Optical Coatings

More documents

Recommendations

Info

Fundamental Optics Material Properties Optical Specifications Gaussian Beam Optics Optical Coatings High-Reflection Coatings Melles Griot offers a wide variety of high-reflection coatings for mirrors, beamsplitters, polarizing beamsplitters, dichroic mirrors, bandpass filters, and rejection filters. Some of these coatings are applied to optics as requested; others are offered only as an integral part of specialized optical elements. High-reflection coatings are ordered in the same way as antireflection coatings, namely by appending the three-digit coating suffix to the catalog number of the part being ordered. High-reflection coatings may be applied to the outside of a component, such as a flat piece of glass, to produce a first-surface mirror. Alternately, they may be applied to an internal surface to produce a second-surface mirror, such as a prism. High-reflection coatings can be categorized as either metallic or dielectric coatings. METALLIC COATINGS Metallic coatings are used primarily for mirrors and are not classified as thin films in the strictest sense. They do not rely on principles of interference, but rather on the optical properties of the coating material. However, metallic coatings are often overcoated with thin dielectric films to increase reflectance over a desired range of wavelengths or angles of incidence. In these cases, the metallic coating is said to be “enhanced.” Overcoating metallic coatings with a hard, single, dielectric layer of half-wave optical thickness improves abrasion and tarnish resistance but only marginally affects optical properties. Depending on the dielectric used, such overcoated metals are referred to as durable, protected, or hard coated. The main advantages of metallic coatings are broadband spectral performance, insensitivity to angle of incidence and polarization, and low cost. Their primary disadvantages are lower durability, lower reflectance, and lower damage threshold. DIELECTRIC COATINGS High-reflectance dielectric layers work on the same principles as dielectric antireflection coatings. Quarter-wave thicknesses of alternately high- and low-refractive index materials are applied to the substrate to form a dielectric multilayer as shown in figure 5.31. By choosing materials of appropriate refractive indices, the various reflected wavefronts can be made to interfere constructively in order to produce a highly efficient reflector. The peak reflectance value is dependent upon the ratio of refractive indices of the two materials, as well as the number of layer pairs. Increasing either increases the reflectance. The width of the reflectance curve (versus wavelength) is also determined by the film index ratio. The larger the ratio, the wider the high-reflectance region. In contrast to antireflection coatings, the inherent shape of a high-reflection coating can be modified in several different ways. The two most effective ways of modifying the performance curve are to use two or more stacks centered at slightly shifted design wavelengths, or to slightly perturb the layer thickness within a stack. Reflectance of such films can easily be made to exceed the highest metallic reflectances over limited wavelength intervals. Such films are effective for both s- and p-polarization components and over a wide angle-of-incidence range. At oblique incidence, reflectance is markedly reduced. Because of the materials chosen for the multilayer, durability and abrasion resistance of such films are normally superior to those of metallic films. air substrate Figure 5.31 quarter-wave thickness of high-index material quarter-wave thickness of low-index material A simple quarter-wave stack 5.24 1 Visit Us OnLine! www.mellesgriot.com
Metallic High-Reflection Coatings We offer eight forms of standard metallic high-reflection coatings formed by vacuum deposition. These coatings, which can be used at any angle of incidence, can be applied to most optical components. Simply append the coating suffix number to the component product number (see figures 5.32 through 5.39). Metallic reflective coatings are delicate and require care during cleaning. Dielectric overcoats substantially improve abrasion resistance, but they are not impervious to abrasive cleaning techniques. Clean, dry pressurized gas can be used to blow off loose particles, then clean, deionized water, a mild detergent, and alcohol can be used. Gentle cleaning with a swab is recommended. ALUMINUM (/016) $ The most widely used metallic mirror coating $ Provides consistently high reflectance throughout the near-ultraviolet, visible, and near-infrared regions $ R avg > 90% from 400 to 1200 nm $ Damage threshold: 0.2 J/cm 2 810%, 10-nsec pulse (12 MW/cm 2 ) at 532 nm; 0.3 J/cm 2 810%, 20-nsec pulse (14 MW/cm 2 ) at 1064 nm Aluminum, the most widely used metal for reflecting films, offers consistently high reflectance throughout the visible, near-infrared, and near-ultraviolet regions of the spectrum. While silver exhibits slightly higher reflectance than aluminum through most of the visible spectrum, the advantage is temporary because of oxidation tarnishing. Aluminum also oxidizes, though more slowly, and its oxide is tough and corrosion resistant. Oxidation significantly reduces aluminum reflectance in the ultraviolet and causes slight scattering throughout the spectrum. PERCENT REFLECTANCE 100 95 90 85 80 normal incidence 400 600 800 1000 WAVELENGTH IN NANOMETERS Figure 5.32 Aluminum coating /016 typical reflectance curve PROTECTED ALUMINUM (/011) $ The best general-purpose metallic reflector for visible to near-infrared $ Protective overcoat extends life of mirror and protects surface $ R avg > 87% from 400 to 800 nm $ Damage threshold: 0.3 J/cm 2 810%, 10-nsec pulse (21 MW/cm 2 ) at 532 nm; 0.5 J/cm 2 810%, 20-nsec pulse (22 MW/cm 2 ) at 1064 nm Protected aluminum is the very best general-purpose, metallic coating for use as an external reflector in the visible and nearinfrared spectra. Unless we specify otherwise or you specifically request a different coating, our mirrors are coated with protected aluminum. Protected aluminum is coated with a dielectric film of disilicon trioxide (Si 2 O 3 ) of half-wavelength optical thickness at 550 nm. The protective film arrests oxidation and helps maintain high reflectance. It is durable enough to protect the aluminum coating from minor abrasions. PERCENT REFLECTANCE 100 95 90 85 normal incidence 80 45° incidence s-plane p-plane 400 450 500 550 600 650 700 750 WAVELENGTH IN NANOMETERS Figure 5.33 Protected aluminum coating /011 typical reflectance curves Fundamental Optics Gaussian Beam Optics Optical Specifications Material Properties Optical Coatings Visit Us Online! www.mellesgriot.com 1 5.25
Page 1 and 2:
Fundamental Optics 1 Introduction 1
Page 3 and 4:
Paraxial Formulas SIGN CONVENTIONS
Page 5 and 6:
object F 2 Figure 1.4 image Example
Page 7 and 8:
Figure 1.6 Figure 1.7 f f 2 object
Page 9 and 10:
Figure 1.8 Figure 1.9 INDIVIDUAL EL
Page 11 and 12:
Performance Factors After paraxial
Page 13 and 14:
third-order, monochromatic, spheric
Page 15 and 16:
Positive lens elements usually have
Page 17 and 18:
Lens Shape Aberrations described in
Page 19 and 20:
ay f-numbers 2.7 3.3 4.4 6.7 13.3 S
Page 21 and 22:
AIRY DISC DIAMETER = 2.44 l f/# Fig
Page 23 and 24:
Lens Selection Having discussed the
Page 25 and 26:
Example 4: Diffraction-Limited Perf
Page 27 and 28:
Aberration Balancing To improve sys
Page 29 and 30:
Definition of Terms FOCAL LENGTH (f
Page 31 and 32:
infinity (which is a comfortable vi
Page 33 and 34:
For symmetric lenses (r 2 = 4r 1 ),
Page 35 and 36:
Separation of Nodal Point from Corr
Page 37 and 38:
Gaussian Beam Optics 2 Introduction
Page 39 and 40:
Even if a Gaussian TEM 00 laser-bea
Page 41 and 42:
eam expander INCORPORATING M 2 INTO
Page 43 and 44:
M 2 AND THE LENS EQUATION For real-
Page 45 and 46:
employed when laser power must be c
Page 47 and 48: Figure 2.13 Keplerian beam expander
Page 49 and 50: Optical Specifications 3 Wavefront
Page 51 and 52: Centration The mechanical axis and
Page 53 and 54: PERFECT RECTANGULAR LENS The MDMTF
Page 55 and 56: The permissible number of maximum-s
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: V-Coatings V-coatings are multilaye
Page 101 and 102: INTERNAL SILVER (/036) $ For intern
Page 103 and 104: Dielectric High-Reflection Coatings
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
show all

Optical Coatings

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?