Optical mirror : a complete guide

&#;Magic mirror on the wall &#; who is the fairest one of all?&#;.

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While this guide won&#;t provide the answer to the Queen&#;s question in Disney&#;s classic Snow White and the Seven Dwarfs, it will give you a glimpse of the optical mirrors available on the market and help you select the best one to suit your needs.

Optical mirror definition

An optical mirror is a precision optics component made of a optical polished substrate material and a high reflection coating. It can be of various shape and dimensions from less than a millimeter to several meters.

Optical mirrors are found in all the Photonics applications : in astronomy inside every telescope, in lasers by design, in vision equipment, and most of the optical assemblies systems. While their usage is quite simple : reflect the light, their range of application is wide, from UV to IR it can be used to transmit light with a change of direction, to focus light (for example in a Cassegrain telescope) or pumping light in a laser.

Types of optical mirrors

Shapes

Under the name &#;mirror&#; are many different types of precision optics that may be sorted as one of below type :

• Flat mirror, it is the simplest form similar to bathroon mirrors with higher precision.
• Spherical mirror, relates to mirrors with a radius of curvature either concave or convex.
• Aspherical mirror, more complex shape mirrors designed to attenuate optical aberration of reflected beam. For example : off-axis parabolic mirrors which are used to collimate a bean à 90° angle.
• Elliptical mirrors, are usually used at a 45° angle where they provide optimum circular clear aperture.
• Concave mirrors, the concave shape increase the reflection that can go more than 99% with an angle of incidence (AOI) of 0°.
• Freeform mirrors
• Prism mirrors, can be used either with outside reflection or inside reflection.

 Substrate material

Mirrors substrate material are usually selected for their good shape resistance to temperatures changes, easiness of coating to bound with it and easiness of polishing to guaranty a good polishing shape. For some high power laser applications, substrate can also be the in LIDT constraint and therefore be selected carefully.

Below table shows most common substrate material for mirror blanks :

Substrate material Remarks Float Very cheap material that comes with acceptable polishing and flatness level for low requirement applications. Fused silica Resistant material with good optical machining properties, very commonly used. N-BK7 / H-K9L Most common optical glass, can be used for high requirement surface aspects. Zerodur Material almost non affected by low temperature, used mainly for space applications. Metals Polished aluminium or brass can be used for some high power laser applications, they are also easier to machined than glass type material and mechanicaly stronger. Plastics Plastic substrates are used in optical applications, but the difficulty to keep a good surface quality and the easy wearing of the parts due to outside environment limit its usage, while this remain the cheapest substrate material.

Remark : for some applications where mirrors maybe of important dimensions and the part weight is an important constraint like in space industry for example, a lightening of the part can be done with a structuring of the back of the substrate.

Mirror coating

The most important part of an optical mirror is its high reflection coating, it is that coating that will be at the interface of incident beam and reflect it to an orthogonal direction.

Broadband dielectric coatings

With the improvement of vacuum deposition equipment in the last 30 years, layer of dielectric material can be deposit of substrates with a nanometric precision.

The material and the thickness used for each layer enable the trimming of  a broadband of wavelengths where the incident beam will be reflected.

It has to be noted that the reflection of such mirror degrades as the AOI increases.

Metal-coated mirrors

Coating metal RefLection Indicative damage threshold Remarks Protected* silver Ravg>97.5% on the visible + NIR, Ravg>96% on MIR+FIR 3J/cm2 Very good reflection in the visible Protected* Aluminium Ravg>90% on the visible + NIR, Ravg>95% on MIR+FIR 0,3J/cm2 A MgF2 layer can be added to improve reflection in the UV (250 to 400nm) Protected* Gold Ravg>96% on the IR 2J/cm2 Without surprise the most expensive of the metal coatings.

*Metal coatings are protected with SiO2 layers in order to avoid corrosion.

**An additional Chromium layer is usually added between the substrate and the metal to improve adhesion.

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Laser mirrors

The critical specification for optical mirrors to be used in laser applications is their LIDT (Laser Induced Damage Threshold, or in short Damage Threshold).

This specification often measure in J/cm2 defines the energy that can be accepted by a surface area.  (for reminder Watts are defining a power so values in Watts should be integrated in the time to be translated to energy).

LIDT should be defined with detail of the type of reference laser : CW or pulse laser, pulse duration, frequency and power.

Usually the coating is the contraint before the substrate, but for high power laser substrate can be wear by laser usage too. The lower the polishing  and the higher of impurities in the material will bring to lower LIDT of the substrates.

Superpolished mirrors

Superpolised mirrors refers to mirrors with very high surface quality (superpolishing) with RMS of less than one Angström and low loss IBS dielectric coatings. They are often used for laser applications.

Deformable mirrors

Deformable mirrors are active optics elements used in adaptative optics. They are typically made of an optical membrane, actuators and an electronic controller.

Their main usage is compensating aberrations, from example of the atmosphère in astronomy, of the eye in ophthalmology or of the cells water in biomicroscopy.

Main suppliers of deformable mirrors are ALPAO and ISP-System for high power applications.

Hot mirrors and cold mirrors

What are Hot mirrors and cold mirrors ? It is rather intuitive (while sometimes confusing) :

• Hot mirrors are reflecting the IR part of an incident beam and transmit the visible part of it.
• Cold mirrors are transmitting the IR part of an incident beam and reflect the visible part of it.

In a nutshell : Hot mirrors reflect warmth, cold mirrors reflect cold signal.

They are used to separate visible to IR parts of an optical signal.

Where to buy optical mirrors ?

Like most optical components optical mirrors are bought from precision optics manufacturers & distributors. It is rather common that the substrate and the coating are made in two different site, therefore buying mirror blank and doing the coating in-house or subcontracting it to other supplier is common practice.

Don&#;t hesitate to contact SINOPTIX for your questions about optical mirrors or to request for a quote.

,Step Feature Specification Characteristics / Benefits Limitations 1 Specify the Quantity Quantity Required &#; The larger the quantity of pieces that can be used in an application, the less expensive each part becomes as material, labor and coating charges can be divided over the total number of parts.

&#; Advanced Optics has the ability to modify catalog/overrun optical mirrors (when possible) to reduce costs and lead times. Small number of prototypes may be more expensive due to lot charges for glass and coating. 2 Select the Material Soda-lime Glass
&#; Commonly known as float glass. &#; Least expensive of all glass types. &#; Can be polished 1-3 waves/inch. &#; May be tempered making it 3 times stronger than non-tempered glass. &#; Softer than borosilicate glass making it easily scribed and broken.
&#; Cannot be precision polished and is available in commercial grade only (1-3 waves/inch).
&#; Has the lowest thermal shock and chemical resistance of all glass materials used to fabricate optics.
&#; Not as scratch resistant as borofloat, quartz or fused silica. BOROFLOAT®33 &#; Borofloat®33 is a borosilicate glass with a low thermal expansion.
&#; Good all around general purpose mirror substrate that is moderately priced.
&#; Easier to polish than harder materials such as fused quartz, fused silica or Zerodur® and is much less costly. &#; May be polished down to λ/10, but is not suitable for polishing down to λ/20.
&#; 2-3 times more costly than float glass (soda-lime glass).
&#; Not as thermally shock resistant as fused quartz or fused silica.
&#; Cannot be fully tempered like soda-lime glass.
&#; Not suitable for extreme high temperature conditions and will not hold its shape over 450° C for long periods of time. N-BK7® &#; Common borosilicate crown glass know for its low bubble and inclusion content.
&#; Economically priced, may be used as an optical mirror substrate, but more commonly used in the manufacture of optical windows. N-BK7 is not recommended for applications where thermal shock is a factor. Viosil &#; Viosil is a synthetic quartz glass substrate manufactured by ShinEtsu.
&#; The absence of bubbles and inclusions make it an excellent window substrate.
&#; It offers excellent chemical resistance, mechanical strength and high heat resistance. Carry glass only up to .250&#; thick. Fused Silica &#; Made from a synthetically derived silicon dioxide that is extremely pure.
&#; It is a colorless, non-crystalline silica glass.
&#; The main difference between fused silica and fused quartz is that the former is composed of a non-crystalline silica glass while the latter is composed of a crystalline silica glass.
&#; Advantages of fused silica over fused quartz include; greater ultraviolet and infrared transmission, a wider thermal operating range, increased hardness and resistance to scratching and a lower CTE which provides resistance to thermal shock over a broad range of temperatures.
&#; As opposed to other less costly glasses, the surface figure (flatness) of optical mirrors made of fused silica are not at risk in applications that expose the material to coatings applied at high temperatures or applications that require the material to remain flat at high and/or varying temperatures.
&#; Fused silica is also chemically resistant and provides superior transmittance in the UV spectrum when compared to fused quartz.
&#; Fused silica comes in many grades with the most common being 2G. Please visit Corning&#;s Quality Grade Selection Chart for further information. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses.
&#; The homogeneity of fused silica exceeds that of crystalline fused quartz, however standard 2G (UV grade) material has a higher OH content which cause dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. Quartz &#; Made from naturally occurring crystalline quartz or silica grains whereas fused silica is entirely synthetic.
&#; Fused quartz and fused silica are both extremely pure materials and have very low thermal expansion rates. However, fused quartz is more cost effective.
&#; Known for its incredible thermal shock resistance, chemical resistance and for being an excellent electrical insulator.
&#; Fused quartz has more metallic impurities and a lower OH content than standard UV grade fused silica which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses, but less expensive than fused silica.
&#; Fused quartz shares many of the same advantages of fused silica with the exception of metallic impurities found in the mined, natural quartz or silica sand. These impurities inhibit the materials ability to transmit well in the UV spectrum. ULE® Low Expansion Glass ULE® is a titania-silicate glass with near zero expansion characteristics that have made it the material of choice in unique applications such as machine tool reference blocks, gratings, interferometer reference mirrors, and telescope mirrors. Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. ClearCeram®-Z ClearCeram®-Z is a glass-ceramic material that offers an ultra low thermal expansion and is Ohara's equivalent to Zerodur® which is manufactured by Schott. Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. ZERODUR® &#; Glass-ceramic material which has a yellowish tint.
&#; Extremely low thermal expansion coefficient which approaches zero allowing it to be used to produce mirrors that retain their surface figures in extremely cold environments such as space.
&#; The CTE of Zerodur® is lower than ULE, fused quartz and fused silica.
&#; Known for its low level of bubbles and striae, internal stress and its excellent chemical resistance. &#; Yellow tint.
&#; Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. 3 Determine the Size/Shape Round
Rectangular
Square
Custom Round provides the best opportunity for obtaining flatness/accuracy. Square, rectangular and custom shapes provide more challenges to maintaining surface flatness. 4 Refine your Mechanical Tolerances Defines the acceptable limits of both size and thickness required for an application. Specified in inches or mm and typically given a +/- value.
&#; Round: Provide tolerance for diameter.
&#; Rectangular/Square: Provide tolerance for LxW.
&#; Thickness: Provide tolerance for thickness. &#; Tighter tolerances for diameter and LxW are typically easier to hold than for thickness.
&#; Extremely tight tolerances available, but may require specialized techniques and can reduce yield leading to increased costs.
&#; Loosening your tolerances can reduce costs. 5 Establish the Correct Accuracy Commercial grade
1-3 waves/inch

Precision polished λ/4 or λ/10


Precision polished λ/10 or λ/20 Commercial grade mirrors are generally made from less expensive materials such as soda-lime glass and borofloat.

Working grade mirrors are polished either λ/4 or λ/10 and most often made of Borofloat®33 or N-BK7.

&#; Precision grade mirrors are polished either λ/10 or λ/20 and are typically made from harder glass materials such as quartz, fused silica or Zerodur®.
&#; To achieve the best accuracy, optical mirrors are polished in a 6:1 aspect ratio (diameter to thickness). The higher the ratio, the greater probability the glass will distort during the manufacturing process. When the glass is deblocked after polishing, mirrors with non-standard aspect ratios may spring as they do not have the stability to hold surface flatness.
&#; Advanced Optics manufactures precision grade mirrors with non-standard aspect ratios. Achievable surface accuracy is dependent on choice of substrate and thickness of material. 6 Specify the Surface Quality Provide the required
Scratch and Dig 80-50: Commercial grade mirrors, suitable for non-critical applications, easily manufactured, lowest cost.

60-40 or 40-20: Working grade mirrors, precision quality, suitable for most scientific and research applications as well as low to medium power lasers, intermediate price point.

20-10 or 10-5: Precision grade, suitable for high power lasers, highest cost. Extremely tight tolerances available, but may require specialized techniques and can reduce yield leading to increased costs. 7 Provide Parallelism (if required) Amount of wedge or variation in thickness allowed over the surface of a part.

It is defined in arc minutes (an angular measurement that is 1/16th of a degree) or arc seconds where 60 arc seconds is equal to 1 arc minute.
Advanced Optics can hold parallelism of < 2 arc seconds.

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Extremely tight requirements for parallelism require specialized manufacturing techniques which may reduce yield and increase manufacturing costs. 8 Define the Clear Aperture/ Edge Bevel Requirements The clear aperture is the percentage of useable area of an optical mirror.

An edge bevel or safety chamfer is applied around the edge of an optical mirror.
Normally 90% or advise requirement.


An edge bevel or safety chamfer is applied around the edge of an optical mirror to eliminate sharp edges and reduce edge chips caused by cutting of the glass. Typically between .010"-.040" face width at 45 degrees depending on size of part, please advise preference and tolerance. Very small edge bevels with tight tolerances will add additional costs. 9 Choose the Proper Coating Metallic and Dielectric coatings available for the UV-VIS-NIR regions. &#; Provide the wavelength(s) of interest and % reflectivity required.
&#; Provide the intended AOI (angle of incidence) for the optical mirror. Custom coatings for a small quantity of parts may add additional expense. 10 Customization The following attributes can be added to customize your mirror. &#; Shapes: Provide drawing of custom shape.
&#; Holes and Notches: Provide location, size with tolerances.
&#; Custom Bevels: Provide location, depth and angle.
&#; Custom Coatings: Provide expected % of reflectivity over wavelength(s) of interest and AOI (angle of incidence). Additional features may add lead time and cost.