Specifying an Optical Coating:

Optical coatings generally fall into three main spectral regions:

• The ultraviolet (UV)
• The visible and near-infrared (VIS-NIR) and
• The Infrared (IR).

The coating materials used differ depending on the wavelength region of operation, though some specialized coatings are multi-spectral. For ease of specifying an optical coating, the following form can be completed and sent to the ELCAN Thin-film Design Engineer, by fax at (705) 526-5831 or Email ELCAN.

The purpose of the intended optic:

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The type of coating proposed:

Anti-Reflective Coatings:

Single-layer anti-reflection (SLAR): This is a single-layer coating and is usually specified for one wavelength, one angle of incidence and one set of reflection/transmission requirements.

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Broadband anti-reflection (BBAR): This is a multi-layer anti-reflection coating that is designed to operate over a wavelength range e.g. 400 – 700 nm, at designated angles of incidence and with specified reflection/transmission characteristics.

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Extended-band anti-reflection: This is a multi-layer anti-reflection coating that is designed to operate over a larger wavelength than BBAR, for example 400 – 1100 nm, and the reflection/transmission requirements are not usually as stringent as the broadband coating.

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Multi-band anti-reflection: As suggested, these are specified over two or more distinct wavelength ranges, e.g. 400 – 700 nm, 1060 – 1070 nm and 1530 – 1150nm.

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V-Coat anti-reflection: This type of coating is used at a specified wavelength with a very low reflection (R) requirement – usually R < 0.2%.

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Wavelength Filters:

Short-wave Pass (SWP): This type of coating is specifically designed to transmit short wavelengths and to reflect longer wavelengths. A sample specification could be a transmission of T>90% absolute at 400 – 520 nm, T=50% at 530±5 nm, T<0.1% at 550 – 690 nm; angle of incidence 0±5°, random polarization. Transmission is maximized if an anti-reflection coating is applied to the reverse face.

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Long-wave Pass (LWP): This coating type is designed to transmit longer wavelengths and reflect short wavelengths. A sample specification could be T>90% at 540 – 700 nm, T=50% at 530±5 nm, T<0.1% at 410 – 510 nm; angle of incidence 0±5°, random polarization. Transmission is maximized if anti-reflection coating is applied to the reverse face.

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Band Pass (BP): These allow the transmission of light within a carefully defined wavelength range, while light outside this range is blocked. Available with narrow or wide wavelength windows of transmission, these filters are usually defined through their center wavelength S, full-width half-maximum (FWHM), peak transmittance and rejection bands. More than one pass band can be specified. For all the parameters tolerances are required.

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Notch: A notch filter is used to reflect a specified wavelength, or a narrow wavelength region with high transmission outside that region. A typical performance might be T=45±2% at 550±3 nm, T>95% at 420 – 520 nm and T>95% at 580 – 770 nm for a specific notch-depth coating. Sometimes notches are specified with a wavelength window, e.g. width<20 nm at 10% points and %<0.0001% at 560 nm. An alternative specification would be optical density OD>5, T>95% at 420 - 540 nm and T>95% at 580 - 770 nm. More than one rejection band can be specified.

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Reflective Filters:

Three types are available:

• Commonly High Reflectors
• Metallic Reflectors and
• Enhanced Metallic Reflectors.

These are all specified with various levels of reflection, angles of incidence, polarization and wavelength range. For a high reflectance over a restricted wavelength range a multi-layer would be best. In contrast, for a wide wavelength range and a lower reflection requirement, the enhanced metallic reflector is suitable and for a very wide wavelength region, potentially from the visible into the far-infrared, a metallic reflector is probably best.

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Beamsplitter Coatings:

Beamsplitters split an incoming light beam into two distinct output beams and often make use of specialized coatings. When purchasing, the transmission and reflection characteristics, X wavelength range(s), angles of incidence and polarization must be specified, e.g. T=70±3%, R=30±3% at 420 – 700 nm, angle of incidence 45±5º, random polarization.

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Diamond-Like Carbon Coatings:

These coatings provide exceptional durability for systems working in the infrared at the cost of a small drop in optical performance.

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High Damage Threshold Coatings:

For use with various lasers, at powers that will destroy an ordinary coating, e.g. >5 Jouls/cm² at 1542 nm in a 20 ns pulse.

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Transparent Conductive Coatings:

These coatings are highly transparent in the visible and near IR, but also electrically conductive. They can be used for heating to eliminate fog or ice. They also protect the interior against electromagnetic interference. In addition to optical performance, electrical resistivity must be specified e.g. 20 ohms per square.

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The wavelength range(s) involved:

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The transmission and reflection requirements (with tolerances) of the filter:

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The angles of incidence, e.g. 45±5%:

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The polarization state or states of the incident light:

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The type of substrate material:

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The dimensions of the part:

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The clear aperture:

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The cosmetic quality required, i.e. the scratch/dig requirements:

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Which side(s) of the optic can be coated:

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The laser damage threshold(s), if required:

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The optic’s environmental requirements:

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Any military (MIL) specifications or similar restraints:

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The quantity required:

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Coatings can be made to meet the most stringent requirements from a technical point of view, but remember that the price depends significantly on the tightness of the specification. So always ask yourself, “Do I really need ±1 nm tolerance, or will ±5 or ±10 nm suffice?” The difference in price is remarkable.

Remember too that your ELCAN Program Manager is always willing to discuss your requirements and assist you in selecting a specification that optimizes both your performance and cost targets.

For more information: Visit www.elcan.com, Email us at sales@elcan.com or Request Your Custom Solution