The idea of the ray- gun has been cen­tral to science-fic­tion for decades. Now it’s science fact. CATHAL O’CON­NELL reports.

Cosmos - - Front Page -

THE TUR­RET EMERGES from the ship and piv­ots, fol­low­ing its tar­get with its tele­scopic eye. Silently it fires, and two kilo­me­tres away, a small, glow­ing spot ap­pears on the fuse­lage of an air­craft—the en­gine bursts into flames.

Not the Mil­len­nium Fal­con. Not the USS En­ter­prise. But the USS Ponce, an Amer­i­can Navy ves­sel in 2015. The ray­gun is here.

In War of the Worlds, the in­vad­ing Mar­tians dev­as­tated Earth’s forces with their ‘heat-ray’. Vari­ants of the ray-gun have been a main­stay of science fic­tion ever since, from the phasers of Star Trek to the blasters of Star Wars.

But though the US mil­i­tary pur­sued the idea for decades—cul­mi­nat­ing in Rea­gan’s pro­posed Star Wars de­fence sys­tem for us­ing satel­lites to blast mis­siles out of the sky—the re­search never bore fruit. Not, at least, un­til now.

In 1960, Amer­i­can physi­cist Ted Maiman built the first laser. He crafted a cylin­der of ruby with re­flec­tive ends, and placed it in­side a pow­er­ful xenon lamp. A flash of light sent the atoms of a ruby into an excited state. Some of th­ese atoms emit­ted a pho­ton—which them­selves went on to hit other excited atoms, caus­ing ‘stim­u­lated emission’ of other pho­tons—iden­ti­cal to the first.

The light beam that emerged was co­her­ent, mean­ing all the peaks and troughs lined up. Th­ese prop­er­ties min­imised the ten­dency for a light beam to spread out and lose power over dis­tance, mak­ing the ray-gun a pos­si­bil­ity. Un­be­knownst to Maiman, the fledg­ling Ad­vanced Re­search Projects Agency was al­ready con­sid­er­ing a pro­posal by an­other physi­cist to cre­ate a laser weapon.

Over the decades, many kinds of pow­er­ful lasers were de­vel­oped. The prob­lem was all of them re­quired huge equip­ment and mas­sive amounts of en­ergy. In the early 2000s, the US Air Force tried pack­ing a chem­i­cal laser, the Air­borne Laser YAL1, onto a Boe­ing 747. It worked, shoot­ing down a cou­ple of train­ing tar­gets, but cost bil­lions of dol­lars and was ul­ti­mately canned.

In the last decade we’ve seen spec­tac­u­lar ad­vances in laser tech­nol­ogy that may make the ray-gun prac­ti­cal again.

The Laser Weapons Sys­tem (LAWS) is one of the first of a new breed of more com­pact sys­tems based on the fi­bre laser. Fi­bre lasers can gen­er­ate laser beams at ef­fi­cien­cies of 40%, far higher than con­ven­tional lasers, and achieve kilo­watt pow­ers. High power fi­bre lasers are al­ready used in in­dus­trial cut­ting and weld­ing ma­chines, some with laser power of 100 kw and ca­pa­ble of weld­ing blocks of metal parts 30 cm thick.

A 100 kw in­fra-red laser is ex­actly the ‘heat-ray’ that Wells imag­ined—equiv­a­lent to us­ing a gi­ant, kilo­me­trewide mag­ni­fy­ing glass to fo­cus the sun’s heat en­ergy onto a sin­gle point the size of your fin­ger­nail.

The ob­jec­tive for LAWS is to af­ford­ably shoot down cheaply made in­sur­gent rock­ets and drones, with­out wast­ing ab­surdly ex­pen­sive mis­siles. While an anti-air cruise mis­sile might cost hun­dreds of thou­sands of dol­lars, a sin­gle shot from LAWS works out at about $1 in en­ergy cost. In 2014, a LAWS pro­to­type in­stalled on the USS Ponce demon­strated it could shoot down drones and dis­able boats. The US Air Force plans to put a sim­i­lar de­vice, de­vel­oped by Lock­heed Martin, on a fighter jet by 2021.

One dif­fer­ence from movie sci-fi, th­ese real ray-guns don’t emit ex­cit­ing ‘Pew! Pew!’ sound ef­fects when they fire. They’re silent. Wells’ omi­nous words are more apt: “this in­vis­i­ble, in­evitable sword of heat.”

SEN­SORS: Sen­sors on the tur­ret track the tar­get’s move­ment. A ra­dio fre­quency sen­sor pro­vides range data, help­ing LAWS to fo­cus its mul­ti­ple beams on the same spot.

BEAM DI­REC­TOR: The laser it­self is gen­er­ated in­side the ship, and then routed through the fi­bre op­tic ca­bles to the ‘beam di­rec­tor’, which looks a bit like an omi­nous ama­teur tele­scope. This pro­to­type com­bined six laser fi­bres, for a to­tal op­ti­cal en­ergy of 30 kw. Other ver­sions of up to 150 kw are in the pipe­line.

FI­BRE LASERS: The core tech­nol­ogy at the heart of LAWS is the op­ti­cal fi­bre laser. The light beam is first gen­er­ated in a diode laser, which is in­cred­i­bly ef­fi­cient, but only works at low power. This low­power laser beam is fed into a spe­cial op­ti­cal fi­bre con­tain­ing lightemit­ting atoms of yt­ter­bium in its core. As the laser light bounces along the fi­bre, it con­tin­u­ally ex­cites more pho­tons from the core— build­ing up an in­tensely pow­er­ful beam.

COM­BIN­ING LASERS: While in­di­vid­ual fi­bre lasers can reach tens of kilo­watts, en­gi­neers re­alised it was more ef­fi­cient to com­bine the power of up to 100 smaller laser beams into one fi­bre. To do it, en­gi­neers bor­row a trick from the com­mu­ni­ca­tions in­dus­try called “spec­tral beam com­bin­ing”. The beams are de­signed with slightly dif­fer­ent wave­lengths so they wouldn’t in­ter­fere with one an­other. In tele­coms, this means higher band­width in­ter­net. Here, it makes for a highly ef­fi­cient and pow­er­ful ray- gun blast.

“How­ever it is done, it is cer­tain that a beam of heat is the essence of the mat­ter. Heat, and in­vis­i­ble, in­stead of vis­i­ble, light. What­ever is com­bustible flashes into flame at its touch, lead runs like wa­ter, it soft­ens iron, cracks and melts glass …” HG Wells, The War of the Worlds, 1897

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