Refractive Optical Elements Research Articles

Imaging with extended focal depth by means of the refractive light sword optical element

The paper presents first experiments with a refractive light sword optical element (LSOE). A refractive version of the LSOE was prepared in photoresist by gray scale photolithography. Then we examined chromatic aberrations of the produced element and compared them with those corresponding to two different lenses. For this purpose we performed two experiments, the first one where white light illumination was used and the latter one by the help of monochromatic illumination with three different wavelengths. The obtained results lead to the conclusion that the refractive LSOE does not exhibit significant chromatic aberrations and can be successfully used for imaging with extended depth of focus in polychromatic illumination.

A gradient method of designing optical elements for forming a specified irradiance on a curved surface

This paper discusses how to design refractive optical elements to form a specified irradiance distribution on a curvilinear surface when a compact light source is used. The calculation is based on representing the surface of the element in terms of the distribution of the eikonal of the light field in an adjacent plane. The eikonal is defined in the form of a polynomial. The design of the refractive surface is based on gradient minimization of an error functional representing the difference of the calculated and specified irradiances of the field. An analytical expression is obtained for the gradient of the error functional. Refractive elements are designed for the formation of a rectangular irradiance distribution on flat and cylindrical surfaces.

New modules for the VITESS software package: Time-gradient magnetic fields and neutron refractive lenses

Neutron spin-handling devices that employ time-dependent magnetic fields are very frequently used in different polarized neutron instruments. We have developed a new VITESS module that allows us to calculate the neutron spin dynamics in a time-dependent magnetic field, time dependent both in direction and in magnitude. Neutron beam focusing by means of a neutron refractive optic principally allows for a very significant enhancement of neutron flux at the sample position of the small-angle scattering instruments. Presently, there is no possibility to simulate the performance of such neutron refractive optical elements. The new VITESS module for the simulation of spherical and parabolic neutron refractive lenses enables the inclusion of a sequence of neutron lenses (the compound lens), and it takes into account the gravity effects and is conveniently supplied with the trajectory visualization option.

A Hybrid Compression Method for Integral Images Using Discrete Wavelet Transform and Discrete Cosine Transform

Integral imaging (II) is a promising three-dimensional (3-D) imaging technique that uses an array of diffractive or refractive optical elements to record the 3-D information on a conventional digital sensor. With II, the object information is recorded in the form of an array of subimages, each representing a slightly different perspective of the object In order to obtain high-quality 3-D images, digital sensors with a large number of pixels are required. Consequently, high-quality II involves recording and processing large amounts of data. In this paper, we present a compression method developed for the particular characteristics of the digitally recorded integral image. The compression algorithm is based on a hybrid technique implementing a four-dimensional transform combining the discrete wavelet transform and the discrete cosine transform. The proposed algorithm outperforms the baseline JPEG compression scheme applied to II and a previous compression method developed for II based on MPEG II.

Novel fabrication method for three-dimensional nanostructuring: an application tomicro-optics

We propose a 3D micro and nanofabrication method with potential applications to severalnanotechnology-related fields. Our approach is based on the combination of lithographicsteps and isotropic wet etchings performed on a quartz or glass substrate to form 3Dstructures with very accurate shape control and nanometer scale surface roughness. Theresulting concavities at the quartz surface are converted into convex plastic elements by hotembossing or casting techniques. Complex all-polymer refractive optical elements have beenrealized by this method. Upon illumination, such micro-optics focus the light intopredetermined 3D distributions of focal lines and spots. The general fabrication schemeexplored here is illustrated through a series of examples in optics, but is expected to offernew solutions to other fields such as medicine, microfluidics and nano-optics.

Gradient method for calculating the eikonal for focusing to a given region

An iterative method for designing a field eikonal that creates conditions for light beam focusing to a given 2D region is considered. The eikonal function that ensures focusing from a circular to rectangular region is calculated. Application of the method to design of refractive optical elements is considered. The refractive element that focuses a plane beam with a circular section to a rectangle is designed.

Image Quality Evaluation and Tolerance Analysis for Camera Lenses with Diffractive Element

A novel image quality evaluation method, which is based on combination of the rigorous grating diffraction theory and the ray-optic method, is proposed. It is applied for design optimization and, tolerance analysis of optical imaging systems implementing diffractive optical elements (DOE). The evaluation method can predict the quality and resolution of the image on the image sensor plane through the optical imaging system. Especially, we can simulate the effect of diffraction efficiencies of DOE in the camera lenses module, which is very effective for predicting different color sense and MTF performance. Using this method, we can effectively determine the fabrication tolerances of diffractive and refractive optical elements such as the variations' in profile thickness, and the shoulder of the DOE, as well as conventional parameters such as decenter and tilt in optical-surface alignments. A DOE-based 2M-resolution camera lens module designed by the optimization process based on the proposed image quality evaluation method shows <TEX>${\sim}15%$</TEX> MTF improvement compared with a design without such an optimization.

Self-aligned spatial filtering using laser optical tweezers

We present an optical spatial filtering device that has been integrated into a microfluidic system and whose motion and alignment is controlled using a laser optical tweezer. The lithographically patterned micro-optical spatial filter device filters out higher frequency additive noise components by automatically aligning itself in three dimensions to the focus of the laser beam. This self-alignment capability is achieved through the attachment of a refractive optical element directly over the circular aperture or pinhole of the spatial filter. A discussion of two different spatial filter designs is presented along with experimental results that demonstrate the effectiveness of the self-aligned micro-optic spatial filter.

Design considerations for efficient planar-optical systems

Planar integrated free-space optics is well suited for various applications, e.g., in telecommunication, optical interconnects and security application. However the overall efficiency of purely diffractive systems is low, i.e., in the range of a few percent. We show that it is possible to raise the efficiency significantly by integration of refractive optical elements (prisms, wedges) for coupling into the substrate. In this case, the systems design is particularly challenging. We compare the impact of different coupling mechanisms on the optical designs of integrated 4f-imaging systems.

High Precision Diamond Machining of Hybrid Sol-Gel Coatings

Flexible and economic production of complex reflective optical elements is achieved by high-precision machining of aluminum and copper with diamond tools. There is also an increasing demand for complex refractive optical elements like micro lens arrays, Fresnel lenses or prismatic surfaces on silicon wafers or metallic surfaces. For the production of these optical elements, hybrid sol-gel coatings based on methacryloxypropyltrimethoxysilane (MATMS) and zirconium (IV) tetra n-propoxide (ZTP) were deposited on aluminum substrates by spin-coating. The influence of the rotational velocity and the chemical sol composition on the coating thickness was determined. The hardness and elastic modulus of these coatings was measured as a function of the chemical composition. The machining characteristics of these coatings were investigated by high precision turning and fly cutting with diamond tools of different geometry. The resulting surface finish obtained was determined as a function of the machining parameters.

Reflective Microoptical Element Array Fabricated by Photofabrication Technique

The authors present reflective microoptical element arrays fabricated by a photofabrication technique. The three-dimensional base of the microoptical element array is photofabricated by exposing photopolymerizable resin to a light beam through a grayscale mask. Reflective film on the photopolymerized base of the optical element is introduced to prevent serious problems in photofabricated refractive optical elements, such as when the optical characteristics of the elements fundamentally depends on the spectroscopic property of the material of the photofabricated optical element. The relationship between the pixel value in the grayscale bitmap image used for making the grayscale transparency mask and the cure depth of the photofabricated base is experimentally clarified for a photopolymerizable resin. 15×15 unit convex and concave micromirror arrays, in which each optical element has a diameter of 930 µm, have been batch fabricated. The focusing capability of the fabricated concave micromirror array is demonstrated.

Electron-beam lithography for thick refractive optical elements in SU-8

We have fabricated refractive optical elements in SU-8 resist using electron-beam lithography. Exposing an 8-μm-thick layer of SU-8 resist with a 30 kV electron beam, we demonstrate fabrication of an f/9 spherical lens, an f/2 spherical lens, and a prism with a 3° tilt. The elements all perform near the diffraction limit, with rms surface variations between 1/10 wave and 1/7 wave. The electron scattering from the thick resist is measured and shown to limit the technique to optical elements with relatively smooth surface figures.

Optical matrix structure for optical interconnection between two groups of points

We propose a new method for optical interconnections between illuminating and receiving points. This method uses optical matrix structures (OMSs) to operate compact, simple, and cheap connections. Matrix structure elements can be reflective, refractive, or diffractive optical elements. The optical features of these elements must be determined one by one according to their light-redirection requirement. From the experiment results, this new OMS method performs better than the other conventional methods.

Confocal microscopy with a refractive microlens–pinhole array

The stable setup of a confocal arrangement consisting of a combination of a refractive microlens and a pinhole array is presented. The focal plane of the microlenses lies at the rear surface of the substrate in the pinhole plane. By using a microscope objective one can image the stop array onto the object at a reduced size. Surface profiles of refractive and diffractive optical elements were measured with the help of this confocal microscope.

Pattern transfer for diffractive and refractive microoptics

Pattern transfer technologies developed for very large and ultra-large scale integrated (VLSI and ULSI) circuit processing can be adapted to fabricate high fidelity microstructures for planar diffractive and refractive optical elements (DOEs and ROEs). Lithographic, dry-etching, and material deposition techniques used to pattern continuous and stepped microoptical surface relief structures in dielectric, metallic, and semiconductor materials are reviewed. Examples are given of multilevel DOEs, analog refractive lenslet arrays, and subwavelength structures.

Diffractive, refractive optics or anything more? Comparative analysis and trends of development

The development of diffractive and refractive optics from ancient times (2000 years ago) to the present is traced from a theoretical and practical point of view. A comparative analysis of the possibilities of the two fields in optics is presented. The possibility for achromatization and corrections that exceed the boundary of diffractive and refractive optical elements is pointed out.

Experimental evaluation of the 3-D optical shuffle interconnection module of the sliding Banyan architecture

A free-space optical interconnection module for the sliding Banyan (SB) multistage interconnection network is experimentally evaluated. This three-dimensional (3-D) optical shuffle topology employs a macro-lens array in a reflective architecture. Interconnections for multiple stages are interleaved across a single two-dimensional (2-D) multichip array of "smart pixels". The experimental module implements five interleaved stages of shuffle interconnections with approximately 10 /spl mu/m resolution and 10 /spl mu/m registration accuracy across a 10/spl times/10 cm, 256 node, simulated optoelectronic (OE) backplane. The experiments demonstrate the use of conventional refractive optical elements to implement the required shuffle interconnection pattern in a SB network. The results suggest that this reflective 3-D shuffle interconnected SB approach may lead to ATM switching fabrics with aggregate throughputs scaleable to >1 Tb/s. Such a system could be implemented with vertical cavity surface emitting laser (VCSEL) based smart pixel OE technology.

The preparation of optical coatings for fusion lasers

Ultraviolet (UV) optical materials and coatings play a key role in laser imaging technology. The optical materials form the basis for manufacturing a wide variety of lenses, mirrors, and other critical optical elements needed to generate and deliver UV energy to the desired surface. The coatings are critical in maintaining the necessary transmission, reflection, and overall imaging quality in optical systems. Coatings are used to protect a variety of refractive and reflective optical elements in beam delivery systems and in laser imaging lenses. They are also used on intracavity optics in excimer and other UV lasers. UV coatings help in maintaining high optical efficiency in laser beam delivery systems. Specially designed coatings are used to reflect and transmit specific wavelengths or wavelengths in a specified band. Interference and antireflection coatings are used to increase peak transmission or reflection in a wide variety of optical imaging systems. They are also used on optical surfaces of laser imaging systems, including mirrors, attenuators, beam splitters, polarizers, windows, and other optics.

Log-frequency variable resolution optical spectrum analysis using holographic mapping techniques

A frequency-variant optical spectrum analyzer for one-dimensional signals is described. The configuration considered provides a log-frequency display of signal spectral content. A special refractive or holographic optical element is used to perform the basic frequency scale warping; modifications provide for frequency dependent resolution of the output spectral display. System operation is analyzed and experimental results presented.

Optical Engineering

The optical design itself is very often a small portion of the engineering effort on an optical instrument The optical engineer, when he sets about to design an optical system, must concern himself with problems in the fields of structures, associated electronic systems, materials, specifications, and test methods. This paper describes some examples of current activities in each of these fields. In addition, this paper considers the influences on design of temperature and mechanical shape, and presents pertinent examples of solutions to specific problems. Various methods are described for supporting refractive and reflective optical elements, both large and small, and the influence of the need for reduced weight is discussed. Requirements for satellite systems are also considered. Furthermore, the importance of electronic subsystems, as well as their interrelation with optical and structural design, is treated. Several examples of guidance systems are discussed as well as some problems of focusing. Sections are also included on performance specifications and quality control by the use of modulation transfer functions.