Diffractive Optical Elements Array Research Articles

Method of fabricating an array of diffractive optical elements by using a direct laser lithography

Recently, aspheric lenses have been developed to correct various aberrations and to produce optical elements with various functions. In particular, efforts have focused on fabrication miniaturized and compact elements that make the use of flat, diffractive optical elements such as zone plates to replace conventional lenses. Direct laser lithography is one of the typical methods used to manufacture such diffraction optical elements. By using a high magnification objective lens, it generates a laser beam with a spot size of less than 1 μm at the focal point. To produce a pattern, such as a diffraction grating, a chromium-coated surface is controllably exposed to this light. In this study, the process conditions for fabricating a zone plate using a direct laser lithography were studied by using an air-bearing type linear XY stage which can easily make various patterns including a zone plate array. The previous studies using a polar coordinate lithographic system have an advantage for fabricating the circular type zone plate because the system uses a rotational stage. It, however, is not proper to fabricate an array type diffractive optics element. In this study, we used a rectangular type lithographic system by using an XY stage and tested the fabrication issues such as a runout error. The optical performance of the zone plate array fabricated by the suggested method was also verified by experimental evaluation.

Investigation of physical properties of quartz after focused ion beam bombardment

Optical properties (transmission and refractive index) and phase change (from amorphous to crystal) of a commonly used glass, quartz, were investigated before and after focused ion beam (FIB) bombardment with ion energy from 30 to 50 keV. We found different influences of FIB bombardment on the optical properties and chemical structure of the quartz in the wavelength region of visible and near infrared, respectively. The quartz still can be used in the infrared wavelength for conventional optical applications. As an application example, an array of diffractive optical elements (DOEs) was directly fabricated on the quartz by the FIB milling. The measured diffraction efficiency of the DOEs is 83.5%, which is acceptable for practical use.

Design of array of diffractive optical elements with inter-element coherent fan-outs

Building on the optimal-rotation-angle method, an algorithm for the design of inter-element coherent arrays of diffractive optical elements (DOEs) was developed. The algorithm is intended for fan-out DOE arrays where the individual elements fan-out to a sub-set of points chosen from a common set of points. By iteratively optimizing the array of elements as a whole the proposed algorithm ensures that the light from neighbouring elements is in-phase in all fan-out points that are common to neighbouring DOEs. This is important in applications where a laser beam scans the DOE array and the fan-out intensities constitute a read-out of information since the in-phase condition ensures a smooth transition in the read-out as the beam moves from one DOE to the next. Simulations show that the inter-element in-phase condition can be imposed at virtually no expense in terms of optical performance, as compared to independently designed DOEs.

The promise of diffractive optics in maskless lithography

For photon-based maskless lithography, a system based on the incoherent addition of scanned, multiplexed, on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We have fabricated binary-phase zone-plate arrays with numerical apertures up to 0.9 with near-ideal performance, and used them to achieve high quality lithography of arbitrary patterns. Using a wavelength of 400 nm we achieve k 1 factors as low as 0.32, without the use of any resolution enhancement techniques. Sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1. Advantages of our diffractive-optical approach include: the feasibility of wavefront engineering by using elements other than binary-phase zone plates; the simplicity of the planar processes needed to manufacture large arrays of high-NA diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

The promise of diffractive optics in maskless lithography

For photon-based maskless lithography, a system based on the incoherent addition of scanned, multiplexed, on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We have fabricated binary-phase zone-plate arrays with numerical apertures up to 0.9 with near-ideal performance, and used them to achieve high quality lithography of arbitrary patterns. Using a wavelength of 400 nm we achieve k 1 factors as low as 0.32, without the use of any resolution enhancement techniques. Sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1. Advantages of our diffractive-optical approach include: the feasibility of wavefront engineering by using elements other than binary-phase zone plates; the simplicity of the planar processes needed to manufacture large arrays of high-NA diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

The case for diffractive optics in maskless lithography

For photon-based maskless lithography we show that a system based on the incoherent addition of scanned, multiplexed, and on-axis focal spots from an array of diffractive-optical elements, such as binary-phase zone plates, has significant advantages over systems based on partially coherent projection of the image of a micromechanical mirror array through a large refractive or reflective lens. We show that zone-plate arrays with numerical apertures up to 0.9 can be manufactured with near-ideal performance, and used to achieve high-quality lithography of arbitrary patterns. Using a wavelength of 400 nm we demonstrate k1 factors as low as 0.32, without the use of any resolution-enhancement techniques; that sufficient contrast is achieved in dense patterns despite the presence of diffracted orders other than +1; and that a process-latitude greater than 10% is achieved for 150 nm lines and spaces. Advantages of our diffractive-optical approach include: the feasibility of wave-front engineering by using elements other than binary-phase zone plates; the simplicity of the processes needed to manufacture large arrays of high-numerical-aperture diffractive-optical elements; and the ease of wavelength scaling, with the promise of ultimately approaching the limits of the lithographic process.

Design of diffractive optical elements array with wide field of view for integration with photodetectors

A special design is proposed to achieve a diffractive optical elements (DOEs) array with wide field of view (WFOV), which integrates with InGaAs/InP p–i–n photodetectors array on the rear surface. It consists of five different DOEs that focus the light from different angles into the corresponding photodetectors. Each DOE will take care of one dedicated direction of the incident light. Consequently, a WFOV of 120° can be obtained. This will increase the detection efficiency of the photodetector array. At the same time, we can also determine the direction of the incident light by probing each photodetector elements response. The structure and parameters of the designed DOEs array have been evaluated. The optimum surface-relief equation of each DOE, which describes an aspheric surface profile, has been obtained. Simulation results of their performance will be presented in this paper. Combining with these efforts, the optimum parameters for fabricating the designed DOE array by direct laser writing are also described.

Investigation of direct milling of micro-optical elements with continuous relief on a substrate by focused ion beam technology

A novel method of manufacturing micro-optical elements with continuous relief by focused ion beam (FIB) is described in detail. Microfabrication of diffractive optical elements (DOEs) with feature sizes as small as the submicronmeter range is realized in this way. Making use of direct milling by FIBs, the microstructure on any substrate (metal and nonmetal) and the mold used for micro-optical element replication are realized easily without any other procedures. The DOEs are duplicated by hot-embossing with relief surface roughness [root mean square (rms) value] of 3 nm measured in a 2 x2 µm2 area, which is higher than the 5 nm achieved with conventional methods. In addition, other examples such as a microlens mold, a microcylindrical lens, and a 3 x3 DOE array are introduced briefly.

Diffractive optical array element for realizing uniform spot on the plane non-perpendicular to the optical axis with geometrical shape transform

In this paper, the problem of transforming laser input of any shape to uniform spots output of any shape on the plane non-perpendicular to the optical axis is studied. A diffractive optical elements (DOE) array is used to obtain such uniform spot. A linear phase-shift is added to the squared cells of the DOE array so as to change the relative position and the overlapped area of every sub-focal spot to realize the geometrical shape transform. An indirect design principle based on long focal depth is adopted, which modifies the phase distribution of the cells to realize the uniform spots on three planes at right angles with the optical axis and a uniform spot is ensured on the inclined plane. A type of hybrid algorithm based on hill-climbing and simulated annealing is utilized for phase design. Simulated results show that the method is easy and valid.

Compensation of the wavelength dependence in diffractive star couplers

Optical systems are described that compensate for the wavelength dispersion that arises in a diffractive fan-out star coupler. Two approaches are investigated, a space-variant one and a space-invariant one. In the space-variant approach an array of diffractive optical elements corrects the wave fronts. In the space-invariant approach a combination of refractive and diffractive lenses compensates for the chromatic focus shifts that are caused by the fan-out element. Examples for such compensating optical systems are presented, along with simulated results. The detailed design for an optimized space-invariant system is described.

Programmable optical logic systems using free-space optical interconnections

A free-space optical logic technique is presented that utilizes a two-dimensional array of diffractive optical elements. Each optical element focuses light to multiple, separate positions in the output focal plane. The focal spots from different optical elements are allowed to overlap spatially, resulting in interference. By changing the phase shift between the optical elements, one can create different optical logic operations in the focal plane. The technique is demonstrated by the use of two input beams incident onto a multiplexed optical element written onto a programmable spatial light modulator. The optical element simultaneously creates both AND and XOR logic functions in the output plane.

Binary Optics Microlens Arrays in CdTe.

ABSTRACTArrays of miniature focusing optics located at the focal plane can improve the performance of focal plane systems. By more completely collecting the light at the focal plane and concentrating it into a smaller spot size on the detector plane, the photodetector area can be substantially reduced. Increased gamma radiation hardening and noise reduction result from the decrease in photodetector surface area. Binary optics technology, a process for fabricating large arrays of diffractive optical elements, is especially attractive for infrared materials. In this paper, diffractive Fresnel microlens arrays containing over six thousand F/0.9 lenslets are patterned in the surface of CdTe substrates by successive photolithographic and Ar+ ion-beam-etching steps. Results on smaller arrays of monolithically integrated binary-optics lenslets with II-VI detectors, demonstrating enhanced photodetector responsivities, are presented for the first time.