Ex­cal­ibur pro­to­type ex­tends reach of high-en­ergy lasers


High-en­ergy lasers (HEL) have the po­ten­tial to ben­e­fit a va­ri­ety of mil­i­tary mis­sions, par­tic­u­larly as weapons or as high-band­width com­mu­ni­ca­tions de­vices. How­ever, the mas­sive size, weight and power re­quire­ments (SWaP) of legacy laser sys­tems limit their use on many mil­i­tary plat­forms. Even if SWaP lim­i­ta­tions can be over­come, tur­bu­lence man­i­fested as den­sity fluc­tu­a­tions in the at­mos­phere in­crease laser beam size at the tar­get, fur­ther lim­it­ing laser tar­get ir­ra­di­ance and ef­fec­tive­ness over long dis­tances.

Re­cently, the De­fense Ad­vanced Re­search Projects Agency’s ( DARPA) Ex­cal­ibur pro­gramme suc­cess­fully de­vel­oped and em­ployed a 21-el­e­ment op­ti­cal phased ar­ray (OPA) with each ar­ray el­e­ment driven by fi­bre laser am­pli­fiers. This low power ar­ray was used to pre­cisely hit a tar­get 7 kilo­me­tres away. The OPA used in these ex­per­i­ments con­sisted of three iden­ti­cal clus­ters of seven tightly packed fiber lasers, with each clus­ter only 10 cen­time­tres across.

“The suc­cess of this real-world test pro­vides ev­i­dence of how far OPA lasers could sur­pass legacy lasers with con­ven­tional op­tics,” said Joseph Mangano, DARPA Pro­gram Man­ager. “It also bol­sters ar­gu­ments for this tech­nol­ogy’s scal­a­bil­ity and its suit­abil­ity for high­power test­ing. DARPA is plan­ning tests over the next three years to demon­strate ca­pa­bil­i­ties at in­creas­ing power lev­els, ul­ti­mately up to 100 kilowatts—power lev­els other­wise dif­fi­cult to achieve in such a small pack­age.”

In ad­di­tion to scal­a­bil­ity, Ex­cal­ibur demon­strated near-per­fect cor­rec­tion of at­mo­spheric tur­bu­lence—at lev­els well above that pos­si­ble with con­ven­tional op­tics. While not typ­i­cally no­tice­able over short dis­tances, the at­mos­phere con­tains tur­bu­lent den­sity fluc­tu­a­tions that can in­crease the di­ver­gence and re­duce the uni­for­mity of laser beams, leading to dif­fuse, shifted and splotchy laser end­points, re­sult­ing in less power on the tar­get. The re­cent Ex­cal­ibur demon­stra­tion used an ul­tra-fast op­ti­mi­sa­tion al­go­rithm to ef­fec­tively “freeze” the deeply tur­bu­lent at­mos­phere, and then cor­rect­ing the re­sult­ing static op­ti­cally aber­rated at­mos­phere in sub­mil­lisec­onds to max­imise the laser ir­ra­di­ance de­liv­ered to the tar­get.

These ex­per­i­ments val­i­dated that the OPA could ac­tively cor­rect for even se­vere at­mo­spheric dis­tor­tion. The demon­stra­tion ran sev­eral tens of me­tres above the ground, where at­mo­spheric ef­fects can be most detri­men­tal for army, navy and ma­rine corp ap­pli­ca­tions. In ad­di­tion, these ex­per­i­ments demon­strated that OPAs might be im­por­tant for cor­rect­ing for the ef­fects of boundary layer tur­bu­lence around air­craft plat­forms car­ry­ing laser sys­tems.

The suc­cess­ful demon­stra­tion helps ad­vance Ex­cal­ibur’s goal of a 100-kilo­watt class laser sys­tem in a scal­able, ul­tra-low SWaP OPA con­fig­u­ra­tion com­pat­i­ble with ex­ist­ing weapon sys­tem plat­forms. Con­tin­ued de­vel­op­ment and test­ing of Ex­cal­ibur fi­bre op­tic laser ar­rays may one day lead to multi-100 kilo­watt class HELs in a pack­age 10 times lighter and more com­pact than legacy high-power laser sys­tems. Fu­ture tests aim to prove the OPA’s ca­pa­bil­i­ties in even more in­tense en­vi­ron­men­tal tur­bu­lence con­di­tions and at higher pow­ers. Such ad­vances may one day of­fer im­proved re­li­a­bil­ity and per­for­mance for ap­pli­ca­tions such as air­craft self-de­fence and bal­lis­tic mis­sile de­fence.

“With power ef­fi­cien­cies of more than 35 per cent and the nearper­fect beam qual­ity of fi­bre laser ar­rays, these sys­tems can achieve the ul­tra-low SWaP re­quired for de­ploy­ment on a broad spec­trum of plat­forms,” said Mangano. “Be­yond laser weapons, this tech­nol­ogy may also ben­e­fit low-power ap­pli­ca­tions such as laser com­mu­ni­ca­tions and the search for, and iden­ti­fi­ca­tion of, tar­gets.”

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