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METALLIC REFLECTIVE COATINGS INTRODUCTION
Most of the first surface reflectors for nonlaser systems are coated with special metallic films in order to produce first surface reflectors. These metals are selected by their performance in the spectral region being used, by their ability to withstand atmospheric corrosion, and their durability while being cleaned. The metals most often used are aluminum, silver, and gold with various protective coatings applied over the metal to enhance the reflectivity or to increase durability. |
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| ALUMINUM MIRRORS/GMP01-00-550
Aluminum is the most commonly used metal for mirrors because it maintains its high reflectance over the spectral region from 2,000Å to 10µ. It is exceeded in reflectivity by silver in the visible band, however, it is unsurpassed as a reflector for UV through near visible applications. This coating uses a quartz overcoat to protect the pure aluminum film from the slow oxidation due to water vapor in the air.
UV Aluminum/GMP 02-00-300
The best reflectors for UV radiation consist of aluminum with an overcoat of magnesium fluoride (MgF2). This dielectric layer provides oxidation and abrasion protection for the aluminum and produces reflectivity of 87% from 2,000Å to 4000Å.
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Deep UV Aluminum/GMP 03-00-200
In order to produce first surface reflectors with high reflectivity below 2,000Å, we make a special aluminum film which is produced by very fast flash evaporation at ultra-high vacuum. This special film is then coated with a very thin layer of MgF2 to yield the very high reflectivity in the deep UV. |
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Enhanced Aluminum/GMP 06-00-550
Aluminum’s reflectivity can be increased over a limited portion of the spectrum by application of a multilayer dielectric stack. This particular coating yields a reflectivity of 96% over the region from 4,500Å to 7,200Å. However, as can be seen from the reflectivity chart, the enhancement effect is paid for by significantly lower reflectivity outside the region of enhancement. Even so, this coating will produce the highest reflectivity within its spectral region. |
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Gold Mirrors/GMP 10-00-150
Gold is the preferred metal for mirrors in the region of 2µ through 11M in the infrared. Gold is very resistant to tarnish and oxidation, but is extremely soft and must be handled with the utmost care. Also, gold films do not adhere well to glass substrates so a chrome seed layer is deposited to ensure good adhesion. We do not particularly recommend that this film be used except in internal reflectors in infrared systems because it is extremely difficult to clean. If for specific reasons of high reflectivity it must be used, then special handling and packaging techniques must be employed. |
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Protected Gold/GMP 11-00-150
This is the recommended coating for the infrared region from 2µ to 11µ. An infrared transmitting dielectric overcoat is put on to the gold in order to yield reflectivity greater than 98% from 1.5µ to 11µ. Although this particular reflectivity is slightly less than pure gold, the great advantage of increased durability allows it to be cleaned by careful optical methods. |
Silver Mirrors/GMP 14-00-500
Silver is indeed more reflective than aluminum in the visible and the near infrared. However, it has a great tendency to tarnish and oxidize even when coated with a protected dielectric film. Therefore, we used silver on internal reflecting surfaces where it is overcoated with quartz and then further treated to preserve its very high initial reflectance. |
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LASER MIRRORS/LMZ01, LMZ10, LMZ20 INTRODUCTION
Mirrors for laser systems cannot accept even the smallest absorption found with the use of metal films. Consequently, the laser mirrors consist of low loss multilayer dielectric stacks producing extremely high reflectance typically 99% or greater at the desired laser wavelength. Specifically, these laser mirrors are divided into three categories by spectral region because the dielectric materials used in making them are different for three regions of the spectrum. The region from 2,600Å to 4,000Å is covered by coating LMZ01, the region from 4,000Å to 7,000Å is covered by coating LMZ10, and the region from 7,000Å to 2.5µ is covered by the coating LMZ20.
Accordingly, the specification for each coating is designated by the number, either LMZ01, LMZ10, or LMZ20, followed by the angle of incidence and then followed by the laser wavelength is microns (µ).
For example, a laser mirror for 45° incidence at 1.06µ would be specified as LMZ20-45-1.06.
Generally, mirrors will show a difference in reflectivity for p and s polarization components. Since the p component is always lower in reflectivity than the s component, we always specify the p component reflectivity. Also, the angle of incidence can be specified from 0° to 70° and typical optical performance of a filter for each of the spectral regions can be found in the next three sections.
Typical Performance of Laser Mirrors in LMZ Series
This shows the reflectance of LMZ20-45-1.06 for both p and s components of polarization. Each coating in this series has a reflectance of 99.7% with very low scatter and absorption.
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Antireflection Coatings
All antireflection coatings at Republic Lens Co., Inc. are made of hard dielectric layers applied in custom coating machines at 250°C. We offer these coatings over the spectral range of 3000Å to 2.5µ and at angles of incidence of up to 60°.
These antireflection coatings are specified by the coating number followed by the angle of incidence, then followed by the wavelength in µ. For example, a two layer “V” coat at 0° incidence at 4880Å is specified as.
and its performance will be shown in the ARQ 02 examples.
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ARQ 01-00-550 Single Layer Magnesium Fluoride
This coating consists of a single layer of hard baked magnesium fluoride for use in general optical systems. It provides a reflectivity of 1.3% per surface at 0° incidence for substrates with an index of refraction of 1.52. |
The above shows the effect of angle in coating ARQ 01-45-550. The reflectivity is now 2% per surface for index of refraction of 1.52.
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| ARZ 02-45-633 Laser “V” Coat
The above reflectivity chart shows that the two laser “V’ coat at 45° incidence now has a reflectivity of 0.4% per surface rather than a reflectivity of 0.3%.
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| ARQ 03-00-550 Broadband Visible Antireflective Coating
This is a multilayer broadband antireflection coating for low index materials like quartz and glass. It will produce a reflectance per surface of less than 0.5% from 4500Å to 7000Å on substrate index of 1.52.
Angles Available 00 to 25°
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| ARQ 04-00-500 Broadband Laser “V” Coat
This special development by Republic Lens Co., Inc., is a broadband coating with the performance of a laser “V” coat simultaneously at 4880 Å, 5145Å, and 6328Å. It, therefore, covers all the popular laser lines at a reflectance per surface of less than 0.3% per surface. This coating on glass or quart then becomes a laser broadband window.
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| ARQ 02-00-663 Laser “V” Coat
This is the classical two laser “V” coat so designated because the shape of the reflectivity curve resembles a letter V. It is used in all high power laser systems and is made with a glassy dielectric material capable of handling large laser power densities. The coating yields a reflectivity of less than 0.3% per surface at 0° incidence at the laser wavelength specified.
Angles Available 00 to 60°
Wavelengths 0.300 to 1.200µ
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ARQ 02-00-1.06 Laser “V” Coat
The above reflectivity chart shows that a reflectance of less than 0.1% per surface at 1.06µ at normal incidence on crown glass at nd = 1.52.
*All above charts are indicative of typical performances. Guaranteed performances are given in the specification of catalog items.
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