Phase-locked optical choppers

Abstract

We have developed a circuit for maintaining a fixed phase relationship between two optical choppers. The basis of this circuit employs a phase-locked loop using both proportional and integral terms in the feedback control voltage. Using additional circuitry, the electronics can compensate for drifts in the phase as well as frequency of both choppers in the frequency range from 200 Hz to 3 kHz. These phased optical choppers have been used to block background light simultaneous with a chopped cw laser in experiments that measure long lived fluorescence.