A team of researchers at the Defense Advanced Research Projects Agency (DARPA) reports that it has made significant progress advancing chip-based integrated photonics and nonlinear optics to miniaturize optical synthesizer components.
In a paper published in Nature, the team details recent success by researchers from the National Institute of Standards and Technology (NIST) and partner institutions, including the University of California at Santa Barbara and the California Institute of Technology in Pasadena, California. The researchers were able to replicate the capabilities of a tabletop-sized optical frequency synthesizer on four microchips, each measuring only about 5 mm by 10 mm, by combining a pair of frequency combs, several miniature lasers, and other compact optoelectronic components. According to the report, the synthesizer can tune over 32 nm and delivers a very high frequency stability after one second of averaging, matching that of the input reference clock.
The report details how researchers created the two miniaturized frequency combs by circulating laser light generated with single-color “pump” lasers around optical racetracks fabricated on silicon chips; successful creation of the combs can produce many additional colors, yielding a spectrum that looks like an actual comb where each “tooth” is an individual color or frequency. The researchers assert that this result is a significant departure from the tabletop versions of frequency synthesizers, which use fiber optics, specialized mirrors, and large mechanical components built by hand to achieve a similar effect.
The report goes on to detail how the two frequency combs guide a programmable laser, which is an output of the synthesizer; one comb, created by NIST, has wider teeth and can calibrate itself by spanning an octave, which represents a doubling of frequency (as in musical octaves). The Caltech team created a second comb with much finer teeth that spans a narrower range. The distance between these very fine teeth is set by a stable and accurate microwave reference clock. When lined up with the NIST comb, the Caltech comb creates a grid of known optical frequencies for the output laser to reference or measure itself against. Taken together, the frequency combs create a synchronized link between the microwave clock and the laser frequency.
The Direct On-Chip Digital Optical Synthesizer (DODOS) program is now entering its final phase, during which researchers will work to integrate the individual components together with electronics and make a compact packaged device suitable for use in future military and commercial optical systems.