An Integrated-Photonics Optical-Frequency Synthesizer (1708.05228v1)

Abstract: 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 $7.0*10^{-13}$ reference-clock instability for a 1 second acquisition, and constrain any synthesis error to $7.7*10^{-15}$ 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.

• The paper introduces an integrated synthesizer that combines III/V-Si tunable lasers with dual dissipative Kerr soliton combs to achieve a 4 THz tuning range and 1 Hz resolution.
• The paper details precise phase stabilization using an octave-bandwidth silicon-nitride comb and a C-band fused-silica comb, realizing fractional instability as low as 7.0×10⁻¹³.
• The paper highlights potential applications in high-precision spectroscopy, coherent LIDAR, and optical communications, paving the way for scalable, low-power photonic systems.

Integrated-Photonics Optical-Frequency Synthesizers: A Technical Summary

The paper "An Integrated-Photonics Optical-Frequency Synthesizer" presents a substantial advancement in the domain of optical-frequency synthesis utilizing integrated photonics. The authors describe the construction and meticulous experimentation of an optical-frequency synthesizer based on integrated photonic technology, which amalgamates tunable lasers and dual dissipative Kerr soliton (DKS) frequency combs on silicon chips. This novel integration yields an overview that is aligned with the SI second and offers highly precise, stable output within the telecommunication C-band.

The integrated-photonics synthesizer demonstrates an impressive frequency tuning capability across a wide 4 THz range near 1550 nm. This modulation is precisely controlled by a microwave clock with a substantial 1 Hz resolution. Such fidelity and traceability are enabled through a heterogeneously integrated III/V-Si tunable laser, phase-locked to DKS frequency combs fabricated on highly efficient silicon microchips. The fractional-frequency instability of the synthesized output matches the reference-clock instability of 7.0×10$^-13$ for a 1-second acquisition time. Moreover, synthesis error constraints are demonstrated to be as low as 7.7×10$^-15$, establishing the robustness and precision of the synthesizer.

In the authors’ analysis, comparisons are drawn between the current state of electronic synthesizers in radio and microwave domains, which have historically created massive technological shifts in navigation and communication systems, and optical domain achievable through this novel integrated technology. Optical frequency combs, derived from mode-locked lasers, provide a solution for phase-coherent links between microwave and optical domains. Despite their established utility, traditional optical comb technology suffers from physical constraints such as bulkiness and high power consumption, limitations that integrated photonics are positioned to overcome.

This synthesizer is based upon two core technologies: heterogeneously integrated silicon photonics and microresonator frequency combs, popularly known as microcombs. Microcombs utilize just a few milliwatts of input power to generate combs through parametric four-wave mixing in microresonators. Notably, the authors leverage recent advances in Kerr soliton discovery, specifically DKS, which vastly improve the signal-to-noise ratio in frequency comb generation, thus advancing the resolution of frequency detection across broad optical spectra.

A significant contribution of this work is the novel dual-comb architecture with phase stabilization utilized to enhance frequency synthesis with reduced power consumption. The DKS dual-comb system includes both an octave-bandwidth silicon-nitride comb with a 1 THz mode spacing and a C-band spanning fused-silica comb with 22 GHz mode spacing. Careful phase stabilization across these combs contributes to the virtually unparalleled performance in fractional frequency error and phase noise control.

The potential implications of this integrated optical-frequency synthesizer are extensive. Applications in fields such as high-precision spectroscopy, optical communications, photonics-based sensing, and coherent LIDAR systems could benefit significantly due to enhanced precision, portability, and lower power requirements. The evolution and maturation of this technology could redefine the landscape of photonics, promoting its integration with classical electronics and enabling new technologies in various scientific fields.

As future work, ongoing enhancements in photonic material engineering and fabrication techniques will potentially further the capabilities and scope of integration, leading toward fully-integrated systems facilitating broader implementations. The reported findings pave the way for subsequent investigational studies focusing on harnessing nanophotonic advances and further refinement of Kerr frequency combs for practical, scalable solutions in real-world applications.