How to Choose Optical Mirrors?

,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.

Mirrors are components found throughout optical systems. They can be utilized to focus and steer light, reject particular wavelengths, and combine wavelengths in imaging and additional applications. Several factors should be considered when selecting a mirror.

Materials

Metallic Mirrors provide a combination of absorbance and reflectance (and transmittance if adequately thin).

They can be employed as neutral density filters, neutral beamsplitters, or wide wavelength-range reflectors. The kind of metal utilized defines its spectral features. The application of these mirrors is mostly outside of angle-of-incidence.

Dielectric Mirrors comprise of delicate layers of non-absorbing materials (normally fluorides and oxides), which vary in refractive index. The composition and thickness of the layers are configured to create reflectance or transmittance in wavelength ranges specified by the application or customer.

These materials absorb little to no light, so dielectric mirrors can frequently be employed as dichroic mirrors (where light of certain colors travels through while reflecting the light of different). Both the angle-of-incidence and the wavelength range must be determined at the design stage.

Function

Imaging - Needs a flatness of λ/10 or better to reduce image distortion. Beam-steering, non-imaging applications do not demand strict specifications on flatness.

Wavelength combining - Dielectric dichroic mirrors are utilized to bring together different laser beams onto one axis. This application requires a flatness of 1/4λ per inch or more.

Wavelength splitting - Desired wavelengths can also be reflected with the use of dielectric dichroic mirrors. Applications involve hot-mirrors that exclude IR and NIR light, transmitting emission light, and reflecting excitation light while identifying alternate wavelength bands with several detectors.

The reflecting and transmitting wavelengths must be thoroughly defined for this kind of application. These are normally utilized at a 45° angle of incidence.

Wavelength rejection - A researcher may want to exclude particular wavelengths from the system in some cases.

Some examples are order-sorting filters (where undesired wavelengths are reflected), cold mirrors (where shorter wavelengths are reflected while longer wavelengths are transmitted, often used in lamp assemblies), and hot mirrors (which reflect IR or NIR).

From a functional perspective, these are dichroic mirrors which are applied in a different respect. They are normally used at near-normal to normal incidence.

Angle of Incidence

Mirrors are mostly configured to be employed at a particular angle of incidence. Hot mirrors are normally utilized at zero or near zero degrees AOI, whereas dichroic mirrors are frequently used at 45°.

The AOI is determined by the optical design of the system. Differences in polarization should be explored when the AOI is more than around 25°. Read the article on angles of incidence to gain more information.

Physical Environment

The requirements for durability should be established according to the physical environment of which the mirror will be exposed.

Temperature cycling is critical for space applications. For applications in the outdoors, temperature and humidity cycling, abrasion resistance, condensation, and salt fog may be considerations.

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Radiative flux (when the filter is put into a highly energetic or intense beam) may lead to a decline in performance over time. There are limited environmental requirements in air-conditioned laboratory spaces or a protected laboratory instrument.

Wavelength Range

UV (180-400 nm) While conventional metal mirrors perform across a wide range of wavelengths, other metals may work better over particular wavelength ranges. First-surface aluminum mirrors protected with Magnesium Fluoride are normally suggested below 430 nm.

Omega has additionally produced dielectric mirrors tailored to this range that comprise of delicate layers of transition metal oxides or silicon dioxide, magnesium fluoride, and lanthanide fluorides for the lower wavelengths.

Visible (400-700 nm) Visible mirrors are traditionally made from silver on the top side (the first-surface) or the backside of a piece of glass. They are frequently shielded with an extra layer of silicon dioxide (for the first-surface) or a plastic material that is not transmissive (for the back surface).

Dielectric mirrors comprise of non-absorbing materials in alternating layers and are produced to enhance reflectance at particular wavelengths and angles while excluding others. Enhanced metal mirrors use both dielectric and metal layers to optimize reflectivity.

NIR &#; IR (700 nm - 10 micron) In the IR and NIR, gold mirrors are commonly employed, which absorb light in certain visible wavelengths but also contain a high reflectance (greater than 95% above 1,500 nm).

A different choice is a transparent conductive oxide mirror (such as ITO) which offers transparency at shorter (visible) wavelengths with high reflectivity at longer wavelengths.

Broadband An application may sometimes need high reflectivity across a wavelength range that covers many of those mentioned earlier. These applications are astronomy, solar photothermal or photovoltaics, and hyperspectral imaging.

For the flattest and highest reflectivity response, dielectric mirrors can be produced (the same as the Ultra Broadband Dielectric Mirror).

This information has been sourced, reviewed and adapted from materials provided by Omega Optical, Inc.

For more information on this source, please visit Omega Optical, Inc.

For more information, please visit Optical Mirrors.