Opticstudio Tutorial
Darios Uclaray
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.
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.
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.
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 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 (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.
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
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.
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.
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).
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