Measuring the photodetector frequency response by a heterodyne system with difference-frequency servo control

A technique for the calibration of photodiodes using standard and cost-effective optical and electronic elements is presented which allows the determination of the photodetector’s frequency response under conditions used in ultrasonic applications.

One of the important quantities describing the properties of photodetectors is the frequency response which can be defined as the voltage at the detector’s output in response to a constant or well-defined optical input signal versus frequency. Photodetectors in ultrasonic applications are applied in very different optical configurations and during photodetector calibration these conditions should be reconstructed to ensure that the frequency response obtained matches the application conditions. So it was advisable to have an in-house calibration set-up which is adapted to the ultrasound application requirements. Since sophisticated and costly optical equipment was not present in a laboratory dealing with ultrasonic applications, a set-up had to be constructed which used only simple components available in a standard optical laboratory.
A heterodyne system (Fig. 1) was realized using two commercially available DFB lasers, and the required frequency stability and resolution was ensured by a difference-frequency servo control scheme. The frequency-sensitive element generating the error signal for the servo loop comprised a delay-line discriminator constructed from electronic elements which converts a frequency deviation into an amplitude deviation. The output signal is used as an error signal for a servo loop controlling the emission frequency of one laser via the driving current.
Measurements were carried out up to 450 MHz and the frequency response of a photodetector used in a multilayer optical hydrophone system is depicted in Fig. 2. 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. The technique initially dedicated to the determination of the frequency response of photodetectors applied in ultrasonic applications can be transferred to other application fields of optical measurements.

Figure 1: Photograph of the optical set-up

Figure 2: Normalized frequency response of a photodetector used in a multilayer optical hydrophone.

Contact person:

Christian Koch, Department 1.6, ultrasonics@ptb.de