Caltech researchers are advancing frequency microcellular technology by overcoming dispersion challenges through innovative designs using ultra-low-loss silicon nitride. Developments in this technology could enable the integration of microcells into compact devices, benefiting from cost-effective manufacturing processes.

Artist's concept of a frequency microcell developed by Caltech's Kerry Vahala and collaborators. Source: Yuan, Bowers, Vahala, et al.

Three years ago, when we last interviewed Caltech's Kerry Vahala, his lab was reporting on the development of a new optical device called a "turnkey frequency microcomb" that could have applications in digital communications, precision timing, spectroscopy, and even astronomy.

The device, built on a silicon wafer, receives a laser input of one frequency and converts it into a uniformly distributed set of light of multiple different frequencies, creating a series of pulses that can be as short as 100 femtoseconds (four billionths of a second) in length. (The "comb" in the name comes from the fact that the frequencies are spaced like the teeth of a comb).

Now, Vahala (BS '80, MS '81, PhD '85), the Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics and executive officer of Applied Physics and Materials Science at Caltech, joins his research group Together with members of John Bowers' research group at the University of California, Santa Barbara, they have made a breakthrough in how to form short pulses in an important new material called ultra-low-loss silicon nitride (ULLnitride), a compound composed of silicon and nitrogen. This silicon nitride is extremely pure and deposited as a thin film.

In principle, short-pulse microcellular devices made of this material require only very low power to operate. Unfortunately, this material cannot properly generate short pulses of light (called solitons) because it has a property called dispersion, which causes light or other electromagnetic waves to travel at different speeds depending on their frequency. ULL has so-called normal dispersion, which makes ULL nitride waveguides unable to support the short pulses required for microcell operation.

Overcome optical limitations

In a paper published in Nature Photonics, the researchers discuss the new microcells they developed that overcome the inherent optical limitations of ULL nitrides by generating pulses in pairs. This is a significant advance because ULL nitride is made using the same technology used to make computer chips. This manufacturing technique means these microcells could one day be integrated into a variety of handheld devices similar to smartphones.

The biggest feature of ordinary microcells is that they have a small halo, which looks a bit like a small race track. During operation, solitons automatically form and circulate around it.

"However, when the ring is made of ULL nitride, dispersion destroys the stability of the soliton pulse," said co-author Yuan Zhiquan (MS '21), a graduate student in applied physics.

Think of the loop as a racing track. If some cars are going faster and some cars are going slower, they will spread out as they go around the track instead of being packed tightly together. Likewise, the normal dispersion of ULL means that the light pulse will spread out in the microcell waveguide and the microcell will stop functioning.

In this animated gif, we can see pulses of light (solitons) spiraling in connected light tracks. Source: Yuan, Bowers, Vahala, et al.

The solution devised by the research team was to create multiple tracks and pair them up so that they look a bit like a figure-eight. In the middle of the "8" shape, the two tracks are parallel to each other, with only a small gap in between.

If we continue with the race track analogy, it's like two tracks sharing a straight. When vehicles on each track converge on a shared stretch of road, they experience a traffic jam-like situation. Just as two lanes of a highway merging into one force a car to slow down, the connecting segment of two microcells forces pairs of laser pulses to cluster together. This aggregation counteracts the tendency of the pulses to scatter, allowing the microcells to function properly.

In traffic jams, cars bunch up as they try to merge. A similar phenomenon occurs with optical tracks developed by Carey-Wahhara and other researchers. This clustering phenomenon is key to the operation of its equipment. Source: Oregon Department of Transportation

Innovative methods and future prospects

"In effect, this cancels out the normal dispersion, leaving the entire composite system with equivalent anomalous dispersion," said graduate student and co-author Gao Maodong (MS '22).

This idea is extended when more racetracks are added, and the team has shown how three racetracks can operate by creating two sets of pulse pairs. Vahara believes this phenomenon can continue to work even with many coupled racetracks (microcells), providing a way to create large arrays of photonic circuits using soliton pulses.

The new microcellular devices work as a pair of connected light rails and can also work when more devices are combined together. Source: Yuan, Bowers, Vahala, et al.

As mentioned above, these ULL microcells are made using the same equipment used to make computer chips based on complementary metal-oxide semiconductor (CMOS) technology. "The manufacturing scalability of the CMOS process means it will now be easier and more economical to make short-pulse microcells and integrate them into existing technologies and applications," said Powers, a professor of electrical and computer engineering who participated in the research.

Regarding these applications, Vahara said: "The comb is like the Swiss Army knife of optics. It has many different functions, which is why it is such a powerful tool."

Compiled source: ScitechDaily