• High Power Laser Science and Engineering
  • Vol. 9, Issue 2, 02000e32 (2021)
Caijian Xie1、*, Tigang Ning1, Jingjing Zheng1, Li Pei1, Jianshuai Wang1, Jing Li1, Haidong You2, Chuangye Wang1, and Xuekai Gao1
Author Affiliations
  • 1Key Laboratory of All Optical Network & Advanced Telecommunication Network of EMC, Institute of Lightwave Technology, Beijing Jiaotong University, Beijing100044, China
  • 2Science and Information College, Qingdao Agricultural University, Qingdao266109, China
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    Abstract

    A kind of tapered segmented cladding fiber (T-SCF) with large mode area (LMA) is proposed, and the mode and amplification characteristics of T-SCFs with concave, linear, and convex tapered structures are investigated based on finite-element method (FEM) and few-mode steady-state rate equation. Simulation results indicate that the concave tapered structure can introduce high loss for high-order modes (HOMs) that is advantageous to achieve single-mode operation, whereas the convex tapered structure provides large effective mode area that can help to mitigate nonlinear effects. Meanwhile, the small-to-large amplification scheme shows further advantages on stripping off HOMs, and the large-to-small amplification scheme decreases the heat load density induced by the high-power pump. Moreover, single-mode propagation performance, effective mode area, and heat load density of the T-SCF are superior to those of tapered step index fiber (T-SIF). These theoretical model and numerical results can provide instructive suggestions for designing high-power fiber lasers and amplifiers.

    1 Introduction

    Over recent decades, fiber lasers and amplifiers have attracted great interest owing to their advantages of high slope efficiency, high beam quality, superior thermal management property, and compactness[1]. Fiber lasers and amplifiers are widely used in the field of laser marking, material processing[2], and many other industrial applications. Although the power of single-mode fiber laser system has already reached a high level of 10 kW[3], scientific and industrial applications seek much higher laser power and light beam quality[4]. However, nonlinear effects induced by high laser power limit further scaling of the output power. For example, single-frequency or narrow-linewidth fiber lasers' power scaling is limited by stimulated Brillouin scattering (SBS), and power scaling for ultrashort pulsed fiber laser systems is limited by stimulated Raman scattering (SRS). Therefore, suppressing nonlinear effects is particularly critical for further scaling of the output power from high-power fiber lasers and amplifiers[5,6]. The most direct and effective approach to increase the threshold of nonlinear effects is to increase the effective mode area (Aeff), and the simplest and most obvious way of increasing Aeff is to increase the core diameter (D) and decrease numerical aperture (NA). Fibers that achieve large mode area (LMA) by increasing the core diameter and decreasing NA are widely applied in high-power fiber laser system[7]. Such LMA fibers can effectively mitigate nonlinear effects[8], but there are two emerging problems in practical applications. One is that they support high-order modes (HOMs) owing to the larger core diameter and lower NA, which results in transverse mode instability (TMI) at high average power levels[9]; the other is that such fibers become highly sensitive to bending, which results in high bending loss and severe mode area compression. Therefore, it is important for LMA fibers to suppress nonlinear effects and ensure transverse mode stability simultaneously.

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    Caijian Xie, Tigang Ning, Jingjing Zheng, Li Pei, Jianshuai Wang, Jing Li, Haidong You, Chuangye Wang, Xuekai Gao. Amplification characteristics in active tapered segmented cladding fiber with large mode area[J]. High Power Laser Science and Engineering, 2021, 9(2): 02000e32
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    Category: Research Articles
    Received: Jan. 5, 2021
    Accepted: Mar. 30, 2021
    Posted: Mar. 30, 2021
    Published Online: Jun. 22, 2021
    The Author Email: Caijian Xie (jjzheng@bjtu.edu.cn)