Power and Scintillation measurements with Avalanche Photodetectors

Abstract

Avalanche photodiode based optical receiver frontends outreach the sensitivity of positive intrinsic negative photodiode receivers and are a key element in free space optical communication. Satellite downlinks are impacted by atmospheric index of refraction turbulence induced fluctuations of signal power and thus must be precisely monitored in order to deploy appropriate countermeasures. Previous methods of power and scintillation monitoring excluded the capabilities of the receiver frontend. The photocurrent is a function of the avalanche multiplication factor M and has a non-linear relation to received optical power. A deviation of received optical power from photodiode current and temperature is presented. The diode’s temperature coefficient is determined and a calculation of the current multiplication factor is implemented. Sensitivity measurements of a receiver frontend for 10 Gbit s−1 verify the found parameters. Optical input power is determined based on direct monitoring of the photodiode current with a high-side current monitor and a power sensor, measuring the output amplitude of the transimpedance amplifier. Results of the power sensor validate the implementations. Synthetic generated power vectors with fixed power scintillation index are used to be measured by the received signal strength indication output of the receiver frontend’s limiting amplifier and the power sensor. The determined power scintillation indices overall match the expectations and show particularly good accuracy when using the power sensor.