Comb on a Chip

Just as a meter stick with hundreds of tick marks can be used to measure distances with great precision, a laser frequency comb, with its hundreds of evenly spaced, sharply defined fre­quencies, can be used to measure the colors of light waves with great precision. Small enough to fit on a chip, miniature versions of these combs are making possible a new gene­ration of atomic clocks, a great increase in the number of signals traveling through optical fibers, and the ability to discern tiny frequency shifts in starlight that hint at the presence of unseen planets.

Schematic setup to generate a set of stable frequencies in a cryogenically cooled laser microresonator frequency comb. (Source: NIST)

The newest version of these chip-based micro­combs, created by researchers at the National Institute of Standards and Technology (NIST) and the University of Cali­fornia at Santa Barbara, is poised to further advance time and frequency measurements by improving and extending the capa­bilities of these tiny devices. At the heart of these frequency microcombs lies an optical micro­resonator, in which light from an external laser races around thousands of times until it builds up high intensity. Micro­combs, often made of glass or silicon nitride, typically require an amplifier for the external laser light, which can make the comb complex, cumber­some and costly to produce.

The scientists have demons­trated that micro­combs created from the semi­conductor aluminum gallium arsenide have two essential properties that make them especially promising. The new combs operate at such low power that they do not need an amplifier, and they can be mani­pulated to produce an extra­ordinarily steady set of frequencies – exactly what is needed to use the microchip comb as a sensitive tool for measuring frequencies with extra­ordinary precision.

The newly developed microcomb tech­nology can help enable engi­neers and scientists to make precision optical frequency measurements outside the labora­tory, said NIST scientist Gregory Moille. In addition, the micro­comb can be mass-produced through nano­fabrication techniques similar to the ones already used to manu­facture micro­electronics. The researchers at UCSB led earlier efforts in examining micro­resonators composed of aluminum gallium arsenide. The frequency combs made from these micro­resonators require only one-hundredth the power of devices fabricated from other materials. However, the scientists had been unable to demons­trate a key property – that a discrete set of unwavering, or highly stable, frequencies could be generated from a micro­resonator made of this semi­conductor.

The NIST team tackled the problem by placing the micro­resonator within a customized cryogenic apparatus that allowed the researchers to probe the device at tempera­tures as low as 4 degrees above absolute zero. The low-tempera­ture experiment revealed that the inter­action between the heat generated by the laser light and the light circu­lating in the micro­resonator was the one and only obstacle preventing the device from generating the highly stable frequencies needed for success­ful operation.

At low temperatures, the team demons­trated that it could reach the soliton regime – where individual pulses of light that never change their shape, frequency or speed circulate within the micro­resonator. With such solitons, all teeth of the frequency comb are in phase with each other, so that they can be used as a ruler to measure the frequencies employed in optical clocks, frequency synthesis, or laser-based distance measure­ments.

Although some recently developed cryogenic systems are small enough that they could be used with the new micro­comb outside the labora­tory, the ulti­mate goal is to operate the device at room temperature. The new findings show that scientists will either have to quench or entirely avoid excess heating to achieve room-tempera­ture operation. (Source: NIST)

Reference: G. Moille et al.: Dissipative Kerr Solitons in a III‐V Microresonator, Laser & Phot. Rev., online 22 June 2020; DOI: 10.1002/lpor.202000022

Link: Photonics and Plasmonics Group, Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg, USA

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