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Variations in materials and physical parameters yield optical coatings
that can act as high reflectors, antireflectors, and optical filters.
By Ranko Galeb, VLOC
ptical thin films have become an inte-
gral part of almost all the optical
components and systems manufactured
today. Their primary function is to gov-
ern the spectral composition and the
intensity of the light transmitted or reflectances of 90% to 98%.
reflected by the optical system. Properly applied to various
optical surfaces in a given system, optical coatings can greatly coating design
enhance image quality and provide a convenient method to Without getting into deep analysis of design methods of optical
spectrally manipulate light. thin films, let us point out that the main building blocks in
Light behaves according to the laws of electromagnetic designing optical coatings are quarter-wave optical thickness
waves. Thus, the interaction of light with the media that it trav- (QWOT) layers of different materials. The QWOT materials
els through or is reflected from is directly related to its wave of high, medium, and low refractive index are usually denoted
nature and manifests as the phenomena of interference and as H, M, and L, respectively. If there are two QWOT layers of
polarization. Whenever light interacts with a thin-film struc- the same material next to each other, they form a half-wave
ture, interference occurs. The degree of polarization imparted optical thickness (HWOT) layer. If only a fraction of QWOT
by the film is a function of the angle of incidence of the light. appears in a design, say one half of H, it is represented either as
HAt normal incidence, no polarization takes place unless the 0.5Hor . 2
light is transmitted through a birefringent material, in which The long expressions for some designs can be represented in
the degree of polarization introduced depends on the axis of concise form. For example, a 15-layer longwave-pass filter on
propagation. At oblique incident angles, polarization is intro- BK7 glass given by
duced; in addition, the reflectance or transmittance
H Hcharacteristics undergo a spectral shift toward shorter wave- BK7 LHLHLHLHLHLHL Air| |
2 2
lengths. This is caused by the optical path difference between
H H 7| |the waves reflected from either side of the film structure. The can be written as BK7(L) Air, where H and L refer to2 2
optical path difference is directly proportional to the cosine of high and low index materials, such as titanium dioxide (TiO )2
the angle of refraction through the coating. and silicon dioxide (SiO ), respectively.2
For an optical coating designer, another important character- Thin-film computer programs enable coating engineers to
istic is the amount of energy loss, or light absorbed in the efficiently determine the best and most economical design—
coating. In general, for any coating there is a relationship once the problem has been properly formulated. Like
between the transmittance T, the reflectance R, and the absorp- lens-design software, coating-design software is only as good as
tance A in the form of the person using it. Successful application of computation tools
to coating design requires engineers with detailed understand-
T + R + A = 1 , ing of coating materials and processes.
where 0 ≤ T, R, A ≤ 1. For dielectrics, absorptance is almost antireflection and reflection
zero—essentially they do not absorb any light. Metals, on the The most widely applied optical coating is the antireflection
other hand, act as light attenuators, and their coefficient of (AR) coating, which is designed to reduce the amount of light
absorption is always greater than zero. They also feature reflected from the optical surface. Its secondary role is to
34 spie’s oemagazine april 2002| High-reflection coatings, such as the alu-
minum coatings used for metallic mirrors,
represent another class of widely used
thin films. Aluminum is a relatively
soft metal, so the coating is often
protected with SiO. The2
reflectance of this coating is
about 90%, but it can be
boosted to 97% or 98% with
the addition of a few more
layer pairs of high- and low-
index materials (e.g., TiO2
and SiO ). 2
Since aluminum is a
metal, there is a slight light
loss associated with its use.
This light loss or absorption
is manifested as heat released
within the coating. In certain
applications, such as those for
high-power lasers, damage con-
siderations mandate the use of
ultralow-absorption mirrors; in such
cases, all-dielectric mirrors are the best
choice.
Dielectric mirrors consist of the sequence
of the alternating high and low QWOT index
materials (e.g., hafnium oxide and SiO ). The2
enhance physical and more layer pairs in the stack, the higher the reflectance.
chemical properties of the surface So-called “cold” mirrors (for visible and ultraviolet light) reflect
to which it is applied. shorter wavelengths and transmit longer wavelengths. “Hot”
Uncoated glass typically has a surface reflection of between mirrors (for infrared light) transmit shorter wavelengths and
4% and 8%. This can be reduced to about 1% at visible wave- reflect longer wavelengths.
lengths by applying a single layer of QWOT low-index
material, usually magnesium fluoride (MgF ). A three-layer filters2
design can reduce the reflection at visible wavelengths even fur- As with electronic circuits, optics requires many different types
ther (see figure 1). The first layer consists of a QWOT of interference filters. Sometimes the goal is to separate one
medium-index material (e.g., aluminum oxide) next to the portion of the spectrum from the other for a beam at normal
glass. The second layer is a HWOT high-index material (e.g., incidence or oblique incidence. Whatever the case, the solution
tantalum oxide). The third layer is a QWOT low-index mater- will be in the form of an edge filter or some kind of dichroic
ial (e.g., MgF ) as a top layer, next to the air. This three-layer beam splitter.2
design falls in the category of the broadband antireflection coat- When the application requires passing just one narrow band-
ing, often denoted as BBAR coating. width and reflecting a portion of the spectrum to either side,
If only one wavelength is considered, a two-layer design of the best choice is a narrow band-pass interference filter, often
high- and low-index materials will bring the reflection down to called a Fabry-Perot filter. This filter became of paramount
nearly zero. With the layer next to the glass fairly thin (high- importance in the production of dense wavelength division
index material) and the layer facing the air side (low-index multiplexing filters for telecommunication applications. To
material) somewhat greater than a QWOT, a relatively broad meet stringent requirements for environmental and spectral sta-
minimum can be obtained. These coatings are usually called V bility, these filters are manufactured using either ion beam
coatings. sputtering (IBS) or plasma ion assisted deposition (PIAD) tech-
A BBAR coating such as one that would apply to both the nologies.
visible and the near-infrared (IR) spectral regions requires many Recently, another class of interference filters has become of
layers of high- and low-index materials. Their thicknesses must great importance in laser and fiber-optic applications: notch fil-
be computer optimized and monitored throughout the deposi- ters, which reflect one or more narrow bands and transmit the
tion process using either a resonant quartz mask monitor or a wider regions around the rejection zone (see figure 2 on page
combination of quartz and optical monitoring. A BBAR coat- 36). To maintain a narrowband characteristic of the rejection
ing that covers 450 nm to 1100 nm would require eight or zone, this filter is often designed using low- and medium-index
more layers to yield less than 1% reflectance at any wavelength materials. This requires many layers to achieve high reflection.
within the region. Essentially, the function of a notch filter is just the opposite of
april 2002 spie’s oemagazine 35|the narrowband filters. account for a 50% of the incident light intensity. Thus, an
With the advent of new polarizing devices in the area of elec- ideal polarizing beam splitter acts as the 50/50 intensity beam
tronic imaging, polarizing beam splitters have become of splitter, where each of the two emerging light beams is 100%
significant importance. Their role is to maximize the reflection linearly polarized (see figure 3).
of s-polarized light and minimize the reflection of p-polarized
light for an unpolarized (randomly polarized) incident beam. coating fabrication
The degree of polarization (P) in transmission is given by Optical coatings are manufactured in high-vacuum coating
chambers. Conventional processes such as thermal evaporationT –Tp s require elevated substrate temperatures, usually around 300°C.P = –––––T
T +Tp s More advanced techniques, such as ion assisted deposition
(IAD), IBS, and PIAD operate at near room temperatures. IAD
and in reflection by processes not only produce coatings with better physical charac-
teristics compared to conventional ones but also can be appliedR –Rps
P = –––––R to plastic substrates.
R +Rsp. Thermal evaporation involves either resistance-heated evapo-
ration sources or electron-beam evaporation. The energies of
The extinction ratio indicates how well the polarizing