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Because of limits on the speed of the photodetector, a nonlinear thresholder is needed at the receiver of a spectral-phase-encoding optical code-division multiple-access system to discriminate between the correctly decoded short pulse and the low-intensity interference. The two most common nonlinear receivers based on second harmonic generation and self-phase modulation effects are analyzed in this paper. Mathematical models are provided, and analytical results are obtained to estimate the receivers' performances. Numerical simulations are carried out for both receivers with different system parameters. Both m-sequences and random binary codes are examined for spreading. The results provide a profile of how these nonlinear receivers perform with various system settings. It is found that, when an m-sequence is used as the spreading code, the encoded signal does not obey Gaussian statistics, and the system performs better than an equivalent system using a random code.