Solitons are persistent, isolated excitations in which the tendency for a wave pulse to disperse is exactly balanced by nonlinear effects in the propagation medium. A classic example is the solitary water wave that was observed traveling down a Scottish canal by John Scott Russell in the 1800s.
Communications networks and optical computing could potentially make use of optical solitons traveling in fibers, but the generation of optical solitons typically requires complex optical arrangements and close tolerances for stability. Reporting in Physical Review Letters, Patrice Genevet, Stephane Barland, Massimo Giudici, and Jorge Tredicce of the University of Nice in France describe their success at building a remarkably simple semiconductor laser system for generating solitons.
The researchers arranged two cylindrical semiconductor lasers to face each other. One of these microresonators acts as an amplifier. The other operates as a nonlinear device that has a high absorption at low light intensity and decreasing absorption at high intensity, causing the emission of light from the coupled lasers to be bistable. Because of the bistability, an incoming “writing” beam can toggle the emission of light at different locations in the cylindrical cross section of the cavities on and off.
The team imaged the spatial distribution of light emitted from the lasers and showed that with the two lasers running in bistable mode, they were able to write soliton structures into the emission of the laser. They could turn on bright soliton structures in the emission pattern selectively and then drag a soliton around by sweeping the writing beam across the diameter of the coupled microresonators.
The authors suggest that this highly controllable soliton generator could be miniaturized into a single semiconductor chip. – David Voss