Freeform Lens Array Research Articles

Freeform Lens Array Design for Precise Control of Irradiance Distribution in VCSEL Array

Freeform Lens Array Design for Precise Control of Irradiance Distribution in VCSEL Array

Design of a high-power collimating optical system based on Fresnel lenses.

In light-emitting diode (LED) illumination (e.g., LED maritime lighting for ships), creating a uniform light environment for optical systems is an important challenge. In this study, we present a high-power collimating system based on Fresnel lenses, which allows high-brightness LED illumination in the earlier-mentioned remote distance. The work presented in this article focuses on improving the power, compacting the optical structure, and promoting the brightness of the spot. To prove the claims, the system with a total power of 1 kW is designed. The system consists of a 27 W LED array, a freeform surface lens array, and a confocal Fresnel lens array. In comparison with the traditional optical system, the optical structure shortens from 390 to 120 mm, and the divergent angle decreases from 3° to 2 . Meanwhile, the illuminance of the system is obtained as high as 230 lx at the near field of 200 m and 3.0lx at the far field of 1.5 nautical miles. This new method provides a practical and effective way to solve the problem of low power, insufficient illuminance, and long optical structure for LED array illumination, which is suitable for remote illumination and guidance of ships.

Waveguide Concentrator Photovoltaic with Spectral Splitting for Dual Land Use

This research presents a highly transparent concentrator photovoltaic system with solar spectral splitting for dual land use applications. The system includes a freeform lens array and a planar waveguide. Sunlight is first concentrated by the lens array and then reaches a flat waveguide. The dichroic mirror with coated prisms is located at each focused area at the bottom of a planar waveguide to split the sunlight spectrum into two spectral bands. The red and blue light, in which photosynthesis occurs at its maximum, passes through the dichroic mirror and is used for agriculture. The remaining spectrums are reflected at the dichroic mirror with coated prisms and collected by the long solar cell attached at one end of the planar waveguide by total internal reflection. Meanwhile, most of the diffused sunlight is transmitted through the system to the ground for agriculture. The system was designed using the commercial optic simulation software LightTools™ (Synopsys Inc., Mountain View, CA, USA). The results show that the proposed system with 200× concentration can achieve optical efficiency above 82.1% for the transmission of blue and red light, 94.5% for diffused sunlight, which is used for agricultural, and 81.5% optical efficiency for planar waveguides used for power generation. This system is suitable for both high Direct Normal Irradiance (DNI) and low DNI areas to provide light for agriculture and electricity generation at the same time on the same land with high efficiency.

Direct laser writing method for micro-lens fabrication on the fiber facet

We present a way for micro-lens and micro-lens array fabrication on the fiber facet by direct laser writing (DLW) method. The proposed setup for DLW printing on the fiber facet can protect objective lens and makes it possible to safely using a wide range of resists. Microlens fabricated on the single-mode optical fiber facet is demonstrated. Based on lens-on-fiber system we proposed concept of the micro-lenses on baseplate array printed on the multi-core fiber, where each lens is rigidly aligned with the core. One of the possible micro-lenses on the baseplate array is presented and its optical properties such as focal spot size and resolution are investigated. The possible application of the proposed micro-lens array is complex optical elements, such as micro-objectives with optimized optical design. Moreover, suggested freeform lens array can find application in high-accuracy wavefront sensing.

Freeform beam splitting system design for generating an array of identical sub-beams.

Laser beam splitting by freeform optics is promising but less studied. Instead of directly forming a target spot array, we propose to first convert the input beam into a closely connected Gaussian sub-beam array. All the Gaussian sub-beams have the same optical field distributions which thus can produce identical discrete spots on the target plane. Such a design concept is very beneficial to ensure the consistency for laser processing. Importantly, the introduction of an intermediate Gaussian sub-beam array can reduce diffraction effects when the size of each Gaussian sub-beam is sufficiently larger than that of the corresponding sub-area within the input beam. The desired transformation can be achieved by two typical systems. The first system consists of two plano-freeform lenses. The second system is composed of a plano-freeform lens and a lens with an entrance freeform surface and an exit surface of freeform lens array. The two freeform beam splitting systems can be determined based on appropriate ray mappings among the input, intermediate and target irradiance distributions and a subsequent double-surface construction. Geometrical and physical simulations verify the effectivenesses of the two beam splitting systems.

Dynamic Aberration Correction for Conformal Dome Based on Free-Form Lens Array

Dynamic Aberration Correction for Conformal Dome Based on Free-Form Lens Array

Integration of On-machine Surface Measurement into Fast Tool Servo Machining

Fast tool servo (FTS) machining is a highly efficient approach to generate optical freeform surfaces and microstructures, where submicrometric form accuracy and nanometric surface finish are often required. The on-machine surface measurement (OMSM) is an enabling technology to improve the metrology efficiency and machining accuracy of FTS. However, the integration of the OMSM with FTS on an ultra-precision machine is a huge challenge due to the complexity of data processing and synchronization. In this paper, a dedicated controller is in-house developed with data processing algorithms for the integration. The effectiveness of the proposed method is demonstrated through the successful fabrication and measurement of a freeform lens array.

Design of a freeform lens array based on an adjustable Cartesian candela distribution

ABSTRACTLight-emitting diode (LED) modules can be flexibly disassembled and assembled and have inherent advantages in large-area lighting applications. In this paper, an efficient method is proposed to design an LED freeform lens array for direct lighting based on an adjustable Cartesian candela distribution. The Cartesian gradient candela distribution (CGCD) is adjusted and determined through the initialized design of a single freeform lens and a diffuser plate. The layout of the LED lens array can then be optimized by controlling the semi-angle of the light intensity distribution (SALI). A design example shows that the uniformity of the lens array increases by 11.63% through only a two-round iterative optimization. Both the simulation and experimental results are discussed to verify the proposed design. The freeform lens array is expected to expand the applications of mini- or micro-LED devices.

Luminance spreading freeform lens arrays with accurate intensity control.

Glare and visual discomfort are important factors that should be taken into account in illumination design. Conventional freeform lenses offer perfect control over the outgoing intensity distribution, thereby allowing optical radiation patterns with sharp cut-offs in order to optimize the unified glare rating index. However, these freeform lenses do not offer control over the near-field luminance distribution. Observing the emitted light distribution from a high-brightness LED through a freeform lens gives a high peak luminance that can result in glare. To reduce this peak luminance, freeform lenses should be used in conjunction with light diffusing structures. However, this diminishes the control over the outgoing intensity distribution what is the main benefit of a freeform lens. Another approach to reduce the observed peak luminance, is by spreading the emitted light over multiple optical channels via freeform lens arrays. This paper proposes a novel method to design luminance spreading freeform lens arrays that offer perfect control over the resulting intensity pattern. The method is based on a non-invertible mapping of a 2D parameter space. This results in a source-target mapping in which multiple ingoing ray directions are mapped onto every position of the target distribution. The case of continuous and discontinuous mappings are both discussed in this paper. Finally, the example of a discontinuous freeform lens array with 7×7 individual lenses is designed and experimentally demonstrated.

Generation of propagation-invariant and intensity-controlled dark hollow beams by a refractive beam shaping system.

Dark hollow beams (DHBs) have great potential in the applications of optical manipulation, and the generation of DHBs is still a challenging and rewarding issue. In this paper, we present a beam shaping system for generating DHBs. The proposed system is composed of a freeform lens array and a non-classical zoom system which has a constant focal length but various image locations. The DHBs with a well-controlled intensity profile generated by the proposed system is not sensitive to the change of the intensity distribution of the incident beam, which allows flexible choices of light sources. Moreover, the annular pattern produced by the DHB remains unchanged when the image plane is moved a long distance of 17mm, and the energy efficiency of the beam shaping system is greater than 90% when Fresnel loss is considered. The proposed beam shaping system endows the generated DHBs with new properties and may have great potential in the field of optical tweezers and atom guides.

Printed freeform lens arrays on multi-core fibers for highly efficient coupling in astrophotonic systems.

Coupling of light into multi-core fibers (MCF) for spatially resolved spectroscopy is of great importance to astronomical instrumentation. To achieve high coupling efficiencies along with fill-fractions close to unity, micro-optical elements are required to concentrate the incoming light to the individual cores of the MCF. In this paper we demonstrate facet-attached lens arrays (LA) fabricated by two-photon polymerization. The LA provide close to 100% fill-fraction along with efficiencies of up to 73% (down to 1.4 dB loss) for coupling of light from free space into an MCF core. We show the viability of the concept for astrophotonic applications by integrating an MCF-LA assembly in an adaptive-optics test bed and by assessing its performance as a tip/tilt sensor.

A laser beam shaper for homogeneous rectangular illumination based on freeform micro lens array

An effective design method of freeform micro lens array is presented for shaping varied laser beams into prescribed rectangular illumination. The variable separation mapping is applied to design concave freeform surfaces for constructing a freeform lens array. Several dedicated examples show that the designed freeform optical lens array can achieve a prescribed rectangular illumination pattern, especially without considering the initial states of incident laser beams. Both high collection efficiency and good spatial uniformity can be available simultaneously. Tolerance analysis is also performed to demonstrate that this optical device can well avoid fabricating difficulty in actual applications.

Deconvolution method in designing freeform lens array for structured light illumination.

We have developed a deconvolution freeform lens array design approach to generate high-contrast structured light illumination patterns. This method constructs the freeform lens array according to the point response obtained by deconvoluting the prescribed illumination pattern with the blur response of the extended light source. This design method is more effective in designing a freeform lens array to achieve accurate structured light patterns. For a sinusoidal fringe pattern, the contrast ratio can be as high as 97%, compared to 62% achieved by the conventional ray mapping method.

Development of a low cost high precision three-layer 3D artificial compound eye

Artificial compound eyes are typically designed on planar substrates due to the limits of current imaging devices and available manufacturing processes. In this study, a high precision, low cost, three-layer 3D artificial compound eye consisting of a 3D microlens array, a freeform lens array, and a field lens array was constructed to mimic an apposition compound eye on a curved substrate. The freeform microlens array was manufactured on a curved substrate to alter incident light beams and steer their respective images onto a flat image plane. The optical design was performed using ZEMAX. The optical simulation shows that the artificial compound eye can form multiple images with aberrations below 11 μm; adequate for many imaging applications. Both the freeform lens array and the field lens array were manufactured using microinjection molding process to reduce cost. Aluminum mold inserts were diamond machined by the slow tool servo method. The performance of the compound eye was tested using a home-built optical setup. The images captured demonstrate that the proposed structures can successfully steer images from a curved surface onto a planar photoreceptor. Experimental results show that the compound eye in this research has a field of view of 87°. In addition, images formed by multiple channels were found to be evenly distributed on the flat photoreceptor. Additionally, overlapping views of the adjacent channels allow higher resolution images to be re-constructed from multiple 3D images taken simultaneously.

Thin LED collimator with free-form lens array for illumination applications

We propose a compact optical system made up of a thin collimator and a free-form lens array to provide the uniform illumination of various-shape regions. Analytical formulas to design the lens array are derived. The simulation results for the designed optical elements demonstrate the high-level illumination uniformity of various regions in the form of a rectangle, a cross, a flag-shaped region, a line segment, and a set of line segments.

Freeform lens arrays for off-axis illumination in an optical lithography system

We propose a method of designing a freeform lens array for off-axis illumination (OAI) in optical lithography to produce desired OAI patterns and improve efficiency. Based on the Snell law and the conservation law of energy, a set of first-order partial differential equations are derived and the coordinate relations for each OAI pattern are established. The contours of the freeform lens unit are calculated numerically by solving the partial differential equations, and the freeform lens array is obtained by arraying the lens units. Moreover, the optical performance for each OAI pattern is simulated and analyzed by software. Simulation results show that the irradiance distribution of each OAI pattern can be well controlled with a maximum uniformity of 92.45% and a maximum efficiency of 99.35%. Also, analysis indicates that this method has the advantages of reducing the complexity of the exposure system and having good tolerance to the input intensity variations of the laser beam.

Diamond Turning Freeform Optics

Freeform optics fabrication has become one of the hottest topics in optics industry in recent years. Although it still remains a challenge, many have tried different ways of manufacturing it. Some have achieved degrees of success. By means of a Nanotech 350-FG five axis diamond turning machine, we too have successfully produced some prototype freeform optics and lens arrays with Slow Tool Servo and Milling method. The produced freeform optics are mainly for automobile LED headlamps and the lens arrays are for LED illumination. In order to produce the freeform optics, we developed our own DT Slow Tool Servo program which is capable of generating a DT program for diamond turning a universal/general 3D freeform surface. Slow Tool Servo technique and Diamond Milling technique were mainly employed to produce these freeform surfaces. The manufacturing process and machining parameter details will be given in the paper. The two main methods we used will be compared and discussed as well. In measuring the freeform surface, a 3D white light interferometer was used to scan and obtain the surface coordinates. The software made by ourselves enabled us to compare the measure results of the work piece with that of the design drawings. The deviation of our finished forms is within 5 um from that of the nominal. The surface quality Rq is about 10 nm. Measuring equipment and methodology will also be discussed in the paper.