An Introduction to Optical Coatings.


Optical coatings are used to manipulate the reflectance and transmittance of an optical component. Typical coating examples include: Antireflection Coatings which reduce the amount of light reflected at a given wavelength or wavelength range; Narrow Band Filters which allow only a given window of wavelengths to transmit; and High Reflector Coatings, used in the production of mirrors.

Coatings are designed for a specific incident angle of light and for a specific polarization of light such as S-polarized, P-polarized, or random polarization. If a coating is designed for light at a 0กใ angle of incidence, but is used with light at a 45กใ angle of incidence, the coating will not perform at the stated transmission/reflection specifications. Similarly, coatings are generally designed for randomly polarized light so using S- or P-polarized light with a coating designed for randomly polarized light will again fail to produce the stated specifications.

Coatings are created by depositing dielectric and metallic materials such as SiO2, TiO2, or Al, in thin layers that are typically equal in thickness to one-quarter the wavelength of the light used in the application. These thin films alternate between layers of high index of refraction, and low index of refraction, thereby inducing the interference effects needed. Most of our optics have some form of coating and we also design and apply custom coating solutions.


DEFINITIONS:

Crystal Thickness Control:
The oscillating frequency of a quartz crystal declines from 5 MHz as coating materials are deposited on the crystal. Consequently, the thickness of the coating on the crystal can be calculated by measuring the fall of the crystal's oscillation frequency. The crystal is positioned at the center of the coating chamber and the coating thickness on the crystal is used to calculate the coating thickness on the optics. (See diagram of coating chamber).

Dielectric:
Material exhibiting very high transmission (non-absorption) across many wavelengths - from the UV through Visible wavelengths and into the IR. (See diagram of light rays).

Optical Monitoring:
Direct measurement of an optic's reflectance, performed by reflecting light off the optic and measuring the intensity of the reflected light. A similar procedure can be applied to measuring transmittance. For certain applications, the Optical Monitoring technique yields greater precision than crystal thickness control. (See diagram of coating chamber).

Spatter:
Material unintentionally ejected in larger particulate form from the coating material chunk. These particles can become embedded in the substrate as surface defects. (See diagram of coating chamber).

Thin Film:
Any film that produces constructive or destructive interference effects when light is transmitted through or reflected from it. (See diagram of light rays).


COATING APPLICATION TECHNIQUES:

IBS: Ion Beam Sputter.
Sputter process in which energetic ions are produced by an ion gun. (See diagram of coating chamber).

PVD: Physical Vapor Deposition. A coating material is heated causing evaporation such that evaporate condenses on the optic, leaving a film. Coating material can be heated by a focused beam of electrons or by heat derived through electrical resistance. (See diagram of coating chamber).

Sputter: Energetic molecules of coating material are created by smashing accelerated ions into a target of coating material. The sputtered material then forms a durable, stable deposit on the optic. The accelerated ions are normally produced from a plasma of inert Argon. (See diagram of coating chamber).

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