Allen Y. Yi

Design and fabrication of a microlens array by use of a slow tool servo

In recent years it has become possible to fabricate free-form optics by use of multiaxis ultraprecision machines. Here a 5 x 5 microlens array is fabricated by using an ultraprecision diamond turning machine equipped with four independent axes. Unlike the conventional process where a single diamond tool is used to machine one lens at a time, this research demonstrates the development of an innovative diamond tool trajectory that allows the entire microlens array to be machined in a single operation. The machined microlens array is measured for both curve conformity and surface roughness. Compared to the conventional approach where indexing the workpiece is difficult and unreliable, this process can produce microlenses with accurate geometry and optical surface finish. This unique process is described in detail from optical measurement to machining process design and development to final results. This research also demonstrates the possibility of fabricating any arbitrary shape with the same approach.

Development of a 3D artificial compound eye

In this research paper, in a major departure from conventional 2D micromachining processes, design and fabrication of a 3D compound eye system consisting of a 3D microprism array, an aperture array, and a microlens array were investigated. Specifically, the 3D microprism array on a curved surface was designed to steer the incident light from all three dimensions to a 2D plane for image formation. For each microprism, there is a corresponding microlens to focus the refracted light on the image plane. An aperture array was also implemented between the microprism array and the microlens array to eliminate cross-talk among the neighboring channels. In this system, 601 individual micro-assemblies consisting of microprisms and microlenses were constructed in a 20 mm diameter area. In this configuration, the maximum light deviation angle was determined to be 18.43 degrees. This research demonstrated an innovative and integrated approach to fabricating true 3D micro and meso scale optical structures. This work also validated the feasibility of using ultraprecision machining process for 3D microoptical device fabrication. The technology demonstrated in this research has high potentials in optical sensing, vision research and many other optical and photonic applications.

Design and fabrication of a freeform microlens array for a compact large-field-of-view compound-eye camera

In this research, a unique freeform microlens array was designed and fabricated for a compact compound-eye camera to achieve a large field of view. This microlens array has a field of view of 48°×48°, with a thickness of only 1.6 mm. The freeform microlens array resides on a flat substrate, and thus can be directly mounted to a commercial 2D image sensor. Freeform surfaces were used to design the microlens profiles, thus allowing the microlenses to steer and focus incident rays simultaneously. The profiles of the freeform microlenses were represented using extended polynomials, the coefficients of which were optimized using ZEMAX. To reduce crosstalk among neighboring channels, a micro aperture array was machined using high-speed micromilling. The molded microlens array was assembled with the micro aperture array, an adjustable fixture, and a board-level image sensor to form a compact compound-eye camera system. The imaging tests using the compound-eye camera showed that the unique freeform microlens array was capable of forming proper images, as suggested by design. The measured field of view of ±23.5° also matches the initial design and is considerably larger compared with most similar camera designs using conventional microlens arrays. To achieve low manufacturing cost without sacrificing image quality, the freeform microlens array was fabricated using a combination of ultraprecision diamond broaching and a microinjection molding process.

Freeform manufacturing of a microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo

We report what is to our knowledge the first approach to diamond turn microoptical lens array on a steep curved substrate by use of a voice coil fast tool servo. In recent years ultraprecision machining has been employed to manufacture accurate optical components with 3D structure for beam shaping, imaging and nonimaging applications. As a result, geometries that are difficult or impossible to manufacture using lithographic techniques might be fabricated using small diamond tools with well defined cutting edges. These 3D structures show no rotational symmetry, but rather high frequency asymmetric features thus can be treated as freeform geometries. To transfer the 3D surface data with the high frequency freeform features into a numerical control code for machining, the commonly piecewise differentiable surfaces are represented as a cloud of individual points. Based on this numeric data, the tool radius correction is calculated to account for the cutting-edge geometry. Discontinuities of the cutting tool locations due to abrupt slope changes on the substrate surface are bridged using cubic spline interpolation.When superimposed with the trajectory of the rotationally symmetric substrate the complete microoptical geometry in 3D space is established. Details of the fabrication process and performance evaluation are described.

Fabrication of diffractive optics by use of slow tool servo diamond turning process

In recent years, it has become possible to fabricate complicated optical surfaces using multi-axis ultraprecision machines. Two diffractive optical designs were fabricated using an ultraprecision diamond turning machine equipped with four independent axes. Unlike the conventional clean-room-based micromachining process, this research demonstrates the development of two innovative diamond tool trajectories that allow the entire diffractive pattern to be machined in a single operation directly, without going through multiple steps, as commonly used in conventional lithography processes. The machined diffractive optical elements were measured for curve geometry and surface roughness. In addition, the optical performance was also evaluated. Finally, a simple welding test setup was utilized to test the 256-level diffractive optical elements (DOEs). Compared to conventional approaches where feature indexing is difficult and unreliable, the slow tool servo (STS) process can be utilized to produce DOEs with accurate geometry and optical surface finish; therefore, the process may be called non-clean-room or maskless micromachining. Unlike its predecessors, this micromachining process which is based on ultraprecision diamond machining can be used to produce true three-dimensional (3D) features in a single operation, thus making it a promising technology for micro-optical, electromechanical component fabrication. Moreover, the 3D micro features can be readily applied to a freeform substrate, making this process a unique approach for fabrication of complicated micro-optical devices.

Numerical Simulation and Experimental Study of Residual Stresses in Compression Molding of Precision Glass Optical Components

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. Residual stresses incurred during postmolding cooling are an important quality indicator for these components. In this research, residual stresses frozen inside molded glass lenses under different cooling conditions were investigated using both experimental approach and numerical simulation with a commercial finite element method program. In addition, optical birefringence method was also employed to verify the residual stress distribution in molded glass lenses. Specifically, optical retardations caused by the residual stresses in the glass lenses that were molded with different cooling rates were measured using a plane polariscope. The measured residual stresses of the molded glass lenses were compared with numerical simulation as a validation of the modeling approach. Furthermore, a methodology for optimizing annealing process was proposed using the residual stress simulation results.

Microfabrication on a curved surface using 3D microlens array projection

Accurate three-dimensional microstructures on silicon or other substrates are becoming increasingly important for optical, electronic, biomedical and medical applications. Traditional microfabrication processes based on cleanroom lithography and dry or wet etching processes are essentially two-dimensional methods. In the past, complicated procedures were designed to create some three-dimensional microstructures; however, these processes were mainly used to create features on planar silicon wafer substrates using the bulk silicon machining technique. In a major departure from previous micromachining processes, a microfabrication process based on microlens projection is presented in this paper. The proposed microfabrication system will have the capabilities of a typical conventional micromachining process plus the unique true three-dimensional replication features based on microlenses that were created on a steep curved substrate. These microlenses were precisely fabricated with a specific pattern on the curved surface that can be used to create microstructures on a pre-defined nonplanar substrate where a layer of photoresist was spin coated. After proper exposure and development, the desired micro patterns are created on the photoresist layer. These micro features can eventually be replicated on the substrate via wet or dry etching processes. The results show that the fabricated three-dimensional microlens array has very high dimensional accuracy and the profile error is less than 6 µm over the entire surface.

Precision compression molding of glass microlenses and microlens arrays—an experimental study

An innovative manufacturing process utilizing high-temperature compression molding to fabricate aspherical microlenses by using optical glasses, such as BK7, K-PG325, and soda-lime glass, is investigated. In a departure from conventional approaches, a unique hollow contactless mold design is adopted. Polished glass substrates and the mold assembly are heated above the glass transition temperature first, followed by initial forming, then annealing. The forming rate is controlled in real time to ensure mold position accuracy. Mold materials used include tungsten carbides, 316 stainless steel, 715 copper nickel, and aluminum alloys. The geometric control of the microlenses or microlens arrays can be precisely controlled by the forming temperature, forming speed, mold design, and annealing time.

Refractive index variation in compression molding of precision glass optical components

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. In a compression molding process, a variation of the refractive index occurs along the radial direction of the glass component due to thermal treatment. The variation of refractive index is an important parameter that can affect the performance of optical lenses, especially lenses used for high precision optical systems. Refractive index variations in molded glass lenses under different cooling conditions were investigated using both an experimental approach and a numerical simulation. Specifically, refractive index variations inside molded glass lenses were evaluated by measuring optical wavefront variations with a Shack-Hartmann sensor system. The measured refractive index variations of the molded glass lenses were compared with the numerical simulation as a validation of the modeling approach.

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.