Pseudo-Lock-in Light Detection Method for a Sinusoidally-Gain-Modulated Photomultiplier Tube

In order to detect a weak signal varied in an intensive background light, we propose a novel pseudo-lock-in light detection scheme used for a photomultiplier tube (PMT) whose gain is modulated sinusoidally. Let the signal light incident on the PMT be modulated sinusoidally at a frequency f. In this situation, one of the dynode voltages applied for the PMT is deeply biased and a sinusoidal reference signal with a frequency f is superimposed on the bias voltage. Then, we detect a frequency component of 2f at the output of the PMT selectively. To pick up only the 2f component, we connect a digital oscilloscope directly to the output of the PMT. Thereafter the digitized waveform stored on a temporal memory of the digital oscilloscope is fast-Fourier-transformed (FFT). Two numerical values obtained from the resultant amplitude and the phase spectrum at the frequency 2f, respectively, correspond to the intensity and the phase of the modulated signal light incident on the PMT. A fundamental performance test of the proposed scheme was carried out.