Xiaohu Guo

Imaging and image restoration of an on-axis three-mirror Cassegrain system with wavefront coding technology

Wavefront coding (WFC) technology is adopted in the space optical system to resolve the problem of defocus caused by temperature difference or vibration of satellite motion. According to the theory of WFC, we calculate and optimize the phase mask parameter of the cubic phase mask plate, which is used in an on-axis three-mirror Cassegrain (TMC) telescope system. The simulation analysis and the experimental results indicate that the defocused modulation transfer function curves and the corresponding blurred images have a perfect consistency in the range of 10 times the depth of focus (DOF) of the original TMC system. After digital image processing by a Wiener filter, the spatial resolution of the restored images is up to 57.14 line pairs/mm. The results demonstrate that the WFC technology in the TMC system has superior performance in extending the DOF and less sensitivity to defocus, which has great value in resolving the problem of defocus in the space optical system.

Research on the controlling thermal defocus aberration for the infrared imaging system based on wavefront coding

We describe the application of wavefront coding technique for infrared imaging system to control thermal defocus. For traditional infrared imaging system, athermalization is necessary to maintain imaging performance which may increase complexity and cost of the imaging system. Wavefront coding includes a phase mask at the pupil which can re-modulate wave front so as to produce an encoded image. After digital processing, the system is insensitive to defocus. In this paper, the combination of wavefront coding technique and infrared imaging system has been discussed. We report here the optic design of the wavefront coding IR system based on Zemax. The phase mask is designed to ensure that the modulation transfer function (MTF) is approximately invariant in the range of working temperature. Meanwhile, we designed three IR systems to put up contrast experiments. System one and two are designed to compare the influence before and after the insertion of phase mask. System three is designed to compare the imaging performance before and after reducing lens in wavefront coding IR system. The simulation results show that the infrared imaging system based on wavefront coding can control thermal defocus in a temperature varying from -60ºC to 60 ºC, at the same time the weight and cost of optical elements are reduced by approximately 40%.

Design of TMC wavefront coding system based on user defined surface mask plate

In wavefront coding optical system, with the traditional cubic mask plate (CMP) which owns several extension times of depth of focus (DOF), it is difficult to manufacture. With the symmetrical surface type mask plate which can be machine relatively easily, but it presents small multiples of the extension of DOF. In this paper, it makes a presentation of mask plate design by user defined surface(UDS) type where it has an easy mechanical process and several multiples of extension of DOF. It presents the analytic expression by calculation and its DLL data file which is adapted to ZEMAX. It also performs a simulation experiment based on the three mirror Cassegrain(TMC) wavefront system. The experiment results indicate that instead of the traditional mask plate, the UDS surface mask plate can obtain a larger ratio of extension of DOF and increase the valuable types of mask plate surfaces. What is more, it decreases the surface mechanical difficulty compared to the asymmetrical surface mask plate. The type of UDS surface has the unique design and the convenience manufacturing, which is of great value in both application and research.

Design of a continuously zoom optical system

With the majority needs of the various observation, such as the public security system including security and traffic monitoring system, we can use the optical system, which can not only satisfy the wide field observation, also meet the needs of monitoring. The continuous zoom optical system, which can realize the wide field of monitoring and remain off the accurate detection would meet the requirements.

Terahertz wave polyethylene lens based on wave front phase modulation

Terahertz(THz) wave modulator technology, due to its important value of imaging and detecting research. In the paper, we discuss the Polyethylene lens based the terahertz wave front modulation, which is benefit the terahertz wave image technology. Simulation results show that the optical system can extent the depth of imaging field of test objects based on continuous terahertz source. The way to get the image has significant meaning for detection and large image quality.

Design and implementation of a large depth-of-field and large aperture optical system

In this paper, it presents a new imaging technology that could realize to extent the depth of field and contribute to receive the large iris aperture optical system. The new technology combines lenses assistance and the aberration modulation. Based on the method, we establish a new optical system. Simulation results indicate that the modulation transfer function (MTF) curves of this optical system have consistency feature during different object distances. According to the feature, we can restore ambiguous images to unambiguous ones. Compared with another normal optical system, the experimental results indicate that the new optical system has the large depth of field and large iris aperture features.

Imaging and image restoration of lens-combined modulated wavefront coding

In this paper, it presents a new wavefront coding (WFC) technology that we call lens-combined modulated wavefront coding (LM-WFC). Based on the LM-WFC design method, we establish a large depth of field and large aperture system. Simulation results indicate that the modulation transfer function of the system has a defocus invariant characteristic, which all typical WFC systems have, with a large depth of field. Experimental results for comparing the LM-WFC imaging system and a traditional imaging system are presented. The comparative results demonstrate that the LM-WFC system has an imaging performance invariant over a wide depth of field range.

Analysis of wavefront coding imaging with cubic phase mask decenter and tilt

This paper has examined how the decenter and tilt of a cubic phase mask plate influence the imaging of a wavefront coding system. The calculated phase term of pupil function with mask decenter and tilt indicates that both decenter and tilt change the shape of the system modulation transfer function in a predictable way by changing the phase and defocus parameters. Simulation in an on-axis three-mirror Cassegrain system is presented to confirm the calculated formula result. Experimental results for mask decenter are also presented. The results demonstrate that the decenter of a phase mask has less effect on the point spread function in the z direction than the x and y directions.

Research on signal-to-noise ratio characteristics and image restoration for wavefront coding

Wavefront coding, a technique of optical-digital hybrid image, can be used to extend the depth of the field. However, it sacrifices the signal-to-noise ratio (SNR) of system at a certain degree, especially on focus situation. The on-focus modulation transfer function (MTF) of wavefront coding system is much lower than that of generally traditional optical system. And the noise will be amplified in the digital image processing. This paper analyzes characteristics of the SNR of the wavefront coding system in the frequency domain and calculates the rate of noise amplification in the digital processing. It also explains the influence of the image detector noise severely reducing the restored quality of images. In order to reduce noise amplification in the process of image restoration, we propose a modified wiener filter which is more suitable for restoration in consideration of noise suppression. The simulation experiment demonstrates that the modified wiener filter, compared with traditional wiener filter, has much better performance for wavefront coding system and the restored images having much higher SNR in the whole depth of the field.

The analysis and design of multiple phase plane for wave front coding system

For large depth of field and large aperture optical system, we design a novel optical system based on a structure of large aperture wave front coding (LAWFC). According to optical aberration theory and information optics theory, we also establish the theoretical model of large aperture wave front coding structure. Applied with the large aperture wave front coding, the optical structure overcomes the contradiction between extension depth of field and size of aperture, and the images have excellent consistency in each field of view. Within the simulation, it shows that the MTF curve of the large aperture wave front coding can achieve 90% consistency in the range of depth of field. Compared with the traditional wave front coding, the experience also shows that not only the depth of field and the size of aperture are extended, but also the LAWFC can significantly improve the illumination and the contrast of image plane, and restrain chromatic aberration. It is concluded that the novel structure can expand not only the depth of field but also the size of aperture.