Tuning Kerr-Soliton Frequency Combs to Atomic Resonances

Frequency combs based on nonlinear optical phenomena in integrated photonics are a versatile light source that can explore new applications, including frequency metrology, optical communications, and sensing. We demonstrate robust frequency-control strategies for near-infrared, octave-bandwidth soliton frequency combs created with nanofabricated silicon nitride ring resonators. Group-velocity-dispersion engineering allows operation with a 1064-nm pump laser and generation of dual-dispersive-wave frequency combs linking wavelengths approximately between 767 and 1556 nm. To tune the mode frequencies of the comb, which are spaced by 1 THz, we design a photonic chip containing 75 ring resonators with systematically varied dimensions and we use a thermo-optic tuning range of $50{}^{\circ }\mathrm{C}$. This single-chip frequency-comb source provides access to every wavelength, including those critical for near-infrared atomic spectroscopy of rubidium, potassium, and cesium. To make this possible, solitons are generated consistently from device to device across a single chip with use of rapid pump-frequency sweeps that are provided by an optical modulator.

Figure 1

Resonator dispersion design and experimental setup. (a) The calculated GVD profile (top) and integrated modal dispersion (middle) for ring resonators with material thickness of 667 nm, radius of 22.5 $\mu \mathrm{m}$, and W of 810, 850, and 890 nm. The experimentally generated single-soliton spectrum from the device with W of 850 nm (bottom) has its strongest SDW line aligned with the $\mathrm{Rb}$ two-photon transition (dashed red line). (b) The experimental setup for soliton generation and comb diagnostics. An external-cavity diode laser (ECDL) is coupled to a Mach-Zehnder intensity modulator, amplified in an ytterbium-doped fiber amplifier (YDFA), and coupled into the photonic chip using lensed fibers and precision positioning stages. The generated comb light is separated into spectral bands centered at 780, 1064, and 1550 nm for subsequent diagnostics. Each branch can be monitored in real time or heterodyned with local oscillators with local oscillators. The notch filter used in the 1064-nm branch to reject pump light when soliton generation is being monitored is not shown. The long-wavelength-DW branch is amplified with an erbium-doped fiber amplifier (EDFA) to provide optical power for further measurement steps. EO, electro-optic; PPLN, periodically poled lithium niobate.

Figure 2

Soliton-microcomb-spectrum control through nanofabrication. (a) Single-soliton spectra obtained from resonators with various waveguide widths on the same chip. Relevant atomic transitions are indicated in the plot. The on-chip, single-sideband pump powers are, from top to bottom, 160, 180, 175, 190, and 160 mW. (b) Comparison between measured dispersive-wave frequencies and the frequency predicted by simulated waveguide dispersion showing good agreement within fabrication imprecision of resonator dimensions. (c) Enlarged plot near the rubidium optical transition frequencies.

Figure 3

Soliton coherence measurements. (a) The filtered and amplified long-wavelength DW before (black) and after (red) being sent to electro-optic (EO) phase modulators. A heterodyne beat is made at the wavelength marked with an arrow to evaluate fluctuations in terahertz comb-mode spacing. This beat is shown in (b). (c,d) Heterodyne beats of each DW with a low-noise auxiliary laser: (c) LDW at 1563 nm and (d) SDW at 777.3 nm. The traces in (b)–(d) are taken with resolution bandwidth of 1, 100, and 300 kHz, respectively.

Figure 4

Thermo-optic tuning of soliton spectrum. (a) The thermal tuning of two modes of the soliton spectrum across the $\mathrm{Rb}$ two-photon optical clock transition (2POC) and $\mathrm{Rb}$ D2 transitions. (b) The repetition rate of the soliton can also be thermally tuned. (c) Overlay of the spectra presented in Fig. 2 plotted over the range from 333 to 337 THz. Any arbitrary frequency can be reached with the current W sweep and a 50-°C thermal tuning range.