SOA-Based Power Equalisation for 100 Gb/s Passive Optical Network

In Passive Optical Networks (PONs), upstream packets arriving at an Optical Line Terminal (OLT) receiver could have up to ~20 dB dynamic range (DR) due to customer-to-OLT differential loss. The conventional approach to equalise upstream packet power variation is to use a burst-mode trans-impedance amplifier (BM-TIA) that resets its electrical gain for every packet. Research attention is currently focusing on spectrally efficient 4-level pulse amplitude modulation (PAM4) as a cost-effective approach to realise 100 Gb/s PON. However, it is challenging to implement BM-TIAs with sufficient linearity, bandwidth and dynamic range to support 50 Gbaud PAM4 transmission, the current state-of-the-art achieving a DR of 15.4 dB [1], for example. Furthermore, such links must meet the 29 dB optical power budget of legacy PON systems, which can be addressed with the use of Semiconductor Optical Amplifiers (SOAs) as a preamplifier in the OLT to increase receiver sensitivity. However, operating the SOA with a constant bias degrades high-power packets due to the gain saturation induced patterning effect. The distortions are nonlinear and hence require Digital Signal Processing (DSP) far more complex than the conventional feed-forward equaliser (FFE) to recover symbols with bit error rates (BERs) below the forward error correction (FEC) limit [2,3]. This work demonstrates the benefits and performance of SOA-based power equalisation in an emulated 100 Gb/s PON upstream (Fig. 1(a)) in a continuous operation mode. We compare two techniques [4,5] to vary the SOA gain according to its input power: (A) variable bias current, and (B) control light injection at a wavelength different from the signal. To the best of our knowledge, this is the first time such techniques have been applied to 100 Gb/s PAM4 for the PON application.