Optical Coatings

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Fundamental Optics BARE GOLD (/045) PROTECTED GOLD (/055) $ Widely used in the near, middle, and far infrared $ Effectively controls thermal radiation $ R avg > 99% from 700 nm to 20 mm $ Damage threshold: 1.1 J/cm 2 810% , 20-nsec pulse (48 MW/cm 2 ) at 1064 nm $ The performance of the durability of dielectrics $ Protective overcoat extends coating life $ R avg > 98% from 650 nm to 16 mm $ Damage threshold: 0.4 J/cm 2 810%, 20-nsec pulse (17 MW/cm 2 ) at 1064 nm Material Properties Optical Specifications Gaussian Beam Optics Optical Coatings Because it combines good tarnish resistance with consistently high reflectance throughout the near, middle, and far infrared, gold is widely used in these regions. While it is possible to construct multilayer films that may surpass the reflectance of gold at specific wavelengths, the useful range of gold is unequaled. Gold is especially effective in controlling thermal radiation. Because bare gold is soft and scratches easily, bare-gold mirrors should be cleaned only by flow-washing with solvents and clean water or by blowing the surface clean with a low-pressure stream of dry air. PERCENT REFLECTANCE 100 80 60 40 20 0 400 800 1200 1600 2000 2400 2800 WAVELENGTH IN NANOMETERS The Melles Griot proprietary protected gold mirror coating combines the natural spectral performance of gold with the durability of hard dielectrics. Protected gold provides over 96% average reflectance from 650 to 1700 nm, and over 98% average reflectance from 2 to 16 mm. As well as the damage threshold listed above, the /055 coating was tested for laser-induced damage and was found to withstand up to 18±2 J/cm 2 with a 260-µs pulse (0.4 MW/cm 2 ) at a wavelength of 3 µm. These mirrors can be cleaned regularly using standard organic solvents, such as alcohol or acetone. PERCENT REFLECTANCE 100 80 60 40 20 0 normal incidence typical reflectance curve 700 800 900 1000 1100 1200 1300 1400 1500 1600 WAVELENGTH IN NANOMETERS Figure 5.38 Bare gold coating /045 Figure 5.39 Protected gold coating /055 Coating Type Aluminum Protected Aluminum Enhanced Aluminum UV-Enhanced Aluminum Internal Silver Protected Silver Bare Gold Protected Gold normal incidence Metallic High-Reflection Coatings To order, append coating suffix to product number. Wavelength Range (nm) 400–1200 400–800 450–750 250–400 400–1200 400–20,000 700–20,000 650–16,000 typical reflectance curve Average Reflectance (%) COATING SUFFIX 90 87 93 86 98 95 99 98 /016 /011 /023 /028 /036 /038 /045 /055 5.28 1 Visit Us OnLine! www.mellesgriot.com
Dielectric High-Reflection Coatings QUARTER-WAVE STACK The basic building block for any coating involving high levels of reflection is the quarter-wave stack— a stack of alternate layers of high- and low-refractive-index material. Each layer in the stack ideally has an optical thickness of a quarter wave at the design wavelength. Alternate reflections are phase shifted by 180 degrees because they occur at low- to high-index interfaces (external reflections). These phase shifts are exactly canceled by the 180-degree phase shifts caused by the path difference between alternate reflecting surfaces. All reflected wavefronts are therefore exactly in phase and undergo only constructive interference. If the difference in the refractive index of the materials is large, then a quarter-wave stack containing only a few layers will have a very high reflectance. PERFORMANCE CURVE The reflection versus wavelength performance curve of a single dielectric stack has a characteristic flat top inverted V shape as shown in figure 5.40. Clearly, reflectance is a maximum at the wavelength for which both the high- and low-index layers of the multilayer are exactly one-quarter-wave thick. Outside the fairly narrow region of high reflectance, the reflectance slowly reduces toward zero in an oscillatory fashion. Width and height (i.e., peak reflectance) of the high-reflectance region are functions of the refractive-index ratio of the two materials used, together with the number of layers actually included in the stack. The peak reflectance can be increased by adding more layers, or by using materials with a higher refractive index ratio. Amplitude reflectivity at a single interface is given by (14p) (1+ p) where ⎛ p = n ⎜ ⎝ n H L n S is the index of the substrate, and n H and n L are the indices of the highand low-index layers. N is the total number of layers in the stack. The width of the high-reflectance part of the curve (versus wavelength) is also determined by the film index ratio. The higher the ratio, the wider is the high-reflectance region. SCATTERING N41 2 ⎞ nH ⎟ × (5.26) ⎠ n , S The main parameters used to describe the performance of a thin film are reflectance and transmittance (plus absorptance, where applicable). Another less well-defined parameter is scattering. This is hard to define because of the inherent granular properties of the materials used in the films. Granularity causes some of the incident light to be “lost” by specular reflection. Often, it is scattering, not mechanical stress and weakness in the coating, that limits the maximum practical thickness of an optical coating. PERCENT REFLECTANCE 100 80 60 40 20 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 BROADBAND COATINGS RELATIVE WAVELENGTH Figure 5.40 Typical reflectance curve of an unmodified quarter-wave stack In contrast to antireflection coatings, the inherent shape of a high-reflectance coating can be modified in several different ways. The two most effective ways of modifying a performance curve are to use two or more stacks centered at slightly shifted design wavelengths, or to perturb the layer thicknesses within a stack. There is a subtle difference between multilayer antireflection coatings and multilayer high-reflection coatings, which allows the performance curves of the latter to be modified by using layer thicknesses designed for different wavelengths within a single coating. Consider a multilayer consisting of pairs, or stacks of layers, which are designed for different wavelengths. At any given wavelength, providing at least one of the layers is highly reflective for that wavelength, the overall coating will be highly reflective at that wavelength. Whether the other components transmit or are partially reflective at that wavelength is immaterial. Transmission of light of that wavelength will be blocked by reflection of a single component. On the other hand, in an antireflection coating, even if one of the stacks is exactly antireflective at a certain wavelength, the overall coating may still be quite reflective because of reflections by the other components (see figure 5.41). This can be summarized by an empirical rule. At any wavelength, the reflection of a multilayer coating consisting of several discrete components will be at least that of the most reflective component. Exceptions to this rule are coatings that have been designed to produce interference effects not just involving the surfaces within the two-layer or multilayer component stack, but also between the stacks themselves. Obvious examples are narrowband interference filters which are described in detail later and in Chapter 13, Filters. Fundamental Optics Gaussian Beam Optics Optical Specifications Material Properties Optical Coatings Visit Us Online! www.mellesgriot.com 1 5.29
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Fundamental Optics 1 Introduction 1
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Paraxial Formulas SIGN CONVENTIONS
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object F 2 Figure 1.4 image Example
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Figure 1.6 Figure 1.7 f f 2 object
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Figure 1.8 Figure 1.9 INDIVIDUAL EL
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Performance Factors After paraxial
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third-order, monochromatic, spheric
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Positive lens elements usually have
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Lens Shape Aberrations described in
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ay f-numbers 2.7 3.3 4.4 6.7 13.3 S
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AIRY DISC DIAMETER = 2.44 l f/# Fig
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Lens Selection Having discussed the
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Example 4: Diffraction-Limited Perf
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Aberration Balancing To improve sys
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Definition of Terms FOCAL LENGTH (f
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infinity (which is a comfortable vi
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For symmetric lenses (r 2 = 4r 1 ),
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Separation of Nodal Point from Corr
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Gaussian Beam Optics 2 Introduction
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Even if a Gaussian TEM 00 laser-bea
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eam expander INCORPORATING M 2 INTO
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M 2 AND THE LENS EQUATION For real-
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employed when laser power must be c
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Figure 2.13 Keplerian beam expander
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Optical Specifications 3 Wavefront
Page 51 and 52: Centration The mechanical axis and
Page 53 and 54: PERFECT RECTANGULAR LENS The MDMTF
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Page 57 and 58: Material Properties 4 Material Prop
Page 59 and 60: Introduction Glass manufacturers pr
Page 61 and 62: maximum value for homogeneity withi
Page 63 and 64: Melles Griot Lens Materials Melles
Page 65 and 66: Refractive Index of Five Schott Gla
Page 67 and 68: Synthetic Fused Silica Fused silica
Page 69 and 70: PERCENT INTERNAL TRANSMITTANCE 99.9
Page 71 and 72: Low-Expansion Borosilicate Glass Th
Page 73 and 74: ZERODUR ® Many optical application
Page 75 and 76: Optical Coatings 5 Optical Coatings
Page 77 and 78: OEM and Special Coatings Melles Gri
Page 79 and 80: PERCENT REFLECTANCE 100 90 80 70 60
Page 81 and 82: will depend on the ratio of optical
Page 83 and 84: v = angle of incidence PERCENT REFL
Page 85 and 86: PERCENT REFLECTANCE PER SURFACE 2.0
Page 87 and 88: Two-Layer Coatings of Arbitrary Thi
Page 89 and 90: ION-ASSISTED BOMBARDMENT Ion-assist
Page 91 and 92: Single-Layer MgF 2 Antireflection C
Page 93 and 94: PERCENT REFLECTANCE 5 4 3 2 1 norma
Page 95 and 96: EXTENDED-RANGE HEBBAR ANTIREFLECTIO
Page 97 and 98: V-Coatings V-coatings are multilaye
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Page 101: INTERNAL SILVER (/036) $ For intern
Page 105 and 106: PERCENT REFLECTANCE 100 80 60 40 20
Page 107 and 108: MAXBRIte Coatings MAXBRIte (multi
Page 109 and 110: Laser-Line MAX-R Coatings These mu
Page 111 and 112: Ultrafast Coating Melles Griot has
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Optical Coatings

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