Broadband point-spread function engineering via a freeform diffractive microlens array.

Conventional integral-imaging systems utilize lenslet arrays with fixed focal lengths and aperture sizes. We propose a time-multiplexed integral-imaging method that enhances both the depth of focus and the resolution of a three-dimensional image by displaying it with an array of lenslets with different focal lengths and aperture sizes. The nonuniform lenslet parameters (focal lengths and aperture sizes) for our method are calculated. Our theoretical analysis indicates that significant improvements in both depth of focus and resolution can be obtained with the proposed technique.

The thermal reflow process is widely used in microlens array fabrication. However, the resulting arrays are commonly criticized for their low fill factor. In this work, caulking is applied to fill the gaps between adjacent lenses. The experimental results prove that a gapless microlens array with a 100% fill factor could be successfully produced and the caulking time precisely controlled. Furthermore, an artificial neural network and genetic algorithm are used to achieve high quality using the thermal reflow and caulking. The L18 orthogonal array is used as the learning data for the artificial neural network to construct a system model that could predict the results (e.g., S/N, focal length, and roughness) for arbitrary parameter settings. The genetic algorithm is then applied to obtain the optimal parameter settings. The major objectives in using the optimal design are to reduce the variation in the focal length and the surface roughness for a microlens array. This allows improved focus and enhanced illumination brightness. The results show that microlens array quality could be significantly improved in comparison with the original design.

Fabrication of bifocal lenses using resin that can be processed by electron beam lithography after ultraviolet-nanoimprint lithography

Fabrication of micro-lens array with antireflection structure

Plasticized polymer gel has emerged as a new material for the fabrication of microlenses with variable focal lengths. To explore a non-toxic and non-irritating gel, we prepared a polyvinyl alcohol (PVA)/glycerol gel, which is sensitive to external direct-current (DC) voltage and exhibits a strong polarization effect upon application of an electric field. Due to the polarization effect, PVA/glycerol gel bends toward cathode like the polyelectrolyte gel. To fabricate the microlens array, PVA/glycerol gel was spin-coated on a glass substrate with diamond Indium Tin Oxides electrode. Upon application of the DC electric field, the surface of the gel membrane followed a cosine wave for creeping motion of PVA particles, hence the membrane exhibited the property of a microlens array. The experiments for measuring imaging quality and focal length of PVA/glycerol-based lenticular microlens array are performed. Our microlens array allows tunable focal length with lower driving voltage compared to the microlens array with polyvinyl chloride gel.

We report on prototype fiber endoscopes with tunable-focus liquid microlenses integrated at their distal ends and actuated through infrared (IR) light. Tunable-focus microlenses allow minimal back-and-forth movements of the scopes themselves and different depths of focus (DOFs), thus having spatial depth perception in the obtained images. The liquid microlens was formed by a water-oil meniscus pinned at a hydrophobic-hydrophilic boundary at an aperture. IR light-responsive hydrogel microstructures were formed by photopatterning thermo-responsive N-isopropylacrylamide hydrogel with entrapped IR light absorbing gold nanoparticles. The volumetric change in the hydrogel microstructures regulated the pressure difference across the water-oil interface and thus varied its focal length. The operations of the microlenses were realized through light transmitted via optical fibers. The images obtained from the microlenses were transferred via image fiber bundles. For two alignments between the hydrogel structures and the fibers, the response times of the microlenses are 65 and 20 s, respectively. Images of the simulated polyps in simulated colons were obtained. The range of focal length of a typical microlens was from 52 to 8 mm. The angle of view of an endoscope was from 77° to 128°. A microlens array could potentially be utilized to simultaneously obtain different DOFs and to increase the field of view.

In order to determine light aberrations, Shack-Hartmann optical wavefront sensors make use of microlens arrays (MLA) to divide the incident light into small parts and focus them onto image planes. In this paper, we present the design and fabrication of long focal length MLA with various shapes and arrangements based on a double layer structure for optical wavefront sensing applications. A longer focal length MLA could provide high sensitivity in determining the average slope across each microlens under a given wavefront, and spatial resolution of a wavefront sensor is increased by numbers of microlenses across a detector. In order to extend focal length, we used polydimethysiloxane (PDMS) above MLA on a glass substrate. Because of small refractive index difference between PDMS and MLA interface (UV-resin), the incident light is less refracted and focused in further distance. Other specific focal lengths could also be realized by modifying the refractive index difference without changing the MLA size. Thus, the wavefront sensor could be improved with better sensitivity and higher spatial resolution.

The microlens array (MLA) scanning system is a relatively new solution for beam scanning, and are used in numerous applications, such as laser radar and optical communications. Excrescent light generated by overlapping divergent beams from adjacent microlenses increases with scanning angle and as a result, the rays do not fill the clear aperture of the MLA. The fill factor is proposed to characterize the fill rate of beams, and its effects on the point spread function (PSF) and modulation transfer function (MTF) are analyzed, this work can benefit the design and evaluation of MLA systems.

Simulation, fabrication and morphological characterization of a PDMS microlens for light collimation on optrodes

Wavefront reconstruction with the adaptive Shack–Hartmann sensor

Novel diffraction limit implied geometric lens imaging model based on a finite number of pixels for integral imaging display

An artificial neural network and genetic algorithm were used to achieve a high quality microlens array fabrication using a UV proximity printing process in this study. The UV proximity printing process can precisely control the geometric profile of the microlens array in the fabrication process without thermal reflow. The major objective in using the robust design is to reduce the variations in the focal length of the microlens array, allowing improved focus and enhanced illumination brightness. The artificial neural network was used first to characterize the nonlinear relationship between the manufacturing parameters and the properties derived from experimental data. The manufacturing parameters which affect microlens array uniformity include: (1) geometrical ratio, (2) printing gap, (3) spin coating revolution speed, (4) exposure time and (5) developing time. It is very important to control these parameters to decrease the sensitivity to noise. The L18 orthogonal array was used as the learning data for the artificial neural network to construct a system model that could predict the focal length for arbitrary setting of parameters. Then, the genetic algorithm was applied to obtain the robust setting of parameters. The results showed that the microlens array quality could be significantly improved in comparison with the original design.

We demonstrate the depth measurement method of holographic images using integral imaging. The depth information of holographic images can be obtained with a single capture by conventional integral imaging pickup system composed of a micro lens array (MLA) and an image sensor. In order to verify the feasibility of our proposed method, an elemental image set of holographic images formed by a MLA was generated by a computer, and then refocused images at different planes were reconstructed numerically using computational integral imaging reconstruction (CIIR) technique for depth measurement. Note that we set the distance between MLA and image sensor as focal length of micro lens for large depth of focus. From the numerical results, we can measure the depth representation of holographic images successfully. However, refocused images from an optically captured elemental image set provide poor depth discrimination due to expected error in distance between MLA and image sensor. Only an object in a particular narrow depth range can be focused clearly when the image sensor is placed out of the MLA focal plane. The simulated results in this condition matched reasonably with the experiment result.

We present a novel photon-acid diffusion method to integrate polymer microlenses (MLs) on a four-channel, high-speed photo-receiver consisting of normal-incidence germanium (Ge) p-i-n photodiodes (PDs) fabricated on a 200 mm Si substrate. For a 29 µm diameter PD capped with a 54 µm diameter ML, its dark current, responsivity, 3 dB bandwidth (BW), and effective aperture size at -3 V bias and 850 nm wavelength are measured to be 138 nA, 0.6 A/W, 21.4 GHz, and 54 µm, respectively. The enlarged aperture size significantly decouples the tradeoff between aperture size and BW and enhances the optical fiber misalignment tolerance from ±5 µm to ±15 µm to ease the module packaging precision. The sensitivity of the photo-receiver is measured to be -9.2 dBm at 25.78 Gb/s with a bit error rate of 10-12 using non-return-to-zero (NRZ) transmission. Reliability tests are performed, and the results show that the fabricated Ge PDs integrated with polymer MLs pass the GR-468 reliability assurance standard. The demonstrated photo-receiver, a first of its kind to the best of our knowledge, features decent performance, high yield, high throughput, low cost, and compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes, and may be further applied to 400 Gb/s pulse-amplitude modulation four-level (PAM4) communication.

In this chapter, we consider the imaging properties of a system with a circular exit pupil by applying the general formulas derived in Chapter 1. We derive expressions for its point-spread function (PSF), optical transfer function (OTF), and encircled, ensquared, and excluded powers. The effect of aberrations on a PSF is first studied in terms of its central value, i.e., its Strehl ratio. An exact expression is obtained for the Strehl ratio and the results for primary aberrations are compared with those obtained from the approximate expressions based on the phase variance of the aberration across the pupil derived in Chapter 1. The tolerances for primary aberrations are given for a certain value of the Strehl ratio. Balanced primary aberrations are also discussed and they are identified with the Zernike circle polynomials. A defocused PSF is considered next, and it is shown that systems with large Fresnel numbers, such as photographic systems, have a very small depth of focus. However, systems with small Fresnel numbers, e.g., a laser transmitter, have a large depth of focus. Focused and collimated beams are discussed, and the concept of near- and far-field distances is introduced. It is shown that if the Fresnel number of the beam focused on a target is small, the axial irradiance is maximum at a point that is closer to the focusing optics. However, maximum central irradiance on a target at a given distance is obtained when a beam is focused on it, even though a larger irradiance is obtained at a closer point. Aberrated PSFs for rotationally symmetric aberrations are considered, and numerical results are obtained that show that the size of their central spot is practically independent of the aberration. Symmetry properties of the aberrated PSFs are considered next. Full PSFs for primary aberrations are shown graphically and pictorially. A brief comparison of the diffraction and geometrical PSFs is also given. The line of sight of an optical system is identified with the centroid of its PSF and discussed for aberrated systems. It is shown that only coma-type aberrations change the line of sight. The OTFs for primary aberrations are discussed, and the concept of contrast reversal is described. The OTF aberrated by coma is complex, and we give its real and imaginary parts as well as its modulus and phase. Aberration tolerances are given for a certain value of the Hopkins ratio. The geometrical OTF is also discussed and approximate expressions for it are obtained for the primary aberrations. It gives reasonably accurate results in the region of very low spatial frequencies, a region of practical interest when the MTF of an aberrated system at high frequencies is practically negligible. It is shown that the MTF averaged over all angular orientations of a spatial frequency vector is maximum when the standard deviation of the ray aberration [i.e., the root mean square (rms) radius of the spot diagram with respect to its centroid] is minimum. The diffraction OTF is shown to approach the geometrical OTF in the limit of zero wavelength. The differences between the diffraction and geometrical OTFs are summarized. The incoherent line- and edge-spread functions are discussed, and numerical results for both the aberration-free and the aberrated systems are given. The aberration tolerances for a certain value of the Struve ratio are also given.