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Highly Linear Integrated Optical Transmitter for Subcarrier Multiplexed Systems

Highly Linear Integrated Optical Transmitter for Subcarrier Multiplexed Systems
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  438 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009 Highly Linear Integrated Optical Transmitter forSubcarrier Multiplexed Systems Ana Ferreira, Tiago Silveira, Daniel Fonseca, Rui Ribeiro, and Paulo Monteiro  Abstract— A highly linear optical transmitter for radio-over-fiber subcarrier-multiplexed systems is presented. An integrateddual Mach–Zehnder modulator (MZM) is utilized to combinean optical carrier suppressed signal with an optical carrier. Theproposed transmitter enables more than 10-dB improvement inthe carrier-to-interference ratio compared to a quadrature biasedMZM, and similar results to low biased MZM considering similarinsertion loss. Negligible radio-frequency power dependence withtemperature-induced bias drift is reported, while the low biasedMZM is penalized by more than 12 dB for a 15% bias drift. Theproposed transmitter reduces the minimum error vector magni-tude from 4.2% to 3.5%, when compared to quadrature biasedMZM, for a 54-Mb/s orthogonal frequency-division-multiplexedsignal.  Index Terms— Carrier suppressed (CS) modulation, modulatorlinearization, radio-over-fiber (RoF), subcarrier multiplexing(SCM). I. I NTRODUCTION T HE consumer demand for broadband wireless servicesis leading to the necessity for seamless interconnectionof wireless and optical systems. The radio-over-fiber (RoF)technique, associated to subcarrier multiplexing (SCM), offersultrahigh capacity by transmission of different radio channelsin each optical carrier. High-performance SCM RoF systemsare enabled by external modulation, due to reduced relativeintensity noise and chirp, and high modulation bandwidth [1].Nevertheless, such systems are still penalized by the nonlineartransmittance response inherent to most modulation schemes.The transmitter nonlinearity generates spurious spectral com-ponents, leading to intermodulation distortion (IMD) in thedetectedsignals.InmostRoFsystems,third-orderIMD(IMD3)is the most determining cause of distortion, since second-orderIMD can be mitigated by suboctave spectral occupation orfrequency mapping of the channels to transmit. Manuscript received July 03, 2008; revised December 13, 2008. Firstpublished February 03, 2009; current version published March 18, 2009. Thiswork was supported in part by the European Commission, in the context of theproject FUTON grant agreement FP7 ICT-2007-215533, and by PortugueseFCT through project THRONE and PhD grant.A. Ferreira, R. Ribeiro are with Instituto de Telecomunicações, Campus Uni-versitário de Santiago, 3810-193 Aveiro, Portugal (e-mail:; Silveira and P. Monteiro are with NokiaSiemens NetworksSA, Alfragide,2720-093 Amadora, Portugal (e-mail:; Fonseca is with Nokia Siemens Networks SA, Alfragide, 2720-093Amadora, Portugal and also with Instituto de Telecomunicações, Instituto Su-perior Técnico, 1049-001, Lisboa, Portugal (e-mail: versions of one or more of the figures in this letter are available onlineat Object Identifier 10.1109/LPT.2009.2012506 The use of low modulation depth has been proposed to im-prove the linearity of systems employing direct detection, atthe expenses of receiver sensitivity penalty. A technique usinga notch optical filter has been employed to reduce the ratiobetween the optical carrier power and the RoF spectral com-ponents, known as carrier suppression ratio (CSR), improvingthe receiver sensitivity [2]. More recently, complete removalof the optical carrier and nonlinear terms lying in its vicinityhas been proposed in [3] to improve the IMD3 results. How-ever, the latter methods rely on the wavelength stability of thesharp filter and optical source; furthermore, a noncommercialinterferometric structure is required in [3]. Finally, biasing aMach–Zehnder modulator (MZM) towards minimum transmis-sion—lowbias—hasbeen proposed [4] to allowwavelength-in-dependent operation with low modulation depth and high re-ceiver sensitivity. However, such a method requires active biascontrol to prevent severe radio-frequency (RF) power penaltiesdue to bias drift, caused by temperature or aging, for example,since the MZM is biased near minimum transmission [5].Recently, we proposed a wavelength-independent opticaltransmitter for high-performance SCM RoF links [6]. Acommercially available integrated dual MZM (dMZM) [7] isemployed. The generated signal does not present even ordersidebands in the optical domain, leading to reduced IMD3 afterdirect detection. In [6], an assessment of the IMD3 reductionby means of carrier-to-interference ratio (CIR) measurementshas been presented. In this letter, the transmitter is comparedwith a conventional push–pull MZM biased at quadrature andlow biased. Measurements of RF power penalty, due to tem-perature-variation-induced bias drifts, and CIR are performed.Error vector magnitude (EVM) measurements are carried usingan orthogonal frequency-division-multiplexed 64 quadratureamplitude modulated data signal (OFDM 64-QAM).II. O PTICAL  T RANSMITTER  O PERATION  P RINCIPLE Fig.1 depictsthe proposed transmitterscheme. An integrated-cut dMZM, with embedded MZM on each arm, is employed[7]. The ac-coupled SCM signal , where stands for time,drivesthe upper MZM, biased at minimum transmission, gener-atinganopticalcarriersuppressed(CS)signal.However,theop-ticalcarrierabsencedoesnot allowrecoveryoftheSCM signalsinformation after direct detection. To overcome such limitation,the lower dMZM arm couples an in-phase optical carrier. Forthis reason, the denomination CS C is employed. The trans-mitter output signal is given by(1) 1041-1135/$25.00 © 2009 IEEE  FERREIRA  et al. : HIGHLY LINEAR INTEGRATED OPTICAL TRANSMITTER FOR SUBCARRIER MULTIPLEXED SYSTEMS 439 Fig. 1. Proposed transmitter scheme. where and are the angular frequency and amplitude of thecontinuous-wave signal at the modulator input. The modulationdepthis representedby ; standsfor theswitching voltageof all Mach–Zehnder structures; and DC is a dc signal biasingthe lower MZM. The modulating signal includes arbitrarySCM channels.The modulation depth determines the voltage swing of thesignal driving the upper MZM, which srcinates the sine termof expression (1). To evaluate the presence of distortion com-ponents at the optical transmitter output, the sine term can beexpanded on a Taylor series, which does not include any evenorder term. Therefore, the proposed transmitter does not srci-nate second-order optical sidebands. The beating of such termswith the fundamental tones during direct detection is reportedin [3] to be the most important contribution for IMD3; hence,reduced signal distortion is expected.DC controlstheopticalcarrierattenuationsufferedatthelower MZM: constant CSR can be obtained for different valuesof optical modulation depth , if DC is tuned accordingly.Thus, the transmitter linearity can be further improved withoutpenalizing the receiver sensitivity, by employing low and ad- justing DC , at the cost of increased insertion loss.To evaluate the performance of the proposed transmitter, itis compared to a conventional push–pull MZM, whose outputsignalisgivenby(2),with representingthedcbias.enables quadrature bias and re-trieves low biased operation of the MZM [4](2)Odd and even order sidebands are observed at the transmitteroutput in the optical domain when performing the Taylor seriesexpansion of the sine and cosine terms presented in (2), for allreferred . Biasing the MZM at quadrature leads to second-order (andall evenorder) IMDcancellationatthe photodetectoroutput,inabsenceofopticalfiber.Nevertheless,IMD3,themostdetermining cause of distortion in the majority of the RoF sys-tems, is not cancelled.III. E XPERIMENTAL  R ESULTS  A. Two-Tone Analysis Thetransmittersunderstudyaredrivenbyasignal com-posed by two RF tones with frequencies and of 2.468and 2.508 GHz, respectively, and similar RF power. The op-tical modulation depth and, consequently, the CSR, are con-trolledbytheRFtones’power.Theexperimentalopticalspectraat the output of the conventional quadrature biased MZM and Fig. 2. Experimental optical spectra for CSR of 10 dB.Fig. 3. Experimental electrical spectra for 10-dB CSR. the proposed transmitter are depicted in Fig. 2, for CSR of ap-proximately 10 dB. The proposed transmitter is tested with thelower MZM biased at maximum and quadrature transmission,corresponding to 0 and 3 dB of additional optical carrier at-tenuation in the lower MZM. These configurations are repre-sented by CS C and CS C , respectively.The signalsspectra and CSR are measured in an optical spectrum analyzer(OSA) with a resolution of 100 MHz. Since the OSA resolu-tion is higher than the frequency separation between the two RFtones (40 MHz), it is not possible to distinguish them and theCSR includes the power of both tones falling within OSA reso-lution.The optical spectrum at the output of the conventional MZMbiased at quadrature, depicted in Fig. 2(a), presents strongsecond-order nonlinear sidebands, as expected from the dis-cussion in Section II. The proposed transmitter output opticalspectra, represented in Fig. 2(b) and (c), present second-orderterms with reduced power. These spurious terms, not presentin the analytic derivation of Section II, are being generated bythe nonlinear response of the electrical circuitry used at themodulator input, and by nonidealities of the dMZM, such asresidual chirp. Nevertheless, second-order optical sidebandspresent lower power than third order sidebands, which are themost prominent nonlinear components. CS C reduces therequired modulation index for identical CSR when comparedto CS C ; therefore, the nonlinear spectral componentsare further reduced.Fig. 3 depicts the experimental electrical spectra after di-rect detection at the transmitter’s output, considering a CSRof approximately 10 dB. Adjacent to the fundamental tones,third-order IMD components are observed at and. To extend such analysis, Fig. 4 presents CIR as afunctionoftheCSR,measuredatthetransmitteroutputwithim-provements of more than 10 and 16 dB granted by CS Cand CS C , respectively, when compared to the conven-tional transmitter biased at quadrature. Fig. 4 also compares theCIR obtained for the common MZM low biased (optical carrierattenuation of 10 dB when compared to maximum transmission[4])andforCS C (opticalcarrierattenuationof4dBinthelower MZM), which present similar insertion losses. The latterconfigurationspresentidenticalCIRimprovementsofmorethan20 dB, compared to quadrature biased MZM.  440 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009 Fig. 4. CIR characterization.Fig.5. (a)ImpactofbiasdriftonthereceivedRFpower,and(b)CS    Cbiasingconfigurations.Fig. 6. Experimental EVM characterization. CS C and low biased MZM present similar CIR; how-ever, significant power penalty may occur in case of bias driftcaused, for example, by temperature variations[5]. ConsideringaCSRof10dB,thetransmitter’sdcbiasandRFinputpowerareadjusted at 20 C. Maintaining such conditions, the RF powerpenalty was experimentally measured at 50 C, and presentedin Fig. 5(a), to evaluate the transmitter’s dependence with biasdrift. The results are presented as a function of the measuredbias drift normalized to , since two different devices are em-ployedandnegligible driftwasverifiedbetweenthetwotem-peratures.Simulationcurvesarealsopresented;constant andsimilardriftsoccurringinthetwoembeddedMZMoftheCS Ctransmitter are assumed. As expected, high sensitivity to the dcbias drift is registered for the low biased MZM, when comparedto quadrature biasing. On the other hand, if the CS C trans-mitter is appropriately biased, the electrical field variations oc-curring in the two dMZM arms approximately compensate atthe modulator output. This is illustrated in bias configuration (i)of Fig. 5(b), which represents the electrical field of the opticalsignal ( ) at each embedded MZM output, as a function of thedc bias voltage ( ). The upper MZM bias is such that the elec-trical field decreases with the dc bias, while the lower MZM isbiased for increasing electrical field, or vice-versa. This biasingconfiguration enables negligible RF power penalty, as depictedin Fig. 5(a). The opposite case is considered in configuration(ii): the electrical fields of the optical signals at the output of theupper and lower MZM increase (or decrease) with dc bias. Forthis configuration, RF power penalties similar to the low biasedMZM are retrieved. The results demonstrate that the CS Ctransmitter with appropriate bias is highly tolerant to bias drifts,avoiding the need for bias stabilization circuitry.  B. Modulation With OFDM 64-QAM  The common MZM biased at quadrature and the proposedtransmitter are fed with a standard wireless local area net-work IEEE 802.11a data signal. For such purpose, an OFDM64-QAM 54 Mb/s data signal, modulated at 2.488 GHz, isconsidered. The measured EVM is presented in Fig. 6, as afunction of the CSR. Constant optical power of 10 dBm isconsidered at the photodiode input in all tests. For high CSR,the performance is mainly impaired by system noise; therefore,the different transmitters present similar EVM degradation. Forlow CSR, the detected signals are distorted by the transmitters’nonlinearity, resulting in EVM penalty. The improved linearityof the proposed transmitter results in reduction of the optimumEVM, since low CSR values are tolerated. Due to experimentalconstraints, the low biased and the CS C transmitters arenot tested; however, from the conclusions of the previous sec-tion, slightly improved EVM is expected for both transmittersin comparison to CS C .IV. C ONCLUSION A wavelength-independent optical transmitter for RoF SCMsystems, with improved modulation linearity, has been experi-mentally reported. A commercial dMZM is employed to com-bine a very linear CS signal and an optical carrier with ad- justable power, allowing control of the CSR.Considering two electrical tones, the CS C transmitter en-abled more than 10-dB improvement of CIR when comparedto a quadrature biased MZM, and results similar to a low bi-ased MZM for the same insertion loss. CS C verified highrobustness to bias drift, enabling stable operation without biasstabilization circuits, which are mandatory for the low biasedtransmitter. The proposed transmitter has also excelled quadra-ture biased transmitter, in terms of EVM measurements for a64-QAM OFDM signal.R EFERENCES[1] T. Darcie and J. Zhang, “High performance microwave-photoniclinks,” in  Proc. IEEE Radio and Wireless Symp. , 2008, pp. 125–128.[2] M. Attygalle, C. Lim, G. Pendock, A. Nirmalathas, and G. Edvell,“Transmission improvement in fiber wireless links using fiber Bragggratings,”  IEEE Photon. Technol. Lett. , vol. 17, no. 1, pp. 190–192,Jan. 2005.[3] C. Lim, A. Nirmalathas, K. Lee, D. Novak, and R. Waterhouse, “Inter-modulation distortion improvement for fiber-radio applications in-cor-porating OSSB+C modulation in an optical integrated-access environ-ment,”  J. Lightw. Technol. , vol. 25, no. 6, pp. 1602–1612, Jun. 2007.[4] M. L. Farwell, W. S. C. Chang, and D. R. Huber, “Increased lineardynamic range by low biasing the Mach–Zehnder modulator,”  IEEE Photon. Technol. Lett. , vol. 5, no. 7, pp. 779–782, Jul. 1993.[5] C. Mueller and J. Coffer, “Temperature dependent bias drift inproton-exchanged lithium niobate Mach–Zehnder modulators,” in Proc. CLEO’99 , May 1999, pp. 291–292.[6] A. Ferreira, T. Silveira, D. Fonseca, P. Monteiro, and R. Ribeiro,“Highly linear radio-over-fiber transmitter for subcarrier multiplexedsystems,” in  Proc. ECOC  , Brussels, Belgium, 2008, Paper Tu.4.F.5.[7] K. Higuma, S. Oikawa, Y. Hashimoto, H. Nagata, and M. Izutsu,“    -cut lithium niobate optical single-sideband modulator,”  Electron. Lett. , vol. 37, no. 8, pp. 515–516, Apr. 2001.
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