Abstract:
Described is an all-optical switch that is significantly insensitive to polarization instabilities. The optical switch can be configured as an ultrafast logic gate, a switch for ultrafast communication systems or a key component of an all-optical regenerator. Performance is independent of the statistical characteristics of the data controlling the switch. The switch includes a birefringent optical channel in communication with one end of a nonlinear optical channel through a coupler and a polarization rotation mirror in communication with the other end of the nonlinear channel. An optical data pulse for controlling the switching function is provided to one port of the coupler.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to provisional U.S. patent application serial No. 60/378,746, filed May 8, 2002, titled “Polarization Stabilized All-Optical Switch,” the entirety of which provisional application is incorporated by reference herein. 
     
    
     GOVERNMENT RIGHTS IN THE INVENTION  
       [0002] This invention was made with United States government support under Contract No. F19628-00-C-002 awarded by the Defense Advanced Research Project Agency (DARPA). The government may have certain rights in the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The invention relates generally to optical switches. More specifically, the invention relates to devices and methods for regeneration and switching in high data rate optical communication systems.  
         BACKGROUND  
         [0004]    Typical optical networks are based on high-speed electronic routers interconnected by optical fiber links utilizing wavelength division multiplexing (WDM) transmission systems. To achieve packet routing, the optical signal entering the router is converted to an electrical signal (O/E conversion) and demultiplexed into lower-rate data streams. The data streams are electronically routed and multiplexed in a high-speed electrical signal that is generated by the router for a specific optical wavelength.  
           [0005]    As the demand for bandwidth in optical networks continues to increase, the router must be able to perform its switching at higher rates. The electronic signal bandwidth limitations incurred during the O/E conversion and the conversion back to an optical signal (E/O conversion), however, often result in router congestion and reduced data throughput. In addition, the low energy optical pulses received at the router after propagation over long distances can be degraded, for example, by chromatic dispersion and nonlinear effects in the transmission fiber. The optical pulses can also exhibit timing variations (i.e., jitter) within a bit interval.  
           [0006]    Conventional electronic signal regenerators provide pulse amplification, pulse shaping and timing jitter correction of an electrical signal. For WDM networks, an electronic signal regenerator is required for each wavelength channel. After the O/E conversion, the single wavelength channel electronic data is demultiplexed into lower-rate data streams and each stream is processed electronically. The lower-rate data streams are multiplexed into a single data stream that is used to modulate the desired wavelength optical carrier. The use of electronic regenerators for re-amplification, re-shaping and re-timing in high-speed optical networks is often prohibited by cost, complexity, and power in many-channel WDM networks and sometimes is limited by processing speed in ultrafast optical time domain multiplexing (OTDM) networks.  
           [0007]    All-optical switches eliminate the need for the O/E conversion and E/O conversion. Consequently, all-optical switches generally support higher data rates than electronic switches. Optical switches utilizing nonlinearities in optical fibers are subject to polarization instability due to temperature variations and acoustic disturbances. Semiconductor materials are used as the nonlinear media in some optical switches, however, the operational speed of these switches is typically limited by multiple physical processes. For example, long-lasting refractive index nonlinearities cause performance degradations that are dependent on the data statistics and the data rate.  
         SUMMARY OF THE INVENTION  
         [0008]    In brief overview, the present invention relates to an all-optical switch that is significantly insensitive to polarization instabilities in the switch and, consequently, does not require active polarization control. The optical switch can be configured as an ultrafast logic gate, a switch for ultrafast communication systems or a key component of an all-optical regenerator. Performance is independent of the statistical characteristics of the data controlling the switch.  
           [0009]    In one aspect, the invention features an optical switch having a birefringent optical channel, a coupler, a nonlinear optical channel and a polarization rotation mirror. The birefringent optical channel has a first port and a second port, and provides a pair of orthogonally polarized optical pulses at the second port in response to a first optical pulse received at the first port. The orthogonally polarized optical pulses are separated by a delay time. In one embodiment, the birefringent optical channel is a polarization maintaining optical fiber. In another embodiment, the birefringent optical channel is a birefringent crystal. The coupler has a first port in optical communication with the second port of the birefringent optical channel, a second port adapted to receive a second optical pulse, and a third port. In one embodiment, the optical switch includes a control optical channel in optical communication with the second port of the coupler. The nonlinear optical channel has a first port in optical communication with the third port of the coupler. In one embodiment, the nonlinear optical channel is a dispersion shifted optical fiber. In another embodiment, the nonlinear optical channel is an optical semiconductor. The polarization rotation mirror is in optical communication with the second port of the nonlinear channel.  
           [0010]    In another aspect, the invention features a method for regenerating an optical data pulse. The method includes transmitting the optical data pulse and a pair of orthogonally polarized optical pulses through a nonlinear optical channel in a forward direction. The optical data pulse and the orthogonally polarized optical pulses each have a forward polarization orientation with respect to the nonlinear optical channel. The orthogonally polarized optical pulses are separated in time, and the optical data pulse and one of the orthogonally polarized optical pulses are substantially temporally coincident. The method also includes transmitting the optical data pulse and the orthogonally polarized optical pulses through the nonlinear optical channel in a reverse direction wherein the optical data pulse and the orthogonally polarized optical pulses each have a reverse polarization orientation with respect to the nonlinear optical channel. Each of the reverse polarization orientations is orthogonal to a respective forward polarization orientation. An optical phase delay is imparted to one of the orthogonally polarized optical pulses relative to the other orthogonally polarized optical pulse in response to the coincident transmission of the optical data pulse and one of the orthogonally polarized optical pulses through the nonlinear optical channel. The method also includes the step of delaying one of the orthogonally polarized optical pulses relative to the other orthogonally polarized optical pulse to generate a temporal coincidence.  
           [0011]    In another aspect, the invention features a method for performing a logical operation of a first optical data bit and a second optical data bit. The first and second optical data bits are separated by a predetermined time. The method includes transmitting the first and second optical data bits, a first optical clock bit and a second optical clock bit through a nonlinear optical channel in a forward direction. The first and second optical data bits and the first and second optical clock bits each have a forward polarization with respect to the nonlinear optical channel. In addition, the first and second optical clock bits are orthogonally polarized and separated in time. The first optical data bit is substantially temporally coincident with the first optical clock bit and the second optical data bit is substantially temporally coincident with the second optical data bit. The method also includes transmitting the optical data bits and the optical clock bits through the nonlinear optical channel in a reverse direction. The first and second optical data bits and the first and second optical clock bits each have a reverse polarization orientation with respect to the nonlinear optical channel. Each of the reverse polarization orientations is orthogonal to a respective forward polarization orientation. An optical phase delay is imparted to the first optical clock bit if the first optical data bit is in an asserted state and an optical phase delay is imparted to the second optical clock bit if the second optical data bit is in an asserted state. The method also includes delaying the first optical clock bit relative to the second optical clock bit to generate a temporal coincidence between the optical clock bits.  
           [0012]    In another aspect, the invention features a method for performing a logical operation on a first optical data bit and a second optical data bit. The method includes generating a pair of orthogonally polarized optical bits separated in time in response to the first optical data bit. One of the orthogonally polarized optical bits is substantially temporally coincident with the second optical data bit. The method also includes transmitting the orthogonally polarized optical bits and the second optical data bit through a nonlinear optical channel in a forward direction wherein the orthogonally polarized optical bits and the second optical data bit each have a forward polarization with respect to the nonlinear optical channel. The orthogonally polarized optical bits and the second optical data bit are transmitted through the nonlinear optical channel in a reverse direction wherein the orthogonally polarized optical bits and the second optical data bit each have a reverse polarization orientation with respect to the nonlinear optical channel. Each of the reverse polarization orientations is orthogonal to its respective forward polarization orientation. An optical phase delay is imparted to the one of the orthogonally polarized optical bits in response to the coincident transmission of the one of the orthogonally polarized optical bits and the second optical bit. One of the orthogonally polarized optical bits is delayed relative to the other of the orthogonally polarized optical bits to generate a temporal coincidence between the orthogonally polarized bits.  
           [0013]    In another embodiment of the method for performing a logical operation on a first optical data bit and a second optical data bit, the method includes transmitting the first and second optical data bits, a first optical clock bit and a second optical clock bit through a nonlinear optical channel in a forward direction wherein the first and second optical data bits and the first and second optical clock bits each have a forward polarization with respect to the nonlinear optical channel. The first and second optical clock bits are orthogonally polarized and separated by a first time. In addition, the first optical data bit and the second optical data bit are substantially temporally coincident with the first optical clock bit. The method also includes transmitting the optical data bits and the optical clock bits through the nonlinear optical channel in a reverse direction wherein the first and second optical data bits and the first and second optical clock bits each have a reverse polarization orientation with respect to the nonlinear optical channel. Each of the reverse polarization orientations is orthogonal to a respective forward polarization orientation. An optical phase delay is imparted to the first optical clock bit if the first optical data bit is in an asserted state, the second optical data bit is in an asserted state or the first optical clock bit and the second optical clock bits are in an asserted state. The method includes delaying the first optical clock bit relative to the second optical clock bit to generate a temporal coincidence between the optical clock bits. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0015]    [0015]FIG. 1 is a diagram of an embodiment of an optical switch in accordance with the invention;  
         [0016]    [0016]FIG. 2 is a plot of the normalized output intensity as a function of delay between a data pulse and the local clock pulse for the optical switch of FIG. 1;  
         [0017]    [0017]FIG. 3 is a plot of the normalized dispersion in the nonlinear medium as a function of wavelength for multiple-wavelength data channels and multiple-wavelength local clock channels for the optical switch of FIG. 1;  
         [0018]    [0018]FIG. 4 is a diagram of another embodiment of an optical switch including a local optical clock source and a clock recovery circuit in accordance with the invention;  
         [0019]    [0019]FIG. 5 is a flowchart representation of an embodiment of a method for regenerating an optical data pulse in accordance with the invention;  
         [0020]    [0020]FIG. 6 is a flowchart representation of an embodiment of a method for performing a logical operation on a first optical data bit and a second optical data bit in accordance with the invention;  
         [0021]    [0021]FIG. 7 is a diagram of another embodiment of an optical switch in accordance with the invention;  
         [0022]    [0022]FIG. 8 is a flowchart representation of another embodiment of a method for performing a logical operation on a first optical data bit and a second optical data bit in accordance with the invention;  
         [0023]    [0023]FIG. 9 is a diagram of another embodiment of an optical switch in accordance with the invention;  
         [0024]    [0024]FIG. 10 is a flowchart representation of another embodiment of a method for performing a logical operation on a first optical data bit and a second optical data bit in accordance with the invention; and  
         [0025]    [0025]FIG. 11 is a diagram of another embodiment of an optical switch in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIG. 1 illustrates an all-optical switch  10  constructed in accordance with the present invention. The switch  10  is configured as a folded ultrafast nonlinear interferometer (FUNI). The switch  10  is a component of an all-optical 3R regenerator that re-amplifies, re-shapes and re-times optical data pulses. Reshaping removes distortion in an optical data pulse caused, for example, by chromatic dispersion and nonlinear properties of the transmission path. Re-timing removes any timing jitter (i.e., variation in pulse position within a bit interval) present in an optical data pulse stream. Effectively, the regenerator recovers the clock of the data pulse stream and generates a stream of local clock pulses to match the data pulse clock. The all-optical switch  10  outputs a local transform-limited optical clock pulse if an optical data pulse is present in the corresponding bit interval of the data pulse stream. Conversely, no local optical clock pulse is output if the optical data pulse is absent from the bit interval. In effect, the switch  10  operates logically as an AND gate with the local optical clock pulses provided as a first input and the optical data pulses provided as a second input.  
         [0027]    The switch  10  includes a birefringent optical channel  14  and a control optical channel  18  in optical communication with ports  22  and  26 , respectively, of an optical coupler  30 . In the illustrated embodiment, the birefringent optical channel  14  is a birefringent optical fiber (BRF)  14  such as a polarization maintaining optical fiber (PMF). The birefringent axes of the BRF  14  are oriented to the polarization of a local optical pulse  34  such that a corresponding pair of orthogonally polarized (OP) optical pulses  38 ,  42  separated by a delay time t, exits the BRF  14 . In another embodiment (not shown), a highly birefringent crystal is used in place of the BRF  14 . The birefringent axes of the crystal are similarly oriented with respect to the polarization of the local optical pulse  34  to provide the OP pulses  38 ,  42 . Use of the birefringent crystal eliminates the need for the longer length of BRF  14  and, therefore, results in a smaller switch package. The control optical channel  18  is a single mode optical fiber coupled to, or part of, an optical communications link. Optical data pulses  46  received at the switch input port  48  are transmitted from the control optical channel  18  to the optical coupler  30 . In one embodiment the optical coupler  30  is a wavelength division multiplexer (WDM) that achieves efficient coupling of different wavelength local and network optical signals received at coupler ports  22  and  26  into a combined optical signal at coupler port  50 . In another embodiment the optical coupler  30  is a beamsplitter-type combiner (e.g., 3 dB combiner).  
         [0028]    The switch  10  also includes a polarization rotation mirror  54  in optical communication with the coupler port  50  of the optical coupler  30  through a nonlinear optical channel  58 . In the illustrated embodiment, the nonlinear optical channel  58  is a dispersion shifted optical fiber (DSF). As described below, the zero dispersion wavelength of the DSF  58  is selected to be approximately the average value of the wavelength of the local optical pulses  34  and the optical data pulses  46 . In an alternative embodiment (not shown) the nonlinear optical channel  58  is an optical semiconductor (e.g., semiconductor optical amplifier Alcatel model no. A1901SOA or OptoSpeed model no. SOA1550MRI/X-250). The polarization rotation mirror  54  is a Faraday mirror (e.g., E-Tek model no. HSFM20A0S4010) or other optical component, or combination of components, that yields a reflected optical pulse having a polarization that is orthogonal to the polarization of the corresponding incident optical pulse. If the optical coupler  30  is not wavelength discriminating (i.e. a WDM coupler), the switch  10  includes a bandpass filter  62  (e.g., DiCon model no. TF-1565-0.8-FC-3.0-1 or JDS model no. TB1570) disposed between the BRF  14  and the data pulse port  22  of the optical coupler  30 . The switch also includes a polarization dependent beamsplitter  66  (e.g., example JDS model no. PMCB2P0781000-001) in optical communication with the BRF  14 , and a three port optical circulator  70  (e.g., JDS model no. CIRM23A231000-1) in optical communication with the beamsplitter  66  and switch output port  78 .  
         [0029]    In operation, the switch  10  receives the stream of local optical pulses  34  generated by a local optical clock source (not shown). The local pulses  34  are routed through the circulator  70  to the polarization dependent beamsplitter  66 . Preferably the local clock pulse  34  polarization is aligned for maximum transmission through the beamsplitter  66  to the BRF  14 . The birefringent axes of the BRF  14  are aligned to the local optical pulses  34  so that the differential propagation for orthogonally polarized components of each local pulse  34  results in a pair of orthogonally polarized (OP), equal-amplitude pulses  38 ,  42  exiting the birefringent channel  14 . For example, if the local pulses  34  are linearly polarized, the birefringent axes of the BRF  14  are aligned at approximately 45° to the plane of polarization.  
         [0030]    The exiting OP pulses  38 ,  42  are separated by a time delay t according to the product of the birefringence magnitude and the length of the BRF  14 . Delay time t determines the temporal width of the desired switching window for the switch  10 . The minimum allowable delay time t is determined by the width of the local optical pulses  34 . The switching window describes the ability of the switch  10  to accommodate timing jitter in the data pulses  46  as described below. OP pulses  38 ,  42  are transmitted through bandpass filter  62  to port  22  of the optical coupler  30 . Data pulses  46  are transmitted over the control channel  18  to the second port  26  of the optical coupler  30 . Both the OP pulses  38 ,  42  and the data pulses  46  are provided at the output port  50  of the optical coupler  30 .  
         [0031]    The polarization of the data pulses  46  is oriented to match the polarization of one of the OP pulses  42 . Because the data pulses  46  are at a different wavelength than the OP pulses  38 ,  42 , the data pulses  46  propagate through the DSF  58  at a different speed than the OP pulses  38 ,  42 . If a data pulse  46  is present during a bit interval, the data pulse  46  overlaps one of the OP pulses  38 ,  42  for a period of time during forward transmission to and reflected (i.e., reverse) transmission from the polarization rotation mirror  54 . As a result of nonlinear cross-phase modulation during the period of temporal overlap, the commonly polarized OP pulse  42  is shifted in optical phase relative to its complementary OP pulse  38 . Advantageously, any polarization instability due to thermal and acoustic effects that occur during forward propagation of the pulses  38 ,  42 ,  46  are reversed, or compensated, during reverse propagation because of the orthogonal polarization rotation induced by the polarization rotation mirror  54 . This compensation is achieved if the polarization instability does not vary over the round trip propagation time through the DSF  58 .  
         [0032]    The spectral bandpass of the bandpass filter  62  is selected so that the OP pulses  38 ,  42  are transmitted but the optical data pulses  46  are rejected. Although all three optical pulses  38 ,  42 ,  46  are transmitted back through the optical coupler  30  to the bandpass filter  62 , only the OP pulses  38 ,  42  are transmitted to the BRF  14 . In other embodiments other types of spectral filters are used in place of the bandpass filter  62 . For example, a short wavelength-pass spectral cutoff filter can be used if the wavelength of the OP pulses  38 ,  42  is less than the wavelength of the data pulses  46 . Conversely, a long wavelength-pass spectral filter can be used if the wavelength of the OP pulses  38 ,  42  is greater than the wavelength of the data pulses.  
         [0033]    The OP pulses  38 ,  42  continue in reverse propagation through the BRF  14 . Because the polarizations of the OP pulses  38 ,  42  during reverse propagation are orthogonal with respect to the original polarization during forward propagation, the delay time t induced during reverse propagation through the BRF  14  is opposite that induced on forward transmission. In other words, the slower OP pulse  38  during forward propagation through the BRF is the faster pulse during reverse propagation through the BRF  14 . Consequently, returned OP pulses  38 ,  42  at the polarization dependent beamsplitter  66  are temporally coincident. Effectively, a single pulse substantially identical to the original local optical pulse  34  with an additional nonlinear phase shift and rotated polarization propagates from the BRF  14  to the beamsplitter  66 .  
         [0034]    If the relative optical phase delay between the coincident OP pulses  38 ,  42  is approximately zero, each pair of coincident OP pulses  38 ,  42  combines at the polarization dependent beamsplitter  66  to generate a single complementary output pulse  82  having a linear polarization oriented for reflection out of the beamsplitter  66 . The complementary output pulse  82  is routed directly by the beamsplitter  66  to the switch complementary output port  86 . Conversely, if the optical phase delay is such that the coincident pulses  38 ,  42  are out of phase (i.e., 180° relative phase difference), the coincident OP pulses  38 ,  42  combine at the polarization dependent beamsplitter  66  to generate a single output pulse  74  having a linear polarization oriented for transmission through the beamsplitter  66 . The output pulse  74  is routed by the circulator  70  to the switch output port  78 .  
         [0035]    The FUNI configured optical switch  10  uses a DSF as its nonlinear medium rather than a BRF. If a BRF were used instead, its high birefringence would cause a single data pulse  46  to pass through multiple OP pulses  38 ,  42 . This “walk-through” results in a symmetric phase shift in the OP pulses  38 ,  42  that is dependent on the number of data pulses  46  encountered. Consequently, the switching contrast is significantly influenced by network data statistics because a differential phase shift between OP pulses  38 ,  42  is required for switching. The use of low-birefringence DSF for the nonlinear optical channel  58 , however, limits single data pulse  46  walk-through to a single OP pulse  42 . The wavelength separation can be large as long as the difference in propagation speeds is small. This goal is accomplished by establishing the two wavelengths symmetrically (approximately) about the dispersion zero. The difference in propagation speeds of the OP pulses  38 ,  42  and the data pulses  46  determines the degree of walk-through in the nonlinear optical channel  58 . A greater difference in propagation speeds permits the switch  10  to operate over a larger timing variation range between the data pulse  46  and its corresponding local pulse  42 . The performance of the switch  10  as a function of time variation between data pulses  46  and local pulses  42  is referred to as the “switching window”. A large switching window is required for a 3R regenerator to accommodate a large timing jitter range.  
         [0036]    [0036]FIG. 2 is a plot  116  of the normalized intensity of the switch output pulses  74  as a function of temporal delay between the data pulse  46  and the optical phase modulated OP pulses  38 ,  42  of the FUNI configured optical switch  10  of FIG. 1. The horizontal axis represents the delay between the data pulse  46  and one of the OP pulses  38 ,  42  and the vertical axis represents the normalized average intensity of the switch output pulse  74 . Each peak  120 ,  124  corresponds to the coincidence of the data pulse  46  and one of the corresponding OP pulses  38 ,  42 . The temporal width t W  of each peak  120 ,  124  is determined by the differential group velocity between the data pulse  46  and the OP pulses  38 ,  42  in the DSF  58 . The temporal separation t of the two peaks  120 ,  124  is determined by the temporal separation between the OP pulses  38 ,  42 .  
         [0037]    In one embodiment, the optical switch  10  of FIG. 1 is configured to operate in a WDM network in which a wavelength multiplexed data pulse stream is provided at input port  48 . Referring also to FIG. 3, the normalized dispersion of the nonlinear optical channel  58  is plotted for each data pulse stream, designated D 1 , D 2  and D 3  (generally D), and its respective local pulse stream, L 1 , L 2  and L 3  (generally L), respectively. Although only three data pulse streams D and three local pulse streams L are plotted for clarity, it should be recognized that the number of data streams D and pulse streams L can be significantly greater. The number of data streams D that can be accommodated by the switch  10  without significant performance degradation is ultimately limited as described below.  
         [0038]    The amount of pulse walk-through is determined according to the differential dispersion of the nonlinear optical channel  58  between the wavelengths of the pulse streams D and L. The D and L wavelength pairs are chosen to provide a small differential group delay such that a small (e.g., few picoseconds) temporal walk-through is achieved and the two pulse trains are tightly coupled. However, the propagation speeds of data pulse D 1  and local pulse L 1  are greater than the propagation speeds of data pulse D 3  and local pulse L 3 . Consequently, data pulse D 1  and local pulse L 1  walk-through multiple data pulses D 3  and local pulses L 3 . As the difference in wavelength range between the first and third D and L pulse streams increases, undesirable cross-phase modulation between distinct D and L pulse streams can impart a sufficient relative phase and the contrast of the output pulse stream  34  begins to degrade.  
         [0039]    [0039]FIG. 4 shows an embodiment of an all-optical switch  10 ′ constructed in accordance with the present invention. The switch  10 ′ includes the components of the optical switch  10  of FIG. 1. In addition, a clock recovery circuit  90  and an optical source  94  enable the switch  10 ′ to maintain, or recover, the clock for the optical data pulses  46 . The clock recovery circuit  90  has an input port  98  in optical communication with the output port  78  of the optical switch  10 ′. The optical source  94  includes a control terminal  102  in communication with an output terminal  106  of the clock recovery circuit  90  and a source output port  110  in optical communication with the optical circulator  70  through an optical time delay module  112 . In an alternative embodiment (not shown) the input port  98  of the clock recovery circuit  90  is in optical communication with the complementary output port  86  of the optical switch  10 ′. In another alternative embodiment (not shown) the input port  98  of the clock recovery circuit  90  is in optical communication with the control input  48  in order to recover the clock directly from the network data stream.  
         [0040]    If the clock recovery circuit  90  is omitted from the switch  10 ′, the local pulse stream  34  from the optical source  94  typically does not remain synchronized to the data pulse stream  46 . The clock recovery circuit  90  serves to “lock” the pulse rate of the local pulse stream  34  to the pulse rate of the data pulse stream  46 . In operation, the clock recovery circuit  90  provides a control signal at its output terminal  106  that is responsive to a timing phase shift between the local optical clock pulses  34  and the optical data pulses  46 . In one embodiment the clock recovery circuit  90  includes a phase lock loop (PLL) circuit (e.g., second-order dithering PLL) and a RF driver circuit, and the optical source  94  includes a mode-locked fiber laser (e.g., mode-locked soliton source). The PLL circuit modulates the RF source to generate a control signal at the output terminal  106  of the clock recovery circuit  90 . In response, the pulse rate of the mode-locked laser is changed to “track” the pulse rate of the optical data pulses  46 .  
         [0041]    The time delay module  112  delays the local optical pulses  34  in response to a delay signal generated by the clock recovery circuit  90 . This delay may be required if, for example, one or more optical path lengths in the switch  10 ′ change in time due to temperature variations in the optical components which can cause an undesirable timing shift between the local pulses and data pulses  34 ,  46 . The time delay module  112  can be, for example, a piezoelectric “stretcher” integrated with an optical fiber that couples the local optical pulses  34  generated by the optical source  94  to the circulator  70 . In an alternative configuration (not shown) the time delay module  112  is in electrical communication with the clock recovery circuit  90  and in optical communication with port  26  of the WDM  30  and the switch input port  48 . In this alternative configuration the time delay module  112  delays the data pulse  46  in response to the delay signal generated by the clock recovery circuit  90 .  
         [0042]    [0042]FIG. 5 shows an embodiment of a method  150  for regenerating an optical data pulse  46  using the optical switch  10  of FIG. 1. A pair of OP pulses separated by a time t are generated (step  152 ) in response to an optical clock pulse. The optical data pulse  46  and the pair of OP optical pulses  38 ,  42  are transmitted (step  154 ) through the nonlinear optical channel  58  in a forward direction. The optical data pulse  46  is substantially temporally coincident with one of the OP pulses  38 ,  42  such that the two optical pulses (e.g.  42 ,  46 ) overlap for at least a portion of the forward transmission and/or subsequent reverse transmission. In one embodiment the wavelength of the data pulse  46  is different from the wavelength of the OP pulses  38 ,  42 . The three pulses  38 ,  42 ,  46  are transmitted (step  158 ) through the nonlinear optical channel  58  in a reverse direction after an orthogonal rotation of their polarization components. Polarization rotation is achieved by the polarization rotation mirror  54  (e.g., reflective Faraday rotator). An optical phase delay is imparted to one of the OP pulses  42  as a result of the walk-through with the data pulse  46  in the nonlinear optical channel  58 . In one embodiment the optical phase delay is an odd integer multiple of 180°.  
         [0043]    In step  162  one of the OP pulses  42  is delayed by a time t relative to the other pulse  38  to “undo” or remove the original delay t so that the pair of pulses  38 ,  42  are temporally coincident. The delay t is imparted during reverse propagation through the birefringent optical channel  14 . A regenerated optical pulse  74  is provided (step  166 ) at the switch output port  78  in response to the imparted optical phase delay. In effect, the regenerated output pulse  74  is the combination of the two OP optical pulses  38 ,  42  into a single pulse having the proper polarization orientation so that the polarization dependent beamsplitter  66  routes the combined pulses  38 ,  42  to the output port  78 .  
         [0044]    [0044]FIG. 6 shows an embodiment of a method  170  for performing a logical operation on a first optical data bit and a second optical data bit according to the optical switch of FIG. 1. It is to be understood that the term “optical bit” as used herein refers to an optical logic event and is generally interchangeable with the term “optical pulse”. Method  170  includes generating (step  174 ) a pair of OP optical bits (pulses  38  and  42 ) in response to the first optical data bit. The OP optical bits  38 ,  42  are separated by a time t. One of the OP optical bits  42  is substantially temporally coincident with the second optical data bit  46 . The OP optical bits  38 ,  42  and the second optical data bit  46  are transmitted (step  178 ) through the nonlinear optical channel  58  in a forward direction. The OP optical bits  38 ,  42  and the second optical bit  46  are then transmitted (step  182 ) through the nonlinear optical channel  58  in a reverse direction. The polarizations of the optical bits  38 ,  42 ,  46  during reverse transmission are orthogonal to the respective polarization of the forward propagating optical bits due to the polarization rotation mirror  54 .  
         [0045]    As previously described, the second optical data bit  46  and OP optical bit  42  overlap for at least a portion of the forward transmission and reverse transmission through the nonlinear optical channel  58 . An optical phase delay is imparted to the overlapped OP optical bit  42  during the pulse walk-through. In one embodiment this phase delay is an odd integer multiple of 180°. After reverse transmission is complete, one of the OP data bits  42  is delayed (step  186 ) relative to the other OP data bit  38  to generate a temporal coincidence. In step  190 , a result of the logical operation is provided at switch output port  78  in response to the phase delay imparted to the overlapped OP optical bit  42  relative to the other OP optical bit  38 . In one embodiment the logical operation is an AND operation. Thus, an output bit (pulse  74 ) is present at the switch output port  78  only if both the first optical bit  34  and the second optical bit  46  are present. In another embodiment the logical operation is a Boolean AND operation between the signal input  110  and the inverse of the control channel  48 . This operation is obtained by taking the complementary output  86  as the switch output.  
         [0046]    [0046]FIG. 7 depicts an all-optical switch  10 ″ configured to operate as an ultrafast Boolean exclusive-OR (XOR) logic gate according to the present invention. All the components of the all-optical switch  10  depicted in FIG. 1 are present in the same configuration except that the bandpass filter  62 ′ is disposed between the circulator  70  and the output port  78  and a second bandpass filter  62 ″ is disposed between the beamsplitter  66  and the complementary output port  86 . The XOR gate  10 ″ also includes a second optical coupler  118  and an optical delay module  122 . The coupler  118  has a first coupler port  126  in optical communication with a first gate input port  130  and a second coupler port  134  in optical communication with the optical delay module  122 . The optical delay module  122  is also in communication with a second gate input port  138 .  
         [0047]    In operation, the logic gate  10 ″ performs the logical XOR operation on logical optical input bit A and optical input bit B (i.e., A XOR B) as summarized in Table 1. The optical delay module  122  delays optical bit B relative to optical bit A by a time t′ approximately equal to the delay time t between the OP pulses  38 ,  42 . To achieve XOR functionality with logic gate  10 ″, the optical input bits A and B are either co-polarized with the OP pulses  38 ,  42  (i.e. same polarizations) or orthogonally aligned to the OP pulse  38 ,  42  polarizations. The combination of optical bit stream A and optical bit stream B is provided at the output port  142  of the coupler  118 . Combined optical bit stream A and B is combined by the coupler  30  with the OP pulse stream  38 ,  42 .  
                                   TABLE 1                                           Complementary               Gate Input   Gate Input   Gate Output   Gate           Port 130   Port 138   Port 86   Output Port 78           A   B   A {overscore (XOR)} B   A XOR B                           0   0   1   0           0   1   0   1           1   0   0   1           1   1   1   0                      
 
         [0048]    If both optical bit A and optical bit B are provided during the same bit interval at the gate input ports  130  and  138 , respectively, optical bit A overlaps (or walks through) one OP pulse  42  and optical bit B overlaps (or walks through) the other OP pulse  38  during forward and reverse propagation through the nonlinear optical channel  58 . Thus, an optical phase shift is induce by cross-phase modulation in each of the OP pulses  38 ,  42 . There is no change, however, to the relative optical phase between the OP pulses  38 ,  42 . Consequently, polarization rotation at the polarization rotation mirror  54  and reverse propagation through the birefringent optical channel  14  yields a single output pulse  82 ′ having a polarization orientation that is substantially orthogonal to the orientation defined by the sum of the polarization vectors of the OP pulses  38 ,  42 . Consequently, the output pulse  82 ′ is reflected out of the polarization dependent beamsplitter  66  and routed through the bandpass filter  62 ″ to the complementary switch output port  86  indicating {overscore (A XOR B)}=1.  
         [0049]    If both data optical bits A and B are absent, there is no optical phase shift imparted to either OP pulse  38 ,  42 . Again, there is no change to the relative optical phase between the optical pulses  38 ,  42  and a single output pulse  82 ′ is provided to the switch complementary output port  86  indicating {overscore (A XOR B)}=1.  
         [0050]    If only one of the optical data bits A and B is present in the bit interval, the corresponding overlapped OP pulse  38  or  42  acquires an optical phase shift during propagation through the nonlinear optical channel  58 . As a result, the relative optical phase between the OP pulses  38 ,  42  is changed. The relative optical phase change that is proportional to the product of the data bit A or B intensity and the DSF  58  length and nonlinear index of refraction is selected to be 180°. After polarization rotation occurs at the polarization rotation mirror  54 , reverse propagation of the OP pulses  38 ,  42  through the birefringent optical channel  14  yields a single complementary output pulse  74 ′ having a polarization orientation that is substantially identical to the polarization orientation defined by the sum of the polarization vectors of the OP pulses  38 ,  42 . Consequently, no output pulse  82 ′ is available at the switch output port  86 . Instead, the complementary output pulse  74 ′ is directed by the circulator  70  through bandpass filter  62 ′ to the switch output port  78  indicating {overscore (A XOR B)}=1.  
         [0051]    In contrast to the optical switch  10  of FIG. 1, the OP optical pulses  38 ,  42  (combined as a logic output pulse  74 ′ or  82 ′, respectively) pass through a bandpass filter  62 ′ or  62 ″ only once. Accordingly, the optical loss imparted to the OP optical pulses  38 ,  42  for the XOR logic gate  10 ″ are less. The two bandpass filters  62 ′,  62 ″ can be replaced by the single bandpass filter  62  positioned as depicted in FIG. 1 if the additional optical loss is acceptable.  
         [0052]    [0052]FIG. 8 shows an embodiment of a method  200  for performing a logical operation on a first optical data bit and a second optical data bit separated by a predetermined time t using the optical switch of FIG. 7. Method  200  includes generating (step  202 ) a first OP data bit A and a second OP data bit B separated by a time t′ in response to a pair of received optical data bits. A first OP clock bit and a second OP clock bit (optical pulses  42  and  38 , respectively) separated by a time t are generated (step  204 ) in response to an optical clock bit. The first and second OP data bits A, B, and the first and second OP clock bits are transmitted (step  206 ) through the nonlinear optical channel  58  in a forward direction. The first OP data bit A is substantially temporally coincident with the first OP clock bit  42  such that the two bits A,  42  overlap during at least a portion of the forward transmission or subsequent reverse transmission through the nonlinear optical channel  58 . Similarly, the second OP data bit B is substantially temporally coincident with the second OP clock bit  38 .  
         [0053]    The method  200  also includes transmitting (step  208 ) the first and second OP data bits A, B and the first and second OP clock bits  42 ,  38  through the nonlinear optical channel  58  in a reverse direction. Because of the presence of the polarization rotation mirror  54 , the polarizations of each of the OP data bits A, B and the OP clock bits  38 ,  42  are orthogonal to their respective polarizations during transmission in the forward direction through the nonlinear optical channel  58 . An optical phase delay is imparted to the first OP clock bit  42  if the first OP data bit A is in an asserted state (i.e., the optical pulse  46  is present for the corresponding bit interval). Similarly, an optical phase delay is imparted to the second OP clock bit  38  if the second OP data bit B is in an asserted state. In one embodiment the phase delay imparted to either or both OP clock bits  38 ,  42  is approximately an odd integer multiple of 180°. The first OP clock bit  42  is delayed (step  212 ) relative to the second OP clock bit  38  so that the bits  38 ,  42  are temporally coincident.  
         [0054]    The result of the logical operation is provided (step  216 ) in response to the optical phase delays imparted to the first and second OP clock bits  42 ,  38 . In one embodiment the logical operation is an XOR operation and the result is represented by optical pulse  74 ′ described above. In another embodiment the logical operation is a complementary XOR operation and is represented by optical pulse  82 ′ described above.  
         [0055]    [0055]FIG. 9 depicts an all-optical switch  10 ′″ configured to operate as a Boolean OR logic gate according to the present invention. The components of the all-optical switch  10 ″ depicted in FIG. 7 are present in the same configuration except that the optical delay module  122  is absent.  
         [0056]    In operation, the all-optical switch  10 ′″ performs the logical OR operation on logical optical bit A and optical bit B (i.e., A OR B) as summarized in Table 2.  
                                   TABLE 2                                           Complementary               Gate Input   Gate Input   Gate Output   Gate           Port 130   Port 138   Port 86   Output Port 78           A   B   A NOR B   A OR B                           0   0   1   0           0   1   0   1           1   0   0   1           1   1   0   1                      
 
         [0057]    If both optical bit A and optical bit B are provided during the same bit interval at the gate input ports  130  and  138 , respectively, the resulting higher amplitude optical pulse A+B shown at the output port  142  of the logic coupler  118  is present. However, the optical powers of the optical data bits A and B are selected so that the switch  10 ′″ operates in saturation. One embodiment of this switch  10 ′″ can be realized if the DSF nonlinear optical channel  58  is replaced with a semiconductor optical amplifier. In such an embodiment, the magnitude of the optical phase shift imparted to the overlapped OP pulse either  38  or  42  during transmission through the nonlinear optical channel  58  is the same regardless of whether one or both optical bits A and B are present. Thus, the presence of at least one optical bit A, B during the bit interval is sufficient to provide an output pulse  74 ′ at the switch output port  78  indicating A OR B=1 (or A NOR B=0). If optical bit A and optical bit B are both absent during the bit interval, an output pulse  82 ′ is provided at the complementary switch output port  86  indicating A OR B=0 (or A NOR B=1).  
         [0058]    [0058]FIG. 10 shows an embodiment of a method  230  for performing a logical operation on a first optical data bit and a second optical data bit with reference to the optical switch  10 ′″ of FIG. 9. The method  230  includes generating (step  232 ) a first and a second OP clock bits  42  and  38 , respectively, in response to a received optical clock pulse  34 . The first and second optical data bits A and B, respectively, and the first and second OP clock bits  42 ,  38  are transmitted (step  234 ) through the nonlinear optical channel  58  in a forward direction. The optical data bits A, B and the OP clock bits  38 ,  42  are then transmitted (step  238 ) in a reverse direction through the nonlinear optical channel  58  in a reverse direction. The polarizations of each of the optical data bits A, B and the OP clock bits  38 ,  42  are rotated by the polarization rotation mirror  54  such that they are orthogonal to their respective polarizations during transmission in the forward direction through the nonlinear optical channel  58 . Because of the temporal coincidence of optical bits A and B, and the first OP clock bit  42 , an optical phase delay is imparted to the first OP clock bit  42  if either optical data bit A or B is in an asserted state. In one embodiment, the phase delay imparted to either or both OP clock bits  38 ,  42  is approximately an odd integer multiple of 180°. The first OP clock bit  42  is delayed (step  242 ) relative to the second OP clock bit  38  so that the OP clock bits  38 ,  42  are temporally coincident. In step  246  the result of the logical operation is provided at switch port  78  (or switch port  86 ) in response to the optical phase delay imparted to the first OP clock bit  42 . In one embodiment the logical operation is an OR operation and is represented by optical pulse  74 ′ as described above. In another embodiment the logical operation is a NOR operation and is represented by optical pulse  82 ′ as described above.  
         [0059]    Many other variations of the all-optical switch  10  of the present invention are possible using the principles described above. For example, FIG. 11 depicts an embodiment of an all-optical switch  10   iv  configured to operate as a 2×2 optical switch according to the present invention. The components of the switch  10  depicted in FIG. 1 are present in the same configuration except that an additional circulator  70 ′ is included. The additional circulator  70 ′ is in optical communication with the polarization dependent beamsplitter  66  and the complementary switch output port  86 . Data channels  1  and  2  provide optical pulses  34 ′ and  34 ″, respectively, having orthogonal polarization orientations. If control pulse  46 ′ is present during a bit interval, the channel  1  optical pulse  34 ′ is routed to the switch output port  78  and the channel  2  optical pulse  34 ″ is routed to the complementary switch output port  86 . Conversely, if control pulse  46 ′ is absent during the bit interval, the channel  2  optical pulse  34 ″ is routed to the switch output port  78  and the channel  1  optical pulse  34 ′ is routed to the complementary switch output port  86 .  
         [0060]    The optical switch of the present invention is not limited to the embodiments described above. Those of ordinary skill in the art will recognize that the optical switch can be configured to perform other logical and processing functions. For example, many data streams can be switched to any number of output ports according to the principles described above. In addition, optical switches can be cascaded or otherwise arranged to provide more complex processing of optical data streams.  
         [0061]    While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.