Semiconductor Lasers and Photonic Integrated Circuits

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Overview

Gain-Clamped SOA

Description

Semiconductor Optical Amplifiers (SOAs) are of great interest for switching and gate applications in WDM based optical networks since they allow very high on-off switching ratios and can provide, in addition, broadband signal amplification. However, the gain saturation in conventional SOAs generates inter-channel crosstalk which severely limits their usage in WDM based systems. Moreover, fast gain dynamics in SOAs in conjunction with gain saturation may generate intersymbol interferences, leading to signal distortions even in single-channel systems. To overcome these limitations gain-clamped SOAs (GC-SOAs) have been proposed and fabricated in the last decade. In GC-SOAs, the optical gain is clamped by a lasing mode generated outside the signal band. The gain-clamping lasing mode can be introduced, for example, by attaching to the ends of the active section either passive distributed Bragg reflectors (DBRs) or active distributed feedback (DFB) structures [1]. As a result, the gain in GC-SOAs remains constant with respect to input power variations, as long as the amplified signal power is less than a certain critical power, leading to a flat gain versus output power response.

Typical Results

The simulation setup illustrating the design of a GC-SOA is shown in Figure 1. The GC-SOA in this example consists of a two-contact active section surrounded by two DBR sections. The first 300 µm of the active layer are injected with 300 mA current while the subsequent 300 µm layers are injected with 280 mA. The length of each DBR region is 100 µm. The idea of such a two-contact GC-SOA has been suggested in [2]; using two contacts in the active layer allows improving the gain flatness of the SOA significantly. Parameters of the active layer correspond to SOA parameters employed in [3]. Figure 2 shows the signal spectrum after passing the GC-SOA. It can be noticed that in addition to the amplified signal at 193.1 THz, the GC-SOA generates a lasing mode at 194.1 THz which is responsible for the desired gain clamping. Figure 3 and Figure 4 represent the dependences of the gain and noise figure of the modeled GC-SOA on the input signal power. Remarkably, the observed variations of the gain do not exceed 0.05 dB for the input signal powers up to 0 dBm. The noise figure of the modeled GC-SOA does not exceed 10.5 dB. It can potentially be reduced to around 7 dB by employing unbalanced Bragg reflectors [4]. The module PhotonicsTLM employed in this setup permits to model various advanced SOA designs with multiple sections of different types, including bulk and MQW active layers, DFB and DBR sections, multiple contacts for injection currents, for instance.

Keywords

Semiconductor Optical Amplifier (SOA), Gain-clamped SOA (GC-SOA), Gain Flatness, SOA in WDM systems