Wavelength-tunable fiber lasers and related components

Abstract

This thesis is concerned with wavelength-tunable erbium-doped fiber lasers and fiber-optic components applicable to the fiber lasers. Firstly, a novel wavelength-tunable single-frequency erbium-doped fiber ring laser incorporating an all-fiber acousto-optic tunable bandpass filter and a saturable absorption grating is successfully demonstrated and characterized. Mode-hopping-free stable single-longitudinal-mode operation was achieved over the wavelength tuning range of 48 nm with the sidemode suppression ratio higher than 50 dB and the output power up to 2 mW. The laser wavelength tuning characteristics, the electrical spectral characteristics of the laser output, and the laser dynamics in wavelength switching and sweeping are investigated, based on the theoretical analysis of the saturable absorption grating and the characteristics of the used acousto-optic tunable bandpass filter. Secondly, methods to suppress the polarization dependence in the interrogation of birefringent fiber Bragg gratings are proposed and successfully demonstrated. A wavelength-swept erbium-doped fiber laser with polarized output is used as the light source. Two polarization-averaging methods, a depolarization scheme and a polarization scrambling scheme, were investigated and compared. The proposed techniques successfully stabilize the reflection spectrum of a birefringent grating regardless of the polarization state of the source laser and birefringence of the lead fiber. The experimental results eliminate one of the major practical difficulties in the current fiber Bragg grating interrogation based on the wavelength-swept fiber laser. Finally, a novel acousto-optic tunable bandpass filter based on intermodal coupling in a two-mode fiber is proposed and successfully demonstrated. The double-pass scheme enables zero frequency shift, and accompanies reduction of bandwidth and enhancement of extinction ratio. A 3-dB bandwidth of 2.0 nm, insertion loss of 5.6 dB, and sidelobe suppression r...