Abstract:
A method introduces variable time offsets into a stream of optical pulses. The method includes receiving a plurality of coherent optical pulses, receiving a plurality of control signals, and forming a coherent pulse array (CPA) from each pulse in response to one of the received control signals. Temporal spacings between pulses of each CPA are responsive to the associated one of the received control signals. For optical control signals, response times can be very short. The method further includes transmitting each pulse through a dispersive optical medium. The act of transmitting makes pulses of each CPA overlap to form an interference pattern.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/117,146, filed Jan. 25, 1999, and U.S. Provisional Application No. 60/126,730, filed Mar. 29, 1999. 
    
    
     The U.S. Government has non-exclusive rights in this invention pursuant to contract number AF19628-95-C-0002 awarded by DARPA. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to optical circuits and networks, and more particularly, to variable optical delays. 
     Variable optical delays have potential applications in both optical data networks and optical logic circuits. The applications involve synchronizing internal components of such networks and logic circuits to external data streams and other internal components, respectively. Synchronizing entails changing the arrival times of optical signals. 
     One potential application of such delays is the construction of packet-switched optical networks. Packet-switched networks need to resynchronize receivers on a pack-by-packet basis. The need for packet-by-packet resynchronization may be met by variable time delays produced by either delay lines or clock recovery techniques. 
     The prior art includes several types of variable optical delay lines. Some such lines use either a stepping motor or a piezo-electric transducer to mechanically change the length of an optical fiber or a gap, carrying the arriving signal. Other delay lines use an acousto-optic modulator or another type of beam scanning crystal to convert changes in arriving beam angles into variable delays. These types of delay lines are typically characterized by response times on the order of milliseconds or longer. 
     The prior art also includes techniques for varying the phase of an optical clock. One clock recovery technique uses electro-optical phase locked loops. Another clock recovery technique uses injection locking of a receiver&#39;s optical clock to the data stream. Both of these techniques have response times in the millisecond range. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a method to introduce variable time offsets into a stream of optical pulses. The method includes receiving a plurality of coherent optical pulses, receiving a plurality of control signals, and forming a coherent pulse array (CPA) from each pulse in response to one of the received control signals. Temporal spacings between pulses of each CPA are responsive to the associated one of the received control signals. The method further includes transmitting each pulse through a dispersive optical medium. The act of transmitting makes pulses of each CPA overlap to form an interference pattern. 
     In some embodiments, the method further includes sending each interference pattern through an intensity discriminator to pass a peak thereof. 
     In some embodiments, the act of forming a CPA for each pulse further includes splitting each received pulse into a plurality of pulses, and delaying at least one of the pulses. The act of delaying includes propagating the one of the pulses and the associated one of the control signals in a nonlinear optical media. 
     Some embodiments further filter the associated one of the control pulses from the nonlinear medium. Other embodiments propagate the pulses and the associated control signals in opposite directions in the nonlinear medium. 
     In general, in a second aspect, the invention features a variable temporal grating generator (TGG). The variable TGG includes an amplitude splitter to split a received optical pulse into a plurality of pulses, a plurality of optical waveguides, and a waveguide coupler connected to receive pulses from the optical waveguides. Each waveguide receives one of the pulses from the splitter. At least one of the waveguides has a variable path element. The variable path element has a control terminal and a optical path length responsive to control signals received at the control terminal. The coupler has an output terminal to transmit CPA&#39;s made of the pulses received. 
     In some embodiments, the variable path element further includes a nonlinear optical medium coupled to receive pulses traveling through the waveguide. The signals received by the control terminal are optical signals. The control terminal transmits a portion of each optical signal to the nonlinear medium. 
     In some embodiments, the variable TGG further includes an optical waveguide coupled to receive the CPA&#39;s from the output terminal and a high frequency signal generator. The generator sends electrical or optical driving signals to a portion of the optical waveguide. The driving signals vary the index of refraction of the portion of the waveguide. 
     In general, in a third aspect, the invention features a variable optical delay line. The optical delay line includes a length of dispersive medium and a TGG having an optical input terminal, an optical output terminal and a control terminal. Either the optical input terminal or the optical output terminal couples to one end of the dispersive medium. The TGG generates a CPA at the optical output terminal from each pulse received at the optical input terminal. Temporal spacings of pulses of each CPA are responsive to control signals received at the control terminal. The dispersive medium causes each CPA to produce an interference pattern. 
     In some embodiments, the dispersive media is a dispersive optical waveguide. The variable TGG may also include an optical clock producing coherent clock pulses. The output terminal of the clock connects either to an end of the waveguide or to the input terminal of the variable TGG. The variable optical delay line may also include an intensity discriminator to receive each interference pattern. 
     In various embodiments, the variable TGG further includes an amplitude splitter and a plurality of optical waveguides. The splitter splits an optical pulse received from the input terminal into a plurality of pulses. Each waveguide connects to receive one of the pulses from the splitter. At least one of the waveguides includes a variable path element coupling to the control terminal. The variable path element has an optical path length responsive to the control signals. The variable TGG also includes a waveguide coupler connected to receive pulses from the optical waveguides. The waveguide coupler has a second output terminal to transmit a portion of the pulses received. 
     The variable path element may further include a nonlinear optical medium coupled to receive pulses traveling through the one of the waveguides. The signals received by the control terminal are optical signals. The control terminal is connected to transmit a portion of each optical signal to the nonlinear medium. 
     In general, in a fourth aspect, the invention features an optical phase locked loop (OPLL). The OPLL includes an optical switch, an optical clock, a dispersive optical waveguide coupled to the optical clock, and a variable TGG having a control terminal. The switch has two input terminals and one output terminal. The variable TGG receives clock pulses from the dispersive waveguide and transmits interference patterns to one input terminal of the optical switch. The output terminal of the switch couples to the control terminal. 
     In various embodiments, the output terminal of the optical switch transmits optical signals to the control terminal. 
     In general, in a fifth aspect, the invention features an antenna array. The array includes a plurality of remote antennae and a control system to produce optical control signals. The array includes a plurality of first optical waveguides that receive the signals from the control system. The array also includes a plurality of variable TGG&#39;s and a plurality of second waveguides. Each TGG couples to one of the first waveguides. Each second waveguide connects one of the TGG&#39;s to one of the remote antennae. Each second waveguide produces an interference pattern from a CPA received from the connected TGG. 
     Other features, and advantages of the invention will be apparent from the following description of the preferred embodiments and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a setup for showing the production of optical time delays with dispersion induced interference; 
     FIGS. 2A-2D show interference patterns produced by the setup of FIG. 1 or various pulse spacings in a CPA; 
     FIG. 3 shows the delayed optical pulses produced by filtering the patterns of FIGS. 2A-2D with a NOLM; 
     FIGS. 4A and 4B show two embodiments of a variable optical delay line; 
     FIG. 5 shows an embodiment of a variable TGG for use in the optical delay line of FIGS. 4A and 4B; 
     FIGS. 6A-6D show alternate constructions of the variable optical path used in the variable TGG of FIG. 5; 
     FIGS. 7A-7B show alternate constructions of the variable optical path of FIG. 5, which use nonlinear optical media; 
     FIG. 8 shows another embodiment of a variable TGG; 
     FIG. 9 shows another embodiment of a variable TGG, which uses Mach Zehnder interferometers; 
     FIG. 10 shows an embodiment of an optical phase locked loop; 
     FIG. 11 shows an all-optical switch for use in the phase locked loop of FIG. 10; and 
     FIG. 12 shows a phased antennae array, which uses variable TGG&#39;s to rephase the array. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a setup  10  for demonstrating that dispersion combined with coherent pulse array (CPA) production can generate an optical delay. The setup  10  includes a mode locked fiber laser  12 , which produces a 2 pico second (ps) coherent optical pulse  14 . Pulse  14  travels through a length of polarization maintaining fiber (PMF)  16 . PMF  16  is a birefringent fiber in which the polarization of pulse  14  is oriented at about 45° with respect to the internal PMF axes. The perpendicular polarization components of the pulse  14  travel at different speeds in the PMF  16 . At an output  18  of PMF  16 , the two components pass through a polarizer  20  that projects the components along the same direction to produce a 2-pulse CPA  22  in which each pulse has the same polarization. If PMF  16  has a length of about 4.6 meters, CPA  22  will contain two mutually coherent pulses, which are still about 2 ps wide and are about 6 ps apart. 
     The pulses of CPA  22  travel through about 10.6 kilometers of dispersive optical fiber  23 . Dispersive fiber  23  broadens each pulse of CPA  22  to a width of about 240 ps. The broadening produces CPA  24  in which the two pulses strongly overlap and interfere. 
     FIGS. 2A-2D show interference patterns produced by the overlapping CPA  24  for different lengths of the birefringent PMF  16 . Each different length of the PMF  16  produces a different temporal spacing of the two pulses of the original CPA  22  and a different interference pattern. 
     Each interference pattern has one larger peak  28  and one or more smaller peaks  35 - 40 . Peaks  28 ,  35 - 40  are located in a 240 ps wide envelope  30 . Envelope  30  is the pattern that would be produced by inserting the original pulse  14  directly into dispersive fiber  23 . Because the pulses of CPA&#39;s  24  producing the patterns for each figure have different temporal spacings, the location of larger peak  28  moves in time. 
     The original pulses of the CPA  22  may have small temporal separations. For example, these separations may be of the order of the time for light to travel a fraction of a wavelength, i.e., a microscopic distance. Such small temporal separations induce phase shifts between the broadened pulses recombined in the CPA  24  and the non-trivial interference patterns of FIGS. 2A-2D. 
     Referring again to FIG. 1, a nonlinear optical loop mirror (NOLM)  26  receives the broadened CPA  24 . NOLM  26  removes portions of the optical signal whose intensities are below a threshold value, e.g., smaller peaks  35 - 40  of FIGS.  2 A,  2 B,  2 C, and  2 D. Portions of the signal above the threshold value, e.g., larger peak  28  of FIGS. 2A,  2 B,  2 C, and  2 D, pass through NOLM  26  without substantial attenuation. 
     FIG. 3 shows the optical signals  43 - 46  produced by NOLM  26  of FIG. 1 for four pulse spacings in the original CPA  22 . The range of timing delays for signals  43 - 46  is about 150 ps. The range of obtainable timing delays is approximately the width of the dispersion envelope, e.g., envelope  30  of FIGS. 2A-2D, that would result in the absence of interference. 
     The timing delay is determined by the temporal spacing between the mutually coherent pulses of the original CPA  22 . Changing the temporal spacing between the original pulses by about λn/2c produces an equivalent optical phase shift of π. This phase shift changes the time delay of the highest peak  28  by about the full-width at half maximum of the dispersion envelope  30  of FIGS. 2A-2D. Here, λ, n, and c are the wavelength of the pulses, the index of refraction of fiber  16 , and the speed of light, respectively. For 1.5 micro-meters wavelength light, changing the separation between the pulses of the CPA  22  by 2.5×10 −15  seconds shifts the time delay of the highest peak in the CPA  24  by about the full-width at half maximum. 
     FIG. 4A shows one embodiment of a variable optical delay line  100 . Variable optical delay line  100  includes a source  102  that produces a stream of optical pulses. The pulses may be mutually incoherent or coherent. Source  102  may be either an input terminal of a receiver or an optical clock or optical data source. Source  102  couples to a variable temporal grating generator (TGG)  104 . Variable TGG  104  couples to an input end of a dispersive optical waveguide  106 . The output end of optical waveguide  106  couples to the input of an optical intensity discriminator  108 , e.g., a NOLM. 
     Herein, waveguides may be optical fibers or other optical conduits that direct light along well-defined paths. The optical conduits may be constructed of linear, non-linear, or electro-optical materials. 
     Variable TGG  104  splits each optical pulse  110  received from source  102  into a CPA  112 , which may include two or more pulses. CPA  112  travels through dispersive waveguide  106 , which broadens the individual pulses so that they overlap. Since the individual pulses of the initial CPA  112  are mutually coherent, their overlap produces an interference pattern  114 . Interference pattern  114  passes through intensity discriminator  108 , which removes subsidiary peaks  116  and  118  from pattern  114  and transmits the largest peak  120 . Thus, each optical pulse  110  from source  102  produces one outgoing optical pulse  122 . 
     The temporal spacing of the pulses of CPA  112  is controlled by a signal applied to control terminal  124  of variable TGG  104 . The pulse spacings also determine the time delay or offset of output pulse  122 , as has been described in relation to FIGS. 2A-2D and  3 . Thus, the time delay or offset produced by the line  100  is controllable by control input  124  of variable TGG  104 . 
     Some embodiments of the variable optical delay line use an optical clock for source  102  to produce an optical clock with a variable offset. 
     FIG. 4B shows a variable optical delay line  130 , which is an alternative embodiment. In variable optical delay line  130 , dispersive waveguide  106  is located between source  102  and variable TGG  104 . Thus, pulse  110  broadens to produce pulse  111  prior to entering variable TGG  104 . Then, the pulses of the CPA which are produced by TGG  104  immediately overlap and interfere at the output of TGG  104 . 
     In delay lines  100  and  130  of FIGS. 4A and 4B, changing the control signal on terminal  124  changes the form of the CPA&#39;s produced by TGG  104 . But, this change does not affect signals produced by lines  100  and  130  until the changed CPA&#39;s propagate to the input terminal of the intensity discriminator  108  and thereafter to the output terminal  123 . In line  100 , the CPA&#39;s have to travel through the long dispersive waveguide  106  to reach input terminal  108 . Thus, line  100  has a longer response time than line  130  where the CPA&#39;s do not have to travel through dispersive waveguide  106  before reaching output terminal  123 . 
     FIG. 5 shows an embodiment of variable TGG  104  used in delay lines  100  and  130  of FIGS. 4A and 4B, respectively. Variable TGG  104  has an optical input terminal  142  and an optical output terminal  144 . Input terminal  142  is an input terminal of a 1×2 waveguide coupler  146 , e.g., an amplitude dividing coupler. The output terminals of the 1×2 waveguide coupler  146  connect to first and second optical waveguides  148  and  150 , e.g., single-mode optical waveguides. First optical waveguide  148  couples to an input terminal of a 2×1 optical combiner  152 . Second waveguide  150  couples to an input terminal of a path element  154  providing a variable optical path length. The output terminal of path element  154  couples to a third waveguide  156 , which in turn couples to the second input terminal of the 2×1 waveguide combiner  152 . The output terminal of the 2×1 combiner  152  is output terminal  144  of the variable TGG  104 . 
     From each received pulse  156 , the TGG  104  produces a CPA having two mutually coherent pulses  158  and  159  at output terminal  144 . The temporal spacing of the pulses  158  and  159  depends on the difference between the optical lengths of waveguide  148  and path  160 . The difference in the optical lengths depends on the control signal applied to control terminal  124  of path element  154 . The control signal controls the optical length of the path element  154 . 
     The path element  154 , which has a variable optical length, can have a variety of different forms. Some forms for element  154  are illustrated in FIGS. 6A-6D and  7 A- 7 B. 
     FIG. 6A shows a path element  170  in which the optical path length is mechanically controllable. Element  170  includes a roll  172  of optical fiber, which is tightly wrapped around a split reel  174 . A voltage applied to control terminal  124  controls a piezo-electric device  176 . Piezo-electric device  176  in turn exerts a pressure on the interior of reel  174  to expand the split reel&#39;s width. Expanding reel  174  stretches the fiber thereby lengthening the optical path length associated with fiber roll  172 . 
     FIG. 6B shows a path element  180  in which the optical path length is thermally controllable. Path element  180  includes an optical medium  182  whose index of refraction changes with applied temperature. The temperature is controlled by a current flowing in an electrical resistor  184 , which wraps tightly around optical medium  182 . Resistor  184  electrically connects between control terminal  124  and ground. 
     FIG. 6C shows a path element  190  in which the optical path length is electrically controllable. Path element  190  is an electro-optical device, which includes an optical waveguide  192  located between a substrate  194  and an electrode  196 . The voltage on electrode  196  controls the index of refraction of waveguide  192 , e.g., a LiNbO 3  material, and the optical path length thereof. The voltage applied to electrode  196  is controllable through control terminal  124 . 
     FIG. 6D shows another path element  200  in which the optical length is mechanically controllable. Path element  200  includes an air gap  201  between collimating lenses  202  and  204 . Lens  204  and waveguide  156 , which couples to the output of path element  200 , are mounted on a mechanical holder  206 . Holder  206  fixes to a base plate  208 . The lateral position of holder  206  on base plate  208  is adjustable through an elector-mechanical device  210  mounted laterally between baseplate  208  and holder  206 . The voltage on control terminal  124  determines the lateral position of holder  206  and the width of air gap  201 . 
     Some embodiments of variable path element  154  of FIG. 5 have optical path lengths, which are adjustable through optical control pulses. Such variable path elements are illustrated in FIGS. 7A and 7B. 
     FIG. 7A shows a variable path element  212  which uses a nonlinear optical media. Path element  212  includes a 2×1 waveguide coupler  214 , e.g., an inverted amplitude splitter or a birefringent coupler, whose input terminals couple to optical waveguide  150  and optical waveguide  215 . Waveguide  215  forms control terminal  124  of FIG. 5, i.e., path element  212  has an optical control. The output terminal of the 2×1 coupler  214  connects to an input end of an optical waveguide  216  constructed of a nonlinear material. The output end of waveguide  216  couples to a filter  218  whose output terminal couples to waveguide  156  of FIG.  5 . 
     The index of refraction of the nonlinear material of waveguide  216  depends on the total light intensity therein. The light intensity is controlled by an optical control pulse of intensity I introduced into waveguide  215  via the control terminal  124 . The control pulse travels through waveguide  216  changing the optical path length for coincident pulses traveling between waveguides  150  and  156 . The change in the optical path length, ΔL, is proportional to the change in the index of refraction times the physical path length. In the nonlinear material, the total change in the optical path length is ΔL=(n 2 ) (I) (L), which shifts a pulse&#39;s phase by Δφ=2π(n 2 ) (I) (L)/λ. Here, n 2  is the nonlinear refractive index (a material parameter) of waveguide  216 , and L is the physical length of waveguide  216 . 
     Since light intensity controls the optical path length of waveguide  216 , selecting an optical path length only constrains the intensity of a control pulse. The wavelength and/or polarization of the control pulse can be chosen freely. Some embodiments chose control pulses whose wavelength and/or polarization are different than those of the pulses coming from waveguide  150 . For such pulses, a bandpass filter  218  reduces contamination of the output of path element  212  by the control pulse. Filter  218  selectively attenuates the control pulse based on the differences between the wavelength and/or polarization of the control pulse and the pulses from waveguide  150 . 
     FIG. 7B shows a variable path element  220 , which uses a nonlinear optical media. Path section  220  is similar to the section  212  of FIG. 7A except that the control pulse travels in one direction, indicated by heavy arrows, and the pulse entering by the waveguide  150  travels in the opposite direction, indicated by light arrows. The opposite flow directions mean that the control pulse does not contaminate the signal at output waveguide  156 . Thus, path element  220  does not need filter  218  to remove the control pulse from the output of element  212  of FIG.  7 A. 
     In some embodiments, the nonlinear waveguide  216  of path element  220  may be waveguide  150  itself. 
     FIG. 8 shows a variable TGG  230  which is another embodiment of variable TGG  104  of FIG.  4 B. Variable TGG  230  receives a input pulse  232  from waveguide  106 . Pulse  232  enters a 1×N waveguide coupler  233  that amplitude splits the pulse into N mutually coherent pulses directed into optical waveguides  236 ,  237 ,  238 , and  239 . The first waveguide  236  carries the received pulse to an input terminal of a N×1 waveguide coupler  240 . The other waveguides  237 - 239  carry the received pulses to variable path elements  241 - 243 , which may be implemented in the various manners illustrated in FIGS. 7A-7B. Variable path elements  241 - 243  delay the pulses and transmit the delayed pulses to waveguides  244 - 246 . Waveguides  244 - 246  carry the delayed pulses to other input terminals of the N×1 waveguide coupler  240 . The N×1 coupler  240  recombines the received pulses to produce an N pulse CPA  234  on output optical waveguide  107 . 
     Each path element  241 ,  242 , and  243  includes a linear, nonlinear or electro-optic material, e.g., variable waveguides  172 ,  182 ,  192 ,  201 , and  216  of FIGS. 6A-7B, through which received control signals travel. Path elements  241 ,  242 , and  243  also have control terminals  247 ,  248 , and  249 , e.g., terminal  124  of FIGS. 6A-7B. Lengths of the waveguides connecting 1×(N−1) waveguide coupler  250  to terminals  247 - 249  are designed to synchronize arrivals of control signals in the linear, nonlinear or electro-optic materials of the different path elements  241 ,  242 , and  243 . 
     Variable TGG  230  produces equal spacings in CPA  234  if the control signals have the same intensity at terminals  247 - 249  and the relative lengths of the nonlinear media of the elements  241 ,  242 , and  243  have the values 1, 2, . . . , N−1. 
     Some embodiments use intensity attenuators (not shown) to adjust the intensities of the individual control signals produced by the 1×N coupler  250 . 
     A signal generator  254  coupled to TGG  230  can be used to modify inter-pulse spacings of the CPA  234 . Signal generator  254  modulates the path length of a path segment  256 . By synchronizing the modulation of the path length of segment  256  with the passage of individual pulses of CPA  234 , changes to the inter-pulse spacings in CPA  234  are achieved. 
     In some embodiments, path segment  256  has nonlinear optical properties. Then, signal generator  254  may be used to generate optical signals that modulate the index of refraction of path segment  256 . 
     FIG. 9 shows a variable TGG  260  employing a Mach Zehnder geometry. TGG  260  includes a sequence of 2×2 waveguide couplers  262 ,  264 , and  266  interconnected by pairs of optical waveguides  267 ,  268 ,  269 , and  270 . One of the waveguides  268  and  270  of each pair includes a variable path element  272  and  274 , respectively, which responds to signals received from a control terminal  124  and  125 . Element  272  produces a time difference, ΔT, between the time required for pulses to travel through waveguide  268  and to travel through waveguide  267 . Similarly, element  274  produces a difference 2ΔT for the time for pulses to travel through waveguide  269  and to travel through waveguide  270 . For these time delays, variable TGG  260  produces a CPA  234  having four equally spaced pulses from the each pulse  156  received from waveguide  106 . 
     U.S. patent application Ser. No. 09/282,880 (&#39;880), filed Mar. 31, 1999 describes other TGG designs, which a person of ordinary skill in the art would be able to combine with the present disclosure to make still other embodiments of variable TGG&#39;s. These other variable TGG&#39;s can produce CPA&#39;s having any desired number of pulses and/or CPA&#39;s having nonuniform pulse spacings. The optical paths of these other variable TGG&#39;s may also include elements that individually modulate amplitudes of the pulses in the CPA&#39;s. Some of these embodiments may not use an intensity discriminator to remove intensity peaks from the output interference pattern produced by the TGG and dispersive medium. For example, some TGG&#39;s do not produce the side peaks  116 ,  118  in the pattern  120  of FIG.  4 A. The &#39;880 patent application is incorporated by reference, in its entirety, in the present application. 
     FIG. 10 shows an embodiment of an optical phase locked loop  300  (OPLL). OPLL  300  receives an externally generated optical pulse stream at a first input terminal  302  of an optical switch  304 . A second input terminal  305  of optical switch  304  receives clock pulses generated by an optical clock having a variable offset  306 . 
     Optical clock  306  includes a standard optical clock  308 . Clock  306  transmits clock pulses to a dispersive optical waveguide  310 . Optical waveguide  310  broadens and transmits each clock pulse to a variable TGG  312 . Variable TGG  312  produces an interference pattern from the each clock pulse. An intensity discriminator  315  recovers an offset clock pulse from each interference pattern. 
     A control terminal  314  of TGG  312  receives optical signals from an output terminal  316  of optical switch  304  via an optical feed back loop  318 . The feedback signal controls the offset of optical clock  306  to continuously resynchronize the pulses on output terminal  320  to the optical pulse stream received from input terminal  302 . Since feedback loop  318  is optical, OPPL  300  can very quickly resynchronize to new pulse streams on input terminal  302 . 
     FIG. 11 illustrates an optical switch  304  that can be used in the OPLL  300  of FIG.  10 . Switch  304  uses a 2×2 waveguide splitter  322  to split an optical pulse from input terminal  305  into first and second pulses propagating in respective clockwise and counterclockwise senses on a loop  324 . The first and second pulses recombine at coupler  322  and exit by the same input terminal  305  unless an external condition modifies the phase of one of the pulses. 
     Input terminal  302  of a 2×2 waveguide coupler  326  receives control pulses, which provide such external conditions. The 2×2 splitter  326  transmits half of each control pulse to loop  324  with a clockwise propagation. The control pulse changes the light intensity in the portion of loop  324  adjacent thereto. The changed light intensity changes the index of refraction of nonlinear material of loop  324 . 
     When the control and first pulses are timed to co-propagate, the first pulse receives an added phase shift due to changed index of refraction. The added phase shift enables the first and second pulses to produce an output pulse on waveguide  330  upon recombining. The amplitude of the pulse produced correlates to the temporal offset between the control and first pulses. 
     A filter  332  removes the control pulse from waveguide  330  so that output terminal  334  is uncontaminated by the control pulse. 
     The variable optical time delays  100  and  130  of FIGS. 4A and 4B have a variety of applications. One application is controlling a phased remote antenna array  338  shown in FIG.  12 . 
     The remote antenna array  338  uses a control system  346  to optically control N remote antennae  340 . Control system  346  sends optical control signals to an input terminal of a 1×N waveguide coupler  350  via an optical waveguide  348 . The output terminals of the 1×N waveguide coupler  350  route the signals to N separate variable TGG&#39;s  352 ,  353 , and  354 . The output of each TGG  352 ,  353 , and  354  couples to a dispersive waveguide  342 ,  343 , and  344 , which carries the control signal to one of the antennae  340 . 
     Control system  346  rephases the array  338  by varying the arrival times of the control signals at the various antennae  340 . To rephase array  388 , the control system  346  sends control pulses to control terminals of the variable TGG&#39;s  352 ,  353 , and  354  via line  356 . The control pulses change the temporal spacings of CPA&#39;s produced by the TGG&#39;s  352 ,  353 , and  354 . At the antennae  340 , the control signals are interference patterns of the pulses of the CPA&#39;s due to dispersion broadening produced by waveguides  342 ,  343 , and  344 . The different temporal spacings affect the arrival times of the control signals at the various remote antennae  340 . 
     Other embodiments are within the scope of the following claims.