There is a general need to detect and measure the concentration of multiple hydrocarbon species in a gas. In this work, we have developed a tunable external cavity interband cascade laser (EC-ICL) that covers the wavelength range λ = 3216 to 3479 nm (Δ λ = 263 nm) in continuous wave (CW) mode. The EC-ICL provides a versatile broadband source for hydrocarbon detection and measurements. To demonstrate this capability, we incorporated the EC-ICL into an absorption spectroscopy sensor that includes a detector, data acquisition electronics, and software for data processing and spectral fitting. The AR/HR-coated laser with 3 mm cavity length is mounted epi-up on a heat spreader with a TEC. The external cavity is formed by a ruled diffraction grating in a Littman-Metcalf configuration to achieve broad tuning. The wavelength is tunable across the entire range with speed exceeding 15 Hz and effective system spectral resolution of approximately Δν = 0.7 cm-1 in a broad tuning mode. In addition, we developed and demonstrated mode-hop-free (MHF) tuning capability of the system for up to 0.4 cm-1 tuning range around an arbitrary user selected central wavelength with estimated spectral resolution significantly lower than Δν = 0.01 cm-1. Using the EC-ICL, we demonstrated direct absorption measurements of mixtures of methane, ethane, and propane inside an absorption cell. Furthermore, we demonstrated high resolution MHF measurements for methane in a low pressure fiber gas cell. The EC-ICL technology demonstrated in this work is appropriate for a variety of tunable laser applications spanning λ = 3 – 6 μm.
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