Ultra-sensitive autocorrelation at 1.5 /spl mu/m using a photon-counting silicon avalanche photodiode

Recently two-photon absorption (TPA) autocorrelation has attracted considerable attention because it achieves quadratic nonlinearity using simple direct electrical detection in a semiconductor material that eliminates the complexities involved with phase matching and polarization sensitivity when using a nonlinear crystal. The functional behavior of TPA depends upon peak power and average power. The remedy for low power situations typically involves boosting the power in an optical amplifier to perform the measurement, even though this degrades signal to noise ratio and distorts the pulseshape. This investigation reports on a novel TPA autocorrelation technique that uses a silicon avalanche photodiode (APD) to achieve enhanced sensitivity over other TPA techniques. The sensitivity of the APD, together with the lack of single-photon background events makes this method ideally suited for characterization of low power pulses.