We examine the limits of receiver sensitivity at capacity for modulation formats compatible with direct detection receivers, using parameters representative of commercial Avalanche Photodiodes (APDs). Considering sources of noise to be shot noise of the signal and dark current, and thermal noise of post-detection electronics (transimpedance amplifier, or TIA), we determine Shannon capacities for On-Off Keying and M-ary Pulse Position Modulation, assuming silicon APDs for 850 nm and InGaAs APDs for 1550 nm. We also explore the Sb-based APDs presently being investigated in many laboratories. While the low k-factor in silicon produces performance approaching that of preamplified receivers, the higher k-factor in InGaAs produces performance gaps ⪆ 10 dB at 1550 nm compared to other mature detection methods. Although several reported Sb-based APDs have low k-factors, their relatively high dark currents still render performance gaps similar to those of InGaAs.
|