An automatic linearity tester for photodetectors

Abstract

The apparatus is designed to measure the properties of photodetectors used for the measurement of pulsed laser power and energy. Repetitively pulsed lasers at 0694, 091, 106 μm are used to give pulse lengths in the range 5-400 ns. The beam power is varied by a slowly rotating polarizer and, after division, produces photocurrents in both the test cell and a standard monitor cell. The signals are integrated electronically and used to produce Y and X deflections of a cathode ray oscilloscope spot. The spot is brightened after a few microseconds to give a point on a plot of photoelectric charge against pulse energy. Alternatively, the test cell output is time-sampled electronically to give a plot of photocurrent against pulse energy (or power). Departures from linearity of 3% can be detected over a range of 10⁵ in photoelectric charge with better accuracy for smaller ranges. For the GaAs laser at 091 μm a single optical pulse, of approximately 30 ps long, can be generated. This is used for rise-time measurements.