An Optical Wavefront Sensor Based on a Double Layer Microlens Array

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.

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.

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.

In this Letter we aim to bring an understanding to the apparition of multiple spots when using a Shack-Hartmann (SH) wavefront sensor behind diffractive lenses. In contrast to previous work, this phenomenon is described in terms of diffractive orders. It is illustrated with Zemax simulations, where three kinds of diffractive lenses (monofocal, bifocal, and trifocal) are set behind a microlens array. The presence of multiple spots is related to the phase jump of the diffractive profile and also to the number of steps seen through the microlens pupil. The possibility of assessing the optical quality of such lenses using SH measurements is discussed, in particular within the field of ophthalmology, where the need for precautions is underlined.

This work presents the fabrication of a high fill factor Fresnel microlens array (MLA) by employing a low-cost home-built maskless exposure lithographic system. A phase relief structure was generated on a photoresist-coated silicon wafer, replicated in Polydimethylsiloxane (PDMS) and electrostatically bonded to a glass substrate. Optical characterization was based on the evaluation of the maximum intensity of each spot generated at the MLA focal plane as well as its full width at half maximum (FWHM) intensity values. The resulting FWHM and maximum intensity spot mean values were 50 ± 8%μm and 0.71 ± 7% a.u , respectively. Such a MLA can be applied as Shack-Hartmann wavefront sensors, in optical interconnects and to enhance the efficiency of detector arrays.

We proposed a resolution enhanced integral imaging display method using two micro-lens arrays (MLA) with different focal lengths for capturing and display respectively. An elemental image array (EIA) is captured with MLA of focal length of f(1) and a processed EIA is displayed with MLA of focal length of f(2) which is larger than f(1). We enlarge the "effective area" in processed EIA to increase the information obtained by viewer, in other words, enhance the viewing resolution. The two micro-lens arrays for capturing and display are g and mg distant from display device respectively, and we can get m(2) times resolution enhancement.

Fabrication of high integrated microlens arrays on a glass substrate for 3D micro-optical systems

We report on a test bed to compare the performance of three different wavefront sensors, the Shack-Hartmann Wavefront Sensor (SHWFS), the Pyramid Wavefront Sensor (PWFS), and the non-linear Curvature Wavefront Sensor (nlCWFS). No single wavefront sensor easily allows for sensing all aspects of atmospheric turbulence. For instance the SHWFS has a large dynamic range and a linear response to input phase aberrations but is not sensitive to low order modes. The PWFS uses the full spatial resolution of the pupil which gives it increased sensitivity to low order modes, however it still treads the line between achieving high dynamic range and high sensitivity. The nlCWFS is the only wavefront sensor designed to sense low and high, spatial frequencies, however this leads to a complex algorithm. We discuss the reconstruction algorithm for each WFS along with simulated comparisons, we present the optical design for the WFS comparison tes tbed, and outline the adaptive optics controls system.

We propose an experimental method consisting of a standard Fizeau interferometer with transmission sphere (TS) for the determination of the focal length of microlens array (MLA) by spherical wavefronts. The TS is axially translated to produce a spherical wavefront of different curvatures with respect to the MLA. The align mode provision of the interferometer helps to monitor the tilt of the MLA with respect to the spherical wavefront. The focal length is determined from the measured distance of adjacent image spots for various spherical wavefronts at the focal plane of the MLA. Error analysis and experimental demonstration with an off-the-shelf MLA are addressed here.

In this paper, we present an array of tunable microlenses on polydimethylsiloxane (PDMS) microfluidic chips as a convenient platform for the study of cell responses to mechanical strains. This array of microlenses is an array of microwells covered with a thin PDMS membrane that are connected by microfluidic channels. The deformation of the PDMS membrane on the microlenses is tuned by external hydraulic pressure that is introduced through the microchannels. The degree of deformation of the PDMS membrane can be estimated from the focal length of the microlens, and the corresponding mechanical strain that is caused can be calculated from finite element simulation. We demonstrate the capability of this array of microlenses as a general platform for studying the influence of mechanical strain on adherent cells by using NIH 3T3 fibroblasts and HeLa cells as our models.

Analysis on Shack–Hartmann wave-front sensor with Fourier optics

A real-time recognition system of three-dimensional (3D) objects by use of microlens array is presented. The proposed system consists of two subsystems: one is a projection system of a 3D object into an array of two-dimensional (2D) images that have different perspectives of the 3D object and the second is an optical processor to perform the correlation. A 3D object is illuminated by incoherent light and then is projected into two-dimensional array of elemental images by use of microlens array. Use of microlens array allows us to make the system compact. Each elemental image corresponds to a different perspective of the 3D object. The set of elemental images contain information of the 3D object. After an optical incoherent-to-coherent conversion by an optically addressed spatial light modulator, an optical processor is used to perform the correlation between the input and the reference 3D objects. All the processes can be implemented optically for real-time recognition. Experimental and numerical results of the recognition of 3D objects are presented. These results show that the proposed system can recognize the 3D object even when an input object is very similar to the reference.

The wavefront measurement technique using the Shack-Hartmann wavefront sensor has been developed for measurement of wavefront distortion of a SR extraction mirror for the beam profile monitor at the Photon Factory. The instrument consists of a multi-lens array (8/spl times/8 array, 250 /spl mu/m pitch and focal length of 4.5 mm), a beam expander (20:1), an image relay lens system (1:1), a He-Ne laser (633 nm) and a digital CCD camera. To analyze positions of focal spot on the CCD in 1/10 of the pixel size, the wave front sensor can measured to be /spl lambda//30. A dynamic range of the wavefront sensor was designed 12.6 /spl mu/m max. Performance of the wavefront sensor was tested with an optical flat. With this wavefront sensor, a wavefront error caused by surface deformation of the SR extraction minor was measured at Beamline 27 of Photon Factory. The correction of the measured wavefront distortion is also described.

An innovative experimental method for studying the influence of supersonic flow on laser beam transmitting has been proposed. The wavefront phase properties and the dynamic wavefront change of laser beam traversing supersonic flow can be detected by Hartmann-Shack (H-S) wavefront sensor. Beam aberrations are constructed by adopting Zernike mode-reconstruction theory. The centroid change of laser beam and the far-field properties can be obtained by further analyzing the experimental data. In the paper, the experimental principle is presented, and wavefront aberrations with several different flow parameters are given. Some beam-quality criteria such as Zernike aberration coefficient, PV and RMS value, circle energy, Strehl ratio, and centroid change of beam, are also calculated. Experimental results show that the stage of flow establishing, stabilizing and ending can be detected by using H-S wavefront sensor. By comparing the result of no flow with that of a given-parameter flow, defocus and astigmatism aberration increase obviously. At the same time, the Strehl ratio is reduced and the beam quality is degraded. The same conclusion can be obtained by comparing no test-model with the given-structural test-mode in same flow. Moreover, the wavefront aberration of laser beam in the condition of different flow parameters can also be measured and compared accurately by using the test method.

This paper compares the closed-loop, adaptive-optics-system performance obtained with a Shack-Hartmann wavefront sensor (WFS) to that of a fixed-pyramid WFS, assuming a point-source beacon. To quantify performance, the analysis uses wave-optics simulations and calculates the peak Strehl in weak to moderately deep turbulence conditions. Initial results show that the Shack-Hartmann WFS outperforms the fixed-pyramid WFS.