Figure 1: Laser-induced damage mechanisms that differ significantly on pulse duration scales. Longer pulses, including those with nanosecond durations, cause damage primarily through thermal effects. As the pulse duration shortens to the femtosecond time scale, carrier absorption and nonlinear effects become the primary damage mechanisms.
As laser technology continues to evolve, so must the optics to meet the demanding specifications required for high precision applications. The power of ultrafast lasers has revolutionized medical procedures, micromachining, basic science research, and many other fields. For industries and applications previously dominated by nanosecond lasers, the adoption of ultrafast lasers presents a number of challenges, including significantly different laser damage thresholds for optical components. To ensure the efficiency and longevity of laser systems, it is critical to understand the differences in laser damage thresholds over nanosecond and femtosecond pulse durations and the reasons for them.
The laser damage threshold (LDT), sometimes referred to as the laser-induced damage threshold (LIDT), is a key parameter to be evaluated when selecting optics for any laser system.ISO 21254 defines the LDT as "the maximum amount of laser radiation incident on an optical element which is presumed to have a probability of damaging the element of zero...". This definition seems simple enough, but the actual LDT value depends on a variety of factors other than the nature of the optical element itself. In particular, the LDT of an optical element may vary by several orders of magnitude when evaluated at nanosecond (10-9s) versus femtosecond (10-15s) pulse durations. This large difference stems from the very different laser damage mechanisms that occur on these different time scales (see Figure 1).
Nanosecond laser damage mechanisms
In contrast to femtosecond pulses, the long pulses of nanosecond lasers cause damage to optical components primarily through thermal mechanisms. The laser deposits a large amount of energy into the material of the optical element, which triggers localized heating within the site of laser incidence. This heating may lead directly to melting, or it may cause some structural changes through thermal expansion and resulting mechanical stress. This stress may go on to cause cracking or even lead to complete separation of the coating from the substrate.
In addition to the direct heating of the coating material, optics under nanosecond laser irradiation are particularly sensitive to defects within the coating. These defects act like small lightning rods within the optical coating, as they have a much higher absorption rate than their surroundings. As a result, these defective regions heat up much more quickly, and in the event of catastrophic laser damage, these defects can explode out of the coating. This drastic damage mechanism typically leaves craters on the surface of the optics, as well as some particulate matter that redeposits on the surface immediately after the damage event (see Figure 2).
Figure 2: Laser damage produced by a 532nm nanosecond pulsed laser. This damage was caused by a defect within the coating of the optical element, resulting in craters and redeposited particulate matter on the surface of the element.
Because these defect sites initiate laser damage, the higher the presence of defects, the lower the LDT is typically for a given optical element. Therefore, for optics used with nanosecond lasers, the focus is on the surface quality of the optics. Moreover, LDT testing on the nanosecond time scale is a highly statistical process. The probability of damage at any given location on an optical surface is due to many related factors, including the size of the incident beam, the distribution and density of defect locations, and inherent material properties. These multiple influences also explain why nanosecond LDT values can vary significantly between batches of the same coating.LDT can be affected by inconsistencies in substrate polishing and preparation, fluctuations in the actual coating deposition process, and even changes in post-coating storage conditions.
The various influences on nanosecond LDT contrast with the main mechanisms responsible for femtosecond laser damage, which is primarily related to the coating material applied.
Femtosecond Laser Damage Mechanisms
Ultrafast pulses of femtosecond lasers cause damage through different mechanisms, in part because of the very high peak power they produce. Even though nanosecond and femtosecond lasers have the same pulse energy, the peak power of a femtosecond laser pulse can be about a million times higher than that of a nanosecond laser due to the shorter pulse duration of a femtosecond laser. These high-intensity laser pulses are capable of directly exciting electrons from the valence band to the conduction band. Even if the photon energy of the incident laser pulse is lower than this jump (known as the material band gap), the peak fluence of the ultrafast laser pulse is so high that electrons can absorb more than one photon at a time. This nonlinear mechanism is known as multiphoton ionization and is a common damage pathway in ultrafast laser optics.
Tunneling ionization may also be a damage pathway in femtosecond laser irradiation. This phenomenon occurs when the ultrafast laser pulse generates a very strong electric field that is so strong that the incident electric field actually distorts the energy in the conduction band, which allows electrons to tunnel through the valence band. Once enough electrons have been excited into the conduction band, the incident radiation begins to couple the energy directly into the free electrons, resulting in the breakdown of the coating material.
Due to these damage pathways, femtosecond LDT is more deterministic than nanosecond LDT. Laser damage is essentially "turned on" at a certain input fluence of the femtosecond laser, which is proportional to the bandgap of the coated dielectric coating material. This contrasts with the probabilistic nature of nanosecond laser damage (see Figure 3).
Image Figure 3: LDT test results obtained at 4ns (left) and 48fs (right) pulse conditions. The flat slope of the nanosecond damage curve reflects the probabilistic nature of the measurements, while the sharp shift towards 100% damage probability reflects the deterministic mechanism of femtosecond laser damage.
In contrast to the nanosecond laser damage pathway, it is important to note that thermal effects do not affect the LDT of an optical element on the femtosecond time scale.This is because the duration of an ultrafast laser pulse is, in fact, faster than the time scale of thermal diffusion within the material structure. As a result, femtosecond pulses do not deposit energy as heat into the coating material, and therefore do not generate thermal expansion and mechanical stress as nanosecond laser pulses do. For these exact reasons, ultrafast lasers are extremely advantageous in many applications that require high-precision cutting and marking, such as in the manufacture of cardiovascular stents.
Choosing the right optics
Like their pulse durations, the typical LDT values for nanosecond and femtosecond pulses can differ by several orders of magnitude. When measured with a 100 fs pulse, the LDT value of an ordinary laser mirror may be about 0.2 J/cm2; however, when measured with a 5 ns pulse, the optic's LDT may be closer to 10 J/cm2 These different values may be worrisome at first, but they are merely indicative of the very different mechanisms of damage on these time scales.
For the same reason, extra care should be taken when using LDT calculators on large time scales. In general, the LDT gets larger as the pulse duration increases. But adjusting the LDT value from adapted femtosecond pulses to adapted nanosecond pulses, or from adapted nanosecond pulses to adapted femtosecond pulses, is likely to result in damage to the optics. It is best practice to select an optic with an appropriate LDT rating that is obtained as close as possible to your actual application conditions (including wavelength, repetition frequency, and pulse duration).
Summary
Laser technology will continue to evolve to meet the need for greater precision. As these new technologies take shape, understanding the differences in laser damage mechanisms (and which damage dominates on a given time scale) will become increasingly important in selecting the right optics for real-world applications. Understanding these differences will not only improve the efficiency and lifetime of in-use laser systems, but will also allow for seamless adaptation to more advanced laser systems of the future.
