LOCK-IN-LIKE LIGHT DETECTION METHOD FOR A GAIN-MODULATED PHOTOMULTIPIER-TUBE

摘要

In order to measure intensity and phase of a weak signal varied in an intensive background light, we propose a novel lock-in-like light detection method using a photomultiplier-tube (PMT) whose gain is modulated sinusoidally. Let assume that an incident light on the PMT is modulated at a frequency f. In this situation, one of the dynode voltages supplied for the PMT is deeply biased and a sine waveform with a frequency f is superimposed on the bias voltage. Then, we detect only a 2f-frequency component from the output signal of the PMT. To pick up the 2f-component, we connect a digital oscilloscope directly to the output of the PMT. The digitized waveform stored on a temporal memory of the digital oscilloscope is fast-Fourier-transformed (FFT). Two values obtained from the resultant power and phase spectra at the frequency 2f give the intensity and the phase of the incident signal light, respectively. In principle, the proposed method has no interference with an intensity level of the direct-current (dc) background light.