# Photonic chip-based soliton frequency combs covering the biological imaging window

## TL;DR

This paper demonstrates the generation of near-infrared soliton Kerr frequency combs on a chip, covering the biological imaging window, with potential applications in biomedical imaging and spectroscopy.

## Contribution

It introduces a method to produce octave-spanning soliton Kerr combs in the near-infrared using silicon nitride microresonators, extending the spectral range for biomedical applications.

## Key findings

- Octave-spanning combs reaching 776 nm achieved
- Soliton states generated in normal dispersion regions
- Potential for biomedical imaging and atomic transition applications

## Abstract

Dissipative Kerr solitons (DKS) in optical microresonators provide a highly miniaturized, chip-scale frequency comb source with unprecedentedly high repetition rates and spectral bandwidth. To date, such soliton frequency comb sources have been successfully applied in the optical telecommunication band for dual comb spectroscopy, coherent telecommunications, counting of optical frequencies and distance measurements. Yet the range of applications could be significantly extended by operating in the near-infrared spectral domain, a prerequisite for biomedical and Raman imaging applications, and a part of the spectrum which hosts commonly used optical atomic transitions. Here we demonstrate the operation of photonic chip-based soliton Kerr combs pumped with 1 micron laser light. By engineering the dispersion properties of a $\rm Si_3N_4$ microring resonator, octave-spanning soliton Kerr combs extending to 776 nm are attained, thereby covering the optical biological imaging window. Moreover, we demonstrate that soliton states can be generated in normal group velocity dispersion regions, when exploiting mode hybridization with other mode families. The reported near-infrared soliton Kerr combs are suitable for a range of studies, including frequency-comb-based optical coherence tomography, coherent anti-Stokes Raman spectro-imaging, and they spectrally overlap with optical atomic transitions in Alkali vapors.