5 Must-Have Features in a Fused Silica Window

Silicon Dioxide – Glass – Quartz – Fused Silica

Silicon Dioxide (SiO2) is the simplest chemical composition of glass. Quartz is the most stable crystal modification at normal temperature and pressure conditions. Quartz is one of the most common minerals in the earth's crust. Glass (from “glasa”, Germanic for amber, the shiny or shimmery) also consists of silicon and oxide, but is a uniform amorphous solid material. Many glass varieties are clear and transparent respectively. This means transmissibility for the visible spectrum of light. In general, such material is associated with the term glass. Transparent materials allow light to pass through them without diffusing (scattering) the light.

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Most common types of glass

At least years ago humans learned how to lower the glass softening temperatures by adding lime and soda before heating, which resulted in a glass containing sodium and calcium oxide.

Glass – Additives and the industrial use of glass

The use of glass as one of the oldest, but also very important materials for the industry is linked with the application of additives. Chemicals like soda (sodium carbonate, Na2CO3) and in the past also potash (potassium carbonate, K2CO3), manganese oxide and metal oxides influence the properties of glass. Manufactured glass is a material formed by heating a mixture of sand, soda and lime to a high temperature and stays in a molten, liquid state. Glass can be made from pure silica, but quartz glass (also referred as quartz) has a high glass transition point at around °C, which makes it difficult to form into panes or bottles.

Quartz glass is the purest form of SiO2 and therefore the most valuable and sophisticated variety. Extremely clear glass can be used for optical fibers. Therefore synthetic quartz glass is used to transmit light across many kilometers. Many glasses block ultraviolet radiation, but only pure fused silica (only SiO2) is transparent for wavelengths < 350 nm (UV). Quartz glass also exists as an opaque variety and with different coloration to change the physical and chemical properties like transmission or absorption for specific wavelengths (filter glass). The opaque material at Heraeus, OM® 100, is also used as a heat barrier or for diffuse scattering of IR radiation.

At first glance, quartz glass appears very simple, both chemically and structurally, since it is made from a single oxide component (silicon dioxide – SiO2).

Chemical structure

Silica, as it is also known, is found throughout the earth’s crust. However, only a small fraction has sufficient purity (> 99.98 %) to be suitable as raw material for quartz glass. Sand at the beach is also SiO2, but isn’t suitable for use in the semiconductor industry.

Structure of quartz and fused silica

In the quartz glass structure all atoms are bonded to at least two others. The strength of the silicon-oxygen (Si-O) chemical bond gives quartz glass high temperature stability and chemical resistance. But the structure is also rather open with wide spaces (interstices) between the structural units. This accounts for higher gas permeability and a much lower thermal expansion coefficient for quartz glass relative to other materials.

Purity is crucial for most industrial applications and processes. Fused silica has an outstandingly high purity and therefore is indispensable in the fabrication of high-tech products.

Despite existing at very low levels, contaminants have subtle yet significant effects. Purity is mostly determined by the raw material, the manufacturing method and subsequent handling procedures. Special precautions must be taken at all stages of manufacture to maintain high purity. Additionally, Heraeus uses different purification steps to improve the quality of the quartz sand as raw material even further.

The most common impurities are metals (such as Al, Na and Fe among others), water (present as OH groups) and chlorine. These contaminants not only affect the viscosity, optical absorption and electrical properties of the quartz glass but they also influence the properties of material processed in contact with the quartz glass during the final use application.

The purities of fused quartz and fused silica are outstandingly high. Synthetic fused silica from Heraeus contains total metallic contamination below 1ppm. For fused quartz the amount is approximately 20 ppm and consists primarily of Al2O3 with much smaller amounts of alkalis, Fe2O3, TiO2, MgO and ZrO2.

OH content

In addition to metallic impurities, fused quartz and fused silica also contain water present as OH units. OH content influences the physical properties like attenuation and viscosity. Generally, high OH content means lower use temperature. Typical values are given in the table. Electrically fused quartz has the lowest hydroxyl content (< 1 – 30 ppm) since it is normally made in vacuum or a dry atmosphere. Hydroxyl content in this range is not fixed in the glass structure. It can go up or down depending on the thermal treatment and amount of moisture the quartz glass is exposed to at elevated temperature. Flame fused quartz has significantly more hydroxyl (150 – 200 ppm) since fusion occurs in a hydrogen/ oxygen flame. Due to the production method, synthetic fused silica has similar high OH contents of up to ppm.

One of the most attractive features of quartz glass is its very low thermal coefficient of expansion (CTE). The average CTE value for quartz glass at about 5.0 × 10 -7/ °C is many times lower than that of other common materials. To put this in perspective, imagine if 1 m3 blocks of stainless steel, borosilicate glass and quartz were placed in a furnace and heated by 500 °C. The volume of the stainless steel block would increase by more than 28 liters and that of the borosilicate block by 5 liters. The quartz block would expand by less than one liter. Such low expansion makes it possible for the material to withstand very severe thermal shock.

It is possible to rapidly quench thin particles of quartz glass from over °C by plunging them into cold water without breakage. However, it is important to realize that the thermal shock resistance depends on factors other than CTE such as surface condition (which defines strength) and geometry. The various types of fused silica and fused quartz have nearly identical CTE’s and thus can be joined together with no added risk of thermally induced breakage.

Mechanical properties, strength and reliability

The theoretical tensile strength of silica glass is greater than 1 million psi. Unfortunately, the strength observed in practice is always far below this value. The reason is that the practical strength of glass is extrinsically determined rather than being solely a result of an intrinsic property like density. It is the surface quality in combination with design considerations and chemical effects of the atmosphere (water vapor in particular) that ultimately determine the strength and reliability of a finished piece of quartz glass. Because of stress concentration on surface flaws, failure most always occurs in tension rather than compression.

In other words: „reliablility depends on the chance“.

This could also be stated as the probability that the piece will experience a mechanical stress greater than the strength of any existing flaws. As a result of this dependence on probability, reliability decreases as the size of the glass article increases. Similarly, if the number of pieces in service increases, so does the chance of experience a failure.

Surface condition is very important. For example, machined surfaces tend to be weaker than fire polished ones. Also, older surfaces are usually weaker than younger ones due to exposure to dust, moisture or general wear and tear. These factors have to be considered thoroughly when comparing the strengths of different “brands” of quartz glass.

This is because these tests in reality often turn out to be just comparisons of surface quality resulting from sample preparation, small differences in which easily overwhelm any differences in intrinsic strength.

Controlled heat management and sustained high temperatures is crucial in many industrial processes, especially in semiconductor industries.

Fused silica is a good electrical insulator, retaining high resistivity at elevated temperatures and excellent high-frequency characteristics. The large band gap inherent in the electronic structure of the silicon-oxygen bond results in electrical conduction being limited to current carried by mobile ionic impurities. Since the level of these impurities is very low, the electrical resistivity is correspondingly high.

Since ionic conduction is related to the diffusion coefficient of the ionic carriers, the resistivity also has a strong exponential temperature dependence. Hence, unlike typical conductors such as metals, the resistivity decreases with increasing temperature.

The dielectric constant of quartz glass has a value of about 4 which is significantly lower than that of other glasses. This value changes little over a wide range of frequencies. The reason for the low dielectric constant is, once again, the lack of highly charged mobile ions but it also results from the stiffness of the silicon-oxygen network which imparts a very low polarizability to the structure.

Because it has very low absorption down into the vacuum ultraviolet spectral range (the cutoff is at about 160 nm for a 1 mm thickness), synthetic fused silica is used for optical lenses in high-energy laser applications and as envelope material for ultraviolet light sources such as excimer or deuterium lamps. Depending on the exact experimental conditions, such as the wavelength, energy density and peak intensities in pulsed laser applications, various kinds of damage can occur to the glass.

At very high laser intensities, photoionization and plasma generation can take place locally at certain positions in the glass. This mechanical damage typically appears at the front or rear surface of the optical element. A related type of mechanical damage which can occur is the generation of fine microchannels within the glass along the propagation direction of a laser beam.

In addition to these visible damage phenomena, a more subtle damage mechanism can take place as defect centers (sometime called color centers) are generated in a photochemical process under irradiation of the glass. These centers cause absorption at characteristic wavelengths. Examples are the E’ center with an absorption maximum at 215 nm and the non-bridging oxygen hole (NBOH) center at 265 nm. The NBOH hole also emits red fluorescence at about 650 nm when excited in its absorption band. These defects also interact with dissolved hydrogen in the glass. Hydrogen can passivate E’ centers to create SiH groups, and NBOH centers to create SiOH groups, thus mitigating the transmission loss at the absorption wavelengths of these defects. Therefore, the hydrogen concentration is often controlled precisely during production processes, and measured by Raman spectroscopy in the analytical lab.

A third type of damage which can take place is a change in the refractive index of the silica glass due to restructuring of the glass network under irradiation. The refractive index can either increase (called compaction), or it can decrease (rarefaction), depending on the silica glass type and the irradiation conditions.

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  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/cm2 ( 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 1

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.

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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:

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 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.

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 CW

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 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 ().

In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the LIDT Calculator button. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.

Figure 71A  A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.

CW Laser Example
Suppose that a CW laser system at nm produces a 0.5 W Gaussian beam that has a 1/e2 diameter of 10 mm. A naive calculation of the average linear power density of this beam would yield a value of 0.5 W/cm, given by the total power divided by the beam diameter:

However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in Figure 71A. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.

An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:

The adjusted LIDT value of 350 W/cm x ( nm / nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.

Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
Suppose that a pulsed Nd:YAG laser system is frequency tripled to produce a 10 Hz output, consisting of 2 ns output pulses at 355 nm, each with 1 J of energy, in a Gaussian beam with a 1.9 cm beam diameter (1/e2). The average energy density of each pulse is found by dividing the pulse energy by the beam area:

As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.

The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:

This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.

Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
Suppose that a pulsed laser system emits 10 ns pulses at 2.5 Hz, each with 100 mJ of energy at nm in a 16 mm diameter beam (1/e2) that must be attenuated with a neutral density filter. For a Gaussian output, these specifications result in a maximum energy density of 0.1 J/cm2. The damage threshold of an NDUV10A Ø25 mm, OD 1.0, reflective neutral density filter is 0.05 J/cm2 for 10 ns pulses at 355 nm, while the damage threshold of the similar NE10A absorptive filter is 10 J/cm2 for 10 ns pulses at 532 nm. As described on the previous tab, the LIDT value of an optic scales with the square root of the wavelength in the nanosecond pulse regime:

This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.

Pulsed Microsecond Laser Example
Consider a laser system that produces 1 µs pulses, each containing 150 µJ of energy at a repetition rate of 50 kHz, resulting in a relatively high duty cycle of 5%. This system falls somewhere between the regimes of CW and pulsed laser induced damage, and could potentially damage an optic by mechanisms associated with either regime. As a result, both CW and pulsed LIDT values must be compared to the properties of the laser system to ensure safe operation.

If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.

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