Measuring Tunable Lasers with Fine-Toothed Combs

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Vahala group and collaborators design vernier spectrometer with a microresonator

Image of microresonator from Vahala group collaboration

June 17, 2019

Kerry Vahala, Ted and Ginger Jenkins Professor of Information Science and Technology and professor of Applied Physics, and his group are pioneers in the field of on-chip ultra-high-Q (UHQ) optical microresonators. They have created the highest Q-factor chip-based resonators of nearly 1 billion and helped launch new areas of study within this field of research.

Their recent publication in Science, “Vernier spectrometer using counterpropagating soliton microcombs,” details the design of their vernier spectrometer which utilizes dual-locked counterpropagating solitons (“bullets” of light) in a microresonator. The novel device allows for high-resolution optical frequency measurements under continuous or abrupt tuning conditions. With this approach Vahala group members and collaborators were able to demonstrate a chip-sized microresonator that yields more accurate measurements over a shorter time span than standard tabletop systems.  

The central feature of the device begins with generating counterpropagating solitons within a high-quality (high-Q) factor silica microresonator. Distinct, controllable repetition rates of the solitons create two mutually phase-locked frequency combs. The “teeth”, or markers, of the frequency combs operate as a vernier scale to measure optical frequencies. Traditionally, a vernier scale consists of two stacked rulers, slightly offset from one another. This simple tool helps reduce human estimation errors and improve the resolution of a measurement.

Using this optical vernier scale, researchers were able to compare the frequency readout from the two phase-locked combs and rapidly map tuned lasers with a high degree of precision. The spectrometer enables characterization of laser tuning at rates over 1 PHz per second.

The next generation of innovative telecommunications applications and laser sensing technology – everything from more reliable self-driving cars to more accurate atomic clocks – will require further advancements in laser frequency measurement and control. This is where device downscaling plays an essential role. The Vahala group’s chip-sized microresonators prove that smaller, more precise, and integrable spectrometer devices are possible.

This project and its outcomes was a collaborative effort among research groups at the California Institute of Technology (Caltech) and the University of California, Santa Barbara. Co-first authors Qi-Fan Yang, Boqiang Shen, and Heming Wang are researchers in the Vahala research group. Additional authors include Vahala group members Chengying Bao and Lue Wu, along with alumnus Ki-Youl Yang (Stanford), Minh Tran (Bowers research group, UCSB) and Zhewei Zhang, a graduate student researcher of Amnon Yariv, Martin and Eileen Summerfield Professor of Applied Physics and Electrical Engineering at Caltech.

Microresonators in this paper were fabricated using the 6000 Series DSW Wafer Stepper in the KNI Laboratory's cleanroom facility.


Vahala microresonator
Image foreground: Microresonator on a silicon chip. Image background: optical spectral analyzer and wavemeter.