Optical Coatings

Fundamental Optics
Material Properties Optical Specifications Gaussian Beam Optics
Optical Coatings
Optical Coatings
A comprehensive survey of all optical components currently in
use would reveal that the vast majority are made of various types
of glass. This survey would also reveal that a majority of these
optics are coated with thin layers of material(s) different from the
substrate. The purpose of these coatings is to modify the reflection
and transmission properties at the surface of the optical element.
Whenever light passes from one medium into a medium of
different optical properties (most notably refractive index), part of
the light (between 0% and 100%) is reflected and part of the light
(between 100% and 0%) is transmitted. The intensity ratio of reflected
and transmitted components is primarily a function of the difference
in refractive index and the angle of incidence. For many uncoated
optical glasses, reflected light typically represents a few percent of
incident radiation. For designs using more than a few components,
losses in transmitted light level can accumulate rapidly. More
important are corresponding losses in image contrast or modulation
caused by weakly reflected ghost images superimposed on the desired
image. Such unwanted images are often defocused beyond recognition
so that contrast reduction (rather than image confusion) is their
primary effect.
Applications generally require that the reflected portion of
incident light approach 0% for transmitting optics (lenses) and
100% for reflective optics (mirrors), or is at some fixed intermediate
value for partial reflectors (beamsplitters). The only applications
that do not require coated optics involve transmitting optics in
which only a few surfaces are in the optical path, where transmission
inefficiencies may be tolerable.
In principle, the surface of any optical element can be coated with
thin layers of various materials (called thin films) in order to ensure
the desired reflection/transmission ratio. Unfortunately, with the
exception of simple metallic coatings, this ratio depends on the
nature of the material from which the optic is fabricated, as well as
the wavelength and angle of incidence. There is also a polarization
dependence to this ratio when the angle of incidence is not 0 degrees.
A multilayer coating (sometimes more than 100 individual layers)
can optimize the reflection/transmission ratio for several sets of
conditions (wavelength and angle of incidence) or optimize it over
a particular range of conditions.
Melles Griot is the leading supplier of precision simple optics.
Because optics for most applications require a coating of some sort,
it would not have been possible to achieve this market-leading position
without our extensive knowledge of thin-film coatings. With
the state-of-the-art coating department located in Irvine, California,
as well as other coating facilities in Japan; Rochester, New York; and
the British Isles, Melles Griot is able not only to coat large volumes
of catalog and special optics, but also to develop and evaluate new
coatings for special customer requirements.
With new and expanded coating capabilities, Melles Griot now
offers the same high-quality coatings as a separate service to
customers wishing to supply their own substrates. As with any
special or OEM order, please contact Melles Griot to discuss your
requirements with one of our qualified applications engineers.
Today, dielectric coatings are remarkably hard and durable.
With proper care and handling, they can have a long life. In fact,
the surface of many high-index glasses that are prone to staining can
be protected with a durable antireflection coating. Several factors
influence coating durability. Coating designs should be optimized
for minimal overall thickness to reduce mechanical stress. The most
resilient materials should be used. Great care should be taken in coating
fabrication to ensure high-quality, nongranular, even layers.
Although we cannot prevent accidental abuse of coated optics,
Melles Griot concentrates on these other factors to produce coatings
that are as durable as possible.
Although the Melles Griot optical-coating departments have
many years of experience in designing and fabricating various types
of dielectric and metallic coatings, the science of thin films is still
developing rapidly. Melles Griot monitors and incorporates new
technology so that we are always able to offer the most advanced
coatings available.
The Melles Griot range of coatings currently includes antireflection,
metallic reflectors, all-dielectric reflectors, hybrid reflectors,
partial reflectors (beamsplitters), and filters for monochromatic,
dichroic, or broadband applications. Many of the coatings can be
applied to the simple optics described in this catalog; some coatings
can be applied only to a specific range of products; and some
of the coatings are supplied only as an integral part of a specific product
(e.g., cube beamsplitters).
If you require a special coating not described in this catalog, please
contact a Melles Griot applications engineer to discuss our special
coating design services.
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