Tunable single-mode chip-scale mid-infrared laser

Portable mid-infrared (mid-IR) spectroscopy and sensing applications require widely tunable, chip-scale, single-mode sources without sacrificing significant output power. However, no such lasers have been demonstrated beyond 3 μm due to the challenge of building tunable, high quality-factor (Q) on-chip cavities. Here we demonstrate a tunable, single-mode mid-IR laser at 3.4 μm using a tunable high-Q silicon microring cavity and a multi-mode Interband Cascade Laser. We achieve single-frequency lasing with 0.4 mW output power via self-injection locking and a wide tuning range of 54 nm with 3 dB output power variation. We further estimate an upper-bound effective linewidth of 9.1 MHz and a side mode suppression ratio of 25 dB from the locked laser using a scanning Fabry-Perot interferometer. Our laser platform based on a tunable high-Q microresonator can be expanded to higher wavelength quantum-cascade lasers and lead to the development of compact, high-performance mid-IR sensors for spectroscopic applications.

Interferometric Technique for the Spectral Characterization of High Frequency Current-Modulated Mid-Infrared Semiconductor Lasers

We describe two methods, based on Michelson interferometery, that enable the determination of the absolute wavelength of current-modulated semiconductor lasers. By non-linear regression of the instantaneous frequency of the interference signal, the rate of change of the wavelength of the radiation can be inferred. Alternatively, the absolute wavelength can be directly calculated from the maxima and minima of the interference signal. In both cases a reference absorption line enables the determination of the absolute wavelength. Both methods offer respective advantages. The methods allow a highly resolved wavelength measurement under lower kilohertz range current-modulation with relatively little effort. As a result, we present the rates of wavelength change and absolute wavelengths exemplarily for a specific interband cascade laser. It is furthermore shown that the spectral dynamic range of the laser decreases with increasing modulation frequency.