The in-vehicle HUD system projects auxiliary driving information into the driver's field of view in front of them by optical components. It can reduce the driver's perspective switching and help to enhance driving safety [1]. Rather than 3D display, traditional HUDs can only provide image information on a single depth plane, which not only fails to accurately display the complex road conditions, but also may cause visual fatigue for drivers [2]. Therefore, the development of multi-depth 3D HUD systems is urgent in this field.

To realize multi-depth 3D HUD, four kinds of techniques including geometric optics-based HUD systems [3], integral imaging-based HUD systems [4], computer-generated holography (CGH)-based HUD systems [5], and HOEs-based HUD systems [[6], [7], [8], [9]] have been currently proposed. The multi-depth information of geometric optics-based HUD systems is projected by multi-focal freeform lenses or auto-focus elements. However, the optical systems of these HUDs are complex and difficult to design and manufacture. Besides, the speed of dynamic focusing is limited [10]. In the integral imaging-based HUD systems, 3D HUD images with low visual fatigue and full parallax are realized using a four-dimensional light field as its image source. But its spatial resolution is low and there is a compromise with the angular resolution [11]. In theory, the CGH-based HUDs are ideal as they can reconstruct the wavefront information of 3D images. Nevertheless, its display performance is constrained by the massive computation of holograms and the limited bandwidth of existing spatial light modulators [12].

Due to the advantages of compact, versatile and convenient production of HOEs, the HUDs of multi-depth plane based on HOEs have attracted much attention. In 2016, Koki et al. successfully expanded the display area and field of view of a digital holographic projection system by using a digitally designed HOE, and they produced a large-area 3D transparent display suitable for in-car HUD [13]. Meanwhile, utilizing holographic display technology, Lee et al. proposed an AR display based on mirror-lens HOE [14]. In 2018, Jackin et al. made a 3D transparent display system by combining holographic micro mirror array sheets and light field display technique [15]. In 2020, using a combination of two 90-degree prisms and a HOE, Wu et al. made a kind of transparent HUD with a wide viewing angle and extended eyebox [16]. In 2021, Liu et al. successfully created an AR HUD system with multi-plane, large area, and multi-color images using volume HOEs [17]. Moreover, they made a multi-plane AR HUD with a real-virtual dual mode and large eyebox in 2022 [9]. In 2023, a large depth of field 3D HUD was made by Lv et al. using a multi-focal HOE and integral imaging technology [2]. Jiang et al. expanded the depth of field of the 3D transparent display system using a dual-mode lens array HOE and integral imaging technique [18].

The imaging principles of the HOEs are based on the diffraction, and chromatic dispersion blur will occur when the HOE is illuminated with a broadband light. To address this issue, the simplest solution is to use narrowband light sources such as lasers to illuminate the HOEs [19]. But lasers have problems of speckle noise and potential safety hazards to the human eye. In addition, the laser is expensive. Another solution is to restore the images generated by HOE through image processing methods such as convolution filtering to optimize their clarity [20]. These methods not only require complex structures, but also have low light energy efficiency.

In this paper, a kind of multi-depth imaging HUD using broadband composite holographic elements is proposed. A composite holographic combiner (CHC) with functions of holographic lens (HOEL), holographic mirror (HOEM), and holographic diffuser (HOED) is designed and fabricated to project the multi-depth driving information onto different depth planes. A composite dispersion compensation element (CDCE) is designed and fabricated to compensate the dispersion of CHC. The principles of dispersion compensation are detailed. A multi-depth imaging HUD is fabricated and tested. 3D images in near, middle, and far depth planes are displayed in high resolution simultaneously.