掺镱大模场光子晶体光纤研究进展（特邀）

掺镱大模场光子晶体光纤在高峰值功率超快激光放大器中有着重要的应用价值，其研究得到了广泛关注。首先简要介绍了国内外掺镱大模场光子晶体光纤的研究进展，阐述了掺镱大模场光子晶体光纤的基本设计思路，对比说明了保偏型掺镱光子晶体光纤的设计制备方法。重点介绍了近十年来中国科学院上海光学精密机械研究所在掺镱大模场光子晶体光纤方面的研究进展。包括掺镱大模场光子晶体光纤的纤芯折射率大小和均匀性控制、光子晶体光纤微结构控制等关键技术。采用自主研制的四种芯径为40~100 μm的掺镱大模场光子晶体光纤开展了皮秒脉冲激光放大实验。利用40 μm芯径的保偏掺镱光子晶体光纤实现了平均功率为100 W、光束质量因子（M²）小于1.4的稳定输出，偏振消光比为12 dB。利用100 μm芯径的保偏掺镱大模场光子晶体光纤实现了M²小于1.5的高光束质量脉冲放大。上述研究为掺镱大模场光子晶体光纤的国产化应用奠定了基础。

Abstract

Significance

Ytterbium-doped large-mode-area photonic crystal fibers (LMA PCFs) have attracted extensive attention owing to their important applications in high-peak-power ultrafast laser amplifiers. Fiber lasers are widely used in advanced manufacturing, medicine, national defense, and scientific research owing to their compact structure, high conversion efficiency, high reliability, and low cost. However, with the development of fiber lasers, particularly the development and application of ultrafast lasers in the field of fine processing in recent years, higher requirements have been placed on the output power and beam quality of fiber lasers. Currently, the output power of internationally commercialized fiber lasers has reached 100 kW. IPG Photonics Corporation uses a double-clad fiber with a core diameter of 30 μm to achieve a 10 kW single-mode single-fiber laser output. However, owing to the limitations of the physical mechanisms, such as nonlinear effects, optical damage, and thermal damage, it is very difficult to further increase the output power of a single laser module. The nonlinear effect of the optical fiber is related to the mode field area of the optical fiber. The larger the mode field area, the weaker the nonlinear effect of the optical fiber, and the higher the threshold of the nonlinear effect. Therefore, large-mode field fibers are one of the most direct and effective ways of overcoming nonlinear effects and fiber laser damage to further increase laser power. However, an increase in the core diameter of large-mode field fibers inevitably causes competition among multiple transverse modes, degrading the beam quality of the laser.

Consequently, various fiber structure designs have been proposed to maintain a satisfactory beam quality with large core diameters, such as rod-type photonic crystal fibers, photonic bandgap fibers, leakage channel fibers, large-pitch fibers, chirally coupled-core fibers, and other microstructure fibers. Among them, the Yb-doped PCF has the most classic architecture, with an ordered array of microscopic air holes. These microscopic air holes favor convenient regulation of the effective refractive index of the cladding. Nevertheless, it is difficult to manipulate the refractive index so that it is close to that of pure silica glass cladding and maintain good uniformity in Yb-doped silica core glass. The commercial method of modified chemical vapor deposition (MCVD) combined with solution doping has some limitations in terms of the core size, refractive index uniformity in the radial and axial directions, and ultralow numerical aperture. Other non-MCVD fabrication technologies have also been developed and reported, including direct nanoparticle deposition (DND), reactive powder sintering of silica (REPUSIL), and sol-gel methods. Heraeus Quarzglas made great progress in the preparation of Yb³⁺/Al³⁺/F^--co-doped silica bulk glasses with the F^--doping-induced refractive index (RI) reduction being evident.

The sol-gel technique is a well-known method for producing centimeter-sized long glassy silica rods. Our group has committed to the preparation of large Yb³⁺-doped silica glass rods with a low refractive index and high optical homogeneity using a modified sol-gel method combined with high-temperature sintering. The sol-gel process ensures dopant mixing in the solution and consequently high doping uniformity, and high-temperature powder sintering allows the preparation of large-sized bulk glass. Fluorine incorporation during the sol-gel process is used to compensate for the increased refractive index caused by ytterbium and aluminum co-doping, and phosphorus is used to suppress the formation of Yb²⁺ and photodarkening. The sol-gel method combined with high-temperature sintering provides a cost-effective method for fabricating the core glass of a Yb-doped LMA PCF.

Progress

In this paper, we briefly introduce the progress of research on ytterbium-doped LMA PCF at home and abroad, as well as the design of ytterbium-doped LMA PCF. The effects of thermal history on the refractive index of the Yb/Al/P/F co-doped silica glass and the beam quality of the PCF are demonstrated. For comparison, the design and preparation methods of the polarization-maintaining ytterbium-doped PCF are presented. This paper focuses on the progress of research on Yb-doped LMA PCF in the past ten years at the Shanghai Institute of Optics and Fine Mechanics (SIOM) (Table 1 and Fig. 20). This includes accurate control of the refractive index value, homogeneity of the fiber core glass, and structure of the PCF. The output laser beam quality is significantly improved owing to the optimization of the Yb-doped core-glass rod.

Conclusions and Prospects

With the rapid development of the domestic ultrafast laser processing industry, the demand for domestically produced Yb-doped LMA PCF by domestic ultrafast laser companies has increased. This paper summarizes the progress of ytterbium-doped large-mode-field photonic crystal fibers in SIOM over the past decade. Ytterbium-doped large-mode field photonic crystal fibers with core diameters of 40 μm, 50 μm, 75 μm, and 100 μm were prepared. Using a 40 μm /200 μm polarization-maintaining ytterbium-doped photonic crystal fiber, we independently designed and prepared an all-fiber amplification module, and achieved picosecond pulse amplification with an average power exceeding one hundred watts and high beam quality. The beam quality factor M² was less than 1.5, the polarization degree was greater than 12 dB, and the power fluctuation was less than 1.3% in 2 h under a 100 W amplification power operation. Using ytterbium-doped LMA PCF with a core diameter of 100 μm as the gain fiber, picosecond pulse amplification with a beam quality factor M²<1.3 and polarization degree greater than 95% was achieved. In the future, the performance of LMA PCFs should be further optimized to meet the requirements for high-average-power ultrafast fiber lasers.

1　引言

近年来，掺镱大模场石英光纤由于在高功率、大能量及高峰值功率脉冲激光及放大器方面的重要应用而得到了广泛的研究^［1-2］。其中，脉冲激光器具有高能量/高峰值功率以及优越的时域特性，在精细加工、生物医疗、科研探索等领域中具有很好的应用前景。无论是连续光纤激光还是脉冲光纤激光，限制激光功率提升的主要因素是非线性效应^［2-3］和模式不稳定效应^［4-5］。抑制非线性效应、提高激光功率的主要解决途径是使用大模场光纤代替传统的小芯径双包层光纤。获取大模场的有效措施是增大纤芯直径，但这会影响光束质量。为了解决大模场和光束质量的矛盾，需要优化光纤的结构设计。为此，国际上相继推出了折射率导引型光子晶体光纤（PCF）、带隙型光子晶体光纤、手性耦合光纤、多坑道微结构光纤等^［6-7］。

掺镱光子晶体光纤自2000年问世以来，英国巴斯大学、德国耶拿大学、丹麦NKT Photonics等国外研究机构和公司对大模场光子晶体光纤的设计、制备以及应用等关键技术进行了大量的研究^［8-9］。目前掺镱大模场光子晶体光纤已能够在多种结构中实现了芯径大于100 μm的单模激光输出^［10-12］。实现掺镱大模场光子晶体光纤产品化的丹麦NKT Photonics公司在2023年将在售85 μm芯径的掺镱光子晶体光纤的建议使用平均功率提升到250 W，4000 h的考核证明其光致暗化性能也有了很大提升^［13-14］。

国内掺镱大模场光子晶体光纤方面的研究起步较晚。燕山大学、武汉烽火通信科技股份有限公司、华中科技大学和中国科学院上海光学精密机械研究所等单位自2010年前后陆续开展了掺镱大模场光子晶体光纤的设计和制备研究。2009年，华中科技大学陈伟等^［15］采用改进化学气相沉积（MCVD）法制备了Yb³⁺掺杂芯棒，拉制了纤芯数值孔径（NA）为0.06、有效模场面积约为1465.7 μm²的掺Yb³⁺光子晶体光纤。2012年，华中科技大学陈瑰等^［16］采用MCVD法结合气相液相混合掺杂技术，制备了大芯径的Yb³⁺掺杂石英光纤预制棒，并以此为纤芯制备了芯径约为90 μm的光子晶体光纤，模场面积约为1330 μm²，纤芯NA为0.065，首次实现了国产Yb³⁺掺杂石英光子晶体光纤的高功率激光输出，在1 m长的光纤中获得了102 W连续激光（CW），斜率效率为76%，但该光纤没有获得单模激光。2013年，燕山大学刘建涛等^［17］采用粉末烧结技术制备了Yb³⁺掺杂的大模场光子晶体光纤，光纤的模场直径为26 μm，模场面积为550 μm²，但未见激光实验结果报道。2016年，华中科技大学李进延课题组研制了50 μm和127 μm芯径的全固态大模场光子晶体光纤^［18］，采用激光脉宽为100 ps、重复频率为500 kHz的种子光，使50 μm芯径的光纤在1064 nm波长处保持准单模传输特性，光束质量因子（M²）为1.37，并获得了8 W的放大激光输出，127 μm芯径的光纤获得了16 W放大激光输出，但光束质量不理想。

中国科学院上海光学精密机械研究所（SIOM）从2011年开始掺镱光子晶体光纤的研发，利用溶胶-凝胶（sol-gel）法结合粉末烧结工艺，开展了掺镱大模场光纤芯棒玻璃的制备研究，制备了Yb³⁺/Al³⁺、Yb³⁺/Al³⁺/P⁵⁺、Yb³⁺/Al³⁺/F^-、Yb³⁺/Al³⁺/P⁵⁺/F^-共掺石英玻璃作为纤芯玻璃，对其光学和光谱性质随成分及热历史的变化规律开展了系统研究^［19-31］。在此基础上，开展了掺镱大模场光子晶体光纤的设计、制备和激光放大性能的研究。本文在简要介绍掺镱光子晶体光纤的设计要点和光纤芯棒玻璃的制备工艺基础上，着重介绍了SIOM在掺镱大模场光子晶体光纤研制方面取得的最新进展。

2　大模场掺镱光子晶体光纤的设计

2.1　基本原理

根据导光原理的不同，光子晶体光纤可分为折射率导引型光子晶体光纤和光子带隙型光子晶体光纤。根据制备材料的不同，光子晶体光纤可以分为空气孔结构光子晶体光纤和全固态结构光子晶体光纤。空气孔结构光子晶体光纤可只用单一材料，通过调整空气孔的分布、大小等参数获得各种不同的光学特性，以下重点讨论空气孔结构掺镱光子晶体光纤的设计。

对于折射率导引型掺镱光子晶体石英光纤，模式在纤芯中的传输条件为

n_{F S M} < n_{e f f} < n_{c o r e}

，（1）

式中 $: n_{F S M}$ 为无限结构范围内第一包层模的有效折射率； $n_{e f f}$ 为模式的有效折射率； $n_{c o r e}$ 为纤芯折射率。当纤芯模式LP₀₁的有效折射率大于 $n_{F S M}$ ，而LP₁₁模的有效折射率小于 $n_{F S M}$ 时，光纤运行在单模状态。

$n_{F S M}$ 与空气孔的占空比d/Λ（d为空气孔的直径，Λ为相邻空气孔中心的距离）有关，占空比d/Λ越小，则说明空气孔所占包层面积越少，则 $n_{F S M}$ 越接近纯石英的有效折射率。对空气孔结构参数d和Λ进行参数化扫描，计算不同取值下的包层折射率，结果如图1所示，其中n_clad为包层等效折射率。表明能够通过对空气孔结构参数的优化，实现对 $n_{F S M}$ 的较大范围调控。