Abstract
The 1.8–2.0 μm waveband contains abundant absorption lines of water, which are much stronger than those in the traditional 1.3–1.5 μm waveband, exhibiting huge potentials for absorption spectrum applications of water. In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used. The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest. With a tunable fiber FP filter and a fiber saturable absorber, a Tm-doped fiber laser is established, featuring broadband tenability and narrow linewidth. Taking advantage of the re-absorption characteristics of Tm-doped silica fibers, a wavelength tuning range covering 60 nm from 1910–1970 nm is obtained through the appropriately designing of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within a tuning range of 1910–1965 nm, while, in alcohol flame, the number of detected lines reaches about 50. Comparison with the HITRAN2016 database gives a laser linewidth of about 0.06 nm which is very close to the static linewidth measured by an OSA. The temperature of the air is derived to be 298 K with a water mole fraction of about 2%, which is consistent with the measurement of the hygrothermograph. And the calculation indicates an alcohol flame temperature of about 1220 K, which is very close to the measurement result of the thermocouple.
Highlights
In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used
The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest
The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum
Summary
可调谐掺铥光纤激光器线宽压缩及其超光谱吸收应用 Linewidth compression of tunable Tm-doped fiber laser and its hyperspectral absorption application 物理学报. 基于二维纳米材料可饱和吸收体的中红外超快光纤激光器 Two-dimensional material as a saturable absorber for mid-infrared ultrafast fiber laser 物理学报. MnPS3可饱和吸收体被动锁模掺铒光纤激光器双波长激光 Dual-wavelength self-starting mode-locking Er-doped fiber laser with MnPS3 saturable absorber 物理学报. 高损伤阈值可饱和吸收体锁模脉冲光纤激光器的研究进展 Research progress of mode-locked pulsed fiber lasers with high damage threshold saturable absorber 物理学报. 2014 年, 陶波等[10] 利用水在 1397.8 nm 附近的吸收线分别建立了基于直接吸收法和二次 谐波法的 TDLAS 测量系统, 并对瞬态高温超声速 流程和甲烷火焰的温度开展了测量实验. 2008 年, Hunsmann 等 [12] 利用水在 1370 nm 处的吸收谱线, 通过 TDLAS 技术对树叶的水蒸发速率进行了精确测量. 2010 年, Caswell 等 [14] 在 1.32—1.38 μm 波长 内选择了 10 条水的吸收谱线, 通过时分复用技术 实现了高压燃气轮机燃烧室内水的温度和摩尔 分数的测量 . 2013 年, Caswell 等 [20] 使用 三台 FDML, 通过水的超光谱吸收实现了对脉冲 爆震燃烧室不同位置的温度、压力和水的摩尔分数 的测量. 近年来, 利用可 调谐法布里-珀罗 (FP) 腔和光纤可饱和吸收体, 课 题组研制了宽带调谐的窄线宽 2 μm 波段的光纤 激光器, 并初步开展了空气中水的超光谱测量实验 研究, 在 1856—1886 nm 约 30 nm 的光谱范围内 分辨了 35 条水的吸收谱线, 且测量结果与 HITRAN 数据库相吻合 [24−26]. 9/125 光纤作为增益光纤. 该光纤长度下, 调节光 纤 FP 滤波器加载电压, 可以测量得到该滤波器的 自由光谱范围 (FSR), 如图 4 所示. 可以看出, 该 FP 滤波器的自由光谱范围约为 97 nm; 在 3.6 m增益 光纤时, FP 滤波器的波长扫描范围理论上可覆盖 1878—1975 nm
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