Investigation of Direction Instability in Fiber Ring Lasers

• Applicant: Dr. Alexander Hartung
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 453562122

The project is about a new form of unidirectional emission which was recently observed in a ring cavity fiber laser. The most uncommon properties 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 laser threshold at full laser operation, has no predefined direction, and is accompanied by optical bistability.Our hypothesis is that we observed a previously unknown form of instability in ring lasers. With increasing pump power the bidirectional steady state becomes gradually more instable regarding small power fluctuations. Fluctuations are mainly damped at small pump power, allowing the system to maintain the bidirectional emission. In contrast fluctuations are increasingly amplified at high pump power until their gain is stronger than the damping mechanism. The bidirectional emission is no longer a steady state of the operation conditions and the system switches into unidirectional mode. A significant amount of nonlinear loss is expected to be mandatory for this phenomenon to occur, but the very nature of this nonlinear loss is expected to be irrelevant. This opens new ways to employ this instability and significantly increases its impact potential. The current standard of fiber-coupled optical isolators to enforce unidirectional operation, well established for typical wavelengths only, limits the exploration of new wavelengths where they have not matured or are not available. The scientifically emerging mid infrared region is considered as a fingerprint region and important for various applications in medicine, life sciences, defense and security. Fiber lasers, already being a billion dollar industry in the near infrared region, had shown better performance compared to other kinds of lasers in this emerging region due to their compact and flexible configuration. Yet they still contain bulky elements which are inconvenient. An all-fiber format, potentially enabled by the present phenomenon, is highly desirable for a most robust and easy operation and has the potential to facilitate the transition of mid infrared fiber lasers from lab demonstrations into widespread applications. There is an even greater potential if this concept can be transferred to pulsed operation. The project aims at revealing the yet unknown conditions that lead to unidirectional light emission and enable utilization by ensuring a predefined final direction. In particular it advances the physical understanding of optical fiber based ring lasers, widens the scope regarding the multitude of possible linear and nonlinear light interactions within, introduces new knowledge to the scientific community, and enables new applications. If successful, this project paves the way for a new class of fiber ring lasers which does not rely on dedicated components to force unidirectional emission.

• DFG Programme: Research Grants