The influence of single-photon absorption on the performance of the two-photon waveguide autocorrelator

The propagation loss and the single-photon absorption coefficients in a two-photon waveguide autocorrelator are measured as a function of wavelength. The propagation loss was as low as 1.37 cm/sup -1/ at a wavelength of 940 nm in a device with an Al/sub 0.2/Ga/sub 0.8/As waveguide core. Using a photocurrent technique, band-to-band absorption was measured for photon energies well below that of the bandgap. It was demonstrated that, although the band-to-band absorption coefficient is small (/spl sim/10/sup -2/ cm/sup -1/ at a wavelength of 1 /spl mu/m), it is responsible for reducing the contrast ratio of the waveguide autocorrelator. It is suggested that the single-photon absorption takes place via deep levels with relatively long carrier lifetimes.