Brief of Sapphire Optical Window
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A sapphire optical window is an optical component with a double parallel plane like other traditional optical windows. It is used in optical systems to isolate two spaces while ensuring that light passes through well. A typical sapphire optical window is made of synthetic sapphire crystals and can follow different crystal orientations. Its geometry can be round, rectangular and other special types. Today, sapphire optical windows have a wide range of applications such as optical imaging systems, observation and inspection systems, and laser output systems, etc. In order to better understanding of sapphire optical windows, we will introduce the following topics:
The transparent area of the sapphire optical window is very wide. It can cover a wave band range from 200 to nm. According to the 1mm thick sapphire optical window sample test, in the range of 250nm~nm, there is more than 82% of its own transmittance.
The hardness of the sapphire optical window is extremely high, which can well resist external damage such as scratches of sharp objects, wind and sand erosion.
The mechanical strength of the sapphire optical window is also high. The drop test under the same thickness conditions showed that the test data of the sapphire optical window was much better than the optical window of other materials.
Sapphire optical windows have excellent heat resistance and can resist high-intensity hot and cold shocks without breaking.
Round Sapphire Optical Windows
Square / Rectanglular Sapphire Optical Windows
T-STYPE Sapphire Optical Windows
Sapphire Conical Optical Windows
Sapphire Wedge Optical Windows
Special Sapphire Optical Windows
High-quality sapphire optical windows have a wide range of applications. Typical applications include industrial and medical equipment, air and gas analyzers/monitors, high intensity xenon lamps, cryogenic viewport, oil pipeline monitoring, spacecraft window, etc.The transmission area of sapphire single crystal covers a wide band range of ultraviolet, visible light, near-infrared and mid-wave infrared from 200nm~nm. To select the appropriate substrate material for a sapphire optical window, it is necessary to first determine its application wave band. For example, the sapphire optical window of 254nm or 308nm application requires strict selection of ultraviolet-grade ultrapure sapphire crystals. Creator Optics uses high-purity KY, CZ sapphire monocrystal ingots as raw materials. Once a material is specified, it must have good transmittance, minimal absorption and residual internal stress. Then, according to the design requirements, the sapphire crystal is oriented cutting and the shape geometry is processed. After these processes, a qualified sapphire optical window blank is obtained. These sapphire optical window blanks can be polished to different degrees to achieve different requirements of sapphire optical windows.Creator Optics selects high-purity optical grade sapphire single crystal (Al2O3) as the windows substrates. These preferred materials are processed into the required precise external dimensional sapphire optical window blanks by laser orientation machines, CNC cutting machines, round grinding machines, multi-wire cutting machines and other equipment. They are then subjected to multi-step CMP to complete the precision polishing of the sapphire flat optical window. Now, the sapphire optical window with excellent surface quality, surface flatness and double-side parallelism characteristics has been completed. Many times, these sapphire optical windows can also be designed and coated with optical films to meet different applications and requirements.At present, Creator Optics can customize the following specifications and series of sapphire optical windows and some optical coating:All coated or uncoated sapphire optical windows supplied by Creator Optics are rigorously inspected. Certified raw material supplier, precise measurement of geometric dimensions, microscopic examination of surface quality, spectral measurement and environmental testing, etc. All of these sapphire optical windows coated with AR or uncoated have factory quality certification.
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The specifications to the right are measured data for a selection of Thorlabs' sapphire windows. Damage threshold specifications are constant for these windows, regardless of the size.
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 and ISO 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/cm( nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
Example Test Data Fluence # of Tested Locations Locations with Damage Locations Without Damage 1.50 J/cm2 10 0 10 1.75 J/cm2 10 0 10 2.00 J/cm2 10 0 10 2.25 J/cm2 10 1 9 3.00 J/cm2 10 1 9 5.00 J/cm2 10 9 1According 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.
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.
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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 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 [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.
Pulse Duration t < 10-9 s 10-9 < t < 10-7 s 10-7 < t < 10-4 s t > 10-4 s Damage Mechanism Avalanche Ionization Dielectric Breakdown Dielectric Breakdown or Thermal Thermal Relevant Damage Specification No Comparison (See Above) Pulsed Pulsed and CW CWWhen 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 [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 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 ().
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, ).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, ().
[4] N. Bloembergen, Appl. Opt. 12, 661 ().