NONLINEAR ULTRASHORT PULSE GENERATION IN OPTICAL FIBER AND ITS APPLICATION TO BIOLOGICAL IMAGING

121 pagesThe development of ultrafast lasers has had a significant impact on science and technology, including in applications as diverse as ultra-high intensity high-field science, laser micromachining, and laser eye surgery. Notably, Gérard Mourou and Donna Strickland shared the 2018 Nobel prize in physics for the development of chirped-pulse amplification, a technique that increased the pulse energy achievable in ultrashort pulses by many orders of magnitude. This achievement has allowed scientists and engineers to harness light's interaction with matter at previously unattainable intensities. While there are many ways to construct a laser that delivers high-energy ultrashort pulses, fiber lasers are frequently a particularly convenient solution. Fiber lasers can be more compact and robust than those constructed from solid-state media and typically deliver a near diffraction-limited spatial mode, which is critical for applications requiring tight focusing. However, the long propagation distance and tight confinement in the fiber waveguide enhances both linear and nonlinear material effects, greatly complicating the designs of ultrafast fiber lasers in particular. Despite these shortcomings, optical fiber is a versatile platform for ultrashort pulse generation, and careful understanding of wave propagation can lead to qualitatively new behavior. This thesis primarily investigates methods of generating short pulses outside the gain bandwidth of typical fiber lasers and the application of lasers with highly nonlinear designs to \textit{in vivo} multiphoton fluorescence microscopy. First, a two-color femtosecond laser that uses the molecular vibrations of silica glass to produce a pulse outside the ytterbium gain bandwidth is demonstrated. The synchronous nature of these pulses allows for a demonstration of simultaneous degenerate and nondegenerate two-photon excitation fluorescence microscopy, and the large frequency separation of the pulses results in fluorophore excitation over a large bandwidth. Next, optical parametric chirped pulse amplification is demonstrated in standard step-index birefringent optical fiber. The versatility of birefringent fiber allows for amplification even further from typical gain bands, and using a highly broadband pump pulse allows for the generation of the shortest pulses demonstrated through fiber optical parametric chirped pulse amplification to date. Lastly, an imaging comparison between a highly nonlinear fiber laser and a commercial fiber laser is presented, demonstrating the promise of such systems for application in multiphoton microscopy in particular. Future speculative directions for enhancing the performance, reliability, and versatility of fiber systems generating pulses outside the gain bandwidth of typical rare-earth dopants are also discussed