Abstract
In this paper, we present a detailed investigation of three types of tapered distributed feedback (DFB) quantum cascade lasers (QCLs) emitting at $7.2~\mu \text{m}$ . The sampling gratings with different sampling duty cycles were fabricated by holographic lithography combined with the conventional optical photolithography. High output power of more than 550mW with excellent beam quality was achieved in continuous-wave operation at 293K for a 2.5-mm-long DFB-QCL with sampling duty cycles of 0.25 and 0.5 in the tapered region and uniform region, respectively. Stable single-mode emission with a side-mode suppression ratio above 25dB was obtained in the temperature range of 283-318K. The demonstrated tapered DFB QCL structure with asymmetric coupling coefficient can effectively achieve high performance with low fabrication cost, which is beneficial for many applications.
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