Introduction to Common Optical Materials

The first step in all optical manufacturing processes is to select the appropriate optical materials. Optical materials' optical parameters (refractive index, Abbe number, transmittance, reflectance), physical properties (hardness, deformation, bubbles, Poisson's ratio), and even temperature characteristics (thermal expansion coefficient, refractive index VS temperature) will affect the performance of optical components and systems. This article will briefly introduce common optical materials and the characteristics of various materials.

Optical materials are typically of three types: optical glass, optical crystals, and special optical materials.

Optical Materials: Optical Glass

Optical glass is an amorphous (glass-like) optical medium material capable of transmitting light. After passing through, it can change the direction, phase, and intensity of light, often used in the manufacture of optical components such as prisms, lenses, mirrors, windows, and filters in optical instruments or optical systems. Optical glass has high transparency, chemical stability, and high uniformity in physical (structural and performance) properties, with specific and accurate optical constants. Optical glass retains its amorphous structure at low temperatures like it has at high temperatures, and ideally, the physical and chemical properties within the glass (such as refractive index, thermal expansion coefficient, hardness, thermal conductivity, electrical conductivity, and elastic modulus) are isotropic.

Optical Materials: Optical Crystals

Optical crystals refer to crystalline materials used as optical media. Due to the structural characteristics of optical crystals, they can be widely used to make windows, lenses, and prisms for various UV and IR applications. According to the crystal structure, they are divided into single crystals and polycrystals. Single crystal materials have higher crystal integrity and light transmittance and lower input losses, so commonly used optical crystals are mainly single crystals.

• Common UV and IR crystal materials include quartz (SiO2), fluorite (CaF2), lithium fluoride (LiF), rock salt (NaCl), silicon (Si), germanium (Ge), and so on.

  
  

• Polarizing crystals: Common polarizing crystals include calcite (CaCO3), quartz (SiO2), and sodium nitrate (niter).

  
  

• Apochromatic crystals: Using the special dispersion properties of crystals to manufacture apochromatic objectives, such as the combination of fluorite (CaF2) and glass to make apochromatic systems, which can eliminate spherical aberration and secondary spectrum.

  
  

• Laser crystals: Can be used as working substances for solid lasers, such as ruby, calcium fluoride, and Nd: YAG crystals.

Crystalline materials are divided into natural and artificial growth. Natural crystals are rare, artificial growth is challenging, sizes are limited, and prices are high. They are generally considered when glass materials cannot meet requirements and can work in non-visible light wavelengths, applied in semi conductor, laser, and other industries.

Specialoptical materials

Microcrystalline glass is a special optical material, intermediate between glass and crystal. The main difference between microcrystalline glass and ordinary optical glass is its crystalline structure, and the primary difference with ceramics is that its crystalline structure is much finer. It has characteristics of a low thermal expansion coefficient, high strength, high hardness, low density, and extremely high stability, widely used in making flat crystals, standard meter rods, large mirrors, and laser-guided gyroscopes.

Silicon carbide is a special ceramic material and can also be used as optical material. Silicon carbide has good stiffness ratio, low thermal deformation coefficient, excellent thermal stability, and significant weight-reducing effects, making it the main choice for large lightweight mirrors, widely used in aerospace, high-power lasers, and semiconductor fields.