Left: Artist impression of the light circulator. Credits: Henk-Jan Boluijt (AMOLF)
Researchers have developed a vibrating glass ring optical circulator that interacts with and routes light. The breakthrough removes the problems associated with miniaturising the centimetre-sized magnets on optical chips presently required to route light. The researchers, at the Netherlands’ Atomic and Molecular Physics (AMOLF) organisation and the University of Texas, say they have created a microscale circulator that directionally routes light on an optical chip without using magnets.
Circulators allow the transmission of information, without loss, among more than two nodes in a network; this is why they are already widely used in optical networks. Circulators have several entrance and exit ports between which they route light in a special way: light entering a particular port is forced to exit in a second port, but light entering that second port exits in a third port and so on.
“Light propagation is symmetric in nature, which means if light can propagate from A to B, the reverse path is equally possible. We need a trick to break the symmetry”, explains AMOLF Group Leader Ewold Verhagen. “Usually this ‘trick’ is using centimetre-sized magnets to impart directionality and break the symmetric nature of light propagation. Such systems are difficult to miniaturise for use on photonic chips.”
Verhagen and his colleagues create circulating behaviour using a microscale glass ring resonator with a different trick. They let light in the ring interact with mechanical vibrations of the same structure. The researchers used this principle in earlier work to demonstrate one-way optical transmission.
“By shining light of a ‘control’ laser in the ring, light of a different colour can excite vibrations through a force known as radiation pressure, but only if it propagates in the same direction as the control light wave,” notes Verhagen. “Since light propagates differently through a vibrating structure than through a structure that is standing still, the optical force breaks symmetry in the same way as a magnetic field would.”
Turning the ‘one-way street for light’ they demonstrated before, into a useful optical ‘roundabout’ was not as straightforward as it may seem, as postdoc John Mathew points out: “The challenge is to dictate the particular exit to which light can be routed, such that it always takes the next port.”
The researchers found the solution in optical interference. Careful control of the optical paths in the structure ensures that light from each input constructively interferes in exactly the right output. “We demonstrated this circulation in experiments, and showed that it can be actively tuned. The frequency and power of the control laser allow the circulation to be turned on and off and change handedness”, says Mathew.
“Devices like this could form building blocks for chips that use light instead of electrons to carry information, as well as for future quantum computers and communication networks,” ventures Verhagen.
The AMOLF/University of Texas research is published in Nature Communications.