Collaborating researchers from Princeton University and the U.S. Naval Research Laboratory demonstrated gas sensor technology capable of detecting very low concentrations of methane gas, at levels as low as 0.1 ppm (parts per million). Methane emission and leaks, detrimental to air quality and a contributor to global warming, are consequential to numerous industries and applications. Agriculture, waste/waste management, and the oil and gas industries are among those that consistently need to detect quantities of methane gas. The newly developed sensor uses an interband-cascade light-emitting device (ICLED), a higher-power LED that emits at mid-IR wavelengths, meaning that it can be used to measure many chemicals. The researchers’ versions, developed by Jerry Meyer’s team at the U.S. Naval Research Lab, emitted roughly 10× more power than commercially available mid-IR LEDs generate. Meyer said the devices could potentially be mass-produced, meaning the team, in its recent work, made strides toward the development of ICLED-based sensors that cost less than $100 per device. To obtain a methane measurement, the sensor measures infrared light transmitted through clean air absent of methane and compares that measurement with transmission through air that does contain methane. The researchers sent the infrared light from the ICLED through a hollow-core fiber measuring just under 40 in. (1 m) that contained an air sample. The fiber’s interior was silver-coated. This caused light to reflect on its interior surface as it traveled down the fiber to the photodetector positioned at the other end. Light interacted with additional methane molecules present in the air, resulting in a higher level of light absorption. Researchers have developed a new sensor that uses an interband-cascade light-emitting device (ICLED) and could allow practical and low-cost detection of low concentrations of methane. Courtesy of Sameer A. Khan via Fotobuddy. Mirrors are commonly used in other architectures to repeatedly bounce light back and forth to increase sensor sensitivity. The fiber design bypasses the use of a mirror, which can be bulky and require precise alignment, said Nathan Li, first author on the paper describing the sensor. “Hollow-core fibers are compact, require low volumes of sample gas, and are mechanically flexible,” he said. The design also provides an alternative to commonly used laser-based sensors. These sensors are effective methane detectors, but they can be expensive — especially as part of a network, such as what might be needed to detect leaks across a landfill, wastewater treatment plant, or farm. To test their device, the researchers introduced known concentrations of methane into the fiber and compared the samples’ infrared transmission with laser-based sensors. In addition to detecting concentrations as low as 0.1 ppm, the researchers’ sensor showed excellent agreement with both calibrated standards and the laser-based sensor. To boost sensitivity, infrared light from the high-power ICLED travels through a 1-m-long, hollow-core fiber containing an air sample. The inside of the fiber is coated with silver, which causes the light to reflect off the the interior surfaces as the light travels down it to the photodetector at the other end. This allows the light to interact with additional molecules of methane in the air, resulting in higher absorption of the light. Courtesy of Nathan Li, Princeton University. “This level of precision is sufficient to monitor emissions near sources of methane pollution,” Li said. “An array of these sensors could be installed to measure methane emissions at large facilities, allowing operators to affordably and quickly detect leaks and mitigate them.” The researchers plan to attempt to increase the mechanical stability of the hollow-core fiber component of the device, increasing the sensor’s practicality for long-term field measurements. They will also study how extreme weather conditions and changes in ambient humidity and temperature might affect the system. The researchers said that because most greenhouse gases and many other chemicals can be identified using mid-IR light, users could adapt the methane-specific sensor to detect other important gases. The research was published in Optics Express (www.doi.org/10.1364/OE.415724).