We demonstrate that direct sequence optical code- division multiple-access (DS-OCDMA) encoders and decoders using sampled fiber Bragg gratings (S-FBGs) behave as multipath interferometers. In that case, chip pulses of the prime sequence codes generated by spreading in time-coherent data pulses can result from multiple reflections in the interferometers that can superimpose within a chip time duration. We show that the autocorrelation function has to be considered as the sum of complex amplitudes of the combined chip as the laser source coherence time is much greater than the integration time of the photodetector. To reduce the sensitivity of the DS-OCDMA system to the coherence time of the laser source, we analyze the use of sparse and nonperiodic quadratic congruence and extended quadratic congruence codes.
© 2007 Optical Society of America
Fiber Optics and Optical Communications
Original Manuscript: July 7, 2006
Manuscript Accepted: September 8, 2006
Ihsan Fsaifes, Catherine Lepers, Mounia Lourdiane, Philippe Gallion, Vincent Beugin, and Philippe Guignard, "Source coherence impairments in a direct detection direct sequence optical code-division multiple-access system," Appl. Opt. 46, 456-462 (2007)