High-energy lasers are pulsed lasers which emit light pulses with relatively high pulse energy. There is no universal definition of how high the pulse energy must be, but usually one compares with ordinary Q-switched lasers, which are mostly solid-state lasers, and considers pulse energies of e.g. 100 mJ or more as high. However, some devices (often not simple lasers, but amplified sources) emit far higher energies of many joules, multiple kilojoules or even megajoules.
In conjunction with nanosecond pulse durations, high pulse energies imply rather high optical peak powers; for example, even just 1 joule delivered within 10 ns implies a peak power of the order of 1 J / 10 ns = 100 MW. However, some high-energy lasers emit relatively long pulses with correspondingly lower peak powers (see below).
The pulse repetition rates of high-energy lasers are usually relatively low, since that way one can achieve the highest pulse energies, and many applications do not require or cannot utilize high pulse repetition rates.
Types of High-energy Lasers
Optical pumping can in principle be done with laser diodes (→ diode-pumped lasers), but this is often not practical for very high pulse energies, since the required pumping energy needs to be supplied within a time which is roughly limited by the upper-state lifetime of the laser gain medium; that is most often between some hundreds of microseconds and a couple of milliseconds. For example, a pulse energy of 1 J may require a pumping energy around 2 J, and if that should be supplied within ≈100 μs, that translates into a required pump power of 2 J / 100 μs = 20 kW. Although laser diodes can be used in quasi-continuous-wave operation for pulse pumping with increased power (beyond what the diodes could deliver in continuous-wave), that increase is usually quite limited. The higher required pump power then translates into a high cost for laser diodes, and also it is difficult to combine the radiation from many laser diodes into the laser gain medium. For those reasons, one often uses lamp-pumped lasers with flash lamps (pulsed gas discharge lamps), which can provide a high pulse energy at quite moderate cost, although that approach leads to a quite low power conversion efficiency of the laser, mostly due to the undirected and broadband lamp emission.
Q switching is the most frequently used technique for pulse generation, and it typically leads to nanosecond pulse durations. However, some high-energy lasers are operated in free-running mode, i.e., without a Q-switch in the laser resonator. This leads to substantially longer pulses with durations of the order of the pump pulse duration; the latter may be determined, for example, by the used flash lamp in conjunction with the driver electronics. The laser output peak power is correspondingly lower.
Amplified Sources (MOPAs)
For further increasing the pulse energy, one often uses an optical amplifier – usually based on one or several more laser crystals or laser glasses which receive even higher pump energies. That approach leads to a device called a master oscillator power amplifier (MOPA), which however is often (somewhat imprecisely) called a laser.
There are also ultrafast laser sources with picosecond or femtosecond pulse durations and relatively high output pulse energies. These are basically always amplified sources, since they originally generate low-energy pulses by mode locking of a laser. A pulse picker and an amplifier system with a very high gain of many dozens of decibels is then required. Usually, one uses a regenerative amplifier and possibly also a booster amplifier of multipass type.
For obtaining highest pulse energies, one sometimes applies techniques of beam combining:
- Spectral beam combining works with multiple laser sources of somewhat different optical wavelength. One may, for example, use a diffraction grating (or some other type of diffractive optics). Unfortunately, many high-energy lasers cannot be easily realized with different output wavelengths, since their gain media have a rather small emission bandwidth. A notable exception are fiber lasers, which however are far more limited in output peak power than solid-state bulk lasers.
- Coherent beam combining requires mutual coherence of the optical fields of multiple lasers. This is also not easy to obtain from high-energy lasers; it generally requires diffraction-limited beams, i.e., with optimal beam quality.
An exotic kind of high-energy lasers are chemical lasers, where the required pumping energy is supplied by a chemical reaction. Such devices can generate pulses with extremely high energies, sometimes several megajoules. They have been developed for certain military applications, but have not become very common.
Free-electron lasers can reach rather high pulse energies, and this in a wide range of spectral regions. However they tend to be rather large and heavy, and are thus basically usable only as stationary devices, or possibly on ships.
Nonlinear Frequency Conversion
In cases, the light pulses are required in a wavelength regime which is not directly accessible with high-energy lasers. Therefore, one sometimes needs to employ methods of nonlinear frequency conversion. For example ultraviolet light is obtained by frequency doubling and possibly sum frequency generation. Long wavelengths are possible with optical parametric oscillators.
Further Technical Details
As already explained above, high-energy lasers generally require relatively high pump powers. However, one usually uses pulsed pumping rather than continuous pumping, since that allows for higher pump powers, and the pulse repetition rate is often far below the inverse upper-state lifetime of the laser gain medium. In that regime, continuous pumping would not make sense, partly because most of the pump power would be lost through radiated fluorescence light.
Due to the often low pulse repetition rate, the average pump power may not be that high. Therefore, the low power conversion efficiency resulting from lamp pumping can often be well tolerated.
Intense pumping causes substantial heating of the laser gain medium, which may not only lead to destruction (e.g. thermally induced crystal fracture), but can also have detrimental effects on laser operation, e.g. through thermal lensing which affects the beam quality and thus the usefulness of the generated laser light. Although the average pump power may not be that high for a low pulse repetition rate, there can be substantial heating within each pump pulse, making it hard to avoid substantial thermal effects. Note also that the pumping time is generally too short to effectively remove a lot of heat from the gain medium by heat conduction; therefore, the generated waste heat will be largely stored in the gain medium during that time, leading to a corresponding temperature increase. In order to limit that increase, one requires a large enough mass of the utilized gain medium. However, the possible energy density may anyway be limited by the possible doping concentration of the gain medium.
Due to the strong thermal effects, it is often not possible to realize a single transverse mode operation, as would be required for optimum beam quality. Rather, the laser resonator is then intentionally designed for relatively small mode areas (well below the beam area in the gain medium), so that strongly multimode laser operation (with many higher-order transfers modes) will result. That way, one obtains substantially wider stability zones of the laser resonator and also a substantially lower alignment sensitivity. However, the poor beam quality will then make it relatively difficult to tightly focus the obtained laser radiation – and focusing is required for many applications.
Very high pulse energies can lead to laser-induced damage of optical components, particularly of those within the laser resonator – particularly in cases where the pulse duration is relatively short and the peak power correspondingly high. Therefore, the laser resonator must be designed such that on no optical component the beam radius will be too small. Unfortunately, a large beam radius in a multimode laser may still lead to “hot spots” in the beam profile with rather high optical intensities; the obtained peak intensity can therefore be substantially higher than the peak power divided by the beam area.
Optical components with a higher laser damage threshold are generally required. This applies, for example, to the laser mirrors, but also to dielectric coatings on laser crystals, to Q switches and to any other components used there.
Pulse durations as short as possible are often desired, basically because they lead to higher peak powers. However, it is more challenging to achieve that specifically for high-energy lasers – for various reasons:
- The high intracavity peak powers increase the risk of optical damage.
- For a Q-switched laser, the round-trip time of the laser resonator should be kept short for achieving short pulses. However, lamp-pumped lasers (or lasers pumped with a large number of laser diodes) require a rather long gain medium and therefore long laser resonators.
- Short pulses require relatively fast operation of the Q-switch. If that is an acousto-optic modulator, that is possible only for relatively tight focusing of the laser radiation in the device, because the utilized sound wave has a limited velocity in the material. That, however, leads to problems with laser-induced damage. In case of an electro-optic modulator (Pockels cell), a problem is the large electrical capacitance which usually results from a large crystal size.
The highest pulse energies are achieved without any Q-switch, but then the pulse durations are far longer.
Typical Applications of High-energy Lasers
In the following, some typical applications are briefly described.
Laser weapons need to deliver substantial amounts of energy in a single pulse to some kind of target, e.g. a flying rocket, a fighter airplane or a tank on the ground.
Due to the generally rather long propagation distance of the laser light in the atmosphere, tight focusing to a target is not possible; therefore, sufficiently high intensities (causing substantial damage) are only possible for correspondingly very high pulse energies of e.g. many kilojoules or even several megajoules. Still, a reasonably high beam quality is important.
Both the required laser apparatus and the beam delivery optics (a kind of telescope) are usually rather large and expensive. Nevertheless, basically all those systems need to be transportable in some way, sometimes even on an airplane.
Extremely high optical intensities are required to drive nuclear fusion, where they create a combination of extremely high temperature and pressure for a short moment (inertial confinement fusion). Huge stationary devices have been constructed for experimental tests of that principle. Most notably, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in Livermore, California, has been constructed since 1997, and after many years of development it succeeded in generating megajoule fusion yields by applying multi-megajoule laser pulses with few-picosecond durations. It is based on huge amplifier chains, and laser beams from a large number of such sources are concentrated on a single small target containing the nuclear fuel. The main purpose is research on nuclear weapons, although the conceivable possibility of future nuclear fusion reactors based on laser ignition is another motivation.
Laser Material Processing
Some methods of laser material processing require relatively high pulse energies, e.g. of multiple joules. The pulse durations are often in the nanosecond regimes, or sometimes much longer. Different wavelengths are applied, e.g. around 1 μm (near infrared), 0.5 μm (green) or in the UV.
High pulse energies may be required for different reasons. For example, one may need to ablate a substantial amount of material with a single laser shot, if the target is moving too fast to be hit several times. A combination of high pulse energy and high repetition rate (i.e., a high average power) is often required for reaching sufficiently high processing speeds.
Various methods of remote sensing can be applied over relatively large distances provided that high enough pulse energies are available. Examples are chemical analysis based on laser-induced breakdown spectroscopy (LIBS) and various LIDAR methods.
Certain medical treatments, e.g. in determatology, require laser pulses with relatively high energy. In that way, the radiation can be applied to relatively large areas e.g. on the skin, and reasonable processing speeds are achieved.
Various areas of physics require laser pulses of rather high energy. The requirements concerning not only the pulse energy, but also pulse duration, wavelength and other parameters differ very much between different applications. Some examples:
- The generation of hard ultraviolet light or X-rays by high harmonic generation requires femtosecond pulses with extremely high power and high beam quality.
- Other applications in high field physics have similar requirements with peak powers reaching multiple terawatts or even petawatts.
- Chemical analysis based on laser-induced breakdown spectroscopy (LIBS) can be done on a relatively large distance, if a sufficiently high pulse energy is available.
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