Sensitivity of Avalanche Photodetector Receivers for Long-Wavelength Optical Communications

In contrast to short wavelength (0.8 jum to 0.9 jtxm) lightwave transmission systems, which generally use an avalanche photodiode (APD) as the detection element, 1 many receivers for long wavelength (1.3 jum to 1.55 jam) lightwave systems use p-i-n detectors. 2,3 The principal reason for using the APD at short wavelengths is the ~15-dB improvement in sensitivity obtained compared with a p-i-n with a nonintegrating front end. The arguments given for using p-i-ns at long wavelengths are: (i) sensitivities comparable to those obtained at short 2929 wavelengths can be achieved using GaAs field-effect transistor (FET) front-end amplifiers; (ii) the ionization coefficients for electrons and holes in InP and related InGaAsP compounds are not significantly different, leading to large excess noise factors and, hence, poor receiver sensitivities; and (iii) the APD is difficult to fabricate, especially with low dark currents. In this paper the subject of receiver sensitivity using APDs is addressed with emphasis placed on the limits imposed by the ionization coefficients and the device dark currents. The principal result is that whereas the ratio of the ionization coefficients determines the maximum sensitivity improvement attainable with an APD, the degree to which this improvement can be achieved in practice is controlled by the dark current. It is concluded that, for sufficiently low dark currents, an APD can yield significant sensitivity improvements over that obtained using a p-i-n detector, resulting in greater regenerator spacings, or permitting the use of less sophisticated amplifiers with greater dynamic range and lower cost.