A Low Noise Receiver For Free Space Optical Links

A 0.8 μm direct detection receiver front end (RFE) for operation in a system test bed (STB) for ESA's SILEX programme is presented. In the RFE a low capacitance high sensitivity silicon avalanche photo diode (APD) converts the weak optical receive signal (down to - 64 dBm average at 15 Mbit/s) to an electrical current. The APD is operated at a reverse bias voltage of 198 V yielding a multiplication factor around M=100 for maximum overall SNR. A low noise transimpedance (LNA) amplifier with GaAs MESFET input stage further amplifies this current. The feedback resistor is switchable with GaAs MESFETs for adaption of the overall response to bit rate and for optimization of sensitivity at the various bit rates. The LNA delivers a 10 mVpp range signal to a gain controlled main amplifier (MA) with a constant 0.5 Vpp output. The MA core is a custom designed monolithic integrated silicon variable gain broadband amplifier with a gain from 0 to 46 dB and a bandwidth from 100 kHz to 500 MHz. Both LNA and MA are realized using unpackaged chips mounted on thin film substrates in order to minimize stray and package capacitances and to provide good reproducibility. They are housed in a common COVAR package prepared for hermetical sealing. Gain control, bandwidth switching and switching-mode high-voltage APD bias supply are realized with specific circuitry and implemented on two separate PCBs contained in separate chambers of an aluminium package. The RFE consumes around 1.3 W of power, measures 51x70x75 mm3 and weighs 280 g. Mass and volume will be further reduced in the flight model design.