Chirped Micro Lens Array For Compound-eye Imaging System On Curved Surface

Compound-eye is a widely existed imaging strategy in nature with the merits of large field of view (FOV), three dimensional images generation, less aberration, and compact size, etc. While the main drawback of compound-eye imaging is the reduced sight distance, about tens to thousands millimeter typically. Artificial compound-eye imaging system have been employed on medical catheter, medical endoscope and machine vision, etc. In these applications, required imaging distance ranges from a few millimeters to several centimeters but large FOV and compact size is critical, which makes the compound-eye systems especially attractive.Compound-eye must be built on a curved surface to obtain a large FOV and its focal plane is curved if the facets in the compound-eye have the same focal length. However, photo detectors (PDs) nowadays are usually fabricated on a flat chip using integrated circuit process. The mismatching between the curved compound-eye and flat PDs creates blur images, hence hinders the use of compound-eye in imaging system. If the diameter of each facet in the compound-eye was changed in order to change the focal length, such a compound-eye is still not compatible with PDs since the pixels in PDs is normally uniform. To solve this problem, an obviously approach is to make the compound-eye chirped. If the focal length of each facet varies with its location and to be equal to the corresponding distances between the center of the facet and the corresponding planar photo detector, the focal plane of the compound-eye will be flatten, and achieve a clear images through the PDs.The main difficulty to realize this idea is the lack of technology to fabricate chirped compound-eye on curved surface. The size of facet is required to match the size of a single or several pixel in a PDs array, normally less than a hundred micron. However, micro lens fabricated by mechanical milling is usually more than a hundred micron and the focal length of polymer micro lenses by reflowing patterned photosensitive or micro ink-jet printing is normally out of precisely control. A new fabrication approach was proposed and demonstrated in this dissertation to address this problem. Electrostatic deformed concave membrane was used as the initial molding template, and the deformation was determined by different applied voltage. By transferring the pattern to another soft template and deforming it by negative pressure, Chirped compound-eye on a curved surface were fabricated by molding process using this curved template successfully.The fabricated compound-eyes were optically characterized, as well as two other control samples, one is the chirped compound-eye on a flat surface, another is the uniform compound-eye on curved surface. The result demonstrated a larger field-of-view than that of flat compound-eye and better imaging performance than that of compound-eye with uniform focal length on curved surfaces.The previous literatures related to artificial compound-eye was cited and discussed in Chapter1. The design and simulation of the chirped compound-eye for a prototype imaging system was presented in Chapter2. Here an two dimensional analysis model was proposed to determine the focal length of each facet in a certain location, which matches the distance from this facet to its corresponding PDs. Then a three dimensional model was simulated by a photo-trace simulation tools to get more precisely parameters. Last, the driving voltage to realize the dimensional parameters was derived by combining finite elements analysis and experimental data.The fabrication of the chirped compound-eye was introduced in Chapter3. A SU-8cavity, a conductive membrane on the top surface of the cavity and electrodes on the bottom were prepared by micro fabrication technology to act as the template. PDMS soft template was obtained by a transfer molding process when different voltages were applied on different pixel of the SU-8template. Negative pressure curved the soft template and UV curing resin were poured on the curved soft template to fabricate chirped compound-eye on curved surface successfully.The fabricated compound-eyes was optically characterized in an imaging system in chapter4. The generated images of chirped compound-eye on curved surface, on flat surface or uniform compound-eye on curved surface were presented and the FOV of each samples were measured.In conclusions, a fabrication method was presented in this dissertation to prepared chirped compound-eye on curved surfaces, and the focal length of the facets in the compound-eye was variable to match the planar photo detectors. This fabrication process is low cost and reproducible mainly because hundreds of the conductive template can be product simultaneously in a single wafer. Generally, this technology is promising for many applications such as catheter, endoscopy, machine vision or3D imaging or chip digital camera to obtain extended field-of-view, better optical performance and compact size simultaneously.