The Adaptive Optics Revolution: A History

12: Sodium Guide Star Laser: The Future

313 T W E L V E Sodium guide Star Laser the Future At the Jasons’ 1982 summer session in La Jolla, Will Happer from Princeton presented the revolutionary concept that the mesospheric sodium layer could be used to generate a high-altitude artificial guide star for use with an adaptive optics telescope. He proposed using a specially tuned yellow laser beam to excite the sodium atoms in the mesosphere to induce resonance fluorescence (glow)—the sodium atom emits a photon at the same wavelength as was absorbed—to create a high-altitude artificial star. The use of this artificial star would make it possible for an adaptive optics system to improve space-object images generated from the collection of light transmitted through the atmosphere by removing the effects of turbulence. The Rayleigh guide star serves the same purpose. But it uses reflected backscatter (not fluorescence), from a laser focused at a lower altitude (10–15 kilometers rather than 90 kilometers). And it is more vulnerable to the problem of focal anisoplanatism. Light from both types of artificial star falls to Earth in a conical path—unlike light from much more distant natural stars, which travels a cylindrical path (see figures 5 and 6). The difference | Twelve 314 between cone and cylinder, and the way that affects a guide star’s ability to mimic a natural star, is called focus anisoplanatism (see chapter 4 for a detailed discussion). Sodium guide stars are less affected by it than Rayleigh guide stars—their light, coming from farther away, falls in a more nearly parallel path. Focus anisoplanatism limits the ability of adaptive optics to sharpen images; the larger the telescope, the worse the problem. With everlarger telescopes appearing on the scene, with apertures as wide as 3 or even 10 meters, a solution was needed.1 Generating a bright enough guide star to produce photons in sufficient numbers from the sodium layer—90 kilometers up and about 10 kilometers thick, with a density of about 5,000 sodium atoms per cubic centimeter—to operate an adaptive optics telescope was easier said than done. To a large degree, it was impractical to make a sodium laser directly using sodium atoms as the laser media. The main problem was the difficulty in achieving efficient pumping of the sodium atom to obtain the required population inversion for lasing.2 Lincoln Laboratory scientists led the way in the 1980s experimenting with dye lasers operating at the sodium wavelength (see chapter 6). But dye was difficult to work with and could be unstable and outright dangerous to handle and store. In the mid-1980s, research moved toward development of solid-state sodium-wavelength lasers using a technique called sumfrequency mixing. Lincoln’s Aram Mooradian led this effort and demonstrated that by combining the two wavelengths at 1.064 and 1.319 microns (the two strongest lasing lines of neodymium-doped: yttrium aluminum garnet or Nd:YAG in a nonlinear crystal (lithium niobate), a third wavelength could be produced for sodium D2a-line resonance (0.589159 microns) excitation. Mooradian and his team built a small laboratory low-power laser to prove his theory, which laid the groundwork for the development of sodium-wavelength laser guide star systems in the 1990s and beyond.3 In 1986 Tom Jeys, who had been on the faculty of Rice University, took a position at Lincoln Laboratory. Jeys’s research took Mooradian’s concepts to the next level. Jeys built the first high-power solid-state laser system that demonstrated a practical source of sodium resonance radiation that could be used for guide star applications. More specifically, Jeys was the first to demonstrate he could get to the “center of the tuning curve”—as opposed to the edges—for generating the 1.064- and 1.319-micron laser wavelengths, Sodium guide Star Laser 315 which combined to produce a quality 0.589-micron sodium-wavelength beam capable of tuning exactly to the sodium D2-line resonance while having sufficient power for mesospheric sodium guide star excitation. At the time, sodium-wavelength laser work was classified. Bob Fugate marveled , “You could not even say the word sodium on the phone!”4 The Air Force funded Jeys to develop a sodium-wavelength laser system that could be used to create a sodium guide star. At that time the Air Force was more interested in using guide stars to perfect ground-based laser antisatellite...