In collaboration with industry, researchers from the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have made the first portable prototype of the Fixed-Length Optical Cavity (FLOC), a device that uses light to measure pressure with higher accuracy and precision than most commercial pressure sensors. NIST says this newest version could revolutionize the way pressure is measured with potential uses by many industries, particularly semiconductor chip and aircraft manufacturing.
In 2017, NIST and MKS Instruments (Andover, MA) signed a Cooperative Research and Development Agreement (CRADA) to take a laboratory-scale version of the FLOC and create a smaller, more robust prototype that more closely resembles a commercial product. Thanks to the CRADA work, the joint NIST and MKS team has now successfully demonstrated a prototype version small enough to fit into two suitcases, Hendricks said.
"MKS Instruments brings over 50 years of pressure measurement, optical and laser experience to this project, and we are honored to have been selected by NIST to work with them on this important and prestigious development," said Phil Sullivan, CTO of MKS's Pressure and Vacuum Measurement Solutions business. "We anticipate that this work will lead to a new wide-range, compact pressure measurement standard."
Robust, portable FLOC sensors could potentially reduce the cost of producing semiconductor chips such as those used in smartphones, as well as decreasing the cost of air travel. This is because both the chip manufacturing and aerospace industries rely on pressure measurements. Conventional pressure sensors are precise, but their readings tend to drift over time, meaning they must be taken out of service regularly to be calibrated. Since the FLOC’s pressure measurements are absolute, no calibration is required. So, FLOCs could be used to check the drift of conventional pressure sensors on the factory floor in real time, reducing the need for downtime.
Conventional pressure sensors are also used in aircraft to measure the plane’s altitude in-flight. A more precise pressure sensor could allow flight controllers to safely arrange planes more densely, saving fuel and potentially lowering the cost to air travelers.
Although it is beyond the scope of the current CRADA project, NIST scientists envision that one day the FLOC could be reduced even further in size, to a chip-scale instrument.
The FLOC measures pressure through subtle differences in the frequency of light passing through two physical channels called optical cavities: a reference channel in vacuum and a test channel filled with a gas whose pressure is being measured.
To measure pressure, the FLOC detects subtle differences in light passing through two channels, one flooded with gas and the other in vacuum. At either end of each channel is a semi-reflective mirror. Laser light enters the channels through one side and reflects back and forth between the mirrors, forming standing waves. Some of this light exits the channels through the other side. The presence of gas in the top channel causes the wavelength to shorten. When the light from the two channels is combined, it creates a wave pattern, a signal that can be used to calculate the gas's pressure in real time.
NIST first made a laboratory-grade version of the FLOC in 2014. It was designed to be sensitive and accurate enough to become a primary calibration standard, an instrument used to calibrate all other pressure measurement devices.
The original standard FLOC fills an entire large-scale laboratory table. The optical cavity containing the gas and vacuum channels is about 15 cm long, roughly the size of a travel mug. The apparatus also includes a vacuum pump, lasers to supply the light and the optics to manipulate them, in addition to a rack of electronics to process the signal. The new portable version is more compact. Its two-channel cavity is only about 2.5 cm long, a little longer than a postage stamp. Both the cavity and its optics fit into a single box, and there is also a smaller electronics rack and a pump for the gas-handling system.
In their partnership, NIST and MKS staff assembled the two-channel cavity at the heart of the prototype, while MKS managed the engineering of a miniaturized version of the system.
"We built the national standard version of the FLOC, which is designed to operate in a high-precision laboratory," Hendricks said. "But we turned to industry under a CRADA to speed up the engineering and miniaturization work that needs to go into making something rugged, stable, transportable, low-power and able to work in a variety of different environments."
Researchers also changed the wavelength of light used from visible red (633 nm) to infrared (1550 nm), used by the telecom industry and therefore a popular wavelength for commercial products.
Vibrations from people's steps in the showroom could have theoretically thrown off the delicate measurements, but the device was robust enough to stand up to that noise. Its range has been demonstrated from ultralow pressures used in vacuum to about 2,000 pascal (the equivalent of about 2 percent of atmospheric pressure, or 0.3 psi), and work is currently underway to test it at much higher pressures.
And there are opportunities for further miniaturization. Future versions can be customized to only include the functions the device needs. Meanwhile, Hendricks' team will compare the performance of the miniature version against that of the standard FLOC. The team will likely also perform shipping tests, where they test the device, pack it up, ship it somewhere, bring it back and then test it again to see if it yields the same results.