Recirculating frequency shifter(RFS)can generate stable multi-carrier,but the carrier-to-noise-ratio(CNR)is limited because noise is accumulated round by round in the recirculation loop structure.A modified RFS with a delay interferometer(DI)is proposed in this paper.The DI can suppress the accumulated noise,and enhance the CNR of multi-carrier.The principle of the generation of high quality multi-carrier based on an RFS with DI is analyzed theoretically.50 stable carriers with spacing of 10.7 GHz are successfully generated.The flatness of 50 stable carriers is 3 dB,and the CNR is 43 dB which is increased by 3 dB compared with the structure without DI.
We study the decoding performance of low-density parity-check （LDPC） code based on Watermark scheme under different percentages of water-mark bits over 100 Gb/s differential quadrature phase shift keying （DQPSK） high-speed optical communication system. We also find the optimum percentage of water-mark bits according to the performance comparison. Simulation result shows an improvement of 0.5 dB net code gain by using the optimum Watermark scheme at a post-forward error correction bit error rate （BER） of 10-9, comparing with the traditional log-likelihood ratios belief propagation decoding algorithm. Also, for the same BER, there is a decrease in the number of iterations used in LDPC decoding.
An optical bandpass filter （OBF） is an important element whose properties considerably influence the output performance of a single side band multicarrier source based on a re-circulating frequency shifter. The influence of the OBF in the loop on the output spectrum is theoretically analyzed. Numerical simulations and experiments are Mso carried out. Results show that the steepness and deepness of a nonflat-top filter influence the stability of the spectrum of an output multicarrier. To obtain multicarrier output with tone to noise ratio （TNR） 〉26 dB and error vector magnitude （EVM） 〈0.25, the steepness ratio of the filter should be greater than 0.75 and deepness should be larger than 0.99.
The laser source is an important element whose properties have influence on the output performance of single sideband（SSB） optical comb based on recirculating frequency shifter（RFS）.The theoretical and experimental results show that the tone-to-noise ratio（TNR）,flatness,and stability of optical comb improve using laser source with narrower linewidth.In order to obtain the optical comb with TNR larger than 42 dB and degree of stability（DOS） smaller than-6.7 dB,external cavity laser（ECL） with linewidth from 100 kHz to 1 MHz is a trade-off choice.
