NRZ and RZ Pulse Forms in WDM Systems with Distributed Fiber Raman Amplifiers

	ABOUT THE AUTHOR Minhui Yan received a B. Sc. Degree from Shanghai Jiao Tong University, where he is now a Master’s Degree candidate. He is engaged in research topics including opto-electronic devices and applications, high-speed optical fiber transmission systems, and ultra-broadband optical amplifiers. He has published several papers in domestic and international conferences and academic journals.

To deal with growing communications traffic, we need to use more wavelengths in optical WDM (Wavelength Division Multiplexing) systems and networks. This need fuels a demand for wide-band optical amplifiers that cover the entire low-loss optical-fiber spectrum.

The optical fiber Raman amplifier, with its wide amplification bandwidth and flexible center wavelength, is a promising candidate for long-haul systems. In addition, fiber Raman amplifiers exhibit low-noise in theoretical and experimental tests. We can therefore expect ultra low-noise for fiber Raman amplifiers used in long-transmission trunks.

Several factors, such as ASE noise and double Rayleigh backscattering limit the noise figure. Another kind of noise arises from the instantaneous pump depletion. In addition to the inherent fiber loss, the pump transfers power to Stocks signal waves through stimulated Raman scattering (SRS), which leads to interaction between pump and signal along the fiber. For multiple wavelength WDM systems, instantaneous pump deletion depends not only on the average signal levels of all channels, but also on the signal formats. In distributed optical fiber Raman amplifiers, instantaneous pump power deletion is related to the patterns of the input WDM signals, especially for high bit-rate applications. The resulted optical noise and inter-channel crosstalk is different for NRZ and RZ pulse forms. An appropriate choice of pulse forms can reduce such phenomena.

In the fiber Raman amplifier, a set of coupled equations describes the amplitudes of the pump and the amplified signal. We can extended these equations to describe WDM systems with n wavelengths:

where:

• A_x = the complex amplitude of pump (x = p) or ith channel signal (x = si)
• g_x = Raman gain
• _x = the self- and cross-phase modulation coefficients
• A_eff = the effective core area
• _x = the fiber attenuation.

In comparison with the equations in , a factor of 2 is applied to the denominators of the first terms at right-hand, accounting for the random polarization of the signals.

The pump power is depleted gradually (for the case of forward pumping) along the fiber length of the Raman amplifier. Due to changing pump power requirements for different signal-code formats, instantaneous pump depletion fluctuates along the fiber causing optical noise and inter-channel crosstalk. Together with other types of crosstalk, such as that resulted from the Raman gain for small frequency shifts, this instantaneous variation of the pump power gives rise to the amplifiers' overall optical noise and crosstalk.

We employed the Split Step Fourier Method (SSFM) to simulate the instantaneous signal amplification and transmission in the distributed fiber Raman amplifier of a WDM system with 64 channels at 10 GB/s bit-rates for each channel. Rounding the edge of perfect rectangular pulses through a low-pass filter generates the NRZ pulse form and the RZ pulse form is generated as conventional Gaussian pulses. In order to simulate the real situation, the time slot of each channel is not synchronized, in other words, the start times for each bit of the different channels is random.

Figure 1:  Comparison of the ith channel's amplified signal between NRZ and RZ

During the simulation, the power of each signal is 0.01 mW for 1s, while the pump is CW with initial power of 600 mW. The fiber is 20-km long with a Raman gain coefficient of 1 x 10^-13 m/W, effective core area of 55 µm², and loss of 0.2 dB/km (0.046/km). From the simulated results, the average Raman gain of RZ is more than that of NRZ and the amplitude of each amplified bit varies around an average value. This variation is due mainly to the instantaneous pump depletion caused by the other 63 channels and results in extra optical noise and crosstalk. NRZ and RZ both tolerate this phenomenon, but the fluctuation of NRZ is more serious than that of RZ.

Figure 2:  Eye diagram comparison of the amplified signals' ith channel between NRZ and RZ

Figure 3:  Pump deletion comparison at the output end between NRZ and RZ

Figure 3 shows the instantaneous pump depletion during the time period when 64 channels are active. Because the pump power required for 1s and 0s is different for NRZ and RZ pulses, pump-depletion fluctuation is different for both pulses. The averaged residual power level is around 120 mW for NRZ and around 150 mW for RZ.

Simulation results of a WDM system with 64 wavelengths show that noise has a more significant effect on NRZ. This factor should impact the future deployment of optical Raman amplifiers.