We propose and numerically demonstrate impairment compensation via digital backward propagation (DBP) for fiber
transmission with distributed Raman amplification. In contrast to EDFA-based systems, signal power evolution in the
distributed amplified system has to be obtained prior to DBP by solving the differential equations for Raman
amplification. The optimum launching power of an unrepeatered Raman link is significantly increased by nonlinearity
To improve the all-optical clock recovery, a novel device termed as code mixer was designed to preprocess the injected data signals. In the time domain, the code mixer can change the probability distribution of the amplitude of the data pulses and reduce the amount of codes "0". In the power spectrum, the code mixer can reduce the continuous spectral component. It was demonstrated experimentally that the code mixer could concentrate the probability distribution of the amplitude of the pulses to half of the maximum amplitude. It was also demonstrated theoretically and experimentally that using a code mixer of more ways or several cascade code mixers the clock signal recovered can be further improved. In the experiment of 40GHz all-optical clock recovery using an injection mode-locked fibre ring laser based on a semiconductor optical amplifier, the amplitude fluctuation and timing jitter of the clock pulses recovered caused by the pattern effect were reduced obviously.
40Gbit/s all-optical 3R regeneration was demonstrated. The wavelength of the data signal was converted with an SOA by cross-gain modulation (XGM). The clock recovery was based on Fabry-Perot (F-P) filter of high finesse (F=1000). The actual bit rate of the data signal was 40.04796Gbit/s, matching the free spectral range (FSR) of the F-P filter. Two SOAs were used in series as the decision gate to make full use of the nonlinearity. Taking advantage of the chirp, one narrowband OBF was used to reshape the waveforms in the clock recovery, so as to reduce the pattern effect. Another narrowband OBF was used after the decision gate to diminish the tails of the pulses. The clock signal had a root mean square (RMS) timing jitter of around 843fs. The output data signal of the regenerator had an optical signal-to-noise ratio (OSNR) of more than 40dB/0.1nm. The maximum timing jitter was 1.439ps.
An improved scheme for all-optical pulse decision gating by cross-gain modulation(XGM) in semiconductor optical amplifier (SOA) is shown in this paper. Influences of the property of optical clock pulses (including the pulse extinction ratio (ER) and the power) on the shape of regeneration pulse of this optical decision gate are studied by means of numerical simulation and experiment. The power and extinction ratio of optical clock pulses are adjusted in the process of numerical simulation. In the beginning, with the power of clock pulses rising or ER dropping, the pattern of the regeneration pulse gets better and the "1" code noise comes down. There are optimal values of the power and ER of clock pulse for the best pattern of the output pulses. If the ER continues to decrease, the ER of output pulses dropped. Similarly, if the power of clock pulse continues to increase the pattern of the regeneration pulse gets worse. In experiment of optical decision, 40Gb/s regenerated pseudo random bit-sequence (PRBS) data signal was got from degraded PRBS input signal and recovery clock pulses by passing the optical decision gating. The result of experiment is uniform in the numerical simulation.