Noise in Optical Comb Sources

Optical comb sources are becoming standard tools in many applications including spectroscopy, sensing, metrology, phase-controlled extreme nonlinear optics, and even astronomy. The issue of comb quality is closely related to the noise properties of the source. There exist a number of practical schemes of comb sources, based on mode-locked laser oscillators: solid-state and fiber lasers, operating in different dispersion regimes. The project aims at developing a unified approach to the modeling and control of the noise in mode-locked laser oscillators that would naturally describe all current and future realizations and allow for various noise sources from technical to quantum. The analytical model, a generalization of quantization procedure to the dissipative solitons, will serve as a basis for numerical modeling in the quantum phase-space. The numerical simulation will be for the first time realized in a full-scale technique, combining the whole oscillator cavity period with fine-grained temporal grid and statistics gathering during long evolution time. As an alternative approach, the simulations in the quantum phase- space based on the Liouville equation will be realized for the first time. This breakthrough technique provides with a direct statistical description of a laser and would allow avoiding the numerical statistics gathering, thus reaching the otherwise inaccessible low-frequency part of the noise spectra. All models will include the measurement scheme and a control loop as integral parts of an experiment. The results will be compared to the practical realizations of the optical comb sources in the collaborating at Hannover University, University of Konstanz, Max- Planck Institute of Quantum Optics, Norwegian University of Science and Technology, and TU Vienna. The project will enhance our knowledge in the field of laser physics, quantum optics, and nonlinear quantum physics, as well as allow developing optimum methods for comb control.

The general idea of the project was to study ultrashort pulsed lasers alias frequency combs, and in particular their noise, i.e. spontaneous changes in output power or frequency. There exist many sources of noise, from external, like temperature changes or vibrations, to inherent, like fundamental quantum noise of photons or Raman scattering of light in the medium. The noise can severely limit the use of frequency combs in applications, and can even cause instability, when the device cannot operate. From the many different results of the project, which allowed significant progress in ultrashort pulsed lasers, we would like to emphasize two as most far-reaching: 1. Discovery of a new type of laser pulses a dissipative Raman soliton which can be quite stable and sustained indefinitely long, despite the noisy effect of Raman scattering. It becomes possible to use a broader choice of materials and make them more powerful. 2. Discovery of chaotic regimes of operation of an ultrashort pulse laser, which have been proven to be a direct analogue to processes found everywhere, from quantum objects in science to turbulent movement in technology, and even wave formation on the sea surface. This allows using fast, compact and easily controlled devices like lasers to study and model other processes in science, which are either not directly measurable or occur very rarely, and will be a basis of another FWF project.