반도체 포화 흡수체 거울을 이용한 극초단 광섬유 레이저의 출력특성 연구

Abstract

Recently, fiber laser systems with ultrashort pulse-widths have been used for fundamental research as well as for medical and industrial applications. One of the methods to generate an ultrashort pulse is the use of a mode-locking technique. The use of the passive mode-locking technique with fiber laser system is popular because of its simple configuration, low fabrication cost, and narrow pulse-width. There are two popular passive mode-locking methods: the first involves the use of nonlinear polarization rotation and a saturable absorber. Although the nonlinear polarization rotation(NLPR) method has several advantages such as long life and narrow short pulse generation, it is sensitive to the surrounding temperature and fiber conditions. In contrast with the NLPR methode, the second method, i.e., the saturable absorber method, is less sensitive to the surrounding environmental conditions, which is important in industrial applications. Semiconductor saturable absorber mirrors(SESAM) comprise one of among the several types of saturable absorbers used for passive mode-locking. SESAMs have a nonlinear reflectance that depends on the intensity incident on their surface. The continuous wave(CW) mode-locking condition of the fiber laser used with a SESAM strongly depends on the laser intensity inside the laser cavity and the SESAM's main parameters, which are the saturation fluence and reflectance modulation depth. In particular, the laser fluence on a SESAM surface intensively affects the laser output characteristics. Hence, to obtain stable mode-locked pulses, we must analyze the mode-locking characteristics by varying the laser fluence incident on the SESAM surface. In this thesis, we design a CW mode-locked Yb-doped PM fiber laser based on a SESAM with a 30% modulation depth and a 9-ps relaxation time. We analyze the output characteristics of the CW mode-locked laser as a function of the laser fluence incident on the SESAM surface by using four aspheric lenses. These four lenses have focal lengths of 6.2 mm, 8 mm, 11 mm, and 15 mm. We found that the dynamic range of pump power required for stable mode-locked operation increased as increasing the focal length of the aspheric lens, while the threshod for CW mode-locking increased. At the optimized condition, the designed laser has a maximum output power of 70.5 mW at a wavelength of 1035 nm, repetition rate of 9.47 MHz, and slope efficiency of 20%. The measured beam quality(M2) is 1.01. We also measured the pulse width by using a FROG and an autocorrelator. The measured pulse shape comprised a short pulse and long background pulse. The short pulse with was about 500 fs, whereas the width of the background pulse was greater than 50 ps.