coherrent Optical RF Beamforming

Abstract

Tho merits of optical rf beamforming an roviowod, together with tho P s r f o n n ~ c ~ and limitations of cumat systoma. Exparimental rmults nre presented on one configusdon operated at 1.3 and 10 GHz in conjunction with lhw and confomd antenna array. Introdaction The inhbiem advantages of phowd May antonnaa over thoir muchanica~ly-mmnod counterparts havo long been rcscognistd [l], but h v o not tmen widely exploited to dato due to &e technological difneultios and co8t of thok implementation. Modem optical camponem and tuchniques appaar to o& sohdonr to many of tho Ceahnok~gicai problem [2-S] but until racently have remahad reln!ively oxpensive. Nwdelcss the “ea of optics in teleco”unidons, technologicrl bmakthroughs, and the pof the mdwtplacr, are &in& prices down, so that optically tontrolbd phaasd a m y antonma are IUreJy to bumno mlhbla and affordable ill the near ftlnIrc!. lhsy will be dopioyod in civil and military cammunications syaSms and radara, and in purely military rpplication~ such as BSM and ECM. Tbis paper begins by summarising the bS0efItt1 of phawrl m y M~BMIM, and the attractions of optid (photonic) techniques, especially in the contcrxt of this application, This is followed by a brief review of one particular twhniquu undw invosdgation in DERA which was Anrt described in MWP’% [6], togaha with mwnt modifications, and measurements in codunction with linear and conformal antenna m y a at 1,3 aad 10 OHz. The pepor concludea with the author’s apeculation on the, future of optid “forming. Pluued army antennas The principal attractions of phased array entonna tochniqueo art sumarisod below! Opthi (Photon&) Twhaiquw The principal aaraccions of optid te~~hniquea in the contact of if baanfarming am: The wmpactnw~s and lightweight of modem apticrl components, which o h derive firom the short wawtengtb (of o& ono micron). 0 The axtrane baudwidth capability, lowlow, and flexibility of optical fibre, which ~ f o spooially valuable for m o t e andlor deployable M-, ag conformal m y s on aim&, and apace-launched antenna 7 8 . Tho immunity of eignalrs in the optical domain to Eled~oMagnetic Interfmnw. Review of DERA optical rl beamfbrmer, 4 t h recent meaeuramentn. The principles of a rather simple and elegant optical rf beamformer were demribed at MWP’96 [6], together with preliminary measurements of radiation pattarns from a 9element 1,3 GHz linear nntennol m y . The buamfoming stru~arrr, used is reproduced in Figure 1, topther with details of the rust of the ante“ test facifity at DERA, Malvern. The most critical optical components of the beamformer are:(a) Two highiy-coherent 40mW didopumped YAG lasers h m Lighlwava Electronics, whose hqusncy separation can be varied from 0 to 100 GHz. By combining the outputs of these lesorsl a 1009’0 amplitudemodulamd sine wave envelop is incident on tho photodetectors, making optimum usa of both the available 1-r power and detector power=handllng capability, (b) A Meadowlark linear-nemadc liquidcrystal Shapeshifbr phase SLM, comprising 128 pixels in ti linea amy on a 2SO-miomn pitch. Each pixol of this SLM is olactrically controllable over 600 degrees of phase, with extreme rqxducibility &om pixol to pixel. This ia illustrated in Figure 2 which ia, incidentally, fat more accurate than M optical phese meesurement, since it is performod h the electrical domain. This phase-only SLh4 effectively fbnctions as an m y of fkequencyindependent microwave phase-shiffers. It is important to note that this scheme is capable of providing an arbitrary phast pattern Bcmss the array, and is not restricted to a veriable linear phase. Similarly an arbitmy amplitude pattern can be introduced via a second (amplitude) SLM, e.g. in the path of the second laser. This provides total antenna