<title>One-dimensional photonic crystals from CDMA</title>

Abstract

1D photonic crystals offer extraordinarily low group velocities and high dispersion near their bandgaps. They therefore have an immediate application in CDMA and optical encoding. In our approach we have chosen photonic crystal implementations using long fiber optic Bragg gratings. This system has been numerically investigated and modeled for experimentally realizable structures. We have developed an experimental technique to measure the group delay, produced by 1D Photonic Bandgap structures, in the time domain. At the core of this technique we used a tunable optical pulse source. It consisted of a 1.55 micrometers , 160 fs mode-locked fiber laser with a bandwidth of 50 nm. One nm spectral slices were taken from this laser to obtain picosecond pulses that were tunable. The group delay was measured using a commercial autocorrelator as an ultrafast optical detector and cross-correlator. This technique allowed us to measure the effective dispersion incurred by optical pulses propagating through the grating. A maximum group delay of 10 ps was measured for a 3mm fiber Bragg grating. We have experimentally demonstrated the sensitivity of the group delay as a function of wavelength in the vicinity of the grating bandgap. Our experimental results were quantitatively confirmed by both theoretical and simulation predictions. We also performed experimental studies on cascaded fiber gratings and showed that group delay was additive for the two gratings measured. The use of 1D photonic crystals for pulse shaping and coding applications is important because of its inherent flexibility. By going to the other side of the photonic bandgap it is possible to make conjugate gratings, reversing the dispersion,and performing the task of decoding. Transmission losses and dispersion compensation in this spread spectrum approach will also be presented.