A distributed-feedback laser diode optimized for sensing methane

Sensing of methane using tunable diode laser absorption spectroscopy (TDLAS) is going to be put to practical use in various fields. Further spread of this technology requires lower costs, especially the cost of the distributed-feedback laser diode (DFB-LD). The light source used for TDLAS requires high single-mode oscillation yield, uniform wavelength distribution, large tunability, and low modulation distortion characteristics. We developed a unique DFB-LD that can be fabricated simply without electron beam (EB) lithography. It achieves high wavelength yield, high tunability and low modulation distortion, as well as sufficiently good reliability. The laser has a flat region without a grating structure between two grating regions, and can be fabricated by simple holographic lithography. Since the propagation constant of the flat region is slightly different from that of the grating region, a phase-shift effect can be obtained effectively. In addition, inserting a highly p-doped layer into the p-cladding achieves modulation over the FWHM of the methane absorption line with a small modulation amplitude for a short chip, and more than 100-mW of output power for a long chip.