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Analysis and comparison of various free-space optical receiver configurations

The paper deals with the analysis of receivers for free-space optical communication systems. The performance of high impedance and transimpedance configurations of front-end amplifiers in combination with an avalanche photodiode and a PIN photodiode are compared. The theoretical sensitivity analysis of optical fiber receivers has been presented in many previous works. But free-space optical systems make somewhat different demands on the receivers. One of them is the relatively large active area of photodiode which is necessary for an optimal coupling to the receiver lens. Large active area causes a large junction capacitance that reduces the bandwidth and increases preamplifier noise as well as photodiode dark current and consequently the photodiode shot noise. These effects decrease the receiver sensitivity, dynamic range and allowable bit rate. The dark current also depends on the photodiode type and the technology and material used. Similarly, bipolar and FET transistors used in the high impedance and transimpedance preamplifiers do not provide the same resulting receiver sensitivity and bandwidth due to their unequal noise and frequency characteristics. Another demand made on the free-space optical receiver is the high dynamic range necessary to limit the influence of varying attenuation in the atmosphere. It can be partially improved by incorporating the automatic gain control loop.