Heterodyne System for Measuring Frequency Response of Photodetectors in Ultrasonic Applications

The heterodyne interferometer using acousto-optic modulation is presented to measure the frequency response of photodetectors (PDs) utilized in ultrasonic applications by optical methods. The heterodyne signal is served as the excitation source. The measurement frequency more than 150 MHz can be achieved by two serial or parallel connected acousto-optic frequency shifters (AOFSs) in the interferometer. The frequency shift of AOFSs can be adjusted continuously, so that the frequency resolution of the measurement better than 500 kHz is easy to realize. The experimental results for two PDs with bandwidth 150 and 50 MHz are discussed. The presented scheme can also be used to characterize the PDs in the frequency range of megahertz for other optical measurement applications.