Lasers for Material Processing
A wide range of different laser sources is used for laser material processing, which is one of the most important areas of laser applications. The involved laser processes span a wide range from low-intensity processes like soldering and hardening over welding, cutting and drilling up to very high-intensity laser ablation processes. Other important application areas are laser marking and cleaning. Very different types of lasers can be suitable, with specific advantages and limitations depending on the specific application requirements. The most important types are discussed in the following sections.
In any case, a well engineered industrial laser system is highly desirable in various practical respects, such as reliability and lifetime, quick availability of possibly needed replacement parts etc. Other practically important aspects are of course the installation cost and the running expenses (electricity consumption, gas consumption, maintenance, repairs).
Gas and Excimer Lasers
In the early days of laser technology, gas lasers where often the only sufficiently powerful laser type available. While many of them have been replaced with different kinds of solid-state lasers (see below), particularly CO2 lasers are still widely used because of their specific advantages for certain processes. That applies in particular to the long emission wavelength of typically 10.6 μm, which leads to superior absorption in various materials (e.g. polymers, wood, ceramics, but usually not in metals). Another advantage is their high output power in combination with high beam quality. Their wall-plug efficiency is usually between 10% and 20% – not as high as for the best solid-state laser systems, and far behind direct-diode lasers, but still in a quite reasonable range.
The operation mode of a CO2 laser can be continuous-wave or pulsed, although not with particularly high pulse energies. Pulsed operation is sometimes just a unavoidable feature of the laser type (transverse excited atmosphere (TEA) lasers), but not essentially used for material processing.
After decades of engineering, CO2 lasers are a highly matured technology, reaching reliable performance and long lifetimes.
Excimer lasers still play an important role in the area of ultraviolet lasers, despite their limitations in terms of wall-plug efficiency. This is mostly because they can generate very intense nanosecond pulses and appreciable average powers (up to hundreds of watts), which are hard to use with other means, for example with frequency-converted solid-state sources. Device lifetimes were originally quite short, but well-engineered excimer lasers can be quite reliable and long-lived. They exhibit some amount of gas consumption, which besides the electricity consumption adds to the operation cost.
Solid-state lasers where initially lamp-pumped. Those types have been largely replaced with diode-pumped lasers in the meantime, but are still widely used in specific areas where a high pulse energy in conjunction with a low pulse repetition rate is needed.
Diode-pumped lasers have been developed with ever-increasing performance. Some types (particularly thin disk lasers and slab lasers) now offer multi-kilowatt continuous-wave pulsed outputs with high beam quality.
While the vast majority of solid-state lasers operates in the spectral region between 1 μm and 1.1 μm, where best laser performance is possible, various other wavelengths can be reached with methods of nonlinear frequency conversion. In particular, there are many green lasers based on frequency doubling, also ultraviolet lasers based on frequency tripling and quadrupling. A wide range of other wavelengths, also with tunability during operation, can be generated with optical parametric oscillators (OPOs), usually with pulsed operation, which is simpler to realize. However, OPOs are not common in material processing.
Solid-state bulk lasers are particularly suitable for generating energetic laser pulses with the method of Q switching. This is because their gain media (mostly, laser crystals, sometimes glasses or ceramics) exhibit good energy storage and are sufficiently robust for rapid extraction of the stored energy. Pulse durations are then typically in the nanosecond regime. There are also various kinds of ultrafast lasers with picosecond or femtosecond pulse durations. Here, in addition to a mode-locked laser, one usually uses a pulse picker for obtaining an appropriately low pulse repetition rate, followed by a high-gain amplifier, e.g. a regenerative amplifier.
Fiber lasers (often in conjunction with high-power fiber amplifiers) are a specific type of solid-state laser which has become quite important. They typically feature a high wall-plug efficiency, high beam quality and (particularly for continuous-wave lasers) high output power. However, compared with bulk lasers they are much more limited in terms of pulse energy. On the other hand, in the typically used master oscillator fiber amplifier configuration, many of them exhibit a high flexibility e.g. concerning the generation of pulse trains with quite variable pulse repetition rate and also options for a burst mode.
Fiber lasers have also become more important in the area of ultrafast lasers. Some of the properties of rare-earth doped fibers are particularly suitable for this operation regime, while on the other hand fibers also introduce substantial limitations, particularly for the reachable pulse energies. Their strengths are most pronounced in areas with low pulse energy requirements, i.e., in micro-machining, while higher-energy applications are generally better served with bulk laser systems. Some laser developers, however, combine both technologies, e.g. using a mode-locked fiber laser, a low-power fiber amplifier and a high-energy bulk amplifier stage.
Direct Diode Lasers
The direct application of diode lasers (→ direct diode lasers), e.g. in the form of diode bars, diode stacks or VCSEL arrays, is very attractive in terms of laser cost and wall-plug efficiency. However, in earlier years it was often not feasible due to their poor beam quality: essentially, it was not possible to sufficiently focus the diode laser radiation to a workpiece. That has changed substantially due to various developments, in particular due to the ever increasing output power per emitter (so that fewer emitters are needed for a given total power), advances in beam shaping and the increasing use of spectral beam combining. Even applications like laser cutting and welding can sometimes be done with such lasers.
Direct diode lasers are usually based on gallium arsenide technology, which allows for emission wavelengths of typically between 0.8 μm and 1 μm, i.e., somewhat shorter than those of diode-pumped doped-insulator lasers. These wavelengths are often similarly suitable for material processing.
In earlier times, doped-insulator solid-state lasers had to be used as brightness converters: they can be optically pumped with diode lasers and emit radiation with substantially higher beam quality and thus higher radiance (brightness). While brightness optimization became somewhat less important, another function in transforming the radiation is still often essential: the ability of such solid-state lasers to generate very intense light pulses. In that area, which is often quite important for material processing, diode lasers have a fundamentally very limited potential due to their very short carrier lifetime, which leads to insufficient energy storage. Although ultrashort pulse generation is possible, it is also generally too limited for applications in material processing.
While the continuous evolution of high-power diode lasers regularly leads to performance enhancements, newer laser types are in development which may in the future deliver even better performance. In particular, there are photonic crystal surface-emitting lasers (PCSELs), which can naturally generate beams with quite high brightness. However, this technology is not yet mature, e.g. not yet reaching the high wall-plug efficiency of edge-emitting laser diodes.
Important Parameters of Laser Sources for Material Processing
- Optical wavelength: In most cases, this is important because light absorption is essential for the used processes, and the strength of absorption can critically depend on the wavelength.
- Optical power and intensity: An appropriately high optical intensity (power per unit area) is needed for many processes. The optical average power is often also very important for achieving an economically viable processing speed.
- Beam quality, quantified e.g. with the M2 factor: that determines how easily the radiation can be focused to a small spot, and how small the working distance (between the laser processing head and the beam focus) can be. Besides, it has an influence on how smooth and consistent the intensity profile is.
- Radiance (often inaccurately called brightness or brilliance): this results from the combination of power and beam quality, and determines what optical intensity level can be achieved on a given spot size and with a limited beam divergence.
- Pulse parameters: In many cases, one uses pulsed lasers, where the pulse durations can differ a lot between different applications – one uses femtosecond lasers, picosecond lasers, nanosecond lasers and in some cases free-running pulsed lasers with microsecond pulse durations. Particularly relevant parameters are the pulse energy and pulse duration, and the peak power is essentially the energy divided by the duration, but with some influence of the pulse shape. The pulse shape itself is important in some of the applications. In many cases, one applies regular pulse trains, or sometimes bursts of pulses.
Often quite irrelevant is the spectral bandwidth (linewidth) of the laser source because the absorption properties of workpieces normally do not vary significantly within the optical bandwidth of a laser, even if it is e.g. several nanometers wide.
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