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First Photonic Pressure Sensor in Focus

Industrial Applications

The aerospace industry could use photonic pressure gauges to build more sensitive pressure measuring capabilities into their aircraft, which might allow them to cut the vertical separation minima between their planes from 2,000 to 1,000 feet. The semiconductor industry could use these devices to increase their control over sputtering and etch rates for chip manufacture.

A variation on the FLOC could even improve semiconductor manufacturers' ability to do dimensional analyses on their chips with lasers, by allowing them to track the index of refraction of air in real time. Minimizing uncertainties in a laser's interaction with air could give them picometer resolution on the factory floor instead of requiring that those measurements be made under vacuum or in a highly controlled laboratory environment.

Another implication: The new method could impact the redefinition of units. Pressure measurements are currently based on the kilogram, the unit for mass, which is the last of the seven base units to have a physical artifact as a standard. This new way of measuring pressure would no longer depend on the kg but rather on the atomic properties of a gas like nitrogen or helium and how light interacts with it.

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FLOC vs. Mercury Standard

The FLOC's resolution is 0.1 mPa (millipascal or thousandths of a pascal) compared to the mercury manometer's 3.6 mPa. The system's stability is also good, the researchers say. "Its repeatability is astounding," Hendricks says, in the range of 2-5 ppm at one atmosphere of pressure. "We think that, once it is calibrated, we can have this device as a transfer standard that's down to part-per-million stability," Hendricks says. "And that's better than anything else out there." This includes a range of non-mercury pressure sensors used commercially.

"We're talking about a technology that could replace a whole bunch of gauge technologies that people have been using," Hendricks says.

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How Tall

To determine the column's height precisely, modern manometers can use ultrasonic transducers or lasers that bounce sound or light off the mercury's surface and listen for the echo.

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A Degree of Control

The temperature inside the cavities is carefully managed. The room temperature is regulated to within about 50 thousandths of a kelvin (+/-50 mK). Shells of temperature control in the device itself bring that variation down to +/- 0.2 mK or less.

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Standing Waves

Each channel is a Fabry-Perot interferometer. The laser light enters each tube and is trapped by partially transparent mirrors on either end. By adjusting the wavelength (i.e. color) using feedback controls, the light in each channel forms itself into a standing wave independent of the light in the other channel. Some of this trapped light from the two standing waves is allowed to exit through the mirrors at one end of the device, where it combines to form an interference pattern that is picked up by readout electronics.

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Lowering Uncertainties via VLOC

To improve the device's accuracy, NIST researchers will need to improve their understanding of how nitrogen's index of refraction should change in response to pressure. They hope to do this using a variation on the FLOC called the VLOC (variable-length optical cavity).

The VLOC uses helium gas instead of nitrogen. Until recently, the uncertainty for helium's index of refraction was unknown but estimated to be as high as 10 ppm. Working with a theorist at the University of Delaware, the FLOC design team was able to drive the uncertainty down to 1 ppm.

Once the VLOC is built and tested, the team will use its performance to lower the uncertainties on nitrogen's index of refraction.

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Created October 27, 2014, Updated April 16, 2024