Efficiency-boosted semiconductor optical amplifiers via mode-division multiplexing

Semiconductor optical amplifiers (SOAs) are a fundamental building block for many photonic systems. However, their power inefficiency has been setting back operational cost reduction, circuit miniaturization, and the realization of more complex photonic functions such as large-scale switches and optical phased arrays. In this work, we demonstrate significant gain and efficiency enhancement using an extra degree of freedom of light—the mode space. This is done without changing the SOA’s material design, and therefore high versatility and compatibility can be obtained. Light is multiplexed in different guided modes and reinjected into the same gain section twice without introducing resonance, doubling the interaction length in a broadband manner. Up to 87% higher gain and 300% higher wall-plug efficiency are obtained in a double-pass SOA compared to a conventional single-pass SOA, at the same operating current, in the wavelength range of 1560–1580 nm.