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ANFILM

Double-sided reflection-increasing coating

quartz glass reflection increasing coating

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Monolayer coating:

Theoretical analysis:
If the medium is vertically incident from n1 to n2
Reflectivity =[ (n2 -n1) / (n1 n2) ]2
Penetrating rate = 4n1n2 / (n1 n2)2
If the refractive index of air is 1.0, the refractive index of coating nc (for example: 1.5), the refractive index of glass n (for example: 1.8)
(1) by the air directly into the glass
Penetrations = 4 × 1.0 × 1.8 / ( 1+1.8 )2=91.84%
(2) by the air into the coating and then into the glass
Penetrating rate =[ 4 × 1.0 × 1.5 / ( 1+1.5 )2] × [ 4 × 1.5 × 1.8 / ( 1.5+1.8 )2]= 95.2%

Antireflection Film Wavelength Selection Table

Standard visible light antireflection film curve

Standard infrared light reflection coating curve

Transparent:

Light (black) hits the antireflection film, and part of it is reflected (blue); part of it is refracted into the antireflection film (cyan), reflected by the second surface of the antireflection film (yellow), and then refracted out (red).
Due to cyan, yellow light travels for two ¼ wavelengths, I .e., 0.5 times the wavelength. Therefore, the phase difference of the two columns of red and blue light is half a wavelength, and they are superimposed and canceled. That is, the light energy enters the lens after entering the antireflection film. Therefore, it is called augmented penetration. The thickness of the single-layer antireflection film is 1/4 of the wavelength that needs antireflection.

Graph

Principle and Application of Monolayer Antireflection Film

Monolayer λ/4 ANO Film

The optical antireflection film of λ/4 (the optical element is replaced by glass and the initial incident medium is replaced by air in the following discussion) is generally coated with a thin film with an optical thickness of λ/4 on the glass, and the refractive index of the thin film is greater than the refractive index of air and less than the refractive index of glass, reflected light has half-wave loss at both the air-film interface and the film-glass interface. set the refractive indexes of air, coating and glass as n0,n1,n2 and n2>n1>n0 respectively to define R01,T01 as the reflectivity and transmittance of the air-film interface, R01,T01 as the reflectivity and transmittance of the film-air interface, R12,T12 as the reflectivity and transmittance of the film-glass interface, R21, t21 is the reflectivity and transmittance of the glass-film interface as shown in fig. 4-1. in order to distinguish human light from reflected light, the incident light line is drawn as oblique incidence here, and the optical path difference of reflected light 1 and 2 in fig. 4-1 is λ/2, so that reflected light can completely cancel out. fresnel formula knows that when light passes vertically through the interface, the relationship between reflectivity r and transmittance t and refractive index n is:

If the light intensity of human light is I0, the light intensity I1 = I0R0 of the reflected light 1, and the light intensity I2 = I0I01R12T10 of the reflected light 2. The square of the reflectivity will appear in the light intensity of the remaining reflected light, because the reflectivity is relatively small, so it can no longer be considered. The optical antireflection film of λ/4 makes the optical path difference between reflected light 1 and reflected light 2 δ = 2n1d1 = λ/2, so the phase difference is л. According to the interference theory, the light intensity after interference is:

Because the refractive indexes n0,n1,n2 are relatively close, for example, n0 = 1,n2=1.5 interface, T = 96%, T01 and T10 can be approximately taken as 1, if Ip is 0, R01 = R12, namely:

by n2>n1>n0

When the above formula is established, the reflectance is minimum and the transmittance is maximum. However, coating a layer of film also has shortcomings, because the refractive index of the commonly used λ/4 optical antireflection film MgF2,MgF2 is 1.38,1.38*1.38=1.9044, while the refractive index of glass is generally between 1.5 and 1.8, so the use of MgF2 antireflection film cannot minimize the light intensity of reflected light, the optical antireflection film with a wavelength of λ Δλ vertically incident on λ/4 reflects light 1 and reflected light 2 like light with a wavelength of λ. The optical path difference is δ = λ/2 and the phase difference is Δ Ψ = 2 л λ/2(λ Δλ) so that the light intensity after interference is:

You can select the appropriate material, so that I1 = I2, so that the above formula becomes

As shown in Figure 4-2, I is the light intensity of the reflected light, Δλ is the line width, and I increases rapidly with the increase of Δλ. The variation curve of the reflected light intensity of the optical antireflection film with optical thickness λ/4 with wavelength is V-shaped, so that the wavelength band with larger transmittance of the optical antireflection film with λ/4 is smaller. We call the frequency bandwidth of the optical antireflection film with λ/4 smaller. The lens of modern camera, the lens of video camera and the screen of color TV do not require high transmittance of light at a certain wavelength, it is desirable that the transmittance is low and consistent in a wide range of wavelength bands, that is, the bandwidth of the antireflection film is required to be large. So we can plate two layers of film, or even multiple layers of film.

The reflection of light on the surface of the lens also causes stray light, which seriously affects the imaging quality of the optical system. In order to reduce the reflection loss of light on the surface of optical components (such as lenses, prisms, etc.), a layer of transparent dielectric film (called antireflection film) is usually coated on the surface of the components.

The principle of antireflection film: "When the thickness of the film is appropriate, the light reflected on the two surfaces of the film, the distance difference is exactly equal to half the wavelength, thus canceling each other out. This greatly reduces the reflection loss of light and enhances the intensity of the transmitted light."

In many optical systems, a very important part is the coating on the lens that can reduce reflection. In many application fields, the antireflection film is indispensable, otherwise, the application requirements cannot be met.

antireflection film principle:

The principle of antireflection film: "When the thickness of the film is appropriate, the light reflected on the two surfaces of the film, the distance difference is exactly equal to half the wavelength, thus canceling each other out. This greatly reduces the reflection loss of light and enhances the intensity of the transmitted light."

One is that when light enters from one medium to another medium, if the difference in refractive index between the two media decreases, the energy of reflected light decreases and the energy of transmitted light increases. When the light hits the interface of two transparent media, if the light is emitted from the light dense medium to the light sparse medium, the light may be totally reflected; when the light is emitted from the light sparse medium to the light dense medium, the reflected light has a half-wave loss. For the anti-reflection film on the glass lens, its refractive index is between the refractive index of glass and air. When the light is emitted from the air to the lens, the reflected light on both sides of the film has a half-wave loss, so that the thickness of the film only The optical path difference between the two reflected lights is half a wavelength. The reflected light on the back surface of the film travels more than the reflected light on the front surface, I .e. twice the thickness of the film. Therefore, the film thickness should be 1/4 of the wavelength of the light in the thin film medium, so that the two reflected lights cancel each other. It can be seen that the thickness of the anti-reflection film is d = λ/4n (where n is the refractive index of the film, and λ is the wavelength of light in the air).

If the surface of the lens is not coated with a thin film, when light is directly incident from air with a refractive index of n1 = 1.0 to the interface of glass with a refractive index of n2 = 1.5, the reflectivity will be 4% of the incident light energy to be reflected and 96% of the incident light energy to enter the glass, which indicates that the reflected light on the surface of the optical device will cause light energy loss. When the light entering the glass enters the air from the glass vertically, the transmitted light will have the same proportion of energy loss compared with the incident light. That is, a simple glass lens, light through its two transparent surfaces, the intensity of the transmitted light I only accounts for the original incident light intensity I0.

It is common to use the objective lens of more advanced cameras, the periscope used in submarines, etc., which are generally composed of multiple lenses, and its purpose is to use the different properties of convex lenses and concave lenses to eliminate phase differences. The greater the loss of light energy, the worse the quality of the image, and the reflected light may be reflected by other surfaces to the vicinity of the image, forming harmful stray light, which will further weaken the quality of the image.

If the surface of the glass lens is coated with a transparent medium with a refractive index between glass and air, the energy of the transmitted light is the energy of the original incident light when there is an anti-reflection film. After adding the magnesium fluoride film, the transmitted light energy is increased by 97.3-92% = 5.3, so the reflected light energy is reduced. The lens composed of 6 lenses coated with anti-reflection film, compared with the same situation when the light directly enters the glass lens from the air, increases the transmitted light energy by 84.8-61% = 23.8, and reduces the reflection loss of light.

Using the principle of thin film interference, the energy of transmitted light is increased. Because when the light from the light sparse medium to the light dense medium, the reflected light has a half-wave loss, that is, the reflected light and the incident light phase is exactly opposite.

If the light directly from the air perpendicular to the surface of the glass lens, the reflected light will directly meet the incident light interference phase, the reflected light offset part of the incident light, so that the energy of the transmitted light is reduced.

If a film is coated on the surface of the glass lens, its thickness is equal to the 1/4 of the wavelength of light in the film.

When the light is emitted from the air to the lens again, because the reflected light on both sides of the film has half-wave loss, the optical path difference between the reflected light on the back surface of the film and the reflected light on the front surface of the film is exactly half a wavelength. At this time, it is not the reflected light and the incident light that cause interference cancellation, but the reflected light on the front and back surfaces of the film, which is equivalent to increasing the energy of transmitted light.

According to the theory of light propagation, the propagation speed and wavelength of light of different frequencies in the same medium are different, but the thickness of the selected material can only be a 1/4 of a certain wavelength, that is, only the reflected light of a certain frequency can be canceled, and the reflected light of other frequencies can not be completely canceled. Therefore, the optical device coated with the antireflection film will appear a certain color under white light irradiation. For example, the camera film is most sensitive to yellow-green light with a wavelength of 5500 angstroms. To eliminate the reflected light of this color light with a wavelength of 5500 angstroms and increase its transmitted light, the thickness of the film can only be the 1/4 of the wavelength of this color light in the film. When the reflected light is less yellow-green in the original white light, the lens will appear lavender.

In summary, we can come to the conclusion that coating a layer of appropriate thickness and material on the surface of the optical lens can increase the energy of the transmitted light and reduce the energy loss of the reflected light. To achieve the effect of "reflection enhancement film.

Optical coating concept and principle

Coating is to use physical or chemical methods to plate a layer of transparent electrolyte film on the surface of the material, or plate a layer of 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, filtering, polarization and other requirements. Commonly used coating methods include vacuum coating (a kind of physical coating) and chemical coating. After the surface of the optical parts is coated, the light is reflected and transmitted multiple times on the film layer to form multi-beam interference, and the refractive index and thickness of the film layer can be controlled to obtain different intensity distributions. This is the basic principle of interference coating.

optical film classification:

The coating: silicon, germanium, zinc sulfide, zinc selenide and other substrates are more, fluoride is rare.
Single wavelength, dual wavelength, broadband
Reflective film: sub-dielectric and metal reflective film, metal reflective film is generally gold-plated plus protective layer.
Semi-reflective, single-wavelength, dual-wavelength, broadband
Hard carbon film: also called DLC film, generally plated on the outer surface of silicon, germanium, chalcogenide glass for protection/antireflection. The other side of the product generally requires antireflection coating.
Split film: some require specific incident angle, visible light band reflection, infrared band through, more used in spectral analysis.
45 degree beam splitter, two-color beam splitter, polarization beam splitter & prism
Filter film: broadband, narrowband
Laser Crystal Film: YAG/YV04/KTP/LBO/BBO/LIND03
UV film-antireflection: 193/248/266/308/340/355, aluminum reflection 180-400nm
Infrared film: CO210.6UM/YAG2940NM/SI & GE & ZNSE & ZNS

high reflective film

Metal Mirror (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.

The presence of partial absorption limits its application in the laser field.

All-dielectric mirror (Dielectric HR coatings)

The cost is higher and the reflection band is narrow.

The reflectivity can be very high.

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

The film layer is thicker, the stress is larger, and there is a risk of film layer falling off.

Coated Substrate

Refers to the coating on what material. The substrate is often determined by the use environment and use. Common coating substrate selection? For example, calcium fluoride substrate is used for gas analysis and protection of gold, float glass for ordinary mirrors, silicon substrate for laser cavity mirrors, silicon germanium for infrared filters, glass for visible and near infrared, nickel and gold for oxygen-free copper.

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

Coating material

　　Attached to the substrate to play the role of transmission, reflection, light and other materials, may be optical materials such as zinc sulfide, magnesium fluoride, etc., may also be metal, such as aluminum gold. At present, mature large quantities of optical coating materials are mostly granular or flaky, and there are also whole crystal coating targets. Metal coating materials are mostly wire and block. The substrate, purpose and coating index determine what coating material to use.

Coating process and equipment

Cleaning equipment:

　　Ultrasonic cleaning machine: refers to the integration of cleaning and drying, can be directly loaded plate coating. At the same time, the machine must be used in clean space;

Ultrasonic Cleaning Technology of Optical Lenses

In optical cold processing, the cleaning of the lens mainly refers to the cleaning of the residual polishing liquid, adhesive and protective material after the lens is polished. Cleaning of grinding oil and glass powder after lens edge grinding; Cleaning of finger marks, saliva and various attachments before lens coating.

The traditional cleaning method is the use of wiping materials (gauze, dust-free paper) with chemical reagents (gasoline, ethanol, acetone, ether) to soak, wipe and other means of manual cleaning.

This method is time-consuming and laborious, has poor cleanliness, and is obviously not suitable for the modern large-scale optical cold processing industry. This forced people to look for a mechanized means of cleaning instead. So ultrasonic cleaning technology gradually into the optical cold processing industry and show their skills, and further promote the development of optical cold processing industry.

The basic principle of ultrasonic cleaning technology can be roughly considered to be a method that uses the huge force generated by the ultrasonic field to promote a series of physical and chemical changes in the washing medium to achieve the purpose of cleaning.

When the high frequency vibration higher than the sound wave (28~40khz) is transmitted to the cleaning medium, the liquid medium produces nearly vacuum cavity bubbles under the high frequency vibration. In the process of collision, merger and extinction of the cavity bubbles, the liquid can instantly generate thousands of atmospheric pressure pressure, and such a large pressure makes a series of physical and chemical changes occur in the surrounding substances.

Process flow:

  
Plasma Enhanced Chemical Vapor Deposition (PECVD):

It is to ionize the gas containing the atoms of the film by means of microwave or radio frequency to form plasma locally, and the plasma has strong chemical activity and is easy to react to deposit the desired film on the substrate. PECVD can be achieved at lower temperatures because the activity of plasma is used to promote chemical reactions.

plasma assisted vapor deposition

At present, the preparation method of DLC film is commonly used. Using radio frequency technology (RF-PACVD) will be introduced into the gas (butane, argon) ionization, in the plate from the bias (negative) attraction, positively charged particles to the substrate impact, deposited on the surface of the substrate.