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An Integrated-Photonics Optical-Frequency Synthesizer
Daryl T. Spencer, Tara Drake, Travis C. Briles, Jordan Stone, Laura C. Sinclair, Connor Fredrick, Qing Li, Daron Westly, B. Robert Ilic, Aaron Bluestone, Nicolas Volet, Tin Komljenovic, Lin Chang, Seung Hoon Lee, Dong Yoon Oh, Myoung-Gyun Suh, Ki Youl Yang, Martin H. P. Pfeiffer, Tobias J. Kippenberg, Erik Norberg, Luke Theogarajan, Kerry Vahala, Nathan R. Newbury, Kartik Srinivasan, John E. Bowers, Scott A. Diddams, Scott B. Papp
TL;DR
Precision optical-frequency synthesis has largely remained confined to tabletop systems. This paper implements it with integrated photonics and demonstrates <1.5 Hz absolute frequency error, showing a route to compact optical-frequency sources.
Problem
Precision and accurate optical-frequency stabilization has been largely confined to tabletop mode-locked laser frequency-comb devices, limiting integrated-photonics demonstrations.
Method
The synthesizer phase-locks a tunable III/V-Si laser to dual silicon-chip dissipative-Kerr-soliton combs stabilized to a microwave clock.
Results
<1.5 Hz absolute frequency error is measured between the output and setpoint, while the synthesizer shows Hz-level tuning and phase-locked operation across its comb frequencies.
Takeaways & Limitations
Integrated photonics can support absolute optical-frequency stabilization and a phase-coherent microwave-to-optical connection at high precision and accuracy.
Takeaways & Limitations
The optical-frequency synthesizer exhibits non-zero synthesis error during operation.
Abstract
from arXiv · showhide
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.
Methods Device and experimental details
The synthesizer combines a heterogeneously integrated III/V-Si tunable laser and SOA with dual microcomb generation, shared pumping, and controlled frequency detuning. An auxiliary Erbium:fiber comb and filtered beat-note counting provide out-of-loop verification of phase locking and frequency comparison.
- Integrated III/V-Si device: The III/V-Si device integrates a tunable laser and SOA, delivering up to ~4 mW CW power and on-chip small-signal gain >10 dB.The laser uses InP-based quantum wells heterogeneously integrated on a Si waveguide, with gain, phase, and high-quality-factor microresonator sections.
- Microcomb generation: A commercial external cavity diode laser shares pump power between the two microcomb generators through a 3 dB splitter, separate LiNbO3 modulators, and erbium-doped fiber amplifiers.Soliton-generation detuning is controlled with amplified voltage-controlled actuators.
- Frequency-offset correction: ≈ 5 GHz of pump-frequency offset arises from the separate single-sideband modulators and is electronically added to or subtracted from heterodyne beat notes.The correction is applied mainly after frep,THz detection and after III/V-Si laser heterodyne detection with the DKS comb.
- Out-of-loop verification: The auxiliary out-of-loop comb is a 250 MHz Erbium:fiber mode-locked laser comb with octave broadening for self-referenced fceo detection.Its 4th harmonic of frep is phase-locked to a reference synthesizer at 999.999 544 MHz, with both synthesizers referenced to the shared fclk.