반사율 가변형 출력경을 갖는 Yb 첨가된 광섬유 레이저의 모드 잠금 출력 특성 연구

Abstract

We design an mode-locked Yb-doped fiber laser coupled by a output coupler with a variable reflectance and investigate its output characteristics. To have a output coupler with a variable reflectance, we utilize a output coupler composed of a polarizing beam splitter (PBS), a 1/4 wave plate(QWP) and a high reflectance (HR) mirror. The reflectance of the output coupler varies with the QWP rotation angle. Hence we measure the output power as a function of the QWP rotation angle. There are the QWP optimum angles to get the maximum output power, which depend on a pumping power in a CW laser mode. From the experiment results, we find that the higher the pumping power, the smaller the QWP rotation angle. The designed laser has a beam quality (M2) of 1∼1.1. To get a mode-locked pulse from our designed laser, we replace a HR mirror of a component of a output coupler with a semiconductor saturable mirror (SESAM). And we investigate the mode-locked laser output characteristics for two kinds of SESAMs with fast and slow relaxation time(500 fs, 9ps). The experiment results show that the larger the QWP rotation angle, the lower the mode-locked output power in our designed laser. And And the results show that only a mode-locking output with multi-pulses lasing can be obtained from the laser with a fast SESAM. On the other hand, the laser with a slow SESAM can operate in a CW mode-locking region without multi-pulses lasing. And its spectral band width (2 nm) is 10 time than one (0.2 nm) for a fast SESAM. Hence we conclude that the output of a laser with a slow SESAM can be more stable than one with a fast SESAM in our designed fiber laser.