Type-II interband cascade lasers (ICLs) based on the GaSb material system represent an enabling technology for laser absorption spectroscopy in the 3-to-5-μm wavelength range. Instruments operating in this spectral regime can precisely match strong absorption lines of several gas molecules of interest in atmospheric science and environmental monitoring, specifically methane, ethane, other alkanes, and inorganic gases. Compared with non-semiconductor-based laser technologies, ICLs can be made more compact and power efficient, ultimately leading to more portable, robust, and manufacturable spectroscopy instruments.

Schematic of the interband cascade laser (ICL) fabrication process, showing (a) etching of the ridge waveguide through the cladding and spatial-confinement heterostructure layers, but not through the laser active region; (b) patterning of the laterally coupled Bragg gratings; and (c) etching of a wider ridge through the laser active region to restrict lateral current spreading.
An alternative fabrication method for single-mode DFB (distributed feedback) ICLs avoids etching gratings through the laser active region, yet does not introduce additional optical loss with deposited metal gratings. A narrow ridge with shallow lateral gratings is etched directly into the semiconductor cladding layers above the laser active region, while an additional etch is used to pattern a wider ridge structure through the active region. This fabrication method ultimately allows for independent patterning of an optical confinement structure, a distributed-feedback grating, and an electric confinement structure, which addresses each fabrication step individually and allows for optimized performance and reliability.

The fabrication process involves three plasma etching steps to define optical and electrical confinement structures in a semiconductor ICL wafer. This technique enables the fabrication of low-loss, low-order gratings without etching high-aspect-ratio corrugations, while facilitating better current confinement by using a straight etch through the ICL active region at a distance far from where the optical mode is generated. This process has resulted in a high yield of lasers with low operating current, above-room-temperature operation, and output powers exceeding 15 mW operating at single-mode emission with at least 25 dB side-mode suppression. The benefit of this fabrication technique is preservation of the critical dimensions of lower-order lateral gratings by separating the ridge waveguide and grating fabrication steps. Furthermore, by removing the active region beyond the distance of optical mode generation, current spreading was minimized and reliability concerns that arise when the active region is exposed close to the sidewall of the ridge waveguide were reduced.

Compact, single-frequency lasers operating in the 3-to-5-μm range can access a wealth of scientifically important gas molecules and their isotopes through the technology of tunable laser absorption spectroscopy. Thus, fabrication techniques that mature the technology of mid-IR semiconductor lasers increase the precision and accuracy in the science field to which they are applied.

This work was done by Clifford F. Frez, Carl E. Borgentun, R yan M. Briggs, Mahmood Bagheri, and Siamak Forouhar of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

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