The quality of a photodetector is characterized by the ultra-low light power that is required to generate a photocurrent that equals the photodetector’s internal noise current. With the photodetector characterisation unit, a photodetector under test can be illuminated by modulated laser diodes of different wavelengths, while light attenuation over 14 orders of magnitude can be achieved down to fW. Using a spectrum analyser, we monitor the photocurrent response at one frequency until the photocurrent response cannot be distinguished from the noise current (Noise Equivalent Power). By doing so, we also obtain the operational window of the photodetector within which the photocurrent response changes linearly with the light power (Linear Dynamic Range). Moreover, by using the Network Analyzer we can identify the frequency window in which the magnitude of the photocurrent response of the photodetector is independent of the light modulation frequency (Frequency Bandwidth).