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.
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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