Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/25—Arrangements specific to fibre transmission
  • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
  • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
  • H04B10/2525—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion using dispersion-compensating fibres
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2210/00—Indexing scheme relating to optical transmission systems
  • H04B2210/25—Distortion or dispersion compensation
  • H04B2210/252—Distortion or dispersion compensation after the transmission line, i.e. post-compensation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2210/00—Indexing scheme relating to optical transmission systems
  • H04B2210/25—Distortion or dispersion compensation
  • H04B2210/254—Distortion or dispersion compensation before the transmission line, i.e. pre-compensation

Definitions

• This invention relates to wavelength division multiplexers (WDMs) systems and, more particularly, to dense WDMs (DWDMs), in which dispersion compensation is utilized at both the receiver and transmitter ends, and further wherein a WDM is optimized overall.
• WDMs wavelength division multiplexers
• DWDMs dense WDMs
• dispersion compensation techniques at either the receiver or transmitter ends of a wavelength division multiplexer have been of interest.
• This invention pertains to the simultaneous use of dispersion compensation at both the receiver (RX) and transmitter (TX) ends of a dense WDM to obtain better performance.
• the invention provides techniques for optimizing dual dispersion compensation, given the characteristics of the overall WDM and its associated network.
• One factor significantly affecting the optimum ratio for dispersion compensation at the RX and TX ends is the chirp value of the transmitter. Other factors include power level, number of channels, channel plan, fiber dispersion, and system length.
• Providing dispersion compensation simultaneously at both the RX and TX ends of the WDM system can produce superior results compared with compensation at a single end.
• the type and distribution of the dispersion (dispersion ratio) between the RX and TX ends must be balanced for the particular system. If not properly balanced, the results can be inferior even to those for single ended compensation.
• the dispersion compensation ratio can be adjusted experimentally by trial and error, but this method is burdensome and painstaking. Optimization can initially be more readily analyzed using a simulation, which describes the propagation of the light wave in the fiber. Software that can perform this analysis is available commercially.
• the analysis can simulate propagation along all channels and account for meaningful nonlinear effects and dispersion effects at the same time.
• a typical long haul broadband DWDM in accordance with this invention, utilizes a system of DFB (distributed feedback) lasers comprising thirty-two channels on an ITU (International Telecommunications Union) grid.
• the lasers are multiplexed and modulated at 10 GBit s by a nominal zero-chirp modulator.
• the signals are directed to a system of five 90 km spans of large effective aperture fiber such as that sold under the registered trademark LEAF ® by Corning Incorporated.
• Optical amplifiers provide signal gain at the input to each span.
• Commercial units of Dispersion Compensating Modules are provided.
• the dispersion compensating modules are applied at the transmitter and receiver ends. Proper selection of the distribution of total dispersion compensation determined by simulation, and verified by experiment, optimizes the DWDM system.
• DWDMs Dense Wavelength Division Multiplexers
• FIGURE 1 illustrates a schematic diagram of a typical fiber optic system in accordance with this invention
• FIGURE 2 depicts a diagrammatic view of the spectra obtained for the fiber optic system shown in FIGURE 1 ;
• FIGURE 3 shows a graph of Q versus wavelength for the fiber optic system depicted in FIGURE 1;
• FIGURE 4 illustrates a graphical view of Q versus channel number compared with the FWM spectrum for the same channels for the fiber optic system depicted in FIGURE 1;
• FIGURE 5a shows a graphical view of Q, as a function of wavelength, for the fully assembled system (squares), together with the Q measured with transmission fiber having been replaced by attenuators (circles); and
• FIGURE 5b depicts a graphical view of the penalty of the fiber (dB) vs. the wavelength (nm).
• the invention features a long haul, broadband, DWDM system that has been optimized by the proper selection of the distribution of total dispersion compensation.
• Dispersion compensation is utilized at both the receiver and transmitter ends.
• System performance is dependent on the ratio of compensation split between the transmitter and the receiver.
• a system operated in the nonlinear regime can be compensated to operate at low BER and with tolerable residual dispersion effects, even when the spread of total accumulated dispersion between the extreme channels in a broadband system exceeds 1,100 ps/nm.
• FIGURE 1 a schematic diagram shows a typical DWDM system in accordance with this invention.
• the system is of the long haul type, and designed to be deployable terrestrially.
• the system utilizes DFB lasers comprising thirty-two channels on an ITU grid.
• the optical amplifiers disposed at the input to each span were characterized by a 25 dB external gain, total power output of 20 dBm (19 dBm directed into the fiber spans), an average noise figure of 5 dB, and an average gain ripple of 1.2 dB.
• Commercial units of Dispersion Compensating Modules were provided as DCM-X, where X was the equivalent length in kilometers of standard single mode fiber dispersion, compensated by the dispersion compensating module.
• the DCM modules were applied at the transmitter and receiver ends.
• rows one and three have the same total compensations and the same is true for rows two and four, yet the system performances are markedly different depending on the rations of the pre- and post-compensation values.
• the lasers were multiplexed with fiber couplers, and modulated with a 2 3l -l , 10 GBit/s pseudorandom bit stream (PRBS) by a Li:NbO 3 , zero-chirp, Mach-Zehnder modulator.
• PRBS pseudorandom bit stream
• the lasers match the ITU-T nominal central frequency grid and minimal channel spacing of 100 GHz.
• the signals were transmitted over a 450 km transmission line consisting of 5 x 90 km spans of LEAF ® large effective aperture fiber and four in-line optical amplifiers.
• LEAF ® fiber has an effective area of 72-78 ⁇ m , which is about 50% larger than typical NZ-DSF.
• the fibers ⁇ o varied between 1506 nm and 1514 nm, and dispersion slope was « 0.1 ps/nm /km.
• VOA Variable Optical Attenuator
• the span loss was increased to 24 dB by adding optical attenuators before each amplifier in order to simulate real system loss margin needed for real systems.
• a variable optical attenuator was used to keep the power substantially constant into the O-E converter.
• the input spectrum before the first VOA and output spectrum before the optical preamplifier are shown in FIGURE 2.
• the same amount of pre- and post-compensation was used for all channels.
• the total accumulated dispersion for the first channel was -454.78 ps/nm and +893.81 ps/nm for the last channel.
• the transmission performance was characterized by measuring the bit error rate as a function of the decision threshold for each channel.
• the system Q was estimated using the full system (fiber + amplifiers) and with fiber spans replaced by attenuators with equivalent loss. The results of the measurements for the full system are illustrated in FIGURE 3. The average Q for the full system was approximately 8.9 dB optical
• FWM Four Wave Mixing
• XPM Cross-Phase Modulation
• SPM Self-Phase Modulation
• the 32 channel system can be optimized with a single, dispersion compensating module design, with no splitting of the signal band, and with negligible variation in dispersion- related system penalty across the channel plan.
• Dense WDM can greatly increase the capacity of transmission at the cost of managing the penalty induced by optical nonlinearities.
• Systems that use non- dispersion shifted fibers must use dispersion compensation on a span by span basis when bit rates greater than, or equal to, 10 Gbit/s/channel are used.
• NZ-DSF non-zero dispersion-shifted fibers
• the signals were transmitted over a 450 km transmission line consisting of five spans of ninety km LEAF ® fiber, and four in-line optical amplifiers.
• the total output power of each amplifier was adjusted to be 16 dBm, for the red band experiment, and 13 dBm for the blue band experiment, which corresponded to an average power of 7 dBm channel.
• the span loss was increased to 24 dB by adding optical attenuators before each amplifier.
• An etalon filter with an FWHM of 0.3 nm at the optical pre-amplifier selected the channel to be measured.
• Optical pre-emphasis was required to equalize the received optical signal-to- noise ratio at the end of the transmission line for the red band experiment.
• Four Wave Mixing (FWM) was not observed in either the red or the blue experiment.
• Error-free transmission was obtained for all channels in both red and blue experiments, when the span loss was adjusted to 24 dB. This corresponds to a received power of -21.9 dBm for channel 1, to -15.1 dBm for channel 8 for red. In other words, more power (14 to 19 dBm) was necessary to obtain a bit-error rate of 10 '9 .
• sensitivity varied between -34.5 dBm and -34.0 dBm. This equates to a power penalty of0.5 to l.0 dB.
• the large effective aperture fiber effectively suppresses FWM in dense WDM systems. Furthermore, due to its large effective area and small dispersion, this fiber allows for the minimization of self- and cross-phase modulation penalties at 10 Gbit s, using dispersion compensation at the terminal. This eliminates the need for dispersion management in the cable or at every amplifier.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Optical Communication System (AREA)

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• 1999
  • 1999-09-21 EP EP99948398A patent/EP1142178A2/de not_active Withdrawn
  • 1999-09-21 AU AU61588/99A patent/AU6158899A/en not_active Abandoned
  • 1999-09-21 CA CA002344543A patent/CA2344543A1/en not_active Abandoned
  • 1999-09-21 JP JP2000571593A patent/JP2002525967A/ja not_active Withdrawn
  • 1999-09-21 WO PCT/US1999/022009 patent/WO2000018047A2/en not_active Application Discontinuation
  • 1999-09-21 CN CN 99811195 patent/CN1323475A/zh active Pending

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