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Optical Design & Engineering

Controlling light

Variations in materials and physical parameters yield optical coatings that can act as high reflectors, antireflectors, and optical filters.

From oemagazine April 2002
31 April 2002, SPIE Newsroom. DOI: 10.1117/2.5200204.0010

Optical thin films have become an integral part of almost all the optical components and systems manufactured today. Their primary function is to govern the spectral composition and the intensity of the light transmitted or reflected by the optical system. Properly applied to various optical surfaces in a given system, optical coatings can greatly enhance image quality and provide a convenient method to spectrally manipulate light.

Light behaves according to the laws of electromagnetic waves. Thus, the interaction of light with the media that it travels through or is reflected from is directly related to its wave nature and manifests as the phenomena of interference and polarization. Whenever light interacts with a thin-film structure, interference occurs. The degree of polarization imparted by the film is a function of the angle of incidence of the light. At normal incidence, no polarization takes place unless the light is transmitted through a birefringent material, in which the degree of polarization introduced depends on the axis of propagation. At oblique incident angles, polarization is introduced; in addition, the reflectance or transmittance characteristics undergo a spectral shift toward shorter wavelengths. This is caused by the optical path difference between the waves reflected from either side of the film structure. The optical path difference is directly proportional to the cosine of the angle of refraction through the coating.

For an optical coating designer, another important characteristic is the amount of energy loss, or light absorbed in the coating. In general, for any coating there is a relationship between the transmittance T, the reflectance R, and the absorptance A in the form of

T + R + A = 1,

where 0 < T, R, A < 1. For dielectrics, absorptance is almost zero—essentially they do not absorb any light. Metals, on the other hand, act as light attenuators, and their coefficient of absorption is always greater than zero. They also feature reflectances of 90% to 98%.

coating design

Without getting into deep analysis of design methods of optical thin films, let us point out that the main building blocks in designing optical coatings are quarter-wave optical thickness (QWOT) layers of different materials. The QWOT materials of high, medium, and low refractive index are usually denoted as H, M, and L, respectively. If there are two QWOT layers of the same material next to each other, they form a half-wave optical thickness (HWOT) layer. If only a fraction of QWOT appears in a design, say one half of H, it is represented either as 0.5 H or .

The long expressions for some designs can be represented in concise form. For example, a 15-layer longwave-pass filter on BK7 glass given by

can be written as

), where H and L refer to high and low index materials, such as titanium dioxide (TiO2) and silicon dioxide (SiO2),respectively.

Thin-film computer programs enable coating engineers to efficiently determine the best and most economical design—once the problem has been properly formulated. Like lens-design software, coating-design software is only as good as the person using it. Successful application of computation tools to coating design requires engineers with detailed understanding of coating materials and processes.

antireflection and reflection

The most widely applied optical coating is the antireflection (AR) coating, which is designed to reduce the amount of light reflected from the optical surface. Its secondary role is to enhance physical and chemical properties of the surface to which it is applied.


Figure 1. A three-layer antireflection coating on BK7 glass (n = 1.52) yields good performance over a 260 nm band. The design is BK7| M2HL |Air at 505 nm for a 0° incident angle (nH=2.126, nM=1.629, nL=1.384).

Uncoated glass typically has a surface reflection of between 4% and 8%. This can be reduced to about 1% at visible wavelengths by applying a single layer of QWOT low-index material, usually magnesium fluoride (MgF2). A three-layer design can reduce the reflection at visible wavelengths even further (see figure 1). The first layer consists of a QWOT medium-index material (e.g., aluminum oxide) next to the glass. The second layer is a HWOT high-index material (e.g., tantalum oxide). The third layer is a QWOT low-index material (e.g., MgF2) as a top layer, next to the air. This three-layer design falls in the category of the broadband antireflection coating, often denoted as BBAR coating.

If only one wavelength is considered, a two-layer design of high- and low-index materials will bring the reflection down to nearly zero. With the layer next to the glass fairly thin (high-index material) and the layer facing the air side (low-index material) somewhat greater than a QWOT, a relatively broad minimum can be obtained. These coatings are usually called V coatings.

A BBAR coating such as one that would apply to both the visible and the near-infrared (IR) spectral regions requires many layers of high- and low-index materials. Their thicknesses must be computer optimized and monitored throughout the deposition process using either a resonant quartz mask monitor or a combination of quartz and optical monitoring. A BBAR coating that covers 450 nm to 1100 nm would require eight or more layers to yield less than 1% reflectance at any wavelength within the region.

High-reflection coatings, such as the aluminum 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 SiO2. The 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 SiO2).

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 considerations 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 SiO2). The more layer pairs in the stack, the higher the reflectance. So-called "cold" mirrors (for visible and ultraviolet light) reflect shorter wavelengths and transmit longer wavelengths. "Hot" mirrors (for infrared light) transmit shorter wavelengths and reflect longer wavelengths.

filters

As with electronic circuits, optics requires many different types of interference filters. Sometimes the goal is to separate one portion of the spectrum from the other for a beam at normal incidence or oblique incidence. Whatever the case, the solution will be in the form of an edge filter or some kind of dichroic beam splitter.

When the application requires passing just one narrow bandwidth and reflecting a portion of the spectrum to either side, the best choice is a narrow band-pass interference filter, often called a Fabry-Perot filter. This filter became of paramount importance in the production of dense wavelength division multiplexing filters for telecommunication applications. To meet stringent requirements for environmental and spectral stability, these filters are manufactured using either ion beam sputtering (IBS) or plasma ion assisted deposition (PIAD) technologies.


Figure 2. The notch filter shows characteristic rejection band. The design is BK7| (L3M)31 4L |Air at 550 nm for a 0° incident angle (nM=1.627, nL=1.460).

Recently, another class of interference filters has become of great importance in laser and fiber-optic applications: notch filters, which reflect one or more narrow bands and transmit the wider regions around the rejection zone (see figure 2). To maintain a narrowband characteristic of the rejection zone, this filter is often designed using low- and medium-index materials. This requires many layers to achieve high reflection. Essentially, the function of a notch filter is just the opposite of the narrowband filters.

With the advent of new polarizing devices in the area of electronic imaging, polarizing beam splitters have become of significant importance. Their role is to maximize the reflection of s-polarized light and minimize the reflection of p-polarized light for an unpolarized (randomly polarized) incident beam. The degree of polarization (P) in transmission is given by

and in reflection by


Figure 3. A polychromatic polarizing cube beam splitter on BK7 glass rejects s-polarization while transmitting p-polarization. The computed reflectance represents a 15-layer design consisting of two materials of high and low refractive index. The incident angle is 52°.

The extinction ratio indicates how well the polarizing beam splitter discriminates between two planes of polarization. In transmission it is given as a ratio of Tp and Ts and in reflection as a ratio of Rs and Rp. When the degree of polarization of an incident beam is very high, the reflected s-polarized component and the transmitted p-polarized component should each account for a 50% of the incident light intensity. Thus, an ideal polarizing beam splitter acts as the 50/50 intensity beam splitter, where each of the two emerging light beams is 100% linearly polarized (see figure 3).

coating fabrication

Optical coatings are manufactured in high-vacuum coating chambers. Conventional processes such as thermal evaporation require elevated substrate temperatures, usually around 300°C. More advanced techniques, such as ion assisted deposition (IAD), IBS, and PIAD operate at near room temperatures. IAD processes not only produce coatings with better physical characteristics compared to conventional ones but also can be applied to plastic substrates.

Thermal evaporation involves either resistance-heated evaporation sources or electron-beam evaporation. The energies of the depositing atoms, typically around 0.1 eV, have the biggest effect on film properties. IAD results in direct deposition of ionized vapor and in adding activation energy to the growing film, typically in the order of 50 eV. Using the ion source, conventional electron-beam evaporation is improved by directing 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 uniform 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 subsequently filled with water when the film is exposed to humid atmosphere.

We define packing density as the ratio of the volume of the solid part of the film to the total volume of film (which includes microvoids and pores). For optical thin films, it is usually 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 monitored 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 evaporated 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 in displacement of air from microvoids and pores, causing an increase in the refractive index. Since the physical thickness of the film remains constant, this refractive index increase is accompanied by a corresponding increase in optical thickness, which results in the spectral shift of the coating characteristic toward a longer wavelength. To minimize this spectral shift caused by the size and overall population of microvoids throughout the growing film, manufacturers perform the coating process with high-energy ions that convey their momentum to the atoms of the depositing material, thereby largely increasing their mobility during the condensation at the substrate surface.

engineering tradeoffs

Production cost per run of a particular coating is primarily determined by the size of the coating chamber, the manufacturing technology, and the complexity of the coating. Since the usable area of the coating chamber is more or less directly proportional to the square of its radius, it follows that the bigger the chamber, the lower the price per coated lens. For example, if the diameter of one chamber is twice the diameter of the other, then approximately four times more lenses can be coated in the first chamber than in the second one.

For some extremely stringent requirements, often found in the production of narrowband and edge filters, it is not always possible to utilize the whole coating area within one chamber but rather one particular segment of it. This is because of the nonuniformity of the coating distribution across the chamber. Essentially, then, the capacity of the coating machine is governed by the tolerances on the spectral characteristics of the coating. For well-designed coating machines, the distribution of the spectral characteristic of evaporated material stays within ±1% of the nominal value.

In addition to the spectral conformity of the coated lens to the prescribed value, its quality is determined by the level of coating voids, the scattering properties, mechanical properties such as adhesion and hardness, environmental stability, and packing density. Complexity of the equipment needed to produce the coatings that would meet high-density requirement, in addition to low scatter and stress, inevitably leads to an increase in price of coated optics. oe

References

1. Jacobson, M. (1986) Deposition and Characterization of Optical Thin Films. New York: Macmillan.

2. Macleod, H.A. (1986) Thin-Film Optical Filters. New York: Macmillan.

3. Thelen, Alfred (1989) Design of Optical Interference Coatings. New York: McGraw-Hill.


putting on a top coat

When it comes to optical 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 techniques but also designs the equipment used to produce the coatings.

"Rarely do you find someone who just designs," says Galeb, originally from Sarajevo, Bosnia. "Architects 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 afford expensive new optical equipment, so he developed his own. For example, while at LaCroix Optical Co. (Batesville, AR), he designed, but never patented, a machine to cement uncentered lenses so they could then be edged as a single compound lens. "This way, you end up with a uniform product," Galeb says. "Conventional methods typically fused two or three individually edged lenses, which were never perfectly aligned."

Engineering keeps him busy. "The thin-film industry is gaining more prominence with the development of new technologies," says Galeb, who cites telecommunications as a key application area. "This market has started to have a huge impact on the evolution of technologies in thin films such as ion beam sputtering and plasma assisted deposition."

Despite the recent downturn in the industry, Galeb expects a strong, healthy market for years to come. "The customers demand better, more advanced coatings every day," he says. "There is a bright future for the optical thin-film industry."

If Galeb ever tires of optical engineering, you may hear one of his musical pieces. Not only does he play classical guitar and piano, but he also composes. Four years ago, Galeb recorded an orchestral arrangement titled "My Life in Paradise: Sarajevo." --Laurie Ann Toupin


Ranko Galeb

Ranko Galeb is the senior coating engineer at VLOC, New Port Richey, FL.