단일 광섬유 레이저에서의 모드잠금 펄스와 잡음 펄스 생성 및 특성 분석

Abstract

ABSTRACT

Generation and analysis of mode-locked and noise-like pulses from the single fiber laser oscillator

Choi mun sung Advisor : Prof. Ki-Nam Joo Co-Advisor : Ph.D. Seungman Kim Department of Photonic Engineering Graduate School of Chosun University Korea Institute of machinery & materials

Ultrafast pulse fiber lasers are promising optical sources for high precise industrial applications such as absolute distance metrology, surface metrology and non-thermal micromachining. In general, ultrafast optical pulses can be generated in a fiber oscillator based on well-known the mode-locking technique which defines the phase relationship between the lasing modes. On the other hand, a new type of pulses has been recently reported in the same oscillator and its temporal and spectral characteristics are entirely different from those of mode-locked pulses. It was reported that this new type of pulses were able to be generated by high optical nonlinear effects and dispersion properties of the optical fiber in the oscillator and it was named as ‘noise-like pulses.’ Manifold researches have been focused on the theory for the mechanism of generating noise-like pulses and analytical models to infer the main characteristics, but the origin or the principle of the pulse generation have not yet been proved. Moreover, the experimental verifications of its temporal and spectral characteristics are not enough. Even though the noise-like pulse is not sufficiently revealed by the fundamentals, it is still very attractive to many industrial applications and its various characteristics should be clearly provided. In addition, mode-locked pulses and noise-like pulses can be generated in the same fiber oscillator by adjusting polarization states of the lasing light, and the automatic selection between two pulses is advantageous to the comparative analysis and, by extension, the various industrial application. My thesis aims at the experimental characterization of mode-locked pulses and noise-like pulses comparatively with a home-built fiber oscillator which can generate two types of pulses selectively and automatically. For the research goal, a ytterbium (Yb)-doped fiber oscillator to selectively generate mode-locked pulses and noise-like pulses was proposed and constructed as an automatic system in order to investigate the various characteristics of each pulses in the time and spectral domain. The oscillator was designed as a turn-key system for convenience and the algorithm to distinguish between two types of pulse was adopted based on the spectral bandwidth difference between the pulses. By using proper spectral band stop filters with fiber Bragg gratings (FBGs), each pulse operation was able to be identified and the result was transferred to the polarization control unit as a feedback. In order to adjust the polarization states of the light, several wave-plates linked to motors in the oscillator were rotated to generate different nonlinearities, which leads to different pulse operations. The comparison between two types of pulses were investigated with the home-built laser system with several equipment such as an optical spectrum analyzer, radio-frequency (RF) spectrum analyzer and autocorrelator for the characterization of the pulses in the time and spectral domain as well as the pulse shape and width. Furthermore, temporal coherence of pulses were verified with Michelson type interferometers with balanced and unbalanced arms. In the result, the mode-locked pulse has relatively narrow bandwidth corresponding to the bandwidth of the interference filter used as a pulse shaper in the oscillator and high temporal coherence between neighborhoods. On the other hand, the noise-like pulse was generated with relatively broad spectrum beyond the spectral bandwidth of the interference filter because of the optical nonlinearity caused by the small core size of the single mode fiber and rarely temporal coherence between neighborhoods. Depending on these characteristics, suitable laser pulses can be selected and generated in the proposed laser system as the industrial demands. By analyzing the optical characteristics of two types of pulses generated in the laser system, it is possible to apply each pulse to the proper application fields such as metrology, imaging and manufacturing..

Key words : selective pulse generation system, mode-locked pulse, noise-like pulse,