反共振空芯光纤中氘气受激拉曼散射实验研究 下载: 1189次
1 引言
1963年Minck等[1]首次报道气体受激拉曼散射,目前,它已被证明是一种产生新型波长激光的有效方法,采用这种方法产生的激光的波长覆盖紫外和红外波段[2-3]。然而,在传统的气体腔中,由于激光与气体的有效作用距离非常短,泵浦阈值非常高,同时容易产生多阶斯托克斯谱线,因此从泵浦激光向目标拉曼谱线的转化效率通常很低。微结构空芯光纤的出现为解决上述问题提供了一条有效的技术途径[4-5]。在空芯光纤中,激光可以被约束在微米量级的空芯内传播很长的距离,大大提高了泵浦强度,极大地增强了激光与气体的相互作用。此外,传输带可设计,空芯光纤具有波长选择特性,这使得获得特定拉曼谱线的高效转化成为可能[6-7]。近年来,随着反共振空芯光纤的出现和发展[8-14],基于空芯光纤的气体拉曼激光器受到了研究人员的极大关注[15-27]。反共振空芯光纤的传输带灵活可控,在中红外波段可以实现很低的传输损耗,大大促进了中红外波段光纤气体拉曼激光器的发展[23-25,27]。在前期研究中,本课题组利用反共振空芯光纤,实现了基于乙烷和甲烷气体的高峰值功率、窄线宽的1.5 μm光纤气体拉曼激光器[18-22],同时通过级联拉曼的方式实现了2.8 μm光纤气体激光输出[24]。氘气振动拉曼频移的大小与甲烷和乙烷接近,可以作为实现1.5 μm 和2.8 μm波段光纤气体拉曼激光输出的替换气体[26],同时具有相对简单、更加稳定的分子结构,在高功率输出情况下具有一定的优势。因此,开展反共振空芯光纤中氘气的受激拉曼散射研究十分必要。
本文对反共振空芯光纤中氘气受激拉曼散射的特性进行了研究,首先采用1064.6 nm高峰值功率激光器作为泵浦源,泵浦一段长为2 m的充氘气的冰激凌型空芯光纤,对不同泵浦功率下的输出光谱、不同气压下主要谱线的功率以及脉冲光斑进行测量分析,结果表明:气压较高时有利于多条谱线的产生,气压较低并且泵浦脉冲峰值功率适当时有利于一阶振动斯托克斯谱线(1561 nm)输出。随后,将该光纤在400 kPa气压下产生的最大输出拉曼光耦合到另一段2.2 m长的无节点型空芯光纤中,与氘气再次发生受激拉曼散射,通过此级联方式获得了包括2.92 μm中红外光在内的拉曼激光输出。
2 实验装置
用于研究空芯光纤中氘气受激拉曼散射的级联结构实验装置如
图 1. 实验装置。(a) 实验装置示意图; (b)冰激凌型空芯光纤横截面的扫描电镜(SEM)图;(c)无节点型空芯光纤横截面的扫描电镜图
Fig. 1. Experimental setup. (a) Schematic of experimental setup; (b) SEM image of cross section of ice-cream type HCF;(c) SEM image of cross section of node-less type HCF
3 分析与讨论
图 2. HCF1的输出光谱、测量损耗谱及输出光谱的能级跃迁图。(a) 400 kPa和800 kPa气压下泵浦功率分别为25,50,90 mW时HCF1输出光谱和测量损耗谱;(b) 800 kPa 气压下泵浦功率为90 mW时HCF1输出光谱的能级跃迁图(括号内的是相应的拉曼频移)
Fig. 2. Output spectra, measured loss spectrum, and energy level transition diagram of output spectrum of HCF1. (a) Output spectra and measured loss spectrum of HCF1 at pressures of 400 kPa and 800 kPa under pump powers of 25, 50, and 90 mW; (b) energy level transition diagram of output spectrum of HCF1 at pressure of 800 kPa under pump power of 90 mW (corresponding Raman frequency shifts are shown within brackets)
图 3. 不同气压下,拉曼光功率随耦合泵浦光功率变化的曲线及最大耦合功率泵浦下空芯光纤的输出光谱。(a)(b) 400 kPa;(c)(d) 600 kPa;(e)(f) 800 kPa;(g)(h) 1000 kPa
Fig. 3. Raman light power as a function of coupled pump power and output spectra of HCF under maximum coupled pump power at different pressures. (a)(b) 400 kPa; (c)(d) 600 kPa; (e)(f) 800 kPa; (g)(h) 1000 kPa
图 4. 光斑图。 (a)泵浦源输出的泵浦光;(b)空芯光纤输出端的泵浦光;(c)低功率下1561 nm拉曼光;(d)高功率下1561 nm拉曼光
Fig. 4. Measured patterns. (a) Pump light from pump source; (b) pump light at output end of HCF; (c) 1561-nm Raman light under low power; (d) 1561-nm Raman light under high power
在氘气和氘气级联的实验研究中,将第一级结构处于400 kPa气压下产生的最高功率拉曼光耦合到充有1400 kPa氘 气的无节点型空芯光纤中去,利用光谱仪(Yokogawa AQ6376D)测量光纤的输出光谱。
图 5. HCF2输出光谱、测量损耗谱及输出光谱的能级跃迁图。 (a) 1400 kPa气压下的输出光谱及HCF2的测量损耗谱,图中S1和S2分别代表一阶和二阶斯托克斯,插入图片是在0.02 nm精度下2924.9 nm的精细谱;(b) 1400 kPa气压下输出光谱的能级跃迁图
Fig. 5. Output spectra, measured loss spectrum, and energy level transition diagram of output spectrum of HCF2. (a) Output spectrum and measured loss spectrum of HCF2 at pressure of 1400 kPa, where S1 and S2 represent first- and second-order Stokes respectively and inset shows fine spectrum near 2924.9 nm with resolution of 0.02 nm; (b) energy level transition diagram of output spectrum of HCF2 at pressure of 1400 kPa
4 结论
本文在由两段充有氘气的不同类型的反共振空芯光纤构成的级联结构实验系统中开展了氘气受激拉曼散射研究。结果表明:相比于氢气和甲烷、乙烷,氘气的受激拉曼散射容易产生三种小频移系数的转动谱线,因此实验结构的第一级在高气压、高功率泵浦下获得了0.6~2 μm空芯光纤传输带范围内的众多拉曼谱线。为获得有效的一阶振动斯托克斯1.5 μm输出,降低气压、选取传输带范围更窄的空芯光纤或者选择峰值功率相对较小的泵浦脉冲是有效的方式。第二级在第一级输出光的泵浦下,产生了相对于1064 nm最初泵浦线的二阶振动斯托克斯谱线(2925 nm),但是由于同时产生了转动谱线,2925 nm激光的输出效率很低。如果使用传输带只含1.5 μm和2.9 μm的空芯光纤,则将有望提高2.9 μm的拉曼转化效率。氘气相对于甲烷具有更加简单和稳定的分子结构,在高功率下具有一定优势。本文为下一步开展高效、高功率的氘气振动拉曼激光器的研究打下了良好的基础。
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