A better way to measure acceleration

You are going at the speed limit down a two-lane road when a car comes out of a pavement on your right. You scream on the brakes, and within a fraction of a second of the impact an airbag delivers, save you from serious injury or even death.

The airbag uses thanks to an accelerator – a sensor that detects sudden changes in speed. Accelerators keep rockets and planes on the right flight path, provide navigation for self-driving cars, and rotate images so they stay on the right side up on cell phones and tablets, among other activities other necessary.

Addressing the growing demand to accurately measure acceleration in smaller navigation systems and other devices, researchers at the National Institute of Standards and Technology (NIST) have made an accelerator just a millimeter thick. uses laser light instead of mechanical snoring to produce a signal.

While a few other accelerators also rely on light, the NIST instrument design simplifies the measurement process, providing higher accuracy. It also works over a larger frequency range and has been proven harder than similar devices.

Not only is the NIST engine, known as an optomechanical accelerometer, far more accurate than the best commercial accelerators, it does not have to go through a time consuming process for occasional calibrations. In fact, because the instrument uses laser light at a known frequency to measure acceleration, it may eventually serve as a portable reference standard to capitalize other accelerators now on the market, making them more accurate.

The acceleratorometer also has the potential to improve inertial navigation in such emergency systems as military aircraft, satellites and submarines, especially when a GPS signal is not available. NIST researchers Jason Gorman, Thomas LeBrun, David Long and colleagues describe their work in the journal Optica.

This animation reflects new accelerator operating principles. This optomechanical accelerator is made up of two silicon chips. In the first chip there is a test mass suspended with a set of silicon conductors, which allows the test mass to move directly. The top of the ornament is covered with a mirror. The second chip has an inset hemispherical mirror. Together, the decorative and hemispherical mirrors form an optical cavity. An infrared laser light is directed into the device. Most of the frequency is fully expressed. However, light that matches a repetitive frequency builds up inside the cave, increasing in intensity, so that the intensity of the light transmitted by the cavity is reflected. match the input. Light transmitted by the cavity can be found on the other side. As the machine accelerates, the length of the cairn changes, shifting the resonant frequency. By continuously matching the laser to the resonant frequency of the caveat, researchers can determine the acceleration of the device. Animation: Sean Kelley / NIST The study is part of NIST on a Chip, a program that brings the institute ‘s advanced measurement science technology and knowledge directly to users in commerce, medicine, defense and academics.

Accelerators, with the introduction of the new NIST engine, record changes in speed by monitoring the position of a freely moving mass, and obtained the “test mass,” compared to reference point located inside the device. The distance between the test mass and the reference point will only change if the accelerator slows down, accelerates or changes direction. The same is true if you are a passenger in a car. If the car is at rest or moving at a constant speed, the distance between you and the dashboard will remain the same. But if the car suddenly brakes, you will be thrown forward and the distance between you and the dashboard will decrease.

The movement of the test mass creates a visible signal. The accelerator developed by NIST researchers relies on infrared light to measure the change in distance between two highly reflective surfaces that take up a small area of ​​empty space. The test mass, which is hung by flexible beams one-fifth the width of a human hair so that it can move freely, supports one of the surfaces with a mirror. The other reflective surface, which is the fixed reference point of the accelerator, is made up of a non-removable microfabricated concave mirror.

Together, the two reflective surfaces and the void space between them create a place where infrared light of the right wave can reverberate, or kick back and forth, between the mirrors, building in intensity. . That wavelength is determined by the distance between the two mirrors, since the level of a plugged guitar depends on the distance between the fret and the bridge of the instrument. If the test magnitude moves in response to acceleration, changing the separation between the mirrors, the resonant wavelength also changes.

To monitor the changes in the resonant wavelength of the cairn with high sensitivity, a single-frequency stable laser is locked to the cavity. As explained in a recent publication in Optics Letters, the researchers have also employed an optical frequency comb – a device that can be used as a controller to measure a light wave – to measure the length of the cave with high accuracy. The signals of the ruler (the teeth of the comb) can be thought of as a series of lasers with waves of equal width. When the test mass moves during an acceleration period, either shortening or widening the cavity, the intensity of the reflected light changes as the wavelengths associated with the teeth of the comb. moving in and out of position with the cavity.

Changing the movement of the test mass to acceleration is a critical step that has been a problem in most optomechanical accelerators. However, the team’s new design ensures that the dynamic relationship between the movement of the test mass and the acceleration is simple and easy to model through the first principles of physics. In short, the test mass and the support beams are designed so that they behave like a simple spring, or harmonic oscillator, which vibrates at one frequency in the operating range of the accelerator.

This simple dynamic response allowed the scientists to achieve low measurement uncertainty over a wide range of acceleration frequencies – 1 kilohertz to 20 kilohertz – without ever capitalizing on the device. This feature is unique because all commercial accelerators need to be calibrated, which is time consuming and expensive. Since their study was published in Optica, the researchers have made several improvements that should reduce the uncertainty of their device to nearly 1%.

Capable of detecting movements of the test mass less than one hundred miles in diameter of hydrogen atoms, the optomechanical accelerator detects accelerations of as little as 32 billion ag, where g is accelerated due to gravity of the Earth. That’s a higher sensitivity than all the accelerators now on the market with equal size and bandwidth.

With further improvements, the NIST optomechanical accelerator could be used as a high-precision reference tool to capitalize other accelerators without introducing them into a laboratory.

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Paper 1: F. Zhou, Y. Bao, R. Madugani, DA Long, JJ Gorman and Thomas W. LeBrun. Thermomechanically limited bandwidth sensing with optomechanical accelerator. Optica. Published March 8, 2021. DOI: 10.1364 / OPTICA.413117

Paper 2: DA Long, BJ Reschovsky, F. Zhou, Y. Bao, TW LeBrun and JJ Gorman. Electro-optic frequency combing for rapid interrogation in cavity optomechanics. Optics Letters. Published January 29, 2021. DOI: 10.1364 / OL.405299