Measurement of the absolute linearity of photodetectors with a diode laser

An automated instrument based on the beam-addition technique has been developed for measurement of the linearity of photodetectors. The system is designed for absolute characterization of a transfer standard photodetector, against which the linearity of other detectors can be measured. The measurement set-up has been made as simple as possible. A diode laser at 633 nm is used as the light source due to the good stability and high power output available with the device. Two measurement beams of 0.9 mm diameter are aligned to intercept on the photodetector. As an example, a silicon photodiode of the type S1227 has been studied and found to be linear within 3 × 10-5 up to 7 mW of optical power.