KEYWORDS: Spectroscopy, Signal to noise ratio, Quantum cascade lasers, Absorption, Methane, Spectral resolution, Optical engineering, Signal detection, Digital filtering, Sensors
While midinfrared radiation can be used to identify and quantify numerous chemical species, contemporary broadband midinfrared spectroscopic systems are often hindered by large footprints, moving parts, and high power consumption. In this work, we demonstrate multiheterodyne spectroscopy (MHS) using interband cascade lasers, which combines broadband spectral coverage with high spectral resolution and energy-efficient operation. The lasers generate up to 30 mW of continuous-wave optical power while consuming <0.5 W of electrical power. A computational phase and timing correction algorithm is used to obtain kHz linewidths of the multiheterodyne beat notes and up to 30 dB improvement in signal-to-noise ratio. The versatility of the multiheterodyne technique is demonstrated by performing both rapidly swept absorption and dispersion spectroscopic assessments of low-pressure ethylene (C2H4) acquired by extracting a single beat note from the multiheterodyne signal, as well as broadband MHS of methane (CH4) acquired with all available beat notes with microsecond temporal resolution and an instantaneous optical bandwidth of ∼240 GHz. The technology shows excellent potential for portable and high-resolution solid-state spectroscopic chemical sensors operating in the midinfrared.
Majority of chemical species of interest in security and safety applications (e.g. explosives) have complex molecular structures that produce unresolved rotational-vibrational spectroscopic signatures in the mid-infrared. This requires spectroscopic techniques that can provide broadband coverage in the mid-IR region to target broadband absorbers and high resolution to address small molecules that exhibit well-resolved spectral lines. On the other hand, many broadband mid-IR absorbers exhibit well-resolved rotational components in the THz spectral region. Thus, development of spectroscopic sensing technologies that can address both spectral regions is of great importance. Here we demonstrate recent progress towards broadband high-resolution spectroscopic sensing applications with Fabry-Perot quantum cascade lasers (QCLs) and frequency combs using multi-heterodyne spectroscopy (MHS) techniques.
In this paper, we will present spectroscopic sensing of large and small molecules in the mid-IR region using QCLs operating at ~8.5µm. An example high-resolution, broadband MHS of ammonia (small molecule) and isobutane (broadband absorber) at atmospheric pressure in the 1165-1190 cm^-1 range will be discussed. We have developed a balanced MHS system for mitigation of the laser intensity fluctuations. Absorption spectroscopy as well as dispersion spectroscopy with minimum fractional absorption down to ~10^-4/Hz1/2 and fast spectral acquisition capabilities down to 10 µs/spectrum range will be demonstrated. In order to mitigate the shortcomings of the limited chemical selectivity in the mid-IR, THz QCL based spectrometer is currently under development to provide spectral de-congestion and thus significantly improve chemical identification. Preliminary characterization of the performance of THZ QCL combs for the THz QCL-MHS will be presented.
Interband cascade lasers (ICLs) have proven to be efficient semiconductor sources of coherent mid -infrared (mid-IR) radiation. Single mode distributed-feedback (DFB) ICLs are excellent high-resolution spectroscopic sources for targeting important molecular species in the mid-IR fingerprint region, but are limited to a narrow spectral tuning range. Recent developments in multi-heterodyne spectroscopy with multi-mode Fabry-Perot (FP) lasers have enabled significant progress towards broadband high-resolution spectroscopic sensing applications in the mid-infrared. Here, we characterize the mode structure and tuning properties of multi-mode FP-ICLs for the purpose of evaluating the feasibility of ICL-based multiheterodyne spectroscopy.
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