Measured is the relative intensity noise (RIN) of a mid-infrared Peltiercooled distributed-feedback (DFB) quantum cascade laser (QCL) with a feedback loop, where optical feedback parameters affecting the RIN were estimated by using the self-mixing method. To that end, a reflecting mirror forming the external cavity was placed on a voice coil and was moved back and forth after the RIN measurements, by which characteristic waveforms appeared. The amount of external feedback could thereby be deduced. There was no appreciable change in the RIN of the DFB-QCL, even with an optical feedback being in the coherence collapse regime in the case of conventional DFB-laser diodes.

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