Dark Solitons Discovered in Ring Semiconductor Lasers

Dark solitons - the extinction region in a bright background - spontaneously form in a ring semiconductor laser. Observations conducted by an international research group may lead to improvements in molecular spectroscopy and integrated optoelectronics.

Frequency comb - a pulse laser that outputs light at equidistant frequencies - is one of the most important achievements in the history of laser physics. Sometimes referred to as optical rulers, they serve as the basis for time and frequency standards, used to define many fundamental quantities in science. However, traditional frequency comb lasers are bulky, complex, and expensive, and laser experts are keen on developing simpler versions that can be integrated into chips.

In a similar attempt in 2020, researchers from the Federico Capasso team at Harvard University unexpectedly discovered that after initially entering a highly turbulent state, the mid infrared "fingerprint" region widely used in molecular spectroscopy in quantum cascade ring lasers stabilized to a stable frequency comb - although only nine teeth.

A ring laser has an optical cavity in which light is guided around the closed loop, and a quantum cascade laser is a semiconductor device that emits infrared radiation.

"All these interesting results come from controlling devices - we didn't expect this to happen," said Marco Piccardo of Harvard University. After several months of confusion, researchers have found that this effect can be understood by describing the instability of nonlinear differential equations in systems - the complex Ginzberg Landau equation.

In this new study, Capasso and his colleagues collaborated with researchers from the Benedikt Schwarz group at the Vienna Institute of Technology. The Austrian team has developed several frequency comb designs based on quantum cascade lasers. Researchers integrated waveguide couplers into the same chip. This makes extracting light easier and achieves greater output power. It also allows scientists to adjust coupling losses by pushing the laser between its frequency comb and the state in which it should operate as a continuous wave laser with continuous output radiation.

However, in the "continuous wave" system, even more strange things have happened. Sometimes, when the laser is turned on, its behavior is only a continuous wave laser, but turning off and on the laser may cause one or more dark solitons to randomly appear.

Solitons are nonlinear, non dispersive, and self enhancing radiation wave packets that can propagate indefinitely in space and effectively transmit to each other. They were first observed in water waves in 1834, but were later discovered in many other physical systems, including optics.

Surprisingly, this latest observation shows that solitons exhibit small gaps in continuous lasers. The seemingly small changes in laser emission cause significant changes in its spectrum.

"When you talk about continuous wave lasers, it means you have a monochromatic peak in the spectral domain," Piccardo explained. This decline means the entire world... These two images are linked by the principle of uncertainty, so when you have very, very narrow things in space or time, it means that in the spectral domain, you have many, many patterns, and many, many patterns mean that you can do spectroscopy to observe molecules emitted over a very, very large spectral range.

I have occasionally seen dark solitons before, but they have never appeared like this in small electrically injected lasers. Picardo said that from a spectral perspective, dark solitons are as useful as bright solitons. However, some applications require bright pulses. The technology required to generate bright solitons from dark solitons will be the theme of further work. Researchers are still studying how to generate solitons with certainty.

A key advantage of this comb like design for integration is that, due to the fact that light circulates only in one direction in a circular waveguide, researchers believe that the laser is essentially unaffected by feedback that may disrupt many other lasers. Therefore, it does not require a magnetic isolator, as magnetic isolators are often not commercially integrated into silicon chips.

Considering integration, researchers hope to extend this technology beyond quantum cascade lasers. "Although chips are very compact, quantum cascade lasers typically require high voltage to operate, so they are not the true way to place electronic devices on the chip," Piccardo said. If this can work in other lasers, such as interband cascade lasers, then we can miniaturize the entire thing and it can really be powered by batteries.

Laser physicist Peter Delphi from the University of Central Florida in Orlando believes that this work brings hope for future work. "Dark pulses in the frequency domain are a set of colors, and although their spectral purity is very good, their precise positioning has not yet been achieved," he said. However, in fact, they can achieve this - using electric pumping equipment to create solitons on chips - which is actually an extremely significant progress. There is no doubt about it.

The study was described in the journal Nature.

Source: Laser Net