Abstract:
The disclosed technology provides a dynamic interconnection system which allows to couple a pair of optical beams carrying modulation information. In accordance with the disclosed technology, two optical beams emanate from transceivers at two different locations. Each beam may not see the other beam point of origin (non-line-of-sight link), but both beams can see a third platform that contains the system of the disclosed technology. Each beam incident on the interconnection system is directed into the reverse direction of the other, so that each transceiver will detect the beam which emanated from the other transceiver. The system dynamically compensates for propagation distortions preferably using closed-loop optical devices, while preserving the information encoded on each beam.

Description:
This application is a continuation in part of U.S. patent application Ser. No. 09/848,563 filed May 3, 2001 now U.S. Pat. No. 7,113,707, the disclosure of which is hereby incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates, in general, to the field of optical telecommunications and compensated imaging. It relates to a system and method for creating an optical link between two stations, each station not necessarily being in the line of sight of the other, with full duplex communications being possible, and more particularly relates to the transmission of reference information with associated data. 
   BACKGROUND 
   The prior art includes systems for relaying optical information between two beacons. This is conventionally accomplished by first detecting and demodulating the optical information received by the first beacon from an optical source, subsequently synthesizing a optical beam by modulating another optical source with this information, and, finally, directing the new optical beam to the second beacon. This multi-element repeater system has application to well-defined relay modules, along optical fiber links for example, or for N×M interconnects for photonic networks, among others. However, in the general case, where propagation errors may be dynamic, and where the incident beams can arrive over a large field-of-view, a more robust interconnection system is required. These problems and limitations are addressed by this invention. 
   The prior art also includes systems comprising a set of tilt-mirror compensators which are used for correcting certain errors. Such systems can only compensate for the lowest-order errors such as tilt or beam wander. Other low-order errors, such as focus and astigmatism, can be corrected with a variable focus element. However, these systems are unable to compensate for higher order propagation errors such as general wavefront distortions due to propagation through turbulent atmospheres, multi-mode optical fibers, etc. Thus, a system and method are needed that provide ways of compensating for these errors. 
   The disclosed technology addresses the general case of phase (wavefront) errors. In this connection, the prior art includes the Double-pumped Phase Conjugate Mirror (DPCM). The DPCM does not require any servo-loop devices, since it proceeds via an all-optical nonlinear interaction. However, the DPCM requires the power carried by the incident laser beam to be above a given threshold, in order to properly function. This threshold generally ranges between a few μW/cm 2  to a few mW/cm 2 , depending on the particular crystal used for the DPCM. Some examples of adequate crystals include BaTiO 3  and InP. Moreover, the response time of a DPCM depends on the intensity of the incident beams, and the intensities of the two incident beams need to have similar values for the device to function optimally (fast response time, stable operation, and suitable wavefront compensation). Finally, the DPCM is lossy and the insertion loss can be large, approaching 30% or more. 
   In contrast, the present device may have a very low insertion loss (it preferably only requires enough light to be sensed by the wavefront error sensor which can approach the shot-noise limit per pixel), and can function with input beams with intensities which need not to be equal (i.e., not necessarily balanced). Similar to conventional adaptive optical systems, the wavefront compensation capability will be a function of the number of equivalent pixels, or phase actuators, relative to the number of resolvable coherent phase patches which need to be phased up or corrected. 
   One object of the present invention is to provide a system and method for relaying optical information from one transceiver to another. Specifically, this invention will direct a first optical beam emanating from a first transceiver and travelling to a second transceiver, into the reverse direction of a second optical beam emanating from the second transceiver and traveling to the first transceiver. The beams can be encoded, so that a communication link is realized with diffraction-limited capability. In its most basic form, a simple pair of tilt mirrors may be used to direct one beam into the reverse direction of the other. However, in general, the beams are not plane waves, and may have undergone time-varying (i.e., dynamically varying) propagation distortions, including atmospheric distortions, multi-mode fiber-induced distortion, etc. Therefore, an adaptive optical element is used to compensate for, and to track out, these undesirable time-varying distortions, and, at the same time, provide a means for coupling the light from one direction into the other, and vice versa (without loss of the desired duplex modulation). Since this system provides for coupling of the two optical beams, no local detector or source is required at the location of the interconnect module. Reference data is preferably transmitted with the desired data to be transmitted for tracking out the aforementioned errors. The optical beams that leave the interconnect module may propagate in precisely the reverse direction of the incident beams (i.e., they are mutually phase-conjugate replicas of the incident beams). Thus, pointing and tracking is realized with this system, so that the system performance is not compromised (i.e., low insertion loss and high directivity). Finally, modulation is preserved on the various beams, so that information can be transferred from one station to another station, with diffraction-limited performance, and subject to typical adaptive optical design issues and constraints, such as diffraction, dispersion, depolarization, the compensation bandwidth, the spatial bandwidth of the system (e.g., the number of resolvable pixels for wavefront reconstruction), etc. 
   Applications of the disclosed technology include optical “relay nodes” for free-space, space-based or terrestrial-based, as well as for guided-wave based (e.g., coupling of the output of a single or multi-mode fiber to another fiber or to a free-space path), optical communication and image relay links, or combinations thereof. Many applications do not afford the luxury of line-of-sight viewing between the stations at the end points of the communication link. For example, a mountain may obstruct the end points for direct viewing, or two satellites may no longer “see” each other. To overcome this problem, an intermediate “relay node” or interconnection system is required, which may be placed on a hilltop or on an intermediate satellite, such that the interconnection system is in the line of sight of both stations. It may also be necessary to optically relay (one-way or two-way) information from one subsystem (e.g., a multimode fiber) into another subsystem (e.g., an array of optical modulators, detectors, etc.). General extensions of this design philosophy follow. For example, a cascade of interconnection modules can be placed on a series of hilltops so that a complete communications link can be established across the chain of hilltops. 
   As shown in  FIG. 1 , the prior art discloses a method to first detect and demodulate the beam (originating from a first station) at the approximate mid-point (e.g., a hill-top or satellite in the case of a non-fibre based communication system) of the link between two stations, then to encode this information onto another laser, and finally direct the encoded data to a second station to complete the link (on the other side of the hill-top). This approach, however, does not compensate for propagation distortions. Hence, the very photons from one end of the link will arrive at the other end of the link in a diffraction limited manner, and, vice versa. 
   The disclosed technology provides for an automatic re-directing of the beam, as it arrives at the hill-top, to the second half of the link, as shown in  FIG. 2 . Moreover, the invention compensates for propagation distortions, so that the beam will arrive at the other end of the link without distortion. The disclosed technology enables such an intermediate node to be realized, without the usual photonic repeater requirements of high-bandwidth photo-detection, modulation and retransmission of the data. In the disclosed technology, the temporal modulation format imposed onto the beam from its initial point of origin is preserved. As it goes through the interconnection system only its wavefronts are modified, while its temporal encoding is maintained. Further, the system can function using mutually incoherent sources (e.g., free-running lasers at each end point of the link). When both of these lasers impinge onto the system, the beam from one of the end-points will be directed into the wavefront-reversed direction of the path that the second beam took, thereby “finding” and arriving to the other end of the link distortion-free (assuming usual time scale of beam formation by the system, range, atmosphere distortion time scale, and motion of the source locations during the optical transit time). Hence, the very photons from one end of the link will arrive at the other end of the link in a diffraction-limited manner, and, visa-versa. 
   Additionally, the system of the disclosed technology provides for “auto-tracking”. Indeed, if the end-point stations are moving, the interconnect can track or follow the moving stations. This assumes that the stations move slowly with respect to the reconfiguration time of the interconnect and the time/spatial scale of the dynamic distortions. The system provides for propagation-distortion compensation as well. 
   A related application is in the area of space-based low-cost relay mirrors. A pair of large-area telescopes are used to collect a weak signal, and then relay the beam to another location. These lightweight mirrors, which may be made of thin membranes (mylar, etc.), often possess optical distortions because the lightweight material they are made of can easily deform. The system performance is thereby degraded. By placing the proposed invention between the pair of large-area relay mirrors, the local mirror aberrations, as well as path distortions experienced by the two incident beams, can be corrected in real-time. Other potential areas of application include stratospheric relay platforms, such as Low Earth Orbit (LEO) and Medium Earth Orbit (MEO) satellites and other airborne systems, with application to backbone feeder lines, as well as dynamic links for optical fibers, laser sources and beam combining systems. In the latter case, a given incident probe beam can be used as a local reference beam, which can, as a result of the interconnection system, phase up a collection of single-frequency, but randomly phased oscillators, including optical fiber amplifiers and oscillators. 
   SUMMARY 
   The disclosed technology provides a novel system that can adaptively interconnect two incoherent optical beams thereby creating an optical link between two stations. The disclosed technology also provides a method of optically interconnecting two stations from which two optical beams emanate, the two optical beams being directed from the two stations to a common location such as a hilltop. 
   The overall scope of the disclosed technology is to provide a dynamic interconnect capability to couple a pair of spectrally narrowband or broadband optical beams, or one in each category, which may carry modulation information. By way of an example, let us suppose that two optical beams emanate from transceivers at two different locations, and are both incident upon the optical interconnection system of the disclosed technology. The system will direct each beam into the reverse (i.e., phase conjugate) direction of the other, so that each transceiver will detect the beam that emanated from the other station without an intermediate detection, demodulation and encoding of another mid-point source with the demodulated information. 
   In general, the incident beams propagate along different paths, and, thus, may experience different propagation distortions, beam wander, etc. The disclosed technology provides an interconnection system for optical beams which may have experienced different Doppler shifts, possess different wavefront distortions, speckle, as well as depolarization (the latter two cases would involve the use of additional SLMs (Spatial Light Modulators). 
   The system architecture comprises a pair of closed-loop Adaptive Optical (AO) modules (or, two regions on a common-focus correction module, the latter for bore-sighting the two beams and adaptive optical element), in conjunction with an optional tilt-focus compensator for low-order aberration errors, if necessary. Also comprised in the system are a number of reflectors and beam splitters. Each AO module is controlled by a given input beam. 
   The disclosed optical system is not a conventional repeater device. That is, it does not merely detect and demodulate the beam, and then encode the information onto another optical source (e.g., as in a relay station). Instead, it re-directs one optical beam into the reverse direction of another by modifying its wavefronts. In this manner, the system compensates for wavefront errors along the paths of the two incident beams, resulting in a well-defined output beam, with near-diffraction-limited performance. Moreover, any global modulation information is preserved on each incident beam, which is redirected into the reverse path of the other beam. Therefore, no demodulation and subsequent repeater-based modulator elements are required, thereby greatly simplifying the basic system architecture. If necessary, however, optical amplifiers (bulk or guided-wave classes) can be placed at any point along the system (including at the interconnect module). The interconnect module will provide compensation for optical distortions in the amplifiers as well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a prior art system for exchanging optical information between two stations, the line of sight between the two stations being obstructed by a hill. 
       FIG. 2  demonstrates that an optical interconnect may be used for providing optical information exchange between two stations, even though the line of sight between the two stations is obstructed by a hill. 
       FIG. 3A  depicts an embodiment wherein the AO modules function in reflection mode and wherein a pair of optical tilt-focus error compensators is used; 
       FIG. 3B  depicts an embodiment wherein the AO modules function in reflection mode and wherein a single optical tilt-focus error compensator is used; 
       FIG. 4  depicts an embodiment wherein the AO modules function in transmission mode. 
       FIG. 5  depicts an embodiment wherein a single AO module is used, the AO module comprising a single AO wavefront corrector having two regions, and a pair of wavefront error sensors. 
       FIG. 6   a  depicts one embodiment of the present invention by which a reference signal can be transmitted along with desired data to be transmitted, the two signals being distinguished by temporal shifting or sequencing. 
       FIG. 6   b  depicts a preferred format for a frame of transmitted data in the embodiment of  FIG. 6   a.    
       FIG. 7  shows a alternative embodiment of the present invention by which a reference signal can be transmitted along with desired data to be transmitted, the two signals being distinguished by polarization of the optical signals. 
       FIG. 8  depicts an embodiment of the present invention by which the reference signal can be transmitted along with desired data to be transmitted, the two signals being distinguished by frequency shifting the optical signals. 
   

   DETAILED DESCRIPTION 
   Basic embodiments of the disclosed technology is illustrated with reference to  FIG. 3A ,  FIG. 3B , and  FIG. 4 . The systems of  FIGS. 3A and 3B  relate to an optical interconnect functioning in reflection mode, whereas the system of  FIG. 4  relates to an optical interconnect functioning in transmission mode. The following description applies equally to both the reflection-mode systems of  FIGS. 3A and 3B  and the transmission-mode system of  FIG. 4 . When appropriate the distinctions between these two systems are made clear.  FIG. 5  depicts an embodiment wherein a single AO module is used, the AO module comprising a single AO wavefront corrector having two regions, and a pair of wavefront error sensors. 
   Improvements are discussed with reference to  FIGS. 6-8  whereby a reference signal can be transmitted along with the desired data to be transmitted. 
   For the purpose of illustration, beam  9  originating from station A, and beam  10  originating from station B, are shown displaced relative to one another. In actuality, the two beams travel on top of one another, in opposite directions. 
   The system allows two stations, A and B, to exchange information via an optical link created between them using an interconnect. The interconnect preferably comprises a pair of Adaptive Optical (AO) modules  3  and  4 , each of which comprising a pair of AO wavefront correctors  3   a  and  4   a , and a pair of Wavefront Error Sensors  3   b  and  4   b  to drive AO wavefront correctors  3   a  and  4   a , respectively. The interconnect further comprises a pair of optical tilt-focus error compensators  7  and  8 , placed upstream and downstream of the AO modules, respectively, and a pair of beam splitters  17  and  18  placed between the AO modules. Tilt-focus error compensator  7  is positioned between station A and AO module  3  such that tilt-focus error compensator  7  is in the light path between station A and AO module  3 . Similarly, tilt-focus error compensator  8  is positioned between station B and AO module  4  such that tilt-focus error compensator  8  is in the light path between station B and AO module  4 . Alternatively, the configuration shown in  FIG. 3B  may be used wherein a single optical tilt-focus error compensator  78  is used instead of two. In this case, the optical tilt-focus error compensator  78  is placed near the midpoint of the overall system, in the light path between AO module  3  and AO module  4 , approximately midway between the two modules. Beam  15 , resulting from the reflection of beam  9  by AO wavefront corrector  3   a , is split by beam splitter  17  into a first part which is directed to AO module  4 , and a second part which is directed to Wavefront Error Sensor (WES)  3   b . WES  3   b  senses the distortions (e.g., its wavefront errors) of the beam, computes the required correction and addresses the AO wavefront corrector  3   a  to drive the input distortions to zero or nearly zero, depending on the servo-loop gain of the system. Similarly, beam  16 , resulting from the reflection of beam  10  by AO wavefront corrector  4   a , is split by beam splitter  18  into a first part which is directed to AO module  3 , and a second part which is directed to WES  4   b . WES  4   b  senses the distortions of the beam, computes the required correction and addresses the AO wavefront corrector  4   a  to drive these input distortions to zero or near zero. This is an example of a servo-loop or closed-loop system. 
   Each AO module  3 , 4  is driven or controlled (i.e., configured, in terms of its pixelated phase map) by the respective incident optical beam. AO module  3  is controlled by optical beam  9  originating from station A, while AO module  4  is controlled by optical beam  10  originating from station B. 
   Preferably, the AO modules  3 , 4  are configured by a reference signal which is subjected to the same path distortions as is the data. The assumption which is made is that if the distortions in the reference signals can be adequately compensated for, that applying the same compensation to the data signals will also compensate for path distortions there as well. Different techniques for transmitting the reference information will be discussed later. 
   In order to optimize the efficiency of the system, beam splitters  17  and  18  are preferably designed to transmit most of the incident light (typically in the range of 90% of the incident light, depending on the signal-to-noise ratio (SNR) achieved) to AO module  4  and AO module  3 , respectively, while reflecting just enough light to WES  3   b  and WES  4   b , respectively, so that the WESs can function with adequate SNR (i.e., SNR&gt;1, preferably in the range of 10 to 100 or more). 
   The purpose of the optical tilt-focus error compensators  7  and  8 , as shown in  FIG. 3A , is to remove overall tilt and/or focus errors between the pair of beams, so that they propagate in exact opposition to each other within the system (i.e., they counter-propagate). These compensators  7  and  8  may be omitted if the field-of-view and the dynamic range of the AO modules  3  and  4  provide sufficient correction for lower-order errors (tilt and focus) without compromising the ability to compensate for higher-order wavefront errors on the respective input beams. This assumes that the AO modules have sufficient dynamic range (i.e, greater than a wave, preferably greater than several waves). 
   For the purpose of illustration, let us suppose that incident input beams  9  and  10 , each possesses an arbitrary wavefront error upon incidence onto the respective AO modules  3  and  4 . Moreover, let us assume that each beam is encoded with information, in the form of either amplitude or phase modulation. The information can be encoded onto a single spatial mode, or, can be in the form a multi-pixel “image” with each resolvable pixel corresponding to an independent channel of information. 
   Assume further that the temporal encoded modulation bandwidth exceeds the adaptive optical closed-loop compensation bandwidth, so that the desired modulation is preserved, after beam error compensation. The compensation bandwidth must equal or exceed the distortion effective bandwidth for the system to function. As an example, atmospheric distortions have a time scale on the order of a millisecond, so the AO compensation bandwidth must be greater than 1 KHz. On the other hand, the desired communication bandwidth (or link data rate) can be very large (1 to 100 GHz, for example). 
   The role of the AO module  3  is to minimize, upon reflection/transmission by/through AO module  3 , the wavefront errors carried by the input beam  9 . For example, AO module  3  will drive the spatial phase error φ res  of incident beam  9  to a small residual value dictated by the closed-loop servo gain G (φ res ≈φ in /(1+G), where φ in  is the input phase error). The gain G usually ranges from about 2 to about 100, with higher values of G giving better system performance. The result of this operation is that a highly aberrated input beam  9 , will, after reflection by/transmission through AO module  3 , emerge as a near-plane wave beam  15 . Note that any global phase or intensity modulation will remain on the planarized (i.e., the wavefront scrubbed) beam  15 . The planarized beam  15  maintains the (desired) globally encoded modulation information. This modulated plane wave beam  15  will then be reflected/transmitted by/through the other AO module, namely AO module  4 . Note that the cleaned-up beam  15  does not affect the spatial phase of AO module  4  since this module is controlled by incident beam  10  originating from station B. 
   By reciprocity, the plane wave beam  15  will, upon reflection off/transmission through AO module  4 , emerge with the same wavefront as beam  10  had before it reflected off/transmitted through AO  4 . The encoded input beam  9  will thus propagate into the precise reverse direction of beam  10  and arrive at station B as a diffraction-limited beam. Diffraction-limited characterizes a beam with highest focusing ability, and is determined by the ratio, λ/D, where λ is the wavelength and D the aperture. Optical distortions increase this ratio by one to several orders of magnitude (˜10 to ˜1000, or more) which in turn degrades performance. 
   The foregoing discussion is also applicable to AO module  4 , input beam  10 , planarized beam  16  and station A. 
   In yet another embodiment of the disclosed technology, the two AO wavefront correctors  3   a  and  4   a  of  FIGS. 3A ,  3 B and  4 , are replaced with two regions on a common-focus correction module, as illustrated in  FIG. 5 . In accordance with this embodiment, the interconnect comprises a common-focus correction module or AO wavefront corrector  34  having a first region  341  and a second region  342 , each region forming a separate AO wavefront corrector. The interconnect further comprises a pair of WESs  34 A and  34 B, to drive AO wavefront corrector regions  341  and  342  respectively, a pair of optical tilt-focus error compensators  7  and  8 , placed upstream of the AO wavefront corrector  34 , a pair of beam splitters  38  and  39 , and seven reflectors  35 ,  36 ,  37 ,  40 ,  41 ,  42  and  43 . Tilt-focus error compensator  7  is positioned between station A and AO wavefront corrector  34  such that station A, tilt-focus error compensator  7 , and region  341  of the AO wavefront corrector  34 , are substantially aligned. Similarly, tilt-focus error compensator  8  is positioned between station B and AO wavefront corrector  34  such that station B, tilt-focus error compensator  8 , and region  342  of the AO wavefront corrector  34 , are substantially aligned. Beam  91 , resulting from the reflection of beam  9  by AO wavefront corrector region  341 , is split, by beam splitter  39 , into a first part which is directed to AO wavefront corrector region  342  after successive reflection by reflectors  37 ,  36  and  35 , and a second part (beam  92 ) which is directed to WES  34 A after successive reflection by reflectors  42  and  43 . WES  34 A senses the distortion of the beam, computes the required correction and addresses AO wavefront Corrector region  341  to drive input distortion to zero or near zero. Corrected beam  91  emerges from AO wavefront corrector region  341 , substantially distortion free or at least with reduced distortions. Part of beam  91 , i.e. beam  92 , is redirected to WES  34 A for further corrections and so on. This illustrates the functioning of a servo-loop or closed-loop system. Similarly, beam  101 , resulting from the reflection of beam  10  by AO wavefront corrector region  342 , is split, by beam splitter  38 , into a first part which is directed to AO wavefront corrector region  341  after successive reflection by reflectors  35 ,  36  and  37 , and a second part (beam  102 ) which is directed to WES  34 B after successive reflection by reflectors  40  and  41 . WES  34 B senses the distortion of the beam, computes the required correction and addresses AO wavefront corrector region  342  to drive input distortion to zero or near zero. Corrected beam  101  emerges from AO wavefront corrector region  342 , substantially distortion free or at least with reduced distortion. Part of corrected beam  101 , i.e., beam  102  is redirected to WES  34 B for further corrections and so on. 
   Each of AO wavefront corrector regions  341  and  342 , is driven or controlled (i.e., configured, in terms of its pixelated phase map) by the respective incident optical beam. AO wavefront corrector region  341  is controlled by optical beam  9  originating from station A, while AO wavefront corrector region  342  is controlled by optical beam  10  originating from station B. 
   In order to optimize the efficiency of the system, beam splitters  38  and  39  are preferably designed to transmit most of the incident light, while reflecting just enough light then sensed by WES  34 B and WES  34 A, respectively, so that the WESs can function with a adequate signal-to-noise ratio. 
   The purpose of the optical tilt-focus error compensators  7  and  8 , is to remove overall tilt and/or focus errors between the pair of compensated beams, so that they propagate in exact opposition to each other within the system (i.e., they counter-propagate). These compensators  7  and  8  may be omitted if the field-of-view and the dynamic range of the AO wavefront corrector  34  provides sufficient correction for these lower-order errors (tilt and focus) without compromising the ability to compensate for the higher-order wavefront errors on the respective input beams. This assumes that the AO wavefront corrector  34  has sufficient dynamic range. 
   For the purpose of illustration, let us suppose that incident input beams  9  and  10  each possesses an arbitrary wavefront error upon incidence onto the respective AO wavefront corrector regions  341  and  342 . Moreover, let us assume that each beam is independently encoded with useful (and different) global information, in the form of either amplitude or phase modulation. We further assume that the encoded modulation bandwidth exceeds the adaptive optical closed-loop compensation bandwidth, so that the desired modulation is preserved, after beam clean-up. The compensation bandwidth must equal or exceed the distortion effective bandwidth for the system to function. As an example, atmospheric distortions have a time scale on the order of a millisecond, so the AO compensation bandwidth must be greater than 1 KHz. On the other hand, the desired communication bandwidth (or link data rate) can be very large (1 to 100 GHz, for example). 
   The role of the AO wavefront corrector region  341  ( 342 , respectively) and WES  34 A ( 34 B, respectively) is to minimize, upon reflection by AO wavefront corrector region  341  ( 342 , respectively), the wavefront errors carried by the input beam  9  ( 10 , respectively). That is, AO wavefront corrector region  341  ( 342 , respectively) will drive the spatial phase error φ res  of incident input beam  9  ( 10 , respectively) to a small residual value dictated by the closed-loop servo gain G(φ res ≈φ in /(1+G), where φ in  is the input phase error). The gain G usually ranges from about 2 to about 100, with higher values giving better system performance. The result of this operation is that a highly aberrated input beam  9  ( 10 , respectively), will, after reflection by AO wavefront corrector region  341  ( 342 , respectively), emerge as a near-plane wave beam  91  ( 101 , respectively). Note that any global phase or intensity modulation will remain on the planarized (i.e., the wavefront scrubbed) beam  91  ( 101 , respectively). The planarized beam  91  ( 101 , respectively) maintains the globally encoded modulation information. This modulated plane wave beam  91  ( 101 , respectively) will then be reflected by reflectors  37 ,  36 , and  35  ( 35 ,  36 , and  37 , respectively) and finally by the other AO wavefront corrector region, namely AO wavefront corrector region  342  ( 341 , respectively) which directs corrected modulated plane wave beam  91  ( 101 , respectively) to its final destination, i.e., station B (station A, respectively). Note that the cleaned-up beam  91  ( 101 , respectively) does not affect the spatial phase of AO wavefront corrector region  342  ( 341 , respectively) since this region is controlled by incident input beam  10  ( 9 , respectively) originating from station B (A, respectively). 
   By reciprocity, plane wave beam  91  ( 101 , respectively) will, upon reflection by AO wavefront corrector region  342  ( 341 , respectively), emerge with the same wavefront as beam  10  ( 9 , respectively) had before it reflected off AO wavefront corrector region  342  ( 341 , respectively). The encoded input beam  9  ( 10 , respectively) will thus propagate into the precise reverse direction of beam  10  ( 9 , respectively) and arrive at station B (A, respectively) as a diffraction-limited beam. Diffraction-limited characterizes a beam with highest focusing ability, and is determined by the ratio, λ/D, where λ is the wavelength and D the aperture. Optical distortions increase this ratio by one to several orders of magnitude (˜10 to ˜1000, or more) which in turn degrades performance. Optional amplifiers  190 ,  191  can be placed in the system (which can also be corrected by the system). The location of the amplifiers also is optional in that a nearly planarized beam will enter each of the amplifiers so that an amplifier with a small FOV will suffice. 
   The different types of AO modules suitable for the embodiments previously described, include reflective devices such as conventional pixelated piston-driven membranes (“rubber mirrors”), Liquid Crystal Light Valves (LCLVs) or LC pixelated phase shifters, which can be optically or electrically driven on a pixel-by-pixel basis, liquid crystal Spatial Light Modulators (SLMs), deformable MEMS devices, or optical MEMS-based SLMs. Suitable AO modules may also include transmission devices such as liquid crystal cells with transparent electrodes or any combination of the these devices. Conventional wavefront error sensors may also be used which drive deformable mirrors (e.g., PZT-activated, etc.). Regardless of which devices are used, an incident beam will emerge from each AO module with its wavefronts planarized. 
   The aforedescribed embodiments basically assume that reference information (i.e., a beam that contains the propagation-path phase errors) accompanies the data to be transmitted. For example, if the reference signal is interleaved with the desired data or if the desired data is FM or PM on a carrier signal whose amplitude is controlled by the disclosed embodiment, then the aforedescribed embodiments should work suitably, but they do have the disadvantage of transmission loss which arises since at least a percentage of the light is lost by beam splitters  17  and  18  for the purpose of configuring the AO modules  3 , 4 . 
     FIG. 6   a  is an embodiment similar to the aforementioned embodiments of  FIGS. 3A ,  3 B and  4 . In this embodiment, the beam splitters  17  and  18  instead of programming AO modules, need only split off sufficient light that an associated detector/processor  17   a ,  18   a  can differentiate between transmitted reference information and the desired data for the purpose of controlling electro-optic switches  17   b  and  18   b . The lines connecting elements  17   a  and  17   b  and connecting elements  18   a  and  18   b  are shown in bold lines to represent the fact that these paths are preferably provided by electrical connections rather than optical connections. 
   The transmitted data preferably includes frames which each comprise a sync pulse, followed by reference data, followed by the desired data. The desired data is the information which is to be transmitted from station A to B or vice versa. The reference data is data which is known in advance at both stations A and B and the real purpose in transmitting it is to detect for errors in the communication channel and to compensate for those errors when the desired data is being transmitted. Compared to the transmitted version of the known-in-advance reference data, the known-in-advance reference data or signal against which the transmitted version is compared may be thought of as a idealized version of the reference data while the transmitted version of the reference data or signal may be thought of as a distorted version due to distortions occurring in the communication channel. 
     FIG. 6   b  shows a frame of data  56  transmitted (including a sync pulse  50 , reference data  52  and desired data  54 ). The desired data  54  can comprise text, pictures, video, multimedia, or data for any nature which needs to be transmitted from point A to B (or vice versa). The nature of reference data is known in advance by the receiving station. It may be single valued or it may varied according to a known algorithm. 
   Aberration in the transmitted signals is induced by regions, for example, Φ A  and Φ B , in the optical path which induce such aberrations. 
   Detector/processor  17   a ,  18   a  senses the sync pulses  50  in each frame. The reference data  52  preferably comes next and the electro-optic switches  17   b ,  18   b  switch the incoming signal (light) to WES  3   b ,  4   b  where the received reference data is compared with its expected value(s) and the wavefront correctors  3   a ,  4   a  are adjusted to track out the path error. Thus, electro-optic switches  17   b ,  18   b  switch close to 100% of the light to the associated WES  3   b ,  4   b  during receipt of reference data  52  and allow close to 100% of the light to the travel over the communications path when desired data  54  is being transmitted. The path error is sampled during the time the reference data  52  is received and then the wavefront correctors  3   a ,  4   a  are held in the state of correction determined during receipt of the reference data  52  throughout receipt of the data of interest  54 . Since the path error is typically caused by various environmental factors and since these factors change relatively slowly, making an assumption that the errors detected during data period  52  with also be in play (and therefore can still be tracked out) during data period  54  is a reasonable assumption to make. 
   In a full duplex embodiment, the sync pulses  50  in each half duplex signal should be in sync with one another so that (i) both electro-optic switches  17   b ,  18   b  will be in a position allowing the desired data  54  to be exchanged at the same time by stations A and B along path  62  and (ii) both electro-optic switches  17   b ,  18   b  will be in a position to divert reference data  52  to their respective WES  3   b ,  4   b  at the same time. Preferably, syncing occurs as follows: One station (A or B) initiates half-duplex communications and the other station (B or A) then starts communicating with the sending station by first timing its frames  56  so that its sync pulses  50  to be in sync with sync pulses  50  from the other station within the optical interconnect. 
   Alternatively, the syncing could occur within the interconnect by adding intentional optical delay in path  62  to superimpose the syncing pulses  50  of each sending station one upon the other. 
   The sync pulse may be a single pulse as graphically shown in  FIG. 6   b , or it may be a multi-bit orthogonal code. It may also be combined with reference signal  52 , if desired, as opposed to being a separate pulse as shown. In that case, the reference data is effectively being interleaved with the desired data to be transmitted. The reference data can also be associated with an encryption algorithm. 
     FIG. 7  shows another way of transmitted both reference data and desired data. Instead of distinguishing Φ A  and Φ B  reference signals from the desired link data temporally as done in the embodiment of  FIG. 6   a , in this embodiment this signals are distinguished by using different polarizations of light for the reference data and for desired link data. This embodiment assumes that each polarization experiences the same distortion or aberration field, which is the case for most typical communication scenarios. The reference data for station A, for example, is added to the desired data using a polarizing mixer  59  while the reference data for station B is added to the desired data using a polarizing mixer  60 . The reference data is orthogonal to the desired data. The two are separated using a polarization beam splitter  17   d ,  18   d  for each leg. In this way, the reference data for station A is separated out by splitter  17   d  to follow path  17 - 1  to wavefront corrector  3 . In a similar vein, the reference data for station B is separated out by splitter  18   d  to follow path  18 - 1  to wavefront corrector  4 . 
   Polarization detectors/correctors  17   c ,  18   c  may be used in each leg in order to compensate for the fact that the optical interconnect may well be receiving signals from arbitrary directions for which the polarization of the signals may not be known in advance. The polarization detectors/correctors  17   c ,  18   c  can be used to detect the polarization of the incoming signals and correct them as needed to work properly with the disclosed optical interconnect. The lines connecting elements  17   c  and  17   e  and connecting elements  18   c  and  18   e  are shown in bold lines to represent the fact that these paths are preferably provided by electrical connections rather than optical connections. 
   In this embodiment, the reference signal preferably is modulated with a unique signal which can be detected. For example, the unique signal could be a pseudo random code. The detectors  17   c ,  18   c  would then control the polarization corrector to rotate the polarization in the legs, for example, such as to maximize the strength of the pseudo random code so that the reference signal would indeed be picked off properly by the downstream polarizing beam splitters  17   d ,  18   d.    
     FIG. 8  depicts another embodiment for separating the reference data from the information data to be transmitted. In this embodiment, the two data sets are distinguished by the optical frequency at which they are transmitted. This embodiment is similar to the polarization distinguishing feature of  FIG. 7 , but instead of using polarization beam splitters  17   d ,  18   d , dichroic beam splitters  17   e ,  18   e  (or a multi-pixel imaging optical channelizer) are used in each leg. 
   The disclosed techniques of separating the reference data from the informational data use temporal, polarization and wavelength techniques. However, there is no reason not to combine these techniques. For example, the two wavelengths of the embodiment of  FIG. 8  can, in addition, be time gated differently as described with reference to  FIG. 6A  and/or polarized differently as described with reference to  FIG. 7 . Using such techniques in combination can make the disclosed system more robust and/or covert. Also, certain techniques may well work better in the face of certain types of propagation errors. For example, birefringent and/or dispersive propagation errors may very well limit the techniques which are used. 
   The wavelength difference of two sets of data does not need to be widely spaced. Indeed, standard commercial grade optical communication equipment should suffice, using, for example, the standard channel separation for telephony in an optical circuit as being an appropriate channel spacing between the desired data and the reference data. This choice of closely-spaced optical carrier wavelengths is also desirable in the face of dispersion along the propagation path. 
   Optical tilt-focus error compensators  7  and  8 , discussed with reference to the embodiments of  FIGS. 3A ,  3 B and  4  may also be used with the embodiments of  FIGS. 6A ,  7  ad  8 . 
   Possible wavefront error sensors include conventional shearing interferometric sensors, a Shack-Hartmann (local tilt) sensor, or a holographic intensity-to-phase sensor. 
   Possible global tilt-focus error compensator (used for bore-sighting) include a pair of tilt mirrors (conventional, optical MEMS, etc.), a pair of real-time liquid crystal gratings, etc., which are driven by a standard closed-loop quad detector-based servo loop. 
   The various elements comprised in the interconnection system are preferably packaged in a compact structure. The distances between the two stations and the interconnection system may be large, however. 
   The system of the present invention acts as an optical interconnect, essentially coupling the two beams that emanate from their respective transceivers (stations), while maintaining their encoded information. Each beam leaves the interconnection system (in a spatial sense) in the form of a phase-conjugate replica of the other beam, yet, the temporal encoding on each beam is preserved. Since the pair of AO modules  3  and  4 , or regions  341  and  342 , are locally controlled by input beams  9  and  10  respectively, the two beams do not need to be coherent or even have the same nominal wavelength (the allowed wavelength difference is governed by the dispersion and diffraction of the system, and the propagation path characteristics for a given range). Thus, the system can function in the presence of differential Doppler shifted beams, emanating from platforms moving at different speeds, as well as with general beam wander and propagation errors. 
   The system can also function in guided-wave architectures, providing dynamic coupling of information from one fiber (or waveguide) channel to another, or to a plurality of channels. All that is required is that a given channel provide a reference or beacon beam so that the AO module can planarize the beam and, at the same time, provide for a phase-conjugate return of the temporally encoded beam back to the reference beam point of origin. 
   The optical paths shown in the disclosed embodiment are depicted for ease of illustration as opposed to necessarily representing the actual paths that light might take. Also, some optical paths can be shortened by using electrical paths instead for a portion of an optical path. For example,  FIG. 5  shows a number of light paths and a number of mirrors to effect changes of light direction. Various classes of Wavefront Error Sensors (WES) which involve detection arrays, processors, etc. that electronically drive the SLM on a pixel-by-pixel basis can be used to reduce the light paths. Further more, instead of using mirrors, light can be fed where desired using a bundle of coherent optical fibers. 
   While the present disclosure describes various embodiments, these embodiments are to be understood as illustrative and do not limit the claim scope. Many variations, modifications, additions and improvements of the described embodiments are possible. For example, those having ordinary skill in the art will understand that the process parameters, materials, and dimensions are given by way of example only. The parameters, materials, and dimensions can be varied to achieve the desired structures as well as modifications, which are within the scope of the claims. Variations and modifications of the embodiments disclosed will now suggest themselves to those skilled in the art. The disclosed embodiments best explain the principles of the invention and its practical application to thereby enable others skilled in the art to utilize the disclosed technology in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.