Subpicosecond electrical pulses have been generated from low temperature grown GaAs (LT-GaAs) coplanar stripline photoconductive switches by illumination of 160 fs optical pulses at 1.55 micrometers , which corresponds to below bandgap radiation. The device optical response was attributed to two photon absorption (TPA) as confirmed by the observed quadratic dependence of the DC photocurrent on average incident illumination. At a bias of 39 V and 5.16 mW average illumination a 2.6 (mu) A photocurrent was measured which corresponded to a 0.5 mA/W switch responsivity. For similar switch geometries fabricated onto LT-GaAs and illuminated by above bandgap radiation, responsivities of a few mA/W have been measured. Therefore TPA switch triggering has retained acceptable responsivity levels. The contribution of TPA to the device photoresponse was also confirmed by the dependence of the photocurrent on incident polarization direction; the TPA coefficient exhibited a strong anisotropy with respect to polarization. The device generated electrical pulses (impulse photoresponse) were measured using double sliding-contact photoconductive sampling techniques. Impulse photoresponses with full widths at half maximum of 451 fs were observed which are, to date, the fastest responses measured from LT-GaAs switches at this wavelength. Fourier transformation of the time domain data revealed electrical transients with 3 dB bandwidths of 190 GHz. The feasibility of back illumination of these devices at 1.55 micrometers was also demonstrated. Based on these highly encouraging results freely positionable photoconductive switches were fabricated and at present they are being tested.
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.