Sensitivity of avalanche photodetector receivers for long-wavelength optical communications

We consider, in detail, the potential improvement in receiver sensitivity that can be realized using an avalanche photodiode (APD) rather than the conventional p-i-n diode in long-wavelength optical communications systems. Numerical computations are used to determine optimum gains and receiver sensitivities for several values of ionization coefficient ratios and dark currents. Sensitivities are considered for transmission bit rates of 45 Mb/s, 90 Mb/s, and 274 Mb/s — values characteristic of present long-wavelength systems. We find that general relationships and scaling laws between receiver sensitivity and the other critical parameters can be formulated if the sensitivity is calculated in units relative to the quantum limit. An important result is that the improvement in APD sensitivity depends strongly on dark current, but only weakly on the ionization coefficient ratio. Our calculations are compared with recent results obtained for In0.53Ga0.47As/InP APDs sensitive in the λ = 0.95 μm to 1.6 μm wavelength region. We also include a brief discussion comparing APD sensitivities with those obtained using phototransistors and majority carrier devices.