<title>Impact of FWM in a dense WDM optical network with CPFSK direct detection receiver</title>

Abstract

Optical multi-wavelength transport networks (MWTN) have generated considerable recent interests because of their inherent capability to achieve higher information capacity, greater flexibility, efficient routing, transparent switching, reconfugarability etc.. The effect of switch cross-talk, adjacent channel cross-talk, filtering effect, accumulated ASE in MWTN has been recently addressed. However, the performance of closely packed WDM systems with dispersion shifted fibers is highly vulnerable to the effect of FWM. The present study investigates the degradation encountered in multi-wavelength switched optical network due to the combined influence of receiver noise, phase noise, accumulated amplified spontaneous emission (ASE), FWM crosstalk and dispersion. For each channel CPFSK modulation is considered with direct detection receiver employing Mach-Zehnder interferometer (MZI). The analysis is carried out to include the combined effect of accumulated optical amplifiers’ spontaneous emission (ASE) noise and the beat noise components viz. signal-spontaneous emission beat noise, ASE-ASE beat noise, adjacent channel-spontaneous beat noise, signal- FWM beat noise, FWM-spontaneous beat noise etc.. An expression for the probability density function (pdf) of the random phase fluctuation due to FWM effect is developed and the bit error rate is estimated at each node for different number of channels. In the present analysis, a node architecture of the optical mesh network is considered in which an interconnection between two consecutive nodes consists of optical in-line amplifiers, optical space-switches, power splitters, wavelength multiplexers and demultiplexers and the fiber protection switch. All the wavelengths are combined in the multiplexer and amplified before feeding to the transmission fiber. The gains of the in-line amplifiers are adjusted to compensate for the losses in the fiber, splitters and other passive components. The demultiplexer separates the desired signal wavelength from the WDM multiplex and is realized by a Febry-Perot filter (FPF) tuned to the desired signal channel. The node uses M transmitters and M receivers if M is the number of wavelength channels. Computations of results are carried out at a bit rate of 2.5 Gb/s for several fiber spans with different sets of system parameters, viz. number of channels, number of nodes, optical SPIE Vol. 3211 • 0277-786X/97/$10.00? wavelength from the WDM multiplex and is realized by a Febry-Perot filter (FPF) tuned to the desired signal channel. The node uses N transmitters and N receivers if N is the number of wavelength channels