0001 The U.S. government has non-exclusive rights in this invention pursuant to contract number AF19628-95-C-0002 awarded by DARPA and AF.

TECHNICAL FIELD 0002 This invention relates to optical devices and optical networks.

BACKGROUND 0003 To route optical signals, an optical network may employ programmable optical add/drop multiplexers (OADM's) and optical cross connects (OXC's). OADM's add optical signals to and drop optical signals from optical trunk lines. OXC's switch optical signals between different optical trunk lines. OADM's and OXC's may perform signal routing without converting optical signals into intermediate electrical signals. 0004 To increase transmission bandwidths, an optical network may also perform wavelength division multiplexing. In wavelength division multiplexing, each optical trunk line can transmit several optical signals simultaneously by transmitting the signals at different wavelengths. In wavelength division multiplexed networks, optical elements select and route optical signals based on wavelength. 0005 In wavelength division multiplexed networks, OADM's and OXC's may shift signal wavelengths to enable routing of signals from one optical line to another. Shifting a signal's wavelength enables a switch to route the signal from one optical line where the signal has one wavelength, to another optical line where the same wavelength is already being used to carry another signal. By shifting signal wavelengths, OADM's and OXC's are able to more completely utilize available transmission bandwidth in optical trunk lines of a wavelength division multiplexed network.

SUMMARY 0006 In general, in a first aspect, the invention features a process of shifting wavelengths of optical pulses. The process includes transmitting an incoming optical pulse through a nonlinear optical material, splitting the transmitted pulse into a plurality of mutually coherent optical pulses, and recombining the mutually coherent pulses with inter-pulse temporal delays. The recombined pulses produce a temporal interference pattern. The pattern has a peak whose wavelength is shifted with respect to the wavelength of the incoming optical pulse. 0007 Other embodiments of the process include one or more of the following features. One feature is that the transmitting chirps the incoming optical pulse, which may bandwidth enhance the pulse. The chirping may result from self-phase or cross-phase modulation of the pulse. The chirped pulse has about the same temporal width as the incoming optical pulse. Another feature is that the splitting amplitude splits the pulse. Another feature is that the interference pattern be sent to an optical amplitude discriminator. 0008 The chirping may include applying a control optical signal to the nonlinear optical material to set a spectral modulation level for the incoming optical pulse. To apply the control signal, a voltage may be generated across the nonlinear optical material or a light control signal may be sent through the nonlinear optical material. The amplitude splitting may include separating the transmitted pulse into a plurality of pulses and sending each pulse of the plurality to a separate optical conduit. The different conduits have different optical path lengths determined in part by a control signal. The control signal may produce a voltage across or an increased light intensity in a section of one or more of the optical conduits. 0009 In a second aspect, the invention features an apparatus. The apparatus includes a nonlinear optical material capable of chirping optical pulses and a temporal grating generator (TGG) capable of producing a series of mutually coherent optical pulses from a received pulse. The TGG is optically coupled to the nonlinear optical material. 0010 Other embodiments include one or more of the following features. One feature is that the TGG is configured to make pulses of the series overlap. Another feature is that the TGG is a variable TGG that produces several temporal inter-pulse spacings. Another feature is that pulses passing through the TGG and the nonlinear optical material are sent to an amplitude discriminator. Another feature is that the nonlinear optical material is a semiconductor or a low dispersion optical fiber. 0011 The TGG may include an optical amplitude splitter with several output terminals, an optical coupler with several input terminals and optical conduits that connect the output terminals to the input terminals. The optical conduits may have different optical path lengths. One or more of the optical conduits may also have a section whose optical path length is responsive to control signals. 0012 The nonlinear optical material may be coupled to receive optical pulses from the TGG, or the TGG may be coupled to receive optical pulses from the nonlinear optical material. 0013 In a third aspect, the invention features an optical switch. The optical switch includes a wavelength division multiplexer (WDM) and a wavelength shifter to shift a wavelenth of an optical pulse. The wavelength shifter is coupled to transmit the optical pulse with a shifted wavelength to the WDM. The wavelength shifter includes a nonlinear optical material capable of chirping pulses and a temporal grating generator (TGG) optically coupled to the nonlinear optical material. 0014 Other embodiments of the switch may include one or more of the following features. One feature is that the wavelength shifter includes an amplitude discriminator coupled to receive optical pulses from the TGG or the nonlinear optical material. Another feature is that the nonlinear optical material is a semiconductor or a low dispersion optical fiber such as a dispersion decreasing fiber. Another feature is that the switch includes an optical coupler having a plurality of input terminals and an output terminal coupled to the wavelength shifter. Another feature is that the optical coupler is a wavelength division multiplexer. The WDM's may be coupled for bi-directional transmission. Another feature is that at least one wavelength shifter connects to an output terminal of the WDM. 0015 In a fourth aspect, the invention features a process for routing optical pulses. The process includes shifting a wavelength of an incoming pulse by transmitting the pulse through both a TGG and a nonlinear optical material. The process also includes routing the pulse with a shifted wavelength to one of a plurality of optical output lines based on the shifted wavelength. 0016 Other embodiments may include one or more of the following features. One feature is that the shifting transmits the incoming pulse through the nonlinear optical medium to chirp the pulse and then sends the chirped pulse through a TGG. Another feature is that the shifting sends the incoming pulse through a TGG to produce a series of mutually coherent pulses and then transmit the series through a nonlinear optical medium to chirp the pulses in the series. Another feature is that the process further shifts the wavelengths of the routed the pulse to another wavelength in response to routing the pulse to an optical output line having an available transmission channel at the other wavelength. 0017 In a fifth aspect, the invention features a process that shifts wavelengths of optical pulses. The process includes splitting an incoming optical pulse into a plurality of mutually coherent optical pulses and recombining the mutually coherent optical pulses through a nonlinear optical material to produce a temporal interference pattern having a peak whose wavelength is shifted with respect to the wavelength of the incoming optical pulse. 0018 Other embodiments include one or more of the following features. One feature is that the transmitting chirps each pulse of the series. Another feature is that the transmitting bandwidth enhances each pulse of the series. Another feature is that the transmitting includes performing one of sel-phase modulation and cross-phase modulation on the pulses of the series. Another feature is that the splitting amplitude splits the incoming pulse into a plurality of pulses and sends each pulse of the plurality to a separate optical conduit. Different ones of the optical conduits have different optical path lengths. 0019 Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS 0020 FIG. 1A shows a variable optical wavelength shifter; 0021 FIG. 1B shows an alternate variable optical wavelength shifter; 0022 FIG. 2 is a flow chart for a process of shifting a pulse's wavelength with the wavelength shifter of FIG. 1A; 0023 FIGS. 3A and 3B show two embodiments of variable temporal grating generators (TGG's) for use in the wavelength shifters of FIGS. 1A and 1B; 0024 FIG. 4A shows one embodiment of an electro-optically controlled optical path section for use in the TGG of FIGS. 3A or 3B; 0025 FIG. 4B shows one embodiment of a light controlled optical path section for use in the TGG of FIG. 3A; 0026 FIG. 5A illustrates spectral modulation produced by a bandwidth enhancer used in the wavelength shifter of FIGS. 1A and 1B; 0027 FIG. 5B illustrates pulse modulation produced by propagation in a dispersive medium; 0028 FIGS. 6A and 6B illustrate spectral modulation and pulse interference; 0029 FIG. 7 shows a 1M optical switch that uses the wavelength shifter of FIG. 1A or of FIG. 1B; 0030 FIG. 8 shows a MN optical switch based on the 1M optical switch of FIG. 7; 0031 FIG. 9 shows a bi-directional MN optical switch that uses wavelength shifters of the type shown in FIG. 1A-1B; 0032 FIG. 10 shows a 1M optical switch based on the switch of FIG. 7; and 0033 FIG. 11 shows an MN optical switch based on the switch of FIG. 8.

0034 Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION 0035 A system 10 that shifts wavelengths of optical pulses is shown in FIG. 1A. The system 10 includes a bandwidth enhancer that receives an incoming pulse 12 from an external source 14, e.g., a trunk fiber of an optical network. The bandwidth enhancer 16 includes an optical conduit 17, which is made of a nonlinear optical material such as a semiconductor or a low dispersion optical fiber, e.g., a dispersion decreasing fiber (DDF). The bandwidth enhancer 16 connects to a temporal grating generator (TGG) 20 via an optical fiber 19. The TGG 20 connects in turn to an amplitude discriminator 28 via an optical fiber 24. The amplitude discriminator 28 produces an outgoing pulse 29 whose wavelength is shifted with respect to the incoming pulse 12. The size of the wavelength shift may be varied through control signals sent to control input 26 of the TGG 20. 0036 A process 50 for shifting the wavelength of incoming pulse 12 is illustrated by a flow chart shown in FIG. 2. To perform the shift, the process 50 transmits the incoming pulse through a nonlinear optical material to chirp the pulse 12 (step 51). Chirping spectrally modulates the pulse 12 to produce an enhanced pulse 18 whose wavelength varies monotonically along the pulse's temporal envelope. Chirping is also equivalent to a frequency modulation of the pulse 12. Chirping results from self-phase or cross-phase modulation of the pulse 12 in a nonlinear optical material 17 of the bandwidth enhancer 16. 0037 After chirping, the TGG 20 splits the enhanced pulse 18 into a plurality of separate pulses (step 52). The splitting may be performed by amplitude splitting the enhanced pulse 18 or by polarization splitting the enhanced pulse 18, e.g., with birefringent elements. The TGG 20 delays each of the separate pulses by a different time (step 53) and then, recombines the delayed pulses to form a series 22 of overlapping and mutually coherent pulses on optical fiber 24 (step 54). The series 22 of overlapping pulses interferes to produce a pattern with one or more intensity peaks. 0038 Optical fiber 24 carries the interference pattern to amplitude discriminator 28. Amplitude discriminator 28 filters the pattern to remove lower intensity peaks and produce an outgoing pulse 29 from the largest intensity peak (step 55). The outgoing pulse 29 has a wavelength that is shifted with respect to the wavelength of the original incoming pulse 12. In embodiments that make the spacing between pulses of the series 22 a small fraction of a wavelength, the filtering may be unneeded, because the interference pattern has a single peak. For 1.5 meter light, the spacing is between about 106 to 1 times the inverse bandwidth. 0039 An alternate system 10 for shifting the wavelength of an incoming pulse 12 is illustrated in FIG. 1B. In the system 10 the order of bandwidth enhancer 16 and TGG 20 are reversed so that the incoming pulse 12 is first split by the TGG 20 into a plurality of separate pulses. The TGG 20 delays each separate pulse by a different time and then, recombines the delayed pulses to form a series 22 of overlapping and mutually coherent pulses on optical fiber 19. The series 22 of mutually coherent pulses goes to the bandwidth enhancer 16, which uses a nonlinear optical material to chirp each pulse of the series 22. Chirping produces a series 22 of enhanced and mutually coherent pulses and may be accomplished through self-phase or cross-phase modulation of the pulse 12 in a nonlinear optical material 17. The series 22 of pulses overlaps and interferes to produce a pattern with one or more intensity peaks. 0040 Optical fiber 24 carries the interference pattern to amplitude discriminator 28, which filters the pattern to remove lower intensity peaks thereby producing an outgoing pulse 29 from the largest intensity peak. The outgoing pulse 29 has a wavelength that is shifted with respect to the wavelength of the original incoming pulse 12. In some embodiments, the filtering may be unneeded, because the interference pattern has a single peak. 0041 A TGG 20 based on optical amplitude splitting is shown in FIG. 3A. The TGG 20 may be used in systems 10 and 10 of FIGS. 1A and 1B. The TGG 20 has a 1N optical splitter 30 that receives an enhanced pulse 18 from the bandwidth enhancer 16 via optical fiber 19. The 1N optical splitter 30 amplitude divides the received pulse 18 into N separate pulses and sends each pulse into one of N separate optical conduits 32-34. The optical conduits 32-34 have different optical lengths and thus, transmit the separate pulses to an N1 optical coupler 36 with different temporal delays. The N1 optical coupler 36 optically recombines the pulses received from the conduits 32-34 to produce a series of mutually coherent and overlapping pulses 22 on optical fiber 24. The series 22 of pulses immediately forms a traveling pattern on the optical fiber 24. 0042 An alternate TGG 20 based on cascaded Mach Zehnder interferometers is shown in FIG. 3B. The TGG 20 may also be used in the systems 10 and 10 of FIGS. 1A and 1B. The TGG 2 includes a series of fiber couplers 56, 56, 57, and 57 that are serially connected by pairs of optical fibers (58, 58), (59, 59), and (60, 60). Each coupler 56, 57, 57 amplitude splits received pulses into a pair of pulses and sends one pulse of each pair down one of the fibers connected to the coupler's outputs. The two pulses of the pair acquire a timing difference, because the fibers of each pair (58, 58), (59, 59), or (60, 60) have unequal optical lengths, e.g., (D, 2D) or (D, 4D). The timing differences between pulses accumulate as the pulses travel through the stages of interferometers, e.g., producing 8 pulses in the illustrated embodiment. In other embodiments, the TGG 20 may have different numbers of stages and produce series with different numbers of pulses. 0043 In the TGG 20, one fiber 58 has a path section 61 whose optical path length is variable. The section's optical path length may be varied electrically or optically to change the spacing between the mutually coherent pulses produced by the TGG 20. 0044 Other constructions for TGG's have been disclosed in U.S. patent application Ser. No. 09/282,880, filed Mar. 31, 1999; U.S. patent application Ser. No. 09/362,349, filed Jul. 27, 1999; and U.S. patent application Ser. No. 09/405,262, filed Sep. 23, 1999. These applications are incorporated by reference in their entirety in the present application. Any disclosed construction for a TGG may be used in the systems 10, 10 of FIGS. 1A and 1B. 0045 Referring again to FIGS. 3A and 3B, the path sections 38, 40, and 61 have optical path lengths that vary in response to control signals received from control input terminal 26. Differences between total optical path lengths of the different optical conduits 32-34 and the different fibers 58-59 determine sizes of inter-pulse spacing between members of the series 22. Changing the inter-pulse spacing changes the positions of intensity peaks in the interference pattern generated by the series 22 of pulses. To change the inter-pulse spacing, the optical path lengths of sections 38, 40, or 61 are changed either electrically or optically. 0046 An electrically controlled path sections 40 is shown in FIG. 4A. The path section 40 has an optical conduit 42 that transmits optical pulses between input and output terminals 43 and 44. A top surface of the optical conduit 42 is partially covered by a metallic conductive layer 45, which connects electrically to control input terminal 26. The optical conduit 42 rests on a substrate 46 containing a second conductive layer 47. The conductive layers 45, 47 form a capacitive sandwich over about the optical conduit 42. 0047 The optical conduit 42 is constructed of a material whose index of refraction depends on the size of a transverse voltage applied across the conduit 42, e.g., lithium niobate. During operation, a control voltage is applied to metal layer 45 via control input terminal 26 and conductive layer 47 is grounded. The size of the control voltage determines the index or refraction of the optical conduit 42. The index of refraction determines the temporal delay resulting in optical pulses that travel through the conduit 42. 0048 An optically controlled path section 40 is shown in FIG. 4B. The path section 40 has an optical conduit 48 that is constructed of an optical nonlinear material. The conduit 48 receives optical pulses from input terminal 43 and transmits the received pulses to output terminal 44 via a 12 fiber coupler 49. Another output terminal of the 12 fiber coupler 49 connects to the control input terminal 26. 0049 In the optically controlled path section 40, the control input terminal 26 receives light control signals. The light control signals set total light intensities in the optical conduit 48 thereby setting the index of refraction of the conduit 48 through the intensity dependence of the index of refraction of the nonlinear optical material. Thus, the light control signals set the optical path length of optical conduit 48. The light control signals travel towards the input terminal 43 in a direction that is opposite to the travel direction of optical pulses in the optical conduit 48. This counter-sense travel reduces wash out of optical pulses at the output terminal 44 by light control signals. 0050 The size of the wavelength shift produced by the system 10 and process 50 of FIGS. 1A and 2 depends on the inter-pulse spacing in the series 22 of overlapping pulses and on the spectral bandwidth of the enhanced pulse 18 produced by chirping. Relationships between the inter-pulse spacing, spectral bandwidth, and produced wavelength shift is illustrated in FIGS. 5A, 5B, 6A, and 6B. 0051 FIG. 5A illustrates exemplary pulses 62, 64 before and after chirping by propagation through a nonlinear optical material. Before chirping, the pulse 62 has a frequency that is substantially constant at different points F, C, and B on the amplitude envelope 66. The pulse frequency is measured by the temporal interval between amplitude crossing points at the point of interest. After chirping, the pulse 64 has a frequency that increases monotonically between points B and F on amplitude envelope 68. Before and after chirping by the nonlinear optical material, the pulses 62, 64 have approximately the same width. 0052 The pulse shaping by a nonlinear optical material can be contrasted with pulse shaping produced by a dispersive medium. FIG. 5B shows exemplary pulses 70 and 72 before and after being shaped by propagation through a dispersive medium. In the original pulse 70 the frequency is substantially constant over the length of the pulse. On the other hand, the pulse 72 has a frequency, which is higher at point F than at point B. Thus, propagation through a dispersive medium also induces frequency chirp of an optical pulse. But, propagation through the dispersive medium does not broaden the pulse's spectrum. 0053 The dispersive medium also temporally broadens optical pulses. The temporal broadening causes neighboring series of mutually coherent pulses to spread and can produce overlap. The spreading can produce power loss, and the overlap canm produce errors in data transmission. Avoiding overlap can put an upper bound on data speeds in optical transmission systems that use dispersion to chirp pulses. Since nonlinear optical materials can chirp optical pulses without producing temporal broadening, using nonlinear materials to chirp pulses may be advantageous in high-speed transmission systems. 0054 Furthermore, using nonlinear optical materials to enhance pulse spectra through self-phase or cross-phase modulation can generate larger spectral bandwidths, which are otherwise unobtainable through dispersion. Very large bandwidths can be obtained by performing the above modulation processes in a semiconductor such as InGaAsP or low dispersion fibers such as DDF's. The bandwidths obtainable are also larger than natural bandwidths associated with pulses of about tens of pico second durations. Larger spectral bandwidths imply that wavelength shifting systems based on these nonlinear optical materials can function in wavelength division multiplexed networks having larger numbers of spectral channels. 0055 The result of combining two pulses 74 and 76 that are mutually coherent and chirped is illustrated in FIGS. 6A and 6B. The pulses 74 and 76 were produced from a single pulse according to steps 51, 52, and 53 of process 50, shown in FIG. 2. The process 50 has chirped the pulses 74 and 76 so that shorter wavelength , is found at forward points F and longer wavelength is found at back points B on the pulse envelopes. The process 50 has also produced peak-to-peak spacing, d, between the pulses 74 and 76 by transmitting two originally in-phase pulses through optical conduits of different lengths, e.g., conduits 32, 33 of FIG. 3A. Transmission through the conduits also gave the pulses 74 and 76 a relative phase difference, which depends on wavelength. Due to the prior position-dependent spectral modulation, the relative phase difference also depends on position along the pulse envelopes. 0056 Upon being recombined, pulses 74 and 76 interfere to produce a pattern with one or more interference peaks P and P as shown in FIG. 6B. The wavelengths of the peaks P and P depend on the peak locations within the envelopes of the pulses 74 and 76 because of the spectral modulation of the pulses 76 and 78. The bandwidth of the modulation of each pulse 76 and 78, i.e., equal to 2 in FIG. 6A, fixes the limit range of wavelengths that the process can provide. 0057 The system 10 for providing variable wavelength shifts enables constructing a 1N optical switch 80 shown in FIG. 7. In the switch 80, the system 10 of FIG. 1A transmits optical pulses to a wavelength division multiplexer (WDM) 84 having N output terminals 86, 88. Other embodiments may use the system 10 of FIG. 1B in place of the system 10 of FIG. 1A. The WDM 84 selectively routes optical signals to output terminals 86, 88 based on wavelength and may, for example, use a diffraction grating to perform this wavelength selective routing. The system 10 produces a wavelength shift for an incoming optical pulse 12 so that the wavelength-sensitive WDM 84 routes the pulse to a desired output terminal. The system 10 is able to produce wavelength shifts that cause the WDM 82 to route the pulse to any of the N output terminals 86, 88. 0058 The 1N optical switch 80 of FIG. 7 enables constructing an MN optical switch 90 shown in FIG. 8. In the MN switch 90, an M1 optical coupler 92, e.g., an M1 optical fiber connector or a reversed 1M WDM, connects M input optical lines 94, 96 to input terminal 98 of the 1N optical switch 80. The M1 coupler 92 enables an optical pulse from any of the M optical conduits 94, 96 to be routed to any of the N output terminals 86, 88 of the WDM 84. 0059 In an optical network, the MN optical switch 90 can perform as either an OXC or an OADM. 0060 A MN bi-directional optical switch 100 is shown in FIG. 9. The bi-directional switch includes a 1M WDM 102 and a 1XN WDM 104. The WDM's 102 and 104 receive incoming optical pulses from any of their output terminals 106, 107 and 108, 109, respectively. In response to receiving an incoming pulse, the WDM 102 or 104 sends the pulse to an optical circulator 110 or 112. The optical circulators 110 and 112 send pulses received from the WDM's 102 and 104 to variable wavelength shifters 114 and 116, respectively. The wavelength shifters 114 and 116 may have the form of the system 10, shown in FIG. 1A, or of the system 10 of FIG. 1B. The variable wavelength shifters 114 and 116 appropriately shift the wavelengths of received pulses and transmit the pulses to the remaining optical circulator 112 and 110, respectively. The optical circulators 112 and 110 transmit pulse received from the wavelength shifters 114 and 116 to the WDM's 104 and 102, respectively. In response to receiving optical pulses from the optical circulators 110 and 112, the WDM's 102 and 104 selectively transmit the pulses to their output terminals 106, 107 and 108, 109 based on the pulse's wavelength. 0061 The optical switches 80, 90, 100 of FIGS. 7, 8, and 9 may be configured for time division multiplexed (TDM) networks in which each trunk line carries optical pulses having a predetermined wavelength. FIG. 10 shows a 1N optical switch 120 that is based on the 1N switch 80 of FIG. 7. In the switch 120, wavelength shifters 122, e.g., system 10 of FIG. 1A or system 10 of FIG. 1B, connect output terminals 86 and 88 of WDM 84. The wavelength shifters 122 shift wavelengths of outgoing pulses to the predetermined wavelengths carried by network lines 124 and 126. For TDM networks, the wavelength shifters 122 may be constructed to make fixed wavelength shifts, because optical pulses on each of the network lines 124, 126, and 128 have predetermined wavelengths in TDM networks. FIG. 11 shows an MN optical switch 130 that is based on the MN switch 90 of FIG. 8. The switch 130 also has wavelength shifters 122 connected to the output terminals 86 and 88 of the WDM 80. The wavelength shifters 122 shift wavelengths of incoming signals to predetermined values for the lines 124 and 126. The switch 130 can function as an OXC or an OADM in a TDM network. 0062 The optical switches 120 and 130 of FIGS. 10 and 11 can also operate in a wavelength division multiplexed network. In such a network, the wavelength shifters 122 are variable and operate to shift wavelengths of outgoing pulses to wavelength values that correspond to unused channels of WDM lines 124 and 126. Thus, the wavelength shifters 122 enable the switches 120 and 130 to better use available bandwidth in WDM networks. The switch 130 can also function as an OXC or an OADM in a WDM network. 0063 In a WDM network the switch 130 of FIG. 11 may also have variable wavelength shifters (not shown) connected input terminals 94, 96 of the M1 coupler 92. These wavelength shifters, e.g., systems 10 or 10 of FIG. 1A and 1B, shift wavelengths of optical pulses, which are incoming from the WDM network, to a preselected wavelength. Then, the 1N switch 80 receives incoming optical pulses having the preselected wavelength even though the pulses had different wavelengths on source WDM lines. 0064 Wavelength shifters, i.e., shifters 10 and 10 of FIGS. 1a and 1B, may also be placed on the output terminals of the MN switch 100 shown in FIG. 9. These external shifters may enable the switch 100 to take advantage of available bandwidth in lines that are targets of optical pulse routing. 0065 A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.