Measuring Gas Pressure with LASERs

I have just returned from the American Vacuum Society Symposium in Tampa, Florida. During the Vacuum Technology Division Sessions, we were updated on the NIST project to develop photonic based vacuum measurement standards. In order to introduce the topic, below is an excerpt from my book Understanding Modern Vacuum Technology, 2nd Ed.

As this work is being compiled, there is groundbreaking work being done at NIST in the Thermodynamics Group. This is in alignment with the “NIST on a Chip” program which has a goal of having reference standards based on quantum phenomena that can be scaled down to extremely small packages.

The pressure standards that are in use today are manometers that are evolutionary advances of the mercury barometer that Torricelli developed in 1643. While the principle behind the primary standards is simple, the manometers are operationally complex, requiring accurate determination of temperature, density, gravity, speed of sound, and ultimately column height. The manometers are large instruments (up to 3m tall) and contain up to 250kg of hazardous mercury. Mercury manometers are still used at eleven National Metrology Institutes, including NIST.

The NIST 13 kPa, 160 kPa and 360 kPa Ultrasonic Interferometer Manometers (UIMs) use ultrasound pulses to determine column heights with a resolution of 10 nm or a pressure resolution of 3.6 mPa and provide the world’s lowest pressure uncertainties. Once the instrument is set up, it takes about a minute to take a data point.

NIST mercury interferometer pressure standards.

Figure 4 45 10 kPa Ultrasonic Interferometer Manometer in the foreground and the 360 kPa UIM in the background. The 260 Pa UIM stands three meters tall and provides the nation with best-in-the-world capabilities for measuring absolute and differential pressure from 1 Pa to 360 kPa.

The goal is to replace the oil and mercury manometers with a standard that can be easily transported between national labs, researchers and industrial users. Once the photonic based standard is developed, NIST will be able to export a primary standard to an end user.

The photonic based method is to make vacuum and pressure measurements using Fabry–Pérot optical cavities to measure the interaction of light with a gas. This represents a disruption in pressure measurement technology and a way of realizing and disseminating the pascal, the SI unit of pressure.

for web FLOC Cartoon

Figure 4‑46 A fixed length optical cavity realizes pressure by comparing the beat-frequency, f1-f2, as the pressure in gas cavity is increased while the vacuum in the reference cavity is held constant. This standard is simple (has no moving parts) and requires that the temperature of the cavity and molar index of the gas in the cavity is known from either measurements (for example nitrogen) or from theory (helium). (Hendricks, et al., 2015)

The basis of the optical technique is a Fabry–Pérot cavity, where two highly-reflecting mirrors face each other. Two Fabry–Pérot cavities built on a single low-expansion glass spacer, with one of the cavities serving as a vacuum reference. Figure 4‑46 shows a schematic for a dual Fabry–Pérot device. A laser shines in the end of one cavity and the transmitted light is detected at the other end, after bouncing back and forth inside the cavity multiple times (depending on reflectance of the mirror coatings). When the laser light entering the cavity has a wavelength such that an integer number of half wavelengths exactly fits between the two mirrors, a resonance occurs along with a detected signal intensity maximum at the detector, which is used in a stabilization feedback loop to lock the laser into resonance with the cavity.

The Fabry–Pérot cavity is used to determine the pressure by measuring the refractive index n or equivalently, the refractivity n-1. The first approximation, the refractivity of a gas is proportional to the number density of molecules

equation.GIF

first photonic sensor.JPG

 

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