The modern smartphone is only possible because of sensors. Their svelte form factor conceals accelerometers, magnetometers, temperature sensors, a GPS unit, and gyroscopes. They all consume volume and power, meaning that each sensor, even as it makes your phone smarter, induces battery-sucking anxiety.
Which makes a report of a very tiny laser gyroscope pretty interesting, even if it still has a way to go before being found in your cellphone. Laser gyroscopes are pretty much the “if only” of tiny sensors. Essentially, they seem like they should rule the roost in terms of providing a clean and clear signal. But, so far at least, they don’t.
Good things go bad
To see why this is so, let’s take a look at how a laser gyroscope works. Essentially, light is sent into a ring; half of the light travels around the ring clockwise, and the other half counter-clockwise. The two light beams meet at the opposite side of the ring, where they exit together.
When the two light beams meet up, they greet each other like long-separated friends and continue on as if nothing had separated them. In other words, if we were to measure the properties of the exiting light beam, it would look like the light never even went through the ring.
But, if the ring is spinning, then the light traveling in one direction travels with the spin, and the other light beam travels against the spin. These light beams do not greet each other as if nothing had ever separated them. They don’t quite match at the exit, and the combined beam is less than the sum of its parts.
Now, if I measure the optical power in the light beam that exits the ring, it looks like some power has been lost. The faster the spin, the bigger the effect. This type of measurement is incredibly sensitive (it is the same sort of measurement used to detect gravitational waves). That means we should have incredibly sensitive laser gyroscopes.
And we do. Laser gyroscopes are sensitive to the spinning of the Earth (as intended). But they’re also (unfortunately) sensitive to changes in temperature, people walking outside the lab, local construction work, and the despairing cries of grad students. Then, just to make it worse, the sensitivity of a laser gyroscope is proportional to its size. Making them bigger is, unfortunately, the only way to make things better, which is a bad fit for cell phones.
Pull the thing and the other thing
Enter our intrepid trio of engineers. The problems, they realized, reside in a couple of areas. The things we make are never perfect: you may try to send half the light in one direction and half the light in another, but it never quite balances. The same on the exit: not all the light exits the ring in the same way from each direction. This imbalance makes life difficult. Then there are the temperature fluctuations that effect the signal globally and cannot be distinguished from changes in rotational speed.
To improve matters, the engineers went back to basics. Instead of putting the clockwise and counterclockwise circulating light in one ring, they put them in separate rings. Then to really balance things out, they rapidly switched the roles of the rings back and forth. One ring starts with clockwise light, shifts to counter-clockwise light, and then back again. The second ring is switched in the opposite manner.
By doing this, all the imperfections of the design are balanced out: if one ring is a bit worse than the other, it doesn’t matter too much, because the effect averages out. The same is true of temperature: all temperature changes are balanced by switching the roles of the rings. The only exception is if one ring is at a different temperature to the other.
Finally, the rings themselves are something special. The light does not go around each ring just once, but several thousand times. The shift due to rotation increases with each lap of the ring, so the sensitivity is increased by several thousand times. That, in turn, allows the engineers to use tiny rings instead of large ones.
The end result is a gyroscope that is only a couple of square millimeters. It is far less sensitive to changes in temperature and manufacturing imperfections than earlier laser gyroscopes.
Still not good enough
That said, you aren’t going to see this in your cellphone anytime soon. First of all, even though this paper proves that a highly sensitive laser gyroscope can be made very small, it does not perform anywhere near as well as commercial gyroscope sensors. The angular drift is about a thousand times worse than what’s already on the market. I’m hoping that this is because commercial versions employ some kind of internal drift correction and not because the drift is inherent in the design of the laser gyroscope.
On the upside, this new laser gyroscope does have the possibility of a smaller physical footprint than a commercial version. And, it should even consume less power. So, not tomorrow, but maybe in ten years.