Optical gyro dwarfed by a grain of rice

November 5, 2018 | 00:51
Optical gyro dwarfed by a grain of rice
Optical gyro dwarfed by a grain of rice
At the Californian Institute of Technology (CalTech), the development of a new type of tiny optical gyroscope has been made possible thanks to improvements in signal-to-noise performance.

Gyroscopes are used to give a measure of movement in three-dimensional space of vehicles, drones, smartphones and other electronic devices. In days gone by, mechanical gyroscopes were expensive, precision-engineered works of art using fast spinning gyros suspended on gimbals to allow ships, aircraft and missile guidance systems, to ascertain vehicle movements in space. Today, cheap microelectromechanical sensors (MEMS) gyros are fitted to almost every smart device produced. They measure changes in the forces acting on two identical vibrating masses moving in opposite directions. MEMS gyroscopes have their limitations including limited sensitivity, which is why optical gyroscopes are used for more demanding applications. They do not require any moving parts and achieve higher accuracy by exploiting the Sagnac effect.

This effect was discovered by the French physicist Georges Sagnac. It is a phenomenon produced using a ring interferometer and exploits a property of light which forms the bedrock of Einstein's theory of general relativity. To measure rotation in one plane, a beam of light is split into two and sent in opposite directions around a circular optical waveguide or interferometer. Any physical rotation of the waveguide around its axis results in the two beams meeting up at a slightly different time. Measuring the phase difference between the two beams gives a measure of rotation in this one plane. Three of these gyros aligned in the X, Y and Z plane respectively produces a 3-axis gyro to provide total spatial orientation.

The smallest high-performance optical gyro used today is about the size of a golf ball which makes it tricky to hide inside a typical smartphone case. It’s also too bulky to easily integrate into the control electronics for vehicle navigation applications. As the physical size of the optical gyro is reduced the phase difference produced by the Sagnac effect also diminishes, which limits the gyro’s precision and has, up till now limited the miniaturization of optical gyroscopes.

The Caltech engineers in the Department of Engineering and Applied Sciences have managed to  develop a new optical gyroscope 500 times smaller than current designs which does not sacrifice accuracy. It is in fact able to detect phase shifts 30-times smaller than is possible with larger systems. This new gyroscope is described in the November issue of the journal Nature Photonics.

The new gyroscope achieves this performance gain through the use of a technique called ‘reciprocal sensitivity enhancement’. In this case, reciprocity refers to the fact that both light beams in the gyroscope are affected identically by any interference (produced by thermal effects and light scattering). Differences in the phase of the two beams however are ‘non-reciprocal’.
The team led by Ali Hajimiri found a way to attenuate unwanted reciprocal noise while leaving the non-reciprocal phase signal information intact. The improvement to the system’s signal-to-noise performance was key to the development of the tiny optical gyro.
 
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