Daron A. Westly

Stably accessing octave-spanning microresonator frequency combs in the soliton regime

Microresonator frequency combs can be an enabling technology for optical frequency synthesis and timekeeping in low size, weight, and power architectures. Such systems require comb operation in low-noise, phase-coherent states such as solitons, with broad spectral bandwidths (e.g., octave-spanning) for self-referencing to detect the carrier-envelope offset frequency. However, accessing such states is complicated by thermo-optic dispersion. For example, in the Si3N4 platform, precisely dispersion-engineered structures can support broadband operation, but microsecond thermal time constants often require fast pump power or frequency control to stabilize the solitons. In contrast, here we consider how broadband soliton states can be accessed with simple pump laser frequency tuning, at a rate much slower than the thermal dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4 microresonators, including the generation of a multi-soliton state with a pump power near 40 mW and a single-soliton state with a pump power near 120 mW. We also develop a simplified two-step analysis to explain how these states are accessed without fast control of the pump laser, and outline the required thermal properties for such operation. Our model agrees with experimental results as well as numerical simulations based on a Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover, it also explains an experimental observation that a member of an adjacent mode family on the red-detuned side of the pump mode can mitigate the thermal requirements for accessing soliton states.

An optical-frequency synthesizer using integrated photonics.

Integrated-photonics microchips now enable a range of advanced functionalities for high-coherence applications such as data transmission, highly optimized physical sensors, and harnessing quantum states, but with cost, efficiency, and portability much beyond tabletop experiments. Through high-volume semiconductor processing built around advanced materials there exists an opportunity for integrated devices to impact applications cutting across disciplines of basic science and technology. Here we show how to synthesize the absolute frequency of a lightwave signal, using integrated photonics to implement lasers, system interconnects, and nonlinear frequency comb generation. The laser frequency output of our synthesizer is programmed by a microwave clock across 4 THz near 1550 nm with 1 Hz resolution and traceability to the SI second. This is accomplished with a heterogeneously integrated III/V-Si tunable laser, which is guided by dual dissipative-Kerr-soliton frequency combs fabricated on silicon chips. Through out-of-loop measurements of the phase-coherent, microwave-to-optical link, we verify that the fractional-frequency instability of the integrated photonics synthesizer matches the 7.0x10^(−13) reference-clock instability for a 1 second acquisition, and constrain any synthesis error to 7.7x10^(−15) while stepping the synthesizer across the telecommunication C band. Any application of an optical frequency source would be enabled by the precision optical synthesis presented here. Building on the ubiquitous capability in the microwave domain, our results demonstrate a first path to synthesis with integrated photonics, leveraging low-cost, low-power, and compact features that will be critical for its widespread use.Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission1, highly optimized physical sensors2 and harnessing quantum states3, to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III–V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10−15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.An optical-frequency synthesizer based on stabilized frequency combs has been developed utilizing chip-scale devices as key components, in a move towards using integrated photonics technology for ultrafast science and metrology.

Octave-spanning microcavity Kerr frequency combs with harmonic dispersive-wave emission on a silicon chip

We engineer dispersion and coupling of a Si3N4 microresonator to achieve an octave-spanning comb with a 200mW pump. Our microcomb features dispersive-wave spectral peaks at 1 μm and 2 μm, which potentially enable on-chip self-referencing.

Photonic-Chip Supercontinuum with Tailored Spectra for Counting Optical Frequencies

Supercontinuum generation using chip-integrated photonic waveguides is a powerful approach for spectrally broadening pulsed laser sources with very low pulse energies and compact form factors. When pumped with a mode-locked laser frequency comb, these waveguides can coherently expand the comb spectrum to more than an octave in bandwidth to enable self-referenced stabilization. However, for applications in frequency metrology and precision spectroscopy, it is desirable to not only support self-referencing, but also to generate low-noise combs with customizable broadband spectra. In this work, we demonstrate dispersion-engineered waveguides based on silicon nitride that are designed to meet these goals and enable precision optical metrology experiments across large wavelength spans. We perform a clock comparison measurement and report a clock-limited relative frequency instability of

NIST on a chip with alkali vapor cells: Initial results

3.8\times10^{-15}

Photonically integrated spectroscopy platform using grating-to-grating coupling (Conference Presentation)

at

Photonic waveguide to free-space Gaussian beam extreme mode converter

\tau = 2

Versatile silicon-waveguide supercontinuum for coherent mid-infrared spectroscopy

seconds between a 1550 nm cavity-stabilized reference laser and NISTs calcium atomic clock laser at 657 nm using a two-octave waveguide-supercontinuum comb.

On-chip waveguides for self-referencing low-power and high-repetition-rate laser frequency combs

We describe progress at NIST to develop a series of “chip-scale”, SI-traceable electromagnetic reference instruments based on microfabricated alkali vapor cells. We anticipate being able to realize the second, the meter, the volt, the ampere and the kelvin using microfabrication techniques and spectroscopic measurement of alkali atom energy levels. We report here on initial fabrication efforts to develop optical wavelength references in which atomic vapor cells are integrated with single-mode photonics.

Nonlinear Si-waveguides for mid-infrared comb generation and dual comb spectroscopy at 5 μm

In the pursuit of developing a portable wavelength reference, a photonically integrated chip (PIC) was developed to perform high resolution spectroscopy in a small package. The PIC outcouples light from one grating into free space where it is reflected and directed into an adjacent grating that couples into a separate waveguide. These gratings are extreme-mode-converters which convert the confined mode with a characteristic mode size of less than a micron to a collimated 100 micron diameter beam in order to mitigate transit time broadening for high resolution spectroscopy as well as reduce the diffraction angle. A miniature atomic vapor cell is inserted in the path of the beam to complete the spectroscopic platform. Preliminary results demonstrate sub-Doppler features. Coupling into the chip is achieved using fiber arrays enabling the spectroscopic signal to be routed back through an optical fiber and monitored. A laser is then locked to these sub-Doppler features completing an integrated wavelength reference. Analysis of the atom-light interactions made available by this platform will be discussed with an emphasis on the application of such structures to portable wavelength metrology.