How to shove 50 meters of optical fiber into a microchip

DARPA program develops two photonic chips for variety of cutting-edge applications

The Defense Advanced Research Projects Agency recently said its program to develop cutting edge photonics products has yielded two chips that can support long optical delays with low loss useful for a number of applications including wideband wireless systems, optical buffers for all-optical routing networks, and ultra-stable optical interferometers for sensing applications.

Working under DARPA's integrated Photonic Delay (iPhoD) program the University of California, Santa Barbara (UCSB) and the California Institute of Technology (CalTech) came up with new microchip-scale, integrated waveguides for photonic delay.  Optical waveguides-any structure that can guide light, like conventional optical fiber-can be used to create a time delay in the transmission of light. Such photonic delays are useful in military applications ranging from small navigation sensors to wideband phased array radar and communication antennas, DARPA said.

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According to DARPA, the new waveguides are built onto microchips and include up to 50 meters of coiled material that is used to delay light. "Conventional fiber optic coils of the same length would be about the size of a small juice glass. These waveguides also employ modern silicon processing to achieve submicron precision and more efficient manufacturing. The result is a new component that is smaller and more precise than anything before in its class.  Chip-based waveguides also eliminate bulky, labor-intensive waveguide-to-fiber couplers, DARPA stated.

"Prior to the start of iPhoD, the best integrated waveguides had a signal loss of about 1 decibel per meter with total lengths of only a few meters," said Josh Conway, DARPA program manager in a statement. "Under iPhoD, two research teams created chips with loss around 0.05 decibels per meter. The submillimeter bend diameter, which describes how tightly the waveguide can coil without significant signal loss, allowed the demonstration of a 50-meter optical delay on a single microchip."

DARPA said that the ultra-low loss, true-time delay chip developed at UCSB is composed of silicon nitride. Selecting this material may allow for integration with a variety of devices and materials-thereby reducing size, weight and power requirements of an overall system. UCSB researchers also demonstrated 3D waveguide stacking, enabling more waveguide length, and thus, longer photonic delays, the agency said.

Meanwhile researchers at CalTech developed a waveguide that was constructed from silicon oxide, or glass, and demonstrated low loss over 27 meters.

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