Optical Fiber Technology 15 (2009) 290295

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Crown Copyright 2009 Published by Elsevier Inc. All rights reserved.

Introduction

Recently radio-over-ber (ROF) techniques have become attrac-e solutions in realizing future broadband wireless networks be-use ROF technique can be used for the distribution of wire-ss signals. Optical millimeter-wave signal generation and simplenguration of base station are key techniques to realize lowst and high transmission performance in the ROF-based opticalireless access networks. To realize optical millimeter-wave gen-ation by frequency up-conversion, many techniques have beenported, such as the frequency up-conversions using four-waveixing [1], optical heterodyne detection with optical interleaving] and cross-gain modulation in a semiconductor amplier [35],equency doubling using an optical carrier suppression modula-on [6,7], and frequency quadrupling and sextupling using opticalequency multiplication technique [8]. Because a low cost RF oscil-tor can be used to generate optical millimeter-wave signal withequency quadrupling and sextupling, it has been considered toa cost-effective solution.In order to simplify the base station, it has proposed central-

ed lightwave at the central station or wavelength reuse in these station. To realize wavelength reuse, different schemes haveen proposed [913]. It employed a FBG to reect the optical car-er and reuse it for uplink connection [9], but no any experiments demonstrated how to realize this function for downstream

Corresponding author.E-mail address: hnu_jhe@hotmail.com (J. He).

data transmission. In [10], it utilized optical carrier suppression(OCS) to generate optical millimeter-wave, and then recombinedthe same optical carrier with the optical millimeter-wave beforethe upstream signals are transmitted to the base station. However,the electrical local oscillator (LO) frequency to generate the opticalmillimeter-wave is half of the spacing between the two rst-ordersidebands. For example, for the 40 GHz optical millimeter-wavegeneration, the LO frequency is 20 GHz.

In the paper, we have experimentally demonstrated two dif-ferent schemes to generate optical millimeter-wave using opticalfrequency quadrupling and wavelength reuse for uplink connectionin the ROF systems. In the two schemes, a 40 GHz millimeter-wavesignal was generated by a 10 GHz RF signal. The MZM driven bya large RF signal will lead to a large modulation depth; thereforethe remained optical carrier is large. Hence this remained opticalcarrier can be reused to carry upstream data. In this way, it can ef-fectively utilize optical power and reduce cost of system. We havealso investigated the quality of the signals and transmission perfor-mance of downstream and upstream data considering the impactof ber chromatic dispersion in the two schemes.

2. Principle

Fig. 1 shows the principle of full-duplex ROF architecture in twodifferent schemes to generate optical millimeter-wave with fre-quency quadruple and to realize wavelength reuse for upstreamconnection. The principle in scheme A based on sub-carrier multi-plexing (SCM) technique is shown as Fig. 1(a). In the CS, the base-band downstream data are up-converted with the RF signal fromContents lists availab

Optical Fiber

www.elsevier.co

ull-duplex radio-over-ber system with pptical millimeter-wave generation

He , L. Chen, Z. Dong, S. Wen, J. Yuy Laboratory for Micro/Nano Opto-Electronic Devices of Ministry of Education, School of Com

r t i c l e i n f o a b s t r a c t

ticle history:ceived 2 September 2008vised 4 December 2008ailable online 31 January 2009

ywords:avelength reuseachZehnder modulatortical millimeter-wave with photonicsquency quadruple

We have experimentally invmillimeter-wave using opticwavelength reuse for uplinkMZM is used for both the opMZMs are used. In this schemquadrupling, and another onthe optical carrier can be reexperimentally comparing thit can be seen that scheme B68-5200/$ see front matter Crown Copyright 2009 Published by Elsevier Inc. All rigi:10.1016/j.yofte.2008.12.006at ScienceDirect

echnology

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r and Communication, Hunan University, Changsha 410082, China

gated two different schemes (schemes A and B) to generate opticalrequency quadrupling with a MachZehnder modulator (MZM), andnection in the radio-over-ber (ROF) systems. For scheme A, only onel millimeter-wave generation and signal modulation. For scheme B, twoone of MZMs is used to generate optical millimeter-wave for frequencyused for signal modulation. In both schemes, at the base station (BS),d to carry upstream data and delivered to the central station (CS). Byrformance of downstream and upstream transmission in two schemes,overcome the crosstalk between the upstream and downstream signals,hts reserved.

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bana10mDnasidebands are suppressed completely. The half-wave voltage of thee LO by an electrical mixer. The continuous wave (CW) lightwavemodulated via a LiNbO3 MachZehnder modulator (MZM) driventhe up-converted signal. The MZM has two functions: (1) to

nerate high-order optical harmonic; (2) to modulate optical sig-l. If the MZM is DC biased at the top peak output power whene LO signal are removed, the odd-order sidebands can be sup-essed. Therefore, the optical millimeter-wave signals with foures of LO frequency is generated after they are separated from

e optical carrier by using a FBG [14,15]. The millimeter-wave sig-l is generated when the two second-order optical sidebands areat at an optical/electrical (O/E) converter at the base station. Theillimeter-wave signal will be broadcasted by an antenna after O/Enversion, while the reected optical carrier from FBG is actedthe continuous wave and modulated by an intensity modula-

r driven by the upstream data. The upstream optical data willsent back to the CS, where a low-cost low-frequency receivertects the upstream data.The principle of scheme B is shown as Fig. 1(b). In scheme B,e CW lightwave is modulated via one MZM driven by the RF sig-l. The MZM transmission curve is shown as Fig. 2. The MZM isC biased at the top peak output power when the LO signal are re-oved, which is the same as that of scheme A. After the MZM, thed-order sidebands can be suppressed. Then an optical circulator

MZM is 7.28 V. The optical spectrum after SCM technique mod-ulation is inserted in Fig. 3(a). The wavelength spacing betweenthe two second-order sidebands is 0.32 nm (40 GHz). The second-order sideband is 12 dB lower than the optical carrier while therst-order sideband is suppressed completely. After transmissionover 20 km SMF with a dispersion of 17 ps/nm/km, a FBG with thecentral wavelength at 1543.72 nm is used to reect the optical car-rier while pass the optical millimeter-wave signal to O/E receiver.The optical spectrum after passing through the FBG is inserted inFig. 3(b). The 40 GHz millimeter-wave signal with four times ofRF frequency is generated by beating the two second-order opticalsidebands at the O/E receiver. The optical spectrum of the opticalcarrier reected by the FBG is inserted in Fig. 3(c). For the uplink,the reected optical carrier from the FBG is modulated by anotherintensity modulator driven by the 2.5-Gb/s upstream data with aPRBS length of 2311. After modulation, the optical spectrum isinserted in Fig. 3(d). Then the upstream optical signal is injectedinto another 20 km SMF and transmitted back to the CS. In realnetworks, one duplexer to connect the antenna can be used tocirculate the transmitting and receiving signal at the BS. The base-band upstream signals would be obtained after down-conversionof end users information coming from the duplexer in the BS.

The measured eye diagrams of the millimeter-wave with down-stream data before and after transmission over 20 km SMF areJ. He et al. / Optical Fiber Tech

Fig. 1. Principle of two schemes of full-duplex ROF system with photoni

Fig. 2. MZM transmission curve.d FBG are used to separate the central optical carrier and thecond-order sidebands. The baseband downstream data is added

shvegy 15 (2009) 290295 291

equency quadruple millimeter-wave. (a) Scheme A, (b) scheme B.

the optical millimeter-wave by using an intensity modulator,hich can be realized by another MZM. The central optical carrierill be combined with the downstream optical millimeter-wavegnal by using a 3 dB optical coupler before they are transmit-d to the BS. The BS is the same as that of scheme A.

Experimental setup and results

Fig. 3 shows the experimental setup of scheme A. The 2.5-Gb/sseband signal of the downstream data with a pseudorandom bi-ry sequence (PRBS) length of 2311 is up-converted with theGHz RF signal by an electrical mixer. The CW at 1543.72 nm is

odulated by a MZM driven by the up-converted signal. When theC bias is 3.45 V and the peak-to-peak voltage of microwave sig-l for the electrical narrowband amplier is 7.62 V, the odd-orderown in Figs. 4(a) and 4(b). After a PIN photodiode, the con-rted electrical signal is amplied by an electrical amplier with

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am20pomFBeyg. 3. Experimental setup for scheme A: optical millimeter-wave generation by using SCM modulation based on frequency-quadrupled. MZM: LiNbO3 MachZehnder modu-or; FBG: ber Bragg grating; Cir: circulator.

Fig. 4. Measured eye diagrams of millimeter-wave signals. (a) Back-to-back, (b) transmission 20 km SMF.

Fig. 5. Measured eye diagrams of downstream data. (a) Back-to-back, (b) transmission 20 km SMF.

Fig. 6. Measured eye diagrams of upstream data. (a) Back-to-back, (b) transmission 20 km SMF.

bandwidth of 10 GHz. The measured eye diagrams of the de-odulated downstream data before and after transmission overkm SMF are shown in Figs. 5(a) and 5(b), respectively. The

wer uctuation of the 40 GHz modulation arises from the chro-atic dispersion. The remained optical carrier reected from the

over 20 km SMF are shown in Figs. 6(a) and 6(b), respectively. Itcan be seen that a part of downstream data are transmitted overthe upstream SMF to the CS. The reason is that a part of down-stream signals have been carried by the optical carrier reectedfrom the FBG at BS. Once the optical carrier is reused to modulate2 J. He et al. / Optical Fiber TechG is re-modulated by a 2.5-Gb/s upstream data. The measurede diagrams of the upstream data before and after transmission

upbigy 15 (2009) 290295stream data, the downstream data will be existence. As the DCas is decreased, the power of the second-order sideband and the

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Filatsting the DC bias of the MZM in scheme A.Fig. 8 shows the experimental setup of scheme B. The optical

illimeter-wave is generated using a MZM along with a FBG. TheZM with a half-wave voltage of 7.28 V is driven by a 10 GHz RFnusoidal wave with a peak-to-peak voltage of 9.51 V. The opticalectrum after modulation is inserted in Fig. 8(a). The power of thetical carrier is 10 dB larger than that of the second-order side-g. 8. Experimental setup for scheme B: optical millimeter-wave generation by using OCS mor; FBG: ber Bragg grating; Cir: circulator.ied by an electrical amplier with a bandwidth of 10 GHz cen-red at 40 GHz. In our experiment, the downstream and upstreamta are two 2.5-Gb/s pseudorandom bit sequence electrical sig-ls with a word length of 2311 generated from different sources.e optical spectrum of the upstream optical signal is shown in in-rted in Fig. 8(d). At the CS, the upstream data is detected by ammercial PIN receiver.J. He et al. / Optical Fiber Technology 15 (2009) 290295 293

Fig. 7. BER curves for both downstream and upstream data.

osstalk effect from the downlink will be decreased. However, weve not realize the frequency quadrupled modulation when theC bias is too far away from the idea value to generate quadru-ed modulation. The measured bit-error-rate (BER) performancer both downstream and upstream signals is shown in Fig. 7. Afterkm transmission, the power penalty caused by ber chromatic

spersion is 1 dB for the downstream data, meanwhile the powernalty after transmission over 20 km SMF for the upstream datasmaller than 1 dB. Therefore, the perfect BER performance forth downstream and upstream can be obtained by properly ad-

band. As the DC bias is 3.6 V, the MZM is biased at the maximumoutput power so that the odd-order sidebands can be suppressed.Two second-order sidebands are obtained after using a circulatorand a FBG to lter out optical carrier. In this way, the opticalmillimeter-wave with two second-order sidebands has four timesthe RF frequency. Then the generated optical millimeter-wave isamplied by an EDFA and modulated by an intensity modulatordriven by 2.5-Gb/s electrical signal. The optical spectrum of theoptical millimeter-wave signal with downstream data is insertedin Fig. 8(b). We can see that the optical carrier and odd-ordersidebands are removed. The remained optical carrier is reectedfrom another output of FBG, and the optical spectrum is insertedin Fig. 8(c). After combined by a 3 dB optical coupler, the opticalmillimeter-wave signal and the remained optical carrier are trans-mitted over 20 km SMF with dispersion of 17 ps/nm/km beforereaching the BS.

At the BS, another circulator and FBG are used to separatethe optical millimeter-wave signal and the remained optical car-rier. The FBG lter has a 3 dB reection bandwidth of 0.15 nmand reection ratio larger than 25 dB at the reection peak wave-length, which is used to separate the optical second-order side-bands (downstream signals) and the optical carrier. The separateddownstream signals are detected via a PIN photodiode with a 3 dBbandwidth of 60 GHz after they are boosted by a regular EDFAwith a small-signal gain of 30 dB, and then ltered by a tunableoptical lter with the bandwidth of 0.5 nm to suppress ampliedspontaneous emission noise. The converted electrical signal is am-odulation based on frequency-quadrupled. MZM: LiNbO3 MachZehnder modu-

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stthbyupFiplupFig. 12. BER curves of upstream data at 2.5-Gb/s.

Figs. 9(a) and 9(b) show the eye diagrams of optical millimeter-ave before and after transmission over 20 km SMF, respec-ely. Fiber dispersion re-shapes the optical millimeter-wave sig-l, which leads a different shape compared with the originalillimeter-wave signal without transmission ber. The down-nverted eye diagrams of downstream data before and afteransmission over 20 km SMF are shown in Fig. 10. It shows theer chromatic dispersion impacts on eye diagram of downstreamta after transmission over 20 km SMF. But it is clearly seen thate eye diagrams still keep open after transmission over 20 kmF. The eye diagram of the upstream data for back-to-back and

ansmission over 20 km SMF are shown in Fig. 11. It can be seenom the eye diagrams that there have small amplitude uctuationused by the wide bandwidth of the FBG. The BER performancethe upstream data for scheme A and scheme B are shown in

g. 12. For upstream data in scheme B, the power penalty is1 dB at a BER of 109. It can be seen that receiver sensitivity ofheme B is better than that of scheme A. Therefore scheme B canercome the crosstalk between the upstream and downstreamgnals obviously. Meanwhile it also show that the millimeter-wave

the eye of the upstream signals still keeps open despite transmis-sion over 20 km SMF in Fig. 11. It means that the remained opticalcarrier separated from the downstream signals in this scheme hasnegligible effect on the transmission performance of millimeter-wave with downstream data.

4. Conclusions

We have experimentally demonstrated two schemes of full-duplex ROF systems with frequency-quadrupled optical millimeter-wave generation and wavelength reuse for uplink connection. Inthe two schemes, as a MZM driven by an RF at 10 GHz, 40 GHzmillimeter-wave can be generated, which can greatly reduce thebandwidth of microwave component and modulator. At the sametime, since the remained optical carrier can be reused to carryupstream data at BS, it can effectively utilize optical power andreduce cost of system. We also compared two schemes consider-ing the impact of ber chromatic dispersion. For scheme A, themillimeter-wave with frequency quadruple was generated only byone MZM. However, the generated millimeter-wave and optical car-rier will be contained in downstream data at CS. Once the opticalcarrier reected from the FBG at BS is reused to modulate up-stream data, the crosstalk effect from downstream data will beexistence. Therefore the millimeter-wave power uctuates due toconstructive and destructive interaction between the two beatingsinduced by chromatic dispersion. For scheme B, only the generatedmillimeter-wave is modulated by downstream data, the optical car-rier is not. So the upstream data carried by the optical carrier have4 J. He et al. / Optical Fiber Tech

Fig. 9. Measured eye diagrams of millimeter-wave sig

Fig. 10. Measured eye diagrams of downstream dat

Fig. 11. Measured eye diagrams of upstream datanerated in scheme B has better quality and is almost robust toer chromatic dispersion.

beofgy 15 (2009) 290295

. (a) Back-to-back, (b) transmission 20 km SMF.

) Back-to-back, (b) transmission 20 km SMF.

Back-to-back, (b) transmission 20 km SMF.

Here we compare the transmission performance of down-ream and upstream data in the two schemes. In scheme A,e millimeter-wave with frequency quadruple is generated onlyone MZM; hence this conguration in CS is simpler. But thestream data contains a part of downstream data as shown ings. 6(a) and 6(b). In scheme B, the conguration is more com-icate and expensive since two MZMs are used. However, thestream signal has better performance, which can be seen thattter performance. By experimentally comparing the performancedownstream and upstream transmission in two schemes, it can

J. He et al. / Optical Fiber Technology 15 (2009) 290295 295

be seen that scheme B is better in the quality of the generatedmillimeter-wave, and the generated millimeter-wave signal withdownstream data is almost robust to ber chromatic dispersion inscheme B.

Acknowledgments

This work is partially supported by National 863 High Tech-nology Research and Development Program of China (Grant No.2007AA01Z263), the Hunan Provincial Natural Science Foundationof China (Grant No. 06JJ50108) and the Open Fund of Key Labo-ratory of Optical Communication and Lightwave Technologies (Bei-jing University of Posts and Telecommunications, Ministry of Edu-cation, PR China).

References

[1] J. Yu, J. Gu, X. Liu, Z. Jia, G. Chang, Seamless integration of an 8 2.5 Gb/sWDM-PON and radio-over-ber using all-optical up-conversion based onRaman-assisted FWM, IEEE Photon. Technol. Lett. 17 (2005) 19861988.

[2] T. Kuri, K. Kitayama, Y. Takahashi, A single light-source conguration for full-duplex 60-GHz-band radio-on ber system, IEEE Trans. Microwave TheoryTech. 51 (2003) 431439.

[3] J. Seo, Y. Seo, W. Choi, 1.244 Gb/s data distribution in 60 GHz remote opticalfrequency upconversion systems, IEEE Photon. Technol. Lett. 18 (2005) 13891391.

[4] J. Yu, Z. Jia, G. Chang, All-optical mixer based on cross-absorption modulationin electroabsorption modulator, IEEE Photon. Technol. Lett. 18 (2005) 24212423.

[5] H. Song, J. Lee, Error-free simultaneous all-optical upconversion of WDM radio-over-ber signals, IEEE Photon. Technol. Lett. 17 (2005) 17311733.

[6] J. Yu, Z. Jia, L. Yi, Y. Su, G. Chang, Optical millimeter-wave generation or up-conversion using external modulators, IEEE Photon. Technol. Lett. 18 (2006)265267.

[7] J. Yu, Z. Jia, L. Xu, L. Chen, T. Wang, G. Chang, DWDM optical millimeter-wavegeneration for radio-over-ber using an optical phase modulator and an opticalinterleaver, IEEE Photon. Technol. Lett. 18 (2006) 14181420.

[8] J.J. OReilly, P.M. Lane, Fiber-supported optical generation and delivery of60 GHz signals, Electron. Lett. 30 (1994) 13291330.

[9] A. Kaszubowska, L. Hu, L.P. Barry, Remote down-conversion with wavelengthreuse for the radio/ber uplink connection, IEEE Photon. Technol. Lett. 18(2006) 562564.

[10] Z. Jia, J. Yu, Chang, G. Chang, A full-duplex radio-over-ber system based onoptical carrier suppression and reuse, IEEE Photon. Technol. Lett. 18 (2006)17261728.

[11] J. Yu, M. Huang, Z. Jia, T. Wang, G.K. Chang, A novel scheme to generate single-sideband millimeter-wave signals by using low-frequency local oscillator signal,IEEE Photon. Technol. Lett. 20 (2008) 478480.

[12] J. Yu, Z. Jia, T. Wang, G.K. Chang, A novel radio-over-ber conguration us-ing optical phase modulator to generate an optical mm-wave and centralizedlightwave for uplink connection, IEEE Photon. Technol. Lett. 19 (2007) 140142.

[13] J. Yu, Z. Jia, T. Wang, G.K. Chang, Centralized lightwave radio-over-ber sys-tem with photonic frequency quadrupling for high-frequency millimeter-wavegeneration, IEEE Photon. Technol. Lett. 19 (2007) 14991501.

[14] M. Attygalle, C. Lim, G. Pendock, A. Nirmalathas, G. Edvell, Transmission im-provement in ber wireless links using ber Bragg gratings, IEEE Photon. Tech-nol. Lett. 17 (2005) 190192.

[15] M. Bakaul, A. Nirmalathas, C. Lim, Ecient multiplexing scheme forwavelength-interleaved DWDM millimeter-wave ber-radio systems, IEEE Pho-ton. Technol. Lett. 17 (2005) 27182720.

Full-duplex radio-over-fiber system with photonics frequency quadruples for optical millimeter-wave generationIntroductionPrincipleExperimental setup and resultsConclusionsAcknowledgmentsReferences