Abstract
DFB quantum cascade laser (DFB-QCL) arrays operating between 8.7 and 9.4 mum are investigated for their performance characteristics-single-mode selection of the DFB grating, and variability in threshold, slope efficiency, and output power of different lasers in the array. Single-mode selection refers to the ability to choose a desired mode/frequency of laser emission with a DFB grating. We apply a theoretical framework developed for general DFB gratings to analyze DFB-QCL arrays. We calculate how the performance characteristics of DFB-QCLs are affected by the coupling strength kappaL of the grating, and the relative position of the mirror facets at the ends of the laser cavity with respect to the grating. We discuss how single-mode selection can be improved by design. Several DFB-QCL arrays are fabricated and their performance examined. We achieve desired improvements in single-mode selection, and we observe the predicted variability in the threshold, slope efficiency, and output power of the DFB-QCLs. As a demonstration of potential applications, the DFB-QCL arrays are used to perform infrared absorption spectroscopy with fluids.
Highlights
M ID-INFRARED quantum cascade lasers (QCLs) are unipolar semiconductor lasers that utilize resonant tunneling and optical transitions between electronic states in the conduction band of a multi-quantum-well heterostructure [1], [2]
QCLs can be grown by metal–organic vapor phase epitaxy (MOVPE) [5] with quality comparable to the best devices grown by molecular beam epitaxy (MBE)
Despite the narrower spectral measurement range compared to Fourier transform infrared spectrometer (FTIR) spectrometers, we believe that a spectrometer based on our DFB quantum cascade laser (DFB-QCL) can provide a portable alternative to FTIR spectrometers in the mid-infrared
Summary
M ID-INFRARED quantum cascade lasers (QCLs) are unipolar semiconductor lasers that utilize resonant tunneling and optical transitions between electronic states in the conduction band of a multi-quantum-well heterostructure [1], [2]. QCLs can be grown by metal–organic vapor phase epitaxy (MOVPE) [5] with quality comparable to the best devices grown by molecular beam epitaxy (MBE) They can be designed with broadband gain, with full-width at half-maximum (FWHM) of more than 300 cm , enabling wide wavelength coverage for spectroscopic applications [6]. Our study has been to develop a broadly tunable single-mode QCL source that combines the advantages of external cavity and DFB devices. We consider the performance characteristics of these DFB-QCL arrays, including single-mode selection, threshold, slope efficiency, and output power. The single-mode selection, threshold, slope efficiency, and output power are a function of the coupling strength of the DFB grating and the position and reflectivity of the end facet mirrors.
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