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Optical Coatings Introduction Precision Optics Optical Coatings Selection of Optical Components for Common Laser Types

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Optical Coatings Introduction Precision Optics Optical Coatings Selection of Optical Components for Common Laser Types FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 25 OPTICAL COATINGS INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES CONTENTS OPTICAL COATINGS OPTICAL INTERFERENCE COATINGS 26 METALLIC COATINGS 31 METAL-DIELECTRIC COATINGS 32 MEASUREMENT TOOLS FOR COATINGS 33 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES 26 OPTICAL INTERFERENCE COATINGS The purpose of optical coatings is to change the face. The transmitted beam (b) is again split into a For normal incidence (α = ß = 0°) the formulae can reflectance of optical surfaces. According to the reflected beam (d) and a transmitted beam (e). The be reduced to the simple term: materials used, metallic and dielectric coatings can reflected beams (c) and (d) can interfere. be distinguished. In fig. 1 the phase is represented by the shading of 2 the reflected beams. The distance from “light-to- n2 – n1 R = Metallic coatings are used for reflectors and neutral light” or “dark-to-dark” is the wavelength. Depend- n( 2 + n1 ) density filters. The achievable reflectance is given by ing on the phase difference between the reflected the properties of the metal. Common metals used beams, constructive or destructive interference may for optical applications are described on page 31. occur. The reflectance of the interface between the The phase difference between the beams (c) and (d) two media depends on the refractive indices of the is given by the optical thickness nt of the layer (the Dielectric coatings use optical interference to media, the angle of incidence and the polarization product of the refractive index n and the geometri- change the reflectance of the coated surfac - of the light. In general, it is described by the Fresnel cal thickness t). Furthermore, a phase jump of π, i.e. es. Another advantage is that the materials used equations. one half-wave, has to be taken into account, if light in these coatings show very low absorption. The 2 coming from a low index medium is reflected at the reflectance of optical surfaces can be varied from n1 cosα – n2 cosβ interface to a high index medium. Rs = approximately zero (antireflection coatings) to near- n( 1 cosα + n2 cosβ ) ly 100 % (low loss mirrors with R > 99.999 %) with Please refer to the literature cited on page 32 for a optical interference coatings. These reflectance val- 2 detailed explanation of the physics of optical inter- n2 cosα – n1 cosβ ues are achieved only for a certain wavelength or a Rp = ference coatings. Below are a few unwritten rules n( 2 cosα + n1 cosβ ) wavelength range. to help customers understand the optical properties of the coatings described in this catalog: BASICS Rs ... reflectance for s-polarization Rp … reflectance for p-polarization • High index layers increase the reflectance of the The influence of a single dielectric layer on the reflec- n1 … refractive index of medium 1 surface. The maximum reflectance for a given tance of a surface is schematically shown in fig. 1. n2 … refractive index of medium 2 wavelength λ is reached for nt = λ / 4. An incident beam (a) is split into a transmitted beam α … angle of incidence (AOI) Only in the case of an optical thickness nt = λ / 2, (b) and a reflected beam (c) at the air-layer inter- β … angle of refraction (AOR) the reflectance of the surface does not change for this wavelength λ. • Low index layers always decrease the reflectance (c) (d) (c) (d) of the surface. The minimum reflectance for a (a) (a) Incident Reflected Incident Reflected given wavelength λ is reached for nt = λ / 4. beam beams beam beams Only in the case of an optical thickness nt = λ / 2, the reflectance of the surface does not change for this wavelength λ. (b) High index layer (b) Low index layer Substrate Substrate (e) (e) Figure 1: Schematic drawing to explain the interference effect of quarter-wave layers of a high index material and a low index material (after [1]) FEMTOSECOND LASER OPTICS SELECTED SPECIAL COMPONENTS METALLIC COATINGS FOR LASER AND ASTRONOMICAL APPLICATIONS 27 ANTIREFLECTION COATINGS MIRRORS AND PARTIAL REFLECTORS • A single low index layer can be used as a simple • The most common mirror design is the so-called • To visualize the effect of different refractive AR coating. The most common material for this quarter-wave stack, i.e. a stack of alternating high index ratios, figure 3b compares the reflectance purpose is magnesium fluoride with a refractive and low index layers with equal optical thick- spectra of quarter-wave stacks consisting of index of n = 1.38 in the VIS and NIR. This material ness of nt = λ / 4 for the desired wavelength. 15 pairs of Ta2O5 / SiO2 and TiO2 / SiO2 for 800 nm reduces the reflectance per surface to R ~ 1.8 % This arrangement results in constructive inter- (n1 / n2 = 2.1 / 1.46 and 2.35 / 1.46, respectively). for fused silica and nearly zero for sapphire. ference of the reflected beams arising at each interface between the layers. The spectral width • The theoretical reflectance will approach • Single wavelength AR coatings consisting of of the reflection band and the maximum reflec- R = 100 % with an increasing number of layer 2 – 3 layers can be designed for all substrate mate- tance for a given number of layer pairs depend pairs, assuming that ideal coatings have zero rials to reduce the reflectance for the given wave- on the ratio of the refractive indices of the layer absorption and scattering losses. Partial reflectors length to nearly zero. These coatings are used materials. A large refractive index ratio results in with several discrete reflectance values between especially in laser physics. AR coatings for several a broad reflection band while a narrow reflection R = 0 % and R = 100 % can be manufactured wavelengths or for broad wavelength ranges are band can be produced using materials with a low using only a small number of layer pairs (see fig. also possible and consist of 4 – 10 layers. refractive index ratio. 4). Adding non-quarter-wave layers to a stack optimizes the reflectance to any desired value. a) a) Thickness [nanometers] 5 • Figure 4 also shows that an increasing number of R [%] 1500 1000 500 0 layer pairs results in steeper edges of the reflec- 4 tance band. This is especially important for edge 3 filters, i.e. mirrors with low reflectance side bands. Extremely steep edges require a large number of 2 layer pairs which also results in a very high reflec- Air Substrate 1 tance. Extremely high reflectance values require very low optical losses. This can be achieved by 0 350 400 450 500 550 650600 700 750 800 using sputtering techniques. wavelength [nm] b) b) TiO2 / SiO2 Ta2O5 / SiO2 15 pairs 10 pairs 5 pairs 3 pairs 2 pairs 1 pair 5 100 100 R [%] R [%] R [%] 90 90 4 80 80 70 70 3 60 60 50 50 2 40 40 30 30 1 20 20 10 10 0 0 0 350 400 450 500 550 650600 700 750 800 500 600 700 800 900 1000 1100 500 600 700 800 900 1000 1100 wavelength [nm] wavelength [nm] wavelength [nm] Figure 2: Schematic reflectance spectra: Figure 3: a) Schematic drawing of a quarter-wave stack consisting of Figure 4: Calculated reflectance of quarter-wave stacks consisting of a) Single wavelength AR coating ("V-coating") layers with equal optical thickness of a high index material 1, 2, 3, 5, 10 and 15 layer pairs of Ta2O5/ SiO2 for 800 nm b) Broadband AR coating (shaded) and a low index material (no shading) (after [1]) b) Reflectance spectra of quarter-wave stacks consisting of 15 pairs of Ta2O5 / SiO2 and TiO2 / SiO2 INTRODUCTION PRECISION OPTICS OPTICAL COATINGS SELECTION OF OPTICAL COMPONENTS FOR COMMON LASER TYPES 28 OPTICAL LOSSES • Light, which impinges on an optical component, eters in the order of λ, S ~ 1 / λ2) and Rayleigh • The amount of all kinds of losses depends on is either reflected, transmitted, absorbed or scat- theory (scattering by particles with diameters < λ, the thickness of the layer system. Each layer pair tered. From this basic point of view, the energy S~1/λ4). Depending on the surface and bulk increases the theoretical reflectance; however, in balance can be written in the simple equation structure, Mie and Rayleigh scattering occur simul- practice, it also increases the optical losses. There is R + T + A + S = 1 taneously. Scattering losses depend critically on an optimum number of layer pairs which generates with R … Reflectance, the microstructure of the coatings and as such the maximum reflectance, especially for evaporated T … Transmittance, on the coating technology used. Usually, coat- coatings with relatively large scattering losses. A … Absorption and ings produced by evaporation techniques show S … Scattering significantly higher scattering losses than coat- STRESS ings produced by magnetron sputtering or ion • In laser physics and precision optics absorption beam sputtering. The strong dependence of the • Another effect which limits the number of layers and scattering are summarized as optical losses scattering losses on the wavelength is the reason is the stress in the coating. This stress results from because the absorbed and scattered part of the why scattering losses are a huge problem in the the structure of the layers but also from differ- incoming light can no longer be used as a carrier UV range while they are less important in the NIR ent thermal expansion coefficients of substrate of information or as an optical tool. In practice, and beyond. and coating. Mechanical stress may deform the the reflectance which can be achieved depends substrate but it may also result in cracks in the on the absorption and scattering losses of the • Absorption in optical coatings and substrates is coating or in a reduced adherence of the coating.
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