The first step in all optical manufacturing processes is the selection of appropriate optical materials. The optical parameters (refractive index, Abbe number, transmittance, reflectivity), physical properties (hardness, deformation, bubble degree, Poisson's ratio), and even temperature characteristics (thermal expansion coefficient, refractive index vs. temperature) of optical materials will affect Performance of optical components and systems. This article will briefly introduce common optical materials and the characteristics of various materials.
Optical materials usually fall into three categories: optical glass, optical crystal, and special optical materials.
01 Optical glass
Optical glass is an amorphous (glassy) optical medium material that can transmit light. After the light passes through it, it can change the propagation direction, phase, intensity, etc., and is often used in the production of optical components such as prisms, lenses, reflectors, windows, and filters in optical instruments or optical systems. Optical glass has a high degree of transparency, chemical stability, and a high degree of uniformity in physics (structure and properties), and has specific and accurate optical constants. Optical glass still retains the amorphous structure of high-temperature liquid in the low-temperature solid state. Ideally, the internal physical and chemical properties of the glass (such as refractive index, thermal expansion coefficient, hardness, thermal conductivity, electrical conductivity, elastic modulus, etc.) are the same in all directions. called isotropic.
At present, the main manufacturers of optical glass include Germany's SCHOTT, the United States' Corning, Japan's OHARA, and China's Chengdu Guangming (CDGM). In the ultraviolet band, our commonly used optical glasses include ultraviolet fused silica (UVFS). Commonly used quartz materials include domestic JGS1, JGS2, JGS3, Corning7980, and Ohara's high-quality quartz glass (SK-1300, SK-1310, SK-1320L, SK-1321, etc.), our optical components generally use JGS1, which has high transmittance starting from 180nm. Corning7980 has good uniformity and low bubble and impurity content, so it is mostly used in laser components and can provide a higher laser damage threshold. In the visible and near-infrared bands, commonly used optical glass materials include SCHOTT's N-BK7, float glass B270, Chengdu Guangming's H-K9L, etc. N-BK7 and H-K9L have similar properties and can be substituted for each other. They provide high transmission in the visible and near-infrared (350 nm - 2.0 µm). H-K9L fine annealed optical glass is the most commonly used glass in our high-quality optical components. H-K9L fine annealed optical glass is a hard glass that can withstand a variety of physical and chemical stimuli. It is relatively scratch-resistant and resistant to chemicals. Because there are few bubbles and low impurity content, it is very suitable for manufacturing precision lenses, windows, prisms and other components.
02 Optical crystal Optical crystal refers to the general term for crystal materials used in optical media materials. Due to the structural characteristics of optical crystals, it can be widely used to make windows, lenses, and prisms in various ultraviolet and infrared applications. According to the crystal structure, it is divided into single crystal and polycrystalline. Single crystal materials have higher crystal integrity and light transmittance, as well as lower input loss, so commonly used optical crystals are mainly single crystals.
◆ Common UV and infrared crystal materials include: quartz (SiO2), fluorite (CaF2), lithium fluoride (LiF), rock salt (NaCl), silicon (Si), germanium (Ge), etc. ◆ Polarizing crystal: Commonly used polarizing crystals include calcite (CaCO3), quartz (SiO2) and sodium nitrate (saltpeter). ◆Apochromatic crystal: The special dispersion characteristics of the crystal are used to manufacture apochromatic objective lenses. For example, fluorite (CaF2) and glass are combined to form an apochromatic system, which can eliminate spherical aberration and secondary spectrum. ◆Laser crystal: can be used as the working material of solid-state laser, such as ruby, calcium fluoride and neodymium-doped yttrium aluminum garnet crystal.
Comparison of common crystal characteristics
Crystal materials are divided into natural and artificial growth. There are few natural crystals, it is difficult to grow artificially, their size is limited, and they are expensive. They are generally considered only when glass materials cannot meet the requirements. They can work in the non-visible light band and are used in semiconductor, laser and other industries.
03 Special optical materials
Glass-ceramic is a non-glass amorphous special optical material, between glass and crystal. The main difference between glass-ceramics and ordinary optical glass is that it has a crystalline structure, while the main difference from ceramics is that its crystalline structure is much finer than that of ceramics. It has the characteristics of small thermal expansion coefficient, high strength, high hardness, low density and extremely high stability. It is widely used in processing flat crystals, standard meter rulers, large mirrors, laser-guided gyroscopes, etc.
The thermal expansion coefficient of microcrystalline optical materials can reach 0.0±0.2x10-7/℃ (0~50℃)
Silicon carbide is a special ceramic material that can also be used as an optical material. Silicon carbide has the characteristics of good stiffness ratio, small thermal deformation coefficient, excellent thermal stability and significant weight reduction effect. It is regarded as the main material of choice for large-sized lightweight reflectors and is widely used in aerospace, strong lasers, semiconductors and other fields.
United Optoelectronics' silicon carbide materials The above categories of optical materials can also be called optical media materials. After light passes through them, the propagation direction, phase and intensity can be changed. In addition to these major types of optical media materials, optical fiber materials, optical film materials, liquid crystal materials, luminescent materials, etc. all belong to optical materials. The development of optical technology is inseparable from optical material technology. We look forward to China's optical material technology taking the next level. building.
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