OPTIMAL DISPERSION COMPENSATION FOR RETURN-TO-ZERO TRANSMISSION OVER NON-ZERO DISPERSION SHIFTED FIBERS

Abstract: Dispersion compensation over a plurality of fiber spans includes providing a dispersion compensation module every N spans, where N>1. In addition to inline dispersion compensation, dispersion compensation modules may be provided in one or both of a transmitter and a receiver that the plurality of fiber spans connect. The dispersion compensation modules may compensate for different amounts of dispersion. Each inline dispersion compensation module may compensate for more dispersion than a dispersion compensation module in the transmitter or the receiver. Non-uniform dispersion compensation is particularly useful when the transmission is in return-to-zero format.

Description: OPTIMAL DISPERSION COMPENSATION FOR RETURN-TO-ZERO TRANSMISSION OVER NON-ZERO DISPERSION SHIFTED FIBERS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to optimal dispersion compensation for return-to- zero (RZ) transmission, particularly for cross-channel non-linearities, and, more particularly, for longer transmission distances. 2. Description of Related Art As the demand for higher transmission capacity over a single fiber increases, several key requirements for dense wavelength division multiplexing (DWDM) emerge. Some of these are higher bandwidth, more wavelength channels, and narrower wavelength channel spacing. Higher bit rates require higher power per channel in order to realize acceptable optical signal-to-noise ratios (OSNR). This increased power per channel coupled with the other two requirements of an increased number of channels and a narrowed channel spacing may result in excessively high cross channel nonlinearities. These nonlinearities most significantly include four-wave mixing (FWM) and cross phase modulation (XPM). Conventionally, in most 10 Gbit s systems, data are transmitted using a non-return- to-zero (NRZ) format. The NRZ format is simple, has a relatively high spectral efficiency and may be used with numerous commercially available radio frequency (RF) components. Recently, the return-to-zero (RZ) format has gained attention. While the RZ format has a lower spectral efficiency than the NRZ format, the RZ format possesses the following advantages. The RZ format sensitivity of the optical preamplifier receiver is 2-3 dB higher than for the NRZ format. The RZ format is more tolerant to self phase modulation (SPM). The RZ format is more tolerant to polarization mode dispersion (PMD). The use of the RZ format has been explored in configurations for submarine transmission. In these systems, the amplifier spacing is relatively short, e.g., around 40 km, allowing low power per channel to be used. Thus, these submarine systems would be exploiting the linear advantages of the RZ format, i.e., higher tolerance to amplified spontaneous emission (ASE) noise and PMD. For terrestrial systems, the span length is typically 80-100 km, thus requiring a much higher power per channel to achieve adequate optical signal-to-noise ratio (OSNR) unless special techniques, such as distributed Raman amplification are used. Thus, cross- channel nonlinearities become the major transmission impairments especially when using relatively low dispersion non-zero shifted (NZDS) fibers and narrow channel spacing. Various techniques have been implemented to suppress cross-channel nonlinearities in DWDM systems as discussed below. One approach used to reduce cross channel nonlinearities is the use of a transmission fiber with a high dispersion and a high effective area. A standard single mode fiber (SMF) can be used to suppress FWM and XPM impairments for channel spacing as low as 50 GHz. However, the use of a SMF requires a large number of dispersion compensating modules (DCMs) to compensate for the high SMF fiber dispersion. Current DCMs have a high insertion loss, which minimizes the number of other network elements that could be inserted in the link. Another approach involves polarization interleaving. Channels are arranged so that polarizations of adjacent channels are orthogonal to each other to reduce FWM and XPM. Both of these cross-channel nonlinearities are dependent on the relative polarization of the channels producing the nonlinear crosstalk. However, polarization interleaving limits optical networking capabilities. Add/drop functionality within the link may be difficult unless the add port automatically aligns the polarization of the added channel to be orthogonal to that of adjacent channels. In yet another approach, distributed Raman amplification launches a strong Raman pump signal in the transmission fiber before the erbium doped fiber amplifier (EDFA). The contra-propagating pump amplifies WDM channels in the transmission fiber, allowing the launch power to be decreased. The lower launch power reduces fiber nonlinearities. SUMMARY OF THE PRESENT INVENTION The present invention is therefore directed to dispersion compensation, particularly for cross-channel nonlinearities, for RZ format transmission which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art. In one aspect of the present invention, dispersion compensation system is provided for an optical communications system having a transmitter and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans, the dispersion compensation system including a first dispersion compensation module in at least one of the transmitter and the receiver, and a second dispersion compensation module before at least one of the plurality of fiber spans, wherein the first and second dispersion compensating modules compensate for different amounts of dispersion. The second dispersion compensation module may compensate for more dispersion than the first dispersion compensation module. The optical communications system may transmit signals in RZ format. The second dispersion compensation module may include an inline dispersion compensation module at every N spans, where N>1, e.g., N = 3 or 4. The fibers of the plurality of fibers may be non-zero dispersion shifted fibers. The above and other objects of the present invention may be realized by providing a dispersion compensation system for an optical communications system having a transmitter and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans, the dispersion compensation system including an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. The optical communications system may transmit signals in RZ format. The dispersion compensation system may include an additional dispersion compensation module in at least one of the transmitter and receiver. N may be 3 or 4. The fibers of the plurality of fibers may be non-zero dispersion shifted fibers. In another aspect of the present invention a dense wavelength division multiplexed optical communication system in return-to-zero (RZ) format is provided. It includes an RZ transmitter, an RZ receiver; a plurality of fiber spans connecting the RZ transmitter and the RZ receiver; and an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. The system may have N = 3 or 4. The may include an additional dispersion compensation module in at least one of the RZ transmitter and RZ receiver. The inline dispersion compensation module and the additional dispersion compensation module may compensate for different amounts of dispersion. The inline dispersion compensation module may compensate for more dispersion than the additional dispersion compensation module. In yet another aspect of the present invention a method for compensating for dispersion in an optical communications system is provided having a transmitter and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans including providing an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. An additional dispersion compensation module may be provided in at least one of the transmitter and receiver. These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which: Figure 1 is schematic diagram of a general transmission system; Figure 2 is a schematic diagram of an RZ transmitter to be used with the present invention; Figure 3 is a plot of bit error rate (BER) versus input power for both NRZ and RZ data formats; Figure 4A is a schematic diagram of a transmission system to be used with the present invention; Figure 4B is a schematic diagram of a transmission system with the dispersion compensation of the present invention; and Figure 5 is a plot of average Q-factor (Qave) in dB versus channel power for the different dispersion maps shown in Tables 1 and 2. Detailed Description of the Preferred Embodiments A schematic of a general communication system is shown in Figure 1. A plurality of fiber spans Si-Sj, joined by a respective plurality of amplifiers Ai-Aj-i, connect a transmitter 2 and a receiver 4. As noted above, while traveling between the transmitter 2 and receiver 4, the light experiences broadening from many different sources which degrades the ability of signals to be communicated accurately. This broadening or dispersion must be compensated at some point in the communications path. For the NRZ format, this compensation is optimal when provided between every span, i.e., at each amplifier. However, for the RZ format, in accordance with the present invention, this compensation is optimal if performed every N spans, where N>1. The details of an example of a configuration verifying the optimization for the RZ format, and the acceptability thereof, are discussed below. While any manner of providing an RZ format signal may be used in conjunction with the present invention, a simple RZ transmitter based on a NRZ transmitter is illustrated in Figure 2. As shown therein, an RZ transmitter 10 includes 32 lasers 14 configured into two lx 16 arrays 16, a 1 x 2 coupler 18, a Mach Zehnder modulator (MZM1) 20, preamplifier 24, e.g., an erbium doped fiber amplifier (EDFA), a dispersion compensating module (DCM) 26, a power amplifier 28, e.g. an EDFA, and a variable optical attenuator 12 which controls the total launched power. The units of commercial DCMs are provided as DCM-X where X is the equivalent length in km of standard single mode fiber dispersion for which the DCM would compensate. RZ transmitter 10 is created by inserting Mach Zehnder modulator (MZM2) 22 between preamplifier 24 and the DCM 26. This MZM2 is driven by an amplified clock signal with its phase adjusted to carve out RZ pulses from the center of each bit in the modulated signal. The duty cycle of the RZ pulses is roughly 50%. In one embodiment, MZM1 and MZM2 are Li:NbO 3 , zero-chirp, Mach-Zender modulators. In another embodiment, lasers 14 match the ITU-T nominal central frequency grid and minimal channel spacing of 100 GHz. The first channel wavelength is λl = 1532.68 nm (195.6 THz) and the last channel wavelength is A32 = 1557.36 nm (192.5 THz). On the receiving end, an NRZ format receiver is used to detect incoming RZ data. No modifications need to be made to a conventional NRZ receiver. All of the electronics have the same bandwidth as in the NRZ configuration. A clock recovery circuit that is specifically designed to operate with NRZ data format functions acceptably with the RZ format. The BER versus input power curves for the RZ and NRZ formats as shown in Figure 3. As can be seen therein, the RZ data format has about a 3dB higher receiver sensitivity at BER = 10 "10 . In the system of Figures 4 A and 4B, the RZ transmitter 10 is connected to a terrestrial system, here five spans of 90 km LEAF® fiber. LEAF® fiber has an effective area of 72-78 μm 2 , which is about 50% larger than typical NZ-DSF. The fibers λo varied between 1506 nm and 1514 nm, and dispersion slope was =0.1 ps/nm 2 km. The specific terrestrial system shown in Figures 4A and 4B is only an example. Currently, terrestrial systems may include large number of spans with total transmission distance exceeding 3000 km. The channel plan currently consists of 32 channels separated by 50 GHz. At this channel spacing, FWM and XPM when using the NRZ format are so large that generally 10 Gbit/s transmission over NZDS fiber is considered impractical. Each pair of adjacent spans has an amplifier, e.g., an EDFA, and a DCM. A variable optical attenuator which controls the total launched power is positioned before and after each amplifier-DCM pair. An attenuator is then provided before the receiver 30. The receiver 30 includes a preamplifier with channel selection, a variable optical attenuator, a DCM, an optical-to- electrical converter, an amplifier and clock recovery unit, and an error detector. The amplifier and clock recovery unit output data and clock signals to the error detector. A span is simply the length of fiber between amplifiers. When using a dispersion compensating fiber (DCF) as a DCM, the longer the span, the longer the DCF, which increases losses. Typically, the best performance in NRZ format systems is to equally distribute the DCM for each span. This is also the manner in which the RZ format systems may be compensated, as shown in Figure 4A. However, for RZ systems, the best compensation period is every few spans, e.g., 3-4, as shown in Figure 4B. Tables 1 and 2 below illustrate various dispersion maps, with the total amount of compensation being roughly the same for each configuration. Table 1 Table 2 Figure 5 shows an average Q-factor as a function of channel power for the different dispersion map-format combinations. Several observations about the performance of the various configurations shown in Tables 1 and 2 may be made from Figure 5 as follows: First, when NRZ format is used, span-by-span compensation, referred to as uniform in Table 2, shows the best performance. This type compensation is considered as the most practical in optical networking, since data are compensated at each amplifier where optical add/drop multiplexers (OADM) are usually inserted. Second, unlike NRZ format, for RZ format dispersion maps where dispersion compensation occurs on a span-by-span basis, referred to as uniform and post in Table 1, shows worse performance than the dispersion map with less frequent dispersion compensation. Therefore, in accordance with the present invention, the optimal period N of dispersion compensation for RZ data transmission is greater than 1 (N>1). The fact that less frequent dispersion compensation shows superior performance is related to the cross-channel and single channel nonlinearities. The performance of the RZ system with dense, e.g., 50 GHz, channel spacing is very dependent of the cross- channel nonlinearities FWM and XPM. Qualitatively, FWM does not depend very much on the dispersion map, but XPM in RZ systems is reduced when the compensation period increases. However, the single channel nonlinearity, which is also a factor in the performance of the system, depends on the amount of overlap between the pulses, which mostly arises from dispersion broadening. Thus, the compensation period cannot be too long. In accordance with insertion losses of currently available DCMs and with balancing these two considerations, optimal dispersion compensation for RZ data format and NZDS fiber should be provided every 3-4 spans. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

Claims: What is claimed is: 1. An RZ format transmitter for transmitting an optical signal carrying RZ formatted data, the transmitter comprising: at least one light source, the at least one light source propagating at least one light signal having a channel wavelength; a first Mach-Zender modulator coupled to the at least one light source, wherein the at least one light source is modulated by a data signal to form a NRZ signal; and a second Mach-Zender modulator coupled to the first Mach-Zender modulator, wherein the NRZ signal is modulated by a clock signal to form an RZ signal. 2. The transmitter of claim 1, wherein the at least one light source includes a laser light source. 3. The transmitter of claim 1, wherein the at least on light source includes a plurality of laser light sources, each laser light source propagating a light signal having a channel wavelength. 4. The transmitter of claim 3, wherein the plurality of laser light sources have a minimal channel spacing 50 GHz or greater. 5. A dispersion compensation system for an optical communications system having the RZ transmitter of claim 1, and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans, said dispersion compensation system comprising: a first dispersion compensation module in at least one of the transmitter and the receiver; and a second dispersion compensation module before at least one of the plurality of fiber spans, wherein said first and second dispersion compensating modules compensate for different amounts of dispersion. 6. The dispersion compensation system of claim 5, wherein said second dispersion compensation module compensates for more dispersion than said first dispersion compensation module. 7. The dispersion compensation system of claim 5, wherein the optical communications system transmits signals in RZ format. 8. The dispersion compensation system of claim 5, wherein said second dispersion compensation module includes an inline dispersion compensation module at every N spans, where N>1. 9. The dispersion compensation system of claim 8, wherein N = 3. 10. The dispersion compensation system of claim 5, wherein said fibers of said plurality of fibers are non-zero dispersion shifted fibers. 11. A dispersion compensation system for an optical communications system having a transmitter and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans, said dispersion compensation system comprising an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. 12. The dispersion compensation system of claim 11, wherein the optical communications system transmits signals in RZ format. 13. The dispersion compensation system of claim 7, further comprising an additional dispersion compensation module in at least one of the transmitter and receiver. 14. The dispersion compensation system of claim 11 , wherein N = 3. 15. The dispersion compensation system of claim 11 , wherein N = 4. 16. The dispersion compensation system of claim 11, wherein said fibers of said plurality of fibers are non-zero dispersion shifted fibers. 17. A dense wavelength division multiplexed optical communication system in return-to-zero (RZ) format comprising: an RZ transmitter; an RZ receiver; a plurality of fiber spans connecting the RZ transmitter and the RZ receiver; and an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. 18. The system of claim 17, wherein N = 3. 19. The system of claim 17, wherein N = 4. 20. The system of claim 17, further comprising an additional dispersion compensation module in at least one of the RZ transmitter and RZ receiver. 21. The system of claim 20, wherein the inline dispersion compensation module and the additional dispersion compensation module compensate for different amounts of dispersion. 22. The system of claim 20, wherein the inline dispersion compensation module compensates for more dispersion than the additional dispersion compensation module. 23. A method for compensating for dispersion in an optical communications system having a transmitter and a receiver, the transmitter and the receiver being connected by a plurality of fiber spans comprising providing an inline dispersion compensation module before every N span of the plurality of fiber spans, wherein N>1. 24. The method of claim 23, wherein said providing further includes providing an additional dispersion compensation module in at least one of the transmitter and receiver.

Inventors: TEN SERGEY Y, TEN, SERGEY, Y.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2001
Main classification: H04B10
IPC codes: H04B10