Laser-mode beating used for detector frequency-response measurements

Abstract

Low-power He-Ne lasers generally oscillate in two spectral cavity modes. We have found that the amplitude of the beat note between the two modes is nearly constant, even though the powers of the two modes may be changing considerably with time. We use this fact to measure the frequency response of an optical detector at the intermode-beat frequency. Many photodetectors have impulse responses that decay approximately exponentially with time. Therefore, we can further show that it is possible to estimate the frequency response of such detectors at all frequencies from the result of a simple measurement at a single frequency.