Ultrashort Fiber Laser Platform for Seeding of 1-um Laser Amplifier Systems

Abstract

Diode-pumped Ytterbium-doped (Yb-doped) amplifier systems at 1-µm wavelength supporting high-energy pulses at high average power are becoming the workhorses for ultrafast science, including soft X-ray generation [1], optical parametric chirped-pulse amplifier pumping [2], terahertz radiation generation and amplification as well as high-brightness photo-injectors for laser-wakefield acceleration or free-electron lasers [3]. Room temperature (RT) Yb: ytterbium aluminum garnet (Yb: YAG) laser systems are commonplace for producing high average power and high energy sub-ps optical pulses due to their quantum-limited emission efficiency, superior thermo-mechanical properties, and broadband emission spectrum [4], [5]. The long fluorescence lifetime and large emission cross-section of Yb: YAG enable a significant over-all gain [6]. At cryogenic temperature, Yb: yttrium lithium fluoride (Yb: YLF) shows its advantages regarding the average power scalability due to the transition from a quasi-three-level to a four-level laser system, and also significant improvements of thermos-optical material properties. It supports sub-ps pulse amplification due to its several-nanometers flat gain bandwidth [7], [8]. To operate an Yb-doped solid-state laser amplifier system reliably and efficiently, a suitable seed laser operating at 1-µm central wavelength is essential. A Ti: sapphire laser can be used as the seeder for its broadband emission bandwidth covering the 1-µm spectral slot [9]. However, spectral filtering the 1-µm signal from its broadband output leads to pulses with low energy and low efficiency. Building an ultrafast seeding source based on Yb-doped crystals would be expensive and hard to maintain [10]. Therefore, a fiber-based seed source at 1-µm is desired for its low-cost and reliability, incorporating the advantage of a small footprint and alignment-free, turn-key operation [11]. In this thesis we investigate and expand the parameter space of the ultrafast Yb-based fiber laser system for the purpose of seeding Yb-doped solid-state laser. The output parameters of the fiber laser system in terms of pulse energy, spectral center wavelength, laser stability and noise performance are customized to match the solid-state laser material. Four essential elements are investigated through this thesis, including an all-polarization maintaining fiber oscillator, nonlinear wavelength conversion methods and its noise characteristics, the linear/ nonlinear amplification of short wavelength ultrafast pulses, and the design of low noise front-end laser system. We achieved an ultrashort fiber laser platform with the merit of alignment-free, turn-key operation, and high stability. As a seed source at 1 µm, it offers a tunable output signal for different types of Yb-based solid-state amplifier systems.