Zhongsheng Zhai

Bio: Zhongsheng Zhai is an academic researcher from Hubei University of Technology. The author has contributed to research in topics: Optics & Holography. The author has an hindex of 5, co-authored 22 publications receiving 91 citations.

Extended depth of field through an axicon

Abstract: One of the main disadvantages of imaging systems is the limited depth of field. We present the ability of a refractive axicon for imaging with an extended depth of field. We design an imaging system with a two-step imaging approach that uses a CCD camera to capture intermediate images and uses a digital process to obtain the final images. The depth of field is analyzed based on the condition of the focal segment. In particular, the point spread functions (PSFs) are discussed in simulation in detail. The performed experiments validate the effect and feasibility of the axicon for imaging with an extending depth of field.

Optical measuring system with multiple degrees of freedom

Abstract: The invention discloses an optical measuring system with multiple degrees of freedom, which can be applied to the measurement of bidimensional small angle (or micro-displacement). The system comprises a laser transmitter, an error sensing unit, a photoelectricity receiving unit and a signal processing unit. The error sensing unit comprises two right-angle prisms (2) and (3); the photoelectricity receiving unit comprises a photoelectricity receiving device (5) and a collimating field lens (4); and when the displacement is measured, the collimating field lens (4) is removed. The photoelectricity receiving device (5) is placed on a focal plane of the collimating field lens (4); after reference beam transmitted by a semiconductor laser (1) is reflected by the right-angle prisms (2) and (3), the facula of the reference beam is imaged on the photosensitive surface of the photoelectricity receiving device (5) through the collimating field lens (4), wherein the semiconductor laser (1) and the photoelectricity receiving unit are arranged in the fixed positions; and the error sensing unit is clamped by a clamp and arranged on a measured object and synchronously moves along the measured object. Optical path of the system in the measuring process is constant, thereby eliminating the influence of different diameters of far and near optical spots and beams caused by the divergent angle of laser. Through the displacement amount of the facula on the photosensitive surface, the change of the small angle (or the micro-displacement) of the measured object in the moving process is calculated. The error sensing unit can be arranged on a guide rail, a worktable and other devices, realizes the real-time dynamic measurement of the angle (or displacement), has large measuring range and high precision and can be widely applied in production, detection and other fields.

Tunable Axicons Generated by Spatial Light Modulator with High-Level Phase Computer-Generated Holograms

Abstract: Axicon is an interesting optical element for its optical properties. This paper presents an approach to dynamically generated tunable axicons with a spatial light modulator (SLM). 256-level phase computer-generated holograms (CGHs) were loaded into the SLM to simulate the positive and negative axicons. The intensity distributions of beams passing through these axicons were analyzed with the principle of blazed grating and Fresnel diffraction; and the diffraction patterns were obtained theoretically in terms of zero-order Bessel beams and annular hollow beams, corresponding to the positive and negative axicons, respectively. Experimental results verified that the diffraction patterns have the same distribution as the real axicon. The types of the axicon and the axicon’s parameters can be easily altered through changing the CGHs.

Accommodation-invariant computational near-eye displays

Abstract: Although emerging virtual and augmented reality (VR/AR) systems can produce highly immersive experiences, they can also cause visual discomfort, eyestrain, and nausea. One of the sources of these symptoms is a mismatch between vergence and focus cues. In current VR/AR near-eye displays, a stereoscopic image pair drives the vergence state of the human visual system to arbitrary distances, but the accommodation, or focus, state of the eyes is optically driven towards a fixed distance. In this work, we introduce a new display technology, dubbed accommodation-invariant (AI) near-eye displays, to improve the consistency of depth cues in near-eye displays. Rather than producing correct focus cues, AI displays are optically engineered to produce visual stimuli that are invariant to the accommodation state of the eye. The accommodation system can then be driven by stereoscopic cues, and the mismatch between vergence and accommodation state of the eyes is significantly reduced. We validate the principle of operation of AI displays using a prototype display that allows for the accommodation state of users to be measured while they view visual stimuli using multiple different display modes.

Depth recovery and refinement from a single image using defocus cues

Abstract: We present a technique to recover and refine the depth map from a single image captured by a conventional camera in this paper. Our method builds on the universal imaging principle: only scene at the focus distance will converge to a single sharp point on imaging sensor but other scene will yield different blur effects varying with its distance from the camera lens. We first estimate depth values at edge locations via spectrum contrast and then recover the full depth map using a depth matting optimization method. Due to the fact that some blur textures such as soft shadows or blur patterns will produce ambiguity results during the procedure of depth estimation, we use a total variation-based image smoothing method to smooth the original image, a smoothed image with detailed texture being suppressed can be generated. Taking this smoothed image as reference image, a guided filter is used to refine the final depth map.

Modern Types of Axicons: New Functions and Applications.

Abstract: Axicon is a versatile optical element for forming a zero-order Bessel beam, including high-power laser radiation schemes. Nevertheless, it has drawbacks such as the produced beam’s parameters being dependent on a particular element, the output beam’s intensity distribution being dependent on the quality of element manufacturing, and uneven axial intensity distribution. To address these issues, extensive research has been undertaken to develop nondiffracting beams using a variety of advanced techniques. We looked at four different and special approaches for creating nondiffracting beams in this article. Diffractive axicons, meta-axicons-flat optics, spatial light modulators, and photonic integrated circuit-based axicons are among these approaches. Lately, there has been noteworthy curiosity in reducing the thickness and weight of axicons by exploiting diffraction. Meta-axicons, which are ultrathin flat optical elements made up of metasurfaces built up of arrays of subwavelength optical antennas, are one way to address such needs. In addition, when compared to their traditional refractive and diffractive equivalents, meta-axicons have a number of distinguishing advantages, including aberration correction, active tunability, and semi-transparency. This paper is not intended to be a critique of any method. We have outlined the most recent advancements in this field and let readers determine which approach best meets their needs based on the ease of fabrication and utilization. Moreover, one section is devoted to applications of axicons utilized as sensors of optical properties of devices and elements as well as singular beams states and wavefront features.

Focus free terahertz reflection imaging and tomography with Bessel beams

Abstract: A broadband terahertz (THz) reflection imaging system with high spatial resolution over a large depth of field is reported. A THz axicon lens producing a truncated THz Bessel beam with a linear focus exceeding 120 mm in length, and with the diameter of the central lobe less than 2 mm was used for imaging in a reflection geometry employing a pulsed THz time-domain spectrometer. With numerical post-processing, it was possible to reconstruct the three dimensional shape of the scanned object.