Measuring the frequency response of photodetectors used in ultrasonic applications by a heterodyne system with difference-frequency servo control

A technique for the calibration of photodiodes in ultrasonic measurement systems using standard and cost-effective optical and electronic components is presented. A heterodyne system was realized using two commercially available DFB lasers, and the required frequency stability and resolution were ensured by a difference-frequency servo control scheme. Measurements were carried out up to 450 MHz and the uncertainties of about 4 % (k = 2) can be further reduced without losing the feature of using only simple elements by improved rf power measurement.