DAHLGREN, Va. -- Dr. Evan Bates of the High Energy Laser Measurement Support (HELMS) Laboratory at the Naval Surface Warfare Center Dahlgren Division (NSWCDD) leads a research project that combines the Navy’s next-generation laser weapons and environmental challenges as old as seafaring itself.
On a recent morning, Bates and his colleagues headed out to the Potomac River Test Range, when the dawn hour affords a brief window of time for laser tests before the weather starts to interfere with their instruments. There’s still a bit of morning fog hanging over the grounds as they finish setting up all the equipment and there is nothing to interrupt the natural silence except for the quiet flick of a switch when they activate the laser emitter. Against that backdrop of slowly-wafting haze, the red beam appears perfectly straight and true, with a bright neon glow that lights up the target board 500meters away.
That’s how it appears to a lay observer. Bates would disagree about the beam’s perfect straightness. He glances at several instruments mounted on tripods, with names like the scintillometer and differential image motion monitor. This leads to a brief lesson on laser-distortion and the causes of beam spread, beam wander, and atmospheric jitter. “When laser radiation leaves the aperture, it travels through the atmosphere like a lens, or many small lenses,” he explains. “Each lens has its own index or refraction, and the laser light will start to bounce through each individual lens. By the time it reaches the target, you will have a very distorted wave.”
The group of effects that Bates is measuring for are collectively known as “optical turbulence.” These are the same conditions that cause the light from distant stars to appear to twinkle in the night sky, and shimmering mirages to appear on sand dunes. For the purposes of naval warfare, Bates adds, “They all reduce power on target. That decreases the lethality of your laser.”
Indeed, parts of the Navy’s approach to laser testing are inspired by the equipment and techniques that astronomers use to peer at far away celestial bodies, according to Dr. Peter Wick, a Lead Senior Scientist at NSWCDD who has been working on several directed energy projects since 2007. One of the tripod-mounted instruments (the differential motion monitor) uses measurement concepts “similar to what has been done with guide stars for telescopes,” Wick said, “so you get good pictures of stars beyond mountain tops. We’re using it another way, to look at [laser beam] propagation.”
The Naval Surface Warfare Center Dahlgren Division has conducted a number of test and evaluation events in recent years in support of programs including the Laser Weapon System (LaWS), and the Optical Dazzling Interdictor, Navy (ODIN). In February, the Navy announced that the first ODIN was installed on the Arleigh Burke-class guided missile destroyer USS Dewey (DDG 105). The LaWS was first installed on The Afloat Forward Staging Base (Interim) USS Ponce (AB(I) 15) in 2014. The Navy’s directed-energy programs counter asymmetric threats such as unmanned vehicles and small attack boats. As the Office of Naval Research wrote in 2014: “lasers offer an affordable and safe way to target these threats at the speed of light with extreme precision and an unlimited magazine.”
Both ODIN and LaWS were supported by NSWCDD engineers specializing in laser technology. Bates and his team conducted a full systems test on ODIN, among other tests on related laser programs. Between those events, the HELMS Laboratory at Dahlgren collects and standardizes data to build what Bates describes as “a historical database of the optical transmission quality from throughout the year. It helps us better explain what is going on at these test events.”
In the future, Bates hopes to take the instruments used for optical turbulence measurements and incorporate them into the shipboard lasers. “You want to be able to get a real-time measurement and a single-ended system, because if we can get that we can look at the environment and dial in certain parameters into the laser for more power onto the target. That’s the end goal of optical turbulence instrumentation.”