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Semiconductor lasers subject to optical feedback usually produce rich nonlinear dynamics beyond a critical feedback level. Below the critical feedback level, the spectral linewidth is either reduced or increased by the feedback depending on the feedback phase. For common quantum well lasers, the critical feedback level is usually below -30 dB, and the spectral linewidth is narrowed by a factor of 2 or 3 for the optimal feedback phase. Interband cascade lasers (ICLs) are power-efficient mid-infrared laser sources. This work experimentally demonstrates that the optical feedback significantly narrows the spectral linewidth of an ICL from 529 kHz down to 27 kHz, without any feedback phase control. The ICL under study is a single-mode distributed feedback laser. The optical feedback is provided by a gold mirror, which is placed 40 cm away from the ICL facet. The spectral linewidth of the ICL is extracted from the measurement of the frequency noise power spectral density. The free-running ICL exhibits a lasing threshold of 24 mA, and a lasing wavelength around 3.38 μm. When the lCL is biased at 53.9 mA, the extracted spectral linewidth of the free-running ICL is 529 kHz, for an observation time of 1 ms. When the ICL is subject to weak optical feedback (feedback ratio<-30 dB), the spectral linewidth is either increased or decreased depending on the feedback phase. However, for moderate optical feedback (feedback ratios from -30 dB up to -13 dB), the spectral linewidth declines with feedback ratio, and is always smaller than the free-running linewidth. The minimum linewidth reaches down to only 27 kHz, which is about 20 times smaller than the free-running one. Beyond the critical level of about -13 dB, the spectral linewidth re-broadens due to the existence of nonlinear pulse oscillations.
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