Studies On High Speed Time Spreading OCDM System And It's Applications

Optical code division multiplexing (OCDM) is a communication mode that introduces frequency spreading technology into optical fiber communication system. Coherent OCDM system is superior to incoherent schemes in overall performance such as bit-error-rate (BER) and volume. Among the coherent schemes proposed so far, coherent time domain phase-encoded OCDM system is the most desirable choice for practical use in terms of the correlation property, frequency efficiency, and dispersion. In this dissertation, theoretical and experimental research on superstructured fiber Bragg grating (SSFBG) based time domain phase encoding OCDM system is presented. The main contents are as follows:(1) Theoretical basis for all optical encoding and decoding based on SSFBG is investigated. The operation principle of SSFBG based time domain phase encoding and decoding is given. Temporal response and frequency response to soliton pulse of encoder/decoder gratings are numerically simulated. The correlation performance of SSFBG encoder/decoder is analyzed considering the influences of the refractive index modulation amplitude and wavelength excursion between encoder grating and decoder grating.Δn(2) 40Gbit/s all optical encoding and decoding is implemented using 3-chip quaternary phase-shift SSFBG. In case the encoder grating is matched with the decoder grating, the oscilloscope trace for the decoded pulse shows that short, distinct pulses are reformed, the actual width of the auto-correlation peak is 5.1ps, in the magnitude of chip duration 4.2ps. On the other hand, in case the encoder is not matched with the decoder, short pulse will not be reformed.(3) 7-chip SSFBG based 2×40Gbit/s OCDM system is experimentally demonstrated firstly. Under two channels operation, the degradation of the decoded signal is obvious, although a pulse with single peak was obtained, the width of the main peak is broadened to 7.7ps and the sidelobe of the auto-correlation trace is apparent.(4) The NOLM subsystem based optical thresholding and the NALM based optical thresholding are experimentally implemented, respectively. The NOLM can act as a nonlinear processing element capable of reducing both the pedestal and the width of the associated code recognition pulse, the original 7.7ps auto-correlation peak of decoded pulse is compressed to 3.8ps. The input signal of the NOLM subsystem should be amplified to an average power of 0.42W by using the high power EDFA. Similar result is acquired by using a NALM to improve the system performance, switching is obtained for peak signal power less than 10mW.(5) Supercontinuum and SSFBG based WDM/OCDM system is experimentally demonstrated. The interchannel interference and noise of the WDM/OCDM system is suppressed apparently by incorporating a NALM, both the sidelobe and the pedestal are reduced, the waveform quality of decoded signal is improved, and the original 8.2ps auto-correlation peak of decoded pulse is compressed to 4.8ps.(6) The bit-error-rate (BER) performance of the coherent time domain phase-encoded OCDMA system is evaluated. The theoretical model of the coherent OCDMA system is described. The BER expressions are derived and calculated considering the influences of beat noise, multiple access interference (MAI) and receiver noise. Theoretical analysis shows that overall system performance is limited by MAI and especially limited by beat noise.