Lingjie Wang

Design and fabrication of imaging optical systems with freeform surfaces

Freeform surfaces provide more degrees of freedom for design of optical systems, and enhance the ability of compensation and correction aberrations. Freeform surfaces are of advantage to balance the unsymmetrical aberrations, especially for the wide-field off-axis optical systems. This paper focus on an off-axis reflective optical system, which focal length is 550mm, F# is 6.5 and field of view (FOV) is 76°. The system adopts some freeform surfaces. We discuss the problems we noticed in processes of design, manufacture, measurement and alignment, and the solutions. At last, the periodical research result and the expected performance are given.

Fast and accurate focusing analysis of large photon sieve using pinhole ring diffraction model.

In this paper, we developed a pinhole ring diffraction model for the focusing analysis of a large photon sieve. Instead of analyzing individual pinholes, we discuss the focusing of all of the pinholes in a single ring. An explicit equation for the diffracted field of individual pinhole ring has been proposed. We investigated the validity range of this generalized model and analytically describe the sufficient conditions for the validity of this pinhole ring diffraction model. A practical example and investigation reveals the high accuracy of the pinhole ring diffraction model. This simulation method could be used for fast and accurate focusing analysis of a large photon sieve.

Multiregion apodized photon sieve with enhanced efficiency and enlarged pinhole sizes

A novel multiregion structure apodized photon sieve is proposed. The number of regions, the apodization window values, and pinhole sizes of each pinhole ring are all optimized to enhance the energy efficiency and enlarge the pinhole sizes. The design theory and principle are thoroughly proposed and discussed. Two numerically designed apodized photon sieves with the same diameter are given as examples. Comparisons have shown that the multiregion apodized photon sieve has a 25.5% higher energy efficiency and the minimum pinhole size is enlarged by 27.5%. Meanwhile, the two apodized photon sieves have the same form of normalized intensity distribution at the focal plane. This method could improve the flexibility of the design and the fabrication the apodized photon sieve.

Sphere-cone-polynomial special window with good aberration characteristic

Optical windows with external surfaces shaped to satisfy operational environment needs are known as special windows. A novel special window, a sphere-cone-polynomial (SCP) window, is proposed. The formulas of this window shape are given. An SCP MgF2 window with a fineness ratio of 1.33 is designed as an example. The field-of-regard (FOR) angle is +/- 75 degrees. From the window system simulation results obtained with the calculated fluid dynamics (CFD) and optical design software, we find that compared to the conventional window forms, the SCP shape can not only introduce relatively less drag in the airflow, but also have the minimal effect on imaging. So the SCP window optical system can achieve a high image quality across a super wide FOR without adding extra aberration correctors. The tolerance analysis results show that the optical performance can be maintained with a reasonable fabricating tolerance to manufacturing errors.

Numerical simulation and design of an apodized diffractive optical element composed of open-ring zones and pinholes

In this paper, we investigate the relationship between open-ring zones of the Fresnel zone plate and the pinhole rings of photon sieves (PSs). Numerical simulations show that the normalized diffraction fields near the focal point of an individual pinhole ring and the circular open-ring zone are the same. It is confirmed that the maximum diffraction efficiency of an open-ring zone is higher than that of the traditional pinhole ring. Meanwhile, pinhole rings have more flexibility for apodization filtering. Based on these key findings, we propose the design theory of an apodized diffractive optical element comprised of open-ring zones and pinholes. To validate the theory, we developed a design example. Compared with traditional apodized PSs, the new apodized diffractive element has a 50.19% higher energy efficiency, and the minimum pinhole size is enlarged by 30.77%.

Design of space-borne imager with wide field of view based on freeform aberration theory

Freeform surfaces have advantages on balancing asymmetric aberration of the unobscured push-broom imager. However, since the conventional paraxial aberration theory is no longer appropriate for the freeform system design, designers are lack of insights on the imaging quality from the freeform aberration distribution. In order to design the freeform optical system efficiently, the contribution and nodal behavior of coma and astigmatism introduced by Standard Zernike polynomial surface are discussed in detail. An unobscured three-mirror optical system with 850 mm effective focal length, 20°× 2° field of view (FOV) is designed. The coma and astigmatism nodal positions are moved into the real-FOV by selecting and optimizing the Zernike terms pointedly, which has balanced the off-axis asymmetric aberration. The results show that the modulation transfer function (MTF) is close to diffraction limit, and the distortion throughout full-FOV is less than 0.25%. At last, a computer-generated hologram (CGH) for freeform surface testing is designed. The CGH design error RMS is lower than λ/1000 at 632.8 nm, which meets the requirements for measurement.

Efficient manufacturing technology of metal optics

The efficient manufacturing technologies greatly accelerate the development and production process. Optical components have higher precision requirements than mechanical parts. This provides great challenge for rapid manufacturing. Metallic optical system is featured high resolution, wide spectral range, light weight, compact design, low cost and short manufacturing period. Reflective mirrors and supporting structures can be made from the same material to improve athermal performance of the system. Common materials for metal mirrors in optical applications include aluminum, copper, beryllium, aluminum beryllium alloy and so on. Their physical characteristics and relative advantages are presented. Most kinds of metals have good machinability and can be manufactured by many kinds of producing methods. This makes metallic optical system saving 30%~60% cost and time than others. The manufacturing process of metal mirror is different due to its working spectral. The metal mirror can be directly manufactured by single point diamond turning. This is an outstanding technique in point of ultra-precision as well as economical manufacture of mirrors. The roughness values and form accuracy of optical surfaces after diamond turning can satisfy the quality level for applications in the near infrared and infrared range. And for visible light spectral the turning structures must be removed with a smoothing procedure in order to minimize the scatter losses. Some smoothing methods to obtain visible quality metal mirrors are given in this paper. Some new manufacturing technology, such as 3D printing, can be used for metallic optical system and several promising techniques are presented.

Optical design of off-axis astronomical space telescope based on freeform surfaces

A large-aperture, wide field-of-view, three-mirror optical system for a space borne astronomical survey telescope has been designed. The unobstructed optical system with a circular pupil has a 2 meter aperture, 1.7 deg2 FOV, and provides remarkably good imagery. Freeform surfaces and decentered/tilt surfaces are introduced to the system, which have the advantage of balancing the unsymmetrical aberrations, especially for the wide-field off-axis optical systems. The evaluation of image quality is that the value of the RMS wavefront error is 21 nm, the ellipticity is less than 6.5% in the main imaging area (about 1.1 deg2), and the maximum radius of Encircled Energy 80% (EE80) is less than 0.09arcsec.

The research for EO imaging system simulation

Electric-Optical (EO) imaging systems are modern systems, especially the one consists of aspheric optics and image processing called wavefront coding system. The design concept of this kind system is totally different from the traditional image system. The trade-off between the complexity of the image deconvolution and the depth of focus extended should be considered. So we establish a simulation model consisting of optical optimization design, system signal-to-noise ratio analysis and recovered image evaluation. The data can be exchanged among them. This simulation tools will be very useful in system design process.

Design of dual-FOV refractive/diffractive LWIR optical system

An infrared-optical zoom system using binary element is proposed in this paper. The two main advantages of the zoom system introducing here are: bigger F-number and lower cost. The primary optical properties are: F/#=1,zoom ratio =1:4,and dual field are 26.6°and 5.6°respectively. Wider field of view is used for search and the smaller one is used for imaging details. This system uses un-cooled infrared detector with 320×240 pixels and 45μm pixel size. The F-number matches the sensitivity range of the detector array. Three aspects are considered during design process to make the system more satisfactory and more achievable. First, the manner of zoom is accomplished by exchanging tow lenses into the smaller field of view system layout. The lens exchange manner faces the requirement of simple system structure and good image quality in both focal points. It can also make the system more feasible in the alignment process than mechanical-zooming manner and optical-zooming manner; Second, binary element is used to correct the chromatical aberration by taking the advantage of negative dispersion characteristics and the cost of the system is lower than that of conventional ones with Zinc Selenide (Znse) material at the same level. In the binary element is rotational symmetric with one step which is easy to fabricate; Others, in order to balance 5th spherical aberration, 5th coma aberration and 5th astigmatic aberration, high-order asphere surfaces with 2th order to 10thorder are also hired in the system. Asphere surface is useful in compressing the system and improving optical system transmittance. This kind asphere surface is on industrial level featuring low cost and easy to fabricate. It is shown that good image quality can achieved by implementing five Germanium lenses and the transmittance of system is 72%. All aberrations are diffraction-limited, both spherical aberration and astigmatic aberration are corrected. When the field of view(FOV) is 26.6°and the focal length is 152mm, MTF at Nyquist frequency(11lp/mm) is great than 0.7. The spherical aberration is -0.0073. The coma aberration is 0.0978 and the astigmatic aberration is -0.013. When the field of view(FOV) is 5.6°and the focal length is 38mm, MTF at Nyquist frequency is great than 0.8 with spherical aberration -0.0046,the coma aberration 0.055 and astigmatic aberration 0.034.