Holographic optical element (HOE) are widely used due to their lightweight, miniaturization, and multifunctional characteristics. However, making HOE that can be applied in the infrared band is a challenging task, especially when HOE has multiple function. This paper proposes a design and fabrication method for a near-infrared multifunctional HOE that can be recorded using visible light. The three recording beams of this multifunctional HOE include a plane beam and two spherical beams. Therefore, on the basis of achieving beam redirection, it can also achieve beam splitting and the convergence of each split beam. To solve the problem of wavelength mismatch while achieving multiple functions of HOE. The decrease in diffraction efficiency caused by wavelength shift is compensated by the angle shift of recording light. The wavefront aberration caused by wavelength mismatch is compensated by introducing a cylindrical lens in the recording optical path. The multifunctional HOE has the potential to be used in infrared optical systems to achieve system miniaturization, and this paper provides an application example for laser Doppler velocimeter (LDV). The designed HOE successfully achieved two sets of stripes with opposite slopes in the measurement volume of LDV at different working wavelengths, proving the feasibility of the design and fabrication methods. Furthermore, the method proposed in this paper can provide reference for the design and fabrication of other HOEs working in the near-infrared band.
Holographic optical element (HOE) have been widely applied in many fields. However, the fixed focal length limits its application in dynamic optical systems. The existing methods have the problems of limited adjustment distance and difficulty in correcting the aberration during the adjustment. This paper proposes a method that can correct the aberration while adjusting the HOE focal length in a large range. This method modulates the illumination wave front by superposing the modulation phase factor to achieve the change of the reconstruction distance, that is, the change of the focal length of the HOE. To design modulation phase factors corresponding to different focal lengths, the relationship between the parameters of the modulation phase factor and the focal length of HOE was derived based on the Fresnel diffraction integral formula. To solve the problem of the aberration cannot be uniformly compensated at different focal length, aberrations of different types, sources, and reconstruction distances are compensated separately. The method proposed in this paper achieves a wide range of adjustment of HOE focal length and correction of aberration during the adjustment process. In the experiment, the focal length of HOE is adjusted to 30cm, 40cm, and 50cm, and the evaluation functions of aberrations have changed towards the direction of aberration reduction. This method can be used for the design and fabrication of adjustable HOE, which is expected to be used in holographic projection, structured light generation and many other fields.
Considering the different needs of the driver and passengers for image information when a vehicle equipped with a headup display (HUD) is driving, we propose a multi-functional dual-view compact HUD with both privacy protection and information sharing. The dual viewing angle display is achieved by stacking two layers of holographic optical elements (HOEs) with different diffraction angles due to the excellent angular selectivity of HOEs. Red-green multiplexing HOE1 and red-blue multiplexing HOE2 correspond to viewing angle 1 and viewing angle 2, respectively. The different diffraction angles of the green and blue images ensure the privacy of the driver and passengers, allowing for customizable displays. Moreover, the two diffraction angles of the red image ensure the sharing of warning information between the driver and the passenger, and improve the driving safety. Furthermore, the proposed HUD requires only one picture generation unit (PGU) and one holographic combiner and thus has a compact structure. The experimental results verify the feasibility of this method. The size of the fabricated HOEs was 20 cm×15 cm. The field of view (FOV) and eye-box (EB) of the two viewing zones are 10°×5° and 8°×4°, 147mm×81mm and 105mm×87mm, respectively.
An augmented reality (AR) head-up display (HUD) system based on holographic optical elements (HOEs) with multiple depths, large area, high diffraction efficiency and a single picture generation unit (PGU) is proposed. Since HOEs has excellent wavelength selectivity and angle selectivity, as well as the ability to modulate complex light waves, HOEs can image the red, green and blue parts of the color image emitted by PGU on different depth planes. The experimental results show that the three HOEs of red, green, and blue clearly display images at different depths of 150cm, 500cm, and 1000cm, and the diffraction efficiencies are 75.2%, 73.1%, and 67.5%, respectively. The size of HOEs is 20cm×15cm. In addition, the field of view (FOV) and eye-box (EB) of the system are 12°×10° and 9.5cm×11.2cm.
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