Abstract:
A method for tunable optical pulse width generation comprises varying chirp of the signal of the pulse and concurrently varying dispersion of the signal. A system for tunable pulse width generation comprises a phase modulator providing the chirp modulation by modulating peak to peak phase of an optical signal and a tunable dispersion element providing the chromatic dispersion to the optical signal. A system for tunable pulse width return to zero optical signal transmission comprises at least one optical signal, a plurality of amplitude modulators for modulating the signal and a tunable pulse width generator comprising a phase modulator providing the chirp modulation and a tunable dispersion element providing the dispersion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of and priority to Provisional U.S. Patent Application Serial No. 60/367,040, filed Mar. 21, 2003, entitled TUNABLE PULSE WIDTH GENERATION OF RETURN-TO-ZERO FORMAT FOR SYSTEM OPTIMIZATION, the disclosure of which is hereby incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention is broadly related to fiber optic communications and specifically to systems and methods for providing tunable pulse width generation of return-to-zero format for system optimization.  
         BACKGROUND OF THE INVENTION  
         [0003]    For 10-Gb/s and greater wavelength division multiplexed (WDM) transmission systems, channels experience a confluence of channel-degrading effects that relate to dispersion and nonlinearity. For example, dispersion can cause pulse broadening because different frequency components of a disperses optical signal will transmit at different speed. Also, there are a variety of nonlinear effects in optical fiber, including but not limited to self-phase-modulation (SPM), cross-phase-modulation (XPM), and four-wave-mixing (FWM). These various nonlinearities have various physical background causes. Typically, these nonlinearities can cause signal degradation in either single channel or WDM systems. Previous prior art attempts to solve these degrading effects typically only consider either dispersion or nonlinearities, primarily focusing on dispersion. The prior art fails to provide a method dealing with the combined effects of these degrading factors.  
           [0004]    Resultantly, prior art higher-speed, longer-distance, or high spectral efficiency systems must be designed very carefully such that the data integrity will remain high at the receiver. Rigorous standards are required for deployment such that extensive fiber-link measurements are required before system design; equipment must be manufactured to exact values to account for various deleterious fiber effects; and the fiber plant cannot change after system deployment for fear of a system outage. Moreover, any system upgrades require the replacement of much of the existing terminal equipment since management of the fiber effects will change significantly. All these factors incur large operational costs for systems providers as well as severe constraints on the network operators.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to a systems and methods which provide tunability in data pulse width for return-to-zero (RZ) formatted signals launched into an optical system. The present systems and methods may be used to address the combined effects of degrading factors such as dispersion and nonlinearities. To date, although some experimental results show the performance of different relative pulse widths. See, G. C. Gupta et al., OFC 2000, paper TuJ7; and A. Sano et al., JLT, vol. 16, pp. 977-985, 1998; the disclosures of which are incorporated herein by reference. No demonstration has shown practical tunable pulse-width management. The present systems and methods enable tunability and flexibility in terminal fiber optic equipment to enable proper system performance even under conditions that may vary over time. Another valuable feature of the present invention is that it provides pulse-width management for several WDM channels simultaneously.  
           [0006]    The present invention provides heretofore undemonstrated generation of tunable pulse width for RZ signal transmissions. Embodiments of the present invention provide tunable pulse width management using a phase modulator and a tunable dispersion element. The only tunable pulse width generation system previously known in the art uses electro-absorption modulation. The present systems and methods are more straightforward and stable.  
           [0007]    Typically, phase modulator, without dispersion compensation has been used to generate CRZ pulses. CRZ signals are discussed in J. -X Cai et al., ECOC 2000, paper PD 1.6, the disclosure of which is incorporated herein by reference. Such CRZ signals can be transformed into a RZ signals following dispersion management. Advantageously, the present invention may be used to alter a RZ transmission system to generate Chirped RZ (CRZ) pulses which converge to a dispersion managed soliton format. Soliton format signals are discussed in R. -M. Mu et al., JLT, 2002, the disclosure of which is incorporated herein by reference.  
           [0008]    When compared to mode-locked lasers commonly used to create short pulses, the stability and tunability of the present systems and methods can enhance the system robustness and increase a system&#39;s power margin for both single channel and WDM systems. At higher data rates such as greater than 40 Gb/s, dynamic pulse compression down to several picoseconds is obtainable, particularly if half-wave voltage of phase modulators can be further reduced, to ensure high spectral efficiency and high capacity transmission. In other words, as long as the driving voltage limit of current phase modulator can be reduced larger chirp values can be obtained and combined with an appropriate tuned dispersion to compress the signal more, through pulse compression.  
           [0009]    Applications for the present invention include, by way of example, high bit-rate digital fiber transmission systems, reconfigurable optical networks, and transmission of analog radio-frequency and millimeter-wave signal over fiber. The present invention is well adapted for use with RZ modulation in both WDM systems and single channel systems. All the components used by the present systems and methods are readily and commercially available for existing systems. Therefore, the present systems and methods are readily applicable to RZ transmission systems, whether single-channel or WDM systems.  
           [0010]    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  
       [0011]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0012]    [0012]FIG. 1 is a diagrammatic illustration of an embodiment of a tunable pulse width generator in accordance with the present invention, shown with a testbed loop;  
         [0013]    [0013]FIG. 2 is a diagrammatic illustration of a system embodiment for tunable pulse width transmission in accordance with the present invention.  
         [0014]    [0014]FIG. 3 is a graph of the interaction of chirp and dispersion at various pulse lengths;  
         [0015]    [0015]FIG. 4 is a graph of measured power penalties for pulse widths at various residual power dispersion values;  
         [0016]    [0016]FIG. 5 is a graph of achievable transmission distances in accordance with the present invention for pulse widths at various residual power dispersion values;  
         [0017]    [0017]FIG. 6 is a graph of power penalties over transmission distances for various pulse widths with a 0.4 ps/nm/km residual power dispersion value;  
         [0018]    [0018]FIG. 6 is a graph of power penalties over transmission distances for various pulse widths with a 0.8 ps/nm/km residual power dispersion value; and  
         [0019]    [0019]FIG. 7 is a graph of power penalties over transmission distances for various pulse widths with a −0.2 ps/nm/km residual power dispersion value.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    An embodiment of method  100  for tunable pulse width generation is shown in FIG. 1. Taking advantage of interaction between chirp and dispersion, a pulse can be either compressed or broadened depending on the values of chirp and dispersion. Chirp may be defined as peak to peak phase change. FIG. 1 shows tunable pulse width generation  100  by interaction of chirp generator  101  and chromatic dispersion provided by dispersion element  102 . Both chirp (amplitude) and dispersion value are preferably tunable in order to provide tunable pulse width generation. In accordance with the present systems and methods, when both chirp and dispersion are tunable, the generation of RZ formatted data is insured. Otherwise, the generated signal may be chirped RZ data, which will decrease spectral efficiency.  
         [0021]    An embodiment of system  200  for tunable pulse width RZ transmission is shown in FIG. 2. In the illustrated example four WDM channels λ 1 , λ 2 , λ 3 , and λ 4  located, in this example, at 1553.4, 1554.2, 1555.0, and 1555.8 nm are multiplexed at  201  and RZ modulated using two cascaded electro-optic amplitude modulators  203  and  204 . First amplitude modulator  203 , of the example of FIG. 2, may be driven by a 2 23 −1 (Pseudo Random Bit Sequence (PRBS), or the like, at a 10-Gb/s data rate provided by source  202 . Source  202  may take the form of a commercially available pattern generator operated at 10 Gb/s to generate a NRZ signal, applied to the first modulator. In the Example of FIG. 2, an RZ signal with a 50-ps pulse width is obtained by driving second amplitude modulator  204  with a clock signal which may also be provided by source  202 . Phase modulator  210 , of FIG. 2, also driven by the clock of source  202 , followed by a multi-channel tunable dispersion element  212  are used to generate tunable pulse widths in the manner discussed above in relation to FIG. 1. Phase modulator  210  and tunable dispersion element  212  make up tunable pulse width generator  215 . Phase modulator  210  is used to provide peak to peak phase changes for the signals. Different tunable dispersion elements may be used as tunable dispersion element  212 , such as, by way of example, fiber Bragg gratings disclosed in K. -M. Feng et al., PTL, vol. 11, pp. 373-375, 1999; and B. J Eggleton et al., JLT, pp. 1418-1832, 2000; the disclosures of which are incorporated herein by reference.  
         [0022]    Employing the example system of FIG. 2, pulse widths ranging from 50 ps to 10 ps can typically be realized via the interaction of different amounts of chirp and dispersion as shown in FIG. 3. The amplitude of the driving signal to the phase modulator can be up to 8 Volts (V), the half-wave voltage of the modulator. Applying the data of FIG. 3 to the example of FIG. 2, ˜60 ps/nm dispersion and 8V on phase modulator  210 , corresponding to a chirp value of π, generates ˜10-ps pulses. The driving voltage limit of existing phase modulators is typically about 8V. In accordance with the present invention, reduction in this driving voltage will provide larger chirp values, and when combined with dispersion as described below, will yield more compresses signal pulses. As discussed below, pulse widths of about 10 ps are obtainable using example system  200  of FIG. 2. However, if the half-wave voltage of the phase modulator decreases, pulses of only several picoseconds can be used to carry out higher data rate transmission s for 40-Gb/s or 160-Gb/s systems. A mechanically stretched, sampled, nonlinearly chirped FBG may be used as tunable dispersion element  212  to tune the dispersion from 600 to 1900 ps/nm for the four channels. Spool of single-mode fiber (SMF)  214  may be used to offset the grating to obtain a desired positive dispersion between 60 and 160 ps/nm. For single channel transmission, a different nonlinearly-chirped FGB may be used with a tuning range between 100 and 500 ps/nm with spool of SMF  214  used to offset the grating. For best performance, the group delay ripple for these example gratings is preferably ˜±10 ps.  
         [0023]    To demonstrate the present invention recirculating loop testbed  217  may be employed. After pulse shaping, the signal may be transmitted, via Acousto-optic (AO) switch  216  through recirculating loop  217 . Recirculating loop  217  may, as in the example of FIG. 2, comprise three Erbium-Doped Fiber Amplifier (EDFAs)  220 ,  221  and  222 , 78 km of SMF  225 , and 12 km of Dispersion Compensating Fiber (DCF)  226 . Other spools of SMF  227  with lengths varying from 0 to 4.4 km may be used to change the residual dispersion while maintaining the Optical Signal Noise Ratio (OSNR) of the link. A long-period grating (LPG) may be used as gain equalizer  230  and to reduce the Amplified Spontaneous Emission (ASE) noise in loop  217 . The signal may exit loop  217  via AO switch  231 . After transmission, optical preamplifier  235  is placed before receiver  240  and Optical Filter  237  to increase the sensitivity of receiver  240 . Power penalties, including the OSNR degradation, are measured by comparing the receiver sensitivity such as at a 10 −9  bit-error-rate (BER) with back-to-back sensitivity (in this example −34 dBm). Further analysis of the signal may be provided by coupled-in Optical Spectrum Analyzer (OSA)  242 . The results of testing using testbed  217  indicate that RZ pulse widths ranging from 50 ps to 10 ps are obtainable in the example of FIG. 2 using different combinations of chirp and dispersion. Attenuators  228  and  233  may be used to balance the optical power for the transmission or measure the transmission performance.  
         [0024]    [0024]FIG. 4 graphs power penalties  401  after 600-km single channel transmission with varying pulse widths  402  and residual dispersion values comparing transmission performance of a single-channel for varying pulse widths. FIG. 4 shows example measured power penalties after 600 km of transmission for three different residual dispersion values, 0.4, 0.08 and −0.2 ps/nm/km. As applied to the example of FIG. 2 testbed loop  217 , the optical input power provided by EDFA  220  is set to 6 dBm at 78 km of SMF  225  and at EDFA  221  the power input to DCF  226  is set to ˜−1 dBm. Three typical residual dispersion values are obtained by changing the length of small spool of SMF  227 . These values, 0.4 ps/nm/km, 0.08 ps/nm/km, and −0.2 ps/nm/km, are generally equivalent to a ˜4% variation in link dispersion. For 50-ps RZ pulses, this amount of dispersion variation reduces the transmission distance at ˜5 dB power penalty from 1200 km to 600 km.  
         [0025]    [0025]FIG. 5 graphs transmission distances  501  achieved at ˜5 dB power penalty after tuning pulse width  502  for residual dispersion values of 0.4, 0.08 and −0.2 ps/nm/km. However, this data shows that this distance can be extended to the range of 2400 km to 1000 km by using pulse widths in the 25 to 35 ps range. As indicated by FIG. 5, single channel transmissions may have an optimum range of pulse widths and pulse widths outside this range may be more sensitive to dispersion variations.  
         [0026]    [0026]FIGS. 6 through 8 graph performance (power penalty  601 ,  701  and  801  versus transmission distance  602 , 702  and  802 ) of a typical channel (1555.0 nm) for different pulse widths in a four channel 10-Gb/s WDM system with 0.8 nm channel spacing under different link residual dispersion values (D). The residual dispersion values are 0.4 ps/nm/km in FIG. 6, 0.08 ps/nm/km in FIG. 7, and −0.2 ps/nm/km in FIG. 8. In FIGS. 6, 7 and  8  the solid circle data points correspond to use of a 50-ps pulse; the open circle data points correspond to use of a 35-ps pulse; and the solid square data points correspond to use of a 25-ps pulse. A sampled grating is stretched to tune the dispersion for all four channels simultaneously in the Examples graphed in FIGS. 6 through 8, consequently obtaining the same pulse width for all the channels. The channels in these examples are decorreleted after propagating through a spool of dispersive fiber. The optical power inputs for 78 km of SMF  225  and 12 km of DCF  226  are set to ˜4 dBm and −2 dBm per channel, respectively by EDFAs  220  and  221 , respectively. The transmission performance of a typical channel at 1555.0 nm is measured. For the residual dispersion value of 0.4 ps/nm/km, the transmission distances achieved at 4 dB power penalty are 1900, 1600, and 800 km for pulse widths of 50, 35, and 25 ps, respectively. As the residual dispersion value approaches zero (0.08-ps/nm/kml), the distances change to 800, 1300 and 1200 km for the above listed pulse widths. Some of the curves, such as the 50-ps pulse width and 0.4 ps/nm/km curves, show the effects of fiber nonlinearities. Since nonlinearities, including both self-phase modulation (SPM) and cross-phase modulation (XPM), are key issues in WDM systems, managing pulse width at the transmitter enables flexibility in terms of system optimization.  
         [0027]    Although the present invention and its advantages have been described 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 application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, 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, compositions of matter, means, methods, or steps.