Paul van Gerven
27 June

Combining the light-generating power of indium phosphide (InP) as a gain medium and the ultra-low light losses of silicon nitride (SiN) as an optical cavity, Dutch startup Chilas has launched an on-chip laser that uniquely combines tunability with a narrow spectral output.

As we learn in high school, a basic laser works by oscillating light between two mirrors through an amplifying medium. As the photons bounce back and forth, they encourage the formation of exact ‘copies’ of themselves. This process, known as stimulated emission, not only serves as an amplification mechanism but is also responsible for the main characteristic that makes laser light so special: it’s coherent, meaning the photons are in phase in space and time. By having one spot on the mirrors partially transparent, some light is allowed to escape so we can use it for one of the many applications that have been developed for it.

In this basic setup, the laser’s optical cavity formed by the mirrors and the gain medium basically coincide. But there’s no reason why that should be. In fact, many different types of external-cavity lasers have been developed, some of which have been commercially available for a long time.

Chilas butterfly
Chilas uses industry-standard ‘butterfly’ packaging. Credit: Chilas

Now, Dutch startup Chilas has put the external-cavity principle on a chip. Measuring just 6 by 4 mm, the high-quality laser features a unique combination of two properties: tunability over a wide wavelength range alongside a narrow linewidth. Additionally, the laser is capable of achieving high power output.

Burn out

The concept for Chilas’ technology was developed almost a decade ago by Twente-based Lionix. This company specializes in silicon nitride (SiN) integrated photonics, a material with many favorable properties but lacking the ability to produce light by itself. SiN photonic integrated circuits, therefore, depend on an external light source based on a direct-bandgap semiconductor such as indium phosphide (InP) or another III-V compound material.

Looking into better light sources, engineers at Lionix realized that they could make a really good one by combining InP as a gain medium and SiN as an external optical cavity. Adding an external SiN cavity narrows the spectral linewidth output while also allowing for wavelength tuning.

This in itself wasn’t a new concept, since external-cavity lasers have been commercially available since the 1990s. They leave something to be desired, however. Wavelength tuning depends on very precise mechanical manipulations of mirrors or gratings, which complicates the design and increases the device footprint. Additionally, vibrations, thermal drift and other perturbations can broaden the linewidth and drift lasing wavelength.

“Putting the same concept on a chip wouldn’t only decrease the footprint but also circumvent these problems,” explains Sami Musa, CEO of Chilas. “SiN is an excellent material for the external cavity because it absorbs very little light and isn’t very temperature sensitive. On top of that, the design can handle much high power than InP alone, which saturates at a certain level. It can even burn out. Hybrid integration of InP and SiN yields the best of both materials.”

Thanks to the unique combination of laser characteristics, the investors ascribed a large market potential to it, which would be best realized in a focused company. In 2018, Chilas was founded to develop the laser into a commercial product and take it to market.

Four hard years

Chilas’ laser consists of an InP semiconductor optical amplifier, coated with a reflective material on one side to direct all light into the external cavity, which is a mini-photonic integrated circuit made of SiN. This circuit consists of three main elements. One: a coupler that’s equivalent to a semi-transparent mirror that lets some of the light out and reflects the rest – as needed for any laser. Two: a heater strip placed on a straight waveguide used to change the optical length of the cavity by changing the refractive index utilizing the thermo-optic effect. By adjusting the length of the cavity, the optical wave (mode) can be made to fit exactly within the cavity. And three: a pair of coupled micro-ring resonators, which together ensure a narrow spectral emission and, through the use of heaters placed on top of the rings, their refractive index can be changed, allowing for wavelength tuning.

Chilas layout
Credit: Chilas

After “four hard years” of moving from demonstrator to prototype to product, the company is ready to start making waves, says Musa. “This technology has been incubating for quite some time, but the presence of an extended integrated-photonics ecosystem has been of great help to commercialize it. We don’t need to build production and assembly facilities, allowing us to focus completely on improving our product. I anticipate swift market penetration and strong growth over the next few years in terms of both sales and personnel.”

Applications particularly suited to Chilas’ laser characteristics include fiber-optic sensing, next-gen optical transceivers, quantum key distribution and lidar.