Generally, distributed feedback (DFB) laser diodes are used in these systems, however, they are characterized by a rather limited tuning range, which in most cases restricts the use of the devices to the detection of just one kind of gas.
Our approach, detailed in Nanotechnology 19 235202, to overcome this limit in tuning range is based on a GaSb-based coupled-cavity laser design, which relies on the incorporation of intracavity mirrors consisting of eight rows of two-dimensional photonic crystals lateral to the ridge waveguides. The superposition of the mode combs of the two monolithically fabricated Fabry-Pérot resonators defined by this results in the amplification of only one single longitudinal mode at a time in conjunction with the laser’s gain profile.
By utilizing our recently developed nanofabrication technique (Nanotechnology 19 015203) we were able to etch high-quality photonic crystal patterns and thus keep optical losses to a minimum. Side-mode suppression ratios of more than 30 dB, threshold currents of less than 20 mA and output powers exceeding 18 mW could be achieved – values that are comparable to those reported for standard DFB lasers at the same wavelength.
With increasing driving current and/or varying temperature the lasers show both discrete and continuous wavelength tuning behavior, which is in-line with theoretical sub-threshold simulations. This behavior could enable quasi-simultaneous spectroscopy of multi-gas mixtures with just one single laser device, in the wavelength region of 2–3 µm accessible by using GaSb-based diode lasers. In fact, CO2 detection has already been successfully shown in the Nanotechnology paper.