Multi-component gas detection in the mid-IR

Accurate measurement of gas concentrations is paramount in many industrial applications, i.e., ranging from emissions control and pollution monitoring to optimization of chemical reactions. The most trivial gas concentration measurement techniques rely on direct absorption spectroscopy. In this type of spectroscopy, a gas is probed with a laser beam and the specific absorption lines are identified and associated with particular species. More advanced methods include integrated cavity output spectroscopy or cavity ring-down spectroscopy. In these methods, the effective absorption experienced by the light beam is much higher. This allows substantially better sensitivity levels to be achieved.^{1, 2} A common feature to all these techniques is the use of a narrowband laser source (i.e., in which the laser wavelength is very precisely matched to the absorption line of the gas being measured), which generally means that the analysis is limited to a single species.

Many gases of interest possess very strong absorption lines in the mid-IR spectral region. In fact, these absorption lines are stronger than in any other part of the electromagnetic spectrum. The recent emergence of novel light sources in the mid-IR has therefore made this spectral window an increasingly popular choice for gas detection. Typical high-brightness light sources that are used in this wavelength range include optical parametric oscillators (based on difference frequency generation)³ or quantum cascade lasers.⁴ Tunable quantum cascade lasers are widely used with multipass cavity lasers. These devices offer good performance levels, but they operate relatively slowly when tuned over a broad spectrum.⁵

Over the past decade, the generation of spatially coherent supercontinuum radiation—laser sources in which narrowband pulses experience massive spectral broadening in a nonlinear optical fiber^{6, 7}—has revolutionized many applications (e.g., frequency metrology, microscopy, and imaging). Such fiber supercontinuum sources have found significant commercial success because of their unmatched bandwidth and brightness characteristics. As such, in our work, we developed a new approach for multi-component gas detection at mid-IR wavelengths in which we use incoherent broadband cavity-enhanced absorption spectroscopy and a supercontinuum light source extending from about 900–3700nm.⁸

Incoherent broadband cavity-enhanced absorption spectroscopy⁹ is a simple and robust method for high-sensitivity gas detection. In this technique, a broadband source is coupled to a high-finesse confocal cavity that is filled with gas, and the transmitted light is analyzed with a wavelength-sensitive detection system. The high reflectivity of the cavity mirrors causes the beam to experience a long effective optical path length, which allows detections of very low gas concentrations. If the reflectivity of the mirrors is high over a large spectral region, it is possible (in principle) to simultaneously detect several gases with absorption lines in different wavelength ranges, using a single light source. An important aspect of incoherent broadband cavity-enhanced absorption spectroscopy, however, is the signal-to-noise ratio. This must be sufficient over the measurement bandwidth. A source such as a supercontinuum—which inherently has a high brightness over a very large bandwidth, and which is perfectly spatially coherent—is therefore ideal for exploiting the full potential of this method. We (and others) have therefore made substantial efforts in the development of supercontinuum sources. Until now, however, their potential as a powerful light source for application in multi-component gas detection techniques has not been verified. We have demonstrated multi-component gas detection, with high sensitivity in the mid-IR, over a bandwidth that extends from 3000–3500nm (i.e., a range that corresponds to the strong absorption region of acetylene and methane). Our results are important because they illustrate the potential of incoherent supercontinuum sources for spectroscopy in the mid-IR and because they represent the largest continuous detection window reported so far with this technique.

We constructed our all-fiber supercontinuum from off-the-shelf components. Specifically, we generate the supercontinuum by injecting 10kW nanosecond pulses from a gain-switched fiber laser (at 1547nm) into a 4m-long dispersion-shifted fiber. This fiber is subsequently connected to a 6m-long fluoride fiber. Our approach allows us to efficiently exploit the nonlinear effects that are responsible for the massive spectral broadening of the injected pulses. The resulting supercontinuum extends into the mid-IR wavelengths, i.e., with a bandwidth ranging from about 900–3700nm (see Figure 1). We then couple the supercontinuum to a 1m-long confocal cavity, which is formed from two high-reflectivity (for wavelengths between 300nm and 3500nm) mirrors, corresponding to an effective optical path on the order of 300m over this bandwidth. Finally, we simply monitor the transmitted spectrum with a monochromator and a photodetector.

Figure 1. Spectrum of the supercontinuum used for incoherent broadband cavity-enhanced absorption spectroscopy in the mid-IR.

We are able to retrieve the concentration of gases present in the cavity with parts-per-million-level accuracy (see Figure 2). To reach this level of sensitivity, we developed a complete model of the measurement system. In this model we account (as is the standard) for the absorption cross-section lines of the gas, which we obtain from the High-Resolution Transmission Molecular Absorption (HITRAN) database. In our model, however, we additionally account for other parameters, such as the wavelength-dependence of the mirror reflectivity, the spectrometer resolution, and any possible system drift that occurs during the measurement acquisition period.

Figure 2. Comparison between the measured and modeled absorption lines of acetylene (from 3000–3150nm) and methane (from 3150–3450nm). This allows the concentration of the two gases to be retrieved with accuracies of 5 parts per million (ppm) and 2ppm, respectively.

We have successfully demonstrated supercontinuum sources for incoherent broadband cavity-enhanced absorption spectroscopy and multi-component gas detection at mid-IR wavelengths. Our results indicate the potential of compact supercontinuum sources for spectroscopic applications in the mid-IR, where several gases exhibit strong absorption lines. Our work also illustrates the benefits of incoherent absorption spectroscopy and thus provides a viable alternative to the use of single wavelength lasers for gas analyses. In our current work, we are developing supercontinuum sources with higher spectral densities. In this way, we hope to achieve sensitivities at the parts-per-billion level.