Abstract:
Optical transponders with reduced sensitivity to PMD and CD are described. In one embodiment, an optical transponder comprises a differential group delay (DGD) mitigator integrated within the transponder and optically coupled to an optical input port of the optical transponder, an optical receiver integrated within the optical transponder and optically coupled to the DGD mitigator and to an electrical output port of the transponder, and a multi-level transmitter integrated within the optical transponder, where the multi-level transmitter is electrically coupled to an electrical input port and optically coupled to an optical output port of the transponder. In another embodiment, a method comprises receiving and processing an optical input signal using a DGD mitigator integrated within an optical transponder, and receiving an electrical input signal, narrowing the spectrum of the electrical input signal, converting the electrical input signal into an optical output signal, and transmitting the optical output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to concurrently filed and commonly assigned U.S. patent application Ser. No. ______, entitled “SYSTEMS AND METHODS FOR VARIABLE POLARIZATION MODE DISPERSION COMPENSATION,” the disclosure of which is hereby incorporated by reference herein. 
     
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to optical systems, and more particularly, to optical transponders. 
       BACKGROUND OF THE INVENTION 
       [0003]    In modern optical networks, signals are typically transmitted over hundreds, or even thousands of kilometers. Optical signals traveling over long-haul and ultra long-haul optical fibers encounter many different obstacles including, for example, attenuation, chromatic dispersion (CD), and polarization mode dispersion (PMD). While attenuation problems have been successfully solved by the use of amplifiers (e.g., Erbium-Doped Fiber Amplifiers), CD and PMD issues have been much more difficult to handle. 
         [0004]    CD is a phenomenon that can lead to loss of data due to broadening of the light pulse on the receiving end of the transmission through an optical fiber. One solution to this problem involves the use of a dispersion compensation fiber (DCF), which has a large negative dispersion coefficient. However, in order to eliminate the CD of a given system, the length of the DCF must be very precise. That is, because the CD of the DCF is very large, any extra fiber length may cause more harm than good. And, apart from the high costs of a DCF, it can be very difficult to design a proper DCF to mitigate the dispersion where the CD of the system is unknown. Moreover, in a varying network, precise CD cancellation is a very complicated undertaking. Thus, the inventors hereof have recognized a need for the use of devices and components that are less sensitive to CD. 
         [0005]    In the case of PMD, the problem is even more severe. PMD occurs when different planes (i.e., polarization directions) of light inside a fiber travel at slightly different speeds (for example, due to random imperfections and asymmetries of the optical fiber), thus making it difficult to reliably transmit data at high rates. Typically, the PMD of a given system cannot characterized by a single parameter (e.g., its length), but rather it must be described by a series of parameters that represent the entire history along the communication line. Unfortunately, most networks were built with poor quality fibers in their underground installations at a time when relatively low bit rates were required and PMD was not yet recognized as a potential issue. And, now that these structures must support bit rates of 40 Gb/s and higher, PMD presents a significant obstacle to network upgrading. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to systems and methods for optical transponders which may be used, for example, to facilitate the communication of data across optical networks. It is an objective of the present invention to provide an integrated optical transponder that is capable of reducing chromatic dispersion (CD) effects and correcting polarization mode dispersion (PMD) inherent to long-haul optical fibers. As such, an integrated optical transponder according to aspects of the present invention may be used in Metro and regional networks. It is another objective of the present invention to provide a small and low-cost differential group delay (DGD) mitigation device integrated into a transponder. Certain embodiments of the present invention comprise an optical transponder having a duo-binary transmitter (or another transponder capable of reducing dispersion effects) integrated with a DGD mitigation device. One of the advantages of the integrated optical transponders described herein is that a transmitter may compensate for deficiencies presented by a DGD mitigator and vice-versa. Moreover, the integration described herein also allows components share some of the same electronic infrastructure, thus reducing design and manufacturing costs. 
         [0007]    In one exemplary embodiment, an optical transponder comprises a differential group delay (DGD) mitigator integrated within the optical transponder and optically coupled to an optical input port of the optical transponder, an optical receiver integrated within the optical transponder and optically coupled to the DGD mitigator and to an electrical output port of the optical transponder, and a multi-level transmitter integrated within the optical transponder, where the multi-level transmitter is electrically coupled to an electrical input port and optically coupled to an optical output port of the optical transponder. 
         [0008]    In another exemplary embodiment, a method comprises receiving and processing an optical input signal using a differential group delay (DGD) mitigator integrated within an optical transponder, converting the processed optical input signal into an electrical output signal using an optical receiver integrated within the optical transponder, and receiving an electrical input signal, narrowing the spectrum of the electrical input signal, converting the electrical input signal into an optical output signal, and transmitting the optical output signal using a transmitter integrated within the optical transponder. 
         [0009]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a block diagram of an integrated optical transponder, according to one exemplary embodiment of the present invention; 
           [0012]      FIG. 2  is a block diagram of a DGD mitigator, according to another exemplary embodiment of the present invention; 
           [0013]      FIG. 3  is a block diagram of a DGD mitigator in a reflective configuration, according to yet another exemplary embodiment of the present invention; 
           [0014]      FIG. 4  is a block diagram of a receiver, according to one exemplary embodiment of the present invention; and 
           [0015]      FIG. 5  is a block diagram of a transmitter, according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable a person of ordinary skill in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical, optical, and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
         [0017]    The inventors hereof have recognized a need for an optical transponder that is less sensitive to chromatic dispersion (CD) while being capable of reducing polarization mode dispersion (PMD) in a varying network. Accordingly, one exemplary embodiment of the present invention integrates two complementary devices: a multi-level transmitter (e.g., a duo-binary transmitter) and a first-order PMD or DGD mitigator. Typically, a duo-binary transmitter is more sensitive to first-order PMD (i.e., differential group delay or DGD). However, because its spectrum is narrower, the duo-binary transmitter is less sensitive to second and higher order PMD. Meanwhile, a PMD mitigator may be designed to reduce mostly first-order PMD or DGD. As a result, integration of a duo-binary transmitter with a DGD mitigator may provide an optical transponder with improved performance and reduced costs. 
         [0018]    Turning now to  FIG. 1 , integrated optical transponder  100  is depicted according to one exemplary embodiment of the present invention. Generally speaking, an “optical transponder” is a device that can both transmit and receive optical signals. In this embodiment, transponder  100  integrates DGD mitigator  110  and transmitter  130  into a single device. The input of DGD mitigator  110  is optically coupled to optical input port  111 . The input of receiver  120  is optically coupled to the output of DGD mitigator  110 , and the output of receiver  120  is coupled to electrical output port  121 . Transmitter  130  is coupled to electrical input port  129  and to optical output port  131 . In one embodiment, transmitter  130 , receiver  120 , and DGD mitigator  115  may share internal controller  125 . In another embodiment, transponder  100  may be used as an optical repeater, and electrical output port  121  may be coupled to electrical input port  129 . 
         [0019]    In operation, an optical input signal enters PMD mitigator  110  within transponder  100  via optical input port  111 , which then corrects or reduces any polarization mode dispersion of the optical input signal. DGD mitigator  110  then provides DGD mitigated signal  115  to receiver  120 , which transforms DGD mitigated signal  115  into a digital signal and transmits the digital signal via electrical output port  121 . Meanwhile, transmitter  130  receives an electrical input signal via input port  129 , transforms the electrical signal into an optical signal, and transmits the optical signal via output port  131 . In one exemplary embodiment, transmitter  130  may be a duo-binary transmitter, which is less susceptible to chromatic dispersion because its signal has a narrower spectrum. In other embodiments, however, other types of multi-level transmitter may be used (i.e., any other device capable of transmitting with a “symbol rate” that is a fraction of the data rate, but where each symbol conveys the information normally conveyed in multiple bits). Meanwhile, transmitter  130  receives an electrical input signal through electrical input port  129  and outputs an optical signal via optical output port  131 . As previously noted, in applications where transponder  100  is used as a repeater, the electrical output signal from receiver  120  may be directly fed into electrical input port  129  of transmitter  130 . 
         [0020]      FIG. 2  shows DGD mitigator  110 , which may integrated into transponder  100  depicted in  FIG. 1  according to one exemplary embodiment of the present invention. DGD mitigator  110  comprises a plurality of free-space optical elements arranged in a cascaded configuration, and it is capable of operating upon each of two principal modes of polarization of an optical signal. Input optical signal  105  may be split into two portions, where one portion reaches first collimator  205  and another portion reaches optical detector  245 . The portion of input optical signal  105  that reaches first collimator  205  passes through polarization controller  210 , first birefringent crystal  215 , first tunable λ/2 plate  220 , second birefringent crystal  225 , second tunable λ/2 plate  230 , third birefringent crystal  235 , and second collimator  240  (all of which are optically coupled to each other) thus producing DGD mitigated signal  115 . 
         [0021]    Optical detector  245  detects a portion of optical input signal  105  and transmits an electrical signal to PMD measuring and controlling unit  250 . PMD measuring and controlling unit  250  measures the first-order PMD (i.e., DGD) of input optical signal  105  and controls first birefringent crystal  215  and first and second tunable λ/2 plates  220  and  230  in order to correct or reduce the DGD of the optical signal. In one exemplary embodiment, PMD measurement and controlling unit  250  may be designed as described in U.S. patent application Ser. No. ______, entitled “SYSTEMS AND METHODS FOR VARIABLE POLARIZATION MODE DISPERSION COMPENSATION,” the disclosure of which is hereby incorporated by reference herein. 
         [0022]    Integration of DGD mitigator  110  within optical transponder  100  makes optical transponder  100  capable of correcting polarization mode dispersion over long-haul optical lines. One of the advantages of DGD mitigator  110  over the prior art is that it uses birefringence crystals  215 ,  225 , and  235  rather than optical fibers, thus simplifying its design. Another advantage of DGD mitigator  110  over the prior art is that it provides and discrete, binary tuning set via tunable plates  220  and  230 , as opposed to continuous tuning which is more complex and subject to errors. As will be readily recognized by a person of ordinary skill in the art, DGD mitigator  110  may be integrated within optical transponder  100  of  FIG. 1 , thus resulting in a high performance, low cost, and compact device. Moreover, when used within an integrated optical transponder  100 , DGD mitigator  110  may reduce first order PMD while a multi-level transmitter may mitigate higher order PMD (because of its narrower spectrum). 
         [0023]    Still referring to  FIG. 2 , DGD mitigator  110  is a tunable device. In addition, DGD mitigator  110  advantageously operates between the two principal states of polarizations of the line, thus providing a relative, tunable optical delay line. Polarization controller  210  is provided in front crystals  215 ,  225 , and  235 , so that the principal states of polarization are oriented along the axes of crystals  215 ,  225 , and  235 . Crystals  215 ,  225 , and  235  then fix the DGD between the two principal states of polarization of the line. 
         [0024]    Turning now to  FIG. 3 , another DGD mitigator in a reflective configuration  300  may substitute DGD mitigator  110  within integrated transponder  100  depicted in  FIG. 1 , according to one exemplary embodiment of the present invention. This particular embodiment is advantageous because it allows DGD mitigator  300  to be completely integrated into transponder  100  without enlarging its case, thus resulting in a small and compact device. Particularly, this embodiment allows crystals  215 ,  225 , and  235  to have half the size of their counterparts in  FIG. 2 . In this case, input optical signal enters DGD mitigator  300  via fiber circulator  305 , which may be positioned anywhere inside the transponder&#39;s 100 case. Mirror  310  may take the form of a highly reflecting coating on crystal  235 . For example, in the case of Yttrium Vanadate (YVO 4 ) crystals, where DGD mitigator  300  is designed to mitigate 30±5 ps, the lengths of crystals  215 ,  225 , and  235  may be 11.25 mm, 7.5 mm, and 3.75 mm, respectively, which means that the entire DGD mitigator  300  may be made smaller than 40 mm. 
         [0025]    With respect to  FIG. 4 , receiver  120  may be integrated into transponder  100  depicted in  FIG. 1  according to one exemplary embodiment of the present invention. DGD mitigated signal  115  may leave DGD mitigator  110  (or  300 ) and reach optical detector  405  of receiver  120 . Optical detector  405  converts DGD mitigated signal  115  into an electrical signal, which is then amplified by radio frequency (RF) amplifier  410  and processed by limiting amplifier  415 . Finally, receiver  120  produces an electrical output signal via output port  121 . 
         [0026]      FIG. 5  shows transmitter  130 , which may be integrated into transponder  100  depicted in  FIG. 1  according to one exemplary embodiment of the present invention. Although transmitter  130  is illustrated here as being a duo-binary transmitter, other types of multi-level transmitters may alternatively be used. In this embodiment, an input digital signal is received via input port  129  and processed by non-return to zero (NRZ) formatter  505 , precoder  510 , and electrical low-pass RF filter  515 , respectively. The output of low-pass filter  515  is fed into Mach-Zehnder (MZ) modulator  525 , which modulates the output of laser  520 , thus outputting an optical signal via optical output port  131 . 
         [0027]    When transmitter  130  is a duo-binary transmitter, rather than transmitting the original digital signal (e.g., 1 0 0 1 1 1 0), it provides the sum of two adjacent bits (e.g., 1 0 1 2 2 1). Along with the filter, the Full-Width Half-Maximum (FWHM) spectrum of the duo-binary transmitted signal is narrower than the original NRZ signal. Further, high-frequencies reduced by the use of low-pass filter  515 , which is possible in part because the spectrum of the duo-binary signal is narrow, and therefore less susceptible to such filtering. Furthermore, MZ modulator  525  and precoder  510  may be arranged in such a way that the intensity (rather than the field) of the transmitted signal is identical to that of the input digital via port  129 , thus allowing simpler operation based on the NRZ format. As a consequence of its narrower spectrum, duo-binary transmitter  130  is considerably less susceptible to chromatic dispersion than other transmitters and is more robust against CD problems. Furthermore, because of its relative simplicity and immunity to CD effects, duo-binary transmitter  130  may be especially useful in varying networks, where perfect CD cancellation is very complicated. Again, while duo-binary transmitter  130  is susceptible to DGD, it is less sensitive to higher-order PMD, therefore a DGD mitigation device such as the ones depicted in  FIG. 2  or  3  can dramatically improve performance of an optical transponder such as transponder  100  of  FIG. 1 . 
         [0028]    Although some exemplary embodiments of present invention and their advantages have been described above in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present invention is not intended to be limited to the particular embodiments of the process, machine, manufacture, means, methods and steps depicted herein. As a person of ordinary skill in the art will readily appreciate from this disclosure other, processes, machines, manufacture, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, means, methods, or steps.