We estimated experimentally the linewidth (0.28 MHz) of a distributed feedback quantum cascade laser emitting at 6.2 μm using optical feedback interferometry. The method is simply based on the analysis of the histogram of laser self-mixing fringe periods measured directly as voltage variation across the laser terminals. We assessed the optimal experimental conditions estimating the influence of the optical feedback strength on the interferometric phase noise and compared our results with those reported using conventional interferometric methods based on the analysis of the frequency noise power spectral density.

Optical feedback interferometry is being increasingly applied to imaging of both inorganic and organic samples, especially in the infrared spectral region where imaging detectors are expensive or bulky. We present an attempt to apply optical feedback interferometry to multispectral analysis of polydimethylsiloxane (PDMS) with organic (starch) inclusions, covering the near-and mid-infrared spectrum. The signal analysis discriminated different starch concentration only at the amide-I band, whereas it was insensitive in the near infrared range, where the starch is not absorptive, although diffusive. Following the demonstration of optical feedback interferometry sensitivity to chemical absorption, chemical imaging of in-vivo tissues would become foreseeable.

Self-mixing based on vertical-cavity surface-emitting lasers (VCSELs) offers a compact and low-cost coherent detection scheme for interferometric accessible measurements. The direct detection of the change in the junction voltage, in contrast to the traditional optical detection method by means of an external photodiode, further simplifies the setup by adding detector-less capability. The linewidth of an NIR VCSEL was estimated by using the method based on the statistical analysis of the laser self-mixing fringe period in the moderate feedback regime. We investigate the junction voltage noise and optical power noise, simultaneously acquired, in order to establish the best operational condition for both detection schemes. When comparing the laser linewidth measured by the traditional optical power modulation with that of the detector-less voltage self-mixing signal, the agreement is within the experimental errors.

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