Background-free intensity autocorrelator employing Si avalanche photodiode as two-photon absorber

A background-free intensity autocorrelator has been developed using two-photon absorption (TPA) in a Si avalanche photodiode (APD). Light passing through two arms of the Mach-Zehnder interferometer is chopped at different frequencies, f1 and f2, and the background level in the TPA signal is eliminated by measuring the output of the Si APD at the difference frequency of | f1−f2| using a lock-in amplifier. This background-free autocorrelator has a dynamic range of 30 dB together with high sensitivity, enabling the waveform of optical pulses at 1550 nm to be determined precisely.