What are the differences in optical materials? What categories are there?
1. Common optical materials

In all optical manufacturing, the first step is to select the appropriate optical materials. Important optical parameters in optical materials, such as refractive index, Abbe number, transmittance, reflectance, etc., as well as physical parameters like hardness, bubble content, deformation, and even temperature and thermal expansion coefficient, are all important components in the selection of materials. This article will provide a brief introduction to these materials.

2. Classification of optical materials

Optical materials are generally divided into three main categories: optical glass, optical crystals, and special optical materials.

Optical glass is an amorphous (glass state) optical medium material that can transmit light. After light passes through it, it can change the direction, phase, and intensity of propagation, and is commonly used in the production of optical components such as prisms, lenses, mirrors, window pieces, and filters in optical instruments or systems. Optical glass has high transparency, chemical stability, and high uniformity in physical (structure and performance) properties, with specific and accurate optical constants. Optical glass retains the amorphous structure of high-temperature liquid at low-temperature solid state, ideally having the same physical and chemical properties (such as refractive index, thermal expansion coefficient, hardness, thermal conductivity, electrical conductivity, elastic modulus, etc.) in all directions, which is called isotropy.

Currently, the main manufacturers of optical glass include Schott from Germany, Corning from the USA, Ohara from Japan, and Chengdu Guangming (CDGM) from China. Common optical glasses used in the ultraviolet band include UV fused silica (UVFS), with commonly used quartz materials being domestic JGS1, JGS2, JGS3, Corning 7980, and Ohara's high-quality quartz glass (SK-1300, SK-1310, SK-1320L, SK-1321, etc.). Our optical components generally use JGS1, which has a high transmittance starting from 180nm. Corning 7980 has good uniformity and low bubble impurity content, making it widely used in laser components, providing a higher laser damage threshold. For visible light and near-infrared bands, commonly used optical glass materials include Schott's N-BK7, float glass B270, and Chengdu Guangming's H-K9L. N-BK7 and H-K9L have similar properties and can replace each other. They provide high transmittance in visible light and near-infrared (350 nm - 2.0 µm). H-K9L precision annealed optical glass is the most commonly used glass in our high-quality optical components. H-K9L precision annealed optical glass is a hard glass that can withstand various physical and chemical stimuli. It is relatively scratch-resistant and resistant to chemicals. Due to its low bubble content and impurity levels, it is very suitable for manufacturing precision lenses, window pieces, prisms, and other components.

2.

Optical crystals

Optical crystals refer to the general term for crystal materials used as optical media. Due to the structural characteristics of optical crystals, they can be widely used to make window pieces, lenses, and prisms for various ultraviolet and infrared applications. According to crystal structure, they are divided into single crystals and polycrystals. Single crystal materials have higher crystal integrity and light transmittance, as well as lower input loss, so commonly used optical crystals are primarily single crystals.

◆ Common ultraviolet and infrared crystal materials include: quartz (SiO2), fluorite (CaF2), lithium fluoride (LiF), rock salt (NaCl), silicon (Si), germanium (Ge), etc.

◆ Polarizing crystals: Common polarizing crystals include calcite (CaCO3), quartz (SiO2), and sodium nitrate (saltpeter), etc.

◆ Achromatic doublet crystals: Achromatic objectives are made using the special dispersion characteristics of crystals, such as combinations of fluorite (CaF2) and glass to create achromatic systems that can eliminate spherical aberration and secondary spectrum.

◆ Laser crystals: Can be used as the working material for solid-state lasers, such as ruby, calcium fluoride, and neodymium-doped yttrium aluminum garnet crystals.

Comparison of common crystal characteristics

Crystal materials are divided into natural and artificially grown. Natural crystals are rare, and artificial growth is difficult, with size limitations and high costs, generally considered only when glass materials are insufficient, and can work in non-visible light bands, applied in industries such as semiconductors and lasers.

3. Special optical materials

Microcrystalline glass is a special optical material that is neither glass nor crystalline, lying between glass and crystal. The main difference between microcrystalline glass and ordinary optical glass is that it has a crystalline structure, while its crystalline structure is much finer than that of ceramics. It has characteristics such as a small thermal expansion coefficient, high strength, high hardness, low density, and extremely high stability, and is widely used in processing flat crystals, standard meter sticks, large mirrors, laser-guided gyroscopes, etc.

Silicon carbide is a special ceramic material that can also be used as an optical material. Silicon carbide has characteristics such as good stiffness, small thermal deformation coefficient, excellent thermal stability, and significant weight reduction, making it a primary choice for large-sized lightweight mirrors, widely used in aerospace, high-power lasers, semiconductors, and other fields. The above major categories of optical materials can also be referred to as optical medium materials, which can change the direction, phase, and intensity of propagation after light passes through them. In addition to these major categories of optical medium 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 materials technology, and we look forward to the advancement of China's optical materials technology.