A temperature-controlled photodetector sub-system is described. The temperature control element allows the operation of the photodetector at a desired temperature. The temperature control element can be a heater or a cooler. In some cases, the photodetector is a germanium photodetector. In some cases a temperature measuring device is provided. In some cases, a control circuit is used to control the temperature of the germanium photodetector within a temperature range, or at a temperature of interest. An advantage provided by the apparatus described is the operation of the photodetector so that the responsivity of the germanium detector can be held at essentially a constant value.