Shouqian Chen

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.

Optimized square-root phase mask to generate defocus-invariant modulation transfer function in hybrid imaging systems

Abstract. Wavefront coding involves the use of an asymmetrical phase mask to extend the depth of field of incoherent imaging systems. The performance of wavefront coding systems depends on designing a suitable phase profile to generate the defocus-invariant imaging characteristic. We proposed a square-root phase mask with two profile factors for achieving a steadier defocused modulation transfer function (MTF). Several evaluation methods are employed for the purpose of performance comparison between the proposed phase mask and the previously suggested phase masks under the constraint condition that the phase parameters are optimized at the same level of noise gain. Numerical results show that the square-root phase mask yields better properties in extended depth of field imaging, especially in acquiring defocus-invariant MTFs and eliminating image artifacts associated with the decoded images.

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.

Multitarget compounding technique based on dimpled mirror

Abstract. The multitarget compounding technique is key in an infrared hardware-in-the-loop simulation system. In this study, the beam-broadening theory and carefully designed experiments are presented to analyze a multitarget compounding system based on a dimpled mirror. By paraxial approximation of geometrical optics, the expression of broadened beamwidth of a dimpled-mirror two-mirror three-reflection concentric system was obtained as the function of system parameters. A multitarget compounding setup was designed and experiments were performed. Excellent matching results were achieved between the beamwidth theory and the experiments. Experimental results demonstrated that the beam-broadening theory is creditable and suitable to design the multitarget compounding system. The theory and experiments are beneficial to the future development and implementation of multitarget compounding technique.

Nonaxial Strehl ratio of wavefront coding systems with a cubic phase mask

An approximate expression of the peak position of the point-spread function (PSF) of wavefront coding systems with a cubic phase mask (CPM) is derived and verified by simulation results. An approach called the nonaxial Strehl ratio (NASR) is used to evaluate the performance of wavefront coding systems with defocus aberrations. The characteristics of the NASR are investigated. The relationships between the NASR and the similarity of out-of-focus modulation transfer function (MTF) and recoverability of blurred encoding images are presented, and some useful guidelines are given for the optimization of phase mask parameters according to these relationships.

Extended depth-of-field imaging through radially symmetrical conjugate phase masks

Abstract. We proposed a radially symmetrical conjugate phase mask (PM) pair to yield an invariant imaging property for extending depth-of-field imaging. This conjugate PM pair is a two-radially symmetrical phase function with opposite orientation of the phase modulation. Compared with a single-radially symmetrical PM, the proposed conjugate PM pair shows a symmetrically imaging property on both sides of the focal plane and high magnitude of modulation transfer function (MTF). The quartic phase mask (QPM) with optimized phase parameters is employed to demonstrate our concept. Several evaluation approaches, including point-spread function, MTF, and image simulation, are used to realize the performance comparison among a traditional imaging system, an original QPM system, and a conjugate QPM. The results are proof that the proposed conjugate PM has a superior performance in extending depth of field imaging.

Autofocus algorithm based on Wavelet Packet Transform for infrared microscopy

A new autofocus algorithm based on Wavelet Packet Transform (WPT) was presented to find the best focus of infrared microscopy, which is applied to test the defocusing amounts of athermalized infrared optical system. According to the statistical analysis of the gradient of image, the focus function was obtained by weighting the decomposition coefficients of wavelet packet. Experimental results demonstrated the algorithm had high sensitivity and accuracy for the true focus position from a series of infrared images. Furthermore, the autofocus algorithm was compared with some well-know algorithm like the Sobel Gradient(SG), the Sum-Modified Laplacian(SML) and the Discrete Cosine Transform(DCT) method in order to prove its performance.

Impact of the temperature gradient on optical system parameters: modeling and analysis

A simplified mathematical model was proposed to describe the relationship between image quality and temperature gradient. It can be used to determine the maximal allowable temperature gradient in which optical system was applicable, as well as the applicability of optical system in given temperature gradient condition. Firstly, under some assumption conditions, the thermal deformation of optical surface and refractive index distribution caused by temperature gradient distribution were analyzed, and a simplified mathematical model was built to describe the impact of radial temperature gradient on optical parameters. Secondly, the model was validated by using finite element analysis software analyze an infrared lens. Finally, an application example was given. The image quality of a refractive infrared optical system in thermal environment with radial temperature gradient was analyzed by using the mathematical model proposed above and optical design software.

To enhance imaging performance of hybrid imaging systems by using two asymmetrical phase masks

We propose the use of two asymmetrical phase masks combined with the subtracted imaging method to enhance the signal-to-noise ratio in wavefront coding systems. This subtracted imaging technique is similar to the variable pinhole diameter in confocal microscopy. Two different phase modulations of same phase masks are employed to promote the magnitude of the optical transfer function (OTF). The ratio factor is used to control the phase variation between two phase masks. The noise of decoded images is suppressed owing to the higher magnitude of the OTF than the wavefront coding systems with a phase mask. A tangent phase mask as an example is used to demonstrate our concept. Simulated results show that the performance promotion controls noise amplification of decoded images while maintaining a depth-of-field extension.