Fabrication of polymer-based reflowed microlenses on optical fibre with control of focal length using differential coating technique

Thermal reflow of polymer to generate spherical profile has been used to fabricate microlenses in this paper. A simple model based on the volume conservation (before and after reflow) and geometrical consideration of lens profile, shows that the focal length of the microlens produced by reflow technique is a function of the initial geometry of microcylinders, i.e. diameter and thickness. This relationship of focal length with diameter and thickness is used as a basis to control focal length. A simple spin coating technique on dual surface is used to achieve differential thickness, to control the focal length of microlenses produced on the same substrate. A biomedical application of such combination of microlenses is endoscopy where the lenses of varying diameter and equal focal length are needed on top of optical fibre bundles to provide independent function of illumination and imaging. This paper incorporates the differential thickness technique to show a micro fabrication process to produce the polymer reflowed microlenses, with a control of focal length based on thickness. The design also helps to integrate these microlenses on top an optical fibre with accurate alignment.     

Rigorous electromagnetic analysis of a microcylindrical axilens with long focal depth and high transverse resolution.

We first present nonparaxial designs for a microcylindrical axilens with different long focal depths and rigorously analyze electromagnetic field distributions of the axilens using integral equations and the boundary-element method. Numerical results show that the designed axilenses indeed have the special feature of attaining a long focal depth while keeping high transverse resolution for numerical apertures of 2.4, 2.0, and 1.0. The ratio between the extended focal depth of the designed axilens and the focal depth of the conventional focal lens is 1.41, the corresponding maximal extended focal depth of the axilens can reach 28 microm, and the spot size of the focal beam is approximately 10 microm over the focal range.     

Actual focal length of a symmetric biconvex microlens and its application in determining the transmitted beam waist position

The actual focal length of a three-dimensional continuous profile symmetric biconvex microlens with normal monochromatic plane wave illumination is theoretically determined using a full-field separation of variables method in the oblate spheroidal coordinate system. The investigations are performed for microlenses of 5, 10, and 20 wavelength diameters by calculating the electromagnetic field distributions inside of and adjacent to the microlenses. The importance and potential application of the microlens actual focal length in the design of microlens optical systems are demonstrated by showing that for normal monochromatic TEM00 mode Gaussian beam illumination, the transmitted beam waist position through a single microlens, calculated using Self's beam waist position transformation formula [Appl. Opt.22, 658 (1983)] with the microlens actual focal length, closely matches the exact value given by the separation of variables method.     

Analysis of a cylindrical microlens array with long focal depth by a rigorous boundary-element method and scalar approximations

We investigated the focal characteristics of open-regional cylindrical microlens arrays with long focal depth by using a rigorous boundary-element method (BEM) and three scalar methods, i.e., a Kirchhoff and two Rayleigh-Sommerfeld diffraction integral forms. Numerical analysis clearly shows that the model cylindrical microlens arrays with different f-numbers can generate focusing beams with both long focal depth and high transverse resolution. The performance of the cylindrical microlens arrays, such as extended focal depth, relative extended focal depth, diffraction efficiency, and focal spot size, is appraised and analyzed. From a comparison of the results obtained by the rigorous BEM and by scalar approximations, we found that the results are quite similar when the f-number equals f/1.6; however, they are quite different for f/0.8. We conclude that the BEM should be adopted to analyze the performance of a microlens array system whose f-number is less than f/1.0.     

Rigorous electromagnetic analysis of the common focusing characteristics of a cylindrical microlens with long focal depth and under multiwavelength illumination

The common focusing characteristics of a cylindrical microlens with a long focal depth and under a given multiple-wavelength illumination are analyzed based on the boundary element method (BEM). The surface-relief profile of a finite-substrate-thickness microlens with a long focal depth is presented. Its focusing performances, such as the common extended focal depth (CEFD), the spot size, and the diffraction efficiency, are numerically studied in the case of TE polarization. The results show that the CEFD of the microlens increases initially, reaches a peak value, and then decreases with increasing preset focal depth. Two modified profiles of a finite-substrate-thickness cylindrical microlens are proposed for enlarging the CEFD. The rigorous numerical results indicate that the modified surface-relief structures of a cylindrical microlens can successfully modulate the optical field distribution to achieve longer CEFD, higher transverse resolution, and higher diffraction efficiency simultaneously, compared with the prototypical microlens. These investigations may provide useful information for the design and application of micro-optical elements in various multiwavelength optical systems.     

Metallic cylindrical focusing micromirrors with long axial focal depth or increased lateral resolution

Using a general focal-length function, two-dimensional long-focal-depth (LFD) metallic cylindrical focusing micromirrors (MCFMs) are designed and the focal performance is systematically investigated based on rigorous electromagnetic theory and the boundary element method. For a positive preset focal depth, simulation results reveal that the designed MCFMs still possess an LFD property and high lateral resolution even when the f-number is reduced to f/0.3. On the other hand, through setting the preset focal depth to be negative, increased lateral resolution is obtained, compared with the conventional MCFM. In addition, under multiwavelength illumination, a large common LFD region is demonstrated for the designed LFD MCFMs, which is due to the intrinsic achromatic property of reflective systems.     

Focal shift and extended focal depth with tunable pupil filter

Extended focal depth and focal shift are very important in microscopy, imaging and optical storage systems, and have attracted much attention in recent years. In order to obtain the extended focal depth and focal shift, a new kind of tunable pupil filter is proposed in this article. It consists of one half-wave plate between two quarter-wave plates, and the half-wave plate is made up of two zones that can rotate with respect to each other. By analyzing the intensity distribution in the focal region of the optical system with such a device, it reveals that focal shift can be realized by rotating any zone of the half-wave plate. When the phase difference of the two zones is π, the extended focal depth and transverse superresolution can be obtained at the same time. Therefore, it may be feasible to use such a tunable pupil filter in optical systems that need focal shift and extended focal depth.     

Evaluation of microlens properties in the presence of high spherical aberration

Microlenses can be generated with various fabrication technologies. Some of these technologies cause large spherical aberrations in the resulting microlenses. We describe an algorithm based on Rayleigh's quarter-wave criterion, which allows the evaluation of lens parameters for those microlenses. Specifically, we investigate numerical aperture, focal length, and space-bandwidth product with respect to applications in optical microsystems. We apply our algorithm to different types of microlenses, three gradient-index lenses, and one surface-relief lens. The experimental results demonstrate that our algorithm provides a helpful characterization method for microlenses with large aberrations.     

Drop‐on‐Demand Inkjet Printing of Thermally Tunable Liquid Crystal Microlenses

In this letter, the authors demonstrate Drop‐on‐Demand printing of variable focus, polarization‐independent, liquid crystal (LC) microlenses. By carefully selecting the surface treatment applied to a glass substrate, the authors are able to deposit droplets with a well‐defined curvature and contact angle, which result in micron‐sized lenses with focal lengths on the order of 300–900 µm. Observations with an optical polarizing microscope confirm the homeotopic alignment of the LC director in the droplets, which is in accordance with the polarization independent focal length. Results show that microlenses of different focal lengths can be fabricated by depositing successive droplets onto the same location on the substrate, which can then be used to build up programmable and arbitrary arrays of microlenses of various lens sizes and focal lengths. Finally, the authors utilize the thermal dependency of the order parameter of the LC to demonstrate facile tuning of the focal length. This technique has the potential to offer a low‐cost solution to the production of variable focus, arbitrary, microlens arrays.

     

Implementation of a hybrid lens

Details are presented of the design, fabrication, and use of a hybrid lens employed to interconnect two-dimensional arrays of optical transceivers. The hybrid lens consists of a custom-designed, 42-mm focal length, ?/5 compound lens followed by an array of afocal telescope compound microlenses.     

Coding for compressive focal tomography

We consider the capabilities and limits of strategies for single-aperture three-dimensional and extended depth of field optical imaging. We show that reduced spatial resolution is implicit in forward models for light field sampling and that reduced modulation transfer efficiency is intrinsic to pupil coding. We propose a novel strategy based on image space modulation and show that this strategy can be sensitive to high-resolution spatial features across an extended focal volume.     

折衍混合系统实现长焦深方法研究

通过对传统能量守恒法设计长焦深元件进行介绍与分析,根据衍射元件设计灵活的特性,提出了改进的能量守恒设计方法,利用折衍混合系统实现长焦深元件,并建立了优化的评价函数分两步对设计结果进行优化。     

Continuous radial superresolution filters with large focal depth

A new set of continuous superresolution filters is proposed which exhibits a radial superresolution performance with an extended depth of focus in an optical system by properly choosing the design parameters. Numerical simulation results of the performance parameters of the superresolution gain, the radial central core size, the Strehl ratio, the side-lobe factor and the depth of focus with different design parameters for the optimized patterns are displayed. We also give a design example for this kind of filter characterized by a birefringent element inserted between two parallel polarizers. This kind of filter would be useful in fields such as optical data storage systems.     

The realization of long focal depth with a linear varied-area zone plate

We report a linear varied-area zone plate, in which arbitrary long focal depth can be achieved by properly adjusting the corresponding parameters. Meanwhile, the lateral focal spot and side lobes can be kept very small. Numeral simulations are carried out to verify the performance of our zone plate through Fresnel–Kirchhoff diffraction theory, and the results are in good accord with the experimental verifications. The influences of our zone plate’s parameters to the intensity distribution in focal region are discussed in detail. Comparisons are made with the behaviour of a linear varied-line-space grating, and we find that the behaviour of our novel zone plate along optical axis is just like a reverse transformation of the focusing behaviour of a linear varied-line-space grating.     

Low f‐Number Diffraction‐Limited Pancharatnam–Berry Microlenses Enabled by Plasmonic Photopatterning of Liquid Crystal Polymers

Microlenses are desired by a wide range of industrial applications while it is always challenging to make them with diffraction‐limited quality. Here, it is shown that high‐quality microlenses based on Pancharatnam–Berry (PB) phases can be made with liquid crystal polymers by using a plasmonic photopatterning technique. Based on the generalized Snell's law for the PB phases, PB microlenses with a range of focal lengths and f‐numbers are designed and fabricated and their point‐spread functions and ability to image micrometer‐sized particles are carefully characterized. The results show that these PB microlenses with f‐number down to 2 are all diffraction‐limited. The capability of arraying these PB microlenses with 100% filling factor with a step‐and‐flash approach is further demonstrated.     

Single-step fabrication of refractive microlens arrays

Arrays of submillimeter microlenses are made from droplets of UV-curable optical adhesive dispensed from a pressurized syringe under computer control. Measurements of the focal length uniformity, the minimum focused spot size, and the spherical aberration are presented. An excellent lens diameter and focal length uniformity are achieved over 100 element arrays.     

Chalcogenide glass microlenses by inkjet printing

We demonstrate micrometer scale mid-IR lenses for integrated optics, using solution-based inkjet printing techniques and subsequent processing. Arsenic sulfide spherical microlenses with diameters of 10-350 μm and focal lengths of 10-700 μm have been fabricated. The baking conditions can be used to tune the precise focal length.     

Liquid tunable lens integrated with a rotational symmetric surface for long depth of focus

A liquid tunable lens with an extended depth of focus (DOF) is proposed. By integrating a phase plate with rotational symmetric quartic function (QF) contour into the liquid lens cavity, the lens can achieve higher tolerance to the defocus aberration. The liquid lens was fabricated with a convenient and low-cost process that combined single-point diamond turning (SPDT) with soft lithography using polydimethylsiloxane (PDMS). Experimental results demonstrate that both focal length tunability and extended DOF can be achieved with the proposed liquid lens.     

Analysis of a closed-boundary axilens with long focal depth and high transverse resolution based on rigorous electromagnetic theory

We find that a microcylindrical axilens with a closed boundary and with an f-number less than 1 still can achieve the properties of long focal depth and high transverse resolution, unlike a microcylindrical axilens with an open boundary, which fails to maintain those properties for low f-numbers. The focusing characteristics of the closed-boundary axilens and the open-boundary axilens are numerically investigated based on the boundary integral method. The numerical results show that the ratio of the extended focal depth of the closed-boundary axilens to the focal depth of the conventional microlens can reach up to 1.26 and 2.12 for the preset focal depths 3 and 5 microm, respectively, even though the f-number is reduced to 1/3.     

Modulation transfer function measurements of microjetted microlenses

We measured the modulation transfer function for microjetted microlenses with diameters ranging between 109 and 400 mum and with focal lengths ranging between 135 and 540 mum. We found that single-drop 109-mum-diameter microlenses perform close to their theoretical cutoff frequency. However, the larger lenses made with multiple droplets have a cutoff frequency that is 35% of the theoretical value. We interpret this as an illustration of the rapid increase in spherical aberration as the diameter of a lens increases.