Scientists use microresonators to create the world's smallest optical frequency comb

The picture shows the micro resonator frequency comb system

(Source: Arslan Raja, Federal Institute of Technology, Lausanne)

Optical frequency combs (OFC) is a laser source whose spectrum consists of a series of discrete, evenly spaced comb-shaped spectral lines that can be used for accurate measurements. Over the past two decades, they have become the primary tools for applications such as precision ranging, spectroscopy and communications.

Most commercial optical frequencies based on mode-lock lasers (modulated laser oscillations with a certain phase relationship between longitudinal modes at different frequencies to obtain ultrashort pulse lasers with narrow pulse width and high peak power) Comb drive sources are bulky and expensive, and these features limit their potential for applications in high volume and portable applications. Although chip-scale optical frequency combs using microresonators appeared for the first time in 2007, the fully integrated form was hampered due to high material losses and complex excitation mechanisms.

Tobias J. of the Federal Institute of Technology (EPFL) in Lausanne Kippenberg and Michael L. of the Russian Quantum Center The research team led by Gorodetsky has now built integrated soliton microcombs driven at 88 GHz repetition rate using chip-level indium phosphide laser diodes and silicon nitride (Si3N4) microresonators. With a footprint of only 1 cubic centimeter, the device is the smallest of its kind to date.

The silicon nitride (Si3N4) microresonator is fabricated using the patented photonic damascene reflow process, which achieves unprecedented low losses in integrated photonics. These ultra-low loss waveguides bridge the gap between the chip-level laser diodes and the power levels required to excite the dissipative Kerr soliton state, which is the basis for generating optical frequency combs.

This method uses a commercial chip-level indium phosphide laser instead of the traditional large laser module. In the study, a small portion of the laser is reflected back to the laser due to the inherent scattering of the microresonator. This direct reflection helps stabilize the laser and create solitons. This shows that the cavity and laser can be integrated on a single chip, a unique improvement over previous technologies.

Kippenberg explained: "People have a strong interest in this optical frequency comb drive source, which is opto-electrically driven and fully integrated by optoelectronics to meet the needs of next-generation applications, especially for laser radar ( LiDAR) and information processing in the data center. This not only represents the technological advancement in the field of dissipative Kerr soliton, but also provides an insight into the nonlinear dynamics with the rapid feedback of the cavity."

The entire system is less than 1 cubic centimeter in volume and can be electrically controlled. Arslan Sajid, the lead author and doctoral student of the study, explained: "This micro-comb system is characterized by compact structure, easy adjustment, low cost and low repetitive operation rate, and is suitable for large-scale manufacturing applications. Its main advantage is optical feedback. Fast, no active electronics or any other on-chip tuning mechanism."

The goal of scientists today is to implement integrated spectrometers and multi-wavelength sources, and to further improve the manufacturing process and integration of micro-combs operating at microwave repetition rates.

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