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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) 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.
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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.
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
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.
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.
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
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Coated Retroreflectors in Tool
Thorlabs uses a selection of research-grade spectroscopy instruments to characterize coating performance from the UV to the Far Infrared. Varian Cary 5000 and PE Lambda 950 spectrophotometers are used to characterize the spectral performance of our coatings in the UV-VIS-NIR ranges and an Olis PE 983 IR spectrophotometer is used for infrared coatings (2 - 55.5 µm). In addition to the spectroscopy tools, we also use a variety of laser and laser diode sources, power meters, detectors, and polarimeters to test the performance of our optics. We build custom setups to test both catalog and OEM parts to ensure every optic we offer performs well within the specified range. All of our metrology instruments are calibrated regularly per ISO 9001:2015 standard.
Laser Line/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 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 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, 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.
Neutral Density Filters
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.
Antireflective (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 (Ravg <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.
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 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.
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Dependence of Reflectance on Angle of Incidence
for a 633 nm V-Coat Designed for 0° AOI
Table of Contents
The tables below give specifications for our broadband and narrowband high-reflectance (HR) coatings. Thorlabs offers dielectric and metallic broadband coatings, as well as dielectric narrowband coatings for laser line applications. We also offer specialty ultrafast mirrors for femtosecond pulsed lasers.
Dielectric HR coatings, available in broadband or narrowband designs, are hard, refractory, oxide coatings that maximize surface reflections within a specific wavelength range and angle of incidence. The coating consists of alternating layers of high and low index materials. The layer's thickness is optimized, using computer models, to produce constructive interference for reflected waves and destructive interference for all other wavelengths. The reflectance of the surface is improved greatly by the addition of an HR coating; however, its performance is dependent on the angle of incidence (AOI). At high AOIs, the reflection band shifts to shorter wavelengths and performs differently for S and P polarizations.
Broadband HR Coatings
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 the 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. Of the metallic coatings, the Protected Silver coating has the highest reflectance in the visible spectrum to prevent oxidization, the silver surface is protected with an SiO2 overcoat. The Ultrafast-Enhanced Silver coating is manufactured such that it retains 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 loses in the MIR that are commonly found in gold mirrors. Each coating in protected by an overcoat that also makes the mirror easy to clean. The Unprotected Gold coating offers higher reflectance than the protected gold coating, but is slightly more delicate.
Narrowband Laser Line HR Coatings
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.
Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.
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) . 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:
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.
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 . 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:
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 . 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 . 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.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1998).