POWER LASERS
8.1 CHARACTERISTICS
Power lasers can be categorized by wavelength, power, type, material (gas, solid state, liquid), fiber, semiconductor, pumping technology, continuous wave, or pulse length.
Selecting a laser for an application must also include weight, cost, beam quality, and efficiency.
8.1.1 Wavelength
Wavelength is one of the most important characteristics. For bound electron lasers, wavelength depends on the material bandgap; only a few easy-to-work-with materials will produce high power economically. According to diffraction (Chapter 3), light of a certain wavelength is relatively unaffected by nonabsorbing particles having a size less than the wavelength while objects larger than the wavelength will be scattered or absorbed. This effect can be observed when waves strike large versus small rocks close to a beach. For some substances where the wavelength is similar to the molecule size, the particle will resonate and absorb the wave energy, for example, water vapor in the atmosphere (Chapter 15). Some applications require visible light, target designators, and spotter lasers, while others prefer invisible infrared (IR) light, range finders, and heat damage lasers.
Sometimes we prefer an eye-safe laser to protect our troops: lasers with emission wavelengths longer than 1.4m are often calledeye safebecause light in this wave- length range is strongly absorbed in the eye’s cornea and lens and therefore cannot reach the significantly more sensitive retina. This makes erbium lasers and erbium- doped fiber amplifiers used in 1.5m telecommunication systems less dangerous than Nd:YAG 1m lasers with similar output powers. At longer wavelengths, a CO2laser at 10.4m, the cornea absorption depth is small, energy is concentrated in a small volume along the surface, and the cornea surface is damaged, apparently a painful injury. Of course, the peak power and energy in a light pulse reaching the eye are also critical. Extensive safety documents and regulations apply to lasers.
8.1.2 Beam quality
For a laser beam to deliver energy efficiently to a distant target it must have good beam quality, which means high temporal and spatial coherence and suitable beam convergence (depends on geometry) (Chapter 3). Damaging vehicles with lasers re- quires high-power lasers similar to those used in manufacturing, where the ability to focus a Gaussian beam to a small spot for cutting and welding has led to a specific quality measurement, spot size times convergence angle (see left side of Figure 8).
CHARACTERISTICS 145
Rod laser InnoSlab Thin-disk laser Fiber laser Diode laser systems Diode-pumped lasers
1 10 100 1000 10000
Average laser power P (W)
Laboratory 1000
100
10
1
0.1
Laser Roadmap 2004
Diode lasers (1998) (2003)
Industrial
Lamp-pumped Nd:YAG-Laser
CO2-laser
Beam-Parameter-Prod. Q (mm mrad)
2w0 θf
Q = θf .W0 Qsym = √ Qx .Qy
FIGURE 8.1 Quality for different types of laser.
Figure 8.1 shows how different types of lasers up to 10 kW power compare in quality [6]. At the top of Figure 8.1, laser diodes arrays are robust, small, and efficient, but have the lowest beam quality (Chapter 7). However, laser diode quality improved by an order of magnitude from 1998 to 2003. Lasers used at very high power, the CO2laser (Section 8.3.1) that propagates well through the atmosphere, have a higher quality than the solid-state Nd:YAG laser (Section 8.2). Fiber lasers, generally at telecommunication wavelengths, 1.5m, are eye safe and have similar quality. They can go to high powers because the fiber lasers may be meters in length and can be placed in a water jacket. Large mode area fibers can carry high power for lasers and power delivery [14, 118]. Oxygen–iodine lasers (Section 8.3.2) were not considered for manufacturing in the past because of the need to store dangerous chemicals. Although this is also a major problem for the military, the need for large power in an airborne platform makes chemical lasers attractive because chemicals provide efficient lightweight energy storage (as in gasoline).
8.1.3 Power
Matching the power or peak power to a military application is critical. However, cost, weight, power requirements, complexity, robustness, power requirements, safety, and efficiency are also important. Table 8.1, augmented from Ref. [36], provides an approximate comparison for key properties of the power lasers considered for military applications.
Notes for Table 8.1 by item number.
1. Nd:YAG is a widely used solid-state laser (Section 8.2).
2. The wavelength in item 1 is converted from IR to visible by doubling the frequency (Section 8.2.2).
146 POWER LASERS
TABLE 8.1 Power Lasers Approximate Properties
Peak Pulse Repetition
Type Wavelength Power (W) Length Rate Efficiency
1. Nd:YAG 1.064m 106–1012 10 ps–100 ns 1–100 Hz 10−3 2. Doubled 532m 106–1012 10 ps–100 ns 1–100 Hz <10−3
3. Extreme 351m 0.5×1015 ps–ns 0.001 Hz <10−3
4. CO2 10.4m 108 10 ns–1s 100–500 Hz 10−1
5. Iodine 1.315m 109–1012 160 ps–50 ns 0.014 Hz 10−2
6. KrF 249 nm 106–1010 30 ns–100 ns 1–100 Hz 10−2
7. LD array 0.475–1.6m 108 105ps 105 0.5
8. FEL 10−6–2 mm 105 10–30 ns 0.1–1 Hz 10−2
9. Fiber 1.018, 1.5m 104 10−15–10−8 1011 0.03
3. Extreme refers to the most powerful lasers in development in the world, the prodigious National Ignition Facility (NIF) in the United States (Chapter 13) and the competitive Megajoule Laser in France. The NIF, currently in test phase, focuses 192 powerful Nd:YAG-doped glass lasers, each with 16 ampli- fiers, onto a single target with wavelength conversion from 1.064m (IR) to 351 nm (UV).
4. The CO2laser in gas dynamic form was used to disable sidewinder and cruise missiles in the Airborne Laser Lab in 1985 (Sections 8.3.1 and 12.1.2).
5. The COIL in gas dynamic form was used in the current ABL program to shoot down nuclear-armed ICBMs entering the atmosphere (Sections 8.3.2 and 12.2).
6. A Krypton fluoride (UV) laser is included in the table because its ability to generate high power has been demonstrated in military applications.
7. LD array refers to laser diode arrays composed of a large number of laser diodes working together while preserving beam quality (Section 7.2.4).
8. The free-electron laser (FEL), an example of a cyclotron-based laser, is most promising for future destructive beams because almost any wavelength can be generated at very high power (Section 11.1).
9. Optical fiber lasers [32] are evolving from optical fiber amplifiers [30]. They can replaces item 1 in some cases and can be frequency doubled as in item 2. Multiple water-cooled fiber lasers may be combined. The power and effi- ciency refer to the 1.018m wavelength. The second wavelength, 1.5m, is less efficient but is eye safe.
8.1.4 Methods of Pumping
Energy is provided for population inversion for lasers and optical amplifiers (Chap- ter 7). Population inversion is achieved by several means or their combinations, such as electrical, optical, thermal, and particle acceleration.
CHARACTERISTICS 147
8.1.4.1 Electrical Pumping Semiconductor lasers are pumped with electrical current (Chapter 7). The electrons from the electric current jump to a higher energy level to produce population inversion. Laser diodes convert electrons to photons very efficiently. High power is achieved by combining hundreds of laser diodes to produce a single beam, but maintaining quality in the beam severely limits total power. Pulsed CO2lasers may be pumped with electrical discharge (Section 8.3.1).
8.1.4.2 Optical Pumping Incoherent light such as from a flashtube can be used to pump a pulsed laser or an optical amplifier. In this case, incoherent light is converted to coherent light for which all the photons are in lockstep. Often lasers are pumped with many lower powered lasers in a cascade to provide very high power.
8.1.4.3 Chemical Pumping Heat is a product of many chemical reactions and the thermal energy can be used to excite particles to a higher energy level (Sec- tion 8.3.2).
8.1.4.4 Gas dynamic Pumping For gas lasers, power can be substantially reduced by flowing the incoming cold gases or gaseous chemicals through the laser and exhausting the hot gases to prevent overheating of the active medium. Population inversion is achieved by heating the gases to high temperature and pressure and allowing them to escape through nozzles at supersonic speeds, low temperature, and low pressure. In this case, thermal energy causes population inversion. This form of energy conversion from thermal is more like the process in a steam engine.
8.1.4.5 Particle Acceleration Cyclotron (or synchrotron) pumping passes a stream of electrons at relativistic speed through an evacuated tube with a periodic field in a free-electron laser (Section 11.1). The flow of relativistic electrons, whose mass and speed depend on each other, causes oscillations with density periodicity that provide high and low energy levels. The externally generated periodic field and the energy of the electron gun select frequency.
8.1.5 Materials for Use with High-Power Lasers
High-power lasers described in this chapter and Chapters 9 and 10 are powerful enough to damage optical components used at lower power levels. Consequently, special ma- terial should be used that can withstand the high-power levels without damage. Mate- rials for high-power lasers in the 1–3m range include fusion-cast calcium fluoride (CaF2), multispectral zinc sulfide (ZnS-MS), zinc selenide (ZnSe), sapphire (Al2O3), and fused silica (SiO2) [35]. All these materials can be obtained in blanks larger than 175 mm diameter. Sapphire and calcium fluoride require more experience to fabricate than other materials, but all these materials can be polished to the minimal beam de- viations required. The final choice of material is made based on the requirements of a system and varies based on maximum laser power, operating environment, durabil- ity requirements, wavelength range, size required, and budget. In Section 12.2.2, we
148 POWER LASERS
discuss the materials used for the high-power optical components and windows (zinc selenide) [34] in the Airborne Laser program.