All-fiber acousto-optic devices and their applications to fiber lasers

Abstract

This dissertation describes principles, fabrications, evaluations, and applications of all-fiber Acousto-Optic(AO) devices based on optical mode coupling by a flexural acoustic wave. Three different types of AO devices are described, which use a two-mode fiber(TMF), a fused tapered null coupler, and a single-mode fiber, respectively. Main emphasis is on their spectral characteristics and polarization properties and applications to fiber lasers. Spectral bandwidths of a few nm and tuning ranges of a few tens nm are typically achieved. Polarization dependence of the TMF device using a slightly elliptical-core fiber is greatly reduced by canceling out the geometrical and the stress-induced birefringences by thermal treatment of the fiber. A double-pass configuration using the null-coupler AO device is employed to realize polarization-independent filtering without optical frequency shift. Using multiple cladding modes, a multi-frequency operation of the notch filter based on the single-mode fiber is introduced for electronically controllable designer spectral profiles. Acousto-optic interaction in the TMF AO device is analyzed by using a coupled mode analysis and the acoustic waveguide equation. A new coaxial acoustic transducer is developed to improve the electrical-to-acoustic conversion efficiency and device``s ruggedness required to make it practical. Wavelength tunable modeless $Nd^{3+}$- and $Er^{3+}$-doped fiber lasers are de-monstrated by incorporating the AO devices in the cavity. Key factors governing the operation of wavelength-swept fiber lasers using swept AO filters are investigated both theoretically and experimentally, which shows that matching the filter sweep rate to the frequency shift per roundtrip gives a reduction in the instantaneous linewidth. It is also shown that a large amplified spontaneous emission noise suppresses the natural tendency of the frequency-shifted fiber lasers to pulsing associated with the nonlinear Kerr effect.