# A Study of High-Efficiency Laser Headlight Design Using Gradient-Index Lens and Liquid Lens
## 2 Concept Design of Laser Car Headlight System  
This study conceives an optical model that directly diffuses light; it does not use a mode of reflected light and it is different from the optical system used in the BMW i8 laser headlights. The purpose of designing this optical system is to minimize the amount of stray light and, therefore, reduce the loss of light energy. The laser headlight optical system comprises a laser source, an optical fiber, a ROD, a gradient-index lens (GRIN lens), a liquid lens and a freeform lens [14; 15; 16]. Figure 2a shows the transmission of laser beams in the optical device. The concept of the optical system is one of beam diffusion. A laser outputs a high intensity, polarized, Gaussian distribution beam, which must be diffused into a surface light source to be suitable for use in lighting applications. The laser was transmitted from the laser diode to the headlight system via optical fibers that have the ability to transmit light over long distances and can eliminate the phenomenon of partially polarized light through total reflection [8]. The laser beam was transmitted to the ROD via optical fibers, whereby the ROD rearranged the laser beam and eliminated the polarized light pattern, thus facilitating the subsequent diffusion process. The profile of the ROD's light output was adjusted using a light shaper, which also removed excess stray light. First, the GRIN lens plays a role in the proposed optical design because the shape of the incident light is modified and optimized simultaneously to meet the specifications. Second, liquid lenses with various focal lengths can converge light from the laser source into the laser surface source to optimize the best performance between the high and low beams of the system specification. In other words, after the optimization work, the liquid lens variations in focal length may provide the best performance for either the low- or high-beam mode. According to the illuminance of the low- and high-beam modes, and the specifications of the illumination area, a freeform lens, with the function of adjusting the light pattern and illumination area, was used to satisfy the requirements of the headlight regulations. The light emitted by the laser diode was first modified by the specially designed ROD through fiber optic cables. Throughout the entire light shape (aperture stop), rays were converged via a GRIN lens, focal length and extremely complex freeform optics to deliver the best high-beam/low-beam model performance. The functions and applications of the optical devices used in this study are described below (see Figure 2b,c for the device numbers). The above description corresponds to a single light source module; a collection of many light source modules can form an automobile light system. The color temperature of the automobile light source is in the range of 4000-6000 K and its penetration power and lighting ability are excellent [17]. Therefore, this study used 5500 K white lasers as the light source. No. 1 in Figure 2 is a diode laser, which exhibits the characteristics of homochromatic, high intensity, high penetration and high-beam parallelism, and has become a new option in automobile lighting. Due to its small size and high energy conversion efficiency, a diode laser is able to detect the light source. The light source is a diode laser, which exhibits the characteristics of homochromatic, high intensity, high penetration and high-beam parallelism, and has become a new option in automobile lighting. Due to its small size and high energy conversion efficiency, a diode laser is able to detect the light source. Figure 2: (**a**) Schematic of the laser automotive headlight system. (**b**) Simulation and ray tracking of ROD (orange), GRIN lens (red) and liquid lens (brown) via LightTools under the education license of ORA, U.S.A; (**c**) simulation and ray tracking of whole system including fiber, ROD, GRIN lens (red), liquid lens (brown), freeform for low beam (green) and high beam (blue).