Wang Zhang

Design of fixed correctors used in conformal optical system based on diffractive optical elements.

A conformal dome was designed and the aberration characteristics of the dome were analyzed using Zernike aberration theory. By deriving the equation used to correct Zernike aberrations, the phase coefficients and the phase orders of diffractive optical elements (DOEs) used to correct primary Zernike aberrations were obtained. DOEs were simulated to correct the aberrations of the conformal dome by using optical design software, and the aberrations of the conformal dome decreased dramatically. Finally, a complete cooled conformal optical system was designed. The results show that the number of the fixed correctors elements decreases by using DOEs, and the optical system has better imaging quality.

Photonic bandgaps of different unit cells in the basic structural unit of germanium-based two-dimensional decagonal photonic quasi-crystals.

Based on the infrared optical material germanium, in the basic structural unit of a two-dimensional decagonal photonic quasi-crystal, photonic bandgaps of four square unit cells with a scattering radius in the range of [0,0.3a] have been calculated within two cases of construction (i.e., air cylinders arranged in germanium and germanium cylinders arranged in air) by using the plane wave expansion method. In considering the Bragg-like scattering effect in two-dimensional photonic quasi-crystals as the elastic collision in physics, we put forward the photonic bandgap impact function F=q(1)q(2)q(3)επr(2) for the first time, to the best of our knowledge. A certain unit cell structure shares some similar photonic bandgap properties with a periodic structure. For a certain structure of the unit cell, the center frequency change trends of the photonic bandgap and the type of photonic bandgap generated are not related with the period of the photonic crystal, but with the relative dielectric constant and the construction, respectively. Different unit cell structures own different photonic bandgap structures. This occurs because the high degree of rotational symmetry of the quasi-periodic structure and weak long-range order of the basic structural unit lead to different Bragg-like scattering effects within the unit cell structures.

Focusing properties of two-dimensional photonic quasicrystal flat lens at different object distances

We report the object-distance effect on the focusing properties of a two-dimensional (2-D), photonic quasicrystal (PQC) flat lens by numerical simulation. For the image power and image quality, the half-thickness of the flat lens is no longer the optimal object distance. The object-image distance is not always double the flat lens thickness, which is apparently different from a 2-D, periodic photonic crystal (PPC) flat lens and left-handed material (LHM) flat lens. With the point-light source located near half-thickness of the lens object distance, a 2-D PQC flat lens can achieve a perfect image in a certain range. With an appropriate object distance, a 2-D PQC flat lens is much better than a 2-D PPC flat lens in the aspect of imaging resolution. The results are beneficial to understand the focusing and imaging of a 2-D PQC flat lens.

Comprehensive analysis of imaging quality degradation of an airborne optical system for aerodynamic flow field around the optical window

We investigated the influences exerted by the nonuniform aerodynamic flow field surrounding the optical window on the imaging quality degradation of an airborne optical system. The density distribution of flow fields around three typical optical windows, including a spherical window, an ellipsoidal window, and a paraboloidal window, were calculated by adopting the Reynolds-averaged Navier-Stokes equations with the Spalart-Allmaras model provided by FLUENT. The fourth-order Runge-Kutta algorithm based ray-tracing program was used to simulate the optical transmission through the aerodynamic flow field. Four kinds of imaging quality evaluation parameters were presented: wave aberration of the entrance pupil, point spread function, encircled energy, and modulation transfer function. The results show that the imaging quality of the airborne optical system was affected by the shape of the optical window and angle of attack of the aircraft.

Joint influences of aerodynamic flow field and aerodynamic heating of the dome on imaging quality degradation of airborne optical systems.

We investigated the joint influences exerted by the nonuniform aerodynamic flow field surrounding the optical dome and the aerodynamic heating of the dome on imaging quality degradation of an airborne optical system. The Spalart-Allmaras model provided by FLUENT was used for flow computations. The fourth-order Runge-Kutta algorithm based ray tracing program was used to simulate optical transmission through the aerodynamic flow field and the dome. Four kinds of imaging quality evaluation parameters were presented: wave aberration of the exit pupil, point spread function, encircled energy, and modulation transfer function. The results show that the aero-optical disturbance of the aerodynamic flow field and the aerodynamic heating of the dome significantly affect the imaging quality of an airborne optical system.

Self-thermal radiation compensation for autofocus algorithm in infrared optical system

Self-thermal radiation is a key factor of reducing image quality of infrared optical system and can cause the autofocus process inefficient. A study on infrared images affected by self-thermal radiation arising from temperatures of system under test was presented to optimize autofocus algorithm. Experimental results showed that conventional autofocus functions like Gray-level Variance(GV), Laplacian of Gaussian (LoG) Operator and Discrete Cosine Transform(DCT) methods had low accuracy and sensitivity for determining the optimal imaging plane from a series of infrared images, when the optical system worked in high temperature environments. Integrating self-thermal radiation compensation algorithm to correct the image data, we put forward a new autofocus algorithm. The autofocus function was based on wavelet packet transform to extract high-frequency information from infrared images, and performances of the new algorithm were proved by Athermalization-Efficiency Assessment System.