The coating is the physical or chemical method of plating a layer of transparent electrolyte film on the surface of the material or plating a layer of the metal film, the purpose is to change the reflection and transmission characteristics of the surface of the material, to reduce or increase the reflection of light, beam separation, color separation, light filtering, polarization, and other requirements.

The commonly used coating methods are vacuum coating (a physical coating) and chemical coating. After the surface of the optical parts is coated, the light is reflected and transmitted on the film layer many times to form multi-beam interference. The refractive index and thickness of the film layer can be controlled to get different intensity distributions, which is the basic principle of interference coating.

1. Optical coating theory

To understand optical coating, it is necessary to understand Fresnel equations of refraction and reflection. Refraction is the change in the direction of a wave as it travels from one optical medium to another. It is determined by Snell’s law of refraction:

(1) n1sin theta 1 = 1 = 2 n1sin n2sin theta theta n2sin theta. 2

Where n1 is the refractive index of the incident medium, θ1 is the Angle of the incident light, n2 is the index of the refraction/reflection medium, and θ2 is the Angle of refraction/reflection light.

Using Snell’s law, we can find the light Angle at any position of the multilayer film coating composed of planar parallel surfaces with different refractive indices. Since Snell’s law applies to each interface, the inner Angle of the light in the film is independent of the film order or the position of the film in the stack:

(2) n1sinθ1=n2sinθ2=n3sinθ3=n4sinθ4n1sin⁡θ1=n2sin⁡θ2=n3sinθ3=n4sinθ4

n=1.0   n=1.45   n=1.75  n=1.0

The outgoing ray in the diagram will be parallel to the incoming ray because n1 = n4. Because of the curvature of the optical element, the optical coating on the curved surface is not a true planar parallel structure. However, the approximation is still valid because of the thinner coating.

The law of reflection states that the Angle of reflected light with respect to the surface normal is equal in magnitude to the Angle of incidence, but opposite in the direction of the Angle with respect to the surface normal.

(3) θ1=−θ2θ1=−θ2

If the Angle of incidence of light transmitted from one medium to another with a lower refractive index is greater than the critical angle of the material (θC) defined by the ratio of the two refractive indexes, the total reflection will occur and the light will reflect completely (Figure 4). When the incident angle is equal to the critical Angle, the refraction Angle is equal to 90 degrees.

(4) θC=n2n1θC=n2n1

The amplitude coefficients of transmission and reflection at the interface of two optical media are determined by Fresnel transmission and reflection equations:

(5) ts=2n1cosθ1n1cosθ1+n2cosθ2ts=2n1cosθ1n1cosθ1+n2cosθ2

(6) rs=n1cosθ1−n2cosθ2n1cosθ1+n2cosθ2rs=n1cosθ1−n2cosθ2n1cosθ1+n2cosθ2

(7) tp=2n1cosθ1n1cosθ2+n2cosθ1tp=2n1cosθ1n1cosθ2+n2cosθ1

(8) rp=n1cosθ2−n2cosθ1n1cosθ2+n2cosθ1rp=n1cosθ2−n2cosθ1n1cosθ2+n2cosθ1

Where ts and tp are the amplitude transmission coefficients for s- and p-polarization, rs, and RP are the amplitude reflection coefficients for s- and p-polarization, n1, and n2 are the refractive indices of the two optical media, θ1 is the incident angle, and θ2 is the transmitted or reflected angle.

At normal incidence, θ1 and θ2 are 0, making all cosine terms 1 and amplitude coefficients the same for both polarization states. And that’s intuitive because there’s no difference between the s and p polarization states at normal incidence.

Reflections occur when light hits electrons on the surface of the material it enters. Electrons absorb and re-emit light that has lost some of its energy. Bright and highly reflective mirror materials have more freely moving electrons, allowing for maximum reflection and minimum transmission.

2. Optical film classification:

Anti-reflection film: silicon, germanium, zinc sulfide, zinc selenide, and other bases, fluoride is rare.

The single-wavelength, dual-wavelength, broadband

Reflection film: divided into medium and metal reflection film, metal reflection film is generally gold plated and protective layer.

Semi-reflective, single wavelength, dual-wavelength, broadband

Hard carbon film: also known as DLC film, generally plated on the outer surface of silicon, germanium, and sulfur glass, for protection/anti-reflection effect, the other side of the product is generally required to be plated anti-reflection film.

Spectrometer: Some require specific incident Angle, visible band reflection, and infrared band through, mostly used in spectral analysis.

45-degree splitter, two-color beam splitter, polarization beam splitter & prism

Filter film: wide band, narrow band

Laser crystal film: YAG/YV04 / KTP/LBO/BBO/LIND03

Uv film – anti-reflection: 193/248/266/308/340/355, aluminum reflection 180-400nm

Infrared film: CO21.6um /YAG2940NM/SI&GE&ZNSE&ZNS

3. Highly reflective film

(1) Metallic Mirror

The cost is low and the reflection band is wide.

It is generally used for applications where the reflectivity requirement is not particularly high, but the band is very wide.

Because of partial absorption, its application in the laser field is limited.

(2) Dielectric HR coatings

The cost is higher and the reflection band is narrow.

The reflectivity can be very high.

The range of the reflection band is limited. If the range of the reflection band is increased, the difficulty of film plating will be increased.

The film layer is thick, the stress is large, and there is the risk of film shedding.

4. Coated substrate

Refers to the material on which the film is coated. The base is often determined by the context and use. A common choice of coating substrate? Such as gas analysis protective gold multi-use calcium fluoride base, ordinary mirror with float glass, laser cavity mirror with a silicon substrate, infrared filter multi-use silicon germanium, visible and near-infrared is mostly glass, oxygen-free copper is mostly nickel and gold, etc.

Calcium fluoride, barium fluoride, magnesium fluoride, sapphire, germanium, silicon, zinc sulfide, zinc selenide, chalcogenide glass, N-BK7, fused quartz, etc

5. Coating material

The materials that are attached to the substrate and play the role of transmission, reflection, and splitting may be optical materials such as zinc sulfide and magnesium fluoride, or metals such as aluminum and gold. At present, a large number of mature optical coating materials are mostly granular or drug sheets, there are also whole crystal coating target materials; Metal coating materials are mostly silk and block; Substrate, use, and coating index determine what coating material to use.