However, with the progress of science and technology, the current instruments and equipment have developed towards the trend of optical, mechanical and electrical integration. Optical components manufactured by traditional methods are not only complicated in manufacturing process, but also bulky and heavy, which can no longer meet the needs of today's scientific and technological development. At present, people have been able to manufacture lenses and lens arrays with very small diameters, which are usually unrecognizable by human eyes and can only be observed through microscopes, scanning electron microscopes, atomic force microscopes and other equipment. These are microlenses and microlens arrays.
Microlenses and microlens arrays manufactured by micro-optics technology have the advantages of small volume, light weight, easy integration and array, and have become a new research and development direction. With the development trend of miniaturization of optical components, many new technologies have been developed to reduce the size of lenses and lens arrays, and now microlenses and microlens arrays with diameters of millimeters, microns or even nanometers can be manufactured.
In 1980s, a new type of micro-optical array device-self-focusing planar microlens array was developed. It used the advanced lithography process at that time to produce a well-arranged and uniform microlens array. Moreover, the surface of microlens array is planar, which is easy to be coupled with other planar elements, and has good stereoscopic functions such as focusing, collimation, branching, imaging, wavelength division multiplexing, switching and isolation. In addition, due to the small diameter and high lens density of a single lens, large-capacity and multi-channel parallel processing of information can be realized. Therefore, it has been widely used in optical sensing, optical calculation, optical fiber communication and other optoelectronic devices.
In 1992, Sony Corporation of Japan reported that a high-sensitivity CCD device was made by integrating microlens array with CCD. The integration of microlens array and CCD can improve the filling coefficient of CCD, and then improve the sensitivity and signal-to-noise ratio of CCD. CCD is composed of many photosensitive elements, which convert the obtained optical signals into electrical signals and then transmit them. Because of the existence of shift register and transmission gate, there is an obvious gap between photosensitive elements, and about 2/3 of the signal light falling on CCD can not be picked up by photosensitive elements. The filling coefficient of CCD is only 20.30%, which leads to the low photosensitivity of CCD. In this way, the signal light incident on other areas of CCD will be wasted, and the utilization rate of signal light is very low. Therefore, the main function of microlens array is to make photons falling on the dielectric layer deflect and fall into the photosensitive area due to the action of microlens, so as to improve the filling coefficient of CCD. By using microlens array on CCD, the light can be focused on the photosensitive element of CCD, and the sensitivity of CCD can be greatly improved. In the visible spectrum range, the quantum efficiency of CCD can be improved twice on average.
1994 Philip R&D center successfully fabricated a two-dimensional large-area image sensing microlens array. The diameter of the microlens is 190 μm, the pitch is 200 μm, and the focal length range is 200-450μ m.. The microlens array improves the response speed of the sensing device without affecting the image resolution.
During the period of 1997, researchers from Lincoln Laboratory of Massachusetts Institute of Technology (MIT) successfully fabricated a refractive aspheric microlens array for collimating the beam of a tapered resonator laser, so that the divergence angle of the diffraction-limited beam is only 0.43. And realizes the coupling with the single-mode optical fiber.
In 2002, researchers at Osaka University integrated the microlens array with the second harmonic generation microscope and proposed the multi-focus scanning technology. Compared with the traditional single-focus scanning method, this technology improves the detection efficiency and image acquisition rate of second harmonic generation by dozens of times.
In 2005, South Korean researchers reported that microlens arrays can be used for super-large three-dimensional imaging display. The microlens array can increase the viewing angle of the display, and the displayed image is very clear and undistorted.
In 2006, researchers at Stanford University in California succeeded in replacing a single lens in a digital camera with a microlens array, which greatly increased the focal depth and field of view of the camera. Cameras equipped with microlens arrays can not only make the near and far images clear, but also make the background very clear, while ordinary cameras can only get near or far images.
In 2007, researchers from LG Company in South Korea reported the use of high fill factor microlens array to improve the light output efficiency of organic light emitting diodes. They used the micromachining process of channel formation and polymer conformal layer vapor deposition to fabricate microlens arrays with high filling factor on the surface of organic light-emitting diodes, which improved the output efficiency of organic light-emitting diodes by 48%.
At home, researchers have also conducted in-depth research on the theory and fabrication technology of microlens array, which has made it widely used. For example, Chengdu Optoelectronic Institute has been successfully applied in practical systems such as pre-Apollo measurement, laser beam diagnosis, laser beam shaping and optical element quality evaluation. Zhejiang University has also conducted in-depth research on its application in dense multi-carrier demultiplexer. Diffraction Microoptics Laboratory of the Institute of Optics of Nankai University has also conducted in-depth research on the fabrication technology of microlenses.
Because microlens array has important and extensive applications in micro-optical systems such as optical information processing, optical calculation, optical interconnection, optical data transmission and generating two-dimensional point light sources, it can also be used in copiers, image scanners, fax machines, cameras, medical and health instruments, etc. In addition, the miniaturization and integration of microlens array devices make them have strong adaptability and can be widely used in communication, display and imaging devices. Elliptical refractive microlens array for semiconductor laser can focus, collimate and shape the laser. It can also be used between optical fiber and optical integrated circuit to realize effective coupling of optical devices. In optical fiber communication, elliptical microlens couples the light from free space into the optical fiber and collimates the light from the optical fiber. At present, microlens array has been applied in the field of atomic optics. It is used to make atomic waveguides, beam splitters and Mach-Zehnder interferometers, or to capture atoms or process quantum information of neutral atoms. Therefore, it is necessary to study the materials, manufacturing processes and uses of microlens arrays.