Surface Relief Diffractive Optical Elements Research Articles

Diffractive optics development using a modified stack-and-draw technique.

We present a novel method for the development of diffractive optical elements (DOEs). Unlike standard surface relief DOEs, the phase shift is introduced through a refractive index variation achieved by using different types of glass. For the fabrication of DOEs we use a modified stack-and-draw technique, originally developed for the fabrication of photonic crystal fibers, resulting in a completely flat element that is easy to integrate with other optical components. A proof-of-concept demonstration of the method is presented-a two-dimensional binary optical phase grating in the form of a square chessboard with a pixel size of 5 μm. Two types of glass are used: low refractive index silicate glass NC21 and high refractive index lead-silicate glass F2. The measured diffraction characteristics of the fabricated component are presented and it is shown numerically and experimentally that such a DOE can be used as a fiber interconnector that couples light from a small-core fiber into the several cores of a multicore fiber.

Broadband Gerchberg-Saxton algorithm for freeform diffractive spectral filter design.

A multi-wavelength expansion of the Gerchberg-Saxton (GS) algorithm is developed to design and optimize a surface relief Diffractive Optical Element (DOE). The DOE simultaneously diffracts distinct wavelength bands into separate target regions. A description of the algorithm is provided, and parameters that affect filter performance are examined. Performance is based on the spectral power collected within specified regions on a receiver plane. The modified GS algorithm is used to design spectrum splitting optics for CdSe and Si photovoltaic (PV) cells. The DOE has average optical efficiency of 87.5% over the spectral bands of interest (400-710 nm and 710-1100 nm). Simulated PV conversion efficiency is 37.7%, which is 29.3% higher than the efficiency of the better performing PV cell without spectrum splitting optics.

Diffractive optical elements with porous silicon layers

We investigate the basic chromatic properties of dispersive surface relief diffractive optical elements with porous silicon (PSi) layers. Rigorous and scalar wavelength-dependent diffraction efficiencies are juxtaposed and compared to reflection coefficients of uniform silicon and PSi layers. The application of the device as an enhanced sensor is discussed. A spectral covariance criterion for efficient evaluation of the spectral changes induced by analyte filling the pores is presented. Experimental results for the device reveal an increased spectral selectivity of the diffractively structured PSi layers compared to uniform PSi layers.

MECHANICAL AND SURFACE TOPOGRAPHY CHANGES DURING MECHANICAL TESTING OF DIFFRACTION OPTICAL ELEMENTS IN POLYMER

D evelopments in modern electronics, optoelectronics, industry of microsystems, and technology of biomedical devices are generating intense research interest. Creating polymer materials with well-defined physical and chemical properties is an important step towards building efficient devices. Replication technologies such as embossing, molding, and casting1,2 are highly attractive for the fabrication of surface-relief Diffractive Optical Element (DOE) microstructures.3,4 They have very high resolution, typically in the nanometer range, and allow the fabrication of large area, complex microstructure by low-cost, high-volume industrial production processes. Their use is already well established for gratings, holograms, and diffractive foils, and the extension to the fabrication of deeper and higher aspect ratio microstructure is underway. The combination of replication technology with other processes such as dry etching and thin film coating can also offer new possibilities in the design of DOEs suitable for mass production. Replication is expected to become a key technology for the microfabrication of DOEs in the future.5–7

Investigation of laser ablation on hybrid sol–gel material applied to kinoform etching

Ultraviolet laser machining of a hybrid organic/inorganic material prepared via a sol–gel process has been studied for the fabrication of kinoforms or surface relief diffractive optical elements. The hybrid mixes silicon and titanium oxides and an organic network in order to improve the mechanical properties. Different material compositions have been investigated. Laser ablation of the hybrid material is observed at low laser fluence (measured threshold fluence of 125 mJ/cm2 at 248 nm/6 ns) and shows that the process is well adapted to micro-patterning by laser machining means. The best observed depth resolution is 60± 20 nm and appears to be limited by the ablation setup. Finally, the fabrication of an effective diffractive optical element and its operation at 1.06 μm are described.

Efficient diffractive optical elements in chalcogenide glass layers fabricated by direct DUV laser writing

Direct laser writing with high power densities at a wavelength of 244 nm is applied to the patterning of surface relief diffractive optical elements (DOEs) with submicron features in thin chalcogenide layers of different compositions. The topology of created features in dependence on exposure conditions is investigated. Potential and limitations of this technique for the fabrication of efficient DOEs for applications from the visible to the near infrared spectral region are discussed on the basis of examples.

High sensitive SiO2/TiO2 hybrid sol-gel material for fabrication of 3 dimensional continuous surface relief diffractive optical elements by electron-beam lithography

A negative-tone sensitive SiO2/TiO2 organic-inorganic hybrid sol-gel material was synthesized and characterized for fabrication of multilevel micro-optical elements by direct electron-beam lithography. The exposure was carried out by an in-house modified electron-beam writing system using LEO SEM-982 with Elphy Quantum exposure beam-blanking control system at 25keV. The hybrid Sol-Gel material demonstrated a superb sensitivity for doses between 0.22microC/cm2 and 0.33microC/cm2.

Photoresist characterization and linearization procedure for the gray-scale fabrication of diffractive optical elements

We present a procedure for the characterization and the linearization of the photoresist response to UV exposure for application to the gray-scale fabrication of diffractive optical elements. A simple and reliable model is presented as part of the characterization procedure. Application to the fabrication of surface-relief diffractive optical elements is presented, and theoretical predictions are shown to agree well with experiments.

Single-step fabrication of surface relief diffractive optical elements on hybrid sol-gel glass

Binary and continuous phase-only surface relief diffractive optical elements can be fabricated on hybrid sol-gel glass based on a single-step microfabrication process. We report on the fabrication and characterization of an inorganic-organic hybrid SiO2/TiO2 sol-gel glass by UV light exposure. Preliminary results of binary and continuous surface relief structures fabricated on sol-gel glass with UV exposure using holographic technique and a high-energy-beam-sensitive (HEBS) glass mask are demonstrated. The effects of the fabrication parameters such as doping concentration, bake temperature, and UV light exposure time on the film characteristics (thickness and refractive index) are studied.

Stripe color separation with diffractive optics

We describe a color separation optical element with potential display applications that is designed to separate light into an array of red, green, and blue stripes formed at a certain distance from the component. The stripe color separation grating is a surface-relief diffractive optical element composed of a repeated pattern of three constituent gratings. We describe the design principles and a numerical analysis of the component, showing that a maximum theoretical efficiency with which the colors are directed into the stripes is approximately 80%. We also examine the dependence of the efficiency on the grating feature size. In addition, we report on the fabrication of four of these components, using a combination of electron-beam lithography and photolithography with a backexposure technique. Measurements are presented to show the color separation property.

On-axis beam extinction through diffraction design and analysis

To optimize the functions of the surface-relief diffractive optical elements for optical limiters and other applications, we present a systematic design and analysis with numerical illustrations of the transmission properties of different surface-relief phase gratings and Fabry-Perot elements. The spectral response and the tolerance of fabrication errors have been included. An analysis shows that the blazed grating and the echelon grating, a multilevel element as a substitute of the blazed grating, can make the on-axis transmittance very low (less than 1% with one grating) over the broad visible band with a large tolerance of fabrication errors. The results are highly significant for the design of optical-limiting devices.

Replication techniques for diffractive optical elements

Replication technologies such as embossing, moulding and casting are highly attractive for the fabrication of surface relief Diffractive Optical Element (DOE) microstructures. They have very high resolution, typically in the nanometer range, and allow the fabrication of large area, complex microstructures by low-cost, high volume industrial production processes. Their use is already well established for gratings, holograms and diffractive foil, and the extension to the fabrication of deeper and higher aspect ratio microstructure is underway. The combination of replication technology with other processes such as dry etching and thin film coating can also offer new possibilities in the design of DOEs suitable for mass-production. Replication is expected to become a key technology for the microfabrication of DOEs in the future. This paper reviews the major DOE replication techniques and describes recent work and results.

Development of Optical Engineering. Diffraction Efficiency and Fabrication Technology of Surface-Relief Diffractive Optical Elements.

In the field of microoptics, surface-relief diffractive optical elements (DOEs) are of great interest. Their thin film structure makes it possible to produce and integrate them with other optical components. Recent development in microfabrication technology including electron-beam lithography and binary optics lithography has demonstrated DOEs with blazed/multilevel relief structure. Such relief structure can improve diffraction efficiency of DOEs. Electron-beam lithography has several advantages such as a potential for submicron patterning and flexibility in design and fabrication. In addition, blazed structure can be formed by one process. Binary optics lithography is also attractive because high efficiency DOEs with multilevel structure can be fabricated by semiconductor techniques used in electronic integrated circuits. Typical DOEs with blazed/multilevel structure and their application to micro infrared sensor are demonstrated. Such surface-relief DOEs are expected to be used in various optical systems.

Continuous-relief diffractive optical elements for two-dimensional array generation

Continuous surface-relief diffractive optical elements for two-dimensional array generation (fan-out) are designed and fabricated. Separable and nonseparable solutions for the two-dimensional element design are compared. The phase-grating microstructures are generated by laser-beam writing lithography in a single exposure step and converted to nickel shims by electroplating, enabling low-cost replicas to be produced by using laboratory and commercial replication processes. Results are presented for a 9 x 9 fan-out diffractive optical element with a measured efficiency of 94% and an overall uniformity within +/-8%; replicas in epoxy have the same efficiency and a uniformity of +/-15%.

Aztec surface-relief volume diffractive structure

Surface-relief diffractive optical elements made interferometrically typically are recorded with the off-axis technique, for which the fringe maxima lie on planes that are predominantly perpendicular to the recording surface and with an intensity variation that is sinusoidal. Such a structure can be readily replicated by mechanical means. Volume diffractive elements, on the other hand, which result from beams propagating in opposite directions, have fringe planes that are predominantly parallel to the surface and, as such, cannot be mechanically replicated. A new type of surface-relief diffractive structure called Aztec is discussed here; it combines features of both off-axis and volume recording geometries, with the result being a phase-quantized, or terraced, surface-relief pattern. The groove profile, instead of being sinusoidal, resembles a stepped pyramid. This structure has been replicated by metal mastering and molding into plastic in the same manner as conventional embossed surface-relief elements, but the diffraction characteristics are typical of volume phase reflective structures. Light of a given wavelength is resonantly diffracted from steps that are a half-wavelength apart and with a bandwidth that is inversely proportional to the number of steps. Color control has been achieved by overcoating the step structure with a clear dielectric that shifts the resonant wavelength to a new value, depending on the index of refraction of the dielectric. Information content is less for the single-layered, but stepped, Aztec structure than for the usual multilayered volume diffractive element. Deep Aztec stepped gratings have also been fabricated by optical lithography, using multiple-mask techniques.