Zhaoqi Wang

Optical switching property of a Ce:KNSBN photorefractive volume grating controlled by the readout beam polarization

The theoretical model of a photorefractive optical switch, which is implemented by adjusting the polarization of the readout beam to control the diffraction efficiency of volume grating in a Ce:KNSBN crystal, is proposed. The experimentally measured diffraction efficiency with an extraordinarily polarized readout beam is about 20 times that with an ordinarily polarized readout beam, which agrees with the theoretical prediction and indicates that Ce:KNSBN possesses excellent optical switching properties.

Investigation of aberration characteristics of eyes at a peripheral visual field by individual eye model

We propose a method of constructing an individual eye model with a large visual field, and then investigate aberration characteristics of eyes in peripheral fields with constructed models. Twelve eyes of different aberrations are selected from 89 myopic eyes. It is shown that astigmatism increases as visual field in a quadratic manner. The variation tendency of defocus can be expressed by the cubic curve for 50% of eyes. For most of the eyes, the variation of spherical aberration shows a quadratic rule within ±24° visual field. Coma exhibits obvious individual differences. The impact of high-order aberrations on vision is mainly at a smaller visual field, and it becomes negligible beyond 24° visual field.

Statistical characteristics of aberrations of human eyes after small incision lenticule extraction surgery and analysis of visual performance with individual eye model

Preoperative and postoperative wavefront aberrations of 73 myopic eyes with small incision lenticule extraction surgery are analyzed in this paper. Twenty-eight postoperative individual eye models are constructed to investigate the visual acuity (VA) of human eyes. Results show that in photopic condition, residual defocus, residual astigmatism, and higher-order aberrations are relatively small. 100% of eyes reach a VA of 0.8 or better, and 89.3% of eyes reach a VA of 1.0 or better. In scotopic condition, the residual defocus and the higher-order aberrations are, respectively, 1.9 and 8.5 times the amount of that in photopic condition, and the defocus becomes the main factor attenuating visual performance.

Real-time edge-enhanced optical correlation via four-wave mixing in a cerium-doped potassium sodium strontium barium niobate crystal

Degenerate four-wave mixing in a cerium doped potassium sodium strontium barium niobate photorefractive crystal is studied. The transmission grating is dominated in our experimental geometry that the bisector of the pump beam and the signal beam is normal to the c-axis of the crystal. As the fringe modulation depth of this transmission grating varies, the variation of the conjugated beam intensity forms a loop. Based on this nonlinear property, real-time optical image edge-enhancement processing as well as real-time edge- enhanced optical correlation is obtained without the reverse of the signal-to-pump beam intensity ratio. The full-width- at-half-maximum intensity of the auto-correlation peak is one fifth of that without the edge-enhancement operation. The computer simulation results are given, which are coincident with the experimental data very well.

Real-time multichannel intensity correlator

Abstract An intensity optical correlator with liquid crystal light valve (LCLV) as the real time input device is studied. The in-plane displacement invariance of the Fourier transform holographic filter (FTHF) and the rotating diffuser in the optical system are the key points to realize multichannel optical pattern recognition without multi-focus holographic lens. By adjusting the work state of the LCLV, the optical intensity correlation of the differential images could be completed. An experimental result of four channel intensity correlation with this system is presented.

A new method to accurately assess post-laser-surgery refractive changes with the intrinsic corneal power changes.

To evaluate the ability of assessing the refractive change induced by myopic laser surgery using the intrinsic data, obtained from a reference intrinsic to the cornea. Twenty-nine eyes with laser in situ keratomileusis for myopia were included in the study. Statistical analyses were performed to determine the relationship between refractive changes and two kinds of corneal power: KIP_value (obtained by keratometric index of 1.3375 and radius of curvature) and RIP_value (obtained by real corneal refraction index of 1.376 and radius of curvature). The comparisons were made between results given by intrinsic data and Pentacam HR. Preoperatively, the intrinsic radii of curvature are significantly less (p≪0.001) than those from Pentacam HR, while the intrinsic radii of curvature are significantly larger (p≪0.001) after surgery. The corneal power changes given by Pentacam HR always underestimate the refractive changes. However, the intrinsic data generally reduce the discrepancies between the corneal power and the refractive changes. The intrinsic KIP_value correlated best with the refractive changes at the corneal plane (p≪0.001) with a prediction error as low as 1.51%. The use of intrinsic data of the cornea can possibly provide an accurate estimation of surgically induced refractive changes.

Construction of special eye models for investigation of chromatic and higher-order aberrations of eyes.

An achromatic element eliminating only longitudinal chromatic aberration (LCA) while maintaining transverse chromatic aberration (TCA) is established for the eye model, which involves the angle formed by the visual and optical axis. To investigate the impacts of higher-order aberrations on vision, the actual data of higher-order aberrations of human eyes with three typical levels are introduced into the eye model along visual axis. Moreover, three kinds of individual eye models are established to investigate the impacts of higher-order aberrations, chromatic aberration (LCA+TCA), LCA and TCA on vision under the photopic condition, respectively. Results show that for most human eyes, the impact of chromatic aberration on vision is much stronger than that of higher-order aberrations, and the impact of LCA in chromatic aberration dominates. The impact of TCA is approximately equal to that of normal level higher-order aberrations and it can be ignored when LCA exists.

Optical design of head-mounted projective display for vein imaging

A novel head-mounted projective display (HMPD) for vein imaging is proposed in this paper. The correspondent optical system is designed, which consists of imaging system and see-through HMPD. The infrared imaging system has a large numerical aperture with F number of 2.6, which is extremely conducive to receive and to image the weak reflected near-infrared light from the patients skin surface. The resolution of the imaging system is QXGA (2048 × 1536). The see-through HMPD possesses an 18mm exit pupil and a 25mm eye relief, with ultra-light in weight, compact in structure and comfort in wearing. Meanwhile, it has a satisfactory performance for vein imaging that the field curvature is less than 0.03D, the distortion is less than 0.32% and the resolution is up to QXGA. The structure of the optical system of HMPD is consistent to that of infrared imaging part, which is conducive to simplify the processing and assembly in manufacture. The proposed visual instrument for vein display is suitable for supplementing existing venipuncture techniques.

Two-wave mixing in a Ce:KNSBN crystal with incoherent erasing technique

Two-wave mixing properties of a Ce:KNSBN photorefractive crystal is experimentally studied while the ihcoherent erasing technique is applied. As the signal beam is with extraordinary polarization and the polarized direcitn of the pump beam has an angle φ with its extraordinarily polarized direction, the gain of the signal beam is measured with respect to the angle φ. Because of the erasing of the fanning by the ordinary component of the pump beam, the gain enhances at small angle φ and obtains its maximum at φ = 30°. The couple-wave theory with considering of the fanning effect is used to fit the experimental data and the fitting curve is coincident wiht the experimental data very well. The calculated results of the gain as a fucntin of the angle φ for different fanning factor are given out. Finally the enhancement of the gain is experimentally measured with respect to the angle between the signal beam and the normal of the crystal face. While this angle takes θ=11°, the enhancement of the gain obtains its minimum value, which means the minimum fanning.

Real-time optical correlator using four-wave mixing in BSO crystal and moving gratings

Based on the nonlinear property of the moving grating in BSO crystal in four-wave mixing architecture at large fringe modulation, i.e. the enhancement of the reflectivity increases as the incident beam ratio increases and the grating with large pump beam ratio can obtain higher enhancement of the reflectivity, we achieve edge-enhancement and edge-enhanced optical correlation of a binary optical image by applying the moving grating in the Fresnel transform four-wave mixing system. The relative intensity of the edge of the object is enhanced nearly 2 times. The Full- Width-at-Half-Maximum of the auto-correlation peak and the fluctuation noise if obviously suppressed, which indicate that a significant improvement in the discrimination capability of the correlator is achieved.