causing the dielectric tunability and the changes in tunability withincreased temperature are also under study. Finally, this capacitivetest structure is being used to investigate the relationship betweenapplied voltage and induced current on biopolymers.

ACKNOWLEDGMENT

The authors thank Mr. Gerry Landis, University of Dayton Re-search Institute, for his help with the fabrication of the test struc-tures. The authors also thank Mr. Harold Scott Axtell, SensorsDirectorate at Wright Patterson Air Force Base, for his help withthe microwave measurements. This work was supported in part bya Dayton Area Graduate Studies Institute assistantship and aNational Science Foundation Graduate Fellowship to the firstauthor. The second author acknowledges the subcontract fromAT&T Government Solutions for this work.

REFERENCES

1. Y. Shi, W. Lin, D.J. Olson, J.H. Bechtel, H. Zhang, W.H. Steier, C.Zhang, and L.R. Dalton, Electro-optic polymer modulators with 0.8 Vhalf-wave voltage, Appl Phys Lett 77 (2000), 1–3.

2. J.G. Grote, J.S. Zetts, R.L. Nelson, F.K. Hopkins, L.R. Dalton, C.Zhang, and W.H. Steier, Effect of conductivity and dielectric constanton the modulation voltage for optoelectronic devices based on non-linear optical polymers, Opt Eng 40 (2001), 2464–2473.

3. J.A. Hagen, W. Li, A. J. Steckl, and J.G. Grote, Enhanced emissionefficiency in organic light-emitting diodes using deoxyribonucleic acidcomplex as an electron blocking layer, Appl Phys Lett 88 (2006),171109.

4. E.M. Heckman, J.A. Hagen, P.P. Yaney, J.G. Grote, and F.K. Hopkins,Processing techniques for DNA: A biopolymer for photonics applica-tions, Appl Phys Lett 87 (2005), 211115.

5. J.G. Grote, E.M. Heckman, J.A. Hagen, P.P. Yaney, D.E. Diggs, G.Subramanyam, R.L. Nelson, J.S. Zetts, D.Y. Zang, B. Singh, N.S.Sariciftci, and F.K. Hopkins, DNA: New class of polymer, OrganicPhotonic Materials and Devices VIII, SPIE (2006), 61170J-6

6. L. Wang, J. Yoshida, N. Ogata, S. Sasaki, and T. Kajiyama, Self-assembled supramolecular films derived from marine deoxyribonu-cleic acid (DNA)-cationic surfactant complexes: Large-scale prepara-tion and optical and thermal properties, Chem Mater 13 (2001), 1273–1281.

7. L.L. Brott, S.M. Rozenzhak, R.R. Naik, S.R. Davidson, R.E. Perrin,and M.O. Stone, A poly(vinyl alcohol)/carbon-black composite film: Aplatform for biological macromolecule incorporation, Adv Mater 16(2004), 592–596.

8. P. Queffelec, P. Gelin, J. Gieraltowski, and J. Loaec, A microstripdevice for the broad band simultaneous measurement of complexpermeability and permittivity, IEEE Trans Magn 30 (1994), 224–231.

9. C. Brosseau, P. Queffelec, and P. Talbot, Microwave characterizationof filled polymers, J Appl Phys 89 (2001), 4532–4540.

10. W.B. Weir, Automatic measurement of complex dielectric constantand permeability at microwave frequencies, Proc IEEE 62 (1974),33–36.

11. M.D. Janezic, D.F. Williams, V. Blaschke, A. Karamcheti, and C.C.Shih, Permittivity characterization of low-k thin films from transmis-sion-line measurements, IEEE Trans Microw Theory Tech 51 (2003),132–136.

12. G. Subramanyam, E. Heckman, J. Grote, F. Hopkins, R. Neidhard, andE. Nykiel, Microwave dielectric properties of marine DNA basedpolymers, Microw Opt Technol Lett 46 (2005), 278–282.

13. G. Subramanyam, E. Heckman, J. Grote, and F. Hopkins, Microwavedielectric properties of DNA based polymers between 10 and 30 GHz,IEEE Microw Wireless Components Lett 15 (2005), 232–234.

14. D. Ghosh, B. Laughlin, J. Nath, A.I. Kingon, M.B. Steer, and J.P.Maria, Tunable high-quality-factor interdigitated (Ba, Sr)TiO3 capac-itors fabricated on low-cost substrates with copper metallization, ThinSolid Films 496 (2006), 669–673.

15. S.W. Kirchoefer, J.M. Pond, A.C. Carter, W. Chang, K.K. Agarwal,J.S. Horwitz, and D.B. Chrisey, Microwave properties of

Sr0.5Ba0.5TiO3 thin-film interdigitated capacitors, Microw Opt Tech-nol Lett 18 (1998), 168–171.

16. F. Purroy and L. Pradell, New theoretical analysis of the LRRMcalibration technique for vector network analyzers, IEEE Trans In-strum Meas, 50 (2001), 1307–1314.

© 2007 Wiley Periodicals, Inc.

ALL-OPTICAL mm-WAVE GENERATIONBY USING DIRECT-MODULATION DFBLASER AND EXTERNAL MODULATOR

Lin Chen, Yazhi Pi, Hong Wen, and Shuangchun WenPhotonics Technology Research Center, School of Computer andCommunication, Hunan University, Changsha, China 410082

Received 18 October 2006

ABSTRACT: We proposed and experimentally demonstrated a novelmethod of optical mm-wave generation using only one Mach-Zehndermodulator and a direct-modulator, based on the method of optical car-rier suppression. Using this proposed method, the 2.5 Gb/s data channelwas successfully transmitted over 20 km for downstream with less than1-dB power penalty. Therefore, the system configure of mm-wave gener-ation can be further simplified and be more cost effective. © 2007 WileyPeriodicals, Inc. Microwave Opt Technol Lett 49: 1265–1267, 2007;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.22449

Key words: radio-over-fiber; external modulation; optical carrier sup-pression (OCS); optical millimeter wave generation; direct-modulationDFB laser

1. INTRODUCTION

Millimeter-wave is a promising frequency resource for futurebroad-band radio communication. Radio-over-fiber (ROF) systemsare attracting a great deal of attention as a promising technique forproviding wireless broad-band service. Optical millimeter-wavegeneration and all-optical up-conversion are key techniques torealize low cost and high-transmission performance in ROF sys-tems. Optical mm-waves can be generated by several all-opticalup-conversion schemes such as utilizing highly nonlinear fiber,based on FWM or XPM, EAM based on cross-absorption modu-lation, and external modulation based on dual (single)-band oroptical carrier suppression (OCS) modulation [1–12].Amongthese, the simplest and the most accurate scheme to generateoptical millimeter-wave is to use external intensity modulation [2].Optical millimeter wave generated by the OCS modulationschemes has been demonstrated to have a few advantages, such asthe lowest spectral occupancy and the lowest band-width require-ment for components [5]. So it would be one of the desirablecandidates for downlink transmission. In the conventional OCSmodulation scheme for millimeter-wave, the base band signal isgenerated by a single-electrode Mach-Zehnder modulator (MZM)biased at quadrature and then up-converted using a dual arm MZMbiased at the minimum transmission point. In this paper, wepropose a novel and simple method to generate the mm-wavesignal by using a direct-modulation and an external modulator, Theproposed system is showed in Figure 1. A direct-modulation laseris employed to generate CW light wave and modulate the basebandsignal. The OCS scheme was employed to generate millimeter-wave and up-convert base-band data signal for the downstream. Adual-arm LN-MZM modulator biased at vp and driven by twocomplementary RF signals is used to realize OCS. The optical

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millimeter-wave is transmitted to the base station (BS). At the BS,the optical millimeter-wave signal was filtered by a tunable opticalfilter (TOF) before O/E conversion via PIN PD. The millimeter-wave signal was down-converted by a mixer with a clock signal.Using this proposed system, we experimentally demonstrated thatthe 2.5 Gb/s data channel was successfully transmitted over 20 kmfor downstream with less than 1 dB power penalty. By our knowl-edge, this is the first time to use the direct-modulation laser andexternal modulator to generate a optical mm-wave that can sim-plify the system configuration.

2. EXPERIMENTAL SETUP AND RESULTS

Figure 2 shows the experiment setup for optical millimeter-wavegeneration by using a direct-modulation laser and a dual-armMach-Zehnder LiNbO3 external modulator. At the central station,a CW lightwave was generated by a DFB laser diode at 1534.4 nmand directly modulated with 2.5 Gb/s pseudorandom bit sequenceelectrical signal with a word length of 231 – 1. Extinction rationand DC bias of the laser is 11 dB and 25 mA, respectively. The RFpower to drive the laser at 2.5 Gb/s is 2 Vp-p. The output averagepower of the laser is 1 dBm. The optical spectrum and the eyediagram of the 2.5 Gb/s baseband signal is showed in Figure 2 asinset (i) and Figure 3(a), respectively. A dual-arm Mach-ZehnderLiNbO3 modulator (D-LN-MZM) biased at vx and driven by twocomplementary 20-GHz clocks was used to generate OCS. AfterOCS modulation, a dual-mode lightwave was generated. The re-

petitive frequency of the generated LO optical signal is 40 GHz.The optical spectrum and eye diagram of optical mm-wave gen-erated by the double-arm MZM and OCS scheme are shown inFigure 2 as inset (ii) and Figure 3(b), respectively. The generatedoptical millimeter-wave was transmitted over 20-km standard sin-gle mode fiber (SMF-28). The eye diagram of optical mm-waveafter transmission over 20 km SSMF is shown in Figure 3(c) andmeasured at point A in Figure 2. At the BS, the millimeter wavewas amplified by a regular EDFA with a small-signal gain of 30dB and filtered by a TOF with a band-width of 0.5 nm before O/Econversion via a PIN PD with a 3-dB band-width of 50 GHz. Theconverted electrical signal was boosted by an electrical amplifier(EA) with a bandwidth of 10 GHz centered at 40 GHz. Anelectrical LO signal at 40 GHz was generated by using a frequencymultiplier from 10 to 40 GHz. We used the electrical LO signaland a mixer to down-convert the electrical mm-wave signal. Thedown-converted 2.5 Gb/s signal was detected by a BER tester,before it was filtered by a low pass electrical filer with a bandwidthof 2.8 GHz. The receiver is the same as that used in Ref. 11.Thedown-converted 2.5-Gb/s signal was detected by a bit-error rate(BER) tester, and its eye diagram after transmission over 20 kmSSMF is shown in Figure 3(d) and measured at point B in Figure2. The BER curves are shown in Figure 4. For a BER of 10�9, thereceiver sensitivity is �35 dBm. The power penalty is less than 0.4dB after transmission.

3. CONCLUSION

We have proposed and experimentally demonstrated a new schemeto generate optical millimeter wave by using a direct-modulationlaser and an external modulator with OCS modulation scheme. Bythis scheme, the 2.5-Gb/s data channel was successfully transmit-ted over 20 km for downstream with less than 1-dB power penalty.We can generate optical mm-wave using only one external mod-ulator, which is compact and cost-effective compared to the pre-vious scheme that needs two external modulators. Therefore, thesystem configure of mm-wave generation can be further simplifiedand be more cost-effective.

Figure 1 Principle of optical mm-wave generation by using direct-modulation laser and external modulator, LD, laser diode; TOF, tunableoptical filter; O/E, optical/electrical converter; EA, electric amplifier.[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com]

Figure 2 The experimental setup for optical millimeter-wave generationby using direct-modulation laser and external modulator. IM, intensitymodulator; TOF, tunable optical filter; EA, electrical amplifier; LPF, lowpass filter. Insets (i) The optical spectrum generated by direct-modulationlaser with base-band data. (ii) The optical spectrum of optical millimeter-wave generated by double-arm MZM and OCS scheme. The resolution forall optical spectra is 0.01 nm in this paper. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com]

Figure 3 Eye diagram at different locations labeled in Figure 2. (a)Optical eye diagram after the direct-modulation laser. (b) Optical eyediagram of optical millimeter-wave before the SMF. (c) Optical eye dia-gram after optical millimeter-wave transmission over 20-km SMF. (d) Eyediagram of the down-converted 2.5Gb/s signal after transmission over20-km SMF (100 ps/div). [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

1266 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 6, June 2007 DOI 10.1002/mop

ACKNOWLEDGMENTS

We thank Dr. Jianjun Yu for his useful discussion and encour-agement, and his assistance in part of the experiments. Thiswork is supported by the National Natural Science Foundationof China (Grant nos. 10576012 and 60538010), the program ofthe Ministry of Education of China for New Century ExcellentTalents in University, and the Specialized Research Fund forthe Doctoral Program of Higher Education of China (Grant no.20040532005).

REFERENCES

1. A. Kaszubowska, L. Hu, and L.P. Barry, Remote downconversion withwavelength reuse for the radio/fiber uplink connection, IEEE PhotonTechnol Lett 18 (2006), 562–564.

2. A. Wiberg, P.P. Millan, M.V. Andres, P.A. Andrekson, and P.O. Hed-kvist, Fiber-optic 40 GHz mm-wave link with 2.5 Gb/s data transmis-sion, IEEE Photon Technol Lett 17 (2005), 1938–1940.

3. M. Attygalle, C. Lim, and A. Nirmalathas, Extending optical transmis-sion distance in fiber wireless links using passive filtering in conjunctionwith optimized modulation, J Lightwave Technol 24 (2006), 1703–1709.

4. J.J. O’Reilly, P.M. Lane, R. Heidemann, and R. Hofstetter, Opticalgeneration of very narrow linewidth millimeter wave signals, ElectronLett 28 (1992), 2309–2311.

5. J. Yu, et al., Optical millimeter-wave generation or up-conversion usingexternal modulators, IEEE Photon Technol Lett 18 (2006), 265–267.

6. J. Yu, et. al., DWDM optical millimeter-wave generation for radio-over-fiber using an optical phase modulator and an optical interleaver,IEEE Photon Technol Lett 18 (2006), 1418–1420.

7. A. Nirmalathas, et al., Wavelength reuse in the WDM optical interfaceof a millimeter-wave fiber-wireless antenna base station, IEEE TransMicrowave Theory Tech 49 (2006), 2001.

8. J. Yu, J. Gu, Z. Jia, and G.K. Chang, Seamless integration of an 8�2.5Gb/s WDM-PON and radio-over-fiber using all-optical up-conver-sion based on Raman-assisted FWM, IEEE Photon Technol Lett 17(2005), 1986–1988.

9. G.H. Smith, D. Novak, and Z. Ahmed, Overcome chromatic-dispersioneffects in fiber-wireless systems incorporating external modulators,IEEE Trans Microwave Theory Tech 45 (1997), 1410–1415.

10. G. Qi, J. Yao, J. Seregelyi, S. Paquet, and C. Belisle, Optical gener-ation and distribution of continuously tunable millimeter-wave signalsusing an optical phase modulator, J Lightwave Technol 23 (2005),2687–2695.

11. L. Chen, H. Wen, and S.H. Wen, A radio-over-fiber system with anovel scheme for millimeter-wave generation and wavelength reuse

for up-link connection, IEEE Photon Technol Lett 18 (2006), 2056–2058.

12. J. Yu, Z. Jia, L. Xu, L.Chen, T. Wang, and G.K. Chang, A DWDMoptical mm-wave generation for ROF downstream link using opticalphase modulator and optical interleaver, IEEE Photon Technol Lett 18(2006), 1418–1420.

© 2007 Wiley Periodicals, Inc.

WIDEBAND FRACTAL PRINTEDMONOPOLE ANTENNAS

Yi-Chieh Lee, Jwo-Shiun Sun, and Syuan-Ci LinDepartment of Electronic Engineering; National Taipei University ofTechnology; Taipei, Taiwan, Republic of China

Received 19 October 2006

ABSTRACT: Study of CPW-fed monopole antenna with circular andrectangular fractal shape radiator element is proposed. By usingfractal slot, new design antennas have the wide measured return lossbandwidth. In addition, the omnidirectional radiation patterns of thedesign antennas covering the entire frequency range have been ob-tained. Several properties of the antennas such as impedance band-width, radiation patterns, and gain have been investigated numeri-cally and experimentally in detail. © 2007 Wiley Periodicals, Inc.Microwave Opt Technol Lett 49: 1267–1272, 2007; Published onlinein Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22448

Key words: CPW-fed; wideband; fractal; slot

1. INTRODUCTION

The increasing demand for wireless communication services spurson the need for antenna’s capability of operating at a broadfrequency range. Planar monopole antennas are good candidatesowing to their wide impedance bandwidth, omnidirectional radia-tion pattern, compact and simple structure, low cost, and ease ofconstruction. Conventionally, monopole antennas are fed througha ground plane by a coaxial probe, since it is structurally simpleand a relatively good match can be readily obtained [1]. Recently,monopole antennas fed by printed transmission lines such asmicrostrip line and coplanar waveguide (CPW) have attractedincreasing attention because of their effortless integration withother circuitries.

Modern and future wireless systems are placing greaterdemands on antenna designs. Many systems now operate in twoor more frequency bands, requiring dual- or triple-band opera-tion of fundamentally narrowband antennas. A variety of tech-niques have been used to create multiband antennas. Severalfractal geometries have been introduced for antenna applicationwith different degrees of success in improving antenna charac-teristics. Some of these geometries have been particularly use-ful in reducing the size of the antenna. These are low profileantennas with moderate gain and can be operative at multiplefrequency bands.

In these 20 years, the engineers combine the fractal structurewith the electromagnetic wave theory, and had successfullyapplied the fractal theory to electromagnetic wave radiation,propagation, and scattering field. Summing up the relevantresearch of the fractal antenna project, there are two maindirections to research in. One is the array effect of researchingthe fractal, as when the stimulation of this fractal occurs, thearray factor of every array unit happens will be displayed.

Figure 4 BER curves after 20-km SSMF

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