applications Defence
In the
stephen Mounsey looks at science fiction becoming science fact in terms of lasers and photonics in military applications
wars F
10 electro optics l APRIL 2011
or many people lasers seem to be synonymous with spacecraft and the weapons of the future. On the all-too-rare occasions upon which the mainstream
media covers the discovery of a new aspect of lasers or photonics, the story will almost always include at least a mention of ray guns and outlandish 1950s sci-fi weaponry. While a Star Wars-style blaster remains (thankfully) a long way in the future, weapons and military equipment in the real world are becoming ever more dependent on lasers and other photonics technologies.
strong countermeasures Star Wars was, of course, pure fiction, and even its unofficial namesake the US Strategic Defence Initiative proved to be closer to sci-fi than reality. Although the Cold War ended before Reagan’s Star Wars programme had a chance to put directed- energy weapons into orbit, at least one aspect of its legacy lives on in the form of the Boeing YAL-1. Last year the US Air Force reported that this $500m converted 747 successfully targeted and scored a hit on two dummy ballistic missiles in the boost phase of their launch. The aircraft carries a chemical oxygen iodine laser (COIL), supplied by Northrop Grumman, capable of 20 full-power shots of infrared radiation (1,315nm) at around 20kW CW before it needs to be refuelled. Chemically generated lasers have, until
recently, been the only viable way of generating
the intensities of light necessary to even have a chance of intercepting a ballistic missile – and even at power levels of up to 100kW the laser is not able to destroy the missile outright. Such a system is unable to sustain a beam for more than a few seconds, leaving only a short window during which the target can be damaged. Electrically powered systems can sustain an output for longer but, until now, they have lacked the raw CW power of chemical lasers. Jörg Neukum, sales and marketing director at Dilas (Mainz, Germany), explains how the company is working with military customers to achieve CW DPSS lasers that will one day be capable of sustained outputs of 50 to 100kW for the purposes of shooting down missiles, mortars and rockets. A 50kW DPSS laser, he says, needs about 200kW of diode power behind it to act as the pumping source, and these diodes are supplied by Dilas. At such high power levels, the most significant challenge to laser design becomes thermal management, as it is difficult to remove heat from a crystalline gain medium (conventional Nd:YAG for example). For this reason, some designers have swapped solid-state crystals for gaseous gain media (rubidium vapours), creating so-called diode-pumped alkaline lasers (DPALs). ‘In a 100kW solid-state crystal laser, the main problem is the heat,’ says Neukum. ‘Gas media are the obvious solution to this problem, because the technology is well understood and the pumps are
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