The Dynamic Gain Modulation Performance of Adjustable Gain-Clamped Semiconductor Optical Amplifiers (AGC-SOA)

The growth in demand for high bandwidth services has stimulated the deployment 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 [1], 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 Optical Network Unit, ONU to the Optical Line Terminal, OLT) 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 ( -15 dBm to -28 dBm-the difference between a very close ONU with a small split ratio and a distant ONU with a high split ratio), places severe demands on the burst mode receiver at the OLT. Conventional fibre amplifiers cannot adjust their gain with 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. The paper reports on an adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) designed to maximize the output saturated power while adjusting gain to regulate power differences between packets without loss of linearity. Theoretical modeling predicts that this device is able to modulate gain at nanosecond rates. The analysis is validated experimentally.