General Introduction to Optical Coatings

Comprehensive Guide to Optical Coatings: Types, Techniques, and Applications

Optical Coatings are thin films deposited on the substrates of optics, such as windows, lenses, and mirrors. Optical coatings are proposed to alter how light interacts with optical media by controlling reflection, transmission, absorption, and scattering, enabling precise manipulation of light for various applications. In some cases, optical coatings also act to provide protection for optical components from environmental wear. The most typical types of optical coatings (as divided by the functionalities of the optical coatings) include anti-reflection optical coatings (AR coatings), high-reflective coatings (HR coatings), beamsplitter coatings, etc. Anti-reflective coatings (AR coatings) are optical coatings that enhance the transmission rates of the wavelengths of interest, where high transmittance is desirable, like in the context of a focusing lens of a laser system. The reflective coating is quite the opposite, which is designed to increase the reflectivity of optical components like mirrors. With advancements in fabrication methods, optical thin film coatings have become more durable, versatile, and precise. This optical coating guide explores the fundamentals, types, materials, and process, providing a detailed introduction to optical coatings for anyone seeking knowledge on this critical topic.

optical coating process

Figure 1. Hangzhou Shalom EO's optical coating workshop

Fundamentals of Optical Coatings-The Snell’s Law and The Fresnel Equations

In the field of optical coatings, two fundamental principles/concepts govern how light interacts with coated surfaces: The Snell’s Law, and the Fresnel Equations. These principles are crucial for understanding how light is transmitted, reflected, and refracted when it encounters an optical coating, such as anti-reflective (AR) coatings, high-reflective (HR) coatings, and beam splitters.

1）Snell's Law (Law of Refraction)

The Snell’s Law is a fundamental equation that formulates refraction. Refraction is the redirection of the light waves, and the cause of refraction is the slowing down of light when it is transmitted from one medium to another denser medium, for example, from air to light, and refraction takes at the interface of the two optical media. Snell's Law describes how light bends (refracts) as it passes from one medium into another, based on the difference in their refractive indices. This law is key to understanding how coatings influence the direction of light at interfaces between different materials.

Snell's Law Formula is stated below:

n1sin(θ1)=n2sin(θ2)

Where:

n1 and n2 are the refractive indices of the first and second media, respectively.

θ1 is the angle of incidence (the angle between the incoming light ray and the normal to the surface).

θ2 is the angle of refraction (the angle between the refracted ray and the normal to the surface).

Snell’s Law determines the angle at which the light refracts into the new material (e.g., from glass to coating material, or between alternating coating layers of different coating materials in the case of a multi-layer dielectric optical coating.)

Figure 2. Refraction

(You can click here to learn more about Refraction and The Nature of Light)

To take a more in-depth discussion into the idea of Snell’s Law and refraction, there’s also the concept of the Critical Angle and Total Internal Reflection (TIR). The critical angle is the angle of incidence where the angle of refraction is 90°. Total internal reflection is the physical phenomenon that occurs when the angle of incidence is greater than the critical angle and when light is propagating from a dense medium into a thin medium, in TIR, no light will be refracted into the thin medium, instead, all of the light rays will be completely reflected back into the dense medium.

The formula of the critical angle is as follows:

Θa=sin-1(n2/n1)

Where n1 is the refractive index of the denser medium and n2 is the refractive index of the thinner medium.

2) The Fresnel Equations

The Fresnel Equations provide a mathematical framework to calculate the amount of light that is reflected and transmitted at an interface between two media with different refractive indices. Unlike Snell's Law, which only describes the angles of refraction, the Fresnel Equations give the amplitude coefficients of transmission and reflection.

The Fresnel Equations are:

fundamentals optical coating

Where:

ts and rs are the amplitude coefficients of transmission and reflection for s-polarization;

tp and rp are the amplitude coefficients of transmission and reflection for p-polarization;

θ1 is the incident angle, and θ2 is the transmission or reflection angle;

n1 and n2 are the indices of refraction of the two optical media.

Optical Coatings Are Angle or Polarization Dependant

optical coatings are angle-dependent and polarization-dependent. The behavior of light when interacting with optical coatings varies depending on both the angle at which it strikes the surface and its polarization state. This means optical coatings are often optimized for certain angles of incidence or polarization states. Deviation from the designed angles or polarization states can seriously degrade the proper functioning of the optical coatings or even lead to complete failure to achieve the expected performance.

Types of Optical Coatings

Optical coatings can be divided by its functions, into reflective, anti-reflective optical coating, beamsplitter/dichroic optical coatings, protective optical coatings, and optical filter coatings. The following texts provide a general exploration of the types of optical coatings.

1) Anti-Reflective (AR) Optical Coatings

Anti-reflective (AR) Coatings are optical coatings designed to reduce the reflection of light from a surface, thus increasing the transmission of light through optical components like optical lenses, optical windows, optical filters, etc. Anti-reflective Coatings coatings can enhance transmission, decrease loss, and glare, and eliminate ghosting.

2) High-reflective (HR) Optical Coatings

High-Reflective (HR) Coatings are the converse of AR coatings, HR optical coatings specialized optical coatings designed to maximize the reflection of light over a specified range of wavelengths and angles of incidence. These coatings are typically applied to mirrors, beam splitters, and other optical components where high reflectivity is essential for performance. HR optical coatings are critical in laser systems, telescopes, and high-power optical devices.

3) Beam Splitter Coatings

Beamsplitter Coatings are optical coatings deposited on the optical surfaces of beamsplitters, used to split a single beam of light into two or more separate beams depending on the polarization states of light. The optical coating allows part of the light to be transmitted and the rest is reflected, one can realize the precise control of the ratio of transmission and reflection (i.e. extinction ratio) by carefully engineering the optical coating. Compared with crystal beamsplitters, which induce polarization in transmitted and reflected beams via the light's interaction with the optical axes of the crystals, beamsplitter coatings are a cheaper choice, but the drawback is the relatively lower laser damage threshold and extinction ratio. These special optical coatings are essential in optical systems that require light division, such as in interferometers, optical communication systems, microscopes, and laser applications. In conclusion, beamsplitter coatings enable precise control over the direction and intensity of light, making them crucial for applications where splitting, combining, or directing light is necessary.

4) Dichroic Coatings

Dichroic Optical Coatings are similar to beamsplitter coatings in that dichroic coatings also split light into two, but the difference is dichroic coatings transmit and reflect light depending on its wavelength, rather than its polarization. These coatings are often applied to filters or mirrors that can divide or manipulate light in precise manners based on its color (wavelength).

5) Protective Optical Coatings

These optical coatings shield optical components from damage caused by moisture, abrasion, and temperature fluctuations. Protective optical coatings are often applied when the application environments are harsh and volatile, a fine example would be for Infrared Thermal Imaging Camera Lenses, where protective coatings made of DLC (Diamond-like Carbon) are often applied on the optical fronts of the infrared lens assemblies to protect the lenses from external injuries.

6) Optical Filter Coatings:

Filter Coatings are essential components deposited on the substrates of optical filters, used to manipulate the light passing through or reflecting from the optical filters. These optical coatings are designed to filter out specific wavelengths of light, allowing only certain parts of the spectrum to be transmitted or to be reflected/absorbed. Filter coatings are commonly used in applications ranging from optical instruments, machine vision (such as in Machine Vision Optical Filters), laser systems, and solar panels. By precisely controlling the transmission or reflection properties, filter coatings can improve image quality, reduce glare, and enhance system efficiency.

Optical Coating Materials

Optical Coating Materials can be categorized into three subdivisions:

Dielectrics (e.g., silicon dioxide, magnesium fluoride): For AR and HR coatings.

Metals (e.g., aluminum, gold, silver): For mirrors and conductive coatings.

Hybrid Materials: Combine dielectric and metallic properties for specialized applications.

This part explains the most commonly used materials in optical coatings—ranging from oxides and fluorides to metals and polymers, highlighting their properties, advantages, and typical applications in industries such as optics, electronics, and scientific instrumentation.

optical coating material

Figure 3. Optical coating materials

Titanium Dioxide (TiO2): Known for its high refractive index and durability, TiO2 is effective in anti-reflection optical coatings and enhancing color. It’s used in multi-layer optical coatings for improved optical efficiency.

Magnesium Fluoride (MgF2): Ideal for anti-reflection coatings in the UV to visible range, MgF2 offers low refractive index, durability, and scratch resistance, making it suitable for lenses, windows, and laser optics.

Silicon Dioxide (SiO2): Widely used for anti-reflection coatings and as a protective layer, SiO2 is transparent, durable, and resistant to environmental factors like humidity and temperature.

Aluminum (Al): Frequently used for reflective coatings due to its excellent reflectivity in the visible and infrared spectra. Aluminum coatings are common in mirrors and beam splitters but require protective layers to prevent oxidation.

Silver (Ag): Silver is highly reflective, especially in the visible and near-infrared spectra, and is used in high-performance optical systems. However, it is prone to tarnishing and often needs additional protective coatings.

Gold (Au): Gold is ideal for infrared reflective coatings due to its high reflectivity and resistance to oxidation, making it perfect for high-precision optical applications, although it is more expensive than other materials.

Aluminum Oxide (Al2O3): Known for its durability, Al2O3 is used for protective coatings that resist abrasion, corrosion, and high temperatures, often applied to optical elements like lenses and mirrors.

Calcium Fluoride (CaF2): Used in optical coatings for high transmission in UV and infrared regions, CaF2 is transparent and resistant to high-energy light, making it ideal for high-power laser and infrared optical systems.

Polyimide (PI): A polymer used for protective coatings, Polyimide is resistant to heat, chemicals, and radiation. It’s commonly used in flexible optical devices like fiber optics due to its lightweight and durable properties.

Optical Coating Process PVD coating (Thin film coating)

Physical Vapour Deposition (PVD) is often used to produce Optical Thin film Coatings. Physical Vapour Deposition (PVD) is a technique that describes various vacuum deposition methods, such as sputtering, and evaporation. PVD coatings are thin film coatings where a solid material is vaporized in a vacuum chamber and deposited onto a target material. It is used to change the surface properties of the object to be coated, where new mechanical, chemical, electrical or optical characteristics are needed. PVD coatings result in extreme surface hardness, low coefficient of friction, anti-corrosion, and wear resistance properties. The process is carried out in a vacuum chamber at a temperature between 50 and 600 degrees Celsius, meaning that the atoms that are vaporized from the solid material travel through the vacuum chamber and embed themselves into whatever object is in its path. The result is a thin film coating that adheres to the surface of the substrate. The thin film can range from a few nanometers to several micrometers in thickness, depending on the application. PVD is often used in optical thin film coatings to create coatings that change the optical properties of lenses, mirrors, filters, beam splitters, and other optical components. These coatings include anti-reflective (AR) coatings, high-reflective (HR) coatings, and dichroic coatings, among others.

Common methods of PVD optical coating include:

Ion-assisted electron-beam Evaporative Deposition/ Ion-Assisted E-Beam Deposition (IAD e-beam)
Plasma Assisted Reactive Magnetron Sputtering (PARMS)
Ion Beam Sputtering (IBS)
Advanced Plasma Sputtering(APS)

Items	IAD e-Beam	PARMS	IBS	APS
Spectral Performance	Stable	Stable	Highly Stable	Stable
Layer Density	Above Average	High	Very High	High
Coating Stress	Below Average	Above Average	High	Above Average
Layer Smoothness	Above Average	High	Very High	High
Repeatability	Above Average	High	Very High	High
Manufacturing Time	Short	Long-Medium	Long	Medium
UV Capability	High	Intermediate	Below Average	Above Average
Substrate Shapes	Highly Diverse	Restricted	Restricted	Diverse
Costs	Lowest	Medium	Highest	Medium

The Ion-Assisted Electron-Beam Evaporative Deposition/ Ion-Assisted E-Beam Deposition (IAD e-beam) Optical Coating Process

Hangzhou Shalom EO utilizes the Ion-Assisted E-Beam Deposition technique. This optical coating process combines the evaporation of the target material via electron beam and ion assistance to enhance the properties of the deposited coating. First, an electron beam is used to vaporize the coating material, which then condenses onto the substrate to form a thin film. Then, An ion beam is directed at the growing film during deposition. This ion bombardment helps promote nucleation (the formation of the initial film structure) and improves film growth by encouraging the film to adhere more strongly to the substrate. The advantages of the ion assistance are that coatings are allowed to be formed at lower temperatures and the ion bombardment improves the coating densities, resulting coating being less prone to spectral shifts when exposed to different environmental conditions.

At Hangzhou Shalom EO, our state-of-the-art in-house coating workshop is equipped with meticulous technologies and precision tools, allowing us to produce optical coatings of unparalleled quality and durability. With a strong focus on innovation and excellence, we utilize advanced techniques to ensure that every coating delivers exceptional optical performance across a wide range of applications, from high-precision optics to demanding industrial environments.

Figure 4. Hangzhou Shalom EO's optical coating process

Our optical coating process includes Cleaning, Coating, and Inspection:

Cleaning:

We have separate workshops with automated ultrasonic cleaning machines. By ensuring that the substrate is clean, the cleaning process sets the stage for successful coating adhesion, resulting in stronger, more durable optical coatings.

Figure 5. Hangzhou Shalom EO's automated ultrasonic cleaning machines for optical coatings

Coating:

Shalom EO harnesses advanced Ion Assisted Electron Beam Deposition (IAD e-beam) optical coating machines. At the current stage, we own two IAD e-beam coating machine module types:

· SHINCRON MIC-1350TBN

· Chengdu Guotai Vacuum Equipment GTF-900