비선형 편광 회전을 이용한 모드 잠금된 광섬유 레이저 제작 및 극초단 펄스의 결맞은 빔 결합에 관한 연구

Abstract

A optical fiber laser with ultrashort pulse width has been used in a industrial field and a fundamental research field. To generate the ultrashort pulse, a mode-locking technique is utilized. Especially, the fiber laser operated by passive mode-locking technique is widely used because it has advantages of low cost fabrication, simple configuration, and short pulse width. Recently, an ANDi(All Normal Dispersion) fiber laser has been developed, which is passive mode-locked by a nonlinear polarization rotation phenomena of a fiber. The developed ANDi fiber lasers have shown the performance of a high output pulse energy and a chirped long output pulse width. The property of a chirped long pulse width is useful when its output pulse is used as a seed signal of a high energy ultrashort pulse amplifier. A well-known amplification method of an ultrashort pulse beam is chirped pulse amplification (CPA) method. Although that method is a promising method to obtain an ultrashort high peak power from a solid state laser system such as a Ti: sapphire laser system, CPA method still has a difficulty in a fiber laser design field due to the power limitation by a small fiber core. Hence many high peak power fiber laser designers have suggested a coherent beam combination (CBC) as a method to overcome the power limitation by a small fiber core. In this thesis, to develop a fiber laser system with ultrashort pulse width and high energy, we fabricate a ANDi fiber laser and propose the new type of a coherent beam combiner. The fabricated ANDi fiber laser has the performance of 5-nJ output energy, 4-ps pulse width, and 27-MHz repetition rate. We also propose new type of a passive CBC system for an ultrashort pulse. Our CBC system is composed of a polarization Michelson interferometer geometry and a double pass fiber amplifier. The proposed passive CBC system is designed to has the attribute that the temporally divided pulses in a forward propagation direction are coherently re-combined during the backward propagation. To investigate the degree of coherent beam combination of the proposed beam combiner, we utilize the fabricated ANDi fiber laser as a seed source of the proposed beam combiner. We measure a spectrum and a intensity autocorrelation signal of a single combined pulse beam. The measured results are compared to a spectrum and a intensity autocorrelation signal of a single pulse beam without a pulse division inside the beam combiner. Comparing with those results shows that there are hardly any differences between a combined pulse and a perfectly combined pulse. So we conclude that the proposed beam combiner have the characteristic of a high efficient beam combination.