Short Pulse Lasers Using Advanced Fiber Technology and Saturable Absorbers

Abstract

Ultrafast fiber lasers with superb pulse and beam quality are becoming important in various fields of applications including high-precision material processing, bio-medicine, fiber optical communications, and ranging. This thesis is concerned with the research and development of ultrafast fiber laser technology. Passively mode-locked and Q-switched fiber lasers operating at 1-2.2 μm IR-spectral range have been developed and studied.

Passively Q-switched Tm-Ho-doped fiber lasers operating at 2 μm were thoroughly investigated. A strong impact of a new effect - dynamic gain induced pulse shortening, on the duration, repetition rate and energy of a Q-switched fiber laser was studied. Using this effect in combination with a short-length fiber cavity and a highly nonlinear antimonidebased semiconductor saturable absorber mirror resulted in generation of record-short 20 ns energetic Q-switched pulses at 2 μm.

Well-controlled chirped fiber Bragg grating (CFBG) technology was developed and used for inscription of high-performance all-fiber dispersion compensators. All-fiber ytterbium (Yb) and bismuth (Bi) lasers generating picosecond soliton pulses at 1 μm and at 1.2 μm were demonstrated using CFBG-dispersion compensators. The powerful Ybdoped laser followed by an all-fiber amplifier was used to generate octave-spanning supercontinuum radiation in a nonlinear photonic crystal fiber.

Various wavelength tuning methods of ultrafast soliton fiber lasers, such as selffrequency shifting of Raman solitons and electronic tuning using an acousto-optic tunable filter, were also investigated. In addition, an environmentally stable 200 fs mode-locked Yb-doped fiber laser with dispersion compensation by an index guiding photonic crystal fiber was demonstrated.

Novel Bi-doped mode-locked fiber sources operating at a wavelength of ~1.2 μm using dilute nitride saturable absorbers were developed and studied. The first Bi-doped fiber laser delivering sub-picosecond soliton pulses was demonstrated. Harmonic modelocking in a Bi-doped mode-locked fiber laser with dispersion compensation by a CFBG was used to increase significantly the initially low repetition rate of the Bi-fiber laser. Pulse dynamics of mode-locked Bi-lasers were also investigated in detail.

Ultrashort wavelength-tunable pulses beyond 2 μm with durations below 200 fs were generated through a soliton self-frequency shift in a Tm-Ho fiber amplifier seeded by a Tm-Ho master oscillator mode-locked by a GaSb-based saturable absorber. Finally, a novel ultra-broadband saturable absorber based on carbon nanotubes layered on top of an Ag-mirror was demonstrated. The same sample of polymer-free carbon nanotube absorber was used to reliably mode-lock ytterbium-, erbium-, and thulium-holmiumdoped soliton fiber lasers producing sub-picosecond pulses at 1 μm, 1.56 μm, and 2 μm, respectively.