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Optical Coating

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An optical coating is a thin-film deposit used to enhance transmission, change reflective properties, or change the polarization of light transmitted through an optical component. An optical coating may be a simple layer of metal such as aluminum, or it may be a complex dielectric coating formed of multiple thin layers of material, where the composition, thickness, and number of layers is carefully controlled for a precise result.

Shanghai Optics is able to provide all types of anti-reflective, high reflective, and partial reflective coatings. Whether you need a single layer anti-reflective coating or complex multilayer dielectric stacks, we can manufacture exactly what you need. Dielectric coatings such as BBAR, V-coatings, dual wavelength coatings, and sharp cut-on and cut-off filters are available in stock.

Examples of Optical Coatings

An anti reflective coating reduces unwanted reflection. For instance, a standard uncoated glass component will reflect about 4% of incident light. The same glass component with an AR coating tailored for the wavelength of light being transmitted might reflect less than 0.1% of incident light. We offer a variety of different dielectric anti reflective coatings, including both broadband and narrow band options. One versatile and hardwearing option is magnesium fluoride, which comes in both single-film and multiple layer options.

A reflective coating causes the surface to which it is applied to reflect all or some of the light which hits it. Consider again the uncoated glass optic with 4% reflection. Metal coatings will entirely change the properties of the optic. An aluminum coating will cause the same optic to reflect 88-92% of visible light. Silver coatings reflect 95-99% into the far infrared regions (reflection will be lower in the UV and some parts of the visible spectrum), and an appropriately chosen dielectric coating could increase reflectivity to 99.9%. Dielectric mirror coatings are often formed of two discrete repeating layers, one with a high index (for instance, ZnS or TiO2), and one with a low index (think MgF or SiO2). These high reflective dielectric coatings feature ultra high reflectance over a highly specific wavelength range, called the band stop.

A polarizing coating can be formed of a very thin film of a birefringent material, or alternately by means of interference effects in a multi-layer dielectric coating. If desired, polarizers can be designed to work with an incidence angle of 45 degrees, leading to a beam reflected at a 90 degree angle. Under certain circumstances, a polarizing coating on a lens or optical window can be used to replace polarizing prisms in an optical assembly.

A transparent conductive coating is a special type of coating that combines high transmission of visible light and electrical conductivity. These coatings can be used to protect the aperture of a device from electromagnetic interference, or to provide electrodes through which light can pass. The resistivity of a transparent conductive coating is specified in ohms per square, and may be anywhere from 4-1,000 ohms per square depending on application. ITO (indium titanium oxide) and AZO (aluminum doped zinc oxide) are two conductive coating options.

[An inside look at Shanghai Optics in-house optical lens coating process.]

Shanghai Optics can provide all types of anti-reflective, high reflective, and partial reflective coatings. We produce a wide variety of coatings from a single layer of anti-reflective coating to complex multi-layer dielectric stacks. Types of dielectric coatings are BBAR, V-coatings, dual wavelength coatings, and sharp cut-on and cut-off filters.

Optical Coating Manufacturing

At Shanghai Optics we use a variety of different methods to produce high quality optical coatings. These methods include plasma sputtering, ion beam sputtering, atomic layer deposition, and evaporative deposition.

Ion beam sputtering and atomic layer deposition provide the highest spectral performance, high durability and high repeatability; but the manufacturing process is slow and expensive. Evaporative deposition is more budget friendly, and plasma sputtering, or plasma assisted reactive magnetron sputtering, provides a good middle ground with reasonable quality and reasonable performance.

Ion Beam Sputtering

In ion beam sputtering (IBS) we use a high energy electric field to accelerate a beam of ions, giving them kinetic energy of 10-100 eV. The ion beam hits the source material, and ions from this material sputter onto the optical surface. A dense film is formed upon contact. Since every step of this process is carefully monitored and precisely controlled, the result is a highly consistent coating that meets design specifications and parameters precisely. IBS coatings can even have less roughness than the original substrate does. This is the method used to make “super mirrors”, special coated mirrors with a reflectivity of more than 99.99%.

Atomic Layer Deposition (ALD)

In atomic layer deposition the optic to be coated is placed in a vacuum chamber, often at elevated temperatures. The coating substance is delivered as a gas, in pulses. During each pulse a single layer adheres, and then the vacuum chamber is evacuated in preparation for the next pulse. The benefit of this method is an extremely high level of control in layer thickness and composition of the layers of material, and the ability to uniformly coat optics with any geometry. The only negative side to this method is that it is slow, and hence expensive to run.

Evaporative Deposition

Evaporative deposition is a fast process that produces medium high layer density and a stable spectral performance. It can be used on almost any substrate geometry, and has medium to high layer smoothness. One type of evaporative deposition is called ion assisted electron-beam evaporative deposition, and is performed in a vacuum chamber where an electron gun bombards and vaporizes source materials. While this method is not ideal for ultra-low or ultra high reflection coatings, it is a good choice where low cost and flexibility are more important than high performance. Since it can be conducted at low temperatures (20-100 C) it may be a good choice for temperature-sensitive substrates.

Plasma Assisted Reactive Magnetron Sputtering

In plasma assisted reactive magnetron sputtering (PARMS), glow discharge plasma, confined by a magnetic field to an area near the target, accelerates positive ions onto the target deposition material. This causes the ejection of atoms which then coat the optical surface. The process can run efficiently even at low chamber pressure, which allows it to be conducted with minimal setup time. The film coatings are typically hard and dense, and though PARMS is not as repeatable as ion beam sputtering it has a high through put and is much more repeatable than evaporative deposition. This balance between good optical performance and good volume throughput makes it an ideal choice for something like a fluorescence optical filter.

Optical Coatings and Metrology

After an optical coating has been applied, each component produced at Shanghai Optics undergoes stringent metrology tests to ensure it functions exactly as intended. Research-grade spectroscopes enable us to fully characterize the reflective properties of coatings from Far Infrared through to UV. Other state of the art metrology equipment, such as optical laser and laser diode sources, power meters, polarimeters and detectors give us the ability to quantize the properties of each component and ensure each optical coating performs according to all applicable standards.

Optical Coatings for Fiber Optics

Fiber optics are particularly reliant on coatings to enhance fiber strength, decrease fatigue and improve attenuation. They provide key damage protection and enable you to extend the lifespan of your device. Standard communication fibers typically utilize a UV-cured acrylate polymer coating applied in two layers: a soft primary coating and a harder secondary coating. Other specialty coatings include fluorescent materials, sensing reagents, silicone, sapphire, nitrides, metal, and carbon. Thin carbon and metal coatings can also be used in conjunction with a polymer coating.

At Shanghai Optics we have the facilities to both produce and coat high performance fiber optics per your specifications. Clean room facilities and highly sensitive equipment enable us to facilitate the highly controlled processes necessary to micro-optical production, and all our fiber optics are carefully inspected before they leave our factory.

Custom Optical Coatings

The final properties of a dielectric coating will depend on the coating materials used in the thin film layers—especially their refractive index. Changing the layer thickness and the number of layers can also completely change the interference effects and function of a thin film coating. For precision coatings, the optical thickness of each layer must be very carefully controlled.

The ideal optical coating designs for your application will depend on the wavelength of the light and work environment as well as on your project budget. Our design team has many years of experience in coating technologies, and we would be happy to help you determine the custom optical coatings that best fit your application.

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