Lawrence Livermore National Laboratory
Final Optics Assembly
The Final Optics Assemblies are the last element of the main laser system and the first of the target area systems. Each FOA contains four integrated optics modules (IOMs) that incorporate beam conditioning, frequency conversion, focusing, diagnostic sampling, and debris shielding capabilities into a single compact assembly. These optics are shown in the figure below.
Schematic layout of NIF's final optics assembly (FOA). This mechanical system mounts to the NIF target chamber and contains the final set of optics for four NIF beamlines. The suite of optics for one beamline is on the right. The same mechanical, optical, and beam control components that are used in the FOA at the target chamber are reproduced for a single beamline in the precision diagnostic system. The IOMs mount to the NIF target chamber and house the final set of optics as line replaceable units, or LRUs. Each optic can be individually replaced as required during operations using individual clean cassettes and clean installation protocols. NIF's 192 beamlines connect to the target chamber in groups of four, or through 48 FOAs (see photo below). The 48 FOAs are symmetrically distributed around the upper and lower hemispheres of the target chamber for optimum inertial confinement fusion (ICF) target irradiation. In this configuration, the laser beams are optimally located to provide the proper orientation as they are directed toward the target. Adjustments of the final turning mirrors allow pointing the focused beams to different locations near the center of the target chamber (+/- 3 cm) to accommodate a variety of proposed experiments.
The integrated optic modules are mounted on the target chamber in groups of four, making up the final optics assemblies. Forty-eight FOAs are symmetrically distributed around the upper and lower hemispheres of the target chamber. In this configuration the laser beams which pass through them are optimally located to provide the proper orientation of the laser beams as they are directed toward the target. View Video. One of the main functions of the FOAs is to change the laser light to a shorter wavelength. NIF´s neodymium glass lasers generate light at a fundamental wavelength of about 1,053 nanometers (one omega, or 1ω) in the infrared region. ICF targets, however, perform more efficiently when they are driven with 351-nm ultraviolet radiation (3ω). To change the laser light frequency, the 1ω light passes through two nonlinear crystal plates made of potassium dihydrogen phosphate (KDP). Note that this is the same type of crystal that is used in NIF´s Pockels cell optical switch. The first plate in the assembly converts two-thirds of the nominal 1,053-nm radiation to the 527-nm second harmonic (2ω) wavelength, which is visible green light. The second crystal then mixes that radiation with the remaining infrared light to produce 351-nm ultraviolet radiation at the third harmonic (3ω). The frequency conversion process has a peak efficiency better than 80 percent, and the efficiency can exceed 60% for the complex pulse shapes used to drive ignition targets.
Other functions of the FOAs are also critical to achieving the required laser performance for use in target experiments. Since the beampath houses an argon environment, while the target chamber operates at vacuum, the first optic in the FOA (the target chamber vacuum window or TCVW) separates the two environments. The remaining optics behind the TCVW are all field-replaceable by removing the hatches on the sides of each IOM, as seen in the photo. The first set of these optics is the conversion crystals discussed previously, which are mounted in a low-vacuum environment and are precisely aligned to the beamline for high conversion efficiency. Next is the lens, which focuses the light to the target location at the center of the target chamber, and finally is a beam-smoothing phase plate that is located with the debris shield for beam energy measurement. With this configuration, the FOAs provide a vacuum barrier for the target chamber, convert 1ω to 3ω light, focus the 3ω light to target center, allow for beam smoothing, allow for 3ω power measurement, and provide a protective shield from target debris. For increased flexibility, there are also several other LRU slots for installing user-defined optical LRUs into the IOMs in the future.
Replacement of each final optic is done using specialized clean cassettes attached to the IOM hatches.
More Information
"Efficiently Changing the Color of Laser Light," Science & Technology Review, October 2006
"NIF final optics system: frequency conversion and beam conditioning"
