Experimental results of a single-photon avalanche diode (SPAD) based optical fiber receiver integrated in 0.35 μm PIN-photodiode CMOS technology are presented. To cope with the parasitic effects of SPADs an array of four receivers is implemented. The SPADs consist of a multiplication zone and a separate thick absorption zone to achieve a high photon detection probability (PDP). In addition cascoded quenchers allow to use a quenching voltage of twice the usual supply voltage, i.e. 6.6 V instead of 3.3 V, in order to increase the PDP further. Measurements result in sensitivities of -55.7 dBm at a data rate of 50 Mbit/s and -51.6 dBm at 100 Mbit/s for a wavelength of 635 nm and a bit-error ratio of 2 x 10-3, which is sufficient to perform error correction. These sensitivities are better than those of linear-mode APD receivers integrated in the same CMOS technology. These results are a major advance towards direct detection optical receivers working close to the quantum limit.
Besides this 4-SPAD receiver, in the FWF project SPOR SPADs with thick absorption zone, active quenching and gating circuits for increasing the excess bias voltage and thereby the PDP of SPADs were successfully investigated. The goal of -55dBm sensitivity of SPAD receivers at 100Mb/s was almost reached with -54.6dBm. Compared to optical receivers exploiting avalanche photodiodes in the linear mode, which leave a gap to the quantum limit set by the Poisson statistics of 20dB, a gating SPAD receiver of SPOR reduced the distance to the quantum limit to 8.1dB at 20Mb/s, to 9.9dB at 50Mb/s and to about 14dB at 100Mb/s. SPAD receivers, therefore, can achieve sensitivities being 6 to 12 dB better than APD receivers and being 16 to 22 dB better than receivers with pin photodiodes. For more details and results click "To publication list".