A chip-based spectrometer that is smaller than a dime has demonstrated the ability to detect methane in concentrations as low as 100 parts-per-million. The spectrometer leverages silicon photonics technology to realize a compact, cost-effective design that provides IR tunable diode-laser absorption spectroscopy (IR-TDLAS) on a CMOS-compatible platform. Developed by scientists at IBM Thomas J. Watson Research Center, the chip-based spectrometer uses an approach similar to absorption spectroscopy; but instead of a free-space setup, the laser travels through a narrow silicon waveguide that follows a 10-centimeter-long serpentine pattern on top of a chip measuring 16 square millimeters. This artistic rendering depicts the new silicon photonic absorption spectrometer, which is smaller than a dime and can be manufactured using high-volume computer chip fabrication techniques. The portion of infrared light protruding outside the waveguide is absorbed by methane molecules, enabling spectroscopic measurement of the methane concentration. Courtesy of Joe Green, Beaverworks Canada. While most of the light is trapped inside the waveguide, about 25 percent of the light extends outside of the silicon into the ambient air, where it can interact with trace gas molecules that are in the vicinity of the waveguide. Using 1650-nm light from a distributed-feedback laser and an uncooled InGaAs detector, the team utilized the optical field of the silicon waveguide to probe ambient methane. The probe yielded Gaussian-noise-limited sub-100 parts-per-million by volume detection limits. To increase the sensitivity of the device, the team measured and controlled the factors that contributed to noise and false absorption signals, fine-tuning the spectrometer’s design and determining the waveguide geometrical parameters that would produce the most favorable results. The chip-based spectrometer’s performance was compared with that of a standard free-space spectrometer. Researchers placed both devices into an environmental chamber and released controlled concentrations of methane. They found that the chip-based spectrometer provided accuracy comparable to the free-space sensor, even though the chip-based device had 75 percent less light interacting with the air than the free-space device did. Researchers quantified the sensitivity of the chip sensor by measuring the smallest discernable change in methane concentration. The team believes that these results demonstrate the feasibility of chip-scale photonic integration for the development of compact, cost-effective gas sensors for a diversity of energy and environmental uses. The same high-volume manufacturing methods used for computer chips could be applied to the manufacture of the chip-based methane spectrometer, significantly lowering its production cost especially if it were produced in large quantities. “Compared with a cost of tens of thousands of dollars for today’s commercially available methane-detecting optical sensors, volume-manufacturing would translate to a significant value proposition for the chip spectrometer,” said IBM research leader William Green. “Moreover, with no moving parts and no fundamental requirement for precise temperature control, this type of sensor could operate for years with almost no maintenance.” The IBM team is working with partners in the oil and gas industry on a project that would use the spectrometers to detect methane leaks. “During natural gas extraction and distribution, methane can leak into the air when equipment on the well malfunctions, valves get stuck, or there’s a crack in the pipeline,” said Green. “We’re developing a way to use this spectrometer-on-a-chip to create a network of sensors that could be distributed over a well pad, for example. Data from these sensors would be processed with IBM’s physical analytics software to automatically pinpoint the location of a leak as well as quantify the leak magnitude.” Although methane detection was the focus of these experiments, the team believes that the chip-based spectrometer could be used for sensing the presence of other individual trace gases and for detecting multiple gases simultaneously. The current version of the spectrometer requires light to enter and exit the chip via optical fibers. The team is working to incorporate the light source and detectors onto the chip. This would create an essentially electrical device with no fiber connections required. Next year, the team plans to start field testing the spectrometers by placing them into a larger network that includes other off-the-shelf sensors. “Our work shows that all of the knowledge behind silicon photonics manufacturing, packaging and component design can be brought into the optical sensor space, to build high-volume manufactured and, in principle, low-cost sensors, ultimately enabling an entirely new set of applications for this technology,” said Green. The research was published in Optica, a publication of OSA, The Optical Society (doi: 10.1364/OPTICA.4.001322).