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Interior of an E-Beam Deposition Chamber

Thorlabs' Coating Capabilities

Thorlabs' state-of-the-art, in-house, optical coating department provides us with coating capabilities ranging from metal coatings and antireflective coatings to cutting edge Ion Beam Sputtered (IBS), Radio Frequency (RF) Magnetron Sputtered, and Plasma Assisted coatings. This full-scale facility not only allows us to produce large numbers of our catalog optics in house but also expands our ability to manufacture custom-coated optics to suit a variety of customer needs.

Several coating chambers are outfitted with optical thickness monitoring and crystal thickness monitoring which enable our coatings to meet the stringent specifications required for sensitive applications.

Coating Design

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Layered Coating Stack

The spectral performance and other key characteristics of optical thin films are determined by the structure and number of layers in the coating, the refractive indices of the materials used, and the optical properties of the substrate.

The structure of most coatings resembles a series of discrete alternating layers of high index and low index materials. Different arrangements of stack structure result in different types of coatings (e.g., Bandpass vs. Edgepass vs. BBAR). Fine tuning of layer thicknesses and refractive indices is done to optimize performance characteristics in the wavelength range of interest. Thorlabs has a selection of thin film modeling tools to design, characterize, and optimize many aspects of an individual coating's performance.

Cleaning

The first and one of the most critical steps of our process is cleaning uncoated substrates with an automated ultrasonic clean line. Using a series of ultrasonic solvent and detergent baths, each step of the cleaning process removes different types of contamination from the surfaces of the substrate. This ensures surface contamination does not interfere with adhesion of coatings to the substrate.

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Fully Automated Optical Cleaning System

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Optics After Cleaning

E-Beam Deposition

Thorlabs' coating facility currently operates four fully automated Electron Beam (E-Beam) deposition systems. These systems use an electron beam source to evaporate a selection of materials such as transition metal oxides (e.g., TiO₂, Ta₂O₅, HfO₂, Nb₂O₅, ZrO₂), metal halides (MgF₂, YF₃), or SiO₂. This type of process must be done at elevated temperatures (200 - 250 °C) to achieve good adhesion to the substrate and acceptable material properties in the final coating.

 

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Ion-Assisted E-Beam Deposition

Ion-Beam Assisted Deposition (IAD) uses the same E-beam method to evaporate coating materials but with the addition of an ion source to promote nucleation and growth of materials at lower temperatures (20 - 100 °C). The ion source allows temperature-sensitive substrates to be coated. This process also results in a denser coating that is less sensitive to spectral shifting in both humid and dry environmental conditions.

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IBS Deposition

Our Ion Beam Sputtering (IBS) deposition chamber is the most recent addition to our line-up of coating tools. This process uses a high energy, radio frequency, plasma source to sputter coating materials and deposit them on substrates while another RF ion source (Assist source) provides IAD function during deposition. The sputtering mechanism can be characterized as momentum transfer between ionized gas molecules from the ion source and the atoms of the target material. This is analogous to a cue ball breaking a rack of billiard balls, only on a molecular scale and with several more balls in play.

Advantages of IBS

• Better Process Control
• Wider Selection of Coating Designs
• Improved Surface Quality and Less Scatter
• Reduced Spectral Shifting
• Thicker Coating in a Single Cycle

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Coated Retroreflectors in Tool

Metrology

Thorlabs uses a selection of research-grade spectroscopy instruments to characterize coating performance from the UV to the Far Infrared. In addition to spectroscopy tools, we employ a variety of laser and laser diode sources, power meters, detectors, and polarimeters to test the performance of our optics. Specific metrology systems include Cary 660 FTIR, Cary 5000, PE Lambda 950, and Olis PE 983 IR spectrophotometers, a KLA Tencor surface profilometer, a J.A. Woollam RC2 ellipsometer, Zygo monochromatic interferometers, an Optoflat broadband interferometer for single surface and multi-surface interferometric measurements, a surface scatter instrument, and a custom-built cavity ring down system. All of these tools help us understand our coatings and materials at a molecular level. We build custom setups to test both catalog and OEM parts to ensure every optic we offer performs within the specified range. All metrology instruments are calibrated regularly per the ISO 9001:2015 standard.

Laser Line/Bandpass Filters

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Thorlabs' Selection of Bandpass Filters

Laser Line and Bandpass filters transmit light in a narrow, well-defined spectral region while rejecting other unwanted radiation. This type of filter displays very high transmission in the bandpass region and blocks a limited spectral range of light on either side of the bandpass region. To compensate for this deficiency, an additional blocking component is added, which is either an all-dielectric or a metal-dielectric depending on the requirements of the filter. Although this additional blocking component eliminates any unwanted out-of-band radiation, it also reduces the filter's overall transmission throughput.

These coatings are formed by vacuum deposition coating techniques and consist of two reflecting stacks, separated by an even-order spacer layer. These reflecting stacks are constructed from alternating layers of high and low refractive index materials, which can have a reflectance in excess of 99.99%. By varying the thickness of the spacer layer and/or the number of reflecting layers, the central wavelength and bandwidth of the filter can be altered.

Edgepass Filters

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Thorlabs' Selection of Edgpass Filters

Edgepass filters are very useful for isolating specific spectral regions. Longpass filters transmit wavelengths longer than the cutoff wavelenght and block wavelengths shorter than the cutoff wavelength. Shortpass filters block wavelengths longer than the cutoff wavelength and transmit those shorter than the cutoff wavelength.

All Thorlabs edgepass filters are constructed of durable dielectric coatings and will withstand the normal cleaning and handling associated with any high-quality optical component. Their film construction is essentially a modified quarter-wave stack, using interference effects rather than absorption to isolate their spectral bands.

Dichroic Beamsplitters

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Thorlabs' Selection of Dichroic Beamsplitters

Dichroic Beamsplitters are used as beam directors at 45° and are either longpass or shortpass. The longpass variety reflects >90% of the incident light below the design wavelength and transmits >90% of the incident light above the design wavelength. The shortpass variety transmits below the design wavelength, and reflects above the design wavelength. Dichroic beamsplitters are used in many applications, the most common one being fluorescence microscopy.

The dielectric coating on dichroic beamsplitters is the source of their functionality. The alternating layers in the coating are designed to cause constructive interference for those wavelengths to be transmitted and destructive interference for those wavelengths to be reflected. The thickness of the coating and the refractive index of the materials in the layers determine the design wavelength for a given beamsplitter.

Notch Filters

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Notch filters, also commonly referred to as band-stop or band-rejection filters, are designed to transmit most wavelengths but attenuate light within a specific wavelength range (the stop band) to a very low level. They are functionally the inverse of bandpass filters and are made in the same way.

Thorlabs' Selection of Notch Filters

Neutral Density Filters

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Neutral Density (ND) filters attenuate all wavelengths within a range by a certain factor to prevent damage to detecting equipment. Fixed ND filters attenuate the spectra by a fixed amount. Variable ND filters have stepped films at discrete locations to allow for various attenuation depending on the application. Continuous ND filters have a film gradient across the entire filter, which allows for a continuous range of attenuation. Thorlabs offers a selection of both linear and circular variable and continuous ND filters.

Thorlabs' Selection of Neutral Density Filters

Thorlabs' Selection of Lenses

Thorlabs' Selection of Windows

Thorlabs' Selection of Fiber and Fiber Patch Cables

Antireflective (AR) Coatings

AR coatings are hard refractory-oxide coatings that minimize surface reflections within specified wavelength ranges when applied to the surface of optical components. Without AR coating, 4% of the light is lost at each optical surface due to reflections. For example, if three uncoated lenses are being used in series, this 4% loss occurs at each of the six optical surfaces. This results in a total loss of 21.7%. If three AR-coated lenses with a "B" coating (R_avg <0.5% per surface) are used instead, the total loss of incident light due to surface reflections is <3%. The use of AR-coated optics improves transmission from 78.3% to greater than 97% in this case. Please note that the 4% loss at the interfaces of uncoated optics is an approximate value that varies greatly with material and angle of incidence (AOI). Please note that the color of the lens does not correlate to the lens’ specifications. The color of each AR coating may vary from batch to batch and is not an indicator of performance.

The AR Coating Range graph below shows the specified wavelength range of Thorlabs' in-house AR coatings. Click on the bars in the graph below to view the performance plot for each coating.

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Broadband antireflective (BBAR) coatings consist of multiple layers, alternating between a high index material and a low index material. The layers are deposited on the substrate via electron-beam deposition. The thickness of the layers is optimized, using modeling software, to produce destructive interference between reflected waves and constructive interference between transmitted waves. This results in an optic that has enhanced performance within a specified wavelength band as well as minimal internal reflections (ghosting). Thorlabs' BBAR coatings provide good performance for angles of incidence between 0° and 30° and a numerical aperture (NA) of 0.5. Thorlabs currently offers BBAR coatings designed to maximize performance within 8 different wavelength ranges.

V-coatings are multilayer, dielectric, thin-film, AR coatings that are designed to minimize reflectance over a short wavelength range. Surface reflectance rises rapidly on either side of this minimum, which gives the reflectance curve a "V" shape. Compared to the broadband AR coatings, V-coatings achieve lower reflectance over a narrower bandwidth when used within their design AOI range. See the graph to the right for an example of the reflectance of a 633 nm V-coat designed for 0° AOI at various angles. We offer a variety of different V-coatings; see the table below for more information.

The tables below give the specifications for Thorlabs' in-house antireflection coatings, which are deposited on the surfaces of many optics and fibers in our catalog. However, we also offer optics that are coated by external vendors. As such, the specifications for some of our antireflection coated optics may be slightly different than the specifications given on this page. The AR coating specifications for any individual item are always included in that item's web presentation.

Broadband Antireflective Coating Specifications
Coating Code	Wavelength Range	Reflectancea	Performance Plotb	Coating Variationc
UV	245 - 400 nm	Ravg < 0.5%	Raw Data	Unavailable
A	350 - 700 nm	Ravg < 0.5%	Raw Data	Raw Data
AB	400 - 1100 nm	Ravg < 1.0%	Raw Data	Unavailable
B	650 - 1050 nm	Ravg < 0.5%	Raw Data	Raw Data
C	1050 - 1700 nmd	Ravg < 0.5%	Raw Data	Raw Data
D	1.65 - 3.0 µm	Ravg < 1.0%	Raw Data	Unavailable
Ee,f	2 - 5 µm	Ravg < 1.25%	Raw Data	Unavailable
E1f	2 - 5 µm	Ravg < 1.50% Rabs < 3.0%
E4	2 - 13 µm	Ravg < 3.5% Rabs < 6%	Raw Data	Unavailable
E2	4.5 - 7.5 µm	Ravg < 1.0% Rabs < 2.0%	Raw Data	Unavailable
E3g	7 - 12 µm	Ravg < 1.0% Rabs < 2.0%	Raw Data	Unavailable
F	8 - 12 µm	Ravg < 1.5%	Raw Data	Unavailable
Gg	7 - 12 µm	Ravg < 1.0%	Raw Data	Unavailable

• To calculate R_avg, the reflectance is measured over the specified wavelength range and then averaged.
• The shaded region in the plots represents the specified wavelength range for optimum performance.
• Optical coatings vary from run to run. The specifications provided throughout our website are true for all coating runs. However, these plots give you some idea of the variation that does occur.
• The C coating wavelength range specification was recently updated from 1050 - 1620 nm to 1050 - 1700 nm. All optics in stock currently have a C coating with a wavelength range of 1050 - 1700 nm. However, we are in the process of updating our documentation, and so some individual optics may still be presented with the old wavelength range on our website.
• Manufacturing processes of E-coated optics have recently changed. Physical appearance (e.g., color) of these optics may vary based on factors including materials and processes. The specified performance will be achieved regardless of color variations.
• The E and E1 coatings offer similar performance, but the -E1 coating guarantees an absolute reflectance of <3.0% over the specified wavelength range.
• The G and E3 coatings offer similar performance, but the -E3 coating guarantees an absolute reflectance of <2.0% over the specified wavelength range.

Narrowband Antireflective (V-) Coating Specifications
Coating Code	Design Wavelength	Reflectance per Surfacea	Performance Plot
405	405 nm	<0.25%	Raw Data
532	532 nm	<0.25%	Raw Data
633	633 nm	<0.25%	Raw Data
780	780 nm	<0.25%	Raw Data
1064	1064 nm	<0.25%	Raw Data
YAG	532/1064 nm	<0.25%	Raw Data
1550	1550 nm	<0.25%	Raw Data

• The reflectance is measured at the design wavelength.

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Dependence of Reflectance on Angle of Incidence
for a 633 nm V-Coat Designed for 0° AOI

Thorlabs' Selection of HR-Coated Optics

Table of Contents

• Broadband HR Coatings
• Narrowband Laser Line HR Coatings
• Crystalline HR Coatings

Thorlabs offers dielectric and metallic broadband coatings, dielectric narrowband coatings for laser line applications, and high-performance crystalline mirror coatings. We also offer a selection of ultrafast mirrors optimized for femtosecond laser pulse applications.

Thorlabs' dielectric HR coatings, available in broadband and narrowband designs, are hard, refractory, oxide coatings that maximize surface reflections within specified wavelength ranges and at specified angles of incidence. These coatings consist of alternating layers of high- and low-index materials. Using computer models, the thickness of each layer is optimized to produce constructive interference for reflected waves and destructive interference for transmitted waves.

The reflectance of surfaces can be greatly improved by the addition of an HR coating; however, performance will depend on both the wavelength and the angle of incidence (AOI). At high AOIs, the performance will also depend on the polarization of the incident light. Information about the reflectance of each HR coating can be found in the tables below.

Broadband HR Coatings
Thorlabs offers a number of broadband HR coatings optimized for various performance parameters. The graph below shows the specified wavelength range of Thorlabs' in-house broadband HR coatings. Click on the bars in the graph below to view the performance plot for each coating. Click here for a raw data file that compares all of our broadband HR Coatings.

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The E01, E02, E03, and E04 dielectric HR coatings offer high reflectance over broad wavelength ranges. Our UV-Enhanced Aluminum coating has an overcoat of MgF₂ to increase its average reflectance in the UV portion of the spectrum. The Protected Aluminum coating is an inexpensive solution and has an overcoat of SiO₂ to make the aluminum coating suitable for laboratory and industrial use. The Protected Silver coating has the highest reflectance in the visible spectrum and is protected with an SiO₂ overcoat to prevent oxidation. The Ultrafast-Enhanced Silver coating is manufactured such that it exhibits a low group delay dispersion. Three gold coating options are offered: protected, MIR enhanced, and unprotected. The Protected Gold coating retains a high reflectance down to 800 nm, while the MIR enhanced gold coating is optimized to reduce losses in the MIR that are commonly found in gold mirrors. Each of these gold coatings is protected by an overcoat that also makes the mirrors easy to clean. The Unprotected Gold coating offers higher reflectance than the protected gold coating, but is slightly more delicate.

Broadband HR Coating Specifications
Coating Namea	Wavelength Range	Avg. Reflectanceb	Angle of Incidence	Graphs	Pulsed Laser Damage Threshold for UV Fused Silicac	CW Laser Damage Threshold for UV Fused Silicad
E01	350 - 400 nm	>99%	0° to 45°		1 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.373 mm)	-
E02	400 - 750 nm	>99%	0° to 45°		0.25 J/cm2 (532 nm, 10 ns, 10 Hz, Ø0.803 mm)	550 W/cm (532 nm, Ø1.000 mm)
E03	750 - 1100 nm	>99%	0° to 45°		0.205 J/cm2 (800 nm, 99 fs, 1 kHz, Ø0.166 mm) 1 J/cm2 (810 nm, 10 ns, 10 Hz, Ø0.133 mm)  0.5 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø0.433 mm)	10 kW/cm (1070 nm, Ø0.971 mm)
E04	1280 - 1600 nm	>99%	0° to 45°		2.5 J/cm2 (1542 nm, 10 ns, 10 Hz, Ø0.181 mm)	350 W/cm (1540 nm, Ø1.030 mm)
UV-Enhanced Aluminum (-F01)	250 - 450 nm	>90%	0° to 45°		0.25 J/cm2 (266 nm, 10 ns, 10 Hz, Ø0.150 mm) 0.3 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.381 mm)	300 W/cm (1.064 µm, Ø0.044 mm) 500 W/cm (10.6 µm, Ø0.339 mm)
Protected Aluminum (-G01)	450 nm - 20 µm	>90% (450 nm - 2 µm) >95% (2 - 20 µm)	0° to 45°		0.3 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)	100 W/cm (1.070 µm, Ø0.098 mm) 350 W/cm (10.6 µm, Ø0.339 mm)
Ultrafast-Enhanced Silver (-AG)	750 nm - 1 µm	Rabsolute,S > 99.0% Rabsolute,P > 98.5%	45°		0.18 J/cm2 (800 nm, 52 fs FWHM S-Pol, 1000 Pulses)e 0.39 J/cm2 (800 nm, 52 fs FWHM S-Pol, Single Pulse)e	-
Protected Silver (-P01)	450 nm - 20 µm	>96.5% (450 nm - 2 µm)f,g >95% (2 - 20 µm)	0° to 45°		0.225 J/cm2 (800 nm, 99 fs, 1 kHz, Ø0.167 mm) 1 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.010 mm)	500 W/cm (1.07 µm, Ø0.974 mm) 1500 W/cm (10.6 µm, Ø0.339 mm)
Protected Silver (-P02)h	450 nm - 20 µm	>97% (450 nm - 2 µm) >95% (2 - 20 µm)	0° to 45°		-	-
Protected Gold (-M01)	800 nm - 20 µm	>96%	0° to 45°		2 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)	500 W/cm (1.070 µm, Ø0.089 mm) 750 W/cm (10.6 µm, Ø0.339 mm)
MIR Enhanced Gold (-M02)	2 - 20 µm	>98%	0° to 45°		0.1 J/cm2 (1.064 µm, 10 ns, 10 Hz, Ø1.06 mm) 3 J/cm2 (10.6 µm, 100 ns, 1 Hz, Ø1.29 mm)	25 W/cm (1.07 µm, Ø1.04 mm) 450 W/cm (10.6 µm, Ø1.18 mm)
Unprotected Gold (-M03)	800 nm - 20 µm	>97%	0° to 45°		4 J/cm2 (10.6 µm, 100 ns, 1 Hz, Ø0.435 mm)	1000 W/cm (10.6 µm, Ø1.18 mm)

• Click each link to view our standard fused silica mirror offerings with that coating. Click here to view our complete line of HR-coated optics.
• The reflectance is measured over the specified wavelength range and then averaged for all coatings, with the exception of the Ultrafast-Enhanced Silver. The -AG coating reflectances are absolute.
• All diameters are the 1/e² values.
• The power density of your beam should be calculated in terms of W/cm. For an explanation of why the linear power density provides the best metric for long pulse and CW sources, please see the "Continuous Wave and Long-Pulse Lasers" section in the Damage Thresholds tab.
• For ultrafast optics, the laser-induced damage threshold (LIDT) is defined as the fluence (per pulse) that produces visible damage after a given number of pulses. LIDT values are not guaranteed in the ultrashort pulse regime. As such, they are provided as a service to customers.
• Our Economy Protected Silver Mirrors offer this average reflectance in the 450 nm - 2 µm wavelength range. All other mirrors with item #s ending in -P01 have a higher average reflectance of >97%.
• Our Reflective Microscope Objectives offer an absolute reflectance >96% over the 450 nm - 20 μm wavelength range. 
• This variation of our protected silver coating is only found on the PFR14-P02 Rectangular Mirror.

Narrowband Laser Line HR Coatings
Thorlabs offers a number of laser line HR coatings optimized for various performance parameters. These dielectric HR coatings offer very high reflectance over specific laser line wavelength ranges.

Narrowband Laser Line HR Coating Specifications
Coating Namea	Wavelength Range	Laser Line	Average Reflectanceb	Angle of  Incidence	Graphs	Pulsed Laser Damage Thresholdc	CW Laser Damage Thresholdd
K04	262 - 266 nm	Nd:YAG, 4th Harmonic	>99.0% (S- and P-Pol)	0° to 45°		2 J/cm2 (266 nm, 10 ns, 10 Hz, Ø0.416 mm)	-
K05	300 - 308 nm	Ar Ion	>99.5% (S-Pol) >98.0% (P-Pol)	0° to 45°		-	-
K07	333 - 364 nm	Ar Ion	>99.5% (S-Pol, 45° AOI)e >99.0% (P-Pol, 45° AOI)e >98.7% (Unpol, 0° AOI)e	0° to 45°		5 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.294 mm)	-
K08	349 - 355 nm	Nd:YAG, 3rd Harmonic	>99.5% (S- and P-Pol)	0° to 45°		3.5 J/cm2 (355 nm, 10 ns, 10 Hz, Ø0.350 mm)	-
K10	458 - 528 nm	Ar Ion	>99.0% (S- and P-Pol)	0° to 45°		-	-
J11	520 - 647 nm	Kr Ion	>99.7% (S- and P-Pol)	45°		-	-
K12	524 - 532 nm	Nd:YAG, 2nd Harmonic	>99.5% (S-Pol) >99.0% (P-Pol)	0° to 45°		8 J/cm2 (532 nm, 10 ns, 10 Hz, Ø0.491 mm)	550 W/cm (532 nm, Ø1.000 mm)
K13f (Dual Coating)	532 nm	Nd:YAG, 2nd Harmonic	>98.0% (S- and P-Pol)	0° to 45°		8 J/cm2 (532 nm, 10 ns, 10 Hz, Ø0.491 mm)	-
	1064 nm	Nd:YAG	>99.0% (S- and P-Pol)	0° to 45°		5 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.010 mm)
K14	1047 - 1064 nm	Nd:YAG	>99.5% (S- and P-Pol)	0° to 45°		25 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø0.552 mm)	20 kW/cm (1070 nm, Ø0.974 mm)
L01	10.6 µm	CO2	>99% (S- and P-Pol)	0° to 45°		6 J/cm2 (10.6 µm, 100 ns, 1 Hz, Ø0.362 mm)	10 kW/cm (Ø0.115 mm)

• Click to view our round mirror offerings.
• Unless otherwise specified, the reflectance is measured over the specified wavelength range and then averaged.
• All diameters are the 1/e² values.
• The power density of your beam should be calculated in terms of W/cm. For an explanation of why the linear power density provides the best metric for long pulse and CW sources, please see the "Continuous Wave and Long-Pulse Lasers" section in the Damage Thresholds tab.
• For our K07 coating, we specify the minimum reflectance at 352 nm.
• We also offer unmounted and cage-cube-mounted right angle prism mirrors.

Crystalline Mirror Coatings
Thorlabs Crystalline Solutions currently offers three different GaAs/AlGaAs crystalline coatings optimized for superior mid-IR performance, as well as the ability to create custom crystalline coatings. These high-reflectance mirror coatings are ideal for high-finesse laser cavities, precision interferometry, and high-power laser systems. The specifications below are typical values. Thorlabs offers a selection of concave and plano xtal stable™-coated mirrors from stock. Mirrors with xtal mir™ or xtal therm™ coatings, as well as custom mirrors with xtal stable™ coatings, are made to order by request through Tech Support.

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The conical nanostructures making up our textured surfaces produce a continuous gradient in the effective index from air to the bulk material.

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An SEM image showing a top-view of the T1 textured AR surface.

Thorlabs' Selection of Textured AR Windows

Textured AR Surfaces 

Thorlabs' textured AR surfaces are created by removing material from the bulk optic substrate using our proprietary process, which has been optimized to fabricate subwavelength structures. The surface that remains consists of roughly conical nanostructures, with irregular spacing and varied height. These nanostructures produce a smooth gradient of the effective refractive index.

Since there are no step changes in the refractive index, the incident light is able to travel from the air to the bulk glass with virtually no Fresnel reflections. As a result, textured optics can achieve significantly higher transmission than un-textured optics. Thorlabs currently offers three surfaces for stock optics (see the table below for details).

Antireflection Performance
Compared to our traditional thin-film broadband antireflective (BBAR) coatings, our textured surfaces exhibit lower reflectance over a broad wavelength range and lower angular sensitivity. The anti-reflection mechanism used in dielectric AR coatings is based on the interference between reflections from subsequent thin film dielectric layers. This behavior is highly dependent on the wavelength of the light, as the refractive indices and thicknesses of each layer are designed to provide the desired phase relationship between reflecting waves. Moreover, the angle of incidence (AOI) affects the effective thickness of each layer that the light interacts with, which leads to a strong dependence on the AOI. As a result, there are limitations on the performance that a BBAR coating can achieve over a broad range of wavelengths and AOIs.

In contrast, the effective index gradient of our textured surfaces is less sensitive to the wavelength and AOI than thin-film interference, allowing for very low reflectance over a broad range of both. See the graphs below for an illustration of this difference.

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Comparison of Thorlabs' -AB thin-film BBAR coating and the T1 textured surface reflectances. While both have a 400 - 1100 nm wavelength range, the T1 surface has much lower reflectance.

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Comparison of Thorlabs' -C thin-film BBAR coating and the T2 textured surface reflectances for unpolarized light at 45° AOI. The T2 surface maintains much lower reflectance at high AOIs. 

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The superior antireflective performance of our textured surfaces can be observed by eye. In this image, four 1.0 mm thick UVFS windows were exposed to fluorescent ceiling lights, and reflections from the surfaces of the uncoated and A-coated windows can be seen in this photo. The windows with TU and T1 surfaces show no visible reflections.

Damage Threshold and Durability
Because the nanostructures comprising Thorlabs' textured surfaces are originally part of the bulk optic, they have substantially higher laser-induced damage thresholds than BBAR coatings. Laser-induced damage at moderate fluence typically originates from the interface between two materials, and the large number of thin-film layers in BBAR coatings increase the likelihood of damage. As our textured surfaces consist of the same material as the bulk, the damage threshold is higher.

When properly handled, textured surfaces can exhibit greater durability than BBAR coatings. Their single-material construction means that they do not suffer from mismatched coefficients of thermal expansion (CTE) under temperature cycling and avoid issues with thin-film adhesion that can occur in some AR coatings.

Textured AR Surface Specifications
Surface Name	TU	T1	T2
Substrate	UV Fused Silica		Infrasil ®†
Wavelength Range	230 - 450 nm	400 - 1100 nm	1000 - 1700 nm
Damage Threshold	>30 J/cm 2 at 355 nm, 10 ns, 10 Hz, Ø0.22 mm	>30 J/cm 2 at 532 nm, 10 ns, 10 Hz, Ø0.4 mm	>30 J/cm 2 at 1064 nm, 10 ns, 10 Hz, Ø0.23 mm
Reflectance Over Wavelength Range a	R avg < 0.25%
Reflectance Data (Click for Graph)	Raw Data	Raw Data	Raw Data

• Specified at 0° Angle of Incidence

Handling Precautions and Cleaning
Thorlabs' Textured Windows could be contaminated or damaged by moisture, fingerprints, aerosols, or contact with any abrasive material. The windows should only be handled when necessary and always held by the sides using our TZ2 or TZ3 tweezers. Latex gloves or a similar protective covering should be worn to prevent oil from fingers from reaching the structured surface.

If the surface is contaminated, the windows may be cleaned by:

• Blowing off dust with clean air or nitrogen.
• Rinsing with solvents, such as isopropyl alcohol, followed by clean air or nitrogen blow-drying.
• Immersing in a basic solution (a mix of ammonium hydroxide and hydrogen peroxide) and/or an acid solution (a mix of hydrogen chloride and hydrogen peroxide) followed by clean air or nitrogen blow-drying.

Standard cleaning methods will result in further contamination and thus should be avoided.

†Infrasil is a registered trademark of Heraeus Quarzglas.

Absolute and Average Reflectance

Thorlabs quotes two types of reflectance values for our optical coatings. Absolute Reflectance (R_abs, or simply R) indicates the maximum or minimum reflectance at a specific wavelength or wavelengths. When specified over a wavelength range, it indicates the maximum or minimum reflectance for each wavelength within the range. The reflectance may increase or decrease sharply near the specified wavelengths or wavelength range, as in our V-coatings.

The other type of reflectance specification is Average Reflectance (R_avg), which is always quoted over a wavelength range. R_avg is found by measuring the reflectance at wavelengths across the specified range and then averaging the reflectance over wavelength. This specification is insensitive to certain variations between individual coating runs that may not be of interest for broadband applications. If a guarantee of performance at a particular wavelength is needed, then a coating specified by R_abs will be preferable.

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Thorlabs' -E1 broadband antireflective coating is specified to have R_avg < 1.5% and R_abs < 3% over the 2 - 5 μm range. The reflectance (R_abs) for the representative coating shown here exceeds 1.5% at some wavelengths within that range, but never exceeds 3%.

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Thorlabs' 405 nm V-coating is specified to have R_abs < 0.25% at 405 nm. The reflectance (R_abs) can be seen to increase rapidly away from 405 nm.

Run-to-Run Variation

The plot below shows reflectance data from four -P01 coatings produced by Thorlabs. While all four meet the average reflectance specification from 450 - 2000 nm, their reflectances differ at specific wavelengths. The variation is largest below 850 nm, where all four coatings have reflectances lower than 96.5%. This kind of behavior is typical of coatings specified by R_avg. Outside of the specified range, the variation can be more pronounced, because the reflectance at those wavelengths is not controlled.

Click to Enlarge
8° AOI reflectance curves for four representative -P01 coating runs. The coating
is specified to have R_avg > 96.5% (dashed line) over the 450 - 2000 nm range (blue-shaded region). While all four coatings meet that specification, they all have absolute reflectances less than 96.5% for wavelengths below 850 nm. The individual coating runs also differ in their absolute reflectances at each wavelength.

Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm² (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.

According to the test, the damage threshold of the mirror was 2.00 J/cm² (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

1. Wavelength of your laser
2. Beam diameter of your beam (1/e²)
3. Approximate intensity profile of your beam (e.g., Gaussian)
4. Linear power density of your beam (total power divided by 1/e² beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10^-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^-7 s and 10^-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e² beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm² at 1064 nm scales to 0.7 J/cm² at 532 nm):

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^-9 s and 10^-7 s. For pulses between 10^-7 s and 10^-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).