Aerosol is the general term for solid, liquid, and other particulate material in the atmosphere. The possible sources of aerosol include the combustion of biological substances and fossil fuels, mineral dust, and other harmful pollutant particles. When the concentration of NaCl aerosol in the atmosphere is higher than a threshold concentration, it inhibits plant growth, corrodes metals, harms the respiratory, cardiovascular, and cerebrovascular systems of human beings.
When a femtosecond laser is transmitted in the air, owing to the nonlinear self-focusing effect, the focused light intensity reaches 1013 W/cm2. At this intensity, any substance in air is ionized and the fluorescence spectrum corresponding to the chemical composition of the substance is released. The composition and concentration of atmospheric pollutants can be detected based on this property by collecting and analyzing the fluorescence spectrum excited by the femtosecond laser filament, realizing air pollution monitoring and providing basic data for the formulation of air pollution control programs.
Compared with the grating spectrometer, the light intensity collected by the Fabry-Perot interferometer (FPI) is 2-3 orders of magnitude larger than that of the grating spectrometer with the same spectral resolution. Simultaneously, because the aperture of the FPI is large, the spectrum of the extended light source can be measured directly, thus reducing the requirement of light-collecting elements. Based on the above characteristics, FPI is an ideal device for remote weak-light spectrum measurement.
A Fresnel lens is considerably lighter and less expensive than the traditional spherical lens, which is suitable for the large-aperture low-light collection system. In this study, a Fresnel lens with an aperture of 1.1 m is used to converge remote low-light sources. Although the size of the focusing spot is large, it still can form a remote low-light spectrum detection system with the large-aperture FPI.
The Fresnel lens used in this study is made of polymethyl methacrylate (PMMA) whose diameter and focal length are 1.1 m and 1.31 m, respectively. The Fresnel lens has a ring spacing of 0.5 mm, and each ring has a different depth. The Fresnel lens used in this study has an average transmittance of 94% in the visible light range. The FPI used in this study consists of two partially reflective mirrors with diameters of 1 inch (1 inch=2.54 cm) and three 2-mm-thick piezoelectric ceramics (PZT). The reflectivity of the mirrors is 90%, the applied voltage range of the PZT is 0-60 V, and the maximum displacement of the PZT is 3 μm. The PZT used in this study has good linearity between the displacement and applied voltage, the resolution of the PZT is 0.5 nm, and the free spectrum range of FPI is 9 nm.
Using the industrial CCD as the detector, the spectrum of the mercury lamp can still be detected even when the distance between the lamp and FPI increases to 30 m (Fig. 10). In this case, the illumination on the Fresnel lens surface is only 6.5 μlx.
The self-built FPI combined with the Fresnel lens is used to detect the fluorescent spectrum of the NaCl aerosol induced by the femtosecond laser optical filament at a distance of 10 m (Fig. 11). In the experiment, the pulse energy of the femtosecond laser is 4 mJ, and the pulse width is 50 fs. The NaCl aerosol with mass fraction of 13×10-6 is generated by the aerosol generator. In this case, an intensified complementary metal-oxide semiconductor (CMOS) camera is employed as the detector. The emission lines of Na+ at 589 nm are detected.
The spectrum of the mercury lamp placed at a distance of 8 m away from the Fresnel lens and FPI is determined when an industrial charge coupled device (CCD) is employed (Fig. 7). By applying different voltages to the PZT, the wavelength of the emission spectral line of the mercury lamp is calculated using the relationship between the wavelength and radius of the interference ring. The wavelength error measured by the FPI is less than 1 nm, which is mainly originated from the radius error of the interference ring obtained from the picture taken by the CCD camera.
In this study, a Fresnel lens with a diameter of 1.1 m and a self-built FPI are used to construct a remote spectral detection system. With the efficient light-collecting characteristics of the Fresnel lens and large light-collecting aperture FPI, an industrial CCD camera can detect the spectrum of the micro-lux light sources. Using an intensified CMOS camera, the system can measure the fluorescence of NaCl aerosol with mass fraction of 13×10-6 excited by the femtosecond laser filament at a distance of 10 m. In the future, by further improving the fitting method of FPI and designing more matching optical elements for the light coupling between the FPI and Fresnel lens, the spectral resolution and low-light detection capability of the spectral detection system can be further improved to play a greater role in the long-range measurement of atmospheric pollutants.
