Time domain characterization of avalanche photo detectors for sub-ns optical pulses

The time domain response of high-speed avalanche photo detectors to an impulse resembling optical pulses was studied. In particular, a laser diode giving an optical pulse width of about 100ps (FWHM) was used to provide excitations. The results show that a commercial APD with a cut-off frequency of about 2 GHz extends the output pulse width to ∼ 270 ps and the effective responsivity is thus reduced to about half the nominal value. A custom CMOS APD is capable of responding to fast laser pulses giving a ∼ 200 ps output current pulse width, but the measured effective responsivity was only ∼ 0.1 A/W at a wavelength of 860 nm.