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tutorialVariations in materials and physical parameters yield optical coatingsthat can act as high reflectors, antireflectors, and optical filters.By Ranko Galeb, VLOCptical thin films have become an inte-gral part of almost all the opticalcomponents and systems manufacturedtoday. Their primary function is to gov-ern the spectral composition and theintensity of the light transmitted or reflectances of 90% to 98%.reflected by the optical system. Properly applied to variousoptical surfaces in a given system, optical coatings can greatly coating designenhance image quality and provide a convenient method to Without getting into deep analysis of design methods of opticalspectrally manipulate light. thin films, let us point out that the main building blocks inLight behaves according to the laws of electromagnetic designing optical coatings are quarter-wave optical thicknesswaves. Thus, the interaction of light with the media that it trav- (QWOT) layers of different materials. The QWOT materialsels through or is reflected from is directly related to its wave of high, medium, and low refractive index are usually denotednature and manifests as the phenomena of interference and as H, M, and L, respectively. If there are two QWOT layers ofpolarization. Whenever light interacts with a thin-film struc- the same material next to each other, they form a half-waveture, interference occurs. The degree of polarization imparted optical thickness (HWOT) layer. If only a ...

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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 beam the depositing atoms, typically around 0.1 eV, have the biggest
splitter discriminates between two planes of polarization. In effect on film properties. IAD results in direct deposition of
transmission it is given as a ratio of T and T and in reflection ionized vapor and in adding activation energy to the growingp s
as a ratio of R and R . When the degree of polarization of an film, typically in the order of 50 eV. Using the ion source, con-ps
incident beam is very high, the reflected s-polarized component ventional electron-beam evaporation is improved by directing
and the transmitted p-polarized component should each the flux from the ion gun to the surface of the substrate and
growing film.
In PIAD, various materials are evaporated using electron
guns in conjunction with a plasma source. The plasma source is
located in the foreground. Substrates to be coated are loaded in
fixtures that form a planetary system, which maintains a uni-
form distribution of the evaporated material across the area of
the fixture. Fixtures turn around their common axis, while the
individual substrates revolve around their own axes.
The optical properties of films, such as refractive index,
absorption, and laser-damage threshold, depend largely on the
microstructure of the coating, which is controlled by the film
material, residual gas pressure, and the substrate temperature. If
the depositing vapor atoms have low mobility on the substrate
surface, the film will contain microvoids, which will be subse-
quently filled with water when the film is exposed to humid
atmosphere.Figure 1 A three-layer antireflection coating on BK7 glass
We define packing density as the ratio of the volume of the(n = 1.52) yields good performance over a 260 nm band.
solid part of the film to the total volume of film (whichThe design is BK7| M2HL |Air at 505 nm for a 0° incident
includes microvoids and pores). For optical thin films, it is usu-angle (n =2.126, n =1.629, n =1.384).H M L
ally in the range 0.75 to 1.0, very often 0.85 to 0.95 and rarely
as great as 1.0. A packing density below unity reduces the
refractive index of evaporated material below the value of its
bulk form. Using deposition techniques such as IAD, IBS, and
PIAD, engineers can increase the packing density of evaporated
material to a value very close to unity.
During the deposition, the thickness of each layer is moni-
tored either optically or by a quartz crystal. Both techniques
have advantages and disadvantages that are not discussed here.
What they have in common is that measurements are done in
vacuum while the material is evaporated. Consequently, the
measured thicknesses are related to the refractive index of evap-
orated material in vacuum, not the index the material will
acquire after being exposed to humid air. When the coating is
removed from vacuum, moisture adsorption in the film results
Figure 2 The notch filter shows characteristic rejection
in displacement of air from microvoids and pores, causing an
31 band. The design is BK7| (L3M) 4L |Air at 550 nm for a 0°
increase in the refractive index. Since the physical thickness of
incident angle (n =1.627, n =1.460).M L the film remains constant, this refractive index increase is
accompanied by a corresponding increase in optical thickness,
36 spie’s oemagazine april 2002| putting on a top coat
hen it comes to opti-W cal coatings, Ranko
Galeb does it all. As senior
coatings engineer of VLOC
(New Port Richey, FL), he not
only designs thin films and
develops new coating tech-
niques but also designs the
equipment used to produce
Figure 3 A polychromatic polarizing cube beam splitter onthe coatings.
BK7 glass rejects s-polarization while transmitting p-polar-“Rarely do you find some-
ization. The computed reflectance represents a 15-layerone who just designs,” says
design consisting of two materials of high and low refrac-Galeb, originally from Sarajevo, Bosnia. “Architects
tive index. The incident angle is 52°.design buildings, but they also have in mind how the
building will be constructed. With me, I design plus do a
lot of the construction work.”
For years, Galeb worked for companies that could not engineering tradeoffs
afford expensive new optical equipment, so he devel- Production cost per run of a particular coating is primarily
oped his own. For example, while at LaCroix Optical Co. determined by the size of the coating chamber, the manufactur-
(Batesville, AR), he designed, but never patented, a ing technology, and the complexity of the coating. Since the
machine to cement uncentered lenses so they could usable area of the coating chamber is more or less directly pro-
then be edged as a single compound lens. “This way, portional to the square of its radius, it follows that the bigger
you end up with a uniform product,” Galeb says. the chamber, the lower the price per coated lens. For example,
“Conventional methods typically fused two or three indi- if the diameter of one chamber is twice the diameter of the
vidually edged lenses, which were never perfectly other, then approximately four times more lenses can be coated
aligned.” in the first chamber than in the second one.
Engineering keeps him busy. “The thin-film industry is For some extremely stringent requirements, often found in
gaining more prominence with the development of new the production of narrowband and edge filters, it is not always
technologies,” says Galeb, who cites telecommunica- possible to utilize the whole coating area within one chamber
tions as a key application area. “This market has started but rather one particular segment of it. This is because of the
to have a huge impact on the evolution of technologies in nonuniformity of the coating distribution across the chamber.
thin films such as ion beam sputtering and plasma Essentially, then, the capacity of the coating machine is gov-
assisted deposition.” erned by the tolerances on the spectral characteristics of the
Despite the recent downturn in the industry, Galeb coating. For well-designed coating machines, the distribution
expects a strong, healthy market for years to come. “The of the spectral characteristic of evaporated material stays within
customers demand better, more advanced coatings every ±1% of the nominal value.
day,” he says. “There is a bright future for the optical thin- In addition to the spectral conformity of the coated lens to
film industry.” the prescribed value, its quality is determined by the level of
If Galeb ever tires of optical engineering, you may hear coating voids, the scattering properties, mechanical properties
one of his musical pieces. Not only does he play classical such as adhesion and hardness, environmental stability, and
guitar and piano, but he also composes. Four years ago, packing density. Complexity of the equipment needed to pro-
Galeb recorded an orchestral arrangement titled “My Life duce the coatings that would meet high-density requirement, in
in Paradise: Sarajevo.” —Laurie Ann Toupin addition to low scatter and stress, inevitably leads to an increase
in price of coated optics. oe
Ranko Galeb is the senior coating engineer at VLOC, New Port Richey,
which results in the spectral shift of the coating characteristic FL. Phone 727-375-8562; fax: 727-375-5300; e-mail: rgaleb@vloc.com.
toward a longer wavelength. To minimize this spectral shift
caused by the size and overall population of microvoids References
throughout the growing film, manufacturers perform the coat- 1. Jacobson, M. (1986) Deposition and Characterization of Optical Thin
ing process with high-energy ions that convey their momentum Films. New York: Macmillan.
to the atoms of the depositing material, thereby largely increas- 2. Macleod, H.A. (1986) Thin-Film Optical Filters. New York:
ing their mobility during the condensation at the substrate Macmillan.
surface. 3. Thelen, Alfred (1989) Design of Optical Interference Coatings. New
York: McGraw-Hill.
april 2002 spie’s oemagazine 37|
VLOC