Laser Induced Damage Threshold
The Laser Induced Damage Threshold (LIDT) of optical components is a critical quality parameter for laser systems and their applications operated at high power levels. Historically optical materials could be optimized to excellent quality as well as power handling capability, and consequently, the problem of laser induced damage shifted from the bulk to the surface of the optical component during the last decades.
During production the optical surface is objected to various influences, which modify its structure and composition. However the thin film coating system, which is deposited onto the optical surface to adapt its reflectance and transmittance to the application, contributes predominantly to the reduction of the LIDT-values. It is considered the most critical element in the development of high power laser components.
The following contains a short overview of some major aspects of laser damage in optics including a summary of the observed damage mechanisms in thin films, an introduction into LIDT measurements according to the recent ISO-Standard series, and a few remarks concerning scaling of LIDT values.
Laser damage mechanisms
Laser damage in optical coatings may be initiated and driven by a variety of interactions of matter with radiation. In many cases optical absorption processes couple thermal energy into the coating structure under irradiation and cause a sharp increase of temperature until a catastrophic failure by mechanical disruption or overheating occurs. A typical damage site attributed to such an absorption induced coating failure is depicted in Figure 1.
In this example the laser induced temperature rise was sufficient to reach the melting point of the material leading to an evaporation of coating material and a change in its crystalline structure. If laser power densities slightly exceed the damage threshold of the material, you often can observe discoloration or increased surface roughness in the center of the laser beam and no removal of material (s. Fig. 2).
Thermal expansion driven by laser induced heating may also result in a mechanical stress level exceeding the adhesion strength of the coating to the substrate. Corresponding typical damage morphologies clearly indicate a delamination of layers from the surface of the optical component in the center of the laser beam area (s. Fig. 3).
These damaging effects are results of instantaneous heating of the coating material in the area of interaction with the laser beam. This can be modeled on the basis of adapted heat diffusion equations and corresponding boundary conditions. For the temperature rise (ΔT) in the center of an irradiated circular component and a Gaussian beam profile with power (P) a first order expression can be derived:
Obviously, the temperature rise is dependent on the thermal properties (k: thermal conductivity, κ: thermal diffusivity), on the surface absorption of the component (βs), and on the beam diameter (w).
For long irradiation times tI compared to the typical heat diffusion time w2/κ equation (1) can be reduced to the following asymptotic approximation:
This asymptotic behavior applies to long irradiation times or cw-operation of the laser. In contrast to this, the short pulse regime is represented by the first equation.
For damages dominated by absorption the following can be stated:
• short pulses: LIDT ~ power density P/w2
• long irradiation times or cw: LIDT ~ linear power density P/w
In practice absorption induced damage of laser components is occasionally observed in coating materials for the DUV/VUV-wavelengths and in the MIR-range.
For the VIS- and NIR-spectral range modern optical coating production processes have been developed resulting in extremely small absorption losses. For these layer systems laser damage is dominated by defects which are embedded in the layer structure and which possess a significantly higher absorption than the surrounding layer material. Under laser irradiation, these absorbing defects heat up much faster than the surrounding material and explode removing the covering layer structure. A typical damage site of such a defect or inclusion dominated damage is characterized by small craters in the beam area (s. Fig. 5).
Since the specific optical, thermal and geometric properties of the defects are usually not known in detail, a theoretical description of defect induced damage can only provide trends in the scaling of damage with laser parameters and the material properties. Precise modelling is even more complicated by the statistical distribution of defects in size and physical properties demanding for defect characterization techniques with extremely high sensitivity down to the nm-scale on the experimental side.
Defect induced breakdown is the type of damage predominantly observed for most laser operation regimes. It is still a topic of intensiv research in laser damage as well as in the development of deposition processes with reduced defect generation.
Besides thermal effects, direct electronic excitation may be considered as driving mechanism for short pulse laser damage. These types of dielectric breakdown mechanisms moved into the focus of research after ultra-short pulse lasers with high output power could be developed. The excitation of electrons into the conduction band of a dielectric material by multiphoton processes becomes more probable at the extremely high power densities of pulses with durations in the ps- and fs-range. These electrons can contribute to a further increase of the free electron density in the conduction band via a stepwise excitation to higher energy levels and a subsequent relaxation with energy transfer to a valence band electron to perform a transition to the conduction band (Avalanche effect).
In this model, damage occurs at a critical electron density in the conduction band in the range of a few 1021 1/cm3. After reaching this state, laser radiation is instantly coupled into the free electron plasma leading to catastrophic damage of the material. During the last decade, theoretical models describing damage in the ultrashort pulse regime have been developed to a state which allows the forecast of the damage behavior as a function of the electronic properties of the involved dielectrics. In many practical applications a variety of additional influences may also impair the power handling capability of a laser component.
Weak points are improper handling and cleaning as well as contamination of the coated surfaces. Under special environmental conditions, the laser radiation may even induce the growth of contamination on the optics via transportation effects or photochemical reactions and initiate laser damage by increased surface absorption. Voids, grooves, pores, or scratches on the optical surface also reduce the damage threshold of the optical element, because they act as concentrators for the electric field.
To avoid laser induced damages:
• Use the right substrates with low absorption, low water content and good polishing if necessary.
• LASEROPTIK uses the best fitting coating materials and coating techniques (low absorption, low defects and best electronic properties) for your applications. And we produce under cleanroom conditions.
• Handle coated optics with care and in appropriate environmental conditions.
Measurement of LIDT-values
After long technical discussion, extending over more than ten years, the standard series ISO 11254 for the measurement of LIDT-values had been published in the time period from 2000 to 2006. In view of new developments in laser technology this standard was revised and is now published as a new series, ISO 21254.
Besides the measurement protocols and data evaluation procedures for damage testing with single pulse and multiple pulses described in the first three parts, this revised standard series also contains an ISO Technical Report on damage detection in modern measurement facilities. The basic arrangement of a damage test facility according to ISO 21254 is illustrated in Figure 6.
Operating as the central element in transversal and longitudinal single mode, the test laser source is used with its beam parameters precisely monitored by a beam diagnostic system. The spatial and temporal profile as well as the energy of the laser radiation in the target plane are recorded with the diagnostic system and evaluated in respect to the error budget of the measurement.
To induce damage of a component an optical imaging system concentrates the laser radiation on a sample surface within a well-defined spot of sufficiently high energy density. To tune the laser fluence in the target plane an adjustable attenuator is installed between the laser source and the imaging device. A positioning system moves the test sites on the sample surface into the test beam. Normally this process is controlled by a computer that, in modern laser damage facilities, at the same time also controls the complete measurement procedure.
In the classical so-called 1 on 1-testing procedure, each test site is exposed only one time to a single laser pulse. The complete 1 on 1-measurement protocol consists of multiple irradiation cycles with pulses of different energies, covering specific low fluence values without damage and high values always causing damage.
After the irradiation sequence, the specimen is inspected in respect to the damage state of the individual sites by Nomarski interference contrast microscopy, which is the standard routine according to ISO 21254, or any other highly resolving surface measurement technique with similar sensitivity. The resulting raw data of the test routine comprises of a set of fluence values with assigned damage states.
For the presentation of the final result, the damage probability as the ratio of the number of damaged sites to the total number of sites exposed is calculated for a representative series of fluence values. Subsequently, these damage probability values are plotted as a function of fluence representing the final result of the test. An example for such a diagram, which is often called the survival curve of the optical component, is illustrated in Figure 7.
The damage threshold is given as the highest quantity of laser radiation for which the extrapolated probability of damage is zero. Even though today 1 on 1-thresholds are mainly of academic interest and of limited importance for practical applications they are still listed in many catalogues of optics manufacturers. A more realistic scenario is simulated in S on 1-tests involving an extended irradiation of the sample with a series of pulses. The test is performed similar to the 1 on 1-protocol with the exception that each test site is exposed to a train of a defined number S of identical pulses of preselected laser fluence. In this test procedure damage can actually occur before delivery of the complete train of pulses to a site.
As a consequence, the test facility has to be equipped with an online damage detection system which ceases the laser irradiation and records the number of pulses N until damage occurred. The generated data is evaluated by deriving survival curves for preselected numbers of pulses providing the so-called characteristic damage curve as the final result of the test. An example for such a plot of the energy density values for selected damage probabilities as a function of the number of pulses is depicted in Figure 8.
An interesting practical aspect is the extrapolation of the characteristic damage curve to high pulse numbers in the order of 109 to 1012 shots which may allow for a rough estimation of the lifetime of test component.
For a certification of optical components in respect to their power handling capability, the third part of ISO 21254 describes testing protocols of a defined surface fraction at highest power levels expected in the application. The fundamental approach of these tests is an overlapping interrogation of the entire test area of the optical component, to identify weak spots on the surface and to achieve a high level of confidence for an error-free operation of the optic in the application.
Accordingly, the original component intended for direct use is probed in the certification procedure instead of representative samples, which are tested in 1 on 1- or S on 1-test protocols. LIDT is measured at the facilities of our partner Laser Zentrum Hannover e.V. (LZH).
Scaling of laser damage
For the application of laser coatings, some rough scaling rules can be derived from the developed models (see also here). The most prominent estimation for the LIDT-values is the square root rule for the pulse duration (τ):
LIDT ~ τx ; x = 0.5.
Typically this can be used in the pulse duration regime between 0.1 to 10ns. The dependency can be extended to other pulse regimes up to pulse lengths in the ms-range, if the exponent is replaced by figures ranging between 0.5 and 2. However, this scaling applies mainly for thermal damage processes and has to be applied with extreme care, particularly for extrapolations spanning more than one order of magnitude in pulse duration. Especially below values of 20 ps, a transition from thermal processes to the described electronic effects has to be accounted for, and the τx-scaling may no longer be applicable.
For most materials and under most operating conditions you can observe a decrease of the LIDT value with decreasing wavelength. Scaling with wavelength always implicates the risk that damage mechanisms may change with wavelength, especially when absorption bands are approached. Often, a delay in the range of several 10 seconds between the start of irradiation and the onset of damage has to be taken into account for thermal processes.
Concerning the variation of LIDT-values with beam diameter, numerous investigations indicate a decrease in thresholds for increasing beam diameters. Specifically for inclusion dominated breakdown, the event of damage for a certain site on the coating will be essentially dependent on the distribution of inclusions at that position. In the case of defect damage, for example, with beam sizes smaller than the mean distance between defects vulnerable to damage in the coating, the probability for interrogating a critical defect will be low, and the threshold will be high depending on the quality of the coated material. The probability of interrogating a critical defect will asymptotically reach unity with increasing beam diameter, and the LIDT will approach its smaller value related to defect damage. In most practical situations, the beam size exceed the mean distance of defects by far, and therefore, the onset of laser induced damage can be estimated as virtually independent of the beam diameter for inclusion dominated breakdown.
As mentioned above for pure absorption induced damage and with shorter pulse durations, approximation for the beam diameter dependence of LIDT on the basis of P/w2 is often acceptable. But with cw-lasers or long pulses in the µs- to ms-range, the scaling of the threshold with the beam diameter should be performed in terms of P/w according to equation 2. This P/w scaling results in lower thresholds than the scaling with intensity, which is often erroneously assumed by users, leading to a fatal overestimation of LIDT-values in these operation regimes, one example:
A mirror shows a maximum safe power of 100 W with a beam diameter of 0.5 mm. Using the power density for scaling to a beam diameter of 5 mm the safe operation power would be 10 kW. Scaling with the linear power density only 1 kW will be safe!
In summary, LIDT-values of optical components and thin films are crucially dependent on the operation conditions of the applied laser system. The mentioned change of damage effects from thermal to electronic mechanisms is only one representative example among a variety of many other observed phenomena. As a consequence, threshold values should be scaled with extreme care and for only small intervals in cases, where the fundamental damage mechanism is clarified. In all other situations, the extrapolation of threshold values is extremely unreliable and may lead to severe hazards in the laser system.