Cavity soliton laser brings all-optical processing closer

02 Jun 2008

A CW or pulsed laser that can be switched on and off using light pulses could play an important role in future photonic systems.

In a bid to use light to store, manipulate and transmit data, European researchers have developed a laser that can be switched on and off using light pulses. The continuous-wave laser is one of the key achievements of the fundamentals, functionalities and applications of cavity solitons (FUNFACS) project. The team hopes to develop a pulsed laser by the end of the project in September.

"The key achievement is that our laser is all-optical and that information can be written wherever and whenever you want and can also be erased," Robert Kuszelewicz, FUNFACS project coordinator, told optics.org. "This gives us the possibility of recording and erasing information in the form of optical bits. These characteristics are important for realizing all-optical switching and routing in modern high-speed communication systems."

The laser forms cavity solitons, defined as localized waves packets in which the tendency to diffract is balanced by nonlinearities. A key property is that they are self-localized, which means that they can exist anywhere within a pumped area. They are generated within the active media of a laser by shining light on the cavity and they can be reset by using an equivalent process. These cavity solitons are bistable, which means that they can be present or absent under the same conditions.

Indeed, key to the laser design is the wide cavity, which enables the solitons to form. The semiconductor cavity is a few microns thick, but is hundreds of times larger in the transverse direction compared with the longitudinal direction.

"A system with a large aspect ratio is susceptible to amplifying spatial and temporal instabilities instead of dampening them. This means that the system diverges from a homogeneous response and evolves towards a structured response, which is more stable for the system," explained Kuszelewicz. "These localised structures are called cavity solitons, which can be written, erased and manipulated using an external pulse."

In the set-up, the AlGaAs semiconductor laser system comprises a cavity that is pumped to exhibit gain. A saturable absorber is included into the system, which prompts competition between the gain and absorber medium. "When the absorber is not saturated, lasing is prevented," explained Kuszelewicz. "When the absorber is saturated, the loss drops completely and the system emits light at a wavelength slightly shorter than 1 µm. These situations can coexist in the same external conditions and also switch from one to the other by firing pulses from an external laser."

According to Kuszelewicz, there is still much work to be done before cavity soliton lasers are used in commercial devices. "We may see cavity solitons used in devices in around 10 years," he concluded. "In the meantime, I believe that very convincing demonstrators can be envisioned within the next few years."