Fiber Laser Based Mid-Infrared, Wavelength-Agile Sensor for Monitoring Trace Gas Concentrations in Harsh Environments

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This project develops an all-fiber-optic sensor for measuring trace-gas concentrations (in the range of 1 - 10,000 ppm) in harsh environments such as piston and aeropropulsion engines. The sensor uses mid-infrared absorption spectroscopy to probe transitions in the fundamental vibrational bands of the target gases. The sensor accesses strong carbon-monoxide transitions near 4.85 mm, and has the potential to tune throughout the 1.6 - 7.5 mm spectral range; it can also access a host of other gases including nitric oxide (5.2 mm), nitrogen dioxide (6.1 mm), nitrous oxide (4.5 mm), sulfur dioxide (7.2 mm), hydrocarbons (3.4 mm) and a variety of radical species. Many of these important species are difficult to access in other spectral ranges; for example, electronic CO transitions exist only in the vacuum ultraviolet and are therefore impossible to access by single-photon techniques. The heart of the proposed sensor is an all-fiber-optic source for generating rapid wavelength scans in the mid-infrared. The development of such a source is a significant portion of the proposed work. The "wavelength-agile" source scans through approximately 100 nm every 20 ns. This source consists of three elements connected in series: an ultrafast fiber laser operating near 1.56 mm, a nonlinear wavelength-conversion element for shifting the ultrafast pulses to the mid-infrared, and a chirped fiber Bragg grating for arranging the mid-infrared pulses into rapid wavelength scans. To measure trace-gas concentrations, the wavelength-agile output is fiber-coupled to the environment of interest (such as an engine), and the transmitted light is monitored by a small-area infrared detector. In this fashion, a 100-nm-wide absorption spectra can be obtained every 20 ns. Consecutive absorption spectra can be averaged to improve sensitivity; depending on the application and the trace-gas concentration, 10 - 10,000 averages may be used to obtain absorption spectra every 0.2 - 200 ms. Each absorption spectrum is reduced to gas concentration, enabling the sensor to continuously monitor trace gases at sufficiently high rates to track most transient chemical phenomena. The sensor capabilities can be extended by using the same ultrafast fiber laser to generate both mid-infrared, as described in this proposal, and near-infrared, using techniques recently developed by the investigator. Such a sensor can monitor the concentrations of minor and major species as well as gas temperature and pressure in harsh environments. Measurements such as these have been generally unavailable owing to the high pressures, high temperatures, and multiple abundant species present in these environments. However, emerging fiber-optic technology not only offers a solution to this sensing challenge, but also provides the potential for very compact, handheld trace-gas sensors. Sensing of carbon monoxide (CO) is emphasized here because of the associated health effects, regulatory standards, and relevance to fundamental chemical problems in combustion, fuel reforming, and other processes.