Optical Ruler’s Expanded Frequencies Support Precision Navigation

Researchers at the National Institute of Standards and Technology (NIST) expanded by nearly two-thirds the frequency range over which a chip-scale device can accurately generate and measure the oscillations of lightwaves. The expanded range of the system, a micro-ring resonator frequency comb (microcomb), could lead to better sensors of greenhouse gases and may also improve global navigation systems.

The microcomb consists of a miniature, rectangular waveguide — a channel that confines lightwaves — coupled to a ring-shaped resonator about 50 µm in diameter. Laser light injected into the waveguide enters the micro-ring resonator and races around the ring.

Measurements of wavelength against the microcomb’s behavior, as well as measurements of the soliton states. Researchers at NIST produced the microcomb by using two lasers, each generating a different frequency of light. Through interactions with the soliton light circulating within a micro-ring resonator, the second laser induced two sets of teeth that replicated the original sets in the system but that were also shifted to higher and lower frequencies. The lower frequency set is in the infrared part of the spectrum, while the other is shifted to much higher frequencies close to the visible band. Courtesy of NIST.

Ordinarily, the circulating light begins to vary in amplitude and can form various patterns. However, by carefully adjusting the laser, the light within the micro-ring forms a soliton — a solitary wave pulse that preserves its shape as it moves around the ring. Each time the soliton completes a trip around the micro-ring, a portion of the pulse splits off and enters the waveguide. Soon, an entire train of wave pulses fills the waveguide, and each wave is separated in time from its neighbor by the same fixed interval — the time it took for the soliton to complete one lap around the ring. The train of wave pulses in the waveguide corresponds to a single set of evenly spaced frequencies and forms the teeth of the frequency comb. The number and amplitude of the teeth are primarily determined by the size and composition of the ring and the power and frequency of the laser.

The researchers produced a microcomb by using two lasers, each generating a different frequency of light. Through a complex series of interactions with the soliton light circulating within the micro-ring resonator, the second laser induced two sets of teeth, or evenly spaced frequencies, that replicated the original set of teeth but that were also shifted to higher and lower frequencies. The lower frequency set was in the infrared part of the spectrum, while the other was at much higher frequencies close to the visible band. The comb also retained its original teeth at near infrared frequencies.

The microcomb’s extended range enables applications at various frequencies, according to the researchers. The system marks the first time that researchers have produced a stable microcomb that ties together such a wide range of frequencies, team leader Kartik Srinivasan said.

Additionally, the induced sets of teeth could be shifted to higher or lower frequencies independent of the shape or composition of the micro-ring resonator. This was accomplished by varying the frequency of the second laser.

The team’s findings could enable a single microcomb to measure the characteristic vibrations of atoms and molecules, including pollutants that both emit and absorb light over a broad range of frequencies, thus enhancing the sensitivity of detectors. The broader coverage could also help to stabilize the microcomb so that its tick marks remain fixed rather than wandering slightly from their original set of colors.

The enhanced stability may spur the development of portable optical atomic clocks that are accurate enough to be employed outside the laboratory, leading to more precise navigation systems, postdoctoral researcher Gregory Moille said.

The research was published Nature Communications (www.doi.org/10.1038/s41467-021-27469-0).