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alpha-BBO Glan-Laser Polarizers
GLB10 Polarizer and
Alpha-BBO Glan-Laser Polarizers Divert Ordinary Rays, Leaving Highly Polarized Extraordinary Rays (Aligned with the "Polarization Axis" Mark on the Housing) Passing Through the Polarizer
Our Glan-Laser α-BBO Polarizers are specifically designed to deal with high-energy, short-wavelength laser light. Like our Glan-Taylor and Glan-Laser Calcite Polarizers, these polarizers are ideal for applications requiring extremely high polarization purity (100,000:1), high damage threshold (5 J/cm2, 10 ns pulse, 10 Hz, 0.347 mm Spot Size, @ 355 nm), and transmisison in the UV (210 - 450 nm). A significant amount of reflected light escapes the polarizers through the side port, including all of the ordinary ray and some of the extraordinary ray. As such, the escape beam is not fully polarized, and only the transmitted extraordinary ray should be used for applications that require a high-quality, polarized beam.
The input and output faces of these polarizers are polished to a laser quality 20-10 scratch-dig surface finish to minimize scattering of the transmitted extraordinary polarization component of the incident laser beam or light field. The ordinary polarization component is reflected and exits the polarizer at a 61° angle (wavelength dependent) through one of the two uncoated side ports, which are provided to allow bidirectional use of the polarizer. The escape ray is not fully polarized and the side escape ports have a lower surface quality of 40-20 scratch-dig. In addition, the hygroscopic nature of α-BBO may cause these uncoated side faces may become hazy over time as the crystal absorbs water from the air, further decreasing the beam quality of the escape ray.
The polarizers are available with three antireflection coatings: a single-layer MgF2 antireflection coating (SLAR-MgF2), a UV AR coating, or a V Coating optimized for 405 nm. The SLAR coating features good broadband UV performance with low reflectance (<1.5%) from 210 to 450 nm while the UV AR coating provides exceptionally low reflectance (<0.5%) from 220 to 370 nm. The 405 nm V coating provides even less reflectance (<0.25%) than the UV coating at 405 nm. These coatings also serve as a protective layer that prevents the hygroscopic α-BBO substrate from interacting with moisture in the environment. Please see the Graphs tab for more information about the reflectance of these coatings.
For compatibility with Thorlabs' selection of rotation mounts and other optomechanics, these polarizers can be mounted in our Polarizing Prism Mounts, which are available for both 5 mm (SM05PM5) and 10 mm polarizers (SM1PM10). Thorlabs also provides Glan-Taylor polarizers.
Note: Since α-BBO is a relatively soft material, care must be taken when cleaning. The coated faces of the polarizer can be gently cleaned with solvent and air. The escape faces (perpendicular to the input / exit faces) are also extremely delicate. Do not touch any faces if possible. Cleaning should be light and at a glancing angle. If these surfaces must be wiped, use only solvent-moistened cotton or untreated facial tissues.
The typical transmission and reflectivity of the α-BBO Glan-Laser polarizers are shown in the graphs below. The blue shading indicates the region for which the AR coating is optimized (see Specs tab for more information). The reflectivity plots represent the performance of the coating only, not including internal losses of the polarizer. The transmission plots include both both reflectivity and transmission through the polarizer (including any internal losses).
The plots below apply to the GLB10 and GLB5 α-BBO Glan-Laser polarizers.
The plots below apply to the GLB10-UV and GLB5-UV α-BBO Glan-Laser polarizers.
The plots below apply to the GLB10-405 and GLB5-405 α-BBO Glan-Laser polarizers.
Note: Short wavelength cutoff is due to the cut angle of the α-BBO prism, not the reflectivity or transmission of the material.
Glan Laser Specifications
Polarization-Dependent Refraction - Glan Laser α-BBO Polarizer
Our Glan-Laser and Glan-Taylor polarizer are designed as polarizer elements that remove the reflected ordinary polarization component of a beam. These polarizers are built out of two prisms, as shown in the drawing to the right. They are only designed to work with well collimated light beams; converging and diverging input beams will not exhibit proper polarization and incidence angle at the internal interface. Since α-BBO is a soft crystal that is easily damaged, all of our α-BBO polarizers are offered in metal housings. With convenient threadings and adapters, these housings can easily be mounted into our opto-mechanical products.
Field of View
Field of View Angle Orientation
Damage Threshold Data for Thorlabs' α-BBO Glan-Laser Polarizers
The specifications to the right are measured data for the coatings used in Thorlabs' α-BBO Glan-Laser polarizers.
Care should be taken to ensure that the polarizer's clear aperture is large enough for your beam, and that the polarizer is well aligned. While each polarizer is air-spaced within the clear aperture, the prisms composing the polarizer are separated by an epoxied photo-etched spacer that is not designed to withstand high laser powers. Using these parts outside of the clear aperture can result in catastrophic damage and failure.
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/DIS11254 specifications. A standard 1-on-1 testing regime is performed to test the damage threshold.
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 a set duration of time (CW) or 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) . Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions. Additionally, when pulse lengths are between 1 ns and 1 µs, LIDT can occur either because of absorption or a dielectric breakdown (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 large 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:
The power density of your beam should be calculated in terms of W/cm. The graph to the right shows why expressing the LIDT as a linear power density provides the best metric for long pulse and CW sources. 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. This calculation 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 (1997).
Polarizer Selection Guide
Thorlabs offers a diverse range of polarizers, including wire grid, film, calcite, alpha-BBO, rutile, and beamsplitting polarizers. Collectively, our line of wire grid polarizers offers coverage from the visible range to the beginning of the Far-IR range. Our nanoparticle linear film polarizers provide extinction ratios as high as 100 000:1. Alternatively, our other film polarizers offer an affordable solution for polarizing light from the visible to the Near-IR. Next, our beamsplitting polarizers allow for use of the reflected beam, as well as the more completely polarized transmitted beam. Finally, our alpha-BBO (UV), calcite (visible to Near-IR), rutile (Near-IR to Mid-IR), and yttrium orthovanadate (YVO4) (Near-IR to Mid-IR) polarizers each offer an exceptional extinction ratio of 100 000:1 within their respective wavelength ranges.
To explore the available types, wavelength ranges, extinction ratios, transmission, and available sizes for each polarizer category, click More [+] in the appropriate row below.