The adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) uses two SOAs in a ring-cavity topology: one to amplify the signal and the other to control the gain. The device was designed to maximize the output saturated power while adjusting gain to regulate power differences between packets without loss of linearity. This type of subsystem can be used for power equalisation and linear amplification in packet-based dynamic systems such as passive optical networks (PONs). A detailed theoretical model is presented in this paper to simulate the operation of the AGC-SOA, which gives a better understanding of the underlying gain clamping mechanics. Simulations and comparisons with steady-state and dynamic gain modulation experimental performance are given which validate the model. 1. Introduction Semiconductor optical amplifiers (SOAs) have attracted considerable attention during the last two decades, for use in evolving optical communication networks. SOAs can be used as not only optical amplifiers, but also signal processing devices such as wavelength converters [1], optical switches [2], and electro-optical mixers [3]. In terms of optical amplification, the key issue of operating SOAs is the management of the input optical signal power, which must be maintained within the linear regime of operation; otherwise the device would be driven into saturation causing unwanted intersymbol interference (ISI) or patterning. In order to solve this implementation problem, many different types of gain-clamped semiconductor optical amplifiers (GC-SOAs) have been proposed [4, 5]. Recently, an adjustable gain-clamped semiconductor optical amplifier (AGC-SOA) designed to maximize the output saturated power at a clamped gain which can be adjusted was reported [6]. 2. AGC-SOA The AGC-SOA is a semiconductor optical amplifier topology which has the unique capability to provide variable gain and maintain linear operation through gain clamping over a wide (40？dB) dynamic range, without compromising the saturable output power of the device [6]. A key advantage of this approach is that there are no mechanical tuning elements, and hence the gain can be adjusted via direct electrical control at ns timescales. While the operation of this device has been presented previously for the static gain case [6], and its behaviour under dynamic gain modulation conditions [7], the underlying mechanics is not well understood. Here, a theoretical model for an AGC-SOA has been established, based on the wideband steady-state numerical model of a SOA [8]. The travelling amplified
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