History and Development of Military Lasers Chapter 2 HISTORY AND DEVELOPMENT OF MILITARY LASERS JACK B. KELLER, JR* INTRODUCTION INVENTING THE LASER MILITARIZING THE LASER SEARCHING FOR HIGH-ENERGY LASER WEAPONS SEARCHING FOR LOW-ENERGY LASER WEAPONS RETURNING TO HIGHER ENERGIES SUMMARY *Lieutenant Colonel, US Army (Retired); formerly, Foreign Science Information Officer, US Army Medical Research Detachment-Walter Reed Army Institute of Research, 7965 Dave Erwin Drive, Brooks City-Base, Texas 78235 25 Biomedical Implications of Military Laser Exposure INTRODUCTION This chapter will examine the history of the laser, Military advantage is greatest when details are con- from theory to demonstration, for its impact upon the US cealed from real or potential adversaries (eg, through military. In the field of military science, there was early classification). Classification can remain in place long recognition that lasers can be visually and cutaneously after a program is aborted, if warranted to conceal hazardous to military personnel—hazards documented technological details or pathways not obvious or easily in detail elsewhere in this volume—and that such hazards deduced but that may be relevant to future develop- must be mitigated to ensure military personnel safety ments. Thus, many details regarding developmental and mission success. At odds with this recognition was military laser systems cannot be made public; their the desire to harness the laser’s potential application to a descriptions here are necessarily vague. wide spectrum of military tasks. This chapter focuses on Once fielded, system details usually, but not always, the history and development of laser systems that, when become public. Laser systems identified here represent used, necessitate highly specialized biomedical research various evolutionary states of the art in laser technol- as described throughout this volume. This presenta- ogy, design, and application during their development. tion is neither exhaustive nor definitive, but describes Emitted beam characteristics vary widely and are numerous developmental and fielded laser systems important to the specific application and assessment that cover a range of militarily important applications. of potential hazards. INVENTING THE LASER “A splendid light has dawned on me about the absorption and emission of radiation.” —Albert Einstein, letter to Michele Angelo Besso, September 6, 19161(p82) Light amplification by stimulated emission of radia- his design. Historical credit for the invention of the tion (“laser”) is the optical demonstration of Einstein’s laser went instead to Bell Labs researchers Charles theorized “splendid light.” Einstein realized that an H. Townes and Arthur L. Schawlow, whose detailed atom in an excited state can be induced to make a and published proposal for building what they called downward transition while emitting a photon, if the an “optical maser” created an instant stir when it ap- atom is irradiated at a frequency that matches the peared in Physical Review on December 15, 1958.3 atomic transition energy of the host material. Even so, The race to build the first laser began immediately, in science, realization without proof is mere theory. but there was no agreement about which of several can- Einstein’s “splendid light” remained theory for sev- didate materials might be an acceptable host.4 Townes eral years. led a team at Columbia to build a potassium-vapor laser. Similarly, Gould, at Technical Research Group, Civilian Efforts worked on alkali metal vapors. In the Soviet Union, Nicolay G. Basov concentrated on semiconductors. In 1928, at Kaiser Wilhelm Institute, Rudolf Lad- Ali Javan, at Bell Labs, worked to build a helium-neon enburg proved negative absorption (stimulated gas laser. Schawlow, also at Bell Labs, considered emission) near resonant wavelengths in neon gas.2 ruby, but then dismissed it as unsuitable. Theodore However, Ladenburg’s demonstration was of an Maiman, at Hughes Research Laboratory, became uncontrolled emission, and nothing practical flowed convinced that Schawlow was wrong to dismiss ruby from his proof. Another 26 years would pass before as a host material. On May 16, 1960, Maiman used demonstration of a controlled stimulated emission. a cylindrical ruby crystal and a xenon flash lamp to In 1954, at Columbia University, Charles Townes, generate a monochromatic beam of coherent radia- Herbert Zeiger, and James Gordon stimulated am- tion.5 The ruby laser emitted a 0.5-millisecond pulse monia gas with microwave radiation and created the that approximated the pump lamp’s emission duration first “maser” (microwave amplification by stimulated with a primary emission wavelength of 694.3 nm. The emission of radiation). pulse on higher-energy ruby lasers could linger from Three years later, Gordon Gould, a Columbia gradu- 1 to 5 milliseconds. These are relatively long pulses ate student, coined the acronym “laser.” In a notarized compared to what is possible through Q-switching. but unpublished paper, Gould described how a laser (Q-switched lasers emit short pulses. “Q” refers to the could be built, and was later awarded a patent just for “quality” factor describing the state of a laser cavity.) 26 History and Development of Military Lasers Military Interest preferred solid-state laser changed the potential not only for laser application, but also for result- Researchers at the Defense Department’s Advanced ing medical hazards. Research Projects Agency (ARPA) and military ser- ARPA and the military services’ R&D laboratories vices’ research and development (R&D) laboratories, naturally hoped that laser technology could be man- including the Army’s R&D labs at Picatinny Arsenal, ageably scaled, powered, and packaged as deployable New Jersey, and Ft Monmouth, New Jersey, quickly combat tools. The ultimate goal was development of a recognized that laser energy possessed a number of “ray gun.” In 1968, Frederick Schollhammer obtained special properties, not least of which are spatial and US Patent Number 3,392,261 for the “Portable Beam temporal coherence. Coherent and almost parallel Generator,” also known as the handheld laser ray gun.6 beams of light can achieve extremely high radiation Of course, owning a patent and making a product are densities when tightly pulsed and highly focused, two different things. creating transitory temperatures exceeding those on Impressive though they are, even today’s most mod- the sun’s surface. ern laser applications still pale in comparison to the Even the low-powered, Q-switched ruby laser power and efficiency of those depicted in science fiction. inspired excitement among those who under- Early military applications were all the more benign by stood its potential. Q-switching produces a more contrast. ARPA awarded seed money for research to intense pulse. While output energy is somewhat develop and test new laser action materials (“lasants”). decreased, the pulse duration is markedly short- The original ruby lasant was applied to a number of ened, resulting in a tremendous increase in emit- applications with military potential. Within a year of ted power density; a 10 ns pulse of 1 J represents a Maiman’s original achievement, the first prototype pulse of about 100 million W. The effect of such a military laser device—an artillery laser rangefinder— pulse on target materials, whether rigid or elastic, had been designed and built. In much the same way is more “explosive” or ablative than long pulses. that radar modernized air operations, some visionaries Thus, the transition from ruby to neodymium- believed the laser could fundamentally change future doped yttrium aluminum garnet (Nd:YAG) as the battlefields across a wide spectrum of military functions. MILITARIZING THE LASER Although many nations have harnessed the poten- within every main battle tank the US Army fielded tial of military lasers, none have done so as extensively until the introduction of the M1 Abrams tank series as the United States. Prior to the development of the in 1978. first laser, American military scientists had envisioned Although the ruby laser system was useful in early light-sourced applications to support distance mea- work, faster and cooler lasers were subsequently made surement, target designation, and wireless guidance. possible through the use of Nd:YAG as a lasant. The Unfortunately, none of these applications could be switch to Nd:YAG was concurrent with the introduc- achieved with noncoherent light sources. The laser tion of the M1 Abrams in 1978 and continued in the provided a tight, collimated, and discrete wavelength M1A1 (1985). A laser rangefinder known as the eye- beam that was immediately applied to support these safe laser rangefinder (ELRF) used erbium-doped glass and other applications: as a lasant that did not exceed the radiation protection exposure limits. It was introduced with the M1A2 in Rangefinders 19867 (Table 2-1). The mainstay for US ground as well as nontank ve- The first successful American military application hicle laser rangefinders has been the neodymium-based of laser technology was for the purpose of distance AN/GVS-5. Since 1977, more than 8,000 such units have measurement, or “rangefinding.” The idea in this case been fielded to Army and Marine Corps forward observ- was to employ the energy density of the laser beam, ers. The AN/GVS-5 delivers one ranging measurement the strength of which guaranteed reflection back from per second and can be operated by battery or vehicular almost any irregular surface