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Fused silica die with photonic crystal. Light scattered by the photonic crystal is visible as a rainbow
spectroscopic gas sensor recently, which is capable of detecting trace gases such as methane, in concentrations as low as 100ppm. The device is the direct result of a programme funded by ARPA-E, a research branch of US Department of Energy. ‘With methane being an incredibly potent
greenhouse gas, its leakage poses hazards to the environment… Oil and gas industry operators therefore have an interest in ensuring they can detect, localise and repair the gas leaks occurring within their production, processing, and distribution operations,’ commented Dr Chi Xiong, a physical sciences research staff member at the Thomas J Watson centre. ‘Our mission
“You are limited to the interaction you can get on a centimetre-scale chip”
is to deliver an entire methane detecting solution, including both the physical sensor and the data analytics that pinpoint the leak and quantify the leak rate.’ The IBM researchers have developed a silicon photonics-based spectroscopic gas sensor that combines a laser, a sensing waveguide and a detector all integrated on a 0.5 x 0.9cm chip, which can be manufactured using the same low-cost, high-volume manufacturing methods of silicon semiconductors. As a result, the new system could cost just a few hundred dollars, including a power source and housing. ‘With as many as one million oil and
gas wells across the US, the volumes that these systems could be deployed in are huge,’ said Xiong. ‘There would generally be multiple sensors within each system, thus the volume of sensors deployed is
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also very large. This is where we think the silicon photonics sensor really has an edge, because it can be manufactured at large scales and therefore at low cost, while maintaining performance.’ In contrast, the benchtop spectroscopic
gas analysers available on the market – featuring large standalone lasers, prisms, mirrors and detectors – can cost tens to hundreds of thousands of dollars, making them unsuitable for high-volume manufacturing. These larger systems can offer parts-per-billion sensitivities. IBM’s new spectroscopic gas sensor uses absorption spectroscopy, which measures how particular wavelengths of laser light are absorbed by molecules. Traditional set ups involve the laser light travelling through air, or free space, until it reaches a detector, which then measures how the light was absorbed by any gas molecules that might have been present, allowing their concentration to be calculated. The new system uses a similar approach;
however, instead of a free-space setup, the laser light – at near-infrared (NIR) wavelengths for methane detection – travels through a narrow silicon waveguide spiralling on the chip, which is 20 to 30cm in total length. While some of the light is trapped inside the waveguide, around 25 per cent of it extends outside the silicon and into the ambient air, where it can interact with any trace gas molecules passing nearby the sensor waveguide. To compare the new device’s
performance with that of a standard free-space spectroscopic gas analyser, the researchers placed the devices into an environmental chamber and released controlled concentrations of methane, finding that the chip-based spectroscopic gas sensor provides accuracy on a par with
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