Fishbone Waveguide Design Flattens Dispersion, Expands Bandwidth of Frequency Combs

A waveguide design that could enable highly precise frequency combs has been created by a research team from the French Centre National de la Recherche Scientifique, Paris-Saclay University, and Zhejiang University. The team’s design produces a graded index waveguide that allows the width of a frequency comb to be more than doubled, compared to a normal waveguide.

Frequency comb generation must balance the material properties that allow light to generate new colors with the behavior of light in the waveguide. This balance is easier to engineer in glass, while for applications and integration with existing devices, silicon is preferred.

To expand frequency comb bandwidth, the researchers focused on the behavior of light and specifically on the dispersion of light.

To achieve a broad frequency comb, the colors that make up the comb must all stay in phase with each other. However, the bandwidth in which the nonlinear phase matching condition is satisfied is typically limited due to the dispersion of the waveguide. This limitation is particularly stringent in high-index-contrast technologies such as silicon-on-insulator. The silicon core has a large refractive index compared to the glass cladding. The large difference between the two creates a strong dispersion that overcompensates for the material dispersion. Researchers can counter material dispersion through their waveguide design.

The researchers’ approach is based on subwavelength engineering of single-mode waveguides with self-adaptive boundaries. They realized that the interface between the glass cladding and the silicon core did not have to be sharp. They designed a waveguide that has a silicon core with a fishbone structure that extends outward into the glass cladding. The effective refractive index in the mixed region is the average of the glass and silicon, which gradually transitions from silicon to glass — a graded index waveguide.

Waveguide design, Zhang et al. doi 10.1117/1.AP.2.4.046001. Courtesy of Zhang et al.

In the graded index, red colors spread out to occupy a wider area of waveguide, while blue colors are more tightly confined. The net effect is that the different wavelengths behave as if they are traveling in different width waveguides, although they are actually traveling together in the same waveguide. The researchers refer to this effect as a self-adaptive boundary. The wideband flattened dispersion operation comes from the property of the waveguide optical mode that automatically self-adapts its spatial profile at different wavelengths to slightly different effective spatial spans determined by its effective index values.

The researchers explored different configurations for the fishbone structure. Each configuration increased the wavelength range, over which the dispersion remained small. To confirm that their graded index waveguides would result in better frequency combs, the team modeled frequency comb generation in standard and graded index waveguides. They showed that the frequency spectrum could be extended from about 20 THz to about 44 THz.

So far, the researchers have only calculated and modeled their structures. However, the structures they propose have been developed with fabrication in mind. Once testing of the structures is completed, the research team’s strategy for stretching the spectra of Kerr frequency combs with self-adaptive boundary silicon waveguides could benefit nonlinear applications in which the manipulation of energy spacing and phase matching is required. For example, a chip-based frequency comb could enable high-precision and high-sensitivity compact spectrometers.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.2.4.046001).