A new technique that isolates light onto a photonic chip could pave the way for faster computers and reduced data loss when connected to the global fiber optic network. California Institute of Technology scientists have designed an optical waveguide – an 0.8-µm-wide silicon device – that allows light to travel in one direction but that changes the mode of the light when it travels in the opposite direction; i.e., light traveling through the waveguide moves in a symmetric mode in one direction, then changes into an asymmetric mode in the other. The two beams pass through one another because different light modes cannot interact with each other. Although optical fibers are increasingly replacing copper wires, carrying information via photons instead of electrons, computer technology still relies on electronic chips. In an electrical circuit, diodes isolate electrical signals by allowing current to travel in only one direction. Scientists have worked for more than 20 years to create a photonic analog of the diode, a device known as an optical isolator. Isolated light signals can travel in only one direction. If the light were not isolated, signals sent and received between different components on a photonic circuit could interfere with one another, causing the chip to become unstable. Normally, a light beam displays the same properties when it moves forward as when it is reflected backward. To isolate light, its properties must change when going in the opposite direction. An optical isolator can then block light that has these changed properties, allowing light signals to travel in only one direction between devices on a chip. Although the work is only a proof-of-principle experiment, the researchers have already begun building an optical isolator to be integrated onto a silicon chip. Once brought to market, these chips could operate at 40 Gb/s, much simpler than their electronic counterparts. The work was reported in the Aug. 5 issue of Science (doi: 10.1126/science.1206038).