Modal Gain of 2.4- $\mu$m InGaAsSb–AlGaAsSb Complex-Coupled Distributed-Feedback Lasers

High-resolution spectroscopy was used to examine gain characteristics of Cr-grating complex-coupled distributed-feedback (DFB) lasers near 2.4 $\mu$m. The single-mode lasers contain InGaAsSb–AlGaAsSb active regions grown by molecular beam epitaxy on GaSb. Modal gain was extracted from the measured amplified spontaneous emission spectra and compared with reference Fabry–PÉrot lasers. The material gain is similar in both cases, having a value near 1300 cm$^{-1}$, while the internal losses are quite different. The DFBs have an additional loss, approximately equal to the lateral Cr grating coupling coefficient. This indicates a fundamental performance limitation for complex-coupled DFBs.