Dissipative spatial solitons in active semiconductor optical amplifiers

Semiconductor optical amplifiers (SOAs) have proved to be very useful elements for networks and communications systems. As well as amplifying signals, SOAs have been also used for a number of network optical processing functions, such as switching, demultiplexing, frequency shifting etc. Usually the light is guided in SOAs by fixed channel waveguides fabricated by some form of microlithography. As a result, controllable routing functions etc. need to be implemented by electro-optic devices such as Mach-Zehnder interferometers. There is another option for guiding in SOA chips which is more flexible. Spatial solitons, i.e. self-trapped beams which propagate without spreading, have now been observed with many different nonlinearities. These are all essentially lossless systems in which spatial diffraction is compensated by some self-focusing mechanism. However, in an amplifying medium like an SOA, loss must also be balanced by gain, which leads to an inherently different soliton than the lossless ones. In this case, the small signal gain determines completely the soliton properties, i.e., there is no trade-off between the peak intensity and the soliton width as is well-known for solitons in lossless media. We discuss the first observation of these "dissipative" solitons.