Microwatt picosecond pulse autocorrelator using photorefractive GaAs:Cr

Abstract

Recently, the 1.06-μm photorefractive response of semi-insulating GaAs and InP has been measured.1,2 Preliminary experiments have now been performed using the photorefractive effect in GaAs:Cr as a sensitive nonlinearity for simple low-power picosecond pulse diagnostics at IR wavelengths. In these beam coupling or two-wave mixing2 experiments, a photorefractive index grating is formed by the interference of two 4-ps duration 1.06-μm beams in a 1,5-mm thick GaAs:Cr crystal. The optical pulses were obtained from a fiber-grating compressed cw mode-locked Nd:YAG laser. The temporal profile of these pulses was obtained by detecting the fraction of chopped pump beam scattered by the index grating into the probe beam as a function of delay between the two beams, using an InGaAs PIN photodiode and lock-in amplifier. The measured temporal profile was identical to that obtained by standard noncollinear background-free correlation using second harmonic generation in a 1-mm LilO3 crystal and photomultiplier tube (PMT). With only 2 μW of average power per beam, photorefractive autocorrelation measurements were made with SNRs greater than 5:1 in this nonoptimized experiment. The beam coupling efficiency can be improved by several orders of magnitude by using a longer crystal, antireflection coatings, and an applied electric field. Beam coupling in GaAstCr is linear in intensity and photosensitive beyond 1.6-μm,1 while the harmonic process is quadratic in intensity and the quantum efficiency of typical PMTs is poor at 0.8-μm and beyond. Therefore, photorefractive auto-correlation may prove to be very attractive for real-time measurement of the temporal diagnostics of low-power long-wavelength mode-locked semiconductor lasers.