Abstract:
An optical interconnect for use with a probe beam and optical signals is disclosed. The interconnect comprises an optical waveguide for propagating the probe beam, an optical transcription material that changes a characteristic of the probe beam at locations where the optical signals interact with the probe beam. A signal processor develops an output signal from the changed characteristic representative of the information contained in the optical signals. The optical signals may be amplitude or phase modulated or polarized. The interconnect can be configured to add and subtract the optical signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is related to co-pending U.S. patent application Ser. No. 09,586,014, filed Jun. 2, 2000, 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. 
     The present application is also related to co-pending U.S. patent application Ser. No. 09/587,124, filed Jun. 2, 2000, 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. 
     The present application is also related to co-pending U.S. patent application Ser. No. 09/586,513, filed Jun. 2, 2000, entitled: “High Bandwidth Large Area Optical Communication Receivers by Stephen Palese, assigned to the same assignee as this present invention, and its contents are incorporated herein by reference. 
     The present application is also related to co-pending U.S. patent application Ser. No. 09/587,126, filed Jun. 2, 2000, 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. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to optical devices, and more particularly to an optical interconnect that can be configured to add and subtract optical signals. 
     2. Description of the Art 
     The telecommunications industry is rapidly switching from electronic systems to hybrid platforms which utilize both electronics and photonics to increase the operational bandwidth. Today&#39;s electronic communication systems consist of electrical networks, microwave amplifiers, microwave transmitters, and high speed semiconductor receivers. There are numerous electrical devices available so this architecture works well in the confines of electronics. There are problems in moving to hybrid platforms, however, because few electro-optical devices are available to convert electrical signals into optical signals. Moreover, eventually as optical systems come into use, purely optical signal processing devices will be required. 
     What is needed, therefore, are more optical interconnects especially one that is capable of adding and subtracting optical signals directly, without conversion between electronic and optical architectures. 
     SUMMARY OF THE INVENTION 
     The present invention involves an optical interconnect that utilizes an optical transcription material, while maintaining a fast temporal response, and thus a high bandwidth. This device is applicable for phase modulated, amplitude modulated and polarized optical signals and responds to a separate signal beam and a probe beam. 
     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 arithmetic 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 polarization rotation, amplitude modulation or phase modulation of the probe beam. The optical interconnect speed is limited by the intrinsic response time of the OTM. 
     Briefly, the present invention comprises an optical interconnect, that can be configured for adding or subtracting two or more optical signals. The optical signals contain information which is amplitude, polarization or phase encoded. A probe laser generates an optical probe beam. The optical device includes an OTM that responds to the optical signals and the probe beam such that a characteristic of the probe beam is changed. A signal processor or detector senses the changed characteristic and develops an output optical signal representative of the summed or subtracted information contained in the input optical signals. 
     In other aspects, the present invention subtracts polarization multiplexed signal and reference beams and employs a time compensation architecture. 
     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 
     FIG. 1 is a diagram illustrating a waveguide optical interconnect in accordance with the present invention. 
     FIG. 2 is a side view diagrammatically illustrating the operation of the waveguide optical interconnect shown in FIG.  1 . 
     FIG. 3 is a diagram illustrating a configuration for providing time compensation for use with a serial reading technique in accordance with the present invention. 
     FIG. 4 is a diagram of another embodiment for providing two dimensional time compensation in accordance with the present invention. 
     FIG. 5 is a diagram illustrating another embodiment of the optical interconnect configured for adding and subtracting optical signals in accordance with the present invention. 
     FIGS. 6A and B are graphs illustrating the absorption characteristics of the optical transcription material. 
     FIG. 7 diagrammatically illustrates yet another alternative embodiment of the waveguide optical interconnect shown in FIG.  1 . 
     FIG. 8 is a graph illustrating the absorption/gain characteristic of the optical transcription material used in FIG.  7 . 
     FIG. 9 is a diagram illustrating yet another embodiment of the present invention with a phase locked loop configuration. 
     FIGS. 10A, B, and C shows the waveforms of the phase locked probe electric fields associated with the optical addition/subtraction technique illustrated in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the diagrams of FIG.  1  and FIG. 2, the present invention provides an optical device or interconnect, generally designated by the numeral  10 . Briefly, the optical interconnect  10  comprises a waveguide optical interconnect  12  that is characterized by generally total internal reflection (TIR). More particularly, the optical waveguide  12 , also referred to as a TIR device, includes a layer  14  of an optical transcription material (OTM) over a top surface  16 . The thickness of the waveguide is selected such that certain modes of a probe beam hit the interaction areas  18  of the OTM  14  when a probe beam  20  is applied to an input port and propagated through the waveguide. Optical fibers  22  configured in an array  23  propagate signal beams  24  to the interaction areas  18 . The optical fibers  22  extend from the signal fiber array  23  having m fibers. A signal processor  33  serves to process, manipulate, display, and store the output signals developed by the optical interconnect. A suitable optical storage device could be provided by Templex Incorporated. 
     A probe laser  30  transmits the probe beam  20  through an optical fiber to the input port of the optical waveguide  12 . 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 beams  24 . 
     The OTM element  14  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. It serves to impart a phase shift or amplitude modulation to the probe upon photo-excitation of the OTM. 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. 
     At the OTM  14  the signal beams  24  interact with the near diffraction limited probe beam  20  through a linear or nonlinear optical process and impresses a 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 modulation of the probe beam. 
     More particularly, and with reference to FIG. 2, the evanescent wave  40  of the probe beam  20  penetrates into the OTM at the interaction areas  18  and introduces either an amplitude or phase change on the probe in response to the index of refraction change, Δn, induced by the signal beam. 
     This optical interconnect is based on amplitude or phase variations which can be induced at TIR interaction surfaces  18 . The exponentially damped evanescent wave  40  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  14  must be physically close to the TIR surface  16 , respectively, such that the probe evanescent field penetrates into the interaction region  18  which is optically excited by the signal beams  24 . This TIR surface can reside in a bulk optic or an optical waveguide. 
     The critical angle , θc, 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  12  and the OTM  14 , respectively. 
     The fiber optic array  23  is constructed such that each of the individual fibers  22  transmit the signal onto one site of the optical interconnect device which is coated with the OTM  14 . The index of refraction variations of the optical transcription are material induced by the signal (pump) beam  24 , 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 the 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          Δ                 φ                =       1     n   1                         ∂   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⊥≈4.4Δ n   2   
       
     
     
       
         ∂φ≈7.6Δ n   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 ordinarily employs 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. 3 shows a one dimensional time compensation system for a serial reading device, generally illustrated by the numeral  50 . Many of the parts of the system  50  are identical in construction to like parts in the apparatus illustrated in FIGS. 1 and 2 described above, and accordingly, there have been applied to each part of the system in FIG. 3 a reference numeral corresponding to the reference numeral that was applied to the like part of the apparatus described above and shown in FIGS. 1 and 2. A collected optical signal arrives at a time compensation element (TCE)  52 . The TCE  52  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  52  lies in front of the optical fiber array  23 . The optical signal at time t s=t   os , transmits through the TCE  52  and propagates to the interaction area  18  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  52 , n f  is the index of refraction of the optical fiber  22 , 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  22  are the same length and no time compensation element is included, then the probe beam  20  and the signal beam  24  accumulate a temporal error equal to n 1 d 11mi /c which degrades the array&#39;s temporal resolution. The time compensation optical element  52  with t cm =n 1 d m1mi  serves to synchronize the probe beam  20  and the optical signal beam  24  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  20 . 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  23 , 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. 4 shows an element by element time compensation architecture. The optical signal beam arrives at the fiber array  23  at time t s =t os  and propagates to the interaction area  18  at 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  22 , d fm  is the length of fiber m, and c is the speed of light. 
     If the fibers  22  are the same length and no time compensation element is included, then the probe beam  20  and signal beam  24  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. 
     Alternatively, active or nonsynchronous passive time compensation architectures can be implemented to provide additional functional capability (for example, dynamically controlled probe sequences) or environmental corrections. An optical interferometer or polarizer may be utilized to convert the phase or polarization rotation modulated signal into an amplitude modulation. The time compensated methodology may be used in all the embodiments of optical interconnects when it is desired to correct time delays. 
     Referring now to FIG. 5, an alternative embodiment of the optical device, generally designated by the numeral  70 , is shown. The device  70  is configured to perform addition and subtraction logic functions, as will be subsequently described. 
     Many of the parts of the optical device  70  are identical in construction to like parts of the device  10  illustrated in FIGS. 1 and 2. Accordingly, there has been applied to each part of the device  70  a reference numeral corresponding to the reference numeral that was applied to a like part of the device described above. The fundamental difference is that a layer of subtractive OTM  15  covers the lower surface  17  of the optical waveguide  12 . 
     The OTM element  15  is similar to the OTM  14  except that it imparts a negative phase shift to the probe upon photo-excitation of the OTM. The OTM absorption characteristics are illustrated in FIG. 6A which shows phase versus wavelength for OTM  14  as +γ and for OTM  15  as −γ. FIG. 6B shows absorption versus wavelength for the subtraction OTM  15  and for the addition OTM  14 . Note that phase is generally considered as the Kramer&#39;s Konig relation of the absorption versus wavelength graph. As shown, at resonance, illustrated by the dashed lines  75 , there is no phase shift. Thus, the wavelength of the probe beam  20  is selected such that the negative OTM  15  provides a negative phase change that is equal to the positive phase change provided by the positive OTM  14 . This occurs when the probe wavelength lies in between the two OTM absorption profiles, shown by the numeral  76 . It is know that the absorption wavelengths can be controlled by structure (for example, in quantum well devices, nanocrystals), chemical composition (for example, changing side chemical groups in conjugated polymers, relative elemental composition), or externally applied electric or magnetic fields. The arithmetic sum of the interconnect is measured by looking at the relative phase shift or polarization rotation of the probe. 
     Also, each fiber path includes an electro-optic modulator  25  and a polarizer  26 . The modulators  25  rotate polarized signal beams. The polarizers  26  serve to orthogonally polarize the signal beams so as to provide polarized signals with one containing the additive information (shown as A l  . . . A m )and the other containing the subtractive information (shown S l  . . . S m ). As illustrated, the optical fibers  22  carrying the optical signals  24  are selectively propagated to the additive OTM  14  at the interaction area  18  and to the subtractive OTM  15  at its corresponding interaction areas. 
     In the event that amplitude modulation of the eprobe beam is required for further signal processing an optical interferometer can be utilized for the conversion. For this a reference beam from the probe laser  30  must be applied to the signal processor  33 . Hence, an optical path  31  (see FIG. 5) is supplied to propagate the reference beam. Note that this is not needed if the probe can carry phase encoded information. 
     The passive time compensation architectures shown in FIGS. 3 and 4 need to be incorporated into the signal fiber array, if it is desired to synchronize the probe and pump at each point in the interconnect  70 . Also, active or nonsynchronous passive time compensation architectures can be implemented to provide additional functional capability (for example dynamically controlled probe sequences) or environmental corrections. 
     Referring now to FIGS. 7 and 8, another embodiment of the optical interconnect  80  is shown. Many of the elements of the interconnect  80  are identical to like parts of the interconnect  70  illustrated in FIGS. 5 and 6 and like reference numerals are applied to the elements. The fundamental difference is that in this embodiment, the OTM  14  is characterized as having additional absorption and gain of the probe upon photo-excitation. This is designated by the numeral  14  on FIG. 8, where the maximum change in absorption, +β, occurs at the operating probe wavelength  82 . The subtractive OTM  15  has a characteristic that exhibits negative absorption and loss that equals −β at the probe wavelength upon excitation and is shown as the numeral  15  in FIG.  8 . The combined signal will be the additive of magnitude of the optical signals  24 . In other words, the arithmetic sum of the interconnect  80  is measured by looking at the relative amplitude of the probe  20 . 
     With reference to FIGS. 9 and 10, an optical architecture  100  for a phase-locked optical interconnect is illustrated. As shown, optical addition and subtraction of signals is accomplished by utilizing a phase-locked signal and probe laser  102 . Phase-locking necessitates that the relative phases of the signals and probe electric fields are constant over the reading time of the interconnect (see FIG.  10 ). FIG. 10 shows the electric fields over time. FIG. 10A shows two separate unsynchronized electric field pulses. With reference to FIG. 7B, addition is accomplished if the signal and probe electric fields are in-phase with each other, which produces constructive interference between the two beams. This is achieved by adding a time delay configuration such that the two pulses overlap, which results in a single pulse having a greater amplitude. Subtraction is accomplished if the signal and probe electric fields are out-of-phase with each other, which produces destructive interference between the two equal amplitude beams. This is shown in FIG.  10 C. The relative phases of the probe and each individual signal electric field can be controlled dynamically by using either an array of phase modulators  25  or active time compensation architectures. In this instance, the arithmetic function can be controlled in real time. 
     For applications that simply require a predetermined operation between specified signal fibers, passive time compensation architectures can be implemented to provide the required phase delay. Partially phased beams are also possible and may be of interest for certain signal processing applications (for example, data encryption), in these instances the relative phase between the probe and signal electric fields may lie between in-phase and out-of-phase conditions. 
     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.