Among the multiple methods for characterising the frequency response of fast photodetectors, using the heterodyne signal produced by the beating of two single-mode laser sources is one of the most common. When both lasers impinge on a biased photodetector, they generate a photocurrent at their difference frequency. Thus, the detector frequency response can be retrieved by shifting the frequency difference and recording the photocurrent signal amplitude as a function of its frequency.
An alternative method for achieving the same goal uses ultra-short optical pulses and a fast oscilloscope to record the electrical impulse response, from which its frequency response can be obtained by means of Fourier transform.
In this regard, Radiantis ORIA IR XT OPOs are outstanding tunable generators of optical femtosecond pulses for this task, as they generate ultra-short pulses centred at any wavelength within the 990 nm – 1550 nm and 1696 nm – 4090 nm ranges, with output powers >1 W. In one of their many applications, they have been successfully used to characterise quantum cascade detectors, ideally suited for high-speed and room temperature operation in the MIR range .
 Hillbrand et al., High-speed quantum cascade detector characterized with a mid-infrared femtosecond, Opt. Express, 29(4), 5774 – 5781 (2021).