Silicon Photonic Chip Detects Gas Leaks

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. (Source: J. Green, Beaverworks Canada)

The process of extrac­ting natural gas from the earth or trans­porting it through pipe­lines can release methane into the atmo­sphere. Methane, the primary component of natural gas, is a green­house gas with a warming potential approxi­mately 25 times larger than carbon dioxide, making it very efficient at trapping atmo­spheric heat energy. A new chip-based methane spectro­meter, that is smaller than a dime, could one day make it easier to monitor for efficiency and leaks over large areas.

Scientists from IBM Thomas J. Watson Research Center in Yorktown Heights, NY, developed the new methane spectro­meter, which is smaller than today’s standard spectro­meters and more economical to manufacture. The researchers show that it can detect methane in con­centra­tions as low as 100 parts-per-million. The spectro­meter is based on silicon photonics tech­nology. Because the same high-volume manu­facturing methods used for computer chips can be applied to make the chip-based methane spectro­meter, the spectro­meter along with a housing and a battery or solar power source might cost as little as a few hundred dollars if 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 William Green, leader of the IBM Research team. “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.” Such low-cost, robust spectro­meters could lead to exciting new appli­cations. For example, 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, saving companies the time and money involved in trying to find and fix leaks using in-person inspec­tion of thousands of sites.

“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 the researchers demonstrated methane detection, the same approach could be used for sensing the presence of other individual trace gases. It could also be used to detect multiple gases simul­taneously.

“Our long-term vision is to incor­porate these types of sensors into the home and things people use every day such as their cell phones or vehicles. They could be useful for detecting pollu­tion, dangerous carbon monoxide levels or other molecules of interest,” said Eric Zhang, a member of the research team. “Because this spectro­meter offers a platform for multi­species detection, it could also one day be used for health moni­toring through breath analysis.”

The new device uses absorp­tion spectro­scopy, which requires laser light at the wave­length uniquely absorbed by the molecule being measured. In a tradi­tional absorp­tion spectro­scopy setup, the laser travels through the air, or free-space, until it reaches a detector. Measuring the light that reaches the detector reveals how much light was absorbed by the molecules of interest in the air and can be used to cal­culate the concen­tration of them present. The new system uses a similar approach, 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. Some of the light is trapped inside the waveguide while about 25 percent of the light extends outside of the silicon into the ambient air, where it can interact with trace gas molecules passing nearby the sensor wave­guide. The researchers used near infrared laser light (1650 nano­meter wave­length) for methane detection.

To increase the sensi­tivity of the device, the inves­tigators carefully measured and controlled factors that contribute to noise and false absorp­tion signals, fine-tuned the spectro­meter’s design and determined the waveguide geome­trical para­meters that would produce favorable results. To compare the new spectro­meter’s per­formance with that of a standard free-space spectro­meter, they placed the devices into an environ­mental chamber and released controlled concentrations of methane. The researchers found that the chip-based spectro­meter provided accuracy on-par with the free-space sensor despite having 75 percent less light inter­acting with the air compared to the free-space design. Furthermore, the funda­mental sensi­tivity of the chip sensor was quantified by measuring the smallest dis­cernable change in methane concen­tration, showing perfor­mance comparable to free-space spectro­meters developed in other labora­tories.

“Although silicon pho­tonics systems – especially those that use refractive index changes for sensing – have been explored previously, the inno­vative part of our work was to use this type of system to detect very weak absorp­tion signals from small concen­trations of methane, and our compre­hensive analysis of the noise and minimum detec­tion limits of our sensor chip,” said Zhang. The current version of the spectro­meter requires light to enter and exit the chip via optical fibers.

However, the researchers are working to incor­porate the light source and detectors onto the chip, which would create an essen­tially electrical device with no fiber connec­tions required. Unlike current free-space sensors, the chip then does not require special sample or optical prepa­ration. Next year, they plan 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. (Source: OSA)

Reference: L. Tombez et al.: Methane absorption spectroscopy on a silicon photonic chip, Optica 4, 1322 (2017); DOI: 10.1364/OPTICA.4.001322

Link: IBM Thomas J. Watson Research Center, Yorktown Heights, USA

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