c

Two-dimensional optical architecturesfor the receive mode of phased-array antennas

Luc Pastur, Sylvie Tonda-Goldstein, Daniel Dolfi, Jean-Pierre Huignard,Thomas Merlet, Olivier Maas, and Jean Chazelas

We propose and experimentally demonstrate two optical architectures that process the receive mode ofa p 3 p element phased-array antenna. The architectures are based on free-space propagation andswitching of the channelized optical carriers of microwave signals. With the first architecture a directtransposition of the received signals in the optical domain is assumed. The second architecture is basedon the optical generation and distribution of a microwave local oscillator matched in frequency anddirection. Preliminary experimental results at microwave frequencies of ;3 GHz are presented.© 1999 Optical Society of America

OCIS codes: 040.1240, 350.4010, 040.2840, 070.6020.

1. Introduction

The far-field pattern of a phased-array antenna iscontrolled by the relative phase and the amplitudedistribution of microwave signals emitted by regu-larly spaced transmit–receive modules. Low ob-servability, jamming robustness, or earlier detectionof targets requires wide instantaneous bandwidth.This imposes a time-delay control on the antenna.1,2

Our aim in this paper is to present two original ar-chitectures dedicated to the processing of the receivemode, using a concept similar to that demonstratedin Ref. 3 for emission. In the direct architecture thereceived microwave signals are optically carried andtravel back through the same time-delay network asthat used for the transmit mode. In the matchedlocal oscillator architecture ~MLOA! a channelizedmicrowave local oscillator, time delayed and opticallycarried, is mixed, at the antenna level, to the receivedsignals. The oscillator provides a heterodyne filter-ing, matched in frequency and direction, to the re-ceived signal. Such an approach was recentlyconsidered with quite different implementations in

L. Pastur, S. Tonda-Goldstein, D. Dolfi ~dolfi@lcr.thomson-sf.com!, and J.-P. Huignard are with Thomson-CSFyLaboratoire

Central de Recherches, Domaine de Corbeville, 91404 Orsay Ce-dex, France. T. Merlet and O. Maas are with Thomson-CSFyAirsys, 91470 Limours, France. J. Chazelas is with Thomson-CSFyRadars & Contre-Mesures, 78852 Elancourt Cedex, France.

Received 12 June 1998; revised manuscript received 8 December1998.

0003-6935y99y143105-07$15.00y0© 1999 Optical Society of America

Refs. 4 and 5. Here we propose a third implementa-tion, fully programmable and based on the optical gen-eration of simultaneous complementary delays forboth the transmit signal and the local oscillator ~LO!.

2. Operating Principles

The operating principle of the direct architecture thatwe first propose is shown in Fig. 1. As described inRef. 3, the emitted signal, channelized on p 3 p com-ponents, is optically carried through a time-delay net-work @Fig. 1~a!#. On each channel the beamintercepts one of the p 3 p pixels of a set of N spatiallight modulators ~SLM’s! @Fig. 1~c!#. Depending onthe applied voltage ~0 or VM!, each pixel acts as apolarization switch ~py2 rotation or 0 rotation!. Foreach delay block a polarizing beam splitter ~PBSi!switches the beam along one of the two paths. Thevalue of the delay is determined by the position of theprism Pi. Owing to time delays in a geometric pro-gression ~t, 2t, . . . , 2N21t!, N blocks provide ~2N 2 1!time-delay values. The maximum delay is tM 5 ~2N

2 1!t, where t is the time increment. The delay lawapplied by the time-delay network controls the scanangle of the emitted signal and the antenna far-fieldpattern.

When we now use this architecture in the receivemode @Fig. 1~b!#, the microwave signal reflected bythe target travels back to the antenna and is detectedby an array of p 3 p microwave receivers. The sig-nals issued from each receiver are used to feed anarray of modulated lasers ~direct or external modu-lation!. For radar detection in the same direction asfor emission, the received signals, optically carried,

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pc

3

have to travel through the same time-delay networkas that used for the emission mode. This permitsin-phase addition over a large frequency bandwidthof all the microwave signals received by the antenna.An array of p 3 p photodiodes then extracts the mi-crowave information from the optical carriers for pro-cessing.

This detection mode leads to some important prob-lems. Indeed, robustness to jamming requires opto-electronic elements able to handle high-powermicrowave signals. At the same time they must beable to detect low-level signals, corresponding to thetarget signatures in the range limit. The ratio jam-mer signal to lower signal requires linearity of optolinks in the 100–120-dB range.6,7 This correspondsto spurious free dynamic range in the 70–80-dByMHz2y3 range.8,9 Such performance is still difficult toattain over large bandwidth with currently availableoptocomponents. This first architecture, by use of di-rect transposition of the received microwave signals,operates with a time-reversal approach. Below wedetail an approach with a matched LO that operates ina way similar to that of phase conjugation ~Fig. 2!.

To overcome this dynamic range limitation, we de-veloped the MLOA, in which a channelized micro-wave LO, optically carried, is used for mixing with

Fig. 1. Principle of the direct architecture. ~a! Transmit mode,~b! receive mode, ~c! optical implementation.

106 APPLIED OPTICS y Vol. 38, No. 14 y 10 May 1999

the received microwave signals.5 The operatingprinciple is shown in Fig. 3. In a way similar to thatin Fig. 1~a! two optical beams are excited by a micro-wave signal at fe and fLO. In the emission mode, forfrequency fe and time-delay tk, the phase of the radi-ating element k is

fke~t! 5 2pfe~t 2 tk!. (1)

After a round-trip time 2T from the antenna to thetarget, receiver k detects a signal of phase

fkr~t! 5 2p~ fe 1 fD!~t 2 2T 1 tk!, (2)

where fD ,, fe is the Doppler frequency that is due tothe target velocity. Below, fr 5 fe 1 fD is the receivedfrequency associated with fe. As for the emitted sig-nal the microwave LO, with ~principal! frequency fLO,is channelized and optically carried through the time-delay network. The carriers are detected by a p 3 p

hotodiode array. Each photodiode provides a mi-rowave signal of frequency fLO, time delayed accord-

ing to the law given by the delay network. Each ofthose channelized microwave signals is then mixed,at the transmit–receive module level, with the corre-

Fig. 2. Principle of the controlled delays.

Fig. 3. Principle of the MLOA.

4

t

b

j

sponding component of the received signal. This re-sults in a low-frequency microwave signal of phase

fks~t! 5 fk

LO~t! 2 fkr~t!, (3)

where fkLO is the phase of the LO. The delay law

experienced by the LO is chosen to perform in-phaseaddition of all the low intermediate frequency signalscoming out of the mixers. To achieve this condition,a remarkable property of an optical architecturebased on liquid-crystal SLM’s can be used here.When two cross-polarized beams travel along thesame channel, their polarizations stay cross polar-ized, and they experience complementary paths ~Fig.!. One of the two beams is delayed by tk, the other

~cross polarized! by ~tM 2 tk!, where tM is the maxi-mum time delay. According to Eq. ~1!, if we choosethe LO complementary to the emitted signal, thephase of the LO will be

fkLO~t! 5 2pfLO@t 2 ~tM 2 tk!#. (4)

According to Eqs. ~2!–~4!, the resulting mixed signalswill have the phase

fks~t! 5 2p@ fi~t 1 tk! 2 fLOtM 1 2fr T#, (5)

where fi 5 fLO 2 fr is the intermediate frequency andhe term 2p~ fit 2 fLOtM 1 2frT! is common to all the

channels. In the case of homodyne detection, wherewe would have fLO 5 fr, the phase, at the output ofeach channel, reduces to fk

s~t! 5 2p~ fLOtM 1 2frT!and ensures that all the channels are added in phase.Note that for an emitted signal with a large frequencybandwidth, the condition fLO 5 fe is satisfied for eachcomponent of the spectrum by use of a large-frequency-bandwidth LO. With this complemen-tary path approach we can therefore generate aperfectly matched LO. To avoid any cross talk be-tween emission and reception, the wavelength of theLO and the emission optical carriers have to be dif-ferent. At the output of the delay network, a di-chroic mirror switches the carriers on two differentphotodiodes ~Fig. 3!. One will provide the signal toe emitted, the other one the microwave LO signal.Unfortunately, in radar applications, because of

amming, the LO frequency fLO must remain out of

Fig. 4. Complementary path. Two cross-polarized b

the radar bandwidth Df. The ideal homodyne pro-cessing must be replaced with heterodyne detection,where fLO Þ fr. The phase difference dfjk 5 fk 2 fjbetween channels j and k becomes

dfjk 5 2pfi~tk 2 tj! 5 2pfidtjk. (6)

If this difference is smaller than the quantizationphase error, it can be neglected. For an antenna ofdimension L, scanning in the direction u, the maxi-mum value dtM of dtjk is given by cdtM 5 L sin u.According to a distance between two of the p 3 pmodules of approximately Ly2,1 with L the emittedwavelength, the maximum phase error is

dfM , p~p 2 1! fi~Lyc!sin u 5 p~p 2 1!~ fiyfe!sin u.

(7)

With a bandwidth Dfyf of ;30% near fe 5 3 GHz, fican be as high as 900 MHz. This results in an pro-hibitive maximum phase error of 4p for an antenna of32 3 32 modules and a 630° scanning range. In thetime-delay control architecture4 the microwave sig-nal driving the antenna elements was transmitted ona single wavelength tunable optical carrier by meansof a bank of dispersive fiber-optic links. In the re-ceive configuration the beam former generated prop-erly phased LO signals for downconverting thesignals received by the antenna elements. The op-tical wavelength settings required for the receivemode steering were complementary to the transmitmode. Frankel and Esman4 achieved such opera-tion in the system by tuning the wavelength to theopposite side of the center wavelength. They dem-onstrated a 635° azimuth steering with two antennaelements over a 6–16-GHz frequency range. TheLO and the rf signals were offset by 20 MHz fromeach other. Considering radar operational con-straints, such an intermediate frequency ~20 MHz!results in bandwidth limitation problems.

To overcome this limitation, we propose two ap-proaches. One is based on frequency translationand uses cascaded double mixing. The other one,using a doubled number of pixels, is based on time-delay scaling to provide in-phase signals. In thedouble-mixing approach the LO is obtained in two

experience complementary paths through the SLM’s.

eams

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pq

Ep

Tr

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successive steps ~Fig. 5!. The first step provides thesame channelized LO’s as described above but withfLO 5 fe. Then those signals are mixed with in-

hase microwave signals at an intermediate fre-uency fi . Df. In channel k this provides a

resulting LO of phase

fkLO~t! 5 2p~ fe 2 fi!t 2 2pfe~tM 2 tk!, (8)

which is mixed with the corresponding component ofthe received signal given by Eq. ~1!. According to

qs. ~2!, ~3!, and ~6! the resulting mixing is a signal ofhase

fks~t! 5 2p~ fD 1 fi!t 1 2pfe~tM 2 2T! 2 2pfD~2T 2 tk!.

(9)

he phase difference between channels j and k is noweduced:

dfjk 5 2pfD~tk 2 tj! # 2pfDtM. (10)

With a realistic value of fD of the order of 3 kHz cor-responding to a target velocity of 150 m s21 and fe 5 3GHz, the difference remains smaller than dfM ; 1024

rad. For comparison a system of N 5 10 SLM’s, withthe delay increment t 5 tMy~2N 2 1!, will induce theintrinsic phase error of discretization, for a frequencyfe ; 3 GHz, of dft ; py32 .. dfM. With this methodof double mixing, the error becomes negligible.

In the second method it is possible to keep only onelevel of mixers. In this case the system contains2~p 3 p! processing channels for ~p 3 p! active mod-ules to control independently the LO and the emittedsignal ~Fig. 6!. The phase obtained at the output ofmixer k is

fks~t! 5 2pfit 1 2p~ fLOtk

LO 2 frtke! 2 2pfLOtM 1 4pfr T,

(11)

where tM 2 tkLO is the delay experienced by the LO

on channel k and tke the delay experienced by the

emission signal on channel k. The frequency fLOstill has to be out of the radar bandwidth ~ fLO 2 fe .Df !. The phase difference between channels j and kis written as

dfjk 5 2pfLO~tkLO 2 tj

LO! 2 2pfr~tke 2 tj

e!, (12)

Fig. 5. Principle of the double mixing.

108 APPLIED OPTICS y Vol. 38, No. 14 y 10 May 1999

where the term ~tke 2 tj

e! is given by the antennadiagram. We can cancel this difference for only onefrequency of the bandwidth, for example, the centralone fe, by satisfying the condition

~tkLO 2 tj

LO! 5 ~tke 2 tj

e! fryfLO. (13)

3. Experimental Setup and Results

To prove the principle we implemented an experi-mental setup ~Fig. 7! equivalent to a time-delay con-rolled array of two antennas in the receive mode. Itonsists of one single-frequency laser ~l 5 532 nm,00 mW, Dl 5 1 MHz!, two acousto-optic Bragg cellsedicated to receiving the signals of each antenna,ne polarization splitting device, and two cascadedelay devices ~composed of SLM’s and polarizationeam splitter PBS’s!, thus enabling the programmingf the delay law. The delayed signals are then de-ected by fiber-pigtailed photodiodes, amplified ~130

dB!, and displayed onto an oscilloscope to be com-pared with the reference signal.

The dual-frequency beams obtained with eachBragg cell @according to the scheme of Fig. 1~c!# arecross polarized. These two beams are combined ac-cording to Fig. 7. The polarization splitting device~PBS 1 prisms! enables us to separate the two opticalcarriers with respect to their polarization. Thisleads to four parallel beams at the output of the po-larization splitting device. Each beam passesthrough one pixel of the SLM. The SLM’s act asdelay switches that let the beams travel through thecascaded delay devices with respect to the delay law.The two cascaded devices are implemented to gener-ate time delays of either t1, t2, or tMAX 5 t1 1 t2.The beams are able to travel through four differentpaths; one of these paths is considered to be the ref-erence path.

For the direct architecture the received signal issimulated by a microwave signal generator that pro-vides a signal of 20 dBm at fr 5 ~2785 6 0.001! GHz.The precision of the frequency depends on the gener-ator we use. The same microwave signal is used forthe excitation of the two Bragg cells. Only the two1 polarized beams, each coming from one of the twocells, are considered in the experiment. Beam 1 ischosen to be the reference beam and travels throughthe reference path. Beam 2 is delayed either by thereference delay ~when it travels the reference path!,by t1 5 214 6 2 ps, by t2 5 140 6 2 ps, or by tMAX 5

Fig. 6. Processing with 2~p 3 p! pixels. This configuration per-mits cancellation of the phase error for one frequency in the band-width.

24mstv

ifrsptp

rba

Fig. 7. Experimental setup. Direct architecture, f1 5 f2. Onlyeceived signals by an antenna of two radiating elements. MLOAeams are used and detected. Double-mixing, only AOM1 is used,cousto-optic modulator; P, 45°-oriented polarizer!. This corresp

Fig. 8. Experimental results for the direct architecture. Top sig-nal is reference. In each picture the bottom signal corresponds tothe four time delays: ~a! dt 5 0, ~b! dt 5 ~221 6 4! ps, ~c! dt 5~150 6 4! ps, ~d! dt 5 ~371 6 4! ps.

354 6 4 ps. The tolerances on the time delays arecalculated from the precision with which we are ableto position the elements of the two-dimensional- ~2D-!time-delay network. We observed the time-delayedsignals with the oscilloscope. The measured timedelays are 0 @Fig. 8~a!, beams 1 and 2 are in phase#,20 6 4 ps @Fig. 8~b!#, 150 6 4 ps @Fig. 8~c!#, and 370 6ps @Fig. 8~d!#, respectively. The tolerances on theeasured time delays are related to the scope preci-

ion. Such results are in good agreement with theheoretical time delays. They demonstrate the re-erse principle of the transmit mode.3For the MLOA we realized two configurations that

correspond to the single and the double mixing.In the single mixing ~cf. Fig. 3! the first Bragg cell

s fed by a microwave signal of power 20 dBm atrequency fr 5 ~2785 6 0.001! GHz to simulate theeceived signals from the two antennas. These twoignals are carried by two optical beams with crossedolarizations. They are then delayed by the 2D-ime-delay network to simulate the time-delay ex-erienced by one antenna with respect to the other.

beams, from the four generated, are used. They simulate thele mixing: f1 5 fr 5 2.785 GHz, f2 5 fLO 5 2.885 GHz. The fourding rf signal and LO at the same frequency f1 5 2.785 GHz ~AOM,to the case in which the Doppler shift is zero.

two, singprovionds

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The second Bragg cell is fed by the LO signal ofpower 20 dBm at frequency fLO 5 2.885 GHz. Thentermediate frequency is then fi 5 ~100 6 2! MHz,orresponding to a bandwidth of Dfyf 5 3%. TheD-time-delay network delivers four optical chan-els. These channels carry the signals to be delayedy either 0, t1 5 320 ps, t2 5 132 ps, or tM 5 t1 1 t2

5 452 ps with respect to the reference path. In theexperimental setup the received signals are not ob-tained from the reflection of an electronically scannedemitted signal on a target ~reflection that inducestime reversal!. The so-called received signals aredirectly generated with time delays that simulatepropagation but not time reversal. According to thisanalysis a matched LO—with complementary timedelays equivalent to time reversal—is obtained whenthe LO and the simulated received signal experiencethe same time delay on each channel before mixing.The maximum time delay tM corresponds to a scanangle of 5° for an antenna of 32 3 32 modules. Ex-

erimental results are reported in Fig. 9. The twoine traces show the time delay when the path isaximum. As expected, when the time difference isaximum, the time delay is actually measured at t 5

460 6 20! ps. With a bandwidth of only 3% and acan angle of 5° the phase error between the twohannels is ;18°. Compared with the phase-

quantization error, equal to 6° for a 10-bits-phaseencoding ~potentially obtained in our case with N 510 SLM’s!, it illustrates the strong limitation of theingle-mixing method.In the double mixing we used only one Bragg cell

excited at f 5 ~2785 6 0.001! GHz. In this case theeceived signal ~simulated by the same mean as forhe single mixing! and the LO have the same fre-uency, corresponding to the case in which fD 5 0.

The optical carrier, separated into two beams, willprovide the simulated received signal and its associ-

110 APPLIED OPTICS y Vol. 38, No. 14 y 10 May 1999

ated LO, cross polarized with respect to the receivedsignal. The received signal is then time delayed bythe 2D-time-delay network with respect to the imple-mented delay law. The LO signal is delayed by thecomplementary delay. To shift the frequency of thisoptically carried LO, a first mixer is used. One of itsinputs is fed with the detected LO channel, the otherwith a microwave signal at frequency fi 5 ~700 6 1!

Hz provided by a uhf generator. The resultingignal ~at frequency fLO 2 fi! is then mixed in a sec-nd mixer with the received signal. This doubleixing leads to a signal at frequency fi 1 fD 5 fi.

The phase difference between two signals, one car-ried by the optical beam on the longest path and oneby the optical beam on the shortest path, was mea-sured ~Fig. 10! to be smaller than 10 ps ~limit of rangeof the oscilloscope we used!. Such a small time valuepermits in-phase addition of received signals. Itdemonstrates the principle of the double mixing ofthe MLOA.

4. Conclusions

To summarize, optical processing of both emissionand receive modes of a phased-array antenna hasbeen described. For its simplicity the direct archi-tecture, with direct transposition of signals onto anoptical carrier, seems well adapted to large-bandwidth applications that are less demanding interms of dynamic range, such as electronic warfaresystems. The MLOA, in which the LO, matched indirection and frequency, is optically carried seems tobe a potentially good solution to the dynamic limita-tion of the direct architecture. In this case it couldachieve the performance required by specific radarapplications. The principle of the double mixing hasbeen experimentally demonstrated. A two-channelarchitecture with four time delays has been imple-mented and provides results in good agreement with

Fig. 9. Experimental results for the single mixing. In this ex-periment fr 5 2.785 GHz, fLO 5 2.885 GHz, fi 5 fLO 2 fr 5 ~100 62! MHz. The time delay between the two rf beams is maximum@tth 5 ~452 6 22! ps#. As expected the time delay is observed at t5 ~460 6 20! ps, corresponding to a phase error of 18°. ~a! t 5 0,Df 5 0, f1 5 100 MHz; ~b! t 5 460 ps, Df 5 18°, f1 5 100 MHz.

Fig. 10. Experimental results for the double mixing. In this ex-periment one rf beam and one LO beam at frequency f 5 2.785 GHzare used. The intermediate frequency is fi 5 700 MHz. Thebeams experience ~a! the shortest path ~t 5 0! or ~b! the longest path~t 5 450 ps, Dt , 10 ps, Df ; 5°!. The time error between these twocases is less than 10 ps ~range limit of the oscilloscope we used!.

mMc

5. R. R. Stephens, J. J. Lee, G. L. Tangonan, I. L. Newberg, and

expected performance. According to preliminary re-sults and discussion,3 it is realistic to extend theseconcepts to wide instantaneous bandwidth antennas,with as many as 103 transmit–receive modules.

We thank the Service Technique des TechnologiesCommunes of the Delegation Generale de l’Arme-

ent for partial support. We also thank Thomson-icrosonics for having provided the acousto-optic

ells and Thomson-LCD for the liquid-crystal SLM’s.

References1. H. Zmuda and E. Toughlian, Photonic Aspects of Modern

Radars ~Artech, Boston, 1995!.2. N. A. Riza, ed., Selected Papers on Photonic Control Systems for

Phased-Array Antennas, Vol. 136 of SPIE Milestone Series~SPIE Press, Bellingham, Wash., 1997!.

3. D. Dolfi, P. Joffre, J. Antoine, J. P. Huignard, D. Philippet, andD. Granger, “Experimental demonstration of a phased-arrayantenna optically controlled with phase and time delays,” Appl.Opt. 35, 5293–5300 ~1996!.

4. M. Y. Frankel and R. D. Esman, “True time-delay fiber-opticcontrol of an ultrawideband array transmitteryreceiver withmultibeam capability,” Trans. Microwave Theory Tech. 43,2387–2394 ~1995!.

H. T. Wang, “Photonic RF mixing feed for multibeam arrays,” inProceedings of the IEEEyLasers & Electro-Optics Society Inter-national Topical Meeting on Microwave Photonics MWP’97, D.Jaeger, ed. ~Duisburg, Germany, 4–6 September 1997!, PaperWE 1-6.

6. D. Dolfi, J. P. Huignard, J. Chazelas, and O. Maas, “Photonicsfor microwave processing in radar systems,” in Proceedings ofthe IEEEyLasers & Electro-Optics Society International TopicalMeeting on Microwave Photonics MWP’97, D. Jaeger, ed. ~Du-isburg, Germany, 4–6 September 1997!, Paper WE 1-5.

7. M. J. Wale, “Component technology for microwave photonicsystems,” in Proceedings of the IEEEyLasers & Electro-OpticsSociety International Topical Meeting on Microwave PhotonicsMWP’97, D. Jaeger, ed. ~Duisburg, Germany, 4–6 September1997!, Paper TH 1-2.

8. J. J. Lee, R. Y. Loo, S. Livingstone, V. I. Jones, J. B. Lewis, H. W.Yen, G. L. Tangonan, and M. Wechsberg, “Photonic widebandarray antennas,” IEEE Trans. Antennas Propag. 43, 966–982~1995!.

9. A. P. Goutzoulis and J. M. Zomp, “Development and field dem-onstration of an eight-element receive wavelength-multiplexedtrue-time-delay steering system,” Appl. Opt. 36, 7315–7326~1997!.

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