Electro-Optical Characteristics of the Single Photon Avalanche Diode (SPAD)

Observation of faint sources is affected by a chronic lack of photons. This problem becomes critical when very short integration times are required (from micro to milliseconds). An example is observations of transient phenomena and when using adaptive optics to correct atmospheric turbulence in real time. To detect weak and fast transients CCDs are inadequate because of readout noise and read technique. SPAD detectors, which benefit from photon counting operating mode, can perform much better. SPAD detectors are very innovative and can offer opportunities in astrophysical observations. Electro-optical characterization of these devices is very important to prove their potential and to test the range of their applicability to astrophysical observational programs. Dark rate and quantum efficiency measurements of various SPADs in different operating conditions have been made by using a "reflection objective" that focuses a beam with a FWHM of about 10 mu m diameter on the detector surface. Non-uniformity of the SPAD was also investigated. Here we discuss the equipment used and the results obtained.