To meet the requirements for using an optical fiber spectrometer to detect weak signals over long distances and across a wide spectrum, and considering the sensitivity of the spectrometer, a large aperture optical system is needed so that sufficient optical signals can be obtained. In addition, the aperture of the receiving surface of the optical fiber bundle is small, making it difficult for the lens to match the numerical aperture of the optical fiber bundle. Therefore, the focal spot size and convergence angle should be as small as possible to efficiently couple optical signals into the optical fiber bundle and improve energy utilization efficiency. The traditional design of an optical system based on imaging optics increases the weight and volume of the system, ultimately affecting the overall performance of system. Therefore, determining the size of the spot and angle of beam convergence is crucial for improving the system's light energy utilization efficiency.

A large aperture Fresnel lens condensing system is designed based on a hybrid design method of imaging and non-imaging optics. The system consists of a 1.1-m-diameter Fresnel lens, a beam homogenizer, a total reflection collimator, and a relay lens group. The receiving surface is a fiber bundle with a diameter of 2 mm and a numerical aperture of 0.22. The problem of traditional large aperture lenses' large volume and weight is solved using large aperture Fresnel lenses. To achieve uniform energy distribution and reduce the convergence angle of the light bundle, the rear group of non-imaging optical elements, composed of a beam homogenizer and a total reflection collimator, are designed to reduce the spherical and chromatic aberrations of large aperture Fresnel lenses. The relay lens group also controls the beam divergence angle and spot size, allowing the optical signal to be efficiently coupled into the optical fiber and improving the light energy utilization efficiency of the system.

First, a large aperture Fresnel lens condensing system based on imaging and non-imaging optics is designed (Fig. 2). Subsequently, the optical device is modeled and designed using the ZEMAX software, and the overall parameters of the condensing system are achieved (Table 1). The optical system model is established in ZEMAX software to verify the focusing performance of the Fresnel lens condensing system, and simulation results are obtained (Fig. 8). The light energy utilization efficiency of the condensing system is 6.5% according to ray tracing, and the light energy utilization efficiency of the condensing system with a rear group is five times that of the system without a rear group. In addition, the experimental system is built based on the condensing system' s design results (Fig. 9). The experimental light energy utilization efficiency of the condensing system is 3.8%, which is lower than the simulation results. This is because the experimental device parameters and spectral power distribution of the light source are inconsistent with the theoretical simulation. In contrast, the light energy utilization efficiency of the Fresnel lens condensing system without the rear group is only 1.8%. The simulation and experimental results indicate that the rear group system based on the hybrid design method of imaging and non-imaging optics can effectively improve the light energy utilization efficiency of the Fresnel lens condensing system.

In this study, based on the hybrid theory of imaging and non-imaging optics, the large aperture Fresnel lens condensing system is examined, and the design scheme of the rear group system composed of a beam homogenizer, total reflection collimator, and relay lens group is devised. Through the simulation and optimization by ZEMAX software, a theoretical light energy utilization efficiency of 6.5% is obtained for the system. The experimental system is built based on the design results. The light energy utilization efficiency of the Fresnel lens condensing system with the rear group is 3.8%, which is 2.1 times that of the system without the rear group. Given that the experimental device parameters are not completely consistent with the theoretical simulation parameters, the experimental result is essentially reasonable. The theoretical simulation and experimental test prove that the rear group can reduce the influence of aberration, and control the spot size and the convergence angle. The hybrid design method efficiently couples the optical signal into the optical fiber bundle, allowing for the detection of weak signals over long distances and across a wide spectrum.