LI Tian-qi, MAO Xiao-jie, LEI Jian, BI Guo-jiang, JIANG Dong-sheng

(Science and Technology on Solid-State Laser Laboratory,North ChinaResearch Institute of Electro-Optics,Beijing 100015,China)

Abstract: The high or low coupling efficiency and the good or bad coupling facula directly affect the amplification effect of the Rod-type photonic crystal fiber. Therefore, it is necessary to research the coupling effect of seed light and choose a suitable laser as a seed source. In this paper, the coupling efficiency of Rod-type photonic crystal fibers in a solid-state laser and a fiber laser was theoretically analyzed. The changing regulation of the coupling efficiency and the effect of the alignment error on the coupling efficiency were calculated with two different lasers. A suitable lens or group of lenses were selected and an experiment was conducted to couple the solid-state laser beam and the fiber laser beam to the Rod-type photonic crystal fiber. Compared with the coupling effect of the two kinds of lasers, the maximum coupling efficiency of the solid laser is only 62.4%, while the coupling efficiency of the fiber laser is more than 80%. In the case of fiber laser coupling, the coupling efficiency at different power injection and the coupling facula were analyzed. The experimental results will guide the amplification experiment of the Rod-type photonic crystal fiber.

Key words: rod-type photonic crystal fiber;solid-state laser;fiber laser;coupling efficiency

1 Introduction

引 言

Rod-type photonic crystal fibers(ROD-PCF), hereinafter referred to as ROD Fiber for their large area, single mode and high gain, are very suitable for ultrashort pulse amplification[1-2]. At present, countries abroad have successfully developed crystal core picosecond pulse laser systems that can have a maximum average output power of up to 2 kW[3]. However, domestic research on optical fibers is rare, where most current research in this field occurs at the Shanghai Institute of Optics and Mechanics, CAS[1,4]. Optical fibers are also a form of photonic crystal fiber, which have a wide range of industrial applications. They might be used in medical fields, scientific research,etc.[5-8].

光子晶体光纤棒(Rod-type photonic crystal fiber，ROD-PCF)下文简称光纤棒(ROD Fiber)因其具有模场面积大、无截止单模、高增益的特点，十分适合于超短脉冲放大[1-2]。目前国外研制成功的光纤棒皮秒脉冲激光系统最大平均输出功率可以高达2 kW[3]。但是国内对光纤棒的研究报道较少见，目前在该领域研究较多的是中科院上海光机所[1,4]。光纤棒也是一种光子晶体光纤，其在工业、医疗、科研等方面有着广泛的应用[5-8]。

A key issue in the experimentation of ROD Fiber ultrashort pulse amplification systems is the method used to efficiently couple seed light into the core of a ROD Fiber. Coupling effect of seed light with ROD Fiber is important as effective coupling can cause seed light to be amplified more powerfully and the ROD Fiber can consequently receive light of higher quality.

在进行光纤棒超短脉冲放大系统的实验中一个十分关键问题就是如何高效地将种子光耦合进入光纤棒的纤芯。种子光与光纤棒之间的耦合效果十分重要，良好的耦合可以使种子光功率在光纤棒中得到充分的放大，并且可以得到高光束质量的信号光。

Since the used fiber is a photonic crystal fiber and has a large mode field area, it has a large diameter and a regular air hole microstructure[9-10]. If the fiber is welded, its air holes collapse. For this reason, the ROD Fiber is usually fitted with end caps at either end and free-space lens is used to couple the seed light with the pump light[11]. Lens coupling is very demanding on the position and focal length of lenses. It requires complex calculations, simulation and practice to obtain a suitable lens focal length[12]. Lens coupling systems have difficulty performing adjustments. They require adjustments in multiple dimensions for a single lens and require adjustment of multiple lenses for best results[13].

由于光纤棒是大模场面积的光子晶体光纤，因此其直径较大，而且具有规则的空气孔微结构[9-10]，如果对其进行光纤熔接会造成空气孔的塌陷，因此光纤棒通常都是在端面接有端帽，并使用自由空间透镜对种子光和泵浦光进行耦合[11]。透镜耦合对透镜的位置、焦距等要求非常苛刻，需要经过较为复杂的计算、仿真与实践才能获得合适的透镜焦距[12]。透镜耦合系统在调整的时候具有不小的难度，需要对单个透镜的多个维度进行调整，以及对多个透镜进行配合调整才能达到最好的效果[13]。

The beam generated by the seed source can be approximated as a Gaussian beam. The basic principle of Gaussian beams in regards to fiber coupling is pattern matching the Gaussian beam mode field with the fiber mode field[14-15]. In the following experiment, photonic crystal fiber is coupled to a solid laser with a larger facula using single lens coupling and the relationship between coupling efficiency, lens position and focal length are calculated. A fiber laser with a smaller facula is then coupled to the photonic crystal fiber with expanding bean coupling and the relationship between coupling efficiency and beam expansion ratio are calculated. A solid-state laser with an output beam waist diameter of 800 μm and a fiber laser with output beam waist diameter of 25 μm(both with a beam quality ofM2≤1.2) were used as seed lasers with a wavelength ofλ=1 030 nm. Experiments were performed by coupling then with a rod-shaped photonic crystal fiber that has a core diameter ofD=85 μm(mode field diameter ofDMF=65 μm).

种子源产生的光束可以近似为高斯光束，高斯光束到光纤耦合的基本原理就是高斯光束模场与光纤模场的模式匹配[14-15]。本文模拟计算了光斑较大的固体激光器在单透镜耦合情况下，耦合效率与透镜位置和焦距的关系，以及光斑较小的光纤激光器在扩束透镜组耦合情况下，耦合效率与扩束倍率的关系。……