CHAPTER 8 POWER LASERS Powerful laser light from 1 W to many gigawatts can be obtained from solids (Nd:YAG-doped glass), liquids (dye lasers) or gases (carbon dioxide, oxygen–iodine). Semiconductor lasers (Chapter 7), can be used fora1kWlaser (or higher power) by merging outputs for an array of laser diode bars in a way that preserves beam quality (Figure 7.10). A high-power solid-state laser is described in Chapter 13 for allowing nuclear weapon development in an age of test-ban treaties. The free-electron laser, rapidly maturing for very high power from kilowatts to gigawatts, does not depend on bound electron states and is described in Section 11.1. In discussing power, we must separate continuous wave lasers that remain on for over a second from pulse lasers that are on for only a small part of a second. Pulsed lasers are characterized by pulse energy: peak power times pulse duration. Both energy and duration affect the nature of the impact on the target, the average power for driving the laser, the weight, and the cost. Methods of creating pulsed lasers from continuous ones are described in Chapter 9. High power is often attained by following the laser with a series of optical amplifiers, which are often identical to the initiating laser except for the absence of a resonating cavity (mirrors removed). In this case, a pulse shaper ensures high temporal coherence and spatial filters (a pinhole) after each amplifier ensure high spatial coherence for good beam quality at the final output (Chapter 3). In Section 12.2.2, we describe adaptive optics methods for improving spatial coherence. In Section 8.1 of this book, we discuss characteristics of high-power lasers. In Section 8.2, we consider solid-state lasers and frequency doubling of solid-state lasers Military Laser Technology for Defense: Technology for Revolutionizing 21st Century Warfare, First Edition. By Alastair D. McAulay. © 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc. 143 144 POWER LASERS for visible green light. In Section 8.3, we describe gas dynamic principles, gas dynamic carbon dioxide power lasers used for shooting down cruise and sidewinder missiles in 1983, and chemical oxygen–iodine lasers (COIL) used in the Airborne Laser (ABL) to target intercontinental ballistic missiles (ICBMs) and in the Advanced Tactical Laser (ATL) to target armored vehicles. 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 1.4 ␮m are often called eye safe because 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 1.5 ␮m telecommunication systems less dangerous than Nd:YAG 1 ␮m lasers with similar output powers. At longer wavelengths, a CO2 laser at 10.4 ␮m, 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 1000 2w0 Laser Roadmap 2004 θ f 100 Diode lasers (mm mrad) (1998) Industrial Q θ . (2003) Q = f W0 rod. P 10 . Qsym = √ Qx Qy CO -laser Diode laser systems arameter- 2 P 1 Diode-pumped lasers Lamp-pumped Rod laser Nd:YAG-Laser InnoSlab Beam- Laboratory Thin-disk laser Fiber laser 0.1 1 10 100 1000 10000 Average laser power P (W) 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 CO2 laser (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, 1.5 ␮m, 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 1.064 ␮m106–1012 10 ps–100 ns 1–100 Hz 10−3 2. Doubled 532 ␮m106–1012 10 ps–100 ns 1–100 Hz < 10−3 3. Extreme 351 ␮m0.5 × 1015 ps–ns 0.001 Hz < 10−3 8 −1 4. CO2 10.4 ␮m10 10 ns–1␮s 100–500 Hz 10 5. Iodine 1.315 ␮m109–1012 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 0.475–1.6 ␮m108 105 ps 105 0.5 8. FEL 10−6–2 mm 105 10–30 ns 0.1–1 Hz 10−2 9. Fiber 1.018, 1.5 ␮m104 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 1.064 ␮m (IR) to 351 nm (UV). 4. The CO2 laser 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 1.018 ␮m wavelength. The second wavelength, 1.5 ␮m, 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).