Navy laser weapon deployment

The U.S. Navy will deploy a high-energy laser weapon on the USS Ponce in fiscal 2014, chief of Naval Research Rear Admiral Matthew Klunder announced April 8, 2013 at the Sea-Air-Space exposition. The Navy Laser Weapon System (LaWS) will be the first high-energy laser deployed for field use by the armed services. The Navy has tested the laser system against its prime targets, moving small surface boats, and remotely piloted vehicles.

The at-sea deployment comes two years earlier than the Navy had planned. That may be a first in laser weapon development, where schedule slippage and cost overruns have been common. The New York Times reports LaWS cost just under $32 million, roughly two orders of magnitude less than the Airborne Laser, dropped from the fiscal 2011 budget after it failed to reach the required 200 km range.

NAVY LaWS on board a ship during tests of the laser weapon. (Image courtesy of the US Navy)

LaWS is part of the new generation of electrically powered solid-state laser weapons, which Navy officials say offer two advantages. One is a "deep magazine," the ability to fire pulses as long as electrical power is available--and ships have plenty of power. The other is cost. Klunder said, "Our conservative data tells us a shot of directed energy costs under $1," compared to $100,000 or more to fire a missile.

The choice of LaWS marks a big success for fiber lasers. When the Pentagon launched the Joint High Power Solid-State Laser (JHPSSL) program in 2002, developers focused on diode-pumped slab lasers, which at the time seemed the technology most likely to reach the 100 kW sought for defense against rockets, artillery, and mortars. JHPSSL reached that level in 2009, but fiber lasers have been catching up. The Naval Sea Systems Command reached 30 kW by combining the beams from six 5.5 kW industrial fiber lasers to shoot down a drone in 2010. LaWS has been upgraded since then, but Navy officials did not disclose the output power of the current system.

The laser is not the only challenge. For the current version of LaWS, L-3 Integrated Optical Systems (Pittsburgh, PA) upgraded the pointing and tracking system, improving accuracy of the fine steering mirror and controls, and improving the software and user interface. "We took scientists out of the loop to make it operable by seamen," said Don Linnell, director of business development and strategy. The Navy considers that a must for fielding laser weapons.

NIF falls short of ignition

The National Ignition Facility (NIF) will not meet its goal of igniting a fusion plasma before the end of September, the Lawrence Livermore National Laboratory (Livermore, CA) said on Friday. A spokeswoman says Livermore "will continue working toward achieving ignition." The laser is delivering the desired energy, but the target shots are not yielding the expected fusion energy.

NIF was declared complete on March 31, 2009, after it had delivered 1.1 MJ pulses at 355 nm. The 192-beam system was designed to deliver 1.8 MJ pulses, which simulations indicated would be sufficient to ignite a pellet of deuterium-tritium fusion fuel, producing fusion reactions that yielded more energy than the input pulse. The Department of Energy set a target of reaching ignition by September 30, 2012--the end of the fiscal year.

Wary of optical damage, Livermore ramped pulse power and energy slowly. The first 1.8 MJ pulse was not fired until March of this year. On July 5, NIF delivered peak power of 500 tW to a target for the first time in a 1.85 MJ pulse. From outside, it looked like NIF should be closing in on ignition.

But now NIF has become the latest in a long list of fusion lasers that yielded experimental results well short of predictions. A news story in the September 21 issue of Science magazine reports that although computer models predict NIF shots should achieve ignition, the yield of fusion energy from NIF experiments has so far reached only 0.1 of the ignition level.

The National Nuclear Security Administration (NNSA) has already begun studying its options. The first draft of a report is due October 1, with a final report due to Congress on November 30.

Meanwhile, NIF continues firing shots that can produce temperatures and pressures far beyond anything previously possible on the surface of the Earth. Livermore fusion researchers will keep pressing for ignition, and NNSA weapon scientists will get additional shots for their simulations of nuclear explosions as part of the agency's Stockpile Stewardship program.

NIF's laser bay, showing 96 of the 192 beamlines.

DARPA PULSE program

Ultrafast laser research has produced some elegant science, from slicing time into incredibly thin slivers to generating combs of frequencies uniformly spaced across a wide band of the spectrum. These capabilities, in turn, have led to a similarly wide range of applications, including transferring time and frequency standards, measuring short intervals of time, and producing pulses so short that they generate extremely high peak powers with only modest amounts of energy.

However, ultrafast lasers traditionally have been bulky and complex things, custom-assembled on optical tables and delicately aligned in a laboratory. That complexity makes it hard to realize many potential practical applications such as putting frequency combs in space to boost the precision of GPS systems or to measure stellar spectra with extreme precision. Now the Defense  Advanced Research Projects Agency (Arlington, VA) is trying to do something about the problem by creating the Program in Ultrafast Laser Science and Engineering.

DARPA is not the first to think of making smaller and more durable ultrafast lasers. I mentioned the need for "robust frequency combs" for telecommunications systems or space-based instruments in the January Photonic Frontiers. A web search four pages which include the phrase "rugged femtosecond laser," but all of them cite an Army contract awarded last year to Arbor Photonics. However, such references are few and far between, and Google could not find a single page using the phrase "rugged frequency comb" (or combs) when I was writing this blog.

Shrinking the size and improving the robustness of ultrafast lasers is a big challenge, but success could pay off in important ways. DARPA cites some potential military applications that require rugged sources. One is using the time stability of the microwave-band repetition rate of a femtosecond laser to greatly reduce the close-to-carrier phase noise in a microwave oscillator. Others include transferring time or frequency measurements across the spectrum, and generating high-flux isolated attosecond pulses. Civilian science and technology also would benefit from compact  sources of ultrashort pulses.

As is normal with DARPA, success is not guaranteed, but the payoff could be high. In fact, somebody at DARPA surely should have already earned credit in the Pentagon bureaucracy for exceptional skill in acronym creation. Program in Ultrafast Laser Science and Engineering neatly translates into an entirely appropriate acronym -- PULSE.