The most useful wireless communication band is located in the frequency

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1 REVIEW ARTICLE NPG Asi Mter. 3, (2011) DOI: /simt Millimeter-wve photoni wireless links or very high dt rte ommunition Jin-Wei Shi 1, Chen-Bin Hung 2 nd Ci-Ling Pn 2,3 * Ntionl Centrl University nd Ntionl Tsing-Hu University, Tiwn In this pper, reent progress in millimeter-wve (MMW) photoni gigit wireless ommunition is reviewed. This tehnique is ttrtive prtly euse the MMW signl n e esily distriuted rom entrl to se sttions through the use o low-loss optil ier. This rdio-over-ier pproh ilittes the trnsmission o MMW signls. An MMW photoni trnsmitter, omprised o highpower photodiodes with integrted ntenns or MMW signl rodsting, is needed or signl genertion only over the lst mile. The development o severl dierent low-noise optil MMW soures nd high-power photoni trnsmitters nd photodiodes or optil MMW wireless links is summrized. The perormne o photoni wireless links with extremely high dt rtes (>10 Git s 1 ) developed sed on these key omponents nd using dierent modultion shemes is lso reviewed. Finlly, some dvned ommerilly ville produts nd the prospets o uture gigit wireless ommunition er re disussed. The most useul wireless ommunition nd is loted in the requeny rnge o GHz or resons o ompt ntenn size, low propgtion loss nd good penetrtion through uildings [1]. However, wireless rrier requenies re eing pushed higher due to emerging high-ndwidth pplitions, whih inlude next-genertion smrt ell phones [1] nd wireless linking or three-dimensionl super high-deinition television [2]. As result, the millimeter-wve (MMW) nds t 60 (V-nd), 120 (D-nd) nd even higher thn 300 GHz re eginning to ttrt ttention due to their unliensed usge [2 8]. Severl key ront-end omponents hve een developed employing the lredy mture silion-sed omplementry metl oxide semiondutor (CMOS) integrted-iruit (IC) tehnology or wireless ommunition in the V (50 75 GHz) nd W ( GHz) nds or even higher operting requenies or wireless ommunition [9]. Reently, 60 GHz CMOS trnseiver module [10] nd system omprised o ntenns, 60 GHz power mpliiers, lol osilltors nd send ICs hs een ommerilly developed y SiBEAM (USA). In ddition, dvned InP-sed high eletron moility trnsistors (InP-HEMTs) nd heterojuntion ipolr trnsistors hve een used to develop some high-speed ICs nd key omponents tht operte t 125 GHz [11,12] or greter thn 300 GHz [13,14] or >1 Git s 1 wireless ommunition systems. However, MMW signls suer sustntil propgtion loss in ree spe [8]. This prolem nd their inherent stright-line pth o propgtion et onnetions nd synhroniztion etween the dierent prts o the whole ommunition system [15]. A promising solution to overome this prolem is the rdio-over-ier (RoF) tehnique [3,4,16,17], in whih the MMW lol-osilltor signl nd dt re oth distriuted through low-loss optil ier nd only rdited over the lst mile to the end user. Figure 1 shows shemti representtion o this pproh, illustrting the motivtion nd working priniples o two MMW-over-ier ommunition systems. Figure 1() shows the dditionl eletril-to-optil (E O) nd optil-to-eletril (O E) onversion proesses used in the entrl oie nd se sttion, Centrl oie E O onversion Centrl oie E O onversion (Very lossy in MMW le) Optil dt + optil LO (Low-loss ier) (Low-loss ier) Optil dt + optil LO Dt Intermediterequeny mpliier MMW mixer Eletril LO O E onversion O E onversion O E nd up-onversion Antenn Photodiode MMW mpliier Bse sttion Figure 1. Coneptul digrms o MMW-over-ier ommunition systems with () ommon optil lol osilltor (LO) MMW soure shred y dierent se sttions nd () dierent eletril LO MMW soures instlled t eh se sttion. respetively. The optil MMW signl is distriuted remotely through low-loss ier rom the entrl oie to severl se sttions, eetively eliminting the huge propgtion loss o the MMW signl tht ours in n eletril trnsmission line or ree spe. Figure 1() shows nother possile solution or the reliztion o photoni wireless link, where only the optil dt signl is distriuted rom the entrl oie to eh se sttion through optil ier links. At the se sttion, widend photodiode, n osilltor, mixer, n mpliier nd n ntenn operting in the MMW nds re used to up-onvert, mix nd mpliy the *Corresponding uthor. Emil: lpn@phys.nthu.edu.tw 1 Deprtment o Eletril Engineering, Ntionl Centrl University, Toyun 320, Tiwn 2 Institute o Photonis Tehnologies, Ntionl Tsing-Hu University, Hsinhu 300, Tiwn 3 Deprtment o Physis, Ntionl Tsing-Hu University, Hsinhu 30013, Tiwn NPG ASIA MATERIALS VOL 3 APRIL

2 J.-W. Shi, C.-B. Hung nd C.-L. Pn inoming optil dt to eletril MMW signls or rdition to the end user over the lst mile. The se sttions in Figure 1() re more expensive nd omplex ompred with those in the rhiteture shown in Figure 1(), where eh se sttion is synhronized nd shres the sme optil MMW lol-osilltor signl. This is euse high-ost MMW ICs, whih inlude the ove-mentioned elements (mixer, osilltor nd mpliier or the MMW nds), re neessry t eh sttion. In ddition, it remins hllenge to synhronize the dierent MMW osilltors t dierent se sttions, whih my e n issue or moile users roming mong dierent se sttions [15]. We n thus onlude tht in order to relize high-perormne photoni-wireless link, high-qulity optil MMW soure in the entrl oie nd high-power photodiode-integrted MMW ntenn (i.e. photoni trnsmitter) t the se sttion re essentil. Reently, numer o reserh groups t NTT reported exellent results or 10 Git s 1 line-o-sight wireless linking t 120 GHz, whih ws hieved using uni-trveling rrier (UTC) photodiode-sed photoni trnsmitter [3] with n on o keying (OOK) dt ormt nd diret modultion on the 120 GHz optil rrier wve. By urther inresing the optil lol osilltor requeny (>300 GHz), higher dt rte (>10 Git s 1 ) n e expeted [6]. In ddition, using the dvned modultion ormts in the RoF system, suh s the 16-qudrture mplitude modultion (16-QAM) nd orthogonl requenydivision multiplexing (OFDM) tehniques, reserh tems in Germny nd Tiwn hve hieved dt rtes o up to round 30 Git s 1 t 60 GHz [18,19]. Two key omponents ply importnt roles in these high-perormne RoF ommunition systems with suh high rrier requeny (i.e. MMW) nd dt rte. One is the photoni MMW soure [20 23], whih should deliver stle, len nd less-dispersive optil signl with high-modultion-depth MMW envelope or remote genertion o eletril MMW signls t the se sttion. The other is the photoni trnsmitter [24 27], whih is omposed o high-speed, high-power photodiode nd high-diretivity ntenn. During opertion, suh devie n onvert the intense optil signl to strong eletril MMW signl tht n e eiiently rdited to the end user. Compred with using the MMW eletril mpliier to urther mpliy wek eletril signl rom the high-speed photodiode, omintion o n erium-doped ier mpliier nd high-power, high-speed photodiode n onvert the intense optil signl to strong MMW signl diretly. This not only releses the urden imposed on the MMW mpliier ut lso provides superior noise perormne [28,29]. With the integrtion o high-diretivity ntenn, it should e possile to hieve high gin (diretivity) in the rod requeny regime nd rdite the widend digitl dt rried on the MMW wve eiiently [30 32]. In the ollowing setion, we review reent progress relted to dierent kinds o photoni MMW soures nd high-power, high-speed photodiodes with dierent mteril systems tht operte up to the su-thz requeny regime. The improvement in the dt rte o modern MMW RoF systems will then e ddressed. Finlly, we will disuss the hllenges nd prospets o uture work on the MMW RoF system. Optil millimeter-wve soures In this setion, the development o severl high-perormne optil MMW soures with modulted entrl requenies rom 60 GHz up to hundreds o GHz is reviewed, nd the dvntges nd disdvntges o dierent pprohes re disussed. Although suhrmoni modeloked lser diodes hve een reported or MMW signl genertion up to 240 GHz [33,34], the mode-loking eletril signl (t 80 GHz) required or opertion t this requeny is itsel not esily essile. This leds to the simplest pproh eing used to relize photoni MMW soure with wide requeny tuning rnge (rom ner 0 Hz to severl THz): the two-lser heterodyne eting system [35,36]. In this pproh, y sweeping the wvelength dierene etween two tunle ontinuous-wve (CW) lsers, photoni MMW rrier n e lexily otined through oherent intererene. Tking entrl wvelength o Intensity (10 db/div.) 30 GHz 60 GHz 120 GHz Suhrmoni mode-loked lser diode t AWG Wvelength (nm) Frequeny (GHz) 1,550 nm s n exmple, 0.8 nm wvelength dierene orresponds to 100 GHz MMW requeny. Using this method, wireless dt trnsmission hs een demonstrted in the 300 GHz to 1 THz MMW rrier requeny regime [2,6]. Although highly gile, the nture o intererene with this method demnds highly synhronized lsers, otherwise lrge phse noise will umulte in the resulting photonilly generted MMW signl. One solution or otining MMW nd THz signls with low phse noise is to synhronize two lsers o nrrow linewidth (hundreds o khz), s hs een demonstrted or the 830 nm rnge [37]. However, suh system is omplex nd stility remins n issue or prtil pplition in ommunition systems. A homodyne eting system, in whih the photoni MMW signl results rom the intererene etween two or more optil requeny omponents derived rom single lser, is good lterntive s photoni MMW soure [38]. Insted o using mode-loked lser tht requires omplited stiliztion shemes nd oten results in low-requeny sping, stle requeny om soure n e otined y externlly modulting CW lser. A ommon pproh is to injet nrrowlinewidth CW lser into n eletro-optil phse/intensity modultor, whih is then pplied with strong rdiorequeny sinusoidl modultion signl or side-nd genertion [39,40]. The resulting requeny om sping is equl to the modultion (rdio)requeny. Photoni MMW signls with requenies o integer multiples o the om sping n e generted ter pplying n optil mplitude or phse ilter to the requeny om. Reently, severl reserh groups [2,3,20,41] hve demonstrted high-qulity photoni MMW signl genertion t 120 GHz to round 300 GHz [20,41] utilizing this tehnique with n rryed wveguide grting (AWG) s the mplitude optil ilter. A oneptul digrm o suh system is shown in Figure 2 [41]. The AWG eetively ilters every other om line, resulting in n optil spetrum with om line sping o 120 GHz nd mesured MMW spetrum with entrl requeny o 120 GHz. As n e seen rom the igure, the resulting photoni MMW signl hs high modultion depth euse the two om lines re o lmost equl height. The MMW signl is lso reltively noisy s indited y the moderte side-mode suppression rtio (SMSR; >25 db). Despite these exellent results, however, there remin hllenges with suh tehnique. In order to otin low-noise MMW optil envelope with 100% modultion depth, the mgnitude o these two om lines must e extly equl. As the output om spetrl proile oming diretly rom the modultor typilly shows mplitude vritions, high-perormne AWG ilter with high extintion rtio nd speilly designed trnser untion [42] is needed in order to ompenste or the unevenness. Plnr lightwve iruits re lso temperture sensitive. Thereore, either tive therml ontrol is required Intensity (10 db/div.) Coupler 100 khz Figure 2. () Shemti representtion o suhrmoni mode-loked semiondutor lser with AWG ilter setup or photoni 120 GHz MMW signl genertion. () Mesured optil spetrum with 120 GHz line sping, nd () mesured MMW spetrum t 120 GHz (dpted rom Re IEEE). t 42 NPG ASIA MATERIALS VOL 3 APRIL

3 Millimeter-wve photoni wireless links or very high dt rte ommunition REVIEW ARTICLE Intensity (dbm) rep π/ CW lser 36 db MZ- MZ- DC is Wvelength (nm) DC is or therml pkging eomes ritil. Another side issue with the use o mplitude iltering is tht the mjority o the CW lser injeted power is lost. Reently, through reul pplition o is nd modultion ontrols to dul-prllel eletro-optil Mh-Zehnder modultor (MZM), ilterless optil MMW soure or requenies up to 300 GHz (ter requeny 12-tupling) hs een suessully demonstrted [43,44]. Figures 3() nd () show oneptul digrm o this system nd the mesured optil spetrum [44]. The dul-prllel MZM serves s key omponent or the 10 GHz sping phse tuned optil requeny om. This pproh not only elimintes the huge optil loss during mplitude iltering, ut lso oers higher SMSR (>30 db) or the resulting photoni MMW signl [43,44], s shown in Figure 3(). As no optil ilter is required, the proposed system n lso e utilized in wvelength division multiplexing up-onverted systems [44]. Another powerul solution is to urther proess the externlly modulted CW lser requeny oms using spetrl line-y-line pulse shping [45]. Compred with pssive AWG, the line-y-line shper is n dptive setup nd n thereore oer omplete gry-level ontrol in terms o optil phse, mplitude nd even polriztion. Figures 4() to () show typil setup inorporting line-y-line pulse shper or photoni MMW genertion, the optil spetrum o the 100 GHz two-line sinusoidl signl nd the optil spetrum o the 100 GHz short-pulse (~2 ps) signl with our om lines, respetively [22]. The detils o typil reletive line-y-line shper re inluded in Figure 4(). Suh line-y-line pulse shper onsists o ier-pigtiled ollimtor with 3.5 mm spot dimeter, whih is used to send the om to 1,200 groove/mm gold-oted grting. Disrete om lines re dirted y the grting nd oused y lens with ol length o 400 mm. A ierized polriztion ontroller is used to djust or horizontl polriztion on the grting. A omputer-ontrollle pixel liquid-rystl modultor rry is pled immeditely eore the ol plne o the lens to independently ontrol the mplitude nd phse o individul spetrl lines. A retro-releting mirror pled on the Fourier plne o the lens gives doule-pss geometry, where ll spetrl lines reomine into single ier nd re seprted rom the input vi n optil irultor. Comined with polrizer pled etween the ollimtor nd the grting, ontrol o gry level intensity n e hieved with mximum extintion rtio o up to ~27 db, limited y the liquidrystl modultor. The ier-to-ier insertion loss o the pulse shper is elow 6 db. Severl pplitions o suh system hve een reported, or exmple, or rpid-reprogrmmle ritrry mirowve wveorm genertion [46] s well s or the tiloring o mirowve power spetr [47]. t MZ- DC is N rep Single-mode ier Figure 3. () Shemti representtion o dul prllel MZM setup or photoni MMW signl genertion. () Mesured optil spetrum with 72 GHz line sping (dpted rom Re IEEE). Optil spetrum (db) CW lser rep (N rep ) -1 rep Phse modultor PA t Single-mode ier Wvelength (nm) Figure 4. () Shemti representtion o line-y-line pulse shper setup. (,) Mesured optil spetr o 100 GHz two-line () nd 100 GHz our-line () oms (dpted rom Re IEEE). Utilizing this setup, ~1 ps optil pulse trins with GHz repetition rtes suitle or remote high-modultion-depth photoni MMW genertion hve een demonstrted [48]. Compred with the use o pure optil sinusoidl wveorms s the photoni MMW soure, short optil pulse exittion ould oer ~7 db enhnement in the resulting MMW power given the sme verge opertion photourrent. In ddition, higher mximum sturtion urrent is otined due to redued devie heting [22]. Furthermore, the line-y-line shper n simultneously provide the dispersion pre-ompenstion required or delivering short optil pulses or remote (>25 km) MMW signl genertion without the need or dditionl dispersion mngement within the ier link [48]. Eorts in lser engineering hve lso omplished vrious interesting photoni MMW signl genertion results. By reting dulwvelength lser outputs, the photonilly generted MMW signl requeny is deined y the requeny dierene etween the two modes. In one typil pproh, tunle dul-wvelength seprtion n e omplished y ontrolling the optil ilters to e used either or lser sel-injetion [49 51] or externl iltering [52]. Another method utilizes nonliner semiondutor lser dynmis, where the output o mster lser diode is injeted into slve lser diode [53 55]. As injetion is unidiretionl, the slve lser does not lter the ehvior o the mster lser. Creul seletion o the injetion opertionl prmeters (is urrent o the slve lser, injetion power o the mster lser into the slve nd requeny detuning etween the mster nd the slve lsers) n then initite the nonliner dynmil stte o the slve lser [53]. Reently, this pproh hs een utilized to demonstrte MMW signl genertion t 120 GHz [55]. Millimeter-wve photoni trnsmitters Line-y-line pulse shper Mirror Spetrl Grting lines Telesope Input Polrizer Collimtor PC Cirultor Free spe Shped output Fier 100 GHz pulse Wvelength (nm) In this setion, we review the development o severl kinds o su-thz photoni trnsmitters nd photomixers or the MMW or THz requeny regime. The photoni trnsmitters or suh pplitions re onstruted y integrting high-power photodiodes with ntenns or onverting n intense optil MMW signl to n eletril signl, nd then rditing tht signl to the end user. Compred with the pproh o using severl MMW mpliiers to urther mpliy the smll photourrent rom high-speed photodiode, the omintion o highspeed, high-power photodiodes with high-power erium-doped ier mpliier is n ttrtive lterntive to ostly, nrrow ndwidth, noisy eletril MMW mpliiers with limited gin nd sturtion power LCM Lens NPG ASIA MATERIALS VOL 3 APRIL

4 J.-W. Shi, C.-B. Hung nd C.-L. Pn perormne [29]. The (pek) sturtion-urrent-ndwidth produt (SCBP) is key prmeter or evluting the perormne o these high-power, high-speed photodiodes [28,29,56 59]. By inresing the sturtion urrent o the photodiode, it is possile to oost the injeted optil power nd urther inrese the mximum ville MMW power produed thererom. The sturtion urrent density o photodiode n e otined y the ollowing eqution: J mx (V is + Φ) εv rrier (1) D 2 where J mx is the mximum output urrent density rom the photodiode, V is is the reverse is voltge, Φ is the uilt-in potentil o the photodiode, ε is the dieletri onstnt in the depletion region, V rrier is the eetive drit veloity o the photogenerted rriers (inluding the drit veloity o eletrons nd holes [28]) nd D is the depletion lyer thikness. As n e seen, thinner depletion lyer, higher eetive rrier drit veloity, or higher reverse is voltge should improve the mximum output urrent density o the photodiode. The mximum V is o the photodiode is limited y the therml ilure o the devie nd the rekdown ield o the depletion lyer, whih n e improved y pproprite devie pkging with reul onsidertion o het-sinking [58] nd y inresing the depletion lyer thikness, respetively. However, the ltter pproh onlits with eqution (1) ove, whih sttes tht J mx n e gretly inresed y downsling D. In most prtil high-power photodiodes, the mximum output urrent rom the photodiode nd its llowle is voltge re more hevily dependent on the therml pkge thn the rekdown ield in the depletion lyer. Downsling the thikness o the depletion lyer thus eomes the most eetive wy to shorten the rrier trnsit time nd ttin high sturtion urrent perormne [58,59]. However, thin depletion lyer thikness is lwys ompnied y lrge juntion pitne, whih uses signiint trde-o etween the sturtion urrent nd the resistne pitne-limited ndwidth o the photodiode. The key mens o hieving the ultimte high SCBP or photodiodes is thereore to downsle the photo-tive sorption re s well s the thikness o the depletion lyer [58,59]. Low-temperture-grown (LTG) GAs mterils nd devies hve een developed speiilly or esing this trde-o etween resistne pitne-limited ndwidth nd rrier drit time. LTG-GAs is prepred through moleulr em epitxil growth t 200 C insted o the usul 600 C. Ater growth, however, proper in situ or ex situ nneling t 600 C is neessry in order inrese the density o rseni preipittes nd hene resistivity o this lyer [60]. A high density (>10 18 m 3 ) o rseni preipittes in LTG-GAs results in exess rseni-relted point deets nd short rrier trpping time [61]. Due to the high resistivity nd extremely short rrier trpping time (~200 s) o LTG- GAs, rrier drit time, drk urrent nd depletion lyer thikness re o onsiderly less importne or the onstrution o high-power, highspeed LTG-GAs-sed photodiodes. LTG-GAs-sed metl semiondutor metl photodiodes [62-64], photoondutive dipole ntenns (PDAs) [8,65,66], nd p i n photodiodes [59,67] with extremely high sturtion pek output power nd wide O E ndwidth hve een demonstrted. High-power LTG-GAs-sed photodiodes hve een used with plnr-iruit rodside ntenns, inluding plnr dipole, slot nd spirl ntenns [68]. Severl su-thz nd THz LTG-GAs-sed photomixers nd PDAs hve een suessully demonstrted. A silion lens is usully integrted onto the sustrte o the photomixer in order to minimize the sustrte mode prolem nd enhne the rditing power (eiieny) o these integrted rodside ntenns, whih re designed to operted in the ner-thz requeny regime [24,68]. Signiintly higher CW nd pulse THz output power hs een reported or LTG-GAs-sed photomixers ompred with tht hieved y trditionl p i n photodiodes [59,67]. Figure 5() shows n LTG-GAs-sed photomixer module with n integrted spirl ntenn nd silion-sustrte lens or THz requenies [68], nd Figure 5() shows the mesured requeny response o 2-turn spirl Normlized output power P slot > P spirl P spirl > P slot 100 1,000 Frequeny (GHz) 20 μm Normlized to spirl ntenn output 20 μm 20 μm Figure 5. () Top view o n LTG-GAs-sed photomixer integrted with spirl ntenn nd silion lens or THz rdition. () Mesured requeny response o n LTG-GAs-sed photomixer with integrted slot ntenn (normlized to the output power o the spirl ntenn) (dpted rom Re IEEE). suh n LTG-GAs photomixer integrted with nother slot ntenn. As n e seen rom the igure, the slot ntenn provides muh higher output power in the 1 2 THz requeny regime ompred with the spirl ntenn due to the hrteristi resonne o the slot ntenn [68]. These LTG-GAs-sed PDAs hve een suessully used or wireless linking t enter requenies o round 0.3 THz [65,66]. However, the dt rte so r hieved is low (~1 Mit s 1 ) [65] nd limited y the repetition rte o the Ti:spphire lser (~80 MHz). In ddition, the internl quntum eiieny o these LTG-GAs-sed devies is poor, usully less thn 10%, due to the short rrier trpping time. Aside rom these issues, the most serious prolem limiting the potentil o LTG- GAs-sed devies or high-dt-rte ommunition is the dierent rrier trpping nd reomintion time onstnts [69]. Compred with the short trpping time (~200 s), the photogenerted rriers inside the tive LTG-GAs lyer re sujet to muh longer reomintion times in the deet sttes (~1 ns vs ~200 s) [69]. Under high-power CW opertion, these non-reomined rriers my umulte in the deet sttes, whih n seriously degrde the power nd speed perormne o LTG-GAs-sed photoni trnsmitters. In ddition to inresing the is voltge nd downsling the depletion lyer thikness s disussed ove, the most ttrtive wy to urther improve the sturtion urrent without sriiing the speed perormne o the photodiode is to inrese the rrier drit veloity (see eqution (1)). Mny III-V-sed semiondutor mterils, suh s In 0.53 G 0.47 As, InAs nd InS, hve een reported to hve extremely high eletron moility in the inrred wvelength regime. However, during photodiode opertion, photogenerted holes with slow drit veloity re lwys mjor tor limiting speed. The slow hole prolem is eliminted in the InP-sed UTC photodiode struture [28,70,71] nd in ner-llisti Eetive impedne (Ω) 44 NPG ASIA MATERIALS VOL 3 APRIL

5 Millimeter-wve photoni wireless links or very high dt rte ommunition REVIEW ARTICLE P i In 0.53 G 0.47 As sorption lyer N P p-contt lyer N n-contt lyer Holes Eletrons P p p-type In 0.53 G 0.47 As sorption lyer Intrinsi InP olletor lyer W-nd wveguide i Fused sili sustrte 1/4-Height wveguide N Bis GND Upper mount P p-type diusion lok lyer p p-type In 0.53 G 0.47 As sorption lyer Intrinsi InP olletor lyer i i p-type hrge lyer Intrinsi In 0.52 Al 0.48 As lyer under eletri ield Photodiode hip λ = 1.55 μm Δ = 100 GHz Optil lens Strip line Antenn Lens Lser 1 Lser 2 Wveguide Coplnr wveguide to strip line trnsition Lower mount N Figure 6. Coneptul nd digrms or () p i n, () UTC nd () NBUTC photodiodes UTC (NBUTC) photodiodes [56 58,72 74], where only eletrons with high drit veloity t s tive rriers. Figure 6 shows oneptul nd digrms or p i n, UTC nd NBUTC photodiodes. Under optil power illumintion, the photogenerted holes in the p i n photodiode (Figure 6()) umulte in the tive sorption lyer due to their muh lower moility ompred with eletrons. This phenomenon will indue signiint spe hrge ield tht sreens the externl pplied eletri ield, resulting in serious speed degrdtion. In suh se, it is neessry to inrese the reverse is voltge to urther inrese the sturtion urrent up to the therml ilure limit. The prolem o hole trnsport is eliminted in the UTC photodiode (Figure 6()) euse the photo-sorption proess only ours in the p-type In 0.53 G 0.47 As lyer insted o in the intrinsi sorption lyer s in the trditionl p i n photodiode. In the p-type sorption lyer, photogenerted holes re the mjority rrier nd should relx diretly to the node o the UTC photodiode without trnsport [28]. Photogenerted eletrons, on the other hnd, will diuse through the p-type sorption lyer nd drit to the InP depletion nd thode lyers. The ndgp o the InP lyer is lrge enough to void the photo-sorption proess under exittion t wvelength o 1.55 μm. It n thus e understood tht eletrons re the only tive rriers in the UTC photodiode struture. This leds to signiint improvement in the eetive rrier drit veloity o the photodiode during opertion nd exellent SCBP perormne [28,70,71]. The mjor dierene etween the UTC photodiode nd the NBUTC photodiode is the insertion o n dditionl p-type hrge lyer in the olletor lyer to ontrol the distriution o the internl eletril ield, whih results in n over-shoot o the eletron drit veloity even under high reverse is voltge ( 3 V) [56 58,71 74]. Figure 6() shows oneptul nd digrm or n NBUTC photodiode. A thin p-type In 0.52 Al 0.48 As hrge lyer in the InP-sed olletor lyer experienes most o the externlly pplied eletri ield due to the higher rekdown ield ompred with tht o InP. Bsed on eqution (1), the higher eletron drit veloity in the NBUTC photodiode suggests tht UTC photodiode hip Trnsition sustrte Tpered-slot ntenn Figure 7. () Coneptul shemti digrms o rodside pth ntenn (top) nd n end-ire tpered slot ntenn (ottom) or retngulr wveguide exittion (dpted rom Re IEEE). (,) Coneptul ross-setionl digrm o UTC photodiode-sed photomixer with integrted pth ntenn () nd tpered slot ntenn () or WR-10 nd WR-08 wveguide exittion, respetively (dpted rom Re IEEE). these devies ould oer superior SCBP perormne ompred with UTC photodiodes. The reported SCBP or single UTC photodiode is usully round the 2,500 ma GHz level [28,70,71]. With this NBUTC devie we hve demonstrted n SCBP o over 4,070 ma GHz [58], nd NBUTC photodiode with liner sde struture hs een reported to hve n SCBP o s high s 7,500 ma GHz, the highest hieved so r [56,57]. The exellent perormne o UTC nd the NBUTC photodiodes in terms o speed nd sturtion power mens tht high-perormne photoni trnsmitters with n end-ire or rodside rdition pttern [24,25,30,75,76] nd operting requeny o THz n e relized, long with muh higher output power (>20 db) ompred with p i n [77] nd LTG-GAs-sed photoni trnsmitters [70]. Figure 7 shows UTC photodiode-sed photoni trnsmitter with rodside (pth) ntenn [76] nd end-ire (tpered slot) ntenn [30] or the exittion o WR-10 nd WR-8 retngulr wveguide-sed horn ntenns. Using these modules, 10 Git s 1 wireless line-o-sight dt trnsmission t enter requeny o 120 GHz hs een demonstrted [3]. Reently, our group hs lso demonstrted the use o n NBUTC photodiode-sed photoni trnsmitter with qusi-ygi rditor or exittion o WR-10 wveguide-sed horn ntenn [75,78 80]. Figure 8 shows oneptul digrm o qusi- Ygi rditor or retngulr wveguide exittion, top view o the NBUTC photodiode-sed photoni trnsmitter nd photogrph NPG ASIA MATERIALS VOL 3 APRIL

6 J.-W. Shi, C.-B. Hung nd C.-L. Pn Inserted into WR-10 wveguide (rdiorequeny output) Optil lol osilltor input Lensed ier Horn ntenn Devie Mirowve proe NBUTC photodiode hip 31 GHz PA Line-y-line shper CW lser Phse modultor Photoni MMW genertor 20 Git s 1 Reeiver module Reeiver ore Optil pth Eletril pth Antenn Wireless link Distriution network Antenn O/E 25 km Pulse Sinusoidl Trnsmitter ore High-power NBUTC photodiode PTM 20 Git s 1 Dt modultor Intermedite requeny input Figure 9. Coneptul digrm or photoni wireless linking system or 20 Git s 1 dt trnsmission using line-y-line pulse shper s the photoni MMW soure. Dipole-sed rditor W-nd ndpss ilter W-nd ndstop ilter Figure 8. () Coneptul shemti digrm o n end-ire qusi-ygi ntenn or retngulr wveguide exittion. () Top view o NBUTC photodiode-sed photoni trnsmitter integrted with qusi-ygi ntenn or WR-10 wveguide exittion. () Photogrph o n tul devie during mesurement. o n tul devie. The qusi-ygi rditor is omprised o hlwvelength dipole element nd ground reletor, mking the devie signiintly more ompt thn the trditionl tpered slot ntenn used or retngulr wveguide eeding (normlly severl wvelengths in length). Antenns o this sort re reltively immune to sustrte modes [31,32,75]. Compred with the trditionl UTC photodiodesed photoni trnsmitter, this devie inorportes n dditionl intermedite requeny input port nd two dierent W-nd (ndpss nd ndstop) ilters or simultneously relizing ultr-wide O E nd is modultion (swithing) ndwidth [79,80]. Under is modultion, error-ree wireless dt trnsmission t 12.5 nd 20 Git s 1 hs een suessully demonstrted [78 80]. In ontrst to UTC photodiodesed photoni trnsmitters under is modultion [81], the vrition in the deteted MMW power o this NBUTC photodiode devie is due to its signiint is-dependent speed perormne rther thn due to vritions in photourrent [78]. For the se o UTC photodiodes, isdependent nonlinerity origintes mostly rom hnges in photourrent when the is voltge is swung into orwrd is, imposing limit on modultion speed [81]. Photoni millimeter-wve wireless links In this setion, we review progress in the development o photoni MMW wireless links with very high dt rtes (>1 Git s 1 ). Severl windows re ommonly used or MMW wireless dt trnsmission, inluding the 60 GHz, W ( GHz) nd 120 GHz nds. The 60, 120 nd >300 GHz nds re liense ree euse these requenies orrespond to the sorption peks in ree spe [5,8]. These nds re thus suitle or indoor wireless dt trnsmission, suh s or onneting household pplines like high-deinition televisions (HDTVs), ell phones nd lptops, t high dt rtes (>1 Git s 1 ). Reently, the 16-QAM or OFDM modultion ormts hve een used to hieve wireless dt trnsmission o round 30 Git s 1 t 60 GHz enter requeny [18,19]. The W-nd hs muh lower propgtion loss nd roder trnsmission window ompred with the 60 nd 120 GHz nds, nd hs een speiied s the window or outdoor gigit wireless dt trnsmission in severl ountries [5]. Reently, numer o reserh groups hve demonstrted high-perormne photoni-wireless linking in this nd [78 80,82,83]. In our reent work, we demonstrted errorree 20 Git s 1 OOK wireless dt trnsmission in the W-nd using n NBUTC photodiode-sed photoni trnsmitter mixer [80]. The system setup is shown in Figure 9 [79,80]. This devie inludes liney-line pulse shper, whih serves s the MMW photoni soure or sinusoidl or pulse signl genertion t enter requeny o round 100 GHz. The dt signl used or is modultion in this photoni trnsmitter n e t nother optil wvelength, whih is onverted to eletril 20 Git s 1 dt through the use o nother low-speed photodiode. Using is modultion tehnique or remote signl up-onversion to the W-nd t the se sttion, long ier trnsmission distne with less dispersion n e hieved [78 80]. Although lrger ville ndwidth or dt trnsmission n e expeted y urther oosting the enter requeny o the MMW to ner 1 THz, the ndwidth nd speed o the other tive/pssive MMW omponents nd instruments or dt trnsmission nnot ollow simultneously [6]. This truly limits the mximum llowle trnsmission dt rte. In this high-requeny regime (>110 GHz), st MMW power detetor, whih hs wide video ndwidth, is usully dopted t the ront-end o the reeiver to down-onvert the inoming MMW signl nd quire the dt envelope. Using suh tehnique (envelope detetion), 20 Git s 1 error-ree wireless dt trnsmission t enter requeny o 93 GHz hs een demonstrted [80]. Comprison is mde with the reported 16-QAM, OFDM or dierentil phse-shit keying modultion ormts [82,83]. The OOK modultion ormts, despite providing extremely high dt trnsmission rtes (>12.5 Git s 1 ) [6,80], inur serious prolems with regrd to the multi-pth eet nd oupy lrger ndwidth or the sme desired dt rte. However, solution ould e to develop reeiver with muh more ompt nd simpler rhiteture or OOK dt detetion [6,80]. Using the dvned InP-HEMT oundry, single-hip omplex modultion nd demodultion proesses or 10 Git s 1 qudrture phse-shit keying dt hve een demonstrted or 125 GHz wireless trnsmission [11]. Compred with the reported OOK 10 Git s 1 dt trnsmission [3], this solution oers the sme dt rte with only hl the wireless ndwidth, redued multi-pth eet, nd more ompt reeiver size. The progress in high-speed IC tehnology thus provides n eetive wy to optimize trde-os etween the totl trnsmission dt rte, the oupied wireless ndwidth, the multi-pth eet nd the size o trnseiver module in photoni wireless linking system. Prospets Prtil pplitions o photoni MMW wireless linking n e ound in the ields o HDTV rodsting [84], s demonstrted y the NTT nd NHK orportions in Jpn. An unompressed high-deinition seril digitl intere (HD-SDI) signl is desirle during live rodsting, requiring dt rte o 1.5 Git s 1 per hnnel. However, this is muh ster thn the rte hievle using stte-o-the-rt mirowve 46 NPG ASIA MATERIALS VOL 3 APRIL

7 Millimeter-wve photoni wireless links or very high dt rte ommunition REVIEW ARTICLE F-nd photomixer module (wveguide ouple type) SC or FC onnetor UG-387/U lnge Optil ier tehnology will e one o the dominting tors in mking the tehnology ommerilly vile or gigit wireless linking. Reently, some o the key omponents nd systems, whih inlude photoni trnsmitters or GHz [85], 60 GHz eletro-sorption modultors, nd ompt 60 GHz photoni-wireless linking systems [86], hve een ommerilized (e.g. IPHOBAC, to meet the hllenges o the oming wireless gigit er. Figure 10 shows photogrph o ommeril UTC photodiode-sed photoni trnsmitter rom NTT (Jpn). As n e seen, the delite MMW wveguide output nd optil input pkge o suh module llows esy integrtion with n MMW ntenn or mpliier or RoF pplitions. MMW output (dbm) Diode hip perormne evluted rom EOS pulse dt (F-nd hip) λ = 1,550 nm I ph = 7 ma SDC is (GPO) W-nd module F-nd module D-nd module Frequeny (GHz) Figure 10. () Photogrph o ommeril UTC photodiode-sed photoni trnsmitter ( NTT). () Mesured O E response under ixed output photourrent (7 ma). ield pik-up units. MPEG or JPEG2000 enoders re thereore dopted in urrent mirowve wireless links to ompress the HD-SDI signl, ut this n use time delys nd signl distortion during live rodsting. The use o photoni MMW wireless linking in suh pplitions involves onneting severl HDTV mers, eh with dt rte o 1.5 Git s 1, to rodst vn vi optil ier. In the rodst vn, the multi-hnnel HDTV optil dt re onverted to n MMW signl entered t round 100 GHz nd then rdited to rely point using photoni trnsmitter with high-diretivity ntenn. Another importnt ommeril pplition o MMW wireless linkge or onsumer eletronis is in the 60 GHz nd (e.g. SiBEAM). Applitions inlude wireless linking or high-deinition multimedi intere (HDMI) nd high-speed dt trnser etween ell phones nd digitl mers. However, to dte, photoni tehnology hs not een essentil in order to relize 60 GHz indoor wireless linking. The mjor hllenge or MMW photoni wireless linking is the ost o the tehnology nd strong ompetition with the ll-eletroni pproh [3]. As the CMOS IC tehnology mtures, it is now possile to instll MMW CMOS lol-osilltor hips (Figure 1()) in eh se sttion without requiring synhronized lol osilltor signl [3]. Furthermore, i the MMW output power rom the se sttion is suiiently strong, the links ould e mde diretly with the se sttion without using optil ier. However, MMW (>60 GHz) hs optil ehvior, suh s stright-line propgtion nd sujet to shding y ostles in the trnsmission pth. Numerous remote ntenn units would thereore e needed t the user end to inrese network overge. This is prtiulrly importnt issue or outdoor wireless linking systems. 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