Heterodyne technique for measuring photodetector frequency response without rf interference

For heterodyning two optical beams it is necessary to obtain two almost coherent waves. Rather than using an extremely stable laser source, it is possible to split the beam from a single source and use a modulator to shift the frequency of one of the waves. The heterodyne signal generated is at the frequency of modulation, thus, it is susceptible to rf interference by the high modulation voltage. Using two electro-optical modulators, one can utilize the signal at the sum or difference frequency at which no high voltage is present. It is shown that using a single source and two modulators one obtains a signal without any interference leakage. Thus it is possible to measure easily the frequency response of photodetectors up to high frequencies. This configuration also doubles the frequency range obtainable from a single modulator. The theoretical expressions for the intensities at the various frequencies are presented. Noise equivalent power (NEP) measurements of heterodyne signals performed using this setup are very close to the theoretical limit.