Nature RadioEng

8/13/2019 Nature RadioEng

1/12

nature photonics |VOL 1 | JUNE 2007 |www.nature.com/naturephotonics 319

REVIEW ARTICLE

JOS CAPMANY1* AND DALMA NOVAK2

1Universidad Politcnica de Valencia, Institute of Telecommunications and

Multimedia Applications, Edificio 8G, Ciudad Politecnica de la Innovacion,

Valencia, Valencia 46023, Spain2Pharad LLC, 797 Cromwell Park Drive, Suite 5, Glen Burnie, Maryland 21061, USA

*e-mail: [email protected]

Microwave photonics (MWP) has been dened as the studyo photonic devices operating at microwave requencies andtheir application to microwave and optical systems13. Its initialrationale was to use the advantages o photonic technologies to

provide unctions in microwave systems that are very complex oreven impossible to carry out directly in the radiorequency (RF)domain. But MWP is also succeeding in incorporating a varietyo techniques used in microwave engineering to improve theperormance o photonic communication networks and systems.

Figure 1 shows a schematic highlighting the undamentalconcept o a simple microwave photonic link. One or more analogelectrical signals at a microwave requency are transported over anoptical bre or photonic link, with electrical-to-optical and optical-to-electrical conversion at the transmitting and receiving side,respectively, o the photonic link. Te key advantages o microwavephotonic links over conventional electrical-transmission systems,such as coaxial cables or waveguides, include reduced size,weight and cost, low and constant attenuation over the entire

microwave and millimetre-wave modulation requency range,immunity to electromagnetic intererence, low dispersion andhigh data-transer capacity. Te weight and attenuation benetso microwave photonic links over coaxial cables are particularlycompelling: typically 1.7 kg km1and 0.5 dB km1 or bre with567 kg km1 and 360 dB km1at 2 GHz or a coaxial cable.

Our aim here is to introduce MWP to the general photonicscommunity. For that purpose we review the basic conceptsand historical developments behind MWP, describe its maintechnologies and eld o applications, and also report on someo the most notable research results obtained in recent years. Weconclude by discussing our perspective on the uture prospectsand challenges or MWP.

In many aspects the historical development o MWP runsin parallel with the eld o optical communications. According

Microwave photonics combines two worlds

Microwave photonics, which brings together the worlds of radiofrequency engineering and

optoelectronics, has attracted great interest from both the research community and the commercial

sector over the past 30 years and is set to have a bright future. The technology makes it possible

to have functions in microwave systems that are complex or even not directly possible in the

radiofrequency domain and also creates new opportunities for telecommunication networks. Here

we introduce the technology to the photonics community and summarize recent research and

important applications.

to Alwyn Seeds at University College London, the developmento the rst semiconductor lasers and electrooptic modulatorssuitable or gigahertz modulation in the 1970s, the availabilityo low-loss silica multimode and single-mode optical bres, andthe development o ast depletion pin and avalanche detectorsgiving useul microwave bandwidth response, are three o theundamental historical developments in MWP2.

Since the early experiments during the late 1970s, the eldo MWP has expanded to address a considerable number oapplications including high-perormance analog microwavephotonic bre links or antenna remoting in radar systems,microwave photonic links or cellular, wireless, satellite and radio-

astronomy applications, cable television systems, optical signalprocessing and high-speed optical packet switched networks.Some o these applications can be considered as commerciallyestablished (or instance, high-perormance microwave photoniclinks have an annual sales market3 above $100 million), whereasothers are still in a phase o research and development.

DEVICES AND MATERIALS

Te undamental elements o a microwave photonic link aredevices that offer signal modulation, or control, or detection atvery high requencies. Since the rst semiconductor laser wasinvented, much progress has been made in creating lasers that canbe directly modulated at microwave and even millimetre-wave

bandwidths. Te modulation bandwidth o a semiconductor laseris an intrinsic parameter set by the relaxation resonance requencyo the device, which depends on a number o actors, includingphoton lietime, differential gain, carrier recombination time andoptical output power. Te achievement o high direct modulationbandwidths requires an optimization o both the internal structureo the semiconductor laser and the design o the laser packagebecause electrical parasitics associated with the laser chip andpackage will also limit the requency response4.

Well over a decade ago5, efforts commenced to increase thedirect modulation bandwidth o semiconductor lasers throughthe incorporation o quantum wells and the addition o strainin the semiconductor material. Since then, 1.55-m InGaAsPquantum-well semiconductor lasers operating at requenciesgreater than 30 GHz have been demonstrated6. Another approach


2/12

REVIEW ARTICLE

320 nature photonics |VOL 1 | JUNE 2007 |www.nature.com/naturephotonics

to increasing the laser modulation bandwidth is to enhance therequency response resonantly, using external cavity lasers7 ormonolithic multisection lasers8. Using such techniques, narrowtransmission windows at millimetre-wave requencies have beenachieved. Recently a multisection, 1.55-m distributed Braggreector laser incorporating a coupled-cavity injection-gratingdesign was reported9, with an intrinsic 3-dB modulation bandwidtho 37 GHz. Te principle behind this device is to extend the length

o the laser cavity, thereby increasing the photon round-trip timeand enabling the photonphoton resonance to interact with theelectronphoton relaxation oscillation.

Optical injection locking is another technique that enhancesthe resonance requency o semiconductor lasers10. Using such ascheme, researchers at the University o Caliornia (UC), Berkeley,have demonstrated a 72-GHz resonance requency using a 1.55-mInGaAsP distributed-eedback (DFB) laser injection-locked toan external cavity diode laser11. Other UC Berkeley researchersare also using strong optical injection locking to enhance theresonance requency o semiconductor lasers, and have reporteda 50-GHz resonance requency in an optically injection-locked1.55-m vertical-cavity surace-emitting laser (VCSEL)12. Underree-running conditions the VCSEL showed a relaxation resonance

o only 7 GHz.Te development o large-bandwidth external modulatorshas also seen intense investigation over the past two decades.For their practical application in MWP systems, it is imperativethat these devices eature the characteristics o broad bandwidth,low drive voltages, good linearity, bias stability, high opticalpower-handling ability and low optical insertion loss. o give therequired electrooptic effect in an external modulator, materialssuch as lithium niobate, semiconductors or polymers can be used,and travelling-wave intererometric structures are generally usedto achieve a broad requency response.

Tere have been a number o demonstrations o lithiumniobate intererometric modulators with bandwidths in excesso 40 GHz and relatively low drive voltages 13,14. Trough special

design o the modulator electrode structure to incorporate a ridgewaveguide, 3-dB electrical bandwidths o 30 and 70 GHz withdrive voltages o 3.5 V and 5.1 V, respectively, have been achieved14.Several examples o broadband electrooptic modulators based onsemiconductor materials have also been reported15,16. Althoughthese devices have bandwidths in excess o 30 GHz, their drivevoltages are rather high (over 10 V at 40 GHz) because o therelatively low bulk electrooptic coeffi cient o semiconductormaterials as well as a poor overlap o the applied electric eldand the optical mode. Te bre-to-bre coupling loss o suchmodulators also tends to be high (up to 10 dB) compared withlithium niobate devices (typically less than 4 dB).

Organic polymers have several attractive eatures or integratedoptical applications and can be made electrooptic using high-temperature poling methods. Several broadband electrooptic

polymer modulators have been developed17with bandwidths anddrive voltages similar to their semiconductor counterparts, butproblems associated with this technology remain, namely thepower-handling capacity and the long-term bias stability.

Electro-absorption modulators (EAMs) are made romIIIV-compound semiconductors and are either based on bulksemiconductor materials incorporating the FranzKeldysh effector make use o quantum-well structures to give the quantum

conned Stark effect. Electro-absorption modulators based onquantum-well structures18have displayed bandwidths in excess o40 GHz with drive voltages o less than 4 V. Because o ree-carrierabsorption and band-to-band absorption, however, the opticalpropagation loss o an EAM is large, typically 1520 dB mm1,restricting the size o the device to less than several hundredmicrometres. As a result EAMs are considered as lumped-elementcomponents and their speed o operation determined by theresistance capacitance time constant o the circuit. Bandwidthsin excess o 60 GHz can be achieved by lowering the terminatingresistance o the EAM, but this comes at the expense o a higherdrive voltage19, although travelling-wave techniques can potentiallylower the required drive voltage and improve the extinction ratio20.Electro-absorption modulators typically suffer rom relatively

poor optical power-handling capabilities and are also verysensitive to wavelength and temperature changes, so strict biascontrol is necessary during operation. A key advantage, however,is their ability to be directly integrated with semiconductor lasers,and a travelling-wave electrode EAM integrated DFB laser with abandwidth in excess o 50 GHz was recently reported 21.

For the detection o optical signals in MWP systems,photodetector technologies with high responsivities, bandwidthsand optical power-handling capability are required. Tephotodetector bandwidth is ultimately limited by both internaltransit-time considerations and extrinsic actors, such as electricalparasitics. High-speed photodetectors operating in the range1.31.55 m have been reported based on surace-illuminatedor vertically illuminated techniques with 3-dB bandwidths

greater than 110 GHz being achieved22

. Surace-illuminatedphotodetectors, however, suffer rom an inherent trade-off betweenbandwidth and device effi ciency. Te edge-illuminated waveguidephotodetector is one device structure that has been demonstratedas being able to achieve both a large bandwidth and effi ciency;a 45-GHz 3-dB bandwidth and 90% internal quantum effi ciencyat 1.55 m has been reported23. By also designing the waveguidephotodetector to be a travelling-wave type, where photonabsorption occurs in a distributed manner along the length o thedevice, the parasitic bandwidth limitations can be reduced, and abandwidth o 172 GHz has been reported24.

High-requency photodetectors with high saturation powers andlarge electrical output powers are also very useul in certain MWPapplications. Te saturation power is limited by the nonlinearity o the

photodetector response, which is caused by a decrease in the electriceld and a reduction in the carrier velocity due to a spacechargeeffect. A photodetector structure that can overcome the drawbacko a low saturated carrier velocity is the uni-travelling-carrierphotodetector (UC-PD) developed by N Electronics almost adecade ago25. In the UC-PD only electrons travelling at a velocitymuch higher than the saturation velocity contribute to the spacecharge effect, and consequently very high output photocurrents canbe achieved. In addition, the UC-PD can be designed to give a veryhigh requency o operation because the depletion layer thickness isindependent o the absorption layer thickness. A UC-PD with abandwidth o 94 GHz and peak photocurrent o 184 mA has beenreported26, and more recently high-output-power UC-PD moduleswith waveguide output ports have been developed or even higher-requency operation27, extending beyond 300 GHz.

Electrical to opticalconverter

Optical to electricalconverter

Microwavesignal(s)

Opticaltransmission

medium

Figure 1Schematic highlighting the fundamental concept of an analog microwavephotonic link. One or more microwave signals are converted into the optical domainbefore transmission through a photonic or optical fibre link. At the other end of thephotonic link, the signal is converted back into the electrical domain.


3/12

REVIEW ARTICLE


Other devices that may have potential applications in MWPsystems are microwave devices that can be controlled using opticalsignals, including ampliers, oscillators, switches and mixers.Here carriers are photogenerated directly within the microwavedevice by the incident optical signal, thereby changing the devicecapacitance or increasing the conductivity o the semiconductormaterial. Te study o optically controlled microwave devicesbegan over two decades ago with a view to their potentialapplication in optical phased-array radars (a good overview canbe ound in re. 28), but there have been ewer developments overrecent years.

RADIO-OVER-FIBRE SYSTEMS

One o the main applications o MWP technologies is thetransport and distribution o radio or wireless signals overoptical ibre29 an area o intense research and investigationover the past 20 years. he integration o ibre-optic andwireless networks orm what is oten reerred to as a hybrid-ibre-radio (HFR) system, which has become an essentialtechnology or the provision o untethered access to broadbandwireless communications. Its uses include last-mile solutions,improvement o radio coverage and capacity, and backhaul ocurrent and next-generation wireless networks. he transport oa number o modulated radio signals over optical ibre in an HFRsystem is very similar in concept to the delivery o video signalsin optical ibre-cable television systems, which use subcarriermultiplexing to combine multiple video signals in the RF domainor transmission over ibre by a single wavelength30.

Te advantages o optical bre as a transmission mediummake it the ideal solution or effi ciently transporting radio signalsrom a central offi ce to remotely located antenna sites. One othe key benets o radio-over-bre systems is that they allow aexible approach or remotely interacing to multiple antennas,with the ability to reduce system complexity by using a centralizedarchitecture that incorporates a simplied antenna module locatedcloser to the customer. By allowing centralized control over a largegeographical area, HFR systems can support large uctuations intraffi c load through the provision o dynamic-channel-allocationcapabilities31. Hybrid-bre-radio technologies also provide auture-proong capability where by proper design, multiple radioservices and standards can be accommodated. Te types o wirelesssystems where HFR technology is now being used include cellular

networks, indoor distributed antenna systems and wireless localarea networks (WLANs), as well as xed and mobile broadbandnetworks that can provide very high bandwidth services to users.

Hybrid bre radio is now well and truly a commercial realityor indoor applications in the orm o distributed antenna systemsas well as outdoor wireless systems. An example o a commercialHFR system manuactured by Andrew Corporation or indoorwireless signal distribution32 is shown in Fig. 2. Tis opticaldistributed antenna system (the ION) provides uniorm radiocoverage and is capable o transporting a variety o radio signalsat requencies ranging rom 800 to 2,500 MHz, covering cellularand WLAN applications. Tis particular system has recently beendeployed at a number o important sporting events, includingthe 2000 Olympic Games in Sydney, Australia, and the 2006International Football Association World Cup held in Germany32.

Equipment hotel

BTS

BTS

BTS

WLAN

Master unit

ION-B

ION-B

ION-BION-M

ION-M

ION-M

ION-B

ION-B

ION-B

ION-B

ION-M

Figure 2A schematic showing an example of a commercially available HFR system: Andrew Corporations ION (intelligent optical network) family of optical distributed antennasystems. (BTS: base transceiver station; ION-B: formerly Britecell ; ION-M: formerly MMRI)


4/12

REVIEW ARTICLE


echnical challenges or the HFR systems are the developmento suitable technologies and architectures or effi cient conversionand distribution o the radio signals, while reducing thecomplexity o the hardware located at the remote antenna site.Tis is particularly important or wireless systems operating in themillimetre-wave requency range, where the deployment o HFRsystems will require the installation o a larger number o antenna

modules. Te radio environment also sets key requirements orthe HFR system, including the spurious ree dynamic range othe link, which denes the usable dynamic range beore spuriousnoise intereres or distorts the undamental signals. Ultimatelythere is a trade-off between the complexity o the electronic/RFand the optoelectronic interaces in the central offi ce and theantenna unit.

Tere are several possible approaches to transporting radiosignals over optical bre, as shown in Fig. 3, each with certaintrade-offs. In an RF-over-bre transport scheme the radio signalsare transported directly over the bre at the wireless transmissionrequency, without the need or any subsequent requency up- ordown-conversion at the remote antenna sites. Radiorequency-over-bre signal transport or HFR systems operating at cellular and

WLAN systems (


5/12

REVIEW ARTICLE


to-the-home), is also being explored, leading to transparentheterogeneous access architectures5456.

New concepts or simpliying the antenna-module architectureare also being explored. Te concept o a completely passive antennaaccess point based on the use o a single EAM or HFR systemsoperating at requencies below 2 GHz was demonstrated a decadeago57. Since then, the use o a single electro-absorption transceiverin a 60-GHz HFR system has been demonstrated58and a variety otechniques have been reported59,60or re-using the optical carrier

to avoid the need or an expensive high-perormance laser at theantenna site. Most recently, the transmission o ultrawideband(UWB) signals in HFR systems is being investigated owing to theemergence o new WPAN standards61.

OPTICAL BEAM FORMING

One o the earliest applications o MWP technology was the useo optical-bre networks or distributing signals, as well as beamorming in phased-array radar systems. Phased-array antennaswork on the principle that by controlling the relative phase o anRF signal between successive radiating elements o an antennaarray, a beam can be created that will radiate in a specic direction.A beam-orming network is then used to create the required phase

shifs at the antenna inputs. Te operating requencies or thesesystems can extend rom the lower microwave region up into themillimetre-wave band.

Optical bre offers considerable potential or replacing thebulky, heavy and lossy coaxial cable or waveguide eed networksin phased-array radar systems. Optically controlled phased-arrayantennas have a number o important benets including size,weight, bandwidth, propagation loss, immunity to electromagneticintererence, remoting capability and simplied transmitting/receiving architectures. Functions, such as the distribution o RFsignals, phase shifer control, true time-delay beam orming andprocessing o RF signals have been actively investigated over thepast two decades6281.

In a phased-array radar, true time-delay (D) beam ormingis needed to obtain wide instantaneous bandwidth and squint-

ree operation o the antenna array. A variety o approaches orthis have been proposed and demonstrated. One o the earliestdemonstrations o an optically steered phased-array antenna useda D network that incorporated xed-length optical bres as thedelay elements and Nsets o switchable bre-optic delay lines withmultiple sources to create an N-bit delay64. Later D networksincorporated high-dispersion optical bres, where the dispersionproperty o the bre-optic link was used to create variable delays orvariable source wavelengths. An example is the single wavelength-

tunable laser used in conjunction with dispersive bre to create abre-optic prism68, shown in Fig. 4.Switching o the effective optical path length in optically

controlled phased-array systems incorporating D has also beendemonstrated using FBGs. Both non-chirped69,74and chirped75,77FBGs have been used as wavelength-selective time-delay elements.With the use o a chirped FBG, the time delay or the RF signal canbe continuously varied by sweeping the wavelength o a tunablelaser, and the required laser tuning range can be very narrow i ahigh-dispersion chirped FBG is used.

Other approaches have also been investigated or makinguse o optics in phased-array beam-orming networks includingcoherent detection techniques65, and optical-signal-processingconcepts62,66,71,76. In these latter approaches, which offer advantages

or eeding large numbers o antenna elements, the beam-ormingand multibeam operation o the phased-array antenna is controlledby an optical processor, which carries out the signal processing bymeans o Fourier optics. Another optical beam-orming schemeis based on a phase-control technique, where a spatial lightmodulator (SLM) is used to replace the microwave phase shifersin the phased array63,66.

Te area o optical phased arrays continues to be ertile orresearch and the main challenges or their practical use remainachieving robustness, system simplicity and low cost. A combinationo D and phase control to reduce system cost has been recentlyreported81, which has applications in phased-array systems withlimited bandwidth. Progress has also been made in the integrationo beam-ormer unctions in InP (re. 78) and silicon79technologies,which could substantially reduce the size and cost o the optical

PD

PD

PD

PD

PDPD

PD

PD

Tunablelaser

EOM

Fibre

splitter

o

Non-dispersive fibre

Dispersive fibre

Fibre-optic prism

Microwave input Antennaelements

Figure 4Schematic showing a fibre-optic prism with a single laser to feed numerous phased-array elements68. A true time-delay feed is formed by splitting the opticalcarrier into a fibre-optic prism, created by connecting varying amounts of highly dispersive and non-dispersive fibre. At a central wavelength,0, the main antenna beam isdirected broadside, whereas at wavelengths less (greater) than0, each of the prism fibres adds (subtracts) a time delay proportional to its dispersion, resulting in element

phasing such that the main antenna beam is steered towards (away from) the non-dispersive fibre side. (PD: photodetector; EOM: electrooptic modulator.) Reproduced withpermission from ref. 68. Copyright (1993) IEEE.


6/12

REVIEW ARTICLE


beam-orming architecture. Te use o WDM techniques to simpliythe optical beam-orming architecture is also promising.

Microwave photonics techniques are also being used inradio-astronomy applications, such as the Square KilometerArray (SKA)82and the Atacama Large Millimeter Array (ALMA)

project83

. Here, optics is being considered or the generation anddistribution o high-requency local oscillator signals to remotelylocated antennas and heterodyne receivers. Te motivation is tomake use o the low bre transmission loss and avoid the need orcostly local oscillator components that would otherwise have to belocated at each o the many antennas in the array.

SIGNAL PROCESSING

Microwave photonics techniques offer unique eatures or theprocessing o microwave, millimetre-wave and RF signals. Treeapplications that have been extensively developed are microwavephotonic ltering, microwave-signal analog-to-digital conversionand the generation o arbitrary waveorms.

FILTERSTe general concept behind microwave photonic lters is to replacethe traditional approach towards RF signal processing, shownin Fig. 5a, by a new technique bringing unique exibility andadded value82,8487. In the common approach towards microwavesignal ltering, an RF signal originating rom an RF source orcoming rom an antenna is ed to an RF circuit that perorms thesignal-processing tasks (usually at an intermediate requency bandafer a down-conversion operation). In the case o the microwavephotonic ltering approach, which is shown in Fig. 5b, the RF signalthat now modulates an optical carrier is directly processed in theoptical domain by a photonic lter based on bre and integratedoptical delay lines, devices and circuits.

Te use o photonic components brings considerableadvantages. For instance, microwave lters based on photonicscan be made tunable and recongurable, a eature that is notpossible with common microwave technologies. Progress in thiseld over the past 10 years has shown the easibility o obtaininglters with tuning ranges rom a ew megahertz to over 20 GHz,dynamic ranges above 40 dB, Q actors above 1,000 in the 2 and10 GHz bands, and the possibility o ast (below 1 ms) arbitrarytranser unction reconguration by means o electronic controlsignals. Many o the above characteristics are simply unachievablewith current microwave technology or may require costly extradown-conversion and analog-to-digital operations i digital signalprocessors (DSPs) are used.

Microwave photonic ltering technology is o interest inmany applications. For example, in radio-over-bre systems,

it provides a means both or channel-rejection and channel-selection applications. In radio-astronomy applications82 thesignal transmission rom several stations to a central site requiresthe removal o strong man-made interering signals rom theastronomy bands. Te ability to reject these interering RF signals

directly in the optical domain is a unique characteristic o thesephotonic lters. Photonic lters or RF signals can also be ointerest or applications where light weight is a prime concern; orexample, analog notch lters are also needed to achieve co-channelintererence suppression in digital satellite communicationssystems88. Finally in moving-target-identication radar systems89,90,the ltering o clutter and noise directly in the optical domaingreatly relaxes the system design.

Several approaches have been reported during the past ewyears. In most cases incoherent operation is preerred because itrenders highly stable lters ree rom environmental uctuations.Earlier work ocused on lter structures, where optical delay lineswere created by the use o bre coils and single broadband sourceswere used to eed the lter. Work at Stanord University in the

1980s demonstrated the implementation o single-mode-bredelay-line networks capable o synthesizing many sophisticatedtime- and requency-domain ltering operations91.

Te advent o FBG technology in the middle o the 1990s openedthe possibility o implementing recongurable and tunable ltersusing both uniorm and linearly chirped gratings. Researchers at theUniversity o Sydney pioneered this work9295. Te rst continuouslytunable notch lter was reported by Hunter and Minasian92 in1995, ollowed later by a bandpass conguration93. Te notch lterconguration was based on a single tunable modulated laser sourceand two long, chirped gratings on separate ports o a coupler astapping elements. Subsequent work by this group led to high-quality-actor (Q= 938) innite-impulse-response (IIR) lters with wideand continuous tunable centre requencies combining passive and

active (that is, erbium-doped) photonic waveguides94,95

. In parallel,researchers at the University o Aston reported more complexstructures capable o providing transer-unction recongurationby means o tap apodization96,97.

In 1999, researchers at the Universidad Politcnica deValencia reported the irst microwave photonic ilter capable osimultaneous transer-unction reconiguration and bandpasstunability98. Figure 6a shows the ilter layout. he principalnovelty arose rom the use o dierent optical sources orthe dierent ilter taps. he modulated optical signal is thensent to a dispersive linearly chirped FBG, which perormsa wavelength-to-time mapping producing equally spaced iltersamples. By controlling the power delivered by the laser sources,it is possible to reconigure the ilter transer unction; bychanging the wavelength separation o the optical carriers, the

Optical

receiver

Optical

c.w. source

Opticalsignal processor

Antenna

RF inputsignal RF output

signal

Opticalinputsignal

Opticaloutputsignal

Modulator

RF inputsignal

RF outputsignal

Antenna

RF circuit

a b

Figure 5Schematics of RF signal processing. a, Traditional approach. b, Microwave photonic approach. (c.w. means continuous wave.) Reproduced with permission fromref. 85. Copyright (2005) IEEE and OSA.


7/12

REVIEW ARTICLE


ilter bandpass can be tuned. Figure 6b shows a typical spectrumcentred around 2.5 GHz. he tuning range can be easi ly extendedover 40 GHz, and although the main-to-sidelobe ratio in this casewas around 13 dB, researchers at ACREO, Sweden, have reported

a ilter with a ratio o over 40 dB in the 10 GHz region99

. Lower-cost tunability using a spectrally sliced broadband source can beachieved by changing the dispersion characteristic o the FBGdevices100,101or by switched delay lines102.

Microwave photonic ilters ace several limitations; orincoherent ilters, one o the main problems is that in principleonly positive-valued samples can be produced (because opticalintensity cannot be negative). Several dierent solutions havebeen proposed, including those based on dierential detection103,phase shit in wavelength conversion104, and phase shit indual input/output electrooptic modulators105. Furthermore,researchers at the Universidad de Navarra and the UniversidadPolitecnica de Valencia have recently demonstrated incoherentilters eaturing complex-valued coeicients, by making use othe stimulated Brillouin scattering eect106. Another limitation o

microwave photonic ilters, especially in structures using singlesources, is related to the eects o source coherence. Severalschemes that provide the possibility o coherence-ree operationhave been proposed107.

A third important limitation o microwave photonic lters isconnected to the act that the spectrum o microwave photoniclters is periodic. Single resonance lters have, however, beenproposed and put into practice. wo technical approaches havebeen proposed, based respectively on the cascade o lters witha slightly different spectral period and the use o tuned (that is,requency-selective) electrooptic modulators108. wo ltersbased on this last approach hold the current highest values oQ actors reported in the literature or transversal (Q= 234)and IIR (Q = 3,300) operation in the UMS (Universal Mobileelecommunications System, f= 1.9 GHz) band. High-Qtransversal lters require a high number o signal taps, and orthis purpose, coherent spatial architectures capable o providing aconsiderable number o spectral samples and yielding promisingresults are under current investigation109.

1,547 1,548 1,549 1,550 1,551Wavelength (nm)

0

10

20

30

40

Re

flectiviy(dB) G

roup

delay

(ns)

4

3

2

1

0

N

k

2

1

o

Tunable source 1

Tunable source 2

Tunable source 3

Tunable source 4

DFB laser

Vectornetworkanalyser

5 1coupler 2 2

couplermodulator Fibre grating

Adaptedterminals

Electro-optical

2.125 GHz12dB

Modulus(dB)

Frequency (GHz)

Experimental result

Theoretical result

0 1 2 3 4 5 6

0

10

20

30

40

50

60

Figure 6 A reconfigurable and tunable microwave photonic filter using a laser source array and a dispersive linearly chirped FBG device. a, Schematic of the device.b, An example of a periodic MWP spectrum implemented by the filter. c, Dispersion characteristics of the linearly chirped FBG device used in the filter. The subscripts of thewavelength,, label the order in the wavelength spectrum. Reproduced with permission from ref. 98. Copyright (1999) IEEE.


8/12

REVIEW ARTICLE


ANALOG-TO-DIGITAL CONVERTERS

A second eld where MWP signal processing brings an importantadvantage is in UWB analog-to-digital converters110123. In certainanalog applications, such as high-perormance broadbandcommunication systems and radar, the use o digital

signal processing provides superior perormance and astreconigurability. he Achilles heel here is the act that theanalog-to-digital conversion (ADC) is the main bottleneck.ypical state-o-the-art electronic CMOS digitizers are limitedin speed by several technology actors, including the jitter inthe sampling clock, the settling time o the sample-and-holdcircuit, the speed o the comparator and inally the mismatchesin the transistor thresholds and passive component values. helimitations imposed by all these actors become more severe athigher requencies. Although several o these limiting actors canbe overcome using parallelism to implement time-interleavedADC architectures, the current perormance limit in the range o100 gigasamples per second cannot be improved at the same rateas that required by the digital signal processors. For instance,

their dynamic range and resolution are limited by mismatchesbetween digitizers, with typical values or state-o-the-artelectronic ADCs embodied by real-time digitizing oscilloscopesin the range o 20 gigasamples per second and a 4-GHz analogbandwidth. he perormance o ADCs operating under arbitrarysources o noise and nonlinear distortion is characterized bythe signal-to-noise and distortion ratio, SINAD, rom which itis customary to deine the eective number o bits (ENOB) oan ADC by ENOB= (SINAD 1.76)/6.02, where SINAD is indecibels. For commercial state-o-the-art ADCs, the ENOB isapproximately 45 bits, measured over the ull bandwidth. Aninteresting option to circumvent these diiculties and extendthe perormance o electronic ADCs is to incorporate photonictechniques. Several approaches have been proposed111116o whichthe photonic time-stretch (PS) technique116,117 has produced

a record value o 480 gigasamples per second and 96 GHzintrinsic bandwidth120.

Photonic time stretch improves the operation o anelectronic ADC by slowing down the electrical signal by meanso three photonic processing steps117: (1) time-to-wavelengthtransormation; (2) wavelength domain processing; and (3)

wavelength-to-time mapping. Figure 7 shows a particular versiono the above concept where a linearly chirped optical pulse isrst generated by propagating broadband transorm limitedpulses (optical bandwidth fopt) in a dispersive bre (dispersionparameter2) o length L1. Te time duration o these pulses is thesystem time aperture A= 2|2|L1fopt. Te electrical signal to beslowed down modulates the intensity o the chirped pulse in anelectrooptic modulator and nally, the envelope o the pulse isstretched in a second bre link (length L2) beore photodetectionand digitization by a lower-speed electronic ADC. Overall, themodulated pulse is stretched in time by a actor M= 1 + L2/L1and the effective sampling rate o the electronic digitizer fs isincreased to Mfs. Another key important system parameter is the3-dB electrical bandwidth fRF. Tis depends on the modulation

technique and in the case o double sideband modulation (DSB) itis given by fRF= [M/(8|2|L2)]1/2.From the operational point o view, the PS system

perormance metric is given by the product o the time apertureand the RF bandwidth (BP)118. I DSB modulation is used, thedispersion penalty limits the system perormance to BP valueso around 100, because there is a trade-o between the timeaperture and RF bandwidth. his limitation can be overcome,in principle, by using single sideband modulation (SSB)119,and BPs o several thousand can be achieved. But using SSBintroduces several practical diiculties that must be taken intoaccount. First o all, there is a residual phase distortion that mustbe corrected using an electronic equalizer. Second, SSB relies onthe use o a microwave hybrid to provide quadrature outputs.he operation o the hybrid device is typically requency-limited,

SCsource Modulator Photodiode

Fibre L1 Fibre L2

DispersionDispersion

Chirpedoptical pulse

Modulated chirpedoptical pulse

Time-stretchedmodulated chirped

optical pulse

Wavelength

Opticalbandwidth

Input RF signalStretched RF signal

Time

Figure 7Block diagram of the photonic stretch processor. A linearly chirped optical pulse is obtained by dispersing an ultrashort, UWB pulse generated by a supercontinuumsource at the first fibre link, length L1. Time-to-wavelength mapping is achieved when the pulse is intensity-modulated by the electrical signal. The pulse is stretched in asecond fibre link, length L2. Reproduced with permission from ref. 117. Copyright (2003) IEEE and OSA


9/12

REVIEW ARTICLE


and thus SSB operation will also have a bandwidth limitation.Recently researchers at the University o Caliornia, Los Angelesproposed a new technique120to overcome the limitation imposedby dispersion, based not on SSB, but on the exploitation o thephase diversity that exists between the two outputs o a dual-output MachZehnder modulator. Photonic-time-stretch ADCshave to solve several technical issues to oer continuous real-time operation, including calibration and channel matching, theimpact o the noise rom the optical source and the distortionsin the dispersive ibre, which have been outlined by Valley121ina comprehensive review paper. Nevertheless, a 4.5-bit ENOB,10 terasamples per second transient has recently been reported122.he Valley paper and the recent workshop held at the IEEEM-S International Microwave Symposium123 provide urther

valuable inormation about this subject to the interested reader.ARBITRARY WAVEFORM GENERATION

he generation o arbitrary microwave and RF waves is veryuseul in a variety o applications, including pulsed radar, UWB,optical and electronic tests and measurements and ground-penetrating radar124135. Current electromagnetic arbitrarywaveorm generation (AWG) is band-limited to the rangebelow 2 GHz. However, MWP signal-processing techniquescan greatly improve the perormance o these systems. Avariety o photonic techniques have been developed during thepast ew years that can generate microwave waveorms in thegigahertz and multiple gigahertz region. For example, photonicAWG has been demonstrated by individually modulating the

intensity and phase o the longitudinal modes o a mode-lockedsemiconductor laser125. Coherent pulse-shaping techniques basedon direct space-to-time mapping126129have been demonstratedor the production o both burst and continuous waveormsin the 3050 GHz requency range. Optical and microwavearbitrary waveorms have also been generated using ixed andprogrammable shaping o the spectrum o a supercontinuumollowed by a wavelength-to-time mapping in a dispersiveibre link130,131. he operation o an equivalent scheme underincoherent operation132 has also been proposed very recently,although no experimental veriication has yet been provided.he potential o combined chromatic dispersion and nonlineareects in optical ibres to generate complex microwave signalshas also been investigated133.

O all the techniques described in the previous paragraphs,those based on the spectral shaping o a supercontinuumsource ollowed by wavelength-to-time mapping have potentialor the widest variety o applications. Figure 8a shows theirst coniguration proposed by Jalali and co-workers130,135.he principle o operation, which has some similarities to thetime-stretching technique described above, is now explained.Supercontinuum generation in nonlinear ibres creates abroadband optical pulse and bandwidths in excess o 200 nm canbe produced. he pulse spectrum is then bandpass-iltered toaround 20 nm and spectrally dispersed by a diraction grating.he dierent spectral components impinge on dierent pixelso a high-resolution SLM. Each pixel can apply precise levels oattenuation to its speciic input spectral component. In this waythe spectrum o the output light rom the SLM can be carved.

Spectrally generatedwaveform

Input (unmodulated)spectrum

Diffractiongrating

SLM

array

Diffractiongrating

Dispersion

FibrePhotodiode

Output RFsignal

Supercontinuumsource

2 1 0 1 2

Time (ns)

RF Output

Input

20

60

1000 1 2 3 4 5 6 7 8 9 10

Frequency (GHz)

(dBm) Ultrabroad RF bandwidth - GHz

Arbitraryelectricalwaveform

Optical-to-electricalconversion

Femtosecondoptical pulse

Frequency-to-timeconverter

Circulator

Reflective Fouriertransform pulse shaper

5O GHzsampling oscilloscope

Broadband hornantenna (receiver)

~ one metre

Broadband hornantenna (transceiver)

Figure 8The photonic AWG comprising the spectral shaping of a supercontinuum source followed by a wavelength to time mapping130,135. a, Block diagram of theconfiguration. (Reproduced with permission from ref. 135. Copyright (2006) IEEE and OSA.) b, Generated ultra-broadband monocycle burst waveform131. (Reproduced withpermission from ref. 131. Copyright (2005) IEEE.) c, Photonic AWG set-up used in ref. 134 to compensate for antenna dispersion effects in UWB communication system.(Reproduced with permission from ref. 134. Copyright (2006) IEEE.)


10/12

REVIEW ARTICLE


Te different spectral components are then ocused onto adispersive optical bre link, which creates a one-to-one mappingbetween wavelength and time, converting the spectral modulationinto a time-domain amplitude modulation. Te length o thedispersive bre link determines the temporal duration o the signal.On photodetection, the time variation o the electrical output signalis proportional to the optical intensity. Te system perormance islimited by the bandwidth o the photodiode, but with components

available at present, bandwidths exceeding 60 GHz are achievable.Te above system required a closed loop to generate an inputcontrol correction signal to the SLM to take into account the non-ideal eature o the dispersive bre. Tis limitation was overcomein an open loop proposed by Andrew Weiner and colleagues 131.Using this conguration, the researchers at Purdue University havedemonstrated the synthesis o arbitrary waveorms in the UWBwireless communication spectral region and the ability to modulatethe requency o the RF carrier on a cycle-by-cycle basis, a eaturethat cannot be achieved with current electronic techniques. Forinstance, Fig. 8b shows a typical UWB signal generated, eaturingmonocycles o the order o 200 ps. Tis time resolution cannot beachieved with current electronic techniques.

wo salient eatures o the conguration in Fig. 8 have

been recently demonstrated that outline the added value thatMWP techniques bring to the synthesis o arbitrary microwavewaveorms. First, the compensation o antenna dispersioneffects134 in impulsive UWB signals (3.110.6 GHz region) hasbeen reported by extracting the RF spectral phase rom a time-domain impulse-response measurement o the antenna and thesubsequent programming o the matched signal in the SLM.Preliminary results have been reported or EM antennas, butit is expected that it will be applicable to a variety o antennasoperating in the UWB band. Figure 8c depicts the experimentalconguration. Second, a photonic AWG has been successullyused to compensate or the gain ripple and phase distortion o RFcascade ampliers required to boost the RF signal in broadbandhigh-power waveorm synthesis135. Te results obtained set the

path or obtaining high-power undistorted arbitrary microwaveand millimetre-wave signals with voltages above 10 V.

FUTURE PROSPECTS AND CHALLENGES

Microwave photonics has succeeded in developing anddemonstrating a wide range o devices, technologies andapplications during the past three decades, some o which(especially those related to communications) have reachedcommercial realization. Te key to accelerating the use o MWPtechniques in real-world systems is to continue to improve theperormance o the devices and links outlined in this review. Inparticular, the challenge is to improve their RF spectral regiono operation, conversion effi ciency and dynamic range, while at

the same time reducing cost. In addition, the unique eatures oMWP devices need to be used to create emerging and uture nicheapplications. Possible applications, such as optical packet switchednetworks and optical probing, are already being researched, whileothers, such as terahertz-wave generation and processing or non-invasive high resolution sensing and MWP-based quantum keydistribution, are just around the corner.

doi:10.1038/nphoton.2007.89

References1. Seeds, A., Lee, C. H., Funk, E. & Nagamura, M. Guest editorial: Microwave photonics.J. Lightwave

echnol.21,29592960 (2003).2. Seeds, A. Microwave photonics. IEEE rans. Microwave Teory ech.50,877887 (2002).3. Seeds, A. J. in Proc. IEEE Int. opical Meeting Microwave Photon. Oqunquit, Maine, USA 1619 (2004).4. ucker, R. S. & Pope, D. J. Microwave circuit models o semiconductor injection lasers. IEEE

rans. Microwave Teory ech.83,289294 (1983).

5. Ralston, J. D. et al.Control o differential gain, nonlinear gain, and damping actor or hig h-speedapplication o GaAs-based MQW lasers. IEEE J. Quant. Electron.29,16481659 (1993).

6. Matsui, Y., Murai, H., Arahira, S., Kutsuzawa, S. & Ogawa, Y. 30-GHz bandwidth 1.55-mstrained-compensated InGaAlAsInGaAsP MQW laser. IEEE Photon. echnol. Lett.9,2527 (1997).

7. Nagarajan, R., Levy, S. & Bowers, J. E. Millimeter wave narrowband optical ber links usingexternal cavity semiconductor lasers.J. Lightwave. echnol.1,127136 (1994).

8. Lim, C., Nirmalathas, A. & Novak, D. echniques or multi-channel data transmission using amulti-section laser in millimeter-wave ber-radio systems. IEEE rans. Microwave Teory. ech.47,13511357 (1999).

9. Bach, L. et al.Enhanced direct-modulated bandwidth o 37 GHz by a multi-section laser with acoupled-cavity-injection-grating design. Electron. Lett.39,15921593 (2003).10. Meng, X. J., Chau, . & Wu, M. C. Experimental demonstration o modulation bandwidth

enhancment in distributed eedback lasers with external light injection. Electron. Lett.34,20312032 (1998).

11. Lau, E. K., Sung, H. K. & Wu, M. C. inProc. IEEE Opt. Fiber Commun. Conf. Anaheim, California,USAOTG2 (2006).

12. Chrostowski, L.et al.50-GHz optically injection-locked 1.55-m VCSELs. IEEE Photon. echnol.Lett.18,367369 (2006).

13. Dol, D. W. & Ranganath, . R. 50 GHz velocity-matched broad wavelength LiNbO3modulatorwith multimode active section. Electron. Lett.28,11971199 (1992).

14. Noguchi, K., Mitomi, O. & Miyazawa, H. Millimeter-wave i: LiNbO3optical modulators.J. Lightwave echnol.16,615619 (1998).

15. Walker, R. G. in Proc. 8th IEEE LEOS Meeting. Sydney, Austrailia118119 (1995).16. Spickermann, R., Sakamoto, S. R., Peters, M. G. & Dagli, N. GaAs/AlGaAs traveling wave

electro-optic modulator with electrical bandwidth greater than 40 GHz. Electron. Lett.32,10951096 (1996).

17. Chen, D. et al. Demonstration o 110 GHz electro-optic polymer modulators.Appl. Phys. Lett.70,

33353337 (1997).18. Ido, . et al.Ultra high-speed multiple quantum well electroabsorption optical mo dulators withintegrated waveguides.J. Lightwave echnol.14,20262034 (1996).

19. Mineo, N., Yamada, K., Nakamura, K., Sakai, S. & Ushikobo, . in Proc. IEEE Opt. Fiber Commun.Conf. San Jose, California, USA287288 (1998).

20. Zhang, S. Z., Chiu, Y. J., Abraham, P. & Bowers, J. E. 25 GHz polarization insensitiveelectroabsorption modulators with travelling wave electrodes. IEEE Photon. echnol. Lett. 11,191193 (1999).

21. Akage, Y. et al.Wide bandwidth o over 50 GHz travelling-wave electrode electroabsorptionmodulator integrated DFB lasers. Electron. Lett.37,299300 (2001).

22. Wey, Y. G. et al.110-GHz GaInAs/InP double heterostructure p-i-n photodetectors.J. Lightwaveechnol.13,14901499 (1995).

23. mbach, A., rommer, D., Mekonnen, G. G., Ebert, W. & Unterbrsch, G. Waveguideintegrated 1.55-m photodetector with 45 GHz bandwidth. Electron. Lett.32,21432145 (1996).

24. Giboney, K. S. et al.ravelling-wave photodetectors with 172-GHz bandwidth and 76-GHzbandwidth-effi ciency product. IEEE Photon. echnol. Lett. 7,412414 (1995).

25. Ishibashi, . et al.Uni-traveling-carrier photodiodes. ech. Dig. Ultrafast Electron. Optoelectron.8387 (1997).

26. Ishibashi, ., Fushimi, H., Ito, H. & Furuta, . High power uni-travelling-carrier photoiodes. Proc.Int. IEEE opical Meeting Microwave Photon. Melbourne, Austrailia 7578 (1999).

27. Ito, H., Furuta, ., Muramoto, Y., Ito, . & Ishibashi, . Photonic millimetre- and sub-millimetre-wave generation using J-band rectangular-waveguide-output uni-travelling-carrier photodiodemodule. Electron. Lett.42,30333034 (2006).

28. Seeds, A. J. & de Salles, A. A. A. Optical control o microwave semiconductor devices. IEEE rans.Microwave Teory ech.38,577585 (1990).

29. Novak, D. et al.. inMicrowave Photonics157184 (CRCaylor and Francis, Boca Raton,Florida, USA, 2007).

30. Chiddix, J. A., Laor, H., Pangrac, D. M., Williamson, L. D. & Wole, R. W. AM video on ber inCAV systems: Need and implementation. IEEE J. Sel. Areas Commun.8,12291239 (1990).

31. Rivas, I. & Lopes, L. B. in Proc. IEEE Vehic. echnol. Conf. Ottawa, Canada13951399 (1998).32. Casini, A. & Faccin, P. inProc. IEEE Int. opical Meeting Microwave Photon. Budapest, Hungary

123128, (2003).33. Qian, X., Hartmann, P., Ingham, J. D., Penty, R. V. & White, I. H. Directly-modulated photonic

devices or microwave applications. Proc. IEEE M-S Intl Microwave Symp. Long Beach,California, USA (2005).

34. Chia, M. Y. W., Luo, B., Lee, M. L. & Hao, E. J. Z. Radio over multimode bre transmission orwireless LAN using VCSELs. Electron. Lett.39,11431144 (2003).

35. Persson, K.-. et al.WCDMA radio-over-ber transmission experiment using singlemode VCSELand multimode bre. Electron. Lett.42,372374 (2006).

36. Smith, G. H., Novak, D. & Ahmed, Z. echnique or optical SSB generation to overcomedispersion penalties in bre-radio systems. Electron. Lett. 33,7475 (1997).

37. Lim, C., Novak, D., Nirmalathas, A. & Smith, G. H. Dispersion-induced power penalties inmillimeter-wave signal transmission using multi-section DBR semiconductor lasers. IEEE rans.

Microwave Teory ech.49,288296 (2001).38. Marti, J., Fuster, J. M. & Laming, R. I. Experimental reduction o chromatic dispersion effects in

lightwave microwave/millimetre-wave transmissions using tapered linearly chirped bre gratings.Electron. Lett. 33,11701171 (1997).

39. Kitayama, K. Fading-ree transport o 60 GHz optical DSB signal in non-dispersion shifed berusing chirped ber grating. Proc. IEEE Int. opical Meeting Microwave Photon. Princeton, New

Jersey, USA223226 (1998).40. OReilly, J. J. et al.RACE R2005: Microwave optical duplex antenna link. IEE Proc. J140,

385391 (1993).41. Park, J. & Lau, K. Y. Millimetre-wave (39 GHz) bre-wireless transmission o broadband

multichannel compressed digital video. Electron. Lett.32,474476 (1996).


11/12

REVIEW ARTICLE


42. Nol, L., Marcenac, D. & Wake, D. 120 Mbps QPSK radio-ber transmission over 100 km ostandard ber at 60 GHz using a master/slave injection locked DFB laser source. Electron. Lett.32,18951897 (1996).

43. Smith, G. H., Novak, D., Lim, C. & Wu, K. Full-duplex broadband millimetre-wave opticaltransport system or ber wireless access. Electron. Lett.33,11591160 (1997).

44. Braun, R. P. et al.in Proc. IEEE M-S Int. Microwave Symp. Denver, Colorado225228 (1997).

45. Von Helmolt, C. H., Krger, U., Krger, K. & Grokop, G. A mobile broad-band communicationsystem based on mode-locked lasers. IEEE rans. Microwave Teory ech.45,14241430 (1997).

46. Sthr, A., Kuri, ., Kitayama, K., Heinzelmann, R. & Jger, D. Full-duplex 60 GHz ber optic

transmission. Electron. Lett.35,

16531655 (1999).47. Lim, C., Nirmalathas, A., Novak, D., Waterhouse, R. & Ghorbani, K. in Proc.IEEE M-S Int.Microwave Symp. Anaheim, California, USA12011204 (1999).

48. Haisch, H. & Peiffer, . in Proc. IEICE Int. opical Workshop Contemp. Photon. echnol.1720 (2000).49. Ogawa, H., suji, H. & Hirakawa, M. in Proc. IEEE M-S Int. Microwave Symp. Anaheim,

California, USA12131216 (1999).50. Choi, C. S. et al.60-GHz bidirectional radio-on-ber links based on InP/InGaAs HP

optoelectronic mixers. IEEE Photon. echnol. Lett.17,27212723 (2005).51. oda, H., Yamashita, ., Kitayama, K. & Kuri, . A DWDM mm-wave ber-radio system by

optical requency interleaving or high spectral effi ciency. Proc. IEEE Int. opical. MeetingMicrowave Photon. Awaji, Japan8588 (2002).

52. Lim, C., Nirmalathas, A., Attygalle, M., Novak, D. & Waterhouse, R. On the merging omillimeter-wave ber-radio backbone with 25 GHz WDM ring networks.J. Lightwave echnol.21,22032210 (2003).

53. Nakasyotani, ., oda, H., Kuri, . & Kitayama, K. Wavelength-division-multiplexed millimeter-waveband radio-on-ber system using a supercontinuum light source.J. Lightwave echnol.24,404410 (2006).

54. Martinez, A., Polo, V. & Marti, J. Simultaneous baseband and RF optical modulation scheme or

eeding wires and wireline heterogeneous access network. IEEE rans. Microwave Teory ech.49,

20182024 (2001).55. Ikeda, K., Kuri, . & Kitayama, K. Simultaneous three-band modulation and ber-optic

transmission o 2.5-Gb/s baseband, microwave-, and 60-GHz-band signals on a single wavelength.J. Lightwave echnol.21,31943202 (2003).

56. Lim, C., Lee, K. L., Nirmalathas, A., Novak, D. & Waterhouse, R. Optical interace or IMDreduction in ber-radio systems with simultaneous baseband transmission or heterogeneousaccess networks. Proc. IEEE Opt. Fib. Commun. Conf. Anaheim, California, USA(2007).

57. Wake, D., Johansson, D. & Moodie, D. G. Passive pico-cell A new concept in wireless networkinrastructure. Electron. Lett.33,404406 (1997).

58. Kitayama, K. et al.An approach to single optical component antenna base stations or broad-band millimeter-wave ber-radio access systems. IEEE rans. Microwave Teory ech.48,25882595 (2000).

59. Kuri, ., Kitayama, K. & akahashi, Y. in Proc. IEEE Int. opical Meeting Microwave Photon.Melbourne, Austrailia123126 (1999).

60. Nirmalathas, A., Novak, D., Lim, C. & Waterhouse, R. B. Wavelength re-use in the WDM opticalinterace o a millimeter-wave ber wireless antenna base-station. IEEE rans. Microwave Teoryech. 49,20062012 (2001).

61. Ong, L. C., Yee, M. L. & Luo, B. in Proc. IEEE LEOS Ann. Meet. Montreal, Canada 522523 (2006).62. Koep, G. A. Optical processor or phased array antenna beamormation. Proc. SPIE 477,7581 (1984).

63. Dol, D., Michel-Gabriel, F., Bann, S. & Huignard, J. P. wo-dimensional optical architecture ortime-delay beam orming in a phased array antenna. Opt. Lett.6,255257 (1991).

64. Ng, W. et al.Te rst demonstration o an optically steered microwave phased array antenna usingtrue-time-delay. IEEE J. Lightwave echnol.9,11241131 (1991).

65. Benjamin, R. & Seeds, A. J. Optical beam orming techniques or phased array antennas. IEE Proc.H 139,526534 (1992).

66. Konishi, Y., Chujo, W. & Fujise, M. Carrier-to-noise ratio and sidelobe level in a two-lasermodel optically controlled array antenna using Fourier optics. IEEE rans. Ant. Propagat. 40,14591465 (1992).

67. Riza, N. Liquid crystal-based optical control o phased-array antenna.J. Lightwave echnol.10,19741984 (1992).

68. Esman, R. D. et al.Fiber-optic prism true time-delay antenna eed. IEEE Photon. echnol. Lett. 5,13471369 (1993).

69. Molony, A., Edge, C. & Bennion, I. Fibre grating time delay element or phased array antennas.Electron. Lett.31,14851486 (1995).

70. Frankel, M. Y. & Esman, R. D. rue time-delay iber-optic control o an ultrawideband arraytransmitter/receiver with multibeam capability. IEEE rans. Microwave heory ech.43,23872394 (1995).

71. Ji, Y., Inagaki, K., Miura, R. & Karasawa, Y. Optical processor or multibeam microwave arrayantennas. Electron. Lett.32,822824 (1996).

72. ong, D. . K. & Wu, M. C. A novel multiwavelength optically controlled phased array antennawith programmable dispersion matrix. IEEE Photon. echnol. Lett.8,812814 (1996).

73. Paul, D. K., Razdan, R., Markey, B. J. & akats, P. Optical beam orming and steeringarchitectures or satcom phased-array antennas. Dig. IEEE Ant. Propagat. Symp. 2,15081511 (1996).

74. Zmuda, H., Sore, R., Payson, P., Johns, S. & oughlian, E. N. Photonic beamormer orphased array antennas using ibre grating prism. IEEE Photon. echnol. Lett.9,241243 (1997).

75. Romn, J. E., Frankel, M. Y., Matthews, P. J. & Esman, R. D. ime-steered array with a chirpedgrating beamormer. Electron. Lett.33,652653 (1997).

76. Ji, Y., Inagaki, K., Shibata, O. & Karasawa, Y. Beam ormation by using optical signal processingtechniques. Dig. IEEE Ant. Propagat. Symp. 2,739742 (1997).

77. Corral, J. L., Mart, J. & Fuster, J. M. in Dig. IEEE Microwave Teory ech. Symp. Baltimore,Maryland, USA13791382 (1998).

78. Stulemeijer, J., Maat, D. H. P., Moerman, I., Van Vliet, F. E. & Smit, M. K. Photonic integratedbeamormer or a phased array antenna. Proc. Europ. Conf. Opt. Commun. Madrid, Spain637638 (1998).

79. Kuhlow, B. et al.in Proc. IEEE Opt. Fiber Commun. Conf. Atlanta, Georgia, USA 732734 (2003).80. Vidal, B. et al. Simplied WDM optical beamorming network or large antenna arrays. IEEE

Photon. echnol. Lett. 18,12001202 (2006).81. Vidal, B., Mengual, ., Ibez-Lpez, C. & Mart, J. Optical beamorming network based on ber-

optical delay lines and spatial light modulators or large antenna arrays. IEEE Photon. echnol.Lett. 18,25902592 (2006).

82. Hall, P. in Proc. Aust. Acad. Sci. Applicat. Radio Sci. Workshop. Beechworth, Austrailia 4146 (2000).

83. Payne, J. P. & Shillue, W. P. in Proc. IEEE Int. opical Meeting Microwave Photon. Awaji, Japan912 (2002).84. Minasian, R. A. Photonic signal processing o microwave signals. IEEE rans. Microwave Teory

ech.54,832846 (2006).85. Capmany, J., Ortega, B., Pastor, D. & Sales, S. Discrete-time optical processing o microwave

signals.J. Lightwave echnol.23,702723 (2005).86. Kitayama, K. Architectural considerations o ber-radio millimeter-wave wireless access systems.

J. Fib. Integ. Opt.19,167186 (2000).87. Pastor, D., Ortega, B., Capmany, J., Fonjillaz, P.-Y. & Popov, M. unable microwave photonic lter

or noise and intererence suppression in UMS base stations. Electron. Lett. 40, 997999 (2004).88. Sugiyama, ., Suzuki, M. & Kubota, S. An integrated intererence supression scheme with adaptative

equalizer or digital satellite communication systems. IEICE rans. Commun.E79-B, 191196 (1996).89. Skolnik, M. I. Introduction to Radar Systems(McGraw-Hill, New York, 1980).90. Zmuda H. & oughlian, E. N. Photonic Aspects of Modern Radar(Artech House, Boston, 1994).91. Jackson, K. et al.Optical ber delay-line signal processing. IEEE rans. Microwave. Teory ech.

33,193204 (1985).92. Hunter, D. B. & Minasian, R. A. unable transversal lter based on chirped gratings. Electron. Lett.

31,22052207 (1995).

93. Hunter, D. B. & Minasian, R. A. unable microwave ber-optic bandpass lters. IEEE Photon.echnol. Lett.11,874876 (1999).94. You, N. & Minasian, R. A. A novel high-Q optical microwave processor using hybrid delay line

lters. IEEE rans. Microwave Teory ech.47,13041308 (1999).95. You, N. & Minasian, R. A. High-Q optical microwave lter. Electron. Lett.35,21252126 (1999).96. Yu, G., Zhang, W. & Williams, J. A. R. High-perormance microwave transversal lter using ber

Bragg grating arrays. IEEE Photon. echnol. Lett.12,11831185 (2000).97. Zhang, W., Williams, J. A. R. & Bennion, I. Optical ber recirculating delay line incorporating a

ber grating array. IEEE Microwave Wireless Compon. Lett.11,217219 (2001).98. Capmany, J., Pastor, D. & Ortega, B. New and exible ber-optic delay line lters using chirped

Bragg gratings and laser arrays. IEEE rans. Microwave Teory ech.47,13211327 (1999).99. Popov, M., Fonjallaz, P. Y. & Gunnarson, O. Compact microwave photonic transversal lter with

40 dB sidelobe suppression. IEEE Photon. echnol. Lett.17,663665 (2005).100. Pastor, D., Capmany, J. & Ortega, B. Broad-band tunable microwave transversal notch lter based on

tunable uniorm ber Bragg gratings as slicing lters. IEEE Photon. echnol. Lett.13,726728 (2001).101. Mora, J. et al.Automatic tunable and reconigurable iberoptic microwave ilters based

on a broadband optical source sliced by uniorm iber Bragg gratings. Opt. Express10,12911298 (2002).

102. Capmany, J., Mora, J., Ortega, B. & Pastor, D. High-quality low-cost online-recongurablemicrowave photonic transversal lter with positive and negative co effi cients. IEEE Photon.echnol. Lett. 17,27302732 (2005).

103. Sales, S., Capmany, J., Mart, J. & Pastor, D. Experimental demonstration o bre-optic delay linelters with negative coeffi cients. Electron. Lett.31,10951096 (1995).

104. Coppinger, F., Yegnanarayanan, S., rinh, P. D. & Jalali, B. All-optical RF lter using amplitudeinversion in a SOA. IEEE rans. Microwave Teory ech. 45,14731477 (1997).

105. Capmany, J., Pastor, D., Martinez, A., Ortega, B. & Sales, S. Microwave photonic lters withnegative coeffi cients based on phase inversion in an elec tro-optic modulator. Opt. Lett.28,14151417 (2003).

106. Loayssa, A., Capmany, J., Sagues, M. & Mora, J. Demonstration o incoherent microwave photoniclters with all-optical complex coeffi cients. IEEE Photon. echnol. Lett. 18,17741776 (2006).

107. Chan, E. H. W. & Minasian, R. A. Photonic notch lter without optical coherence limitations.J. Lightwave echnol.22,18111817 (2004).

108. Ortega, B., Mora, J., Capmany, J., Pastor, D. & Garcia-Olcina, R. Highly selective microwavephotonic lters based on an active optical recirculating cavity and tuned modulator hybridstructure. Electron. Lett.41,11331135 (2005).

109. Xiao, S. & Weiner, A. M. Coherent photonic processing o microwave signals using

spatial light modulators: Programmable amplitude ilters. J. Lightwave echnol.24,

25232529 (2006).110. Walden, R. H. Analog-to-digital converter survey and analysis. IEEE J. Sel. Areas Commun.17,

539550 (1999).111. akara, H., Kawanishi, S., Morioka, ., Mori, K. & Saruwatari, M. 100 Gbit/s optical waveorm

measurement with 0.6 ps resolution optical sampling subpicosecond supercontinuum pulses.Electron. Lett.30,11521154 (1994).

112. Jalali, B. & Xie, Y. M. Optical olding-ash analog-to-digital converter with analog encoding. Opt.Lett.20,19011903 (1995).

113. Frankel, M. Y., Kang, J. U. & Esman, R. D. High-perormance photonic analogue-digital converter.Electron. Lett.33,20962097 (1997).

114. Juodawl kis, P. W.et al.505-MS/s photonic analog-to-digital converter. Dig. Conf. LasersElectro-opt. Baltimore, Maryland, USA6364 (2001).

115. Clark, . R. & Dennis, M. L. oward a 100-Gsample/s photonic A-D converter. IEEE Photon.echnol. Lett.13,236238 (2001).

116. Coppinger, F., Bhushan, A. S. & Jalali, B. Photonic time stretch and its application to analog-to-digital conversion. IEEE rans. Microwave Teory ech.49,18401853 (2001).

117. Han, Y. & Jalali, B. Photonic time-stretched analog-to-digital converter: Fundamental conceptsand practical considerations.J. Lightwave echnol. 21,30853103 (2003).


12/12

REVIEW ARTICLE

330 h i | VOL 1 | JUNE 2007 | / h i

118. Han, Y. & Jalali, B. ime-bandwidth product o the photonic time-stretched analog-to-digital

converter. IEEE rans. Microwave Teory ech.51,18861892 (2003).119. Fuster, J. M., Novak, D., Nirmalathas, A. & Marti, J. Single sideband modulation in photonic time-

stretch analogue-to-digital conversion. Electron. Lett.37,6768 (2001).

120. Han, Y., Boyraz, O. & Jalali, B. Ultrawide-band photonic time stretch A/D converter employingphase diversity. IEEE rans. Microwave Teory ech.53,14041408 (2005).

121. Valley, C., Photonic analog-to-digital converters, Opt. Express15,19551982 (2007).

122. Chou, J., Boyraz, O. & Jalali, B. Femtosecond real-time single-shot digitizer, Proc. Meeting Am.

Phys. Soc. Baltimore, Maryland, USA(2006).

123. Workshop on ultraast analog-to-digital (A/D) converters., Proc. IEEE M-S Int. Microwave

Symp.(2004).124. Jalali, B., Kelvar, P. & Saxena, V. in Proc. IEEE LEOS Ann. Meeting. La Jolla, California, USA 253 (2001).

125. Yilmaz, ., DePriest, C. M., urpin, ., Abeles, J. H. & Delyett, P. J. oward a photonic arbitrarywaveorm generator using modelocked external cavity semiconductor laser. IEEE Photon. echnol.Lett.14,16081610 (2002).

126. McKinney, J. D., Leaird, D. E. & Weiner, A. M. Millimeter-wave arbitrary waveorm generationwith a direct space-to-time pulse shaper. Opt. Lett.27,13451347 (2002).

127. McKinney, J. D., Seo, D. S. & Weiner, A. M. Photonically assisted generation o continuous

arbitrary millimetre electromagnetic waveorms. Electron. Lett.39,309310 (2003).128. McKinney, J. D., Seo, D., Leaird, D. E. & Weiner, A. M. Photonically assisted generation o arbitrary

millimeter-wave and microwave electromagnetic waveorms via direct space-to-time optical pulseshaping. IEEE J. Lightwave echnol.21,30203028 (2003).

129. Xiao, S., McKinney, J. D. & Weiner, A. M. Photonic microwave arbitrary waveorm generationusing a virtually-imaged phased-array (VIPA) direct space-to-time pulse shapers. IEEE Photon.echnol. Lett.16,19361938 (2004).

130. Chou, J., Han, Y. & Jalali, B. Adaptive RF-Photonic arbitrary waveorm generator. IEEE Photon.echnol. Lett.15,581583 (2003).

131. Lin, I. S., McKinney, J. D. & Weiner, A. Photonic synthesis o broadband microwave arbitrarywaveorms applicable to ultra-wideband communication. IEEE Microwave Wireless Compon. Lett.15,226228 (2005).

132. orres-Company, V., Lancis, J. & Andrs, P. Arbitrary waveorm generator based on all-incoherentpulse shaping. IEEE Photon. echnol. Lett.18,26262628 (2006).

133. Levinson, O. & Horowitz, M. Generation o complex microwave and millimeter-wave pulses

using dispersion and Kerr eect in optical iber systems.J. Lightwave echnol.21,11791187 (2003).

134. McKinney, J. D. & Weiner, A. M. Compensation o the effects o antenna dispersion on UWBwaveorms via optical pulse-shaping techniques. IEEE rans. Microwave Teory ech.54,16811685 (2006).

135. Bortnik, B., Poberezhskiy, I., Chou, J., Jalali, B. & Fetterman, H. Predistortion technique or RF-photonic generation o high-power ultrawideband arbitrary waveorms.J. Lightwave echnol.24,27522759 (2006).

Competing financial interestsThe authors declare that they have no competing financial interests.

Documents

Nature RadioEng