Today I would like to present the turbine-powered optical isolator, a steampunk-sounding bit of hardware that only allows light to pass in one direction. That, I must admit, is a sentence I never imagined writing.
It turns out that having components that only allow light to flow one way is really important. I’ve personally managed to destroy more than one laser because some of the light it emitted ended up returning to the hardware and wreaking havoc. Even if you don’t destroy anything, back reflections can still mess experiments up. So optical isolators, as they are called, are really important.
They are also big and expensive. To reduce the cost and size, researchers have demonstrated an optical oscillator that only requires an operating turbine to function. Despite being the least practical isolator ever, I still want one, maybe even two. In fact, give me three since I happen to be here anyway.
Whispered conversations
The isolator effectively relies on the Doppler effect to allow light to flow in one direction and not the other. You are probably familiar with the Doppler effect: it is why an approaching ambulance siren has a higher pitch than an ambulance that is receding. Light is also subject to the Doppler effect: a source of light that is receding has a redder color than one that is approaching.
Note that it's the apparent motion that matters. A moving source and a stationary receiver are exactly the same as a stationary source and a moving receiver.
This is important because the critical piece of the hardware is a little glass ball, called a whispering gallery mode resonator. You may have experienced a whispering gallery mode resonator yourself, or at least you've heard of one. In some cathedrals, sound travels cleanly around the walls. Place your head next to a wall, talk quietly, and your friend on the other side of the cathedral will hear you clearly. (Or, they would if it weren’t for the 10 billion other tourists trying the same trick.)
The point is that the sound will travel around the walls through a series of grazing reflections without passing through the center space of the cathedral. Light does the same thing in one of these little glass balls, traveling in a ring around the very edges.
Unlike a cathedral, the light doesn’t start inside the ball. Instead, the ball is placed within a few nanometers of an optical fiber, which allows the light to leak across the gap to the ball. In fact, this leakage can be designed so perfectly that all the light goes into the ball.
Our ball is also a bit more selective than a cathedral: it will only allow certain colors (or wavelengths) of light to travel the ring around the circumference. The rule is this: the circumference of the ball should be equal to a half integer number of wavelengths.
That seems a simple enough selection rule, except light doesn’t measure things like we do. When light passes from air to glass, it slows down (by about 30 percent), so the light kind of bunches up, shortening its wavelength. I won’t go into detail about how this happens, but essentially, for light to move through a material, it has to move electrons (this is what gives glass a refractive index greater than one). Electrons are heavy and don’t like to move, which slows the light down. Light moving electrons is the key to the story here because it bring the Doppler effect into play.
Are you going my way?
To get an idea of how the Doppler effect changes things, let’s think about light entering a plate of glass that is moving very quickly toward the light source.
The electrons in the glass see the light as having bluer color than we see. The electrons also respond as if the light were bluer. Blue light has to work harder to shift the electrons (the refractive index is larger). As a result, the light slows down even more than it would have if the glass was standing still.
From the light’s perspective, the plate of glass seems be just that little bit thicker, and more wavelengths fit inside.
If the glass moved with the light, the opposite would occur. The wavelength would appear to be redder. The electrons respond as if the light were redder (the refractive index decreases). And the light slows down less. From the light’s perspective, the plate also appears a little thinner, and fewer wavelengths fit inside.
Spin me up
The researchers achieve the same effect by spinning the ball of glass. Light traveling down the fiber in one direction encounters glass traveling toward it. The light has the wrong wavelength to fit into the glass resonator, so it continues down the optical fiber.
But light traveling the other direction has exactly the right wavelength to fit into the resonator, so it gets sucked in. Once in the resonator, the light cannot return to the fiber without somehow changing direction. Instead, it stays in the ball until it is absorbed or lost to the outside world through imperfections in the glass.
Matching the light's wavelength is why the researchers needed a turbine with some pretty high speeds: between 120,000 and 400,000 revolutions per minute. This speed is fast enough to ensure that light traveling with the direction of spin ends up in the ball and lost to the fiber, while light traveling in the opposite direction cannot enter the ball.
The spinning also had a nice side effect. To get light into the glass ball, the fiber has to be positioned just the right distance from the ball. The drag from the spinning ball creates an air cushion that stabilizes the fiber at a fixed distance. The researchers can tune the distance by increasing or decreasing the tension in the fiber. This seems to be more stable than for a non-spinning glass ball, which is pretty cool.
And now for the downside
The isolator works, but it only works for a specific color of light. Or more precisely, there is a set of colors that can only propagate in one direction down the fiber. There is a second set of colors that can only propagate in the opposite direction. All other colors can propagate in both directions.
The colors are also very precisely defined. For context, in an optical communications system, a laser with a very narrow color range is modulated very fast to carry information. This modulation isn't large; if the laser light were in the visible range, you would be unlikely to be able to perceive that the light color was less pure after modulation. However, the color impurity induced by modulation is enough to prevent the isolator from doing its job. The isolator would work for the light before modulation very nicely, and not at all after modulation. Put differently, we can't use this hardware on light that's carrying data.
Still, this is the beginning of a new approach to a problem that has previously relied on big magnets and special crystals. So it's nice to see a new idea or two.
Nature, 2018, DOI: 10.1038/s41586-018-0245-5
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