Measurements of Ultrashort Light Pulses with a Multichannel Two-Photon Photoconducitvity Detector

Abstract

Since the first successful generation of picosecond optical pulses [1], there has been a lot of progress made in the measurement technique for such ultra-short pulses [2–8]. So far a number of instruments are capable of displaying a picosecond optical signal in a laser shot with picosecond resolution, but each one suffers some degree of shortcoming. On of the promising techniques for the measurement of sub-nanosecond optical pulses involves using the multi-photon conductivity effect in semiconductors as proposed by PATEL [9]. LEE and JAYARAMAN [10,11], after observing two-photon and three-photon conductivity effects in semiconductors using nanosecond or picosecond laser pulse excitation, used this technique to measure the pulse width of picosecond pulses from a mode-locked Nd:glass laser. The multiphoton conductivity technique is analogous to the two-photon fluorescence (TPF) technique. The major difference between these two techniques is the method of detection. In the TPF method the fluorescent signal from a dye is measured, while in the two- photon conductivity (TPC) or three-photon conductivity methods the photo- conductive signal from a biased semiconductor is measured. This difference is naturally reflected in the construction of the detector. We report here the operation of a seven-channel prototype TPC detector. It is a second order laser intensity autocorrelator which can measure the pulse width of mode-locked Nd:glass laser pulses in a single laser firing. A simple peak detector was used to relay the signals from the two-photon detector to a displaying system. The successful demonstration of the TPC detector has led us to believe that, due to the simplicity of the device, a refined version of it which may consist of a layered thin film semiconductor and/or charge coupled device with multichannel data processing capability can be built with relatively low cost.