Abstract:
A method and system for AO 3 R functionality is presented. The system includes an AO 2 R device followed by an AOCR clock recovery module and an AOR retiming device. The AOR retiming device takes as input a recovered clock signal extracted from the output of the AO 2 R by the AOCR clock recovery module. The output is the recovered clock signal gated by the regenerated and reshaped input signal, and a monitor circuit is used to set the optimum operations of the retiming device. In a first embodiment the output of the AOR retiming device is fed to an AOC code and wavelength conversion output stage, which returns the signal to the NRZ coding, on a service wavelength converted to match the fixed wavelength connection with the DWDM transmission system. In a second embodiment the code conversion is incorporated into the AOR retiming device, and wavelength conversion is accomplished in the AOCR clock recovery device.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/238,296 filed Oct. 6, 2000. This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/849,411 filed May 4, 2001 and U.S. patent application Ser. No. 09/843,968 filed May 4, 2000 now U.S. Pat. Nos. 6,563,621 and 6,570,697 (the “Parent Applications”), the complete specification of each of which is hereby incorporated herein by this reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to telecommunications, and more specifically, to an improved system and method for the all optical reshaping, regeneration and retiming (“AO 3 R”) of optical signals in a data network, precluding any need for Optical-Electrical-Optical (“OEO”) conversion. 
     BACKGROUND OF THE INVENTION 
     Noise, timing jitter, and attenuation in long-haul optical line systems result in the deterioration of the transmitted signal. Consequently, one of the fundamental requirements of nodal equipment in optical networks is the capability to regenerate, reshape and retime (3R Regeneration) the optical pulses. Notwithstanding the plethora of claims by various companies to have implemented “all-optical” systems, presently retiming of the optical pulses is achieved by converting the incoming optical signal into an electrical signal. This is followed by full regeneration and retiming of the electrical signal using Application Specific Integrated Circuits (ASICs). A laser source is then modulated using this fully regenerated and retimed electrical signal. However, there are certain drawbacks to converting an optical signal into an electrical one and back again. First, electrical processing of data signals is not transparent to bit rate and is format sensitive. Thus, an OEO system could not process an arbitrary incoming data signal; the bit rate, format and coding would need to be known a priori. Second, there is a significant power loss in converting to the electrical domain, and back again therefrom to the optical domain. 
     As optical networks become increasingly transparent, there is thus a need to regenerate the signal without resorting to Optical-to-electrical, or OEO, conversion of the signal. 
     At present, signal coding in optical networks generally takes the Non-Return to Zero (NRZ) code. In such coding, the signal level of the high bit does not return to zero during a portion of the incoming signal, which consists of a series of high bits. NRZ coding is the coding of choice today in optical communications systems due to the signal bandwidth efficiency associated with it. This code has thus been used for optical line systems operating at line rates up to 10 Gb/s. However, as service line rates increase to 40 Gb/s, for reasons associated with fiber dispersion and reduction in inter-symbol interference (ISI) penalties, Return-to-Zero (RZ) coding is generally favored. In RZ coding, the signal returns to zero in each and every bit period. 
     Future optical networking line systems will incorporate service signals at both 10 Gb/s as well as 40 Gb/s along with their associated Forward Error Corrected (FEC) overhead. The FEC rates related to 10 Gb/s data transport include the 64/63 coding for 10 Gb/s Ethernet, the 15/14 encoding of SONET-OC192 FEC and the strong-FEC rate of 12.25 Gb/s, as well as numerous potential coding schemes yet to be developed. Effectively, to support multiple FEC—and other coding related—protocols, an optical network node must be able to process numerous line rates. 
     As the networks tend towards optical transparency, the nodal devices in the optical network must work with all available line rates independent of their coding. One of the fundamental functions of these devices is the capability to extract the clock from the signal wholly in the optical domain. The RF spectrum of an RZ signal reveals a strong spectral component at the line rate. Consequently, the incoming signal can be used directly to extract the clock signal. In the case of the NRZ signal format, the RF spectrum reveals no spectral component at the line rate. The RF spectrum of an ideal NRZ signal looks like a sinc function with the first zero at the line rate. As described in the Parent Applications, the fundamental problem of all-optical clock recovery from an NRZ signal is the generation of a RF spectral component at the line rate. As therein described, an NRZ/PRZ (Non-return to Zero/Pseudo Return to Zero) converter is used to generate the strong line rate frequency component by converting the incoming NRZ signal into an RZ-like signal. 
     As a consequence of the above, the clock recovery in these network elements must be tunable over a wide range of frequencies. What is needed therefore, is an AO 3 R system, that is truly all-optical, and that is tunable over a wide range of bit-rate frequencies and works in the carrier frequency range (wavelength range) of the modern telecommunications systems, the C and L wavelength bands. 
     SUMMARY OF THE INVENTION 
     A method and system for AO 3 R functionality is presented. The system includes an AO 2 R device followed by an AOCR clock recovery module and an AOR retiming device. The AOR retiming device takes as input a recovered clock signal extracted from the output of the AO 2 R by the AOCR clock recovery module. The output is the recovered clock signal gated by the regenerated and reshaped input signal, and a monitor circuit is used to set the optimum operations of the retiming device. In a first embodiment the output of the AOR retiming device is fed to an AOC code and wavelength conversion output stage, which returns the signal to the NRZ coding, on a service wavelength converted to match the fixed wavelength connection with the DWDM transmission system. In a second embodiment the code conversion is incorporated into the AOR retiming device, and wavelength conversion is accomplished in the AOCR clock recovery device. 
     Previous schemes for performing the O 3 R functionality use some level of Optical-to-electronic (OEO) conversion to generate the clock signal. The AO 3 R scheme presented here carries out all three functions in the optical domain, and returns a clean output signal using identical coding as the input, on a wavelength of choice. 
     A lossy component, such as an optical cross-connect switch can be placed either before the AO 3 R device or inside of it after the AO 2 R device and before the signal is split to the AOCR clock recovery and the AOR retiming devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a schematic diagram of the system of the present invention according to a first embodiment; 
     FIG. 2 depicts a schematic diagram of the all-optical clock recovery stage depicted in FIG. 1; 
     FIG. 3 depicts the feedback circuit for data/clock phase alignment according to the present invention; 
     FIG. 4 depicts the cross section of an exemplary SOA-AMZI device according to the present invention; 
     FIG. 5 depicts a schematic view of a fully integrated AO 3 R subsystem according to a first embodiment of the present invention, indicating the various blocks; 
     FIG. 6 depicts the subsystem of FIG. 5, without the block identifiers; 
     FIG. 7 depicts a schematic diagram according to a second embodiment of the present invention; 
     FIG. 8 depicts a first instance of a second embodiment of the fully integrated AO 3 R subsystem; and 
     FIG. 9 depicts a second instance of a second embodiment of the fully integrated AO 3 R subsystem. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A schematic diagram of the AO 3 R device is shown in FIG.  1 . The input stage  110  comprises an optical amplifier to boost the incoming signal. The all-optical 2R device that follows  120  essentially removes the noise from the boosted signal and reshapes it into a square wave with a high extinction ratio. This device can be implemented in many ways. Commercially available devices that use a Semiconductor Optical Amplifier-Mach Zehnder Interferometer (SOA-MZI) can be utilized for this purpose. Other embodiments that take advantage of four-wave mixing non-linearities in fiber and SOA can also be used for this purpose. 
     The signal is then split into two parts, as depicted at point  150  in the figure. The splitting ratio can range from −3 dB to −10 dB. One part of the signal  150 A is input into a clock recovery module  130 , and the other part of the signal  150 B is input into an all-optical retiming (AOR) module  131 . The clock recovery module  130  is an all-optical device. There is no conversion of the optical signal into the electrical domain. The device and method of such all-optical clock recovery are discussed in detail in U.S. Pat. No. 6,563,621. The preprocessor extracting the line rate comprising the first stage of the clock recovery module is discussed in detail in U.S. patent application No. 09/848,968. 
     A schematic description of the clock recovery is shown in FIG.  2 . The NRZ to PRZ line rate recovery pre-processor  250  forms the first stage of the AOCR scheme. This consists of a path-delayed Asymmetric Mach-Zehnder Interferometer (AMZI). The AMZI incorporates Semiconductor Optical Amplifiers (SOAs)  205 ,  206  in each of the arms and a phase delay  207  in one of the arms. The line delay is set so that the phase difference between the arms is π. The AMZI is set for destructive interference of the signals in the two paths. Consequently, the interference of a high bit with its path delayed and π-phase inverted copy, generates an RZ-like bit, termed a PRZ bit, at both the leading and falling edges of the original high bit. This latter signal, with a bit rate effectively double that of the original NRZ bit rate, is the PRZ output signal. 
     This effective doubling of the bit rate leads to the generation of a large component of the line rate frequency in the RF spectrum of the output signal  210  of the AMZI  250 . Generally, unless the input signal is exceptionally aberrant, this line rate frequency will be the far and away dominant frequency in the spectrum. Since the preprocessor does not need to know a priori the actual bit rate or coding of the input data to operate, the extraction of the line rate is data rate and format insensitive. For obvious reasons, it is wavelength insensitive as well. 
     Thus the preprocessor has the ability to reshape the PRZ signal as well as adjust its duty cycle. The output  210  of the first stage  250  becomes the input to the second stage  260 . In a preferred embodiment, the second stage  260  comprises a symmetric Mach-Zehnder Interferometer, where each arm contains a semiconductor optical amplifier  211  and  212 , respectively. 
     The principle of clock recovery is based on inducing oscillations between the two lasers DFB 1   213  and DFB 2   214 . The oscillations are triggered by the output of the first stage  210 . As described above, this output can be either RZ or PRZ. The current to DFB 2   214  is tuned close to its lasing threshold, with DFB 1   213  energized so as to be in lasing mode. Thus the trigger pulse  210  induces lasing in DFB 2   214 . The feedback from DFB 2   214  turns off the lasing in DFB 1   213  resulting in DFB 2   214  itself turning off. The reduced feedback from DFB 2   214  now returns DFB 1   213  to lasing. In this manner the two lasers mutually stimulate one another in oscillation. Recalling that the dominant frequency in the input signal  210  is the original signal&#39;s 200 clock rate, pulses from the input  210  are sufficient to lock the oscillation of the DFB lasers at that rate, and, in general, to hold for quite a number of low bits (such as would appear where the original signal  200  had a long run of high bits). Thus, the forced triggering by the PRZ/RZ input  210  locks the phase of the oscillations at the original signal&#39;s 200 clock rate. 
     The interferometer improves the control of the phase input to DFB 2   214 . The use of the SOA-MZI facilitates the tuning of the oscillation rate by adjusting the input signal phase into DFB 2   214 . As the phase of the MZI output is tuned, the gain recovery time of DFB 2   214  is adjusted. This results in the oscillation rate being altered. In this manner the clock frequency can be further tuned to the desired line rate. Using non-linear SOA elements also allows shaping of the output clock with a lesser energy expenditure. Moreover, by adjusting the currents in each of the two SOAs in the second stage interferometer, the refractive index of each SOA&#39;s waveguide can be manipulated, thus altering the phase of the pulse entering DFB 2   214  thus adjusting the phase of the oscillations to align it to the phase of the retimed input signal,  152  in FIG.  1 . Thus, the oscillation rate and phase of the circuit can be altered. The identical circuit can be tuned to the various bit rates available in the network, thus rendering a system that is bit rate independent. 
     Referring again to FIG. 1, as above, the second part of the signal  150 B derived from the AO 2 R  120  is input into the AOR retiming device  131  along with the line rate clocking signal  151 , which was recovered in the AOCR clock recovery module  130  and output therefrom, the process of which being as depicted in FIG.  2 . In the AOR the clocking signal  151  is AND gated with the regenerated and reshaped input signal  150 B, to give the output  152  of the AOR  131 . A feedback circuit  145  ensures that the clock signal  151  and the data signal  152  are phase aligned. This feedback circuit  145  can be implemented, for example, by a simple photodetector-based circuit that monitors the DC power level at the output of the AOR  152  to ensure that the signal level is maximized, as shown in FIG.  3 . The monitor signal  155 , seen as  355  in FIG. 3, passes to the photodetector and peak detector,  345  in FIG. 3 (corresponding to the feedback circuit  145  in FIG.  1 ), generating a negative feedback signal  354 , corresponding to signal  154  in FIG.  1 . The negative feedback from this feedback circuit tunes the static phase condition of the AOR (i.e. by adjusting the tuning currents controlling the SOAs in the MZI of the AOR, as described below) such that the detected photocurrent is a maximum. This indicates an optimum phase shift between the original signal  100  and the recovered clock  151  in the AOR retiming circuit. 
     The output of the AOR  131  is fed into the all-optical RZ to NRZ and wavelength converter  140 . A CW (continuous wavelength) laser source in the coding converter is utilized to execute wavelength conversion. This functionality is depicted by the Lambda Conversion module  142  in FIG.  1 . 
     Recalling the functionality of the AOCR module, as described in U.S. patent application Nos. 09/849,441 and 09/848,968, the clock recovery transforms an NRZ input signal to a PRZ signal. If the network is set up to run NRZ coded data, the output has to be transformed back to NRZ coding. As well, network conditions and provisioning may desire that the input data signal be carried on a different outgoing wavelength than the one that brought it in. Thus, wavelength conversion is supplied at the output stage. In an alternative embodiment, as described below, the wavelength conversion can be accomplished in the AOCR device  130 , and the code conversion integrated into the AOR device  131 , obviating devices  140  and  142  in FIG.  1 . 
     A lossy component, such as an optical cross-connect switch, can be placed either before the AO 3 R device or inside of it after the AO 2 R device and before the signal is split to the AOCR clock recovery and the AOR retiming devices. The use of a commercial AOCR device  130  predicates a modular structure to the overall AO 3 R scheme as shown in FIG.  1 . One embodiment of this assembly can be a multi-chip module (MCM) based on the Silicon Optical Bench (SiOB) technology. In such an embodiment the interconnection between the individual chips that make up the four main components, i.e. AO 2 R  120 , AOCR  130 , AOR  131  and AOC RZ/NRZ and Wavelength Converter  140 , each of which utilizes the same symmetrical MZI with SOAs in each arm structure, is provided by silica waveguides on a silicon substrate. 
     A preferred embodiment of the AO 3 R can be a completely integrated subsystem on an InP substrate. This would imply that the structure of the AO 2 R would consist of an SOA-MZI integrated with a laser, and the similar structures would be composed of SOA-MZls integrated with lasers as required by their function (e.g., AOCR, Wavelength Converter). Such an integration is similar to the implementation of an AOCR as discussed in U.S. patent application No. 09/849,441. 
     As discussed above, the method of the invention can be implemented using either discrete components, or in a preferred embodiment, as an integrated device in InP-based semiconductors. The latter embodiment will next be described with reference to FIG.  4 . 
     FIG. 4 depicts a cross section of an exemplary integrated circuit SOA. With reference to FIG. 2, FIG. 4 depicts a cross section of any of the depicted SOAs taken perpendicular to the direction of optical signal flow in the interferometer arms. Numerous devices of the type depicted in FIG. 4 can easily be integrated with the interferometers of the preprocessor, the closck recovery so that the entire circuit can be fabricated on one IC. The device consists of a buried sandwich structure  450  with an active Strained Multiple Quantum Well region  411  sandwiched between two waveguide layers  410  and  412  made of InGaAsP. In an exemplary embodiment, the λ g  of the InGaAsP in layers  410  and  412  is 1.17 μm. The sandwich structure does not extend laterally along the width of the device, but rather is also surrounded on each side by the InP region  404  in which it is buried. 
     The active Strained MQW layer is used to insure a constant gain and phase characteristic for the SOA, independent of the polarization of the input signal polarization. The SMQW layer is made up of pairs of InGaAsP and InGaAs layers, one disposed on top of the other such that there is strain between layer interfaces, as is known in the art. In a preferred embodiment, there are three such pairs, for a total of six layers. The active region/waveguide sandwich structure  450  is buried in an undoped InP layer  404 , and is laterally disposed above an undoped InP layer  403 . This latter layer  403  is laterally disposed above an n-type InP layer  402  which is grown on top of a substantially doped n-type InP substrate. The substrate layer  401  has, in a preferred embodiment, a doping of 4-6×10 18 /cm −3 . The doping of the grown layer  402  is precisely controlled, and in a preferred embodiment is on the order of 5×10 18 /cm −3 . On top of the buried active region/waveguide sandwich structure  450  and the undoped InP layer covering it  304  is a laterally disposed p-type InP region  421 . In a preferred embodiment this region will have a doping of 5×10 17 /cm −3 . On top of the p-type InP region  421  is a highly doped p+-type InGaAs layer. In a preferred embodiment this latter region will have a doping of 1×10 19 /cm −3 . The p-type layers  420  and  421 , respectively, have a width equal to that of the active region/waveguide sandwich structure, as shown in FIG.  4 . As described above, the optical signal path is perpendicular to and heading into the plane of FIG.  4 . 
     Utilizing the SOA described above, the entire all-optical 3R device can be integrated in one circuit. With reference to FIG. 5, a schematic layout of an exemplary fully integrated AO 3 R device is shown. It is noted that for ease of viewing FIG. 5 only shows the active parts of the circuit, Thus, devices with redundant structures could be used in any of the depicted modules. As well, FIG. 5 has blocks drawn around the portions of the circuit comprising the various devices and modules depicted schematically in FIG.  1 . Thus, the two figures can be easily correlated. The integrated device depicted in FIG. 5 implements all of the various functionalities of FIG. 1, as will next be discussed. 
     There are four stages in the integrated device, corresponding to the AO 2 R stage  120 , the Clock Recovery stage  130  (which includes the pre-processor stage), the AOR stage  131 , and the AOC RZ/NRZ code and wavelength converter stage  140 , of FIG.  1 . In general the reference numbers in FIGS. 1 and 5 are identical in the tens and digits places, again for ease of correlation. 
     At the top of FIG. 5 appear the input signal  500 , the pre-amplifier  510  and the AO2R stage  520 . The incoming signal  500  enters at the top right of the figure, and passes through SOA  510 . From there it enters the MZI, with integrated laser, of stage  520 . The output from the AO2R stage  550  then bifurcates, into signals  550 A and  550 B. Output  550 B, now a regenerated and reshaped optical pulse train goes into the clock recovery stage  530 , comprising the preprocessor  530 PP and the clock extraction  530 CE sub-stages. As described in the Parent Applications, if the original input was RZ coded the gain of the upper arm of the AMZI in stage  520  is set to zero, and intermediate signal  550 I is RZ coded as well. If the original was NRZ coded, intermediate signal  550 I is PRZ coded. The intermediate output from the preprocessor  550 I is fed into the clock extraction sub-stage  530 CE, which outputs the now RZ coded clock signal  551  (also possibly having undergone wavelength conversion via DFB-2R laser  560 , according to a second embodiment of the invention, described below). This latter signal  551  is input to the AOR stage  531 , along with the split output  550 A from stage  520 , which is the data signal, and is input to the AOR at SCA  570 . This input  550 A gates, through phase modulation in the MZI containing SOAs  571  and  572 , the clock signal  551  to generate the retimed output of this stage,  552 . 
     AOR output signal  552  is an RZ coded signal. This signal  552  is input to the code and wavelength conversion module  540 . When the input signal  500  is NRZ coded, the AOR output signal  552  is fed to the MZI  590 , comprising SOAs  575  and  576 , through both SOAs  573  and  574 . An undelayed (via SOA  573 ) high bit phase modulates the continuous wavelength light from DFB-2R  542  for constructive interference (as the SOAs  575  and  576  are initially set to a relative phase shift of π (in general all SOAs in opposite arms of MZIs are so set); thus a high bit in the upper arm changes the phase difference between the two SOAs to zero, and a high bit on each SOA changes the relative phase shift back to π), and the CW light combines at the output  553  to generate a “1.” When the high bit through the upper arm of MZI  590  has passed, SOA  574  then passes the delayed copy of that same bit to the MZI via delay element  580 . 
     Using an appropriate delay, depending, inter alia, on the phase shift latency in the SOAs and the full period bit-rate of the recovered clock signal, the RZ signal is converted to an NRZ coded signal. 
     In this manner an NRZ pulse is generated from an incoming RZ pulse  552 . When the input signal  552  is RZ coded, SOA  574  is turned off, thus blocking the delayed signal to the MZI  590  code converter. The converter thus passes the RZ pulses unchanged to the system output  553 . 
     Wavelength conversion of the regenerated, reshaped and retimed signal  552  is achieved by tuning the frequency of the DFB laser  542 . The sampled power monitor, PM  545 , is sent to the feedback controller, as described above, and used to set the tuning current in SOAs  571  and  572 . 
     The net result is the final output  553  of the entire AO 3 R device, which is a clean, regenerated, reshaped, and retimed optical pulse train, on a wavelength chosen by the user. 
     FIG. 6 is identical to FIG. 5, but is made more readable by removal of the blocks denoting the various stages. The reference numbers are identical to those in FIG. 5, except that the hundreds place digit is a “6” in FIG. 6, replacing the “ 5 ” in such index numbers from their FIG. 5 counterparts. 
     An exemplary method of effecting such an integrated AO 3 R device is next described. 
     After an epiwafer is grown with the waveguide and the SOA active regions, the wafer is patterned to delineate the SOAs, the AMZI and the various MZIs. In a preferred embodiment the path length difference between the two arms of the AMZI in the clock-extraction sub-stage is approximately 1 mm. 
     Next, the DFB regions of the second stage of the device are created using either a holographic or a non-contact interference lithographic technique. The periodicity of the grating in a preferred embodiment is approximately 2850A. 
     The grating is of Order  1  and provides optical feedback through second-order diffraction. The undoped InP top cladding layer, the p-type InP layers, and the contact layer are then regrown on the patterned substrate. This step is then followed by photolithography for top-contact metallization. The device is then cleaved and packaged. 
     A second embodiment of the invention is depicted in FIGS. 7-9. In a second embodiment of the invention the RZ to NRZ conversion is implemented by the AOR retiming device. This eliminates the need for the AOC device  540  in FIG.  5 . In this case the wavelength conversion is achieved by tuning carrier frequencies of the DFB lasers#1 and #2 in the clock extraction device of the AOCR,  530 CE. FIG. 7 shows a functional block diagram for this case. Similar index numbers (in the tens and units digits) in FIG.  7  and FIG. 1 correlate to similar functionalities. In FIG. 7 lambda conversion  742  is now done in the AOCR module  730 , and RZ/NRZ conversion in the AOR retiming module  731 . 
     There are two instances, or versions of this second embodiemtn, depicted in FIGS. 8 and 9, respectively. FIG. 8 shows the counter-propagating implementation of the AOR/RZ-to-NRZ-conversion device. In this configuration the delayed input signal  850 A and the delayed recovered clock signal  851  inputs to the AOR  831  must be both delayed by the same amount for the RZ-to-NRZ conversion, as described above, thus delay elements  880  and  880 ′. 
     FIG. 9 shows the co-propagating implementation of the AOR/RZ-to-NRZ-conversion device. In this configuration only one delay  980  is required for the coupled input and recovered clock signals. In this implementation both the retimed and converted output  951 , and the regenerated input signal  950 A are transferred to the output of the AOR device. In this configuration an optical filter  999  is required to filter-out the regenerated input signal  950 A,  950 AA,  950 AB. 
     While the above describes the preferred embodiments of the invention, various modifications or additions will be apparent to those of skill in the art. Such modifications and additions are intended to be covered by the following claims.