Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec Full length article Paving way for fabrication of silica-based single -frequency seed laser: Ultrahighly Yb-doped optical fibers via sol-gel method combined with silica tube inner wall coating and fusion-tapering technique Lisi Xia a,b , Meng Wang a , Pei-Wen Kuan a , Qiubai Yang a,b , Chongyun Shao a , Qinling Zhou a , Chunlei Yu a,c, ⁎ , Lili Hu a,c , Yang Zhang d , Qiuhong Yang b a Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China c Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China d Shanghai Institute of Satellite Engineering, Shanghai 201109, China HIGHLIGHTS • Sol-gel method combined with silica tube inner wall coating and fusion-tapering. • Overcomes rare-earth dissolution rate limit of silica fibers and splicing problem. • A ~ 1.03 μm laser with 70 dB-SNR is achieved. • Fiber core can extend to any heavily rare-earth-doped system. ARTICLE INFO Keywords: ultrahighly Yb-doped fibers Single-frequency seed laser Silica-based Splicing ABSTRACT A method based on sol-gel preparation combined with silica tube inner wall coating and fusion-tapering tech- nique for fabrication of ultrahighly rare-earth-doped fibers without fluorescent quenching is provided. Ultrahighly Yb-doped high SiO 2 content glass fiber is developed and is easily spliced to commercial fiber- components. A ~ 1.03 μm laser with 70 dB-SNR and 0.2 nm-linewidth is achieved. The problem of low dis- solution rate to rare-earth ions in silica fibers and the splicing problem between soft glass fibers and commercial fiber components have been overcome, indicating the method has potential in fabrication of short-cavity high- gain silica fiber which is applicable to single-frequency seed source. The method is worth exploring in that the core can extend to any heavily rare-earth-doped system as long as it can be realized in the sol form with good dispersion, therefore paving way for fabrication of silica-based single-frequency seed laser. 1. Introduction Single-frequency lasers have been undergoing prominent advances and finding increasing applications since its creation at 1990 s [1]. Benefiting from the merit of high purity in optical spectrum, single- frequency fiber lasers have been harnessed in areas such as coherent optical communication, optical metrology, high precision optical sen- sing, interferometry and spectroscopy [2]. Single-frequency lasers usually require high power generation in short length [3–11], and in order to obtain higher output power, a low-power single-frequency laser can be utilized as a seed source for power amplification through a master oscillation power amplifier (MOPA) configuration. Since the output laser performances such as linewidth, noise and power stability are greatly restricted by the seed laser itself [2]. To realize linearly polarized operation of single-frequency seed laser, high optical gain and short cavity, the length of which is usually centimeter level, are essentially required. While the doping level of rare-earth ions in commercial active fibers is commonly less than 1 wt% owing to the dissolution rate limit of silica glass to rare-earth ions and unwanted cluster formation phenomena [2,12]. In the field of single- frequency laser, rare-earth doped multicomponent (non-silica) glass matrix fibers especially Er 3+ /Yb 3+ -doped phosphate glass fibers ex- hibit excellent laser performances, the basic reason of which is owning to the high dissolution rate of phosphate glass to rare-earth ions, even https://doi.org/10.1016/j.optlastec.2020.106425 Received 23 March 2020; Received in revised form 1 June 2020; Accepted 11 June 2020 ⁎ Corresponding author. E-mail addresses: sdycllcy@163.com (C. Yu), zhangyang-457845@163.com (Y. Zhang). Optics and Laser Technology 131 (2020) 106425 0030-3992/ © 2020 Elsevier Ltd. All rights reserved. T