Several algorithms can be used in order to calculate a phase function that shapes a light beam into a certain intensity distribution. In cases of smooth phase functions, e.g. for the conversion of a Gaussian beam into a flat top distribution, the optical element can be realized by a smooth surface profile without 2 pi jumps. Even though the disadvantages of diffractive elements are well known, often the limitations of the fabrication technologies permit the realization of only diffractive elements. For the beam shaping elements in micro optics, methods exist which allow the fabrication of smooth surface profiles. Such refractive elements have the advantages of a wavelength independent behavior and a conversion efficiency of nearly 100% as known from refractive lenses. In some cases, the design of refractive result in thick surface profiles and the fabrication technique do not fit to this. Therefore, the objective of the paper is to show some possibilities to make the design meet the fabrication capabilities. We used lithographic and etch techniques as well as replication technologies for the fabrication. The basis of the presented technology is gray tone technique, the generation of a proper pre-form, and its correction in order to achieve maximum coincidence with the desired surface profile. Several refractive beam shaping elements have been realized.
The fabrication of refractive optical elements faces the demand of an absolute accuracy of the profile which depends on the wavelength of light. Analogue lithography is a suitable technology for the fabrication of such smooth profiles up to a profile depth of about 10 micrometers . The realization of larger profile depths requires new fabrication techniques. Our approach to solve this problem is to use the similarity of refractive optical surface profiles and certain minimal surfaces. The objective of this paper is to describe design and fabrication of optical profiles which can be realized by combining minimum surfaces and analogue lithography. As a result, with refractive beam shaping elements a wavelength independent conversion efficiency of more than 99% was realized.
Mid-infrared optical microelements, like lenses or array of them, can be used to couple light or to form images. We present simple methods to fabricate mid-infrared microlenses and other optical elements by means of irradiation of a polymer substrate with CO2 laser or by the melting method. Application of these methods can lead to a mass production, low cost mid-infrared elements. It is analyzed the influence of some fabrication process variables in the final parameters of the elements. The quality of the microlenses was characterized by direct measurement of their surface profile through mechanical (surface analyzer) and optical (interference) methods. Capability of microelements to form infrared images is shown.
Growing interest in miniaturized optical components for various applications such as optical interconnection systems and telecommunication industry have led to the development of several techniques that are used in the fabrication of micro-optical elements. One approach involves the use of polymers as recording materials: these are flexible, highly transparent and cheap. The technique described in the present paper is founded on the ability of self-processing photopolymers to generate refractive microlens arrays. Spatially controlled illumination of a photosensitive layer induces an inhomogeneous photopolymerization involving a mass-transport process of reactive species and generating a relief in the photopolymer layer. The presentation focuses on the fabrication of microlens arrays through photopolymerization with the green line of an argon-ion laser. Surface tension and differential volume shrinkage turned the illuminated area into good quality lenses. The fabricated lens arrays exhibit diameters ranging from less than 20 micrometers to more than 500 micrometers and focal lengths from 100 micrometers to a few millimeters, depending on photonic, optical and physico-chemical parameters. This imaging technique is highly flexible as regards height, shape and optical properties of the lenses that are produced. By starting from the same background, one can also fabricate diffractive optical elements such as gratings and duplicate computer-generated holograms that come increasingly into prominence as the micro-opto-electro-mechanical field expands.
In this paper, we report our success in the fabrication of optical diffractive gratings by means of direct embossing on sol-gel derived glasses. The sol-gel glass film is derived from composite organic-inorganic material with spin-coating. No baking is required during the whole processing. As a preliminary result, the grating with a period of 1.102 micrometers and a thickness of 57.2 micrometers has been successfully fabricated. Our experiments provide a simple and low temperature approach for fabricating optical gratings useful in integrated optics.
We report on recent progress in the fabrication of fused silica micro-optical elements, such as blazed gratings, refractive microlenses and microprisms. The elements are first made in photoresist and then they are transferred into fused silica by reactive ion etching. High selectivity etching is needed to realize structures with a high aspect ratio. Results are shown using various metallic etch masks. The shaping of optimized profiles is also presented to generate microlenses which are aspheric, or which have a low numerical aperture.
High aspect ratio gratings are of interest for a couple of applications. Especially artificial birefringence based on zero order dielectric gratings are calling for small pitches (e.g. < 450 nm) and a high aspect ratio (e.g. > 10 in fused silica). One of the most difficult problems is to reach the optical parameters that are demanded. Based on e- beam writing, ion beam etching, reactive ion beam etching and chromium coating we developed a technique for the fabrication of such gratings. In result, we reached phase retardation between TM and TE polarization of more than 90 degree(s). An iterative step technique allows realization of the phase retardation of accuracy better than 1%. The highest aspect ratios of the gratings, we fabricated, were in the range of about 25 at 440 nm period and 100 nm gap.
Multilevel diffractive lenses give much higher efficiencies than simple binary structures by introducing a blaze. We have developed a novel micro-machining process that allows complex multi-level optics to be fabricated in silicon for use at terahertz frequencies. In this paper we demonstrate the fabrication of a four level (quaternary) lens designed for use at 1 THz. The process required two highly anisotropic dry etch stages that both used a SF6/O2 plasma at 173 K. The etch produced smooth etch surfaces with a vertical etch rate of 1 micrometers /min. The lithography for the first etch stage used a conventional positive photoresist process followed by NiCr deposition and lift-off to form an etch mask. For the second etch step the substrate was planarized and over-coated using SU-8, which was also used as a negative resist. The planarized specimen was then exposed to produce the etch mask for the next phase level etch of the lens. The process yielded a silicon:SU-8 etch selectivity of 4:1 which was more than adequate. Between the two etches a total etch depth of 115 micrometers was obtained, with steps at 38 and 77 micrometers and a minimum feature of 84 micrometers . It is anticipated that the process can be extended to give more phase levels with greater optical efficiency.
In this paper, we developed a nonsilicon surface- micromachining technique that uses a thick photoresist film and a spattered copper layer as two sacrificial layers and uses the electroplated ferronickel (FeNi) as the structure material. The proposed nonsilicon micromachining process is simpler, with relatively low temperature, and more flexible for various materials. By using such technique, silica or glass can be used as an optical material. Several out-of- plane multilevel diffractive optical elements (DOE's) including gratings, phase Fresnel lenses and some other optical components supported by FeNi microstructures are successfully constructed on the silicon wafer. Those DOE's are fabricated by several steps both of photolithography and reactive ion etching on the spattered silica layer. This technology offers a new approach to fabricate high quality phase micro-optical elements for free-space integrated micro-optics and other applications.
The fabrication of 3D-microstructures with well-defined curved surface contours is of great importance for various mechanical, optical and electronic devices and subsystems. Complex geometrical structures or topographies are necessary to obtain a certain mechanical stability, a specific surface property or a predetermined electrostatic field configuration. Obviously in the micro-optic domain, there is a great demand to produce sophisticated surface topographies for refractive or diffractive optical elements, e.g. Fresnel lenses. This paper reports on progress in graytone lithography using subresolution pixeled chromium glass masks and introduces some replication techniques for different materials. In continuation of our work on graytone lithography, reported elsewhere, detailed view on reproducibility, fidelity and process latitude will be presented. Based on this results infrared diffractive optical elements have been fabricated in silicon using an 1:1 dry etching process, where the surface roughness of the shaped areas after etching has an 1 (sigma) value of 18 nm. For low cost application in the visible wavelength region a replication technique in polycarbonat by injection modeling is described. First results are shown.
Based on the analysis of the effect of fabrication errors on diffraction efficiency of a diffractive optical element and image quality of a binary optical lens (BOL), a novel quasi- continuous mask-coding (QCMC) method is presented in this paper. This coding method puts forward a series of new mask patterns, making it possible to fabricate quasi-continuous micro profile by using the same number of masks as employed in the traditional `binary' fabrication technology. The QCMC technique breaks through the current convention for coding m masks to get the micro profile with 2m steps in one zone. This paper presents the theory of the QCMC method and provides examples of practical mask patterns. by using QCMC, BOLs with enhanced diffraction efficiency can be fabricated by traditional multilayer micro fabrication method or by laser writing technology without any additional equipment or processing procedure. Employing this method will enhance the diffraction efficiency, the optical performance, and in particular the image quality of a hybrid optical imaging system.
Micro Stereo Lithography (MSL) is a poor man's LIGA for fabricating high aspect ratio MEMS devices in UV curable semiconducting polymers using either two computer-controlled low inertia galvanometric mirrors with the aid of focusing lens or an array of optical fibers. For 3D MEMS devices, the polymers need to have conductive and possibly piezoelectric or ferroelectric properties. Such polymers are being developed at Penn State resulting in microdevices for fluid and drug delivery. Applications may include implanted medical delivery systems, chemical and biological instruments, fluid delivery in engines, pump coolants and refrigerants for local cooling of electronic components. With the invention of organic thin film transistor, now it is possible to fabricate 3D polymeric MEMS devices with built-in-electronics similar to silicon based microelectronics. In this paper, a brief introduction of MSL system is presented followed by a detailed design and development of micro pumps using this approach.
Interferometric lithography is a maturing technology for patterning large area, periodic sub-micron features. The interference of two or more coherent optical waves is recorded in photoresist to produce a variety of structures including gratings, holes, posts, cones, and grids. This lithographic technique allows maskless patterning of large area substrates using short exposure times. Applications for periodic patterns include distributed feedback lasers, field emission displays, liquid crystal displays, advanced data storage applications, optical gratings, metrology standards, and sub-wavelength structures. Our work is motivated by interest in bringing interferometric patterning out of research laboratories and into mainstream production facilities for high volume applications, by automating and simplifying the exposure process. The concept and operational principles of two automated interference lithography systems, the HLS model 1000, and the HLS model PC2, are introduced.
Binary gratings with feature sizes smaller than the illumination wavelength were fabricated in quartz glass by means of microstructuring techniques. Using rigorous coupled wave analysis polarization elements like polarizing beam splitters and phase retardation plates were designed for operation in transmission at the wavelength of 650 nm. High frequency polarization gratings with feature sizes down to 140 nm and aspect ratios up to 7 were realized. For the polarization selective beam splitting elements we measured diffraction efficiencies of about 80% in the -1st order for TE polarization, and 90% in the 0th order for TM polarization. The values are in good agreement with the theoretical values. Furthermore we realized phase retarding elements e.g. (lambda) /8-plates which showed a phase difference of (Phi) equals 44.8 degree(s) ((Phi) theor. equals 45 degree(s)) between TE and TM polarized light. The design and the fabrication process as well as the optical properties of our high frequency binary phase gratings will be presented. Experimental results will be compared with theoretical values.
Various diffractive optical elements have been fabricated for the visible wavelength region, mainly for laser beam splitting purposes, but also for the generation of arbitrary but predetermined intensity patterns (e.g. spirals, logos etc.). To obtain high efficiencies the computer-generated holograms were realized as transmissive diffractive phase elements (DPE). More detailed we report on a beam splitter which was intended to distribute an incoming laser beam into 40 partial beams of equal intensities arranged equidistantly on a circle. These circular beam splitters--designated to be used in a measuring system--were realized as binary phase elements. In addition DPEs, that generate a given arbitrary intensity pattern, were produced in 2- and 8-level approximation. The computer-generated phase elements with feature sizes down to the sub-micrometer range were fabricated in quartz-glass by means of microstructuring techniques. Due to our precise and well developed processes we realized binary- and multilevel microstructures of high optical quality. For the binary 1:40 beam splitters we reached diffraction efficiencies of about 60% and uniform spot intensities of better than +/- 2.5% of the average intensity value. The measured efficiency of the eight-level pattern generators was higher than 80%. The optical characterization of our components showed a good agreement with the results expected from simulations. Using simple embossing techniques we were able to replicate first test samples in organic polymers which showed good optical performances.
The study of planar imaging system is an interesting topic because its compact structure and lightweight are consistent with the trend of production miniaturization. This paper presents our study of designing and fabricating planar imaging elements. Theoretical analysis of grating and off- axis imaging diffractive elements was conducted. A low- aberration symmetrical imaging optical system was designed and evaluated with CODE V. Using a variable intensity laser- writing system, diffractive surface relief patterns were written on photoresist and were later transferred to a fused quartz plate.
In this paper we present the results of the first fabrication and micro-assembly experiments of a silicon- wafer based micro-optical table (MOT). Based on these experiments, estimates of position accuracy are reported. We also report on progress in fabrication of lens elements in a hybrid sol-gel material (HSGM). Diffractive optical elements have been patterned in a 13-micron thick HSGM layer on a 150-micron thick soda-lime glass substrate. The measured rms surface roughness was 20 nm. Finally, we describe modeling of MOT systems using non-sequential ray tracing.
It is a relatively simple and effective method to incorporate a monolithic refractive microlens array with a linear high power semiconductor laser array. The refractive microlens array of a corresponding periodicity behaves as optical collimating lens and is used to collimate the highly divergent beam out of the semiconductor laser array. Just a few papers were reported for the use of diffractive and refractive microlens to improve the performance of diode lasers. This paper describes the design and analysis of the refractive cylinder microlens array, and the integration of a linear semiconductor laser array and a refractive cylinder microlens array with different sag and diameter. The theoretical results and computer simulation show that the refractive cylinder microlens array can be effectively used to collimate the light beam out of semiconductor lasers from several tens of degrees to several tens of mrad. The major challenge is the accurate profile control of refractive cylinder microlens, good optical coupling between semiconductor lasers and refractive microlens array, and the critical alignment of lasers and microlens array.
At present, large-area staring infrared (IR) focal plane arrays (FPAs) with CCD multiplexer readout arrays have been used in many fields. Because the filling factor of the detector is poor (generally not more than 50%), which reduces the opto-electronic sensitivity of image sensor, refractive microlens array has been used to increase the filling factor of the large-area IR focal plane array and then improve the opto-electronic sensitivity of the imaging device. Taking into account the situation as above, the characteristics of the nonuniformity of the opto-electronic signal of IR focal plane array with refractive microlens array should be considered. The opto-electronic signal from an IR focal plane array with a microlens array as the function of the incident radiation power falling on it differs from pixel to pixel. This nonuniformity will degrade image IRCCD device. In this paper, we deal with the nonuniformity of the large-area PtSi IRCCD with refractive microlens array, calculate the change of nonuniformity of the imaging device with micro-optical component, and analyze the influence of refractive microlens array on the nonuniformity of PtSi IRCCD device. The theoretical model is developed to determine the maximum opto-electronic sensitivity of the detectors with microlens array.
In high power laser system, such as inertial confinement fusion and laser forming, the requirement for the intensity distribution of the focused laser profile must be top head, steep edge, low side lobe and concentrated high power performance in the main lobe. Diffractive Optical Element (DOE) is used to produce an arbitrary radiation distribution on the focal plane. It has the features of high diffraction efficiency, steep edge, small side lobes and high flexibility. A kind of hybrid algorithm based on hill- climbing and simulated annealing is utilized for phase design because of the ability of strong convergence of the hill-climbing and the global optimization potential of the simulated annealing. Continuous phase should be adopted to increase the light efficiency and decrease wide-angle scattering. The continuous phase DOE with diameter 100 mm has been obtained by gray-level mask and ion-etching on the K9-glass substrate. The DOE's interferogram is given to detect the precision of manufacturing. The manufacturing error, including depth error and alignment error, are analyzed. The intensity distribution of the focal spot is measured by a general CCD, and a series of attenuators are used to increase the dynamic range of the CCD. The results after data-processing show that the uniform illumination has been obtained.
In high power laser system, such as inertial confinement fusion and laser forming, a uniform focal spot is required with top head, steep edge, low side lobe and concentrated high power performance in the main lobe. Diffractive Optical Element (DOE) is a promising technique to obtain such uniform focal spot. It has the features of high diffraction efficiency, steep edge, small side lobes and high flexibility. On the basis of the analysis of the existed optimization algorithms, three kinds of hybrid algorithms are developed for phase design of the DOE to realize uniform focal spot, including global/local united searching algorithm (GLUSA), the modified GLUSA, and the modified GLUSA with multi-resolution. Continuous phase should be adopted to increase the light efficiency and decrease wide- angle scattering. The continuous phase DOE with diameter 100 mm with more than 95% light efficiency and less than 5% top non-uniformity is designed.
The design, realization and characterization of a miniaturized objective are presented. The goal was to obtain an objective with a wide angular field in water, dedicated to noninvasive surgery. This demonstrator proves the usefulness of polymer microlenses (obtained by hot embossing) and outlines the critical steps of the manufacturing process. The objective is 4 mm long, has a diameter of 0.9 mm and an angular object field of 90 degrees (in water). It consists of one pair of concave glass microlenses and two polymethyl methacrylate (PMMA)/glass doublets. The starting design data were imposed by both the previously obtained PMMA microlens characteristics and by the application itself. The small-size-adapted classical technological flux used to manufacture the glass microlens is described and compared to the specific LIGA technological flux used to obtain the PMMA microlenses. Qualitative and quantitative functional characterizations of the components and of the obtained miniaturized optical system (e.g. object field, distortion, MTF, etc.) are performed and discussed. The problems encountered during manufacturing, assembling and characterization processes are outlined and their influence upon the functional characteristics is revealed.
The original diffraction grating was fabricated using a well known Si anisotropic etching technique. By means of this micromachined Si stamper it is possible to transfer its surface-relief profile into a dry negative resist film. The transfer is performed during an embossing process which contains the following steps: In the first step, the dry photosensitive film having 1.5 mils thick, commonly used in PCB fabrication, is laminated onto a quartz UV transparent glass. After that, the protective cover sheet is removed from the laminated substrate, and the Si stamper is placed on the free surface of the photopolymer film. Next, the shimmed sandwich is loaded at the embossing temperature of 115 degree(s)C into a vacuum UV contact printing unit, while a moderate external pressure is applied, during aprox 5 second. The embossed information layer is then firmly bonded to the substrate, and the embossing is made permanent by ultraviolet radiation curing. This treatment consists of exposing the photopolymer, through the transparent substrate, at a UV broad band light source, with a hardening dose of at least 2500 mJ/cm2. Finally, the vacuum is stopped, the sandwich is removed from the exposing unit, and the assembly is separated by slightly flexing, obtaining the permanent complementary replica of the stamper. This fabrication method, tested only for diffraction grating replicas, has also great potential in batch production of many other low-cost integrated optical components.
The lithography of gratings or structures using photoresist holographic masks is very critical, in particular when high selectivity etching processes were employed. In this paper we study the effect of the mask profile and of the phase perturbations during the holographic exposure in the noise of the photoresist masks. It is shown that the use of appropriate conditions of development and exposure may reduce significantly this noise allowing the recording of high aspect ratio structures and the use of selective deposition techniques.