Laser Induced Damage Threshold Tutorial
Introduction:
Laser Induced Damage Threshold, abbreviated as LIDT, is a measure of the optic’s capabilities to handle high and intense power. As defined in ISO 21254, the laser-induced damage threshold is the highest fluence or power density of laser radiation incident upon the optical component before any unacceptable damage occurs, quantified in J/cm2 for pulsed laser, or W/cm2 for Continuous Wave (CW) lasers, given the wavelength and fluence (energy/area) or intensity (power P/effective beam area) of your lasers. Essentially, the LIDT defines the point at which the laser's energy causes the material to break down or suffer damage, which can change depending on the laser's characteristics and the material's properties. When using optics that do not meet the necessary LIDT for your laser systems, the consequences can be severe. The component might degrade or break down, reducing the effectiveness of the system and even causing permanent damage. The LIDT of an optical element sets the limit on the maximum power it can handle without failing.
Laser-induced damage thresholds are often applied in the application context of laser optics (e.g. laser mirrors, laser lenses). Lasers, particularly high-power lasers, often have very high optical intensities. This is because laser beams are usually focused into small diameters or emitted in ultra-short pulses. As a result, optical components used with lasers must have a high LIDT. In some circumstances, other high power optical sources other than lasers also require a high laser-induced damage threshold.
Optics, especially those designed for lasers are often specified with a certain value of laser-induced damage threshold when offered by suppliers. But how to use the given, specified LIDT to evaluate the potential of causing actual damage in your application scenario is the critical problem. Therefore, building a clear concept and having an in-depth understanding of laser-induced damage threshold (LIDT) is essential. This article offers a basic tutorial and explanation of the laser damage threshold, including:
- The definition of LIDT
- The test method of LIDT
- Laser induced damage threshold for CW laser and the calculation formula of LIDT
- Laser induced damage threshold for pulsed laser and the calculation formula of LIDT
- How to scale the LIDT
- Different mechanisms of laser damage
- The factors and causes of laser damage
- High power laser optics and coatings
The Testing Method of Laser Induced Damage Threshold
The most common method of determining the LIDT of optical components is destructive testing. In this test, a laser with a known power level is directed onto a sample optical component. The energy of the laser is gradually increased until visible damage appears. At that point, the power density, wavelength, and other factors such as pulse duration and pulse repetition rate (for pulsed lasers) are recorded. This data establishes the LIDT for that particular component.
An alternative method involves testing optical coatings by exposing them to specific conditions, either as dictated by the manufacturer or based on customer requirements. The optical coatings' performance is then observed under these conditions to determine whether the optics are eligible.
Optical Intensity Profile:
The Optical Intensity Profile refers to the distribution of light intensities across the cross-section of the laser beam. The laser beam profile significantly influences the uniformity of the laser intensities. Two common types of laser beams are Gaussian beams and top-hat beams.
A Gaussian beam has its intensity highest at the center and decreases gradually towards the edges. The peak power intensity is typically twice as high as that of a top-hat beam under the same power conditions.
A top-hat beam, or a flat top beam has a uniform intensity profile across the beam diameter. The flat-top beam profile is less common but can be achieved with specialized optics or beam-shaping techniques. It is particularly useful when uniform energy distribution is required across a specific area, such as in certain manufacturing processes.
Figure 1. Optical Intensity Profile of Flat-Top Beam and Gaussian Beam
The effective diameter of a laser beam is an important factor in determining the laser damage threshold. For Gaussian beams, the effective diameter increases with the laser fluence, which can have an impact on the ability of the optical component to withstand laser damage. For example, as the fluence increases, the risk of damage due to intensity spikes also increases.
Types of Laser Induced Damage Threshold Based on Laser Regime
The principle based on which the laser causes damage is in general fundamentally different for CW lasers and pulsed lasers.
LIDT for Continuous Wave (CW) Lasers
A continuous wave laser, or CW laser delivers a continuous stream of light. The primary damage caused by CW lasers is thermal in nature, as the laser energies are absorbed by the material and transferred as heat these continuous energies are absorbed and can’t be dissipated quickly, causing the temperature of the material to rise. This can cause overheating, chemical degradation, and melting of optical coatings. Chemical degradation weakens the material's structure or affects its optical properties, such as causing discoloration or contamination of the surface. And melting takes place when the CW laser heats the coating beyond its melting point, the coating can begin to melt or vaporize, which can permanently damage the optical component. This damage can also cause the laser light to scatter or refract in undesirable ways, reducing the quality of the beam or image.
When choosing optical components for CW lasers, it is essential to compare the optic’s specified laser induce damage threshold with your laser's linear power density. This requires knowing:
1. Your laser’s wavelength
2. Beam diameter
3. Optical intensity profile (e.g. flat top or Guassian.)
Then, the linear power density P can be calculated using the formula below:
P=Power/Area=Power/πr^2
Where r is the effective beam diameter of your laser.
Note that for a top-hat beam, this formula can be directly applied. However, for a Gaussian beam, the intensity at the center will need to be multiplied by a factor of two due to the higher peak intensity.
Then you could compare your laser’s maximum power density to the given LIDT of the optics. For example, a 10mW, 800nm Ti: sapphire laser source with a flat top beam profile is used with a beam diameter of 0.5mm, then the power density would be:
P=Power/Area=Power/πr^2=10mW/π(0.5mm)^2=1.2739W/cm^2
The laser source should incorporate optics with a laser damage threshold of more than this 1.2739W/cm^2 value to prevent potential damage.
LIDT for Pulsed Lasers
Pulsed lasers, on the other hand, emit intense light in discrete bursts. There are several important parameters for pulsed lasers, average power is the total amount of energy the laser delivers over a period of time, energy per pulse is the amount of energy delivered in a single laser pulse, typically measured in Joule (J), the repetition rate refers to how often the laser pulses are emitted, typically measured in hertz (Hz), which indicates the number of pulses per second.
Figure 2. Pulsed Lasers
The relationship between average power, pulse energy, and repetition rate can be calculated using the following formula:
Average Power=Pulse Energy x Repetition rate
Therefore the pulse energy increases if the average power increases and decreases as the repetition rate increases.
The main damage mechanism for pulsed lasers is dielectric breakdown, which occurs when the intensity of the laser pulse is strong enough to ionize the material, overcoming its insulating properties. When the laser energies are concentrated in a very short time (typically femtoseconds to nanoseconds), the electric field inside the material becomes extremely high. If this electric field exceeds the material’s ability to resist the electric stress, it causes the electrons in the material to be stripped away, leading to a plasma (a state where electrons and ions are separated). This breakdown of the dielectric material’s insulation can lead to permanent damage, such as cracking, chipping, or even complete material failure.
The pulse duration of a laser has a strong impact on the type of damage it can cause. For instance, Pulse durations between 10^-9s and 10^-7s are typically associated with dielectric breakdown. Pulse durations between 10^-9s and 10^-12s can cause nonlinear effects like multiphoton absorption where the material absorbs more than one photon at the same time, leading to photoionization of atoms in the material. For longer pulse durations (10^-7s to 10^-4s), thermal effects become more prominent, often mixing with dielectric breakdown.
For pulsed lasers, the LIDT is typically measured influence, which is the energy per unit area (J/cm²). The fluence can be calculated when given:
1. Your laser’s wavelength
2. Beam diameter
3. Pulse duration
4. Pulse repetition rate
5. Pulse Energy
6. Optical intensity profile (e.g. flat top or Guassian.)
The formula for calculation of the fluence of pulsed laser is stated below:
Fluence= Pulse Energy/Beam Area=Pulse Energy/πr^2, where r is the beam diameter of your pulsed laser.
This formula also assumes a uniform optical intensity profile, for a Gaussian beam, the user needs to adjust the calculation results.
The tricky part is that laser damage from pulsed lasers can be either deterministic or non-deterministic (uncertain). For example, in the case of femtosecond pulses, the laser-induced damage is generally quite deterministic, meaning once the damage threshold is reached, damage will reliably occur. However, for nanoseconds or longer pulses, damage is often observed to be stochastic, meaning the probability of damage increases gradually as the energy approaches the damage threshold. In the latter case, a larger safety margin is required to ensure damage is avoided.
One also needs to be aware that pulse repetition frequency (PRF) can influence the laser-induced damage threshold of optical components used in pulsed lasers. At high PRFs, the behaviors of pulse lasers can become comparable to that of a CW laser. Therefore for a pulsed laser with high repetition frequency, its average power and peak powers must be compared to the equivalent CW power. Specifically, this similarity between high RPF laser and CW laser is mainly manifested in thermal effects, especially related to factors such as laser absorption and thermal diffusivity (i.e., the speed at which heat propagates through the material). Currently, there is no completely reliable method to accurately determine whether high-PRF lasers will cause damage to optical components due to thermal effects.
Another issue is that even though the laser damage threshold for pulsed lasers is often quantified in J/cm^2, independent of the spot size, larger beam sizes can illuminate a larger area of the optical surface, which increases the chance of encountering defects (like scratches, inclusions, or other imperfections) that naturally exist on the material surface. Defects act as weak points where damage can begin to occur at lower energy levels than the ideal LIDT for the material. For example, If you have a laser beam with a spot size of 1 cm^2 and the optic’s LIDT is 10 J/cm^2, it will withstand up to 10 joules of energy in that area without damage. If you increase the spot size to 10 cm^2, the optics still can handle 10 J/cm^2, but now a much larger area is illuminated. If this larger area contains more defects, the actual threshold of damage could occur at a lower energy level, because defects are prone to damage.
Laser Damage Mechanisms
The causes and mechanisms of laser damage are manifold. The manner in which laser light interacts with optics varies as the laser regimes and pulse lengths differ. The laser damage mechanisms can be classified into:
1. Pulsed Laser-Pulse Duration <10^-9s-femtosecond or picosecond range-Nonlinear Effects, Avalanche Ionization
Avalanche Ionization is a process where free electrons gain enough energy from the laser pulse to ionize atoms in the material. These freed electrons can then ionize more atoms, creating a cascade or avalanche of ionization.
2. Pulsed Laser-10^-9s<Pulse Duration <10^-7s--Nanosecond (ns) to sub-nanosecond range-Dielectric Breakdown
Dielectric breakdown is when the electric field becomes strong enough to pull electrons from atoms, creating a plasma channel and leading to localized breakdown of the material. This often results in permanent damage like surface ablation or the creation of cracks in optical materials.
3. Pulsed Laser-10^-7s<Pulse Duration <10^-4s-Microseconds (µs) to millisecond (ms) range-Dielectric Breakdown+Thermal Effects
The combination of dielectric breakdown and thermal damage leads to a more gradual and deeper penetration of the damage into the material, compared to the abrupt damage caused by shorter pulses.
4. Continuous Wave Laser-Continuous, or very long duration (constant laser emission)-Thermal Effects
The laser beam continuously deposits energy into the optical components, causing it to heat up. If the material can't dissipate heat fast enough, this will lead to thermal expansion, stress, and potentially melting, cracking, or burning of the material. The temperature rise is not as sudden as with pulsed lasers, but over time, the accumulated heat can cause material degradation. The heat can also lead to a change in the refractive index of the material, creating thermal lensing.
Scaling Laser Induced Damage Threshold (LIDT)
The LIDT given might have a different wavelength or pulse duration from your laser, in such cases, you need to scale the specified laser-induced damage threshold so that it complies with your laser wavelength and pulse duration. Regarding CW lasers and pulsed lasers, the scaling calculations are different.
For continuous wave lasers, the wavelength is linearly proportional to the LIDT. To estimate the LIDT at a specific wavelength, the scaled LIDT for CW laser can be calculated using:
Scaled LIDT=Specified LIDT x (Your Laser Wavelength/LIDT Wavelength)
Note that the scaling calculation methods of LIDT given above are all rough rule-of-thumb estimations and the results gained should not be considered the absolute values. For example, for CW laser optical components of different wavelengths, different substrates and coating materials may be used, which will have a greater impact on LIDT.
For pulsed lasers, the LIDT is proportional to the square root of wavelength. As the wavelength gets longer, the damage threshold gets greater. For pulsed laser, the adjusted LIDT can be calculated as:
Scaled LIDT=Specified LIDT x (Your Laser Wavelength/LIDT Wavelength)^1/2
The laser damage threshold is also proportional to the square root of pulse length, as:
Scaled LIDT=Specified LIDT x (Your Laser Wavelength/LIDT Wavelength)^1/2
The calculations are also estimations, and are only suitable for pulse durations between 10^-9s and 10-7s, for longer pulses, you must also consider the continuous wave damage threshold.
Possible Reasons Affecting the Laser Damage Threshold
1. Coating may reduce the laser damage threshold. For example, the dielectric film of a dispersive mirror may greatly reduce the laser damage threshold. The inter-layer structure of the coating will also affect the laser damage threshold.
2. Cemented optics using glue may have a lower damage threshold. Compared with optics of cemented construction, optics with adhesive-free construction will have a higher laser damage threshold. For example, a zero order waveplate consists of two plates with their optical axis crossed, an optically contacted zero order waveplate where the two plates are joined via optical bonding is much more durable to laser damage than a cemented zero order waveplate where the two plates are joined using adhesives.
3. Surface treatment. The surface damage threshold of optical components after processing is often not as good as that of bulk material. This is because the optical surface often has more defects and impurities at the microscopic level. For example, the residue after the polishing process will have a negative impact on the damage threshold. High-precision surfaces usually have higher laser damage thresholds. This also requires manufacturers to maintain a clean environment when producing optical components and clean optical components.
4. Other reasons: For example, the plastic mounts with low damage threshold melt under exposure to lasers and deposits on the optical components, causing laser damage.
High Power Laser Optics and High Power Optical Coating
There’s no definite standard for high-power lasers, but it is generally agreed that high power laser refers to lasers with average power levels higher than hundreds of watts. Regarding pulsed lasers with ultrashort pulse duration, enormous pulse peak power of billions of watts can be generated.
High power lasers optics, like high power deep UV (193nm-248nm), vacuum UV (150nm-200nm) optics find versatile usage in applications like LASIK, KrF/ArF excimer lasers, semi-conductor chip manufacturing and astronomical observations.
Optical coating, which is deposited on the optical substrate is an important determinant to the damage threshold of the optics. Failures of optical coatings due to laser damage are manifold, such as absorption sites on coating, which are gross defects that absorb laser energies and lead to localized melting or thermal stress factors, or plasma burn. Production and design of high power optical coatings is a precise and challenging technique that requires a combination of proper material selection, coating structure design, and meticulous control of optical substrate preparation and environmental cleanliness in the coating process. Click here to learn more about optical coating.
Hangzhou Shalom EO offers high-power optical coatings with enhanced laser induced damage threshold, the coatings are tailored and optimized for high-power lasers. The high-power optical coatings are coated using our ion-assisted deposition (IAD) e-beam techniques, in our in-house class 1000 clean room coating workshop, leveraging top-class facilities like SHINCRON MIC-1350TBN optical coating machine, PerkinElmer Lambda 1050+ spectrometer, Ultrafast Innovations GOBI white light interferometer. Our diverse product range runs from high-power laser mirror coating (LIDT>20J/cm2@1064nm, 10ns, 10Hz pulses), high-power AR coating, high-power beamsplitter coating, etc. Contact us to learn more.
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