可调谐掺铥光纤激光器线宽压缩及其超光谱吸收应用

可调谐二极管激光吸收光谱技术是一种应用非常广泛的吸收光谱测量技术.利用宽带可调谐窄线宽光源进行吸收光谱测量的超光谱吸收技术可以在单次扫描中获取一段连续光谱的所有吸收数据,可大大提高可调谐二极管激光吸收光谱技术的数据信息容量和光谱诊断能力.分析了在2μm波段对水进行超光谱吸收测量时对激光器输出线宽的具体要求.利用掺铥光纤在2μm波段较宽的发射谱,采用可调谐法布里-珀罗滤波器和光纤可饱和吸收体相结合的技术方案搭建了一台宽带调谐窄线宽的2μm光纤激光器.获得了1840—1900 nm约60 nm范围的调谐光谱输出,激光器静态线宽仅为0.05 nm.利用该光源对空气中水在2μm波段的吸收光谱数据进行了超光谱吸收测量,在1856—1886 nm约30 nm的光谱范围内分辨了35条水的吸收谱线.通过对不同线宽条件下1870—1880 nm范围内的理论吸收光谱数据进行对比发现,测量数据无法有效分辨分别位于1873 nm和1877 nm处与强吸收线相邻的两条吸收谱线,且测量结果与激光线宽在0.08 nm条件下的HITRAN2012光谱数据库最为接近.这表明,在动态扫描过程中激光器的线宽得到了展宽.

Linewidth compression of tunable Tm-doped fiber laser and its hyperspectral absorption application

Tunable diode laser absorption spectroscopy(TDLAS)is a widely used technology for measuring absorption spectrum.However,the measurement efficiency of TDLAS is greatly limited by the narrow tuning range of conventional tunable laser diode.Exploiting a wideband,narrow linewidth tuning laser source,hyperspectral absorption spectroscopy possesses the ability to provide the overall absorption information over a continuous waveband in a single scan,which would significantly improve the data volume and diagnostic capability of TDLAS.With profound and strong absorption lines of water and carbon dioxide,the 2μm waveband is an ideal candidate for water and carbon dioxide related absorption spectrum.An absorption line recognition threshold of 0.07 nm is derived for the absorption spectrum measurement of water around 2μm through theoretical analysis.Utilizing the wideband emission spectrum of Tm-doped fiber,a wideband tunable,narrow linewidth fiber laser operating at 2μm is built by combining a tunable FP filter with a fiber saturable absorber.The tunable FP filter is responsible for the wavelength control of the laser system,with which a 60 nm wideband tuning range from 1840 nm to 1900 nm is achieved.With a section of Tm-Ho codoped fiber as the fiber saturable absorber which is used for linewidth compression,a static linewidth of 0.05 nm is attained.This wideband tunable,narrow linewidth fiber laser is tested for the hyperspectral absorption spectrum measurement of water around 2μm.Drived with a 0–10 V triangle wave at a repetition rate of 50 Hz,the output spectrum of the laser spans over a wavelength range of about 30 nm from 1856 nm to 1886 nm.The laser beam propagates about 50 cm through an open air,and then enters into the detectors for direct measurement.The 35 absorption lines of water are recognized after processing the data.Within the 1870–1880 nm range,comparisons with the theoretical absorption spectra at different laser linewidths,derived from the HITRAN2012 absorption database,show that the measured data cannot effectively distinguish two absorption lines adjacent to the strong absorption line at 1873 nm and 1877 nm.And,the measured results can be best fitted to a laser linewidth of about 0.08 nm,demonstrating that in the dynamic scanning process,the linewidth of the laser is expanded beyond the absorption line recognition threshold.Thus,when operating in a fast wideband scanning mode,the laser system should further compress its linewidth.