Comparison of available detectors for digital optical fiber systems for the 1.2-1.55 µm wavelength range

Receiver sensitivity is estimated at 1.3 and 1.5 μm for commercial Ge APD's for bit rates between 8 and 1200 MBd, for a variety of APD diameters and operating temperatures. Although both holes and electrons are injected into the depletion region at these wavelengths, the measured photocurrent excess multiplication noise is found empirically to be well described by the simple expression for unilateral carrier injection into the depletion region, while the measured noise on the bulk leakage current can be characterized by the photocurrent parameters for wavelengths
\sim1.8 \mu
m. Measurements at BTRL on a 140 Mbit/s system receiver using an Optitron GA-1 Ge APD at temperatures in the range
20-60\deg
C agree within 1 dB with the theoretical model using these data. The performance of Ge APD-based receivers is strongly influenced by noise on the leakage current and is therefore susceptible to temperature fluctuations. The ionization rates for holes and electrons are comparable in Ge, resulting in a high excess noise factor and a strong dependence of multiplication factor on bias voltage. Thus, APD's for long-wavelength digital optical receivers operating below ∼1 GBd require a bulk leakage current density
\ll 10^{-4}
A/cm
²
and markedly different ionization rates for holes and electrons, in order to match the otherwise superior performance of the present-day high-impedance p-i-n/FET hybrid.