam emitted by the transmitter ( 104 ,  204 ) passes through the wave-front corrector to obtain a wave-front phase modulation that reduces the effects of turbulence-induced phase distortions in the optical communication channel. 
   After reading the foregoing specification, one of ordinary skill will be able to effect various changes, substitutions of equivalents and various other aspects of the present invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof. 
   Having thus shown and described what is at present considered to be the preferred embodiment of the present invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the present invention are herein meant to be included.