# Self-referenced frequency combs using high-efficiency silicon-nitride waveguides

## TL;DR

This paper demonstrates a low-power, silicon-nitride waveguide-based method for self-referencing telecom-wavelength frequency combs, enabling compact, efficient, and potentially portable frequency comb systems for various applications.

## Contribution

The authors introduce a novel silicon-nitride waveguide approach for self-referencing frequency combs at significantly lower power levels than traditional fiber-based methods, including direct f-to-3f self-referencing.

## Key findings

- Achieved supercontinuum generation with only 11.3 mW incident power.
- Demonstrated self-referencing using dispersive waves and second harmonic generation.
- Enabled direct f-to-3f self-referencing without external doubling crystals.

## Abstract

We utilize silicon-nitride waveguides to self-reference a telecom-wavelength fiber frequency comb through supercontinuum generation, using 11.3 mW of optical power incident on the chip. This is approximately ten times lower than conventional approaches using nonlinear fibers and is enabled by low-loss (<2 dB) input coupling and the high nonlinearity of silicon nitride, which can provide two octaves of spectral broadening with incident energies of only 110 pJ. Following supercontinuum generation, self-referencing is accomplished by mixing 780-nm dispersive-wave light with the frequency-doubled output of the fiber laser. In addition, at higher optical powers, we demonstrate f-to-3f self-referencing directly from the waveguide output by the interference of simultaneous supercontinuum and third harmonic generation, without the use of an external doubling crystal or interferometer. These hybrid comb systems combine the performance of fiber-laser frequency combs with the high nonlinearity and compactness of photonic waveguides, and should lead to low-cost, fully stabilized frequency combs for portable and space-borne applications.