Stretching the Spectra of Frequency Combs

This kind of a silicon core fishbone waveguide extends frequency comb. (Source: Zhang et al., SPIE)

Frequency combs are becoming one of the great enabling techno­logies of the 21st century. High-precision atomic clocks, and high-precision spectro­scopy are just two techno­logies that have benefited from the develop­ment of highly precise frequency combs. However, the original frequency comb sources required a room full of equipment. And it turns out that if you suggest that a room full of delicate equipment is perfect for a commercial appli­cation, the development engineer makes a beeline for the nearest exit.

These disadvantages would be solved by making chip-based devices that are actually robust enough to withstand the rigors of everyday use. To do that, scientists have to balance material properties with the behavior of light in a waveguide. This balance is easier to engineer in glass, while for appli­cations and integration with existing devices, it would be better to use silicon. It is difficult to make very wide frequency combs from silicon wave­guides, but clever waveguide engi­neering may be about to make that task a bit easier. Zhang and col­leagues have now shown a way to make 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 is a delicate balance between the material properties that allow light to generate new colors of light referred to as the optical non­linearity, the confi­guration of the path the light follows, and the dispersion. The last item, dispersion, is usually the killer, and this is where the work of Zhang and colleagues focuses. To generate a very broad frequency comb, the colors that make up the comb must all stay in phase with each other. Put con­cretely: if two waves at one point have their peaks lined up, then at some point further along in space and time, those peaks should still line up. But, ordinarily, this never happens, and the peaks slip past each other, preventing any new frequencies from being generated.

To compen­sate for the material dis­persion, researchers often turn to waveguide engi­neering. Since waveguides are made of materials, they have dispersion, and the confinement of the waveguide itself introduces another type of dispersion. This dispersion depends on the shape of the waveguide, the dimensions, as well as the materials that are used. This allows engineers to counter material dispersion through their wave­guide design. But, this is tough work in silicon. 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.

The insight of Zhang and colleagues is that the interface between the glass cladding and the silicon core doesn’t have to be sharp. They have designed a waveguide that has a silicon core with a fishbone structure that extends outwards 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.

In the graded index, red colors spread out to occupy a wider area of wave­guide, while bluer colors are more tightly confined. The net effect is that the different wavelengths behave as if they are traveling in different width wave­guides, while they are actually traveling together in the same waveguide. The researchers refer to this effect as a self-adaptive boundary. They explored different configurations for the fishbone structure. Each confi­guration increased the wave­length range over which the dispersion was 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 was extended from about 20 THz to about 44 THz.

So far the researchers have only calcu­lated and modeled their structures. However, the proposed structures have all been chosen with fabri­cation in mind, so once they get their bunny suits, test devices should be on their way. Then silicon frequency combs can really strut their stuff. A good example: silicon is transparent over a broad range of the infrared, which is also the wavelength range needed for spectro­scopic identi­fication of molecules. A chip-based frequency comb will enable high precision and high sensitivity compact spectro­meters. (Source: SPIE)

Reference: J. Zhang et al.: Stretching the spectra of Kerr frequency combs with self-adaptive boundary silicon waveguides, Adv. Phot. 2, 046001 (2020); DOI: 10.1117/1.AP.2.4.046001

Link: Centre de Nanosciences et de Nanotechnologies C2N, CNRS, Paris, France

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