Optical element

Conventionally, an optical system comprises a pliuahty of lenses for refracting a light beam. The chromatic aberration is decreased by combining glass materials with different dispersion characteristics. 

Objective lenses of a telescope, for example, comprise a positive lens using a low dispersive glass material and a negative lens using a high dispersive glass material. These lenses are combined to correct chromatic aberration appearing on an axis. 

However, when the lens configuration is restricted or glass materials to be used are limited, sometimes the chromatic aberration cannot be corrected fully. On the other hand, the chromatic aberration can be decreased by mounting a diffraction grating to a lens. The grating can be formed from PVK. PVK as such can be bonded onto a diffraction grating. 

Since PVK is a very fragile material, even a small load can easily crack it. Moreover, the method is time-consuming and thus not suitable in industrial processes. 

Another method is to apply PVK by thermoplastic molding. Further, the polymer can be dissolved in a solvent, which is evaporated to form the optical element. Still another method is supplying the monomer, NVK to a mold. Then the monomer is polymerized in the mold by means of a polymerization reaction to convert it into PVK. The polymerization reaction is a thermal polymerization between 70 °C and 130 °C. Photopolymerization is also possible, e.g. by means of 1-hydroxycyclohexyl-phenyl-ketone.  

The purpose of optical elements is to channel radiation as efficiently as possible from the observed area on the planetary object onto the detector. The simplest optical elements capable of doing so are lenses and curved reflectors (Fig. 5.2.1). Mirrors 

The azimuth angle is / . The relationship between and 0max is shown in Fig. 5.2.2. The same figure indicates the /-number of the lens /-number = focal length diameter. For small values of 0max, the cosine term may be approximated by (1 — 0 /2), which is equivalent to the paraxial approximation, sin 0 and 

This approximation, also shown in Fig. 5.2.2, is good for values of up to 40° (80° full cone angle) or an /-number of 0.6. Another often used and convenient approximation, area of aperture divided by the square of the distance between aperture and detector, yields 

Thus the product AS2 is invariant in an optical system. Actually the term is invariant, n being the refractive index, but as long as we measure A and Q in the same medium (air or vacuum, for example) it is sufficient to consider A 2 to be invariant. An optical system may be compared to an electrical transformer, where the product current times voltage (power) is also invariant. The telescope and the transformer are not without losses optical and electrical efficiencies are less than unity. 

Two variations of the field lens concept are mentioned the immersion lens and the Winston cone. Both are designed to reduce the required detector area and, thereby, improve the signal-to-noise ratio. The immersion lens (Fig. 5.2.5) increases the speed of a condensing optic beyond that of a conventional field lens by coupling the detector directly to a material of a high refractive index. The immersion lens is the analog to the immersion objective of high-power microscopes. Instead of a 

---

This article presents methods of synthesis and objective control of special flat optical elements (computer-synthesized holograms) as elements of protection. Due to the limited number of pages, we cannot go into great detail to explain the technology of how computer-synthesized holograms are created. The goal of this publication is to discuss following subjects ... 

Let us briefly discuss the mathematical problems of the synthesis of flat computer optical elements Let

[Pg.265]

We need to point out that, if the wavelengths of laser radiation are less than the size of typical structures on the optical element, the Fresnel model gives a satisfactory approximation for the diffraction of the wave on a flat optical element If we have to work with super-high resolution e-beam generators when the size of a typical structure on the element is less than the wavelengths, in principle, we need to use the Maxwell equations. Now, the calculation of direct problems of diffraction, using the Maxwell equations, are used only in cases when the element has special symmetry (for example circular symmetry). As a rule, the purpose of this calculation in this case is to define the boundary of the Fresnel model approximation. In common cases, the calculation of the diffraction using the Maxwell equation is an extremely complicated problem, even if we use a super computer. 

The approximation of Fresnel is scalar approximation. Let u(, r],0-0) be the scalar wave function of the laser beam falling onto the optical element, and u( X,y,Cl) will the be scalar wave function in the plane Z = Cl. Then [3,4]... 

Here G is the area of localization of the flat optical element, z = a is the plane of the... 

In the first approximation, the flat optical element may be described as an element which transforms laser radiation as it passes through the element, as is shown by the following formula ... 

Complex function if) for the phase optical element may be described as... 

Where function characterizing the flat optical element Without giving details of technology, we can say it is possible to create flat computer element if the function (, 77) is known. In real problems of synthesis, we know only the absolute... 

A V Goncharsky and V.V.Stepanov, Inverse Problems in Synthesis of Optical Elements, Ill-Posed Problems in the Natural Sciences, MIR Publishers, Moscow, (1987), pp.318-340. 

Fig. 40. Schematic of an euv exposure tool. Key features are the excimer laser-driven x-ray source and the redective optical elements (including the mask) in...

Optically Functional. Laser optics, layer architectural glass panels (up to 3 x 4.3 m), lenses, TV-camera optical elements, and similar apphcations require optically functional coatings. 

There are other groups of optical elements that use plastics in very fine patterns to make special optical elements. One of these is the Fresnel lens that is a collapsed lens structure that has the effect of a strong magnifier... 

One of the major advantages of plastics optical elements is that they can me made by different processes such as injection molding, casting, or extrusion to good accuracy at low cost. Some glass lenses are pressed from hot glass but the majority of the lenses... 

To be successful, molded optical elements of plastics must be produced with careful control of the fabricating process. In the case of these optical products it is particularly important that the molding conditions be carefully controlled to minimize molded-in stress. In addition to these stresses reducing the dimensional stability of the products leading to distorted images, the stresses themselves affect the quality of the image. This is a result of the fact that the stresses/strained areas have a different refractive index from that of the... 

The function of an antireflection (AR) coating is to reduce the surface reflection of optical elements and increase the amount of light transmitted. The glare and ghost images from secondary reflections are minimized. Multiple coatings are now produced that can... 

There are many ways one can imagine dividing up a primary mirror into smaller optical elements. A few include ... 

Adapting the system to a mobile unit involved redesigning the instrumentation for portability and mounting it into a 3 A x 5-ft steel cart. A major requirement for this unit was containment of the Line-Lite laser, optical elements, and detection module in a clean laboratory environment. Other requirements included high ground clearance, accessibility to electricity within 100 ft, and durable construction. The entire mobile unit can be powered by a 5-kW gasoline generator. 

Accurate experimental determinations require that not only the handedness of the produced light, but its exact degree of polarization, are known. The theoretical performance of an undulator may be in practice be degraded by magnetic defects, and the optical beam can be further depolarized by reflections along the beamline. Again, the dephasing on optical elements can in principle be... 

---