They were everywhere in Star Wars. Remember the flashing towers of laser cannons mounted on the surface of the Death Star firing away at the puny handful of Rebel fighters and how they would make blasting sounds even in the deep vacuum of interplanetary space?
If it wasn’t for the Force, poor Luke Skywalker wouldn’t have had a chance. Well that’s about the odds given to any drone or rocket trying to attack US warships in the future. Just last week the Navy tested a new high energy laser system that can instantly shoot down any incoming enemy missile with a beam of high intensity light.
The test was performed in the Pacific a little ways from Pearl Harbor where the USS Portland, an amphibious transport dock ship, successfully disabled an unmanned drone using a fiber optic based solid state laser, developed by Northrup Grumman. Information listed on the www.navy.mil website gives information on the test along with dramatic video of the laser in action.
In announcing the results of the test, commanding officer of the Portland, Captain Karrey Sanders said: “By conducting advanced at sea tests against UAVs (Unmanned Aerial Vehicles) and small craft, we will gain valuable information on the capabilities of the Solid State Laser Weapons System Demonstrator against potential threats.”
It is a good bet that the laser will be used on Iran’s gunboats in the future as they continue to threaten our patrol in the strategic Strait of Hormuz, gateway to 25% of the world’s oil flow.
According to the website, the U.S. Navy has been developing directed-energy weapons since the 1960s when optical lasers first emerged. Their goal for the past six decades has been to devise an electromagnetic system (and light is an electromagnetic wave) capable of converting chemical or electrical energy to radiated energy and focusing it on an incoming target, resulting in physical damage that degrades, neutralizes, defeats, or destroys any adversarial capability.
It seems with this latest test we are close, at long last, into something paying off. “The Solid State Laser Weapons System Demonstrator is paving the way for future weapons systems,” said Sanders. “With this new advanced capability, we are redefining war at sea for the Navy.”
The progress on laser weapon systems has been rapid in the last several years, moving from just 30 kilowatts in 2014 to 150 kilowatts in 2020, a five-fold jump in power. But, as you will see, this hasn’t been easy for several reasons.
First, high power laser beams begin to cause plasma breakdown of the atmosphere in the path of the beam. When energy densities rise above one mega-joule per cubic centimeter, the air just simply ignites. This effect, called “blooming,” causes the laser to defocus and disperse energy, heating the air and not the incoming missile.
As one would expect, blooming can be more severe if there is smoke, dust, rain, snow, smog, or even fog in the air. This problem can be mitigated to some degree by several methods. By spreading the beam across a large, curved mirror and focusing the power on the target the energy density can be kept below the threshold for blooming. But this requires a large and precise mirror, much like a searchlight. And in combat, glass mirrors are fragile things.
Another technique to prevent blooming is to use a very short pulse that finishes before blooming cab even start. This requires a tremendously high power laser to concentrate large amounts of energy in the one pulse and these are still a way off. Lastly, another idea is to focus multiple lasers of relatively low power scattered around ship board, on the target. Redundant systems such as this have distinct advantages during battle.
To develop the light output for the test just conducted, the new laser system uses a combination of solid state devices probably based on gallium nitride materials, along with an optical fiber amplifier. This component, around for about 20 years, can increase an optical signal directly, without the need to first convert it to an electrical signal. It works by using a rare-earth optical fiber, either erbium or ytterbium, and can boost the power of up to 20 watts per fiber strand, allowing a high power density when all of the fibers are coupled coherently together. Amplification in the fiber is achieved by stimulated emission of photons from dopant ions in the glass. The pump laser excites these ions into a higher energy from where they can release photons via stimulated emission, much like a typical laser but in the fiber itself.
An advantage of using fiber amplifiers instead of one large laser, is that the light is delivered by an inherently flexible medium, which allows easier delivery to the focusing mechanism. In addition, by using long fibers, say, up to a kilometer long, wound on a spool, the output power compared to other types of laser is very high. They can support kilowatt levels continuously because of the fiber’s high surface area to volume ratio, which allows efficient cooling. Getting the heat out is always a problem with any weapon system.
You will see more fiber amplifiers and laser systems in the future, already they are being used to cut and weld metal in industrial applications.
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