In order to realize the superresolution imaging with a telescope optical system, a two- dimensional polynomial function phase pupil filter with a high strehl ratio is designed, and its improvement for the light distribution around the focal point of the optical imaging system is demonstrated. The result of the theory analysis shows that, when the filter is added into the system, the system optical resolution is increased 1.33 times, and the strehl ratio is 0.75 time as much as that of the system without this pupil filter. The performance of two-dimensional polynomial filter is compared with the other typical phase pupil filters, including the three-zone, the four-zone and the one-dimensional polynomial phase filter. The comparison results show that the strehl ratio of the two-dimensional polynomial filter is the highest in these filters, and the transverse superresolution ratio of this filter is next only to that of the four-zone filter. But the strehl ratio of the four-zone filter is too low to be applied into the telescope.
Pupil off-axis optical system has the advantages of imaging unobscured and restraining the stray light for the next generation space telescope. However, non-rotationally asymmetric aberrations lead by the pupil off-axis structure becomes the main influencing factor of imaging quality. Since the freeform surface can provide more degrees of freedom for optical design, it is widely used to control the non-rotationally asymmetric aberrations. In order to guide designer optimize the pupil off-axis optical systems with freeform surfaces efficiently, an aberration analysis method for pupil off-axis optical system with Fringe Zernike polynomial freeform surfaces is proposed. The analysis expressions of third-order astigmatism and coma of the pupil off-axis freeform system are deduced. The freeform impacts on aberration nodal properties are analyzed. According to the aberration distribution, a pupil off-axis space telescope freeform optical system is designed by optimizing the Zernike terms pointedly. The effective focal length is 25m, the pupil diameter is 2m. By stitching multi-detectors to realize the field of view (FOV) of 1.2°×1.2°. Because of introducing freeform surfaces, the third-order astigmatism and coma nodes are both moved back to the FOV. Therefore, the asymmetric aberrations lead by pupil off-axis are balanced. The final imaging quality is closed to the diffraction limit, which meets the requirements.
When the center of the dome inner and outer concentric spherical surfaces and the gimbal joint of the airborne laser communication antenna are not coincidence, the influence of asymmetry dome to communication beam far-field divergence angle for different transmitting angles is analyzed. The simulations shows that, at a maximum off-axis transmitting angle 54°, a K9 spherical dome with a 200mm diameter and 5mm thickness makes the communication beam far-field divergence angle expands to 5.27~6.46mrad from 126μrad. The divergence angle variation purely introduced by the dome manufacturing precision is very tiny, and can be ignored compared the variation introduced by the dome optical power. A lens with spherical surfaces is used to compensate the far-field divergence angle change, and after the compensation the far-field divergence angle of the beam on the whole range of transmitting angles is reduced to no more than 600μrad.
The star sensor is one of the sensors which are used to determine the spatial attitude of the space vehicle. An optical system of star sensor with large aperture and wide field of view was designed in this paper. The effective focal length of the optics was 16mm, and the F-number is 1.2, the field of view of the optical system is 20°.The working spectrum is 500 to 800 nanometer. The lens system selects a similar complicated Petzval structure and special glass-couple, and get a high imaging quality in the whole spectrum range. For each field-of-view point, the values of the modulation transfer function at 50 cycles/mm is higher than 0.3. On the detecting plane, the encircled energy in a circle of 14μm diameter could be up to 80% of the total energy. In the whole range of the field of view, the dispersion spot diameter in the imaging plane is no larger than 13μm. The full field distortion was less than 0.1%, which was helpful to obtain the accurate location of the reference star through the picture gotten by the star sensor. The lateral chromatic aberration is less than 2μm in the whole spectrum range.
The design and analysis of achromatic double-prism scanners for free space laser communication is presented. Double prism canner consists of two independently rotating prisms; the two prisms are parallel and adjacent to each other. Compared with conventional two-axis optical electro-gimbals, the double –prism system has the advantages of high accuracy, compactness and good dynamic performance. But the wavelength of beacon and communication laser is different; the achromatic aberration caused by double prisms will result in alignment error between beacon link and communication link. Design results for the scanner, when used in 800nm and 1550nm demonstrated total fields of view 13 degrees with almost no aberration were presented. This will provide a new lightweight canner for free space laser communication.
According to the transverse intensity distribution of the TEM00 Gaussian light field and character of an irradiance redistribution element, we proposed a novel method, which has the advantages of high light-energy-utilization-ratio (LEUR). The current laser communication (LASCOM) antenna frequently employs the Cassegrain reflective optical structure, in which the secondary mirror will introduce a center obscuration, leading to high ratio of transmitting power loss. To solve this problem, we make the transmitting beam pass through double convex axicons. The Gaussian peak of incident light coming into the central part of such element, will go out from near the peripheral part of the second axicon, and the edge part of Gaussian beam will go out from the central part. When the changed beam passes the Cassegrain structure, the utilizing efficiency will be raised obviously. In the paper, on different obscuration rate, the LEURs of LASCOM system before and after using the axicons are compared. In addition, the far-field intensity distribution of the laser beam changed by the axicon pair and transmitted by the antenna is calculated. The simulation result shows that the LEURs of antenna with and without an axicon pair are 91.7% and 28.9% on a Line obscuration ratio of 1/4. After a propagation of 1000 km, the far-field energy distribution of the hollow beam translated by the high LEUR antenna is closer to a flattop beam than that for the common Gaussian beam.
KEYWORDS: Far-field diffraction, Super resolution, Free space optical communications, Telecommunications, Laser systems engineering, Diffraction, Free space optics
In the free-space laser communication, there is a strong need for a technology that can decrease the size of the diffraction spot in the receiver port, because a smaller diffraction spot in the receive port makes the transmit data more secure. In this paper, instead of the usage of the larger size aperture lens in the free-space laser communication system, we introduce a diffractive superresolution technology that changing the received information laser beam into radially polarized beam which is focused on the detector array. In the paper, firstly, the conversion method of the information natural light which the optical antenna received to the radially polarized beam is discussed in detail. Then, in the focal plane, the transverse intensity distribution expression near the focal point for the radially polarized laser beam are presented, and the numerical simulation results of the intensity distributions around the focal point on different numerical apertures (NA) are given. The full width at half-maximum (FWHM) values of the main lobe are considered for the standard of the spot size. Through a comparison of the focal point FWHM values with the natural light and radially polarized beam, we judge the superresolution performance of the receiver optical system with radially polarized beam on different NA of 0.4, 0.6 and 0.75. We find that the method of focusing with radially polarized beam generates a smaller spot size than the Airy spot size when the NA is no less than 0.6; when the NA reach to 0.75, the resolution is 1.5 times than the diffraction limit. But it will decrease the light power in the process of natural light converted to radially polarized beam. When the communication laser is polarized laser, the energy loss can be reduced to around 20%. This technology can be applied when the laser energy is not the main concern in the communication.
A single view-point catadioptric long infrared panoramic lens was designed with codeV. The system consists of a hyperboloid mirror and a conventional perspective imaging lens. The hyperboloid mirror enables the lens to obtain a large field of view, the conventional perspective imaging lens projects the virtual image formed by the mirror group onto the detector. The effective focal length is 1.4mm,the relative aperture is 1:1, field of view is ±10°~±90° in vertical direction and 360° in horizontal, the wavelength spectrum is 8-12μm, only Znse and Ge were used in this system. The lens has the characteristics of single view-point, large field view and getting perspective image straightly. Result shows that the modulation transformation function at 17/mm is greater than 0.7, which proves that the image quality of the optical system is good.
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