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该节以无源双包层氟化物光纤作为合束器的信号光纤,在无源氟化物双包层光纤(DC-ZBLAN)上制得侧面泵浦合束器,重点研究合束器的制作工艺流程,研究泵浦光纤拉锥参数对合束器耦合效率的影响,研究合束器高功率泵浦下的输出功率、温度特性。
实验中首先利用拉锥机将一段包层直径为125 µm的无芯石英光纤(NA=0.46)进行拉锥,拉锥光纤示意图如图1(a)所示,拉锥后的无芯光纤包含三个参量,锥区长度(taper length)、腰区长度(waist length)、腰区直径(waist diameter),在拉锥后对无芯光纤的另一边锥区进行切割,不再保留。拉锥光纤为侧面泵浦合束器的泵浦光纤,合束器的信号光纤为无源双包层氟化物光纤,其纤芯及内包层直径分别为12/125 µm,数值孔径分别为0.23/0.4,长度约2 m。信号光纤后向输出端切平角,前向输出端切12°角。实验中先将信号光纤涂覆层剥除约10 cm,然后将拉锥光纤最细的腰区及锥区尾端部分贴紧在泵浦光纤上,再轻轻拉直拉锥光纤,使其剩余锥区能够直直地贴合在泵浦光纤上,由于腰区及锥区尾端部分很细,贴合到信号光纤上呈现出的是一个自然卷曲的状态。整个过程用酒精固定以提高两光纤间的粘附力,示意图如图1(b)所示。当酒精挥发后,两光纤很容易互相脱落。实验中自制了合束器夹具,用来承载贴合了泵浦纤的信号纤,然后浸入胶水固化,进行封装。与此同时,用一热释电功率探测器(power detector, PD)实时监测合束器前向输出功率,确保胶水在浸入过程中,两光纤接触良好、不会脱落。为进一步测试合束器耦合效率及发热情况,拉锥光纤泵浦端与实测最大输出功率87.5 W的976 nm半导体激光器多模尾纤(105/125, 0.22)相熔接,合束器前向输出功率由热释电功率探测器实时测得,合束器光纤温度由热像仪实时监测,测试过程如图1(c)所示。在合束器温度监测过程中,并未加主动冷却装置。
图 1 (a) 拉锥泵浦光纤参数示意图;(b) 泵浦-信号耦合过程示意图;(c) 合束器测试装置图
Figure 1. (a) Schematic diagram of tapered pump fiber parameters; (b) Schematic diagram of the pump-signal coupling process; (c) Diagram of combiner test device
在未浸胶水的状态下,测得低功率(1 W)泵浦下,不同拉锥参数的光纤作为泵浦纤时合束器的泵浦光耦合效率,如表1所示。合束器耦合效率由功率计测得的前向输出稳定功率除以泵浦功率得到。由于无源氟化物光纤损耗低,在计算耦合效率时,忽略了信号光纤传输损耗的影响。
表 1 不同泵浦光纤拉锥参数下合束器耦合效率测试结果
Table 1. Test results of combiner’s coupling efficiencies under different pump fiber tapering para-meters
Taper length/cm Waist length/cm Waist diameter/μm Coupling efficiency 1.5 1.5 ~15 55.9% 3 1.5 ~15 69.5% 4 1.5 ~15 75.3% 5 1.5 ~15 80.2% 5 2.5 ~15 75.6% 5 1.5 ~30 70.8% 6 1.5 ~15 78.1% 从表1可以看出,在锥区长度、腰区长度和腰区直径为1.5/3 cm、1.5 cm和15 µm时,合束器耦合效率较低,为55.9%和69.5%。在锥区长度、腰区长度和腰区直径为5 cm、1.5 cm和30 µm时,合束器耦合效率较低,为70.8%。这是因为锥区长度过短、腰区直径过大,均会导致部分泵浦光没能泄露出去,残留在拉锥光纤内。因此,为实现较高耦合效率的合束器,应保证锥区长度大于4 cm,腰区直径小于20 µm。最后,根据7组实验结果中测得的最高耦合效率,将封装合束器的泵浦光纤参数定为锥区长度5 cm、腰区长度1.5 cm、腰区直径15 µm。
封装后的合束器输出功率随泵浦功率的演化如图2(a)所示,在0~87.5 W的泵浦功率范围内,合束器输出功率基本随泵浦功率线性增加,耦合效率约为82.3%。在最大泵浦功率下,合束器输出功率达到71.3 W。图2(b)为合束器夹具(未盖夹具盖)最高温度点随泵浦功率的演变图,合束器封装实物图及温度热像图如图2(b)内插图所示。由图2(b)可以看出,当泵浦功率大于10 W后,合束器温度随泵浦功率线性增加,在最大泵浦功率87.5 W下为105 ℃,虽未达到氟化物光纤软化温度(~250 ℃),但已接近固化胶水的工作温度上限(~130 ℃)。未来合束器功率的进一步提升主要受固化胶水工作温度的限制。合束器夹具发热最严重的地方主要在拉锥光纤耦合区域尾端,这是因为大部分泵浦光功率在腰区部分耦合入信号光纤,同时未进入信号光纤的残余泵浦光也主要在此位置泄露,这加剧了合束器夹具的发热。未来可以通过提高合束器泵浦光耦合效率并对其进行主动冷却处理,以有效降低其温升。
图 2 (a) 合束器输出功率随泵浦功率演变图;(b) 合束器夹具温度随泵浦功率演变图
Figure 2. (a) Evolution diagram of combiner output power with pump power increasing; (b) Evolution diagram of combiner holder’s temperature with pump power increasing
与以往报道的250 µm包层直径的信号光纤相比,实验所用信号光纤包层直径为125 µm,其涂覆层的剥除、制作工艺难度相对更大,且耦合效率小于250 µm包层直径的信号光纤侧面泵浦合束器[14]。
Mid-infrared side-pumping combiner and all-fiber superfluorescent fiber source (invited)
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摘要: 中红外超荧光光源具有光谱范围宽、空间相干性好、时域稳定性高等特点,应用前景广泛,但受限于中红外侧面泵浦合束器,目前普遍利用空间结构泵浦产生。文中根据拉锥光纤侧面耦合的原理,在125 μm包层直径的无源双包层氟化物光纤上实现了中红外光纤侧面泵浦合束器的研制,该合束器泵浦光耦合效率达82.3%,可承受的最大泵浦功率达87.5 W。通过在中红外增益光纤上制得侧面泵浦合束器,实现了全光纤中红外超荧光光源产生,前后向输出的中红外超荧光最高功率和为91.09 mW (后向输出53.67 mW,前向输出37.42 mW),输出光谱范围从2702 nm覆盖至2830 nm。在中红外超荧光总输出功率为33.03 mW时,获得了108 nm的最宽20 dB带宽。文中实现的中红外全光纤超荧光光源克服了以往空间泵浦复杂度高、调节难的问题,对推动中红外超荧光光源的进一步功率放大具有重要意义。Abstract:
Objective Mid-infrared superfluorescent fiber sources (SFS), working between fluorescent and laser, not only have good spatial coherence, wide emission spectrum and high brightness, but also have no mode competition, no relaxation oscillation and high temporal stability compared to laser. It has been applied to gas detection, optical coherence tomography, and optical fiber sensing. At present, most of the reports on SFS focus on the near infrared band of 1-2 µm, while in the mid-infrared band, there are few reports. Besides, all the works on mid-infrared SFS is based on the space pumping structure, which is mainly caused by the lack of mid-infrared side-pump combiner. In the scheme of space pumping, pump laser is collimated and then focused into the end face of the gain fiber to realize the coupling, and the mid-infrared SFS filtering is realized through a dichroic mirror and a long-wave filter. The structure of this scheme is relatively complex and the system stability is poor. Besides, the end face of the fluoride fiber is easy to be damaged due to the end face pumping, so the injected pump power is limited. Therefore, the development of the mid-infrared side pump combiner can not only realize all fiber structure of mid-infrared SFS and overcome the problems caused by space pumping, but also realize high-power mid-infrared SFS output through backward pump. For this purpose, a home-made mid-infrared fiber side-pumping combiner and mid-infrared all fiber SFS are designed and realized in this paper. Methods Firstly, a home-made mid-infrared fiber side pumping combiner is developed on a passive double clad fluoride fiber with 125 μm cladding diameter by tapered fiber side coupling principle (Fig.1). Influence of different tapering fiber profiles on combiner’s coupling efficiency has been studied. The output power and heating condition of the combiner as the pump power increasing have also been studied. Secondly, a home-made mid-infrared fiber side pumping combiner is developed directly on a Er3+-doped double clad fluoride fiber to obtain mid-infrared all-fiber SFS by the same way. Both ends of the gain fiber are cut at 12° to reduce the threshold of laser self-excited oscillation and increase the output power of mid-infrared SFS. When the output is measured, the output is collimated through a calcium fluoride lens at first and then filtered through a 2.4 µm long wave filter (LF) to remove the residual pump light at 976 nm and SFS near 1550 nm to obtain pure mid-infrared SFS (Fig.2). Both forward and backward output power and spectrum are measured. Results and Discussions Optimized tapering parameters of 5 cm taper length, 1.5 cm waist length and 15 µm waist diameter has been chosen. At the maximum pump power of 87.5 W, the output power of the combiner reaches 71.3 W, and the highest hot spot of combiner reaches 105 ℃. The corresponding coupling efficiency and the maximum pump power are up to 82.3% and 87.5 W, respectively. By fabricating side-pumping combiner on the Er3+-doped double clad fluoride fiber directly, the generation of all-fiber mid-infrared SFS source is achieved. The mid-infrared SFS power sum is 91.09 mW (backward output of 53.67 mW, forward output of 37.42 mW), and the output spectrum ranges from 2702 nm to 2830 nm. The maximum 20 dB bandwidth reaches 108 nm when SFS power is 33.03 mW. This proposed scheme overcomes the problems of spatial pump’s high complexity and difficult adjustment, and is of great significance for further power amplification of mid-infrared SFS. However, the gain fiber used in this paper has a low doping concentration and a short fiber length, which limits the improvement of the output power and efficiency of mid-infrared SFS. In the future, the power can be further improved by increasing the doping concentration of fiber to alleviate the self-terminating phenomenon of Er3+ level and optimizing the length of gain fiber to improve the absorption efficiency of pump laser. Conclusions This paper reports the development of mid-infrared side-pumping combiner and all-fiber SFS source. Based on the side-coupling principle of tapered fiber, a mid-infrared side-pumping combiner is developed on the passive double-clad fluoride fiber with a cladding diameter of 125 μm. The coupling efficiency of the combiner is up to 82.3%, and the maximum available pump power is 87.5 W. By directly fabricating the combiner on the gain fiber, this paper realizes the generation of all-fiber mid-infrared SFS source for the first time. The maximum power sum of mid-infrared SFS output forward and backward is 91.09 mW (backward output of 53.67 mW, forward output of 37.42 mW), and the output spectrum ranges from 2702 nm to 2830 nm. When the total output power of mid-infrared SFS is 33.03 mW, the maximum bandwidth of 20 dB at 108 nm is obtained. The mid-infrared side-pumping combiner and the all-fiber SFS source developed in this paper can not only improve the compactness and reliability of the mid-infrared SFS source, but also provide a good solution for further power amplification of the mid-infrared SFS source. -
表 1 不同泵浦光纤拉锥参数下合束器耦合效率测试结果
Table 1. Test results of combiner’s coupling efficiencies under different pump fiber tapering para-meters
Taper length/cm Waist length/cm Waist diameter/μm Coupling efficiency 1.5 1.5 ~15 55.9% 3 1.5 ~15 69.5% 4 1.5 ~15 75.3% 5 1.5 ~15 80.2% 5 2.5 ~15 75.6% 5 1.5 ~30 70.8% 6 1.5 ~15 78.1% -
[1] Jackson S D. Towards high-power mid-infrared emission from a fibre laser [J]. Nature Photonics, 2012, 6(7): 423-431. doi: 10.1038/nphoton.2012.149 [2] Ye J, Xu J, Zhang Y, et al. Spectrum-manipulable hundred-watt-level high-power superfluorescent fiber source [J]. Journal of Lightwave Technology, 2019, 37(13): 3113-3118. doi: 10.1109/JLT.2019.2911007 [3] Oh K, Morse T F, Kilian A. A new gas detection technique utilizing amplified spontaneous emission light source from a Tm+3/Ho+3 co-doped silica fibre in the 2.0 μm region [J]. Measurement Science and Technology, 1998, 9: 1409-1412. doi: 10.1088/0957-0233/9/9/007 [4] Bouma B E, Nelson L E, Tearney G J, et al. Optical coherence tomographic imaging of human tissue at 1.55 μm and 1.81 mum using Er- and Tm-doped fiber sources [J]. Journal of Biomedical Optics, 1998, 3(1): 76-79. doi: 10.1117/1.429898 [5] Martin-Lopez S, Gonzalez-Herraez M, Carrasco-Sanz A, et al. Broadband spectrally flat and high power density light source for fibre sensing purposes [J]. Measurement Science and Technology, 2006, 17(5): 1014-1019. doi: 10.1088/0957-0233/17/5/S13 [6] Iwanus N, Hudson D D, Hu T, et al. Aim at the bottom: directly exciting the lower level of a laser transition for additional functionality [J]. Opt Lett, 2014, 39(5): 1153-1156. doi: 10.1364/OL.39.001153 [7] Luo H, Li J, Wang L, et al. High power broadband amplified spontaneous emission source near 3 µm [J]. IEEE Photonics Technology Letters, 2014, 26(22): 2287-2290. doi: 10.1109/LPT.2014.2352312 [8] Goya K, Mori A, Tokita S, et al. Broadband mid-infrared amplified spontaneous emission from Er/Dy co-doped fluoride fiber with a simple diode-pumped configuration [J]. Sci Rep, 2021, 11(1): 5432. doi: 10.1038/s41598-021-84950-y [9] Yang L, Wu J, Li N, et al. Watt-level superfluorescent fiber source near 3 µm [J]. Opt Lett, 2021, 46(11): 2778-2781. doi: 10.1364/OL.428310 [10] Wang Y, Luo H, Gong H, et al. 2.3 W, linearly-polarized superfluorescent generation from a polarization-maintaining Er3+-doped fluoride fiber amplifier around 2.8 μm [J]. Journal of Lightwave Technology, 2022, 40(17): 6001-6005. doi: 10.1109/JLT.2022.3183067 [11] Long P, Soltanian M R K, Comanici M I, et al. All-fiber 600 nm amplified spontaneous emission (ASE) source covering the spectral range of 2.75 µm to 3.35 µm [C]//Fiber Lasers XVII: Technology and Systems, 2020, 11260: 112601N. [12] Lei C, Li Z, Zhang H, et al. Taper-fused side pump combiner for all-fiber lasers and amplifiers: A review [J]. Optics and Laser Technology, 2020, 130: 106353. doi: https://doi.org/10.1016/j.optlastec.2020.106353 [13] Schäfer C A, Uehara H, Konishi D, et al. Fluoride-fiber-based side-pump coupler for high-power fiber lasers at 2.8 μm [J]. Opt Lett, 2018, 43(10): 2340-2343. doi: 10.1364/OL.43.002340 [14] Magnan-saucier S, Duval S, Matte-breton C, et al. Fuseless side-pump combiner for efficient fluoride-based double-clad fiber pumping [J]. Opt Lett, 2020, 45(20): 5828-5831. doi: 10.1364/OL.409174