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 corrector's elements decreases by using DOEs, and the optical system has better imaging quality.

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.

Optical distortion evaluation of an aerodynamically heated window, based on the higher-order singular value decomposition with the influence of elasto-optical effect excluded.

The higher-order singular value decomposition (HOSVD) was used to reconstruct the three-dimensional (3D) refractive index field of an aerodynamically heated window. The numerical 3D optical distortion was evaluated for both the reconstructed and the exact refractive index fields of the window, excluding the influence of the elasto-optical effect. The method based on the HOSVD truncation was shown to reduce the refractive index information required to capture the major optical distortion of the window. The refractive index information was reduced by reconstructing the refractive index field of the window using the truncated n-mode singular matrices. The method can also be used to evaluate the optical distortion of the window.

Wavefront coding technique for controlling thermal defocus aberration in an infrared imaging system.

We use a wavefront coding approach to control thermal defocus aberration in an IR imaging system. The design method of athermalized system using a wavefront coding technique is discussed. An athermalized long wave IR optical system, which works at temperatures ranging from -40 °C to 60 °C, is designed by employing a cubic phase mask. Computer simulations and the first experimental demonstration are executed to verify the performance of this wavefront coded athermalized system and to clarify the issues related to its implementation.

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)뵹r(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.

Optical distortion evaluation of an aerodynamically heated window using the interfacial fluid thickness concept.

The interfacial fluid thickness (IFT) concept was used to develop a harmonic-mean refractive index gradient magnitude threshold to retrieve the high refractive index gradient regions of an aerodynamically heated window. The retrieved high-gradient regions were used to reconstruct the refractive index field of the window. The numerical three-dimensional optical distortion evaluation was conducted for both the reconstructed and the original refractive index fields of the window using the ray-tracing program based on a recursive algorithm. Wave aberration results show that the methodology based on the IFT concept reduces the refractive index information required to capture the essential optical distortion of the window. The method can also be used for numerically evaluating the optical distortion of the window.

Imaging quality evaluation of aerodynamically heated optical dome using ray tracing.

An irregular grid model was employed to describe the refractive index distribution of an aerodynamically heated optical dome according to the theories of thermo-optical and elasto-optical effects. Optical transmission through the dome was simulated using the ray-tracing program based on a fourth-order Runge-Kutta algorithm. Two kinds of imaging quality evaluation parameters were presented, wave aberration of the exit pupil and a modulation transfer function. To validate the ray-tracing program, a ray trace through a regular gradient medium was performed. Results were compared with those obtained from the analytic solution. The program was shown to possess great accuracy by using the appropriate parameters.