Performance analysis of a directly modulated semiconductor optical amplifiers using non-return-to-zero, duobinary and quaternary pulse amplitude modulation signalling

Abstract

Numerical investigation was done for the response of two semiconductor optical amplifiers (SOA) to direct modulation using three of the most commonly employed intensity modulation formats: non-return-to-zero (NRZ), duobinary (DB) and quaternary pulse amplitude modulation (PAM-4). It was shown that an optimized SOA not only delivers an average optical power of 18 dBm, but can also be modulated at 25.4, 28.0 and 29.3 Gb/s with the aforementioned formats, respectively. No optical or electronic assistance is considered. The use of spectral-efficient PAM-4 thus leads to an improvement of almost 4 Gb/s with respect to NRZ. Slightly faster modulation is reached by operating the SOA deeper into saturation. These figures fall to 12.8, 15.0 and 16.3 Gb/s after 20 km conventional single-mode fibre propagation in C-band. Calculation of 10 Gb/s back-to-back sensitivities at the low-density parity-check forward error correction bit-error ratio (BER) threshold of 1×10−2 resulted in −24.5, −20.5 and −20.0 dBm for NRZ, DB and PAM-4 modulations, with a 50 km transmission penalty lower than 3 dB for PAM-4. This enables the use of directly modulated SOAs as transmitter alternative in high-loss access networks, such as super-passive optical networks, provided that further technical improvements can be performed to meet its challenging loss budget.