이터븀 광섬유 모드잠금용 반도체 포화흡수체 거울 설계 및 제작

Abstract

A semiconductor saturable absorption mirror (SESAM) has been developed as a passive mode-locking device used in a laser. SESAMs are ideally suited as mode-locking devices because they can cover a broad wavelength range and yield short recovery times, while supporting the generation of femtosecond to picosecond pulse durations. Therefore, SESAMs have been utilized in numerous mode-locked lasers, including solid state lasers, fiber lasers, and semiconductor lasers. SESAMs can be classified according to the number of quantum wells. In general, a single quantum well (SQW) with a low absorption modulation depth has been used to achieve a mode-locking pulse from a solid state laser. However, SESAMs with multiple quantum wells (MQW) that induce a modulation absorption depth of several tens of percentage have been utilized in a fiber laser to achieve the mode locking pulse. To get a stable mode-locked pulse train with a SESAM, the SESAM has to be well designed to achieve enough modulation depth. The modulation of a SESAM depends on the thickness of the quantum well (QW), the number of quantum wells, and the electric field distribution inside the QWs structures. Hence, to design and fabricate a SESAM efficiently, it is necessary to investigate the effects of the physical sizes of a SESAM and the electric field distribution inside it. In this paper, we present the design and fabrication of a SESAM used in a passively mode-locked Yb-doped fiber laser. To design the SESAM, we investigate the dependence of the transition wavelength of an InxGa1-xAs QW on its thickness and the barrier between the QWs, as well as the electric field distribution inside the designed SESAM structure. We fabricated two types of SESAMs (A and B). We measured the optical parameters of the fabricated SESAMs viz recovery time, modulation depth of reflection, damage threshold and saturation fluence. The optical parameters of SESAM (A) are 90% reflectivity at 1035 nm wavelength, 80.8 mJ/cm2 damage threshold, 114 ps recovery time, 9.2% modulation depth, and 1791 μJ/cm2 saturation fluence. The optical parameters of SESAM (B) are 77% reflectivity at 1035 nm wavelength, 202 mJ/cm2 damage threshold, 470 ps recovery time, 23.5% modulation depth, and 684 μJ/cm2 saturation fluence. Using the fabricated SESAMs, we have successfully obtained a mode-locked pulse train from a Yb-doped fiber laser.