Longitudinal-mode controlled lasers and application to imaging system

Abstract

In this thesis, control techniques of operation wavelength and longitudinal-mode of fiber laser were proposed. Various wavelength filtering techniques and single-longitudinal-mode operation techniques were investigated. A swept-source optical coherence tomography (SS-OCT) based on the longitudinal-mode controlled laser was demonstrated. In addition, to overcome previous limitation, time-multiplexed multifocal multiphoton microscopy technique based on a fiber bundle was proposed. Wavelength control technique for the laser is investigated. For control of the operation wavelength of the laser, techniques with wavelength switching, interleaving, wavelength spacing tuning, and sweeping were demonstrated. Transmission of the filtering wavelength can be switched based on a hybrid Sagnac interferometer incorporating a sampled fiber Bragg grating. In addition, a resonance peak wavelength of Sagnac interferometer based on polarization maintaining fiber can be controlled. The transmission wavelength can be interleaved by cascading the Sagnac interferometer with the sampled fiber Bragg grating. The longitudinal-mode of the fiber laser cannot be controlled sufficiently because the longitudinal-modes of the laser cavity is very dense. The transmission wavelength of the wavelength filter generally contains multi-longitudinal-modes. To suppress the multi-longitudinal mode, the performance of a sub-ring cavity was enhanced based on microfiber technique. For extremely wide free spectral range (FSR), a microfiber knot resonator (MKR) was proposed. The FSR was measured to be 365 GHz. The wavelength filtering techniques were applied to fiber lasers. A single-wavelength fiber laser was fabricated and applied for simultaneous measurement of radiation dose and strain. The multiwavelength laser was fabricated with wavelength switching, interleaving, and wavelength spacing tuning. A nonlinear polarization rotation technique was exploited for stable operation and wavelength switching. The multiwavelength laser based on polarization-differential delay line was utilized for the continuous tunable microwave filter. The swept laser based on a Fourier-domain mode-locking (FDML) was investigated. A novel single-longitudinal-mode FDML laser based on the MKR was proposed and experimentally demonstrated for the SS-OCT system. Axial resolution was 10 μm due to the wavelength swept range of 116.2 nm. The sensitivity roll-off of point-spread function was well matched with a theoretical value. In addition, a time-multiplexing multifocal multiphoton microscope based on a fiber bundle was proposed. Previous limitation was overcome by maximizing temporal separation between the multiple foci. The axial resolution was the same with a confocal microscope based on a single fiber. In addition, an illumination system for ophthalmic microscope was fabricated based on the multifocal configuration. The performance of the system was compared to the conventional system by animal test and clinical trial.