1 Introduction

The modern communication systems areall constructed based on the optical fiber net-work. Not only the traditional wire communi-cation systems but recently a wireless system,which uses the optical fiber to transmit theradio wave, are under developing [1]. Thissystem is a fusion of the fiber network inwhich optical wave is used as a carrier and the

wireless system in which the radio wave is themajor communication media. Optical fibercommunication system is of great advantages.It can be high speed and large capacity. Thewireless communication system, on the otherhand, taking the mobile phone as a goodexample, has been becoming more and morepopular in last decade because its mobility andconvenience. The new system, sometimescalled radio-over-fiber system, is trying to

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5-3 Photonic Antennas and its Application toRadio-over-Fiber Wireless Communica-tion Systems

LI Keren, MATSUI Toshiaki, and IZUTSU Masayuki

In this paper, we presented our recent works on development of photonic feeding copla-nar patch antennas for microwave and millimeter-wave wireless communication system. Anexperiment setup for optical modulation of sub-carrier, photodetection of the sub-carrier-modulated optical wave and integration of the photodetector with a coplanar patch antennahave been described. Experimental results of optical modulation using a traveling-waveLiNbO3 optical modulator, RF output from a photodetector: UTC-PD, and the RF outputdependence on modulation index have been presented and discussed. The experimentshowed that the photodetector can generate relatively large RF power at microwave andmillimeter-wave frequencies, for example, more than 10 mW at both 10 GHz and 20 GHz andaround 10 mW at 38 GHz and 7 mW at 60 GHz. Based on this experimental fact, we intro-duced a concept of direct integration of an antenna with the photodector to realize a simplephotonic feeding RF radiation unit to avoid serious transmission loss and simplify the RFsystem especially in high frequency wireless system. A planar antenna: coplanar patchantenna was newly proposed and designed for the direct integration with the phototectorwhich is of a coplanar wavegide output structure. Simulation, design, fabrication and meas-urement have been done for the antennas. Experiment on a hybrid integration of the pho-todetector and the coplanar patch antenna demonstrated a good performance of photonicmicrowave generating, direct feeding, transmitting and receiving. The results clearlyshowed the effectiveness and the potential application of our integration configuration tothe future microwave and millimeter-wave wireless communication system based on theoptical fiber network.

Keywords High output photodetection, Photodetector, Photonic feeding antenna, Coplanar patchantenna (CPA), Radio-over-fiber

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take the advantages of the above two systems.To realize the system, in additional to exploit-ing the technologies developed in the two sys-tems, novel technologies which makes the sys-tem really attractive by taking both advantagesof the radio wave and optical wave are strong-ly required. This is actually the majorresearch issue of the relative new field, so-called microwave photonics or millimeter-wave photonics. There are several keydevices in the system: modulator, which mod-ulates the radio wave on the optical wave;photodetector, which detects the radio waveback from the optical carrier; and antenna,which radiates and receives the radio waveinto/from the space. The radio-over-fiber sys-tem consists of these key devices.

The main objective of this research workis to develop above devices, especially thedevices which can operate efficiently at highfrequencies as millimeter-wave. In this work,we have successfully completed a photodetec-tion experiment where we have obtained highRF (Radio Frequency) from a photodetector(PD). After that we started the research ofplanar antenna, trying to integrate the antennaand the photodetector. Based on the experi-mental fact, we now introduced a concept:photonic feeding antenna, where the antennais fed, not by a RF source as usual, but by aphotonic device which receives optical waveand generated RF signal from the optical wave[2][3]. This concept is now receiving the atten-tion of the researchers in the microwave pho-tonics and the device, photonic feeding anten-na, is expected as a novel key functionaldevice of the next generation microwave andmillimeter-wave wireless communication sys-tem. In a system with the photonic feedingantenna, the RF signal, as sub-carrier of realsignals, is modulated on optical wave by anoptical modulator and propagates though anoptical fiber. The optical wave reaches a pho-todetector and is detected with a RF output atthe device. The RF signal feeds the integratedantenna and is then radiated by the antenna. Itis not necessary to implement transmissionlines and RF circuits such as amplifier in the

system when the detected RF power is highenough for a system. This avoids serioustransmission and processing loss and compli-cated RF configuration in the system especial-ly at the millimeter-wave frequencies and alsomakes the system very simple and then poten-tially low cost. The photodetector used in thiswork is UTC-PD (Uni-Traveling Carrier Pho-todiode) [4]. In this paper, we first present ourrecently obtained experimental results onmodulation and photodetection using UTC-PD. Then we describe a new planar antenna,coplanar patch antenna (CPA) [5][6], which isrecently developed for the direct integrationwith the UTC-PD, including its principle, sim-ulation and measured results. Finally, wepresent the results of a system experimentwhere we use the concept of photonic feedingantenna to demonstrate the system perform-ance.

2 High RF output photodetection

2.1 Experimental setup for opticalmodulation, photodetection andphotonic feeding antenna

Figure 1 shows an experimental setup foroptical modulation using an optical modulator,detection using a photodiode, and radiationand transmission using a CPA and a standardhorn antenna. The light source is a laser diode(LD) at 1.55μm. The optical modulator is aLiNbO3 traveling-wave optical modulator with3 dB bandwidth of 40 GHz, and can also oper-ate extent to 60 GHz but with much lowerresponse. The photodiode used in the pho-todetection is an wide band device called uni-traveling-carrier photodiode (UTC-PD) [4].This photodiode can not only operate over 100GHz but also can handle a relatively largeinput optical power. The optical wave is mod-ulated by the microwave or millimeter-wave(MMW) sub-carrier, which is fed from an RFgenerator and amplified up to about 20 dBm,through the optical modulator and then ampli-fied by an optical amplifier (EDFA) with anoutput up to 20 dBm. Bias voltage applied toPD is kept at -2.5 V. The RF output from the

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PD and power received at the standard hornare measured on a spectrum analyzer. Thedistance between the radiating CPA to thereceiving horn is fixed at 75 cm in this experi-ment.

2.2 Optical modulation and photode-tection results

Figure 2 shows the optical spectra of mod-ulated optical waves at 10 GHz and 20 GHz.The modulation indices for each sub-carrierare adjusted by changing the bias voltageapplied to the modulator. The photodetectionoutput directly depends on the modulationindex, and in our experiment, modulationsidelobes about 3 dB lower than the centralcarrier peak give a maximum output. This isconsistent with the theoretical analysis for aMach-Zehnder interferometer type modulator.The RF outputs at 10 and 20 GHz versus inputoptical power into the PD are shown in Fig.3.These results show a relatively large power ofmore than 10 dBm at both 10 GHz and 20 GHzfrom the PD, which are close to or even largerthan the required power for direct feeding atypical indoor millimeter-wave radiation sys-tem. At millimeter-wave frequencies, we havealso obtained the relatively larger RF power inexperiment as 10 mW at 38 GHz and 7 mW at60 GHz, respectively. This provides a possi-bility to construct a simple microwave andmillimeter-wave radio-over-fiber systems.

3 Consideration of photonic feed-ing antenna

With the relatively large RF output powerobtained from the experiment, we now consid-er a direct integration with antenna in order toconstruct a simple photonic feeding and RFradiation antenna system. Figure 4 shows abasic configuration of the photonic feedingantenna where the PD generates RF signalfrom the optical wave, and then feeds a planarantenna. Since the UTC-PD has a outputstructure of coplanar waveguide, so very natu-rally this system requires a planar antennawith a coplanar fed structure in order to direct-ly connect to the device. We have developed anew antenna, as shown in Fig.5, called copla-nar patch antenna (CPA) for the system, whichhas a coplanar waveguide fed line to connectto the PD and a coplanar patch for radiation.In next sections, we will describe the principleof the CPA, simulation and measured resultsof the CPA performance, including a two-ele-ment CPA array.

3.1 Coplanar patch antenna (CPA)and array [5][6]

Figure 5 shows the configuration of theproposed CPA which consists of a patch sur-rounded by closely spaced ground conductorand a coplanar fed line with a back ground

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Experimental setup of optical modulation using an optical modulator, detection using aphotodiode, radiation using coplanar patch antenna and receiving using a standard hornantenna.

Fig.1

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conductor for obtaining unidirectional radia-tion pattern. The antenna looks very similar tothe loop slot antenna as given in [7]-[9]. Infact, the antennas with similar configurationwere called as loop slot antennas in [8][9].From our intensive electromagnetic field sim-ulations of the structure, however, we discov-ered that the antenna behaves more like amicrostrip patch antenna than a loop slotantenna. Particularly, the resonant frequencyof the antenna is primarily determined by thepatch length (L) of about a half guided wave-

length instead of the total loop size. Electro-magnetic simulation also demonstrated thesimilar distribution of the electric fieldsaround the slots as the distribution around themicrostrip patch edges. Figure 6 shows theelectric field distribution along the slots at theresonant frequency point of 10 GHz. Thefields along the feed side slot and outer sideslot are in phase and of almost uniform distri-bution in the horizontal direction. No fieldchanges to be out phase along both the inputside and outer side slots though an equivalent

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Optical spectra of optical wave mod-ulated at (a) 10 GHz and (b) 20 GHz

Fig.2 RF output from photodetectors at (a)10 GHz and (b) 20 GHz

Fig.3

total length of the slot W + S at outer side isabout 1.6λ0/√εr, whereλ0 is the wavelength infree space at 10 GHz, is longer than one and ahalf guided wavelength, while the fields alongthe left and right side slots show an out phasevariation where the equivalent length of theslots S/2 + L + S/2 is about 0.52λ0/√εr, closeto a half of guided wavelength. This field dis-tribution clearly demonstrates that, with thefield distribution of the microstrip patch in themind, the coplanar patch antenna at the reso-nant point is much more like a “patch” than a“loop slot”. The resonant length of coplanarpatch L ≈ 0.47λ0/√εr ≈ 1/2λ0/√εr also followsthe same rule as for the microstrip patchantenna. This is why we called the antennashown in Fig.5 a “coplanar patch” antenna nota “loop slot” antenna.

Similar tendency of the input impedanceversus the length (L) of the patch has beenalso observed. This makes possible to realizean impedance matching by only adjusting thewidth (W ) of the patch. Based on above facts,we introduced a new concept of “coplanar

patch antenna (CPA)” in our previous paper[5][6]. By introducing this concept, one canexploit techniques well developed for themicrostrip patch antennas by using the simi-larities between the CPA and microstrip patchantenna.

3.2 Simulation and experimentalresults

To demonstrate the antenna performance

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Coplanar patch antenna (CPA) witha coplanar feeding structure

Fig.5

Configuration of a photodetector and a planar antenna to be integrated with the PDFig.4

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of the proposed CPA, we have designed, fabri-cated and measured CPAs at X-band. An two-element CPA array has been also developed toimprove the gain of the CPA. The parametersfor the substrate and the CPA and array usedin this work are listed in Table 1.

Figure 7 and 8 show the return loss andradiation patterns of the CPA. The antennahas about 3.4 % relative bandwidth and 7.8dBi measured gain.

Figure 9 is the return loss and radiationpatterns of the CPA array. The bandwidth isalmost the same as a CPA while the measuredgain is about 10.5 dBi, 2.7 dB improved com-paring with the single CPA. This CPA arraywas used in the photonic feeding and transmit-ting experiment shown in Fig.1.

4 Photonic feeding antenna andtransmitting experiment

Following the experimental configurationshown in Fig.1, we directly fed the CPA arrayusing the photodetector without any RF ampli-fier or extra circuit between the antenna andPD. The RF power was then generated by thePD from input optical waves and radiated bythe CPA array. The radiated power wasreceived by a standard horn antenna, whichhas a gain of 11.2 dB at 10 GHz and waslocated at distance 75 cm from the array. Thereceived power as well as the power after the

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Electric field distribution along the slotsin CPA

Fig.6

Parameters of dielectric substrate and geometrical dimensions of CPA and CPA array at10 GHz

Table 1

Simulated and measured return loss ofCPA (at 10 GHz)

Fig.7

PD are shown in Fig.10. At 10 dBm opticalinput, for example, the received power isabout -22 dBm, which is a large enough valuefor a conventional wireless system. Thisresult confirms our consideration again thatwe can use the optical power to overcome theweakness of the microwave and millimeter-wave and construct a simple radio-over-fibersystem by using the concept of photonic feed-ing antenna. Figure 11 shows two measuredwaveforms of the modulating signal (CW)from the RF source and the output signal fromthe PD. From this result, we can see that thereis no significant signal distortion due to thesystem.

5 Conclusion

In this paper, we have presented an experi-ment on optical modulation and photodetec-tion at microwave and millimeter-wave fre-quencies. The experimental results showedthat we can obtain a relatively large power ofmore than 10 dBm at both 10 GHz and 20GHz from the photodetector and we can thenconstruct a simple photonic feeding antennasystem. A CPA has been developed and usedas radiating antenna, which has a CPW fedline matching to the PD and has been designed

based on a concept of coplanar patch intro-duced in [5][6]. The transmitting experimentdemonstrated the effectiveness and usefulnessof the concept of the photonic feeding antennaand its potential application to the real radio-over-fiber system. This simple configurationwould provide a good solution and become akey technology for the future photonic andmicrowave/millimeter-wave wireless commu-nication systems.

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Simulated and measured patterns ofCPA (at 10 GHz)

Fig.8

Simulated and measured return lossand patterns of CPA array (at 10 GHz)

Fig.9

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Acknowledgment

The authors are very thankful to Dr. T.Itabe, director of Basic and Advanced ResearchDivision, Dr. Y. Suzuki, director of WirelessCommunications Division, for their encour-

agement and advice on the research project,Dr. Kenneth K. F. Tong, Dr. K. Shinohara,Communications Research Laboratory, fortheir helps in fabrication process and measure-ment of the antennas.

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RF power generated by PD andreceived by horn antenna

Fig.10 Waveform of the modulating signal(CW) and output signal from the PD

Fig.11

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LI Keren

Senior Researcher, Millimeter-waveDevices Group, Wireless Communica-tions Department

Optical Communication, MicrowavePhotonics, Microwave Engineering

IZUTSU Masayuki, Dr. Eng. Distinguished Researcher

High-speed Lightwave Modulation

MATSUI Toshiaki

Group Leader, Millimeter-WaveDevices Group, Wireless Communica-tions Department

High Frequency Measurement, Micro-wave and Millimeter-Wave Devices andCircuits

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