The project is about a new form of unidirectional emission recently observed in an annular cavity fiber laser. The most unusual features of this new form of unidirectional light emission are that it occurs in an optically reciprocal ring cavity, has a threshold-like appearance well above the laser threshold at full laser operation, has no predefined direction, and is accompanied by optical bistability.
Our hypothesis is that we have observed a previously unknown form of instability in ring lasers. As the pump power increases, the bidirectional steady state gradually becomes more unstable at small power fluctuations. Fluctuations are mainly damped at small pump powers, allowing the system to maintain bidirectional emission. In contrast, fluctuations are increasingly amplified at high pump powers until their amplification is stronger than the damping mechanism. This opens up new ways to exploit this instability and greatly increases its potential impact. In its present manifestation, an interplay of linear and nonlinear loss is expected. The nonlinear loss at the laser wavelength due to stimulated Raman scattering is expected to be influenced by the linear loss at the Stokes-Raman components of the spectrum. Moreover, a directional imbalance with respect to the linear and/or nonlinear loss seems to favor the instability and lower its threshold. As with previously observed laser instabilities, we expect this directional instability to be a new phenomenon based on the interaction of known physical effects, rather than a fundamentally new physical effect.

The project is supported by DFG grant number HA 9026/2-1; AOBJ: 675090.