As a method of near-field diffraction in the condition of the paraxial approximation, the Fresnel convolution (FR-CV) method is widely used in hologram generation and other applications. However, it is applicable to near-field diffraction, and the quality of holographic reconstruction degrades seriously with the increase of diffraction distance. Moreover, its hologram generation speed is limited due to the use of three fast Fourier transforms in the convolution operation. Nevertheless, there are also many application scenarios that need longer distance diffraction. To achieve a holographic display in broadened distance with high generation speed and reconstruction quality, an optical computational Fresnel convolution method is proposed in this paper. Since an optical Fourier lens is used to perform optical calculations for Fourier transforms in our proposed method, the hologram generation speed of the proposed method is approximately 8 times faster than that of the FR-CV method. Moreover, the reconstructed image with our proposed method can be successfully and clearly displayed at both short and longer diffraction distance by changing focal lengths of the Fourier lens. The effectiveness and superiority of the proposed method have been validated by both numerical simulations and optical experiments.
Cylindrical holography, as a promising 360° display technology, has already attracted a lot of attention. In a previous study, an optical 360° cylindrical holography has been achieved in the visible spectrum using a planar spatial light modulator (SLM) and a 45° conical mirror. Although the 360° viewing zone is successfully achieved in the horizontal direction, in the previous study, the vertical viewing zone remains as narrow as the planar holography, and its expansion is not only necessary but also potential due to the waste of vertical viewing zone in application scenarios such as tabletop and ceiling. In this paper, we propose a method of expanding the vertical effective viewing zone for optical 360° holographic display by using a conical mirror with a base angle of less than 45°. The proposed method can expand the vertical effective viewing zone by shifting the wasted vertical viewing zone into an effective vertical viewing zone from the base to the top angle direction of the conical mirror, which is up to two times theoretically. The feasibility and effectiveness of the proposed method are demonstrated by optical experiments. We believe that it would be promising in the field of augmented reality.
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