Development of visible ultrafast mode-locked fiber lasers

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Abstract Title: Development of visible wavelength fiber lasers Non-Technical Description: Laser light pulses with femtosecond durations have a variety of industrial, medical and scientific applications. Notable applications are machining micro-scale devices, imaging biological samples, medical diagnostics and therapy, and materials spectroscopy. Lasers generating ultrashort optical pulses are referred as mode-locked lasers. Today, mode-locked lasers are mainly based on solid state amplifiers and optical fiber amplifiers. In comparison with solid-state lasers, mode-locked fiber lasers have the advantages of stability, compactness, efficiency and low cost. This project is to develop novel approaches to making mode-locked fiber lasers generating ultrashort femtosecond scale optical pulses at visible wavelengths. Even though mode-locked femtosecond fiber lasers have many practical advantages, a novel pulse forming principle is necessary in developing one in the visible wavelength regime. Most mode-locked lasers shape pulses require that the refractive index dispersion, called the group velocity dispersion, and fiber nonlinearity work together. However, an optical fiber in the visible regime has a very high group velocity dispersion relative to the fiber nonlinearity that hinders the formation of pulses. In this project, mode-locked femtosecond fiber lasers will be developed by manipulating unique pulse propagation phenomena at normal group velocity dispersion to form ultrashort pulses. A mode-locked fiber laser operating at visible colors will be a valuable tool for medical applications such as general microsurgeries, eye macular degeneration, etc. Graduate students will be trained for careers in laser science and technology and actively be involved in the research project. An effort will be made to recruit diverse groups to the project and to provide them with significant scientific research experiences. Undergraduate students will also participate and be exposed to the research process. With this experience they may consider pursuing future careers in science and technology. Technical Description: Mode-locked fiber lasers operating at visible wavelengths will be designed using Praseodymium (Pr)-doped fluoride fibers by manipulating the self-similar evolution and the dissipative soliton propagation in all-normal group velocity dispersion lasers. While the main theme of the project is to generate visible wavelength mode-locked pulses, femtosecond pulse generation beyond the gain bandwidth (BW) limitation will be studied. A Pr-doped fiber has a very narrow gain BW around the wavelength at 635 nm, which is insufficient to support femtosecond pulses. However, by adopting self-similar evolution, pulses with spectra much broader than the gain BW can be created and much shorter pulses can be formed than the gain BW limitation. Visible wavelength mode-locked fiber lasers are relevant for valuable scientific applications such as fluorescence lifetime imaging microscopy, time-resolved photoluminescence spectroscopy, and visible frequency combs. The successful outcome of the project will have a significant impact. The self-similar pulse formation technique can be exploited to generate ultrashort pulses at wavelengths where only continuous-wave lasers are available due to the narrow gain BW limitations. This project has the potential to extend the wavelength range of mode-locked lasers and to open the way for future applications.