Solid-State Lasers Hit the Mark

April 2005
By Robert K. Ackerman
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Lawrence Livermore National Laboratory’s solid-state heat-capacity laser, or SSHCL, provides powerful bursts of laser light from arrays of diodes. In this unit, two banks of diodes—the blocks with gold edges—are generating a single pulse lasting one two-hundredth of a second. Deeper inside the unit are other banks of diodes that are not firing. Diode banks can be arrayed in series with crystal optics to provide enough power to shoot down incoming munitions such as artillery or mortar shells.

Smaller, lighter and more powerful beam systems fit right into military’s new footprint.

Researchers in the long quest for battlefield laser weapons are closing on their objective with the development of a new type of solid-state laser system at a U.S. government laboratory. This laser can be mounted in a small vehicle and can draw from battery power to shoot down difficult-to-hit projectiles such as mortar rounds, or it can be aimed downward to neutralize the threat of buried mines and other explosive devices.

The laser improves on earlier technology by packing a lot of power in a small footprint, an advantage that is becoming more important as the military transforms into a smaller, lighter and more mobile force. The U.S. Army’s Future Combat Systems, for example, will introduce a whole new family of mobile platforms. This laser is being designed with those types of platforms in mind.

But potential applications are not limited to ground platforms. Helicopters, fixed-wing aircraft or even unmanned aerial vehicles could carry the solid-state lasers. Navy ships could carry the lasers and power them directly from ship systems. In all cases, the platforms would not need to carry the exhaustible supplies required to power chemical lasers, which is one of the key determining factors in the size of the footprint each type of laser brings to the field.

This new type of laser has emerged from research at Lawrence Livermore National Laboratory, Livermore, California. The program, known as the solid-state heat-capacity laser (SSHCL), is funded by the U.S. Army Space and Missile Defense Command in Huntsville, Alabama.

Robert M. Yamamoto, program manager for the SSHCL program at Lawrence Livermore, explains that another advantage of the solid-state electric laser is that it can fire at a target immediately without any warm-up. A chemical laser requires mixing of chemicals immediately prior to the laser discharging its energy burst. On the other hand, the solid-state laser effectively can wait in a pause mode for some time until the fire order is given, at which time it will fire instantly for several seconds.

Chemical lasers tend to be much more powerful than solid-state lasers, but the SSHCL makes up for this shortfall through its firing pattern. The SSHCL destroys its target not by hurling a single massive blast of laser energy but by firing sustained pulses until the target is destroyed. Unlike a continuous wave laser, this laser pulses at a rate of about 200 per second. The peak power of each little pulse is much higher than the average power of the laser, Yamamoto reports.

This peak-power pulsing heats the target quicker than a continuous wave, he adds, because the series of pulses delivers more peak energy to the target. About five seconds of sustained pulsing is usually enough to ensure destruction of the target. This time has been confirmed in modeling with mortar rounds where the laser heats the projectile’s explosive warhead to the point of self-detonation.

Another benefit of the pulsed laser is that it can overcome passive countermeasures such as reflective coatings. If the pulsed beam strikes a mortar shell that is coated with a reflective substance to spoof laser beams, the 200 pulses per second will vaporize that coating quickly and lay bare the substrate for heating by subsequent laser pulses.

In the SSHCL, electric-powered diode arrays pump a gain medium to generate laser light. The laser’s optics are fabricated of neodymium:gatalinium-gallium-garnet crystal, or Nd:GGG. Each optic is a square measuring 10 centimeters on a side and 2 centimeters thick.

This size optics produces a square laser beam measuring 10 centimeters on a side, which removes the need for beam combining to create a single large powerful beam. So, coherency problems inherent in beam combining are avoided.

A major element in the laser’s construction addresses optic cooling. The temperature in each Nd:GGG crystal rises from room temperature to about 100 degrees Celsius (212 degrees Fahrenheit) during the several seconds the system is lasing. At this peak temperature, an exchange system removes the hot crystal from the laser system and replaces it with an identical optic at room temperature.

The laboratory addresses this change in a number of manners ranging from simple linear swap-outs to a prototype system of rotating optics that resembles a Gatling gun. In this approach, crystals are replaced in a circular sequence that ensures that, by the time the original optic has returned to replace another, it has cooled sufficiently. Generally, each laser crystal must be exchanged for cooling after about two kills, Yamamoto relates, and it takes about a minute of cooling before each optic is ready to be re-inserted into the beam. Each swap takes about one second. The laboratory is experimenting with five linear swap systems along with the Gatling gun system under design.

The typical SSHCL does not rely on just one layer of optics, however. Yamamoto explains that the laser places several crystals one behind another in a linear construct. Doubling the number of Nd:GGG slabs doubles the amount of power emerging from the laser with a commensurate increase in the number of diodes.

The laser’s power also can be boosted by increasing the size of each slab. Doubling the dimensions would increase the power by a factor of four. Along with the number of slabs, this provides two ways for designers to scale the laser.

These optical constructs need not be in a single straight line, however. A typical 100-kilowatt laser design would feature five Nd:GGG slabs in a line separated from each other by one foot. Then, the system makes a U-turn, and five more slabs are arrayed next to the first group. The beam passes through a total of 10 crystals, but the laser is more compact than if they all were lined up in a single straight line. This U-turn construct becomes especially useful in cramped settings such as armored vehicles.

The laboratory model laser uses lithium-ion batteries to generate power. The laboratory has demonstrated pulses with this power system to ensure the laser’s use in a mobile environment. A recharging system would be integral to the vehicle that serves as a platform, and the hybrid-electric vehicles under development for the military offer potential as a recharging source for the batteries, Yamamoto notes.

Yamamoto explains that, from the battery power source to the output laser beam, the SSHCL has an efficiency of about 10 percent. In fact, the battery system provides the primary limitation on the laser’s use. With the optic cooling system able to keep the system running continuously, the only logistic that could bring the laser to a halt would be the need to recharge the batteries if they were drained from repeated firings. A steady direct-current power source, such as a generator, would remove that drawback, Yamamoto offers.

The laser’s pulsing effect offers new applications in addition to intercepting mortar shells. A variety of airborne targets can fall prey to the 100-square-centimeter beam and its constant pulsing. Any projectile containing an explosive substance—such as artillery shells or Katusha rockets—would find itself enveloped in a swath of heat that would cook off its munitions or its fuel.

Shipboard protection is another application. In addition to airborne threats, terrorists attacking in small watercraft or even on jetskis could find themselves bathed in an instantaneous heat ray that ends their attack far short of their target.

 
Robert M. Yamamoto, program manager for the SSHCL program, adjusts precision equipment behind the aiming lens for the laser. In the background is the actual laser unit. The hose assemblies feed into the diodes and provide cooling. The vertical metal strips between each diode construct are the edges of the frames that hold the square Nd:GGG optic slabs.
Fixed-wing aircraft and helicopters could use the space-efficient laser and provide the necessary power to overcome battery limitations. And, an airborne platform will be able to wield a laser more effectively because it will operate above much of the fine particulate matter that drifts in the lower atmosphere. Yamamoto notes that the laboratory has looked at incorporating the technology into the Joint Strike Fighter. The system’s modularity permits its installation in a number of platforms, he offers.

One down-to-Earth application is mine clearing. The pulsing allows the laser to drill a hole into the soil where a land mine is buried, and its heating characteristics will detonate the explosive. The pulses’ peak energy vaporizes the moisture inherent in the soil, which clears the way for the beams to blast their way to the buried device. Yamamoto offers that the same average power from a continuous wave laser would heat up the soil only nominally, as that type of laser would not have enough peak energy in its average power.

“When we get this device into the hands of the soldiers who eat and breath combat, the applications [they will uncover] will be astounding,” Yamamoto predicts.

He notes that the SSHCL does have limitations. The quality of the air through which the laser propagates can limit its performance. A severe dust storm, for example, will curb its effectiveness.

Battlefield applications will require extra efforts to ensure that the laser’s critical parts remain clean and undamaged in the harsh environment of combat. Researchers are looking at the engineering necessary to ensure cleanliness, Yamamoto notes.

Lawrence Livermore National Laboratory currently has two laser systems using the heat-capacity approach, Yamamoto reports. One is the solid-state diode-pumped laser, but another uses a flash lamp to pump the laser. The flash-lamp-pumped device served to validate the heat-capacity approach, Yamamoto relates. This year, scientists will take this older flash lamp model to the Army’s High-Energy Laser Strategic Test Facility (HELSTF) at White Sands, New Mexico, for field-testing against static targets.

The laboratory’s High Explosive Applications Facility, or HEAF, will be the site of tests against mortar rounds. However, instead of actually using the laser against incoming mortar rounds, researchers will simulate the effect of the pulsed laser beam heating a mortar round. To validate the extensive modeling done at the laboratory, thermal data accrued during these tests will be combined with information gathered during materials testing for the effects of the lasers. Researchers already have used the laser to burn through carbon steel, aluminum and other materials.

Yamamoto explains that the flash lamp system already has been down to White Sands, and the infrastructure to support it remains in place. The diode system will remain at the laboratory for continued engineering work. “The diode system is where the power is now,” he says. “The diode system is the one that we are going to sell.”

The laboratory’s work on these lasers began in 1999 with the development of a three-slab, flash-lamp-pumped system that delivered 1.5 kilowatts of power. The three neodymium glass slabs measured 10 inches on a side. The diode-pumped system that the laboratory tested last year uses four Nd:GGG slabs to deliver more than 30 kilowatts of power. Yamamoto reports that the laboratory could achieve significantly higher power levels if funding became available.

Researchers are working on a couple of technologies that would enhance the nominal overall beam quality of the laser. Since the first diode-pumped laser was built three years ago, diode technologies have improved in terms of cooling and power. The more powerful diodes can run for longer periods of time, and the laboratory seeks to incorporate those improvements into its lasers. The result would be more power and flexibility in the operation and the performance of the laser.

Yamamoto allows that researchers are developing an adaptive optic system to enhance the beam quality. Scientists have placed a state-of-the-art adaptive optic system on both the flash-lamp- and diode-pumped lasers, and work continues on improving those technologies. A more advanced adaptive optic SSHCL system also may be able to overcome some of the limitations imposed by atmospheric attenuation by adjusting the beam as it propagates to the target.

New lithium-ion battery technologies also can improve the laser’s performance. Yamamoto relates that the laboratory’s new lithium-ion batteries are smaller, lighter and more powerful than their predecessors, which will help in designing a system for a mobile platform.

Plans call for designing and building a 100-kilowatt SSHCL system in the laboratory late next year. This laser would be “very prototypical” of what might be installed on a mobile battlefield platform, Yamamoto says. This laser would be less complicated and cleaner in terms of design than current laboratory models, he says.

Ultimately, the laboratory seeks to build a megawatt-class SSHCL laser system, Yamamoto offers. “We have looked at the scaling and have done a lot of modeling, and we believe that it is very doable, especially with the enhancements offered by the new technologies and materials that have surfaced over the past year,” he declares. With the necessary funding, scientists could produce this laser by 2010.

The laboratory is working with several industry teams on subsystem components. General Atomics, San Diego, is supporting the thermal management system. Diode-array technology is being provided by Decade Optical Systems, Albuquerque, New Mexico. The lithium-ion batteries come from Saft America, Cockeysville, Maryland. The power management system is provided by DRS, Huntsville, Alabama. The Boeing Company, Chicago, is the overall systems integrator.

 

Web Resources
Lawrence Livermore National Laboratory: www.llnl.gov
Lawrence Livermore report on mine-clearing laser: www.llnl.gov/str/October04/Rotter.html
Lawrence Livermore HELSTF project: www.llnl.gov/nif/lst/helstf.html