Fresnel Microlenses Research Articles

Single-step fabrication of Fresnel microlens array on sapphire substrate of flip-chip gallium nitride light emitting diode by focused ion beam

A simple one-step focused ion beam milling technique was used to fabricate a Fresnel microlens array on the backside of sapphire substrate of a gallium nitride blue light emitting diode. The optical output power from the flip-chip gallium nitride blue light emitting diode is enhanced to about 1.68 times at the injection current of 20mA. The spatial light distribution from the backside of flip-chip gallium nitride blue light emitting diode with a Fresnel microlens array shows a uniform and stronger light emission.

FIB-milling of photonic structures and sputtering simulation

Focused ion beam milling is used first to sputter simple geometric structures to investigate sputter depth dependence on parameters like ion fluence, beam diameter, and scanning strategy and second to structure photonic crystal etching masks and Fresnel micro-lenses in various materials. Computer simulation of specific scanning strategies is performed to achieve the highest possible precision with respect to the initial design and to obtain controllable and reproducible milling processes.

280 nm Emission Deep UV LEDs with Integrated Fresnel Microlenses

AbstractWe report on the integration of flowable oxide based Fresnel microlenses with AlGaN based 280 nm light emitting diodes (LED). The lenses were fabricated on the back side of the LED sapphire substrates using direct electron beam writing. Ten concentric rings with different width and variable thickness were designed for 360 degree phase correction. Within each ring the thickness was varied in five steps to approximate a linear profile. The width of each thickness step varied from 100 nm to several microns. Outer diameter of the lens was 65μm. A focal distance of 68 μm was measured for the fabricated microlenses. At the focal plane a FWHM of intensity profile as small as 14 μm was measured for lenses integrated with 30 μm diameter UV LEDs . The maximum intensity at focal plane exceeded the background radiation by a factor of 50. Comparison of the LED performance before and after the lens fabrication did not reveal any degradation of integral efficiency of devices. These results demonstrate the feasibility of using flowable oxide Fresnel microlenses in optical systems based on micro-pixel deep UV AlGaN LEDs.

Micro Fresnel Lens Array를 이용한 Backlight Unit 설계

LCD Backlight에서 Micro Fresnel Lens Array (MFLA)가 결합된 도광판을 설계<TEX>${\cdot}$</TEX>분석하였다. MFLA가 적용된 도광판과 기존의 프리즘 형태의 도광판의 광학적 특성을 전산모의하여 비교한 결과를 나타내었다. MFLA를 이용한 도광판을 사용할 경우 기존의 LCD backlight 장치에서 필요로 했던 프리즘 필름을 대체할 수 있음을 보였다. A light-guide plate of an LCD backlight coupled with a microfresnel lens array (MFLA) is designed and analyzed. Computer simulated results of optical characteristics of our MFLA-type light-guided plate compared with the conventional prism-type one are presented. We show that the MFLA-type light-guide plate can replace well the conventional prism-type plate.

Multi-step electron beam technology for the fabrication of high performance diffractive optical elements

Optical performance, i.e. the diffraction efficiency, numerical aperture and wave front quality is of great importance for further successful application of diffractive optical elements in a wide range of research and commercial applications. For the fabrication process, simultaneously large numerical aperture and high diffraction efficiency result in a pattern with sub-wavelength resolution and a multi-level phase profile of nanometer accuracy. To fulfill such high requirements, a multi-step fabrication technique is proposed, in which the electron beam is used both for pattern writing and layout alignment. This method enables the fabrication of a structure with the L = 2 n discrete phase levels in n sequential lithographic steps. In each step, the pattern written in a thin, high contrast resist layer is transferred after development into the substrate by reactive ion etching to form the phase profile. In comparison to the multi-mask binary optics technology, the proposed method offers a higher resolution and reduces the alignment errors to insignificant values. On the other hand, in comparison to the direct write analog technique, our approach allows for a much better control of the phase profile and ensures larger process tolerances. The method has been successfully applied in the fabrication of several types of diffractive elements, including micro-Fresnel lens arrays, phase sampling filters and diffraction gratings on quartz and GaAs wafers. The diffraction efficiencies of these elements were found to be up to 92%. The lens exhibited diffraction-limited focusing characteristics and an insignificant wave front aberration (rms 0.01 λ).

Wavefront reconstruction with the adaptive Shack–Hartmann sensor

In a Shack–Hartmann sensor the wavefront to be measured is sampled by a microlens array. The beams are directed to a spot in the focal plane. The displacement of the spots is a measure for the local gradient of the wavefront over one microlens. In the adaptive Shack–Hartmann sensor the static microlens array of the conventional sensor is replaced by a dynamic liquid crystal display (LCD). The LCD is used to display an array of Fresnel microlenses. Thereby, the Fresnel microlenses can be adapted to the wavefront and the design parameters like focal length, aperture size, position and number of lenses can be changed very quickly. We present an investigation about the wavefront reconstruction with the adaptive Shack–Hartmann sensor. Because of the large pixelsize of the LCD in comparison to static manufactured holograms the microlens diameter of the adaptive Shack–Hartmann sensor is larger than in a conventional sensor. This requires considerations in the reconstruction algorithm to minimize reconstruction errors. We compare Zernike and B-Spline wavefront reconstruction on the basis of fit accuracy, reconstruction time and the effect of missing measurement data and noise. We come to the conclusion that Zernike reconstruction is better for fitting simple wavefronts while B-Spline reconstruction is the better choice for more complex wavefronts.

Surface micromachining of UV transparent materials

A method which utilizes XeCl excimer laser and an absorbing liquid in contact with the material for precise structuring of UV transparent materials is presented. This one step micromachining process enables the fabrication of micro-optical elements with continuous profiles such as Fresnel micro-lenses in CaF2 and quartz with fluences well below the damage threshold of these materials. The roughness of the etched features varies from 10 nm to 3 μm depending on the laser fluence and material. The etch rates of different UV transparent materials (such as CaF2, BaF2, sapphire and quartz) at various laser fluences suggest that several different parameters influence the etching process.

Micromachining of circular ring microstructure by femtosecond laser pulses

In this paper, integration of interference phenomenon into femtosecond laser micromachining was reported as the femtosecond laser pulses were reshaped spatially to perform ablation. The generation of circular interference pattern was demonstrated by overlapping infrared femtosecond laser pulses. The interference pattern was subsequently focused on a copper substrate to ablate microstructures of concentric circular rings. The present technique is expected to open up new applications in the areas of rapid fabrication of micro-Fresnel lenses, hybrid microlenses and lens arrays.

The adaptive Shack–Hartmann sensor

Replacing the static microlens array in a Shack–Hartmann sensor by a liquid-crystal display offers a very flexible measurement approach. The liquid-crystal display is used for the generation of an array of Fresnel microlenses. The design parameters of the microlenses like focal length, number of lenses, position, and aperture size can be quickly adapted to solve difficult measurement tasks. This adaptive Shack–Hartmann sensor allows a pre-correction of the wavefront to take care of larger aberrations and to achieve a higher measurement accuracy. This makes new strategies possible to determine the wavefront.

Zonal diffraction efficiencies and imaging of micro-Fresnel lenses

The dependence of the zonal efficiency of a transmission Fresnel lens on the number of quantization levels and on the polarization of the light, without and with an antireflection (AR) overcoating is investigated. The height of the grooves with a blaze structure is locally optimized for maximum efficiency. A decreasing structure height over the zonal radius from the centre to the outer zones improves the efficiency because of a better-adapted blaze. An additional increase in the efficiency can be obtained by increasing the AR coating thickness over the zonal radius from the centre outwards. Furthermore, the point spread functions (PSFs) for several quantization levels are compared for both polarization cases. While the AR overcoating has nearly no effect on the shape of the x polarization PSF, the y polarization PSFs show differences between both AR coating and no coating and is reduced with an increasing number of levels. Finally, the comparison between the measured efficiency of coated silicon Fresnel lenses and a calculation lead to acceptable results. All calculations are performed with the integral equation system method with parametrization of the grating profile, an exact electromagnetic grating diffraction method for arbitrary profile shapes.

Zonal diffraction efficiencies and imaging of micro-Fresnel lenses

Abstract The dependence of the zonal efficiency of a transmission Fresnel lens on the number of quantization levels and on the polarization of the light, without and with an antireflection (AR) overcoating is investigated. The height of the grooves with a blaze structure is locally optimized for maximum efficiency. A decreasing structure height over the zonal radius from the centre to the outer zones improves the efficiency because of a better-adapted blaze. An additional increase in the efficiency can be obtained by increasing the AR coating thickness over the zonal radius from the centre outwards. Furthermore, the point spread functions (PSFs) for several quantization levels are compared for both polarization cases. While the AR overcoating has nearly no effect on the shape of the x polarization PSF, the y polarization PSFs show differences between both AR coating and no coating and is reduced with an increasing number of levels. Finally, the comparison between the measured efficiency of coated silicon Fresnel lenses and a calculation lead to acceptable results. All calculations are performed with the integral equation system method with parametrization of the grating profile, an exact electromagnetic grating diffraction method for arbitrary profile shapes.

Surface-micromachined free-space micro-optical systems containing three-dimensional microgratings

Free-space micro-optical systems on a chip containing three-dimensional microgratings have been demonstrated using surface-micromachining technique. The micrograting is integrated with a rotary stage, a collimating micro-Fresnel lens, and an edge-emitting laser held by three-dimensional alignment structures on a single Si substrate. Diffraction patterns for various grating rotation angles are observed. Another optical interconnect module consisting of three cascaded microgratings is also demonstrated. The micrograting is a basic building block for many micro-optical systems and is very attractive for applications in microspectrometers, free-space optical interconnect, optoelectronic packaging, and wavelength-division multiplexed integrated micro-optical systems.

Passively aligned hybrid integration of 8 x 1 micromachined micro-Fresnel lens arrays and 8 x 1 vertical-cavity surface-emitting laser arrays for free-space optical interconnect

Surface micromachining technique has been successfully applied to the fabrications of a three-dimensional 8/spl times/1 micro-Fresnel lens array and other novel three-dimensional alignment structures. With the help of these three-dimensional structures, self-aligned integration of the micro-Fresnel lens array and an 8/spl times/1 vertical-cavity surface-emitting laser (VCSEL) array is realized for the first time with passive alignment. Individual addressing of the VCSEL/micro-lens element is also successfully demonstrated. With their three-dimensional and array structural characteristics, they are very attractive for free-space optical interconnect and other integrated micro-optical systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spectrometer employing a micro-Fresnel lens

The dispersion characteristics of a diffractive micro-Fresnel lens (MFL) which is mass-produced for a laser diode collimator are examined for novel applications. The MFL's resolving power (Rayleigh criterion for resolution) at the design wavelength is calculated to be comparable to that of conventional dispersing prisms of the same size. A simple spectrometer with an MFL (4.5 mm diameter, 14.83 mm focal length at 0.78 μm design wavelength) and a movable pinhole (PH), is fabricated. The optimum PH diameter for both resolving power and light efficiency is 5 m, and the resolution and efficiency at the design wavelength are estimated to be 3.7 nm and 85%, respectively. The measurable spectral range determined from both the diffraction efficiency and the free spectral range of the MFL almost covers the entire visible region. The absorption spectrum of KMnO₄ solution is roughly obtained using the spectrometer. The space between the MFL and the PH, and the PH's movable length, which are required for spectroscopy, are as short as 20 and 7 mm, respectively. The simple and straight-line-structured spectroscopic system utilizing a more cost-efficient MFL may lead to the realization of less expensive and compact spectrometers.

Laser-diode collimating light sources using micro-Fresnel lenses

Compact and low-cost laser-diode collimating light sources that employ a thin and mass-producible diffractive micro-Fresnel lens have been developed for optical information-processing equipment such as laser-beam printers. For the compensation of the defocusing of an output beam caused by temperature dependence of the laser-diode wavelength, engineering plastics with different thermal expansion coefficients are adopted for the components. The fabricated light source emits stably up to 60°C. The present laser light sources exhibit allowable optical characteristics for optical information-processing equipment as well as cost-efficient mass production.

Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists

A laser writing system for the fabrication of continuous-relief micro-optical elements in photoresist is described. The technology enables a wide range of planar micro-optical elements to be fabricated and replicated into polymer film using Ni shims electroformed from the photo-resist originals. The advantages and limitations of laser writing technology for micro-optics fabrication are discussed. Examples of fabricated micro-optical elements include Fresnel microlenses and microlens arrays, kinoforms, and other continuous-relief phase elements.

Electron-beam writing system and its application to large and high-density diffractive optic elements

An electron-beam writing system that is chartacterized by a stationary electron beam and a continuously spinning table is proposed and developed for diffractive optic elements with axisymmetric patterns. This system allows us to fabricate continuously concentric or radialized high-density microgratings with effective areas over the electron-beam deflection limits. For a fine electron-dose distribution, we adopted multiple-revolution writing and electron-beam modulation. Several test gratings, including a rotary encoder disk with a 1.57-µm grating pitch and a 15-mm diameter and a blazed micro-Fresnel lens with a 4.5-mm diameter and a N.A. of 0.45, are successfully demonstrated with real-time data processing.

One-step 3D shaping using a gray-tone mask for optical and microelectronic applications

Abstract A novel approach based on the use of gray-tone masks to shape arbitrary profiles is described. Tapered structures in thick polyimide as well as blazed gratings and Fresnel microlenses in fused silica substrates have been fabricated. The 3D shaping capability of this technology is illustrated by a high efficiency of 75% obtained for the lenses. This technology involves one appropriate mask, a photolithographic exposure and an etching step; it is realized on conventional equipment and is therefore cost effective.

Active-fiber star coupler that uses arrays of microlenses and liquid-crystal modulators

An active-fiber star coupler that uses arrays of N x N microlenses and liquid-crystal modulators is presented. A simplified implementation of this device uses an 8 x 8 array of Fresnel microlenses fan out an incident beam into 64 focused spots (an 18 dB fan-out loss); on-off capability (270:1 extinction ratio) at each element is provided by liquid-crystal spatial light modulators. A row of focused spots is coupled into an array of 1 x 8 multimode fibers with a measured excess loss of 12 dB along each path (with an estimated overall loss of 30 dB in an 8 x 8 device). A modification of this device that is capable of wavelength selection at individual output fibers is proposed for wavelength division multiplexing applications.

Integration of planar Fresnel microlenses with vertical-cavity surface-emitting laser arrays

We report a technique of integrating planar Fresnel microlenses with InGaAs/GaAs-based vertical-cavity surfacee-mitting laser arrays by selectively ion-beam milling the substrate. Depending on the application, one can focus, collimate, and bend the individual laser beams using such microlenses. An example is presented where a 32 x 32 array of microlenses, each with an aperture of 80microm and a focal length of 108 microm is integrated with a laser array. As expected, arrays of focused beams, each with a 2-microm spot size, are generated at a distance of approximately 110 microm.