Sensitivity Improvement in 100 Gb/s-per-Wavelength Links Using Semiconductor Optical Amplifiers or Avalanche Photodiodes

Intra-data-center links are scaling to 100 Gb/s per wavelength using four-level pulse-amplitude modulation (4-PAM) and direct detection, but a restrictive link budget makes it difficult to support wavelength-division multiplexing or optical circuit switching. We investigate receiver sensitivity improvements achievable using semiconductor optical amplifiers (SOAs) or avalanche photodiodes (APDs). We analyze and simulate the impact of modulator bandwidth limitations and chirp, fiber dispersion, transient SOA gain saturation, excess shot noise, and intersymbol interference caused by the APD. Using the 4-PAM and linear equalization, an SOA with 20-dB fiber-to-fiber gain and 6-dB noise figure can improve the receiver sensitivity up to 6 dB over a thermal noise-limited receiver, whereas an APD with k(A) = 0.18 and responsivity R = 0.74A/W provides 4.5-dB sensitivity improvement over the same reference system. We further present a simple algorithm to optimize the intensity levels and decision thresholds for a generic non-Gaussian noise distribution, which can provide an additional sensitivity improvement of 1-2 dB. For SOAs, transient gain saturation effects cause negligible sensitivity penalties, but limit the receiver dynamic range to about 15 dB for the worst case of single-wavelength pre-amplification. For APDs, we show that the APD responsivity, impact ionization factor, low-gain bandwidth, and gain-bandwidth product are all critical parameters governing the system performance.