To collect long-distance and wide-spectrum signals of laser filament-induced plasma spectroscopy, a high-efficiency collecting system with a large-aperture Fresnel lens is necessary so that the fluorescence spectrum signals can be converged and coupled into a fiber spectrometer. Although conventional non-imaging Fresnel lenses have superior energy collection efficiencies, the focusing performance of the lens is affected by spherical aberration and dispersion, which are induced by the increased aperture. This leads to difficulties in spectrum analysis at high resolution. Therefore, optimizing the ring shape of the Fresnel lens and improving the performance by considering source parameters, volume size, and lens specifications are crucial to the design of the collecting system.

A large-aperture Fresnel lens collecting system is designed using loop optimization of each ring with multiple software programs. The system consists of seven Fresnel lenses with a diameter of 300 mm and a focal length of 670 mm. To optimize the energy collection efficiency and the focusing spot size in the Zemax sequence mode, a multi-ring aspheric Fresnel lens is designed using Code V and MATLAB softwares, and the lens is modeled and analyzed using the LightTools software. This method reduces the spherical and chromatic aberrations of the large-aperture Fresnel lens, making the efficiency of energy coupling into the optical fiber improved and the convergence spot size reduced. Therefore, the signal intensity of the fiber spectrometer can be raised effectively.

The surface shape of the single-ring band is optimized in the Code V software by controlling the light spot radius and the convergence angle (Fig. 4). The loop optimization of multi-ring bands is carried out in the MATLAB software using the same optimization method (Table 2) and the focusing spot size is analyzed in the Code V software. In addition, the Fresnel lens model is constructed in the LightTools software (Fig. 7), and the full width at half maximum of the focus spot and the energy collection efficiency are calculated (Fig. 8). Finally, the tolerance of the lens is analyzed according to the manufacturing process (Fig. 9). In the simulation, the Fresnel lens energy collection efficiency and the spot diameter are 52.2% and 2.051 mm, respectively. In contrast, the energy collection efficiency measured in the experiment and the light spot diameter are 34.9% and 2.260 mm, respectively. The differences between the simulation and experiment results are possibly owing to the errors in manufacturing and assembly.

In this study, the large-aperture Fresnel lens is investigated according to the requirements of the collecting system of laser filament-induced plasma spectroscopy. A design method of large-aperture Fresnel lenses is proposed using several software platforms, which improve the collection efficiency and reduce the influence of aberration. After the design and optimization of the multi-ring using Code V and MATLAB softwares, along with modeling and analysis using LightTools software, a collection efficiency of 52.2% for the lens and a focusing spot size of 2.051 mm are achieved. Due to the errors in manufacturing and measurement, the collection efficiency is 34.9% in the experiment, which meets the application requirements of the system. The simulation and experiment results show that the design method can reduce the aberration of a large-aperture Fresnel lens and facilitate collection of long-distance and wide-spectrum signals efficiently.