ABSTRACT
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.
ABSTRACT
Bragg Gratings (BGs) have been written within either H2-loaded or UV-hypersensitized phosphorous-germanium co-doped silica planar waveguides through exposure to light at 248 nm. The stability of these BGs has been investigated by means of isochronal annealing experiments. It appears that the stability of both the modulation and the Bragg wavelength is higher in the hydrogenated waveguides than in the hypersensitized counterparts. Moreover, in the case of BGs in the UV-hypersensitized waveguides, the rate of the strength decay depends on the initial amplitude of the refractive index modulation whereas no significant difference could be observed in H2-loaded waveguides. It is shown that the master-curve formalism can be used to predicting an isothermal decay of both reflectivity and Bragg wavelengths of BGs written in the H2-loaded waveguides. This conclusion is illustrated by the fairly nice agreement that exits between the results of an isothermal annealing experiment and those predicted by use of the master-curve.
