Optics is an ancient science. Since Galileo invented the telescope, optics has gone through a long way for hundreds of years. The appearance of laser in 1960s promoted the rapid development of optical technology. However, the traditional optical elements (devices) based on catadioptric principle, such as lenses and prisms, are all made by mechanical milling, grinding and polishing, which are not only complicated in manufacturing process, but also large in size and heavy in weight. At present, under the trend of optical, mechanical and electrical integration, instruments become bloated and cumbersome. It is an urgent task for the optical community to develop small, efficient and arrayed optical elements. In the mid-1980s, the research team led by Veldkamp of Lincoln Laboratory of Massachusetts Institute of Technology in the United States took the lead in putting forward the concept of "binary optics" in designing a new sensing system. At that time, he described: "Now there is a branch of optics, which is almost completely different from the traditional manufacturing method. This is diffraction optics, and the surface of its optical element has a relief structure; Because the original method of making integrated circuits is used, the mask used is binary, and the mask is layered in the form of binary coding, so the concept of binary optics is introduced. " Subsequently, binary optics, not only as a technology, but also as a discipline, quickly attracted the attention of academia and industry, and set off a worldwide research upsurge of binary optics.
In the early 1990s, binary optics rose in the international research upsurge, and attracted great interest and favor from academia and industry. The two main branches of micro-optics development are:
(1) Gradient refractive index optics based on refraction principle,
(2) Binary optics based on diffraction principle. In the mid-1980s, DARPA sponsored a project called "Binary Optics" for Lincoln Laboratory of Massachusetts Institute of Technology. Its research objectives were:
(1) Develop an optical technology based on microelectronics manufacturing process to save money and labor, gain more freedom in design and material selection, and develop new optical functional components;
(2) Promoting the computer-aided design of the whole photoelectric system;
(3) Diffraction optics technology is widely used in American industry. With the development of binary optical technology, binary optical elements have been widely used in optical sensing, optical communication, optical calculation, data storage, laser medicine, entertainment consumption and other special systems. Perhaps it can be said that its development has experienced three generations. In the first generation, people used binary optical technology to improve the traditional refractive optical elements in order to improve their conventional performance and realize special functions that ordinary optical elements could not achieve. This element is mainly used for phase difference correction and achromatic. Usually, the diffraction pattern is etched on one surface of a spherical refractive lens to achieve refractive/diffractive compound achromatic and achromatic in a wider wave band. For example, Perkin-Elmer Company of the United States has successfully used Schmidt telescope to eliminate spherical aberration; In the far infrared system, American Honey-well Company has achieved apochromatic effect, and they have also made a small optical disk read-write head by using binary optical technology. In addition, binary optical elements can generate arbitrary wave front to realize many special functions, which has important application value. Such as beam shaping elements in material processing and surface heat treatment, He-Ne laser focusing corrector in medical instruments, optical interconnection elements (equal intensity beamsplitting Dammann grating) in optical parallel processing systems, radiation focusing devices, etc.
The first generation application technology of binary optical elements has matured, and more than 50 companies in the world are designing new optical systems using mixed special functional elements.
The second generation is mainly used for micro-optical elements and micro-optical arrays. At the end of 1980s, binary optics entered the field of micro-optics, developing towards miniaturization and array, with the element size ranging from a dozen microns to1mm. The high-density microlens array made by binary optics method has high diffraction efficiency and can realize diffraction-limited imaging. In addition, when the etching depth exceeds several wavelengths, the microlens array shows the characteristics of ordinary refractive elements, which has unique advantages: the array structure is flexible and can be arranged in a matrix, a circle or a closely packed hexagon; It can produce lens surfaces with various contour shapes, such as paraboloid, ellipse and synthetic surface. The "dead zone" of the array lens can be reduced to zero (that is, the fill factor reaches 100%). This high-quality diffractive or refractive microlens array has important applications in optical communication, optical information processing, optical storage and laser beam scanning. For example, binary micro-optical elements can be used as adaptive optical interconnects in telescopic hybrid optical systems, intelligent beam control, multi-channel processing, detector arrays and multi-channel micro-sensing systems. The third generation, which is developing at present, aims at multi-layer or three-dimensional integrated micro-optics, and performs beam transformation and control in imaging and complex optical interconnection. Multi-layer micro-optics can integrate the conversion, detection and processing of light to form a multifunctional integrated photoelectric processor. This development will enable the image sensor to adjust adaptively according to different light intensity, detect the movement of the target and automatically determine the position of the target in the background. Veldkamp combines this new binary optical technology with quantum well laser array or seed device and CMOS analog electronic technology, and puts forward the idea of "Amacronic", which couples the focal plane structure with the local processing unit to simulate the close-range detection of axonless neurons on the retinal membrane. The system has the functions of edge enhancement, dynamic range compression and neural network. The typical application of this generation of micro-optics technology is multilayer photoelectric network processor. This is a focal plane preprocessing technology, which uses binary optical elements to provide flexible feedback and nonlinear preprocessing ability. The microlens array on the silicon substrate of the detector focuses the incident signal light on the active area of the array detector, and the integrated circuit of the substrate excites the gallium arsenide indium diode to emit light with the converged light. When the second plane emits light waves, the diffractive elements on both sides of the substrate are directed to the array detector on the third plane silicon substrate, and the diode is excited to emit light after being processed by the integrated circuit, etc., so as to obtain the processed signal. Each layer of this multi-layer focal plane preprocessor adopts micro-optical array to realize interconnection and coupling, which opens up a new way for miniaturization, integration and intelligence of sensors. Development Trend Binary optics is one of the frontier sciences in the optical field based on diffraction theory, computer-aided design and micromachining technology. The design and processing of diffractive elements with ultra-fine structure is the key technology to develop binary optics. The development of binary optics not only makes profound changes in the design and processing technology of optical system, but also its overall development trend is the integration technology and high-performance integrated system of micro-optics, microelectronics and micromechanics in the future.