Abstract:
A tunable duty cycle electrical return-to-zero (RZ) modulation method is realized through tuning of some electrical parameters of an encoder without the need for expensive and/or bulky optical pulse carver, therefore providing a universal RZ apparatus suitable for various high speed applications such as at 10 Gb/s, 40 Gb/s and 100 Gb/s. The electrical RZ modulation scheme is readily combined with other known modulation technologies on the transmitter side to support low cost RZ modulation for metro, long haul and submarine systems.

Description:
[0001]    This U.S. application Ser. No. 12/133,373 is the official continuation filing of the previously filed provisional U.S. Patent Application No. 60/933,077, filed on Jun. 4, 2007, entitled “Tunable duty cycle universal electrical return-to-zero (TDC-ERZ) modulation method and apparatus for low cost optical communication”, therefore claims the priority date of Jun. 4, 2007 of the provisional application U.S. 60/933,077, which is incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to an optical communication system with a return-to-zero (RZ) modulation, and particularly to a method and apparatus generating a widely tunable duty cycle RZ pulse data stream through electrical means without the need for expensive and/or bulky optical RZ pulse carver. The invention provides means to a low cost, small form factor, high performance optical system with great flexibility to support various transmission applications. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical fiber transmission systems are subject to distortion related to loss, noise, and nonlinearities in both the fiber and the modulation and amplification devices. One of the deleterious forms of signal distortion is that due-to fiber nonlinearities and polarization mode dispersion (PMD). The main attraction of the retuen-to-zero (RZ) modulation is its demonstrated improved immunity to fiber nonlinearities and PMD relative to non return-to-zero (NRZ) modulation. 
         [0004]    The RZ format is being used in commercial 10 Gb/s ultra long haul systems, see P. Hoffman, E. B. Basch, S. Gringeri, R. Egorov, D. Fishman, and W. Thompson, “DWDM long haul network deployment for the Verizon nationwide network,” presented at the OFC 2005, Anaheim, Calif., Paper OtuP5. For 40 Gb/s systems, the addition of phase modulation to the RZ format reduces intra-channel nonlinear effects, see S. Appathurai, V. Mikhailov, R. I. Killey, and P. Batvel, “Investigation of the optimum alternative-phase RZ modulation format and its effectiveness in the suppression of intra-channel nonlinear distortion in 40 Gb/s transmission over the standard single mode fiber,” IEEE J. Sel. Topics Quantum Electron., vol. 10, no. 2, pp. 239-249, March-April 2004. Those RZ techniques, variously referred to as alternative phase RZ (AP-RZ) or carrier suppressed RZ (CS-RZ) have been successfully used in early 40 Gb/s applications, see D. Chen, T. J. Xia, G. Wellbrock, D. Petersen, S. Y. Park, E. Thoen, C. Burton, J. Zyskynd, S. J. Penticost, and P. Mamyshev, “Long span 10×160 km 40 Gb/s line side, OC-768c client side field trial using hybrid Raman/EDFA amplifiers,” in Proc. ECOC 2005, vol. 1, pp. 15-16. 
         [0005]    In other modulation schemes, RZ is also commonly used to improve the system performance. For example, in phase shifted key (PSK) modulation schemes suitable for high bit rate applications such as for 40 Gb/s and 100 Gb/s systems, RZ version of differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK) have been shown to provide improved PMD tolerance and approximately 1 to 2-dB improvement in OSNR sensitivity relative to their current NRZ implementation but also requires more bandwidth, see E. Bert Basch, R. Egorov, S. Gringeri, and S. Elby, “Architectural tradeoffs for reconfigurable dense wavelength-division multiplexing systems,” IEEE J. of Selected Topics in Quantum Electronics, vol. 12, no. 4, July/August 2006. 
         [0006]    The two most commonly used techniques to generate optical RZ data streams either employ a sinusoidal driven intensity modulator or an actively mode locked laser, in addition to a NRZ data modulator, see, A. Ougazzaden et al, “40 Gb/s tandem electron-absorption modulator,” in Proc. OFC&#39;01, 2001, Post-deadline paper PD14. Apart from the need for two or more high power RF components, these techniques need the accurate synchronization between the data modulator and the pulse source. 
         [0007]    Yet another technique is the use of the a single NRZ driven phase modulator followed by a passive optical delay interferometer, eliminating the need for any synchronization between the two signals and considerably alleviates the requirements on the driver amplifiers, see P. J. Winzer and J. Leuthold, “Return to Zero modulator using a single NRZ drive signal and an optical delay interferometer,” IEEE Photon. Technol. Lett., vol. 13, no. 12, pp. 1298-1300, December 2001 
         [0008]    In yet another approach, a variable duty cycle RZ pulse can be generated using cascaded optical modulators, see J. C. Mauro, S. Raghavan, S. Ten, “Generation and system impact of variable duty cycle alpha-RZ pulses,” J. Opt. Commun. Vol. 26, pp. 1015, 2005. This is different from all other RZ pulse generation scheme in that the duty cycle of the pulse is variable. However it is implemented in optical domain and therefore expensive to the systems. 
         [0009]    In summary, optical RZ pulses are mostly generated by optical means, and commonly implemented by the separate cascaded Mach-Zehnder modulators driven by an NRZ data stream for one section and a clock pulse carver for the second section. The approach requires precise control of amplitude and phase, as well as separate microwave amplifiers for the two sections. In all cases, RZ format is more complex and costly to implement in its current optical format. 
         [0010]    Therefore, due to the advantages RZ has over NRZ modulation, there is a need for cost effective solutions to generate the RZ modulation with less cost, less size, less power and better performance so that it can be more readily integrated into more compact form factors for the transmitters, such as for the small form factor modules for 40 Gb/s and 100 Gb/s. 
         [0011]    To the best knowledge of the inventors, there is not any RZ pulse generator that is implemented in pure electrical domain and at the same time has a widely tunable duty cycle. It is therefore the objective of the present invention to generate a tunable duty cycle RZ pulse with a universal electrical means, reducing the size, cost, and increasing the flexibility of the systems to adapt to various applications in metro and long haul networks. 
       SUMMARY OF THE INVENTION 
       [0012]    This U.S. application Ser. No. 12/133,373 is the official continuation filing of the previously filed provisional U.S. Patent Application No. 60/933,077, filed on Jun. 4, 2007, entitled “Tunable duty cycle universal electrical return-to-zero (TDC-ERZ) modulation method and apparatus for low cost optical communication”, and incorporated herein by reference. 
         [0013]    The present invention is an electrical RZ pulse generating method and apparatus that has a widely tunable duty cycle that covers the most desirable duty cycle of 33%, 50% and 67% in the single apparatus for high speed 10 Gb/s, 40 Gb/s and 100 Gb/s signals and low cost RZ modulation. 
         [0014]    Briefly, as shown in  FIG. 1(   a ) and  FIG. 1(   b ), a preferred embodiment of the present invention includes an AC coupled dual differential input limiting amplifier with a DC-driven bias tee. The AC coupled dual input ports  103  are drive by the incoming NRZ data  102  and the input clock  101  respectively. The DC bias voltage  110  is used for the continuous adjustment of bias voltage of the limiting amplifier and therefore the resulting duty cycle of the generated RZ pulse. In addition, in  FIG. 2(   a ),  FIG. 2(   b ) and  FIG. 2(   c ), some typical examples are shown for a RZ transmitter with present invention.
       1)  FIG. 2(   a ) shows a tunable duty cycle electrical RZ driven optical differential coded binary modulation, where a duobinary encoder  109  is inserted into the NRZ data input port and the output port of the encoder is then connected into the input port  102  of the apparatus, in such a tunable duty cycle optical duo-binary transmitter is obtained, without the use of more expensive optical MZ modulator for RZ modulation, as is normally implemented. The present invention offers more features, functions, flexibilities, but less cost and smaller size.   2)  FIG. 2(   b ) shows a traditional NRZ based optical duobinary (NRZ-ODB) transmitter, where both data and the inverted data (data_bar) are fed into the input ports  101  and  102  of the differential limiting amplifier  105 , followed by a duobinary encoder  109  to drive a MZ modulator  107 . This is a very simple implementation of NRZ ODB transmitter, using the similar architectural design as those in the design in  FIG. 2(   a ).   3)  FIG. 2(   c ) shows a traditional NRZ transmitter using the same design as in  FIG. 2(   a ) and  FIG. 2(   b ) with differential limiting amplifier  105 .       
 
         [0018]    Several other preferred embodiments and some application examples for combining this type of RZ pulse generating apparatus with other modulation formats are also shown in  FIG. 3 ,  FIG. 4 , and  FIG. 5 . 
         [0019]    In summary, a universal design can be implemented based on the present invention such that not only a widely tunable duty cycle electrical RZ pulse generating apparatus is produced cost effectively, but also, other types of transmitters can also be produced by populating or depopulating the building blocks of present invention (duobinary encoder in this case). All in the same design with some by-pass functions to the encoders. 
         [0020]    One of the advantages of the present invention is that it can be used to convert many different modulation formats from other pulse formats such as NRZ to RZ cost effectively, and with smaller size for further package integration. For example, the traditional NRZ modulation, the NRZ duobinary modulation, the optical single side band (OSSB) NRZ modulation, the DPSK modulation, and the DQPSK modulation, can be converted into their corresponding RZ format. 
         [0021]    The other advantage is its smaller size of the present invention, since it eliminates some of the bulky and/or expensive optical components, such as LiNbO3 or InP Mach-Zehnder modulator. Because of this, many transmitters that employs RZ format can be integrated into the small form factor modules or XFP pluggable package for 10 Gb/s, 40 Gb/s and 100 Gb/s applications. 
         [0022]    The other advantage of the present invention is its widely tunable duty cycle, which is suitable for many different applications, such as for metro, long haul, and submarine optical transmission systems due to the needs for different duty cycles. 
         [0023]    The other advantage of the present invention is its unique implementation, which produces nearly identical RZ pulse with zero chirps, compared with the optical RZ pulse generation for high speed transmitters. 
         [0024]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the best mode operations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0025]    The invention will now be described in great details with reference to the drawings, in which 
           [0026]      FIG. 1  is one of the preferred embodiments of the present invention for transmitter with tunable duty cycle electrical RZ modulation (ERZ). 
           [0027]      FIG. 2 : Several other preferred embodiments and application examples for combining the present invention with other modulation formats. 
           [0028]      FIG. 2(   a ): Tunable duty cycle electrical RZ (ERZ) driven optical differential coded binary modulation (this invention) 
           [0029]      FIG. 2(   b ): Tunable duty cycle electrical RZ (ERZ) transmitter with also electrical duobinary modulation (DB) generation. 
           [0030]      FIG. 2(   c ): Tunable duty cycle electrical RZ transmitter (ERZ). 
           [0031]      FIG. 3 : One of the embodiments of electrical based RZ-DPSK and electrical RZ-DQPSK utilizing the present invention in the electrical domain to remove the need for expensive and/or bulky optical RZ pulse carver such as MZ modulator in their traditional optical domain implementation. 
           [0032]      FIG. 4 : One of the embodiments for NRZ-DQPSK transmitter using single MZ modulator with dual drive in order to further reduce the cost of DQPSK transmitter implementation, in addition to the benefit resulting from the present invention as shown in  FIG. 1 . The implementation of DQPSK itself is also further simplified utilizing a single MZ modulator with dual electrical drive, instead of the two parallel MZ modulators. 
           [0033]      FIG. 5 : One of the embodiments for electrical RZ-DQPSK transmitter using single MZ modulator with dual drive in order to reduce the cost of RZ-DQPSK transmitter implementation. The present invention as shown in  FIG. 1  is used to produce electrical RZ-DQPSK transmitter. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]    Accordingly to one of the preferred embodiments of the present invention, a tunable duty cycle electrical RZ pulse generating apparatus to convert NRZ data stream to RZ data stream is shown in  FIG. 1(   a ) and  FIG. 1(   b ). The incoming NRZ data input  102  and the clock input  101  are both fed through AC coupling  103 , into the electrical RZ (ERZ) encoder and driver, which is made of a differential limiting amplifier  105  and a Bias Tee  104  for the limiting amplifier. The DC voltage  110  of the Bias-Tee  104  can be used as the single parameter to adjust the RZ pulse duty cycle as shown in the diagram in  FIG. 1(   b ). The different limiting amplifier  105  is driven by the NRZ data  102  and the associated clock  101 , therefore generates a RZ pulse output with a tunable duty cycle based on the Bias Tee  103 &#39;s DC bias voltage  110 , which can be continuously fine tuned.  FIG. 1(   b ) shows the input NRZ data stream, the input clock, and the DC bias on the Bias-Tee, and the resulting electrical RS pulse stream. The benefits of this implementation are multi-folded. Firstly, a single RF driver to provide RZ encoding and RZ pulse amplification is needed. There is no need for a 2nd MZ modulator and clock driver for RZ pulse generation in optical domain. Secondly, comparing to the conventional RZ generation, the high-speed AND gate can be removed, which results in further power and cost reduction. Thirdly, using a simple bias-tee and DC offset for the input clock, the duty cycle of the RZ data can be adjusted by greater than ±15% from its default 50% value. Thus this RZ transmitter design is also capable for submarine (33% duty cycle) and terrestrial ultra-long haul, e.g. CSRZ (67%) application. Fourthly, this scheme can be combined with differential encoder to be used for RZ-DPSK applications. 
         [0035]    According to one of the other embodiments, several preferred implementations and applications for, combining the present invention with other modulation formats are shown in  FIG. 2 . As shown in  FIG. 2(   a ), a tunable duty cycle electrical RZ driven optical differential coded binary modulation is presented based on the current invention, where an duobinary encoder  109  is inserted into the NRZ data input port and the output port of the encoder is then connected into the input port  102  of the data input port, in such a tunable duty cycle RZ duobinary transmitter is obtained, without the use of more expensive optical MZ modulator, as is normally implemented. The present invention offers more features, functions, flexibilities, but less cost and smaller size. The encoder portion is a typical data pass summed up with its one-bit delay line (the delay time can be optimized to be less or more than one bit period) with the use of exclusive OR gate logic (XOR gate), or a FIR filter (finite impulse response filter), or a FFE based EDC (electrical dispersion compensator) chip with 3 taps and each tap has a half bit period of delay. As shown in  FIG. 2(   b ), a tunable duty cycle transmitter with an electrical RZ modulation and also an electrical duobinary signal encoding is presented, where both data and the inverted data (data_bar) are fed into the input ports  101  and  102  of the differential limiting amplifier, followed by an electrical duobinary encoder (also the present invention) to drive a MZ modulator. This is a very simple implementation of RZ duobinary transmitter, using the similar architectural design as those in the design in  FIG. 2(   a ). As shown in  FIG. 2(   c ), a tunable duty cycle transmitter with electrical RZ modulation, but without the duobinary encoding is presented using the same design with differential limiting amplifier. In summary, a universal design is implemented based on the present invention such that not only a widely tunable duty cycle electrical RZ pulse generating apparatus is produced cost effectively, but also, other types of transmitters can be produced by populating or depopulating the building blocks (pre-coder and en-coder in this case) in the same design with some by-pass functions to the pre-coders/encoder  109  placed either in the NRZ data incoming path right before the input to the AC coupling port  104  of the differential limiting amplifier  105 , or right after the output of the differential limiting amplifier  105 . 
         [0036]    According to another embodiment of the present invention, an electrical RZ-DPSK transmitter and an electrical RZ-DQPSK transmitter utilizing the present invention in the electrical domain to remove the need for expensive and/or bulky optical RZ pulse carver such as MZ modulator in their traditional optical domain implementation is shown in  FIG. 3 . For electrical RZ-DPSK, instead of using NRZ “Data” and “Data_Bar” to drive the first MZ modulator, the embodiment as shown in  FIG. 1  is used to generate two output RZ pulse streams of the respective differential limiting amplifiers from individual “Data” signal and “Data_Bar” signal sampled and limited by the input NRZ clock, to drive the first MZ modulator. Since the resulting pulse is now an optical RZ stream, there is no need for the second MZ modulator as the RZ pulse carver and therefore, it can be removed. In this case, the MZ modulator can be the standard MZ normally used for DPSK modulation. It can also be the dual drive MZ modulator designed for DQPSK modulation. For the electrical RZ-DQPSK transmitter, similarly, instead of using NRZ “Data” and “Data1” to drive the two parallel MZ modulators, the embodiment as shown in  FIG. 1  is used to generate two output RZ pulse streams of the respective differential limiting amplifiers  105  from individual “Data” signal and “Data1” signal sampled and limited by the input NRZ clock, to drive the first two MZ modulators in parallel. Since the resulting pulse stream is now an optical RZ stream, there is no need for the third MZ modulator cascaded after as the RZ pulse carver and therefore, it can be removed. 
         [0037]    According to another embodiment of the present invention, a NRZ-DQPSK transmitter using single MZ modulator with dual drive is shown in  FIG. 4 . In order to further reduce the cost of DQPSK transmitter implementation, in addition to the benefit resulting from the present invention as shown in  FIG. 1 , the implementation of DQPSK itself can also be further simplified utilizing a single MZ modulator with dual electrical drive, instead of the two parallel-MZ modulators. The diagram is shown for the preferred embodiment for low cost DQPSK modulator, where a single MZ modulator is used. Firstly, the differential pre-coder  202  is used convert the input data stream into two tributary data stream “Data1” and “Data2”. Then each of the data streams is used to drive independently one of the arms of the single MZ modulator with its own bias voltage. In each of the MZ modulator arms, there is an independent phase control section that can be used to set the phase delay in each of the arms independently. Normally, for DQPSK application, the phase in one arm is set to zero and in another is set to 90 degrees. If the input data into the pre-coder is the NRZ stream, and then this embodiment of DQPSK is the NRZ-DQPSK. It has the advantage over traditional implementation that it reduces the cost, size, power consumption, and therefore allows for the integration into a much smaller package, such as XFP and small form factor modules. This implementation is flexible and versatile in that the two independent drives into the MZ modulator arms can be of various amplitude and phase relationship in order to produce various types of phase or amplitude modulation formats. The details will not be discussed here, but anyone with ordinary skills can derive obvious alterations based on this. 
         [0038]    According to another embodiment of the present invention, an electrical RZ-DQPSK transmitter using single MZ modulator with dual drive is shown in  FIG. 5 . 
         [0039]    In order to further reduce the cost of RZ-DQPSK transmitter implementation, the present invention as shown in  FIG. 1  is used to produce an electrical RZ-DQPSK transmitter. Firstly, the differential pre-coder is used to convert the input data stream into two tributary data stream “Data1” and “Data2”. Then each of the data streams is then paired with the input clock signal to feed into one AC-coupled differential limiting amplifier  105  to produce the electrical RZ pulse stream based on the input data stream “Data1” or “Data2”. The bias voltage  110  of the two limiting amplifiers  105  are set as the same in order to produce the same output ERZ pulse duty cycle for input data stream“Data1” and “Data2”. The two output electrical RZ pulse streams from the two limiting amplifiers are then fed into the single dual drive MZ modulator  107  to drive the two arms independently. In each of the MZ modulator arms, there is an independent phase control section that can be used to set the phase delay in each of the arms independently. Normally, for DQPSK application, the phase in one arm is set to zero and in another is set to 90 degrees. This type of electrical RZ-DQPSK transmitter has the advantage that it is much simpler in design, smaller in size, much less expensive in cost, and offers similar or better performance, and can be easily integrated into a much smaller package, such as XFP and small form factor package, which cannot be achieved currently with the exiting solutions due to its bulky size and need of more optical components. 
         [0040]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the present invention.