Abstract:
An optical communication system for communicating through a turbulent medium is disclosed. It includes an optical transmitter and an optical receiver. The optical receiver receives an optical signal containing information that fluctuates as it passes through a turbulent medium. It comprises a reflector for collecting the optical signal and for focusing it, a probe laser for generating an optical probe beam, an optical device having an OTM responsive to the focused optical signal and the probe beam and operative to change a characteristic of the probe beam, and optoelectronic detector means responsive to the changed characteristic and, operative to develop an output electrical signal representative of the information contained in the received optical signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. 09/587,124, presently U.S. Pat. No. 6,585,532, issued Jul. 1, 2003, filed concurrently herewith, entitled: “Optoelectronic Communication System in Turbulent Medium Having Array of Photodetectors and Time Compensation” by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to U.S. patent application Ser. No. 09/587,126, presently U.S. Pat. No. 6,580,540, issued Jun. 17, 2003, filed concurrently herewith, entitled: “Time Compensation Architectures For Controlling Timing Of Optical Signals” by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to co-pending U.S. patent application Ser. No. 09/586,014, filed concurrently herewith, entitled: “Electro-Optic Device For Adding/Subtracting Optical Signals” by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     Also related to U.S. patent application Ser. No, 09/587,125, present U.S. Pat. No. 6,516,103, issued Feb. 4, 2003, filed concurrently herewith, entitled: “Optical Interconnect Capable of Performing Addition/Subtraction” by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to optical communication systems, and more particularly to an optical communication system that employs an optical transcription material in the receiver and that communicates through a turbulent medium. 
     2. Description of the Art 
     The telecommunications industry is rapidly switching to a hybrid platform which utilizes both electronics and photonics to increase the operational bandwidth. Today&#39;s communication systems consist of optical fiber networks, fiber amplifiers, optical diode transmitters, and high speed semiconductor receivers. This architecture works well in the confines of optical fibers. There are problems in matching optical fiber network bandwidths, however, when propagating these signals in free space, which is necessary for remote applications. 
     Free space propagation of the signal through the atmosphere, water or other turbulent media will introduce fluctuating distortions and aberrations. These fluctuations prevent continuous focusing of the signal beam onto the small area high speed detectors typically utilized in optical communication systems. 
     A realistic example indicates that 0.5 m collection dish with a focal length of 1 m would concentrate the light into a diameter of 200 μm for a 100 times diffraction limited beam at a wavelength of 1.5 μm. This area is approximately 300 times larger than the high speed semiconductor photodiode detectors employed in communication systems. Aberrations similar to this could be incurred by atmospheric propagation. One approach is to correct for the distortions with adaptive optics, active tracking systems, or phase conjugation techniques in order to obtain a near diffraction limited signal beam which allows focusing onto the small area high speed detectors. These techniques suffer from slow response times, limited phase front correction or high signal intensities required for efficient conjugation. The other approach is to use a large area detector so that a significant fraction of the distorted signal beam can be collected by the receiver. This method has many advantages but has proven difficult to implement since the detector bandwidth (temporal response) and the detector area are often inherently coupled. 
     What is needed, therefore, is an optical communication system that is capable of communicating through a turbulent medium and which retains a large intrinsic bandwidth. 
     SUMMARY OF THE INVENTION 
     Transmission of an optical signal through a turbulent media, such as the atmosphere, produces a fluctuating spatial intensity pattern due to optical distortions and aberrations. With respect to FIGS. 1A,  1 B, and  1 C, three views are shown of an optical signal being transmitted through a turbulent media at three instants of time, t 1 , t 2 , and t 3 , respectively. These time varying distortions make it impossible to focus the signal beam onto a single small optical detector illustrated by the numeral  10  typically utilized in optical communication systems. The present invention involves collecting either a large enough subarray of the distorted signal (shown by the numeral  12 ) or the entire distorted signal (shown by the numeral  14  and encompassing the periphery in FIGS. 1A,  1 B, and  1 C) with an optical collector. The collected signal is invariant to the fluctuating distortions, thereby eliminating problems in free space propagation of optically transmitted high bandwidth signals. 
     The present invention involves combining an optical transcription material and an optical interconnect into an optical receiver while maintaining a fast temporal response, and thus a high bandwidth. This receiver directly measures temporal pulses, however, it is applicable to both temporally, phase, or frequency encoded signal sources. A separate transmission beam and probe beam architecture allows each of the optical wavelengths to be optimized for their own individual function. For example, the transmission wavelength could be chosen to increase signal throughput through a turbulent media while the probe beam could allow for dispersion free fiber propagation, or the fastest temporal response. 
     The optical transcription material (OTM) which will be described subsequently in more detail, utilizes a linear or nonlinear optical pump-probe mechanism to relay the information from the signal beam to the probe beam. The signal beam, also referred to as the pump beam, induces a time dependent index of refraction change, which is interrogated by a probe beam, also referred to as the reading beam. Through this mechanism, information which is encoded onto the signal beam is transcribed into amplitude, polarization rotation or phase modulation of the probe beam. The optical interconnect speed is limited by the intrinsic response time of the OTM as well as propagation delay time mismatches between the signal and the probe beams. These occur through two main sources, the delay time between the pump and the probe beams over the pumped volume (recognizing there is a 1 element delay) and the accumulated delays which occur over the entire optical interconnect. 
     Briefly, the present invention comprises an optical communication system for communicating through a turbulent medium. It includes an optical transmitter and an optical receiver. The optical receiver receives an optical signal containing information that fluctuates as it passes through a turbulent medium. It comprises a reflector for collecting the optical signal and for focusing it; a probe laser for generating an optical probe beam; an optical device having an OTM responsive to the focused optical signal and the probe beam and operative to change a characteristic of the probe beam, and optoelectronic detector means responsive to said changed characteristic and operative to develop an output electrical signal representative of the information contained in the received optical signal. 
     In another aspect, the present invention involves a time compensated probe methodology. Operational bandwidths in excess of 10 THz could ultimately be supported by this optical receiver methodology. 
     The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, B and C are drawings that show an optical signal pattern as it is transmitted through a turbulent media, such as the atmosphere, at three different times. 
     FIG. 2 is a block diagram of the optical communication system in accordance with the present invention. 
     FIG. 3 is a block diagram of the optical transmitter and receiver. 
     FIG. 4 is a perspective view diagrammatically illustrating transmitter and receiver optics in accordance with the present invention. 
     FIG. 5 is a diagram illustrating a free space optical interconnect used in the optical receiver in accordance with the present invention. 
     FIG. 6 is a diagram illustrating a waveguide optical interconnect used in the optical receiver in accordance with the present invention. 
     FIG. 7 is a side view diagrammatically illustrating the operation of the waveguide optical interconnect shown in FIG.  6 . 
     FIG. 8 is a diagram illustrating a configuration for providing time compensation for use with a serial reading technique in accordance with the present invention. 
     FIG. 9 is a diagram of another embodiment for providing two dimensional time compensation in accordance with the present invention. 
     FIG. 10 is another embodiment using the optical transcription material in the optoelectronic receiver for spectral multiplexing in accordance with the present invention. 
     FIG. 11 illustrates the wavelength multiplexed signal beam used in FIG.  8 . 
     FIG. 12 illustrates the probe spectrum applied to the optical transcription material used in FIG.  8 . 
     FIG. 13 illustrates the probe spectrum developed by the optical transcription material used in FIG.  8 . 
     FIG. 14 is a diagram illustrating an optical subtraction technique for use in an optoelectronic receiver in accordance with the present invention. 
     FIG. 15 shows the waveforms of the signal and reference beams associated with the optical subtraction technique illustrated in FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the block diagram of FIG. 2, the present invention provides an optical communication system, generally designated by the numeral  20 . Briefly, the system  20  communicates between a satellite  22  and ground stations  24  and  26  through a turbulent medium  28 , e.g., the atmosphere. Alternatively, the communication could be between submarines where the turbulent medium is water or between aircraft or ground stations where the turbulent medium is air as long as there is a line of sight path between the transmitting and receiving stations. 
     The system  20  includes an optical transmitter  30  and an optical receiver  32  at each ground station  24 ,  26  and two transmitter/receiver assemblies on board the satellite  22 . 
     As previously described, transmission through the atmosphere  28  produces a fluctuating spatial intensity pattern due to optical distortions and aberrations. These time varying distortions (see FIG. 1) make it impossible to focus the signal beam onto a single small high speed detector, shown by the numeral  10  in FIGS. 1A, B, and C, typically utilized in current optical communication systems. This invention involves collecting either a large enough subarray  12  of the optical signal or the entire distorted signal  14  with a small optical collector. The subarray  12  would collect about  140  times the power that could be collected, for example, by a single detector, as shown by the numeral  10 . The collected signal is invariant to the fluctuating distortions, thereby eliminating problem in free space propagation of optically transmitted high bandwidth signals. In particular, the fluctuation is calculated to be reduced from a 60% mean fluctuation (collected by  10 ) to less than 5% mean fluctuation. 
     Also, with reference to FIGS. 3 and 4, the optical transmitter  30  comprises a laser source  36  for producing an optical signal, a modulator  38  for modulating the optical signal, an amplifier  40  for amplifying the modulated optical signal and an optical reflector  42  for directing and radiating the optical signal to a ground station. 
     In the preferred embodiment, the laser source  36  is an erbium fiber laser and/or semiconductor laser which would operate in the atmospheric transmission regions in the ultraviolet, visible and infrared portions of the spectrum. An erbium fiber laser operating at 1.5 μm in the infrared with these characteristics and that produces a 10 watt signal is sold by Lucent Inc. and IRE-Polus. The modulator  38  is a lithium niobate (LiNbO 3 ) electro-optic modulator sold by New Focus Inc. and the amplifier  40  is a fiber optical amplifier. The optical transmitting antenna  42  comprises one lens, or lenslet, in a lens array  62  or a satellite transmitting mirror, such as one made by Kodak Corporation. 
     As will be described in more detail subsequently with respect to the embodiment shown in FIG. 14, when polarized optical signals are transmitted, the optical transmitter  30  also comprises a reference arm  46  supplying an unmodulated reference signal to the optical transmitting antenna  42 . The reference arm  46  comprises an amplifier  48  for amplifying a portion of the optical signal and a one-half wavelength waveplate  50  to flip the polarization of the amplified optical signal. This allows two orthogonally polarized optical signals both at the same wavelength with one containing the information and the other containing the reference, to be transmitted to the receiver. 
     As shown in FIG. 3, the optical receiver  32  includes receiver optics  41 , an optical amplifier  43 , an optical interconnect  45  and a signal processor  47 . 
     Referring now to FIG. 4, in the transmitter  30  the amplified modulated optical beam generated by the laser source  36  is propagated through an optical transmitter fiber array  56  attached to openings in an interconnect  58 . The individual fibers include Bragg gratings  59  (shown diagrammatically by the numeral) for amplifying (as appropriate), separating, the transmission beam  64  from the received beam  68  by wavelength. Alternatively, Faraday rotators can be employed to separate the transmission and receiving beams based on direction of beam propagation. Each propagated beam diverges in the free space from the opening until it reaches its lenslet  60  or lens in a lenslet array  62 . Typically for a single mode fiber the divergence is 10°. The lenslets  60  serve as the transmitting antenna  42  and. collimate the beam and transmit it as an optical signal  64  into free space through the turbulent medium  28  and toward the selected optical receiver  32 . 
     In the preferred embodiment, the lenslet array  62  is fabricated from plastic and comprises a square matrix of 15×15 lenslets having a total side dimension of between 1 and 100 centimeters. Alternatively, the matrix could be hexagonal or octagonal to obtain better fill factors and the number of lenslets could be selected based on the distance between the transmitting and receiving stations. Also the lenslet array  62  and interconnect  58  are separated by less than 1 centimeter of free space. 
     With reference now to FIG. 5, an embodiment of a free space optical interconnect, generally illustrated by the numeral  61  incorporated into the optical receiver  32 , is illustrated. An optical reflector  62  is positioned to collect the fluctuating and distorted transmitted optical beam  64 , also referred to as a signal beam, from the transmitter  30  at a distant station  22 . The reflector  62  serves to focus the collected beam on an element  70  comprising an optical transcription material (OTM). In the preferred embodiment for a free space optical interconnect the reflector is a 1 meter spherical dish that collects a signal beam with a power level in the milliwatt range. Alternatively, the collector may have a parabolic curvature. 
     The OTM element  70  is formed of a photoactive material, such as a polymer film that has a fast response (both rise time and recover time) to the optical signal beam. It interacts with a preselected frequency and has an index of refraction, n, that varies over time when it is probed with the probe beam. Alternatively, it can be a wafer formed of a semiconductor material, such as silicon or gallium arsenide, that is sensitive to infrared radiation, other polymers (undoped or doped with donor or acceptor molecules), molecular crystals, biological and synthetic chromophore systems, or a superconductive material. 
     A probe laser  72  transmits a probe beam  74  through an optical fiber  76  and a collimating lens  78  to the OTM element  70 . The probe beam is diffraction limited or near diffraction limited, may be characterized as having a Gaussian or Bessel waveform, and has a power in the microwatt range. This power is significantly lower than that of the signal beam. 
     At the OTM the signal beam  64  interacts with the near diffraction limited probe beam  74  through a linear or nonlinear optical process and impresses a phase or amplitude modulation onto the probe beam through index of refraction variations in the OTM. Through this signal-probe mechanism, information which is encoded onto the signal beam is transcribed into amplitude, polarization rotation or phase modulation or the probe beam. 
     The modulated optical signal, thus produced and identified by the numeral  80 , is focused by lens  82  onto an optical fiber  84  and propagated to a high speed photodetector  86 . The photodetector  86  converts the modulated optical signal into an electrical signal  90  representative of the information contained in the transmitted and the received optical beams. A signal processor  92  serves to process, manipulate and display the information. 
     Alternatively, the optical signal can be amplified and sent directly through a ground based optical fiber network (not shown) such that the resultant output signal is optical. 
     In operation, with the OTM  70  placed in between a free space optical interconnect  60 , the distorted signal beam  64  is focused onto the OTM which is interrogated by the probe beam  74 . This causes the index of refraction, n, of the OTM to vary over time corresponding to the information contained in the signal beam. Accordingly, the signal beam is modulated. This modulated beam  80  is propagated to the photodetector  86  which converts it to an electrical signal  90  containing the transmitted information. A signal processor  92  processes the electrical signal. 
     It should be recognized that the probe and signal beam overlap would be variable, trading off increased modulation depth with smaller probe sizes and increased tolerance to signal fluctuations with larger probe beam areas. 
     Another embodiment of the optical interconnect of the present invention is illustrated in FIGS. 6 and 7. As shown, this is represented by a waveguide optical interconnect  100  that is characterized by generally total internal reflection (TIR). More particularly, an optical waveguide  102 , also referred to as a TIR device, includes a layer  104  of an optical transcription material (OTM) over a surface  106 . The thickness of the waveguide is selected such that certain modes of a probe beam hit the interaction areas  109  of the OTM when the probe beam  74  is propagated through the waveguide. A plurality of optical fibers  108  arranged in an array  110  propagate signal beams  120  from the optical receiver (as shown in FIG. 4) to the interaction areas  109 . The signal beams  120  transmitted through the fiber array  110  to the interaction areas  109  of the optical transcription material (OTM)  104  and serve to pump the OTM. The evanescent wave  130  of the probe beam  74  penetrates into the OTM (see FIG. 7) at the interaction areas  109  and introduces either an amplitude or phase change on the probe in response to the index of refraction Δn, change, induced by the signal beam. 
     As will be described regarding FIG. 8, a time compensated architecture for the fiber array will synchronize the signal and probe beams at each point of the OTM minimizing accumulated propagation delay errors. 
     This optical interconnect is based on amplitude or phase variations which can be induced at TIR interaction surfaces  109  (see FIG.  7 ). The exponentially damped evanescent wave  130  exists at a TIR interface and therefore can monitor index of refraction changes which occur on the other side of the boundary. This mechanism forms the basis for many chemical and biological fiber sensors. In these representations, the OTM  104  must be physically close to the TIR surface  109  such that the probe evanescent field penetrates into the interaction region  109  which is optically excited by the signal beam  120 . This TIR surface can reside in a bulk optic or an optical waveguide. 
     The critical angle, θ c , for the TIR is          θ   c     =       sin     -   1            (       n   1       n   2       )                              
     where n 1  and n 2  are the index of refraction of the two mediums comprising the waveguide  102  and the OTM  104 , respectively. 
     Referring also to FIG. 6, the signal beam  64  will initially be collected with a receiver array (see FIG.  4 ). This receiver may be a lenslet array, a diffractive optic, or any combination of these elements and serves to focus the signal  64  onto a fiber optic array  110 . This array  110  is constructed such that each of the individual fibers  108  transmit the signal onto one site of the optical interconnect device which is coated with the OTM  104 . The index of refraction variations of the optical transcription are material induced by the signal (pump) beam  120 , and are translated into either amplitude or phase modulation of the probe. Polarization rotation of the probe can be accomplished by differential phase modulation along orthogonal OTM directions. If the index change at the probe wavelength is large enough that the critical angle requirement is no longer met by the probe, then a portion of the beam will be transmitted through he boundary layer and amplitude modulation will result. Phase modulation occurs if the index variation on the OTM is such that the requirement for critical angle at the TIR interface is still satisfied by the probe beam. This change Δφ can be expressed as        Δφ   =       1   n            ∂   φ       ∂   n              Δ      n     2                              
     where n 1  is the index of the TIR optic or waveguide and n 2  is the index of the optical transcription material. For the two cases where the electric field is either perpendicular or parallel to the plane of incidence            ∂     φ   ⊥         ∂   n       =       -   sin                       φ   ⊥          (     n         sin   2                   θ     -     n   2         )                     ∂     φ   ∥         ∂   n       =       -   sin                       φ   ∥          (       n         sin   2                   θ     -     n   2         +     2   n       )                                
     For a typical TIR optical interconnect θ=55°, n 1 =1.8 and n 2 =1.3 the phase shifts are approximately 
     
       
         ∂φ ⊥ ≈4.4Δn 2   
       
     
     
       
         ∂φ | ≈7.6Δn 2   
       
     
     for parallel and perpendicular electric fields respectively. The probe cumulative phase change through the TIR optic would add in a root mean squared manner (100 bounces will induce 10 times the phase shift) unless the signal and probe beams are optically phase locked. 
     The TIR based interconnect approach would employ a time compensated reading methodology which provides a means for correcting optical delays which occur in a serial reading beam architecture due to the time required for the probe beam to propagate. The probe optical beam has a propagation time of nd/c, where n is the material index of refraction, d is the distance and c is the speed of light. For a large array this propagation delay can seriously degrade the receiver temporal response. For example, a thousand element fiber array of 100 μm diameter fibers would take 600 ps to read. With a time compensated architecture the arrival of the signal beam at the transcription site is made to be synchronous (either on a row by row or element by element basis) with the arrival of the reading (probe) beam so that propagation delays do not accumulate. This is accomplished by inserting an equivalent (compensating) optical delay in a prescribed manner into the signal beam. 
     FIG. 8 shows a one dimensional time compensation system for a serial reading device, generally illustrated by the numeral  200 . Many of the parts of the system  200  are identical in construction to like parts in the apparatus illustrated in FIGS. 5 and 6 described above, and accordingly, there have been applied to each part of the system in FIG. 8 a reference numeral corresponding to the reference numeral that was applied to the like part of the apparatus described above and shown in FIGS. 5 and 6. The collected optical signal  64  arrives at a time compensation element (TCE)  202 . The TCE  202  comprises an optical wedge of the kind that is commercially available from several optical device manufacturers. The optical wedge is configured as an m x i matrix. As the TCE  202  lies in front of the optical fiber array  110 . The optical signal at time t s =t os , transmits through the TCE  202  and propagates to the optoelectronic detector (not shown) at the end of the fiber at time 
     
       
         
           t 
           s 
           =t 
           os 
           +t 
           cm 
           +n 
           f 
           d 
           fmi 
           /c 
         
       
     
     where t cm  is the time delay in row m of the TCE  202 , n f  is the index of refraction of the optical fiber  108 , d fmi  is the length of the optical fiber in row m and column i, and c is the speed of light. 
     If the fibers  108  are the same length and no time compensation element is included, then the probe beam  74  and the signal beam  120  accumulate a temporal error equal to n 1 d 11mi /c which degrades the array&#39;s temporal resolution. The time compensation optical element  202  with t cm =n 1 d m1mi  serves to synchronize the probe beam  74  and the optical signal beam  120  at a point in each row of the array so that the temporal degradation is reduced. 
     Thus, each row is compensated. More particularly, the center element in each row is synchronized with the probe beam  74 . Note the other elements in each row are not synchronized. Thus, this technique has some temporal resolution error associated with it, which would be analogous to time jitter. For a square fiber bundle  110 , the one dimensional time compensation scheme allows the number of fibers to be squared (i.e., 10 fibers becomes 100 fibers) with the same temporal resolution (frequency bandwidth). 
     FIG. 9 shows an element by element time compensation architecture. The optical signal beam  64  arrives at the fiber array  110  at time t s =t os  and propagates to the end of the fiber at time 
     
       
         
           t 
           s 
           =t 
           os 
           +n 
           f 
           d 
           fm 
           /c 
         
       
     
     where n f  is the index of refraction of the fiber  108 , d fm  is the length of fiber m, and c is the speed of light. 
     If the fibers  108  are the same length and no time compensation element is included, then the probe beam  74  and signal beam  64  accumulate a temporal error equal to n 1 d 1m /c which degrades the array&#39;s temporal resolution. As, however, in accordance with this invention the fiber lengths are tailored such that n f (d fm −d f1 )=n 1 d 1m  the probe and signal beam are synchronized at each point in the array. Thus, this embodiment does not degrade the optical reading of the probe beam. 
     There are a number of techniques which can be utilized to increase the operational bandwidth even further. As illustrated in FIG. 10, a dynamic spectral multiplexing configuration is shown. Wavelength multiplexing akin to that used in fiber optic systems can also be applied to increase this receiver&#39;s operational bandwidth. A spectrally selective optical element  240  (Bulk or fiber Bragg grating, acousto-optic or electro-optic deflector, prism, interference filter) is utilized to route four different wavelengths, λ 1 , λ 2 , λ 3 , and λ 4  to different optical interconnects  250 . The receiver bandwidth therefore increases linearly with the number of signal wavelengths. This technique, however, does not take full advantage of the optical transcription material. Instead, dynamic spectral multiplexing/demultiplexing in which a broad bandwidth probe beam  210  (i.e., femtosecond or picosecond pulse, frequency swept (chirped) pulse, broadband continuous wave) monitors the transient response of an OTM  220  across a number of wavelength channels is used to decode a wavelength multiplexed signal beam  230 . FIG. 11 illustrates the graph of a wavelength multiplexed probe beam  210 , with the abscissa being in wavelengths and the ordinate showing the amplitude of the several probes. The abcissa is shown for probes having a wavelength from 1.0 μm to 1.5 μm. FIG. 12 illustrates an alternative probe showing a three-dimensional configuration of the broadband probe beam  210  showing amplitude versus wavelength over time. This technique is based on the phenomena of transient spectral hole burning, where each component of the multi-wavelength signal beam induces an index of refraction change at a specific probe wavelength. Persistent spectral hole burning, which exists at low temperatures, has been utilized for high density optical storage media. Here, the intrinsic relaxation time of the media from an inhomogeneous to a homogeneous state is utilized to accomplish real time signal processing. Physically, the dynamic spectrally multiplexed OTM  220  could be one photoactive medium, or alternatively, a composite photoactive device with a series of carefully controlled layers with each layer&#39;s optical properties tuned by chemical structure. Dynamic spectral multiplexing would reduce the complexity of the system by requiring only one optical interconnect with the probe decomposed into its constituent wavelength channels afterwards by a series of fiber Bragg gratings  240  or other spectrally selective components shows the beam spectrum  80  after passage through the OTM  220  with the information displayed for 3 wavelength channels. 
     In particular, a spectrally broadband probe beam  210  interrogates the wavelength channels of the optical transcription material  220 . These wavelength channels are formed through the dynamic spectral hole burning mechanism. The information encoded on the broadband probe is decomposed into its constituent wavelengths with a spectrally selective element  240  (or elements), such as a series of fiber Bragg gratings, and sent to signal interconnect  250  and then to a signal processor  92 . Alternatively, the optical information may be directly distributed through an optical fiber network. 
     There are also a number of methods to increase the signal to noise ratio for the optical receivers. The signal beam can be optically amplified in a solid state gain media or the fibers themselves could serve as the optical amplifiers. With reference to FIGS. 14 and 15, an optical architecture  300  for subtraction and with polarization multiplexing is illustrated. As shown and also referring to FIG. 3, a polarized optical source beam  64  is generated by the optical transmitter  30  via the reference arm  46  at the satellite  22 . Hence, the optical source beam  64  produces two orthogonally polarized optical signals, both at the same wavelength with one signal beam  302  containing the information and the other reference beam  304  containing the reference. The receiver  32  includes a half wavelength waveplate to adjust the orthogonality of the polarization. These optical interconnect constructs are also amenable to differential signal transmission methods utilizing a reference beam  304  and a polarization multiplexed signal beam  304  (see FIG.  14  and  15 ). The reference could be separated from the signal with a λ/2 waveplate (either a static or dynamic element). When adjusted properly, no mixing of the polarized signal beams occurs. This aligns the receiver polarization to the transmitter and polarizer and is sent to an OTM detector  306 . A polarizer serves to split the received optical source beam into its signal component  305  and its reference component  307 . The signal beam component is supplied to the individual fibers  84  and in turn to the subtraction device  320 . The reference beam component  307  is supplied to a reference optical fiber  332  and in turn to a reference port of the subtraction device  320 . This configuration allows receiving a signal and a reference beam simultaneously. Both beams contain the same aberrations and distortions since they propagate along the same optical path. This configuration serves to remove unmodulated background and enables modulation schemes which can increase the receiver bandwidth. The subtraction device  320  serves to reconstruct the modulated signal. The subtraction device  320  is configured to provide optical or electrical subtraction. 
     An optical subtraction method  300  is illustrated in FIG.  14 . This method utilizes polarization sensitive techniques including transient grating, dichroism, anisotropy, parametric generation, and/or the optical Kerr effect. These techniques could also be employed to optically eliminate the unmodulated background which remains in quadrature phase modulation schemes. The polarization multiplexed signal beam  302  and the reference beam  304  are sent into the OTM  306  with a probe configuration which monitors differences in the orthogonal signal and reference beams. The probe configuration comprises a probe laser  72  that delivers a probe beam  74  via an optical fiber  76 , a collimating lens  78 , and a λ/2 waveplate  322  and a polarizer  324  to provide a probe beam that is oriented at 45° to the reference and signal beams. These optical subtraction techniques offer advantages in terms of operational bandwidth and signal to noise levels from electronic subtraction methods. 
     In operation, a reference beam  304  and signal beam  302  which have orthogonal polarizations (see FIG. 15) can be optically subtracted by using polarization based optical techniques. The signal beams  302 , as shown in FIG. 15B, and the reference beam  304 , as shown in FIG. 15C, have the same fluctuating distortions since they propagate along the same optical path. Illustrated in FIG. 14 is one representative example of optical dephasing which includes the optical Kerr effect and also dichroism where the probe polarization is oriented 45 degrees from both the signal and the reference beams. This configuration monitors differences in the two beams by providing a signal path via focusing lens  330  and a signal fiber  332  and a reference path through focusing lens  82  and a reference fiber  84  to the subtraction device  320 . This is shown in FIG.  15 D and eliminates the unmodulated background which remains in quadrature phase modulated transmission schemes. This optical subtraction method has advantages over electrical techniques in terms of operations bandwidth and signal to noise levels. 
     Alternatively, the polarizer and reference path can be eliminated when an anisotropic OTM is oriented to perform the subtraction function. 
     Optical measurements based on anisotropic OTM signal induced properties, such as optical dephasing including optical Kerr effect and dichroism, could also be utilized to increase operational bandwidths even further for linear optical techniques. In these instances, the relaxation time of the OTM would not be limited by excited state lifetimes (typically 1-100 ps). Instead optical dephasing rates which are one to two orders of magnitude faster (10-100 fs) produce ultrafast OTM response times (&gt;10 THz bandwidth). 
     The receiver bandwidth would therefore increase linearly with the number of signal wavelengths. Fiber based wavelength multiplexing for example has expanded bandwidths by factors of &gt;100. 
     Amplitude modulation of the reference beam could be utilized along with the temporal or phase modulation of the signal beam to encode information. For example, with a signal to noise level of &gt;64:1, 6 bits of information could be encoded instead of 1 bit per pulse allowing an increase in the total bandwidth by a factor of 6. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practices otherwise than as specifically described above.