Packet equalisation in PONs using adjustable gain-clamped semiconductor optical amplifiers (AGC-SOA)

The growth in demand for high bandwidth services stimulated use of Passive Optical Networks (PONs), directly to the home or to the kerb. In many cases, particularly extended reach PONs which may cover distances of 100 km or more, there is the need for low cost reach extension technologies. Semiconductor Optical Amplifiers (SOAs) have a key role in this context, particularly because upstream traffic is commonly carried at 1.3 μm. Upstream traffic in a PON (from the user to the Optical Network Unit, ONU) is normally Time Division Multiplexed (TDM) with a wide variation in path loss arising from differences in transmission distances and splitting losses. The bursty nature of this traffic combined with a wide dynamic range of signal strength (-15dB to -28dB - the difference between a very close Optical Network Terminal (ONT) with a small split ratio and a distant ONT with a high split ratio), places severe demands on the burst mode receiver at the ONU. Conventional fibre amplifiers cannot adjust their gain in response to packet to packet variations due to their response time. Similarly, conventional SOAs suffer loss of linearity if their bias current and hence gain is rapidly reduced. We report here on an adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) designed to maximize the output saturated power while adjusting gain to regulate the power differences between packets without loss of linearity. This device is shown to be able to modulate gain at rates that are compatible with packet to packet equalisation. Through theoretical analysis coupled with experimental verification, this paper demonstrates for the first time that this is entirely feasible.