852 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 8, APRIL 15, 2016

A Full-Duplex Radio-Over-Fiber Link Based on aDual-Polarization Mach–Zehnder Modulator

Zhenzhou Tang, Student Member, IEEE, and Shilong Pan, Senior Member, IEEE

Abstract— A simple and low-cost full-duplex radio-over-fiber (RoF) link implementing wavelength reuse for short-range applications is proposed and demonstrated based ona dual-polarization Mach–Zehnder modulator (DPol-MZM).In the center unit, a continuous-wave optical carrier is sent toa DPol-MZM to generate an optical signal with data modulationalong one polarization direction and an unmodulated opticalcarrier along the other orthogonal polarization direction. In theremote antenna unit, a polarization beam splitter is used toseparate the modulated signal for downstream service and theunmodulated optical carrier for upstream signal remodulation.A proof-of-concept experiment is carried out. Performance ofthe established full-duplex RoF link operated at 18 GHz isinvestigated. The measured bit error rate and the error vectormagnitude confirm that the proposed architecture is a promis-ing candidate for future high-speed and low-cost short-rangeapplications.

Index Terms— Wavelength reuse, dual-polarization modulator,radio over fiber, full duplex, microwave photonics.

I. INTRODUCTION

RADIO-OVER-FIBER (RoF) systems have attracted a lotof interests in recent years thanks to the advantages in

terms of low loss, light weight, large bandwidth and low costas compared to the conventional electrical transmission sys-tems [1]. Although a lot of attention was paid to the RoF linkwith long transmission distance [2], there are still considerableefforts devoted to the RoF systems for short-range applica-tions, such as aircraft attitude determination system basedon GPS-over-fiber [3], distributed antenna system (DAS) inaircraft cabin [4], optical interconnection in satellites [5], [6],and phased array antenna system with short feeder length [7].In these systems, a lot of remote antenna units (RAUs) areconnected to a center unit (CU). To reduce the cost, it is highlydesirable that the RAU is as simple as possible. To do so,wavelength reuse together with bidirectional transmission canbe applied to avoid the extra light sources and their drivers

Manuscript received October 4, 2015; revised December 30, 2015; acceptedJanuary 3, 2016. Date of publication January 5, 2016; date of currentversion March 10, 2016. This work was supported in part by the NationalBasic Research Program of China under Grant 2012CB315705, in part bythe Fundamental Research Funds for the Central Universities under GrantNE2012002 and Grant NP2015404, in part by the National Natural ScienceFoundation of China under Grant 61422108 and Grant 61527820, in part bythe Aviation Science Foundation of China under Grant 2013ZC52050, andin part by the Priority Academic Program Development of Jiangsu HigherEducation Institutions.

The authors are with the Key Laboratory of Radar Imaging and MicrowavePhotonics, Ministry of Education, Nanjing University of Aeronautics andAstronautics, Nanjing 210016, China (e-mail: pans@ieee.org).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2016.2514612

in the RAUs [8]. In addition, since the RAUs are wavelengthindependent, no wavelength management is required in thesystem. Up to now, many approaches have been proposedto achieve the wavelength reuse, among which the mostwidely-used one is based on the signal erasing effect in areflective semiconductor optical amplifier (RSOA) [9], [10].However, to effectively remove the downstream information,the extinction ratio of the optical downstream signal should bevery low and the frequency should be lower than 10 GHz [11].Wavelength-dependent optical filters or interleavers can alsobe employed to realize the wavelength reuse [12]–[14], butprecisely selecting the optical carrier could be very difficultwhen the frequency of the downstream signal is not veryhigh (e.g., <20 GHz). Moreover, this kind of methods willincrease the complexity and cost, which violates the origi-nal intention for simplifying the RAU. Another wavelength-reuse approach is realized by using phase modulation [15]or polarization modulation [16], [17] in the downlink andintensity modulation in the uplink. However, the downstreaminformation is still carried in the optical carrier, which maydeteriorate the upstream signal in case phase modulation-to-intensity modulation (PM-IM) or polarization modulation-to-intensity modulation (PolM-to-IM) conversion effect exists.

In this letter, we propose a simple and low-costfull-duplex RoF link with a centralized light source usinga dual-polarization Mach-Zehnder modulator (DPol-MZM).In the CU, a continuous wave (CW) optical carrier is sentto the DPol-MZM, which integrates a polarization beamsplitter (PBS), two sub-MZMs and a polarization beam com-biner (PBC). In the DPol-MZM, the optical carrier is equallysplit into two branches along the two orthogonal polarizationstates by the PBS and send to the two sub-MZMs. One ofthe two sub-MZMs is modulated by the downstream sig-nal while the other one is left unused, which generates anoptical signal with data modulation along one polarizationdirection and a clear optical carrier along the other orthogonalpolarization direction. Then the modulated optical signal andthe unmodulated optical carrier are combined by the PBCand transmitted to the RAU. In the RAU, a PBS is usedto separate the modulated signal for downstream service andthe unmodulated optical carrier for upstream signal remodula-tion. As compared to the previously reported RoF links withwavelength reuse, the recovered optical carrier in the RAU isideally pure, carrying no downstream information. In addition,the proposed RoF system is simple, flexible and wideband,since only a single modulator is used in the CU and noadditional wavelength-dependent optical devices are required

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TANG AND PAN: FULL-DUPLEX RoF LINK BASED ON A DPol-MZM 853

Fig. 1. Schematic diagram of the proposed full-duplex RoF link. (a)-(c): optical spectra measured at points a, b and c. LD: laser diode; PC: polarizationcontroller; PBS: polarization beam splitter; PBC: polarization beam combiner; MZM: Mach-Zehnder modulator; PD: photodetector; CU: center unit;RAU: remote antenna unit; OC: optical circulator; PRBS: pseudo-random binary sequence; BERT: bit error rate tester.

in the RAUs. A proof-of-concept experiment is carried out.A downstream 18-GHz RF carrier carrying a 50-Mbaud16-QAM data and an upstream 18-GHz RF carrier carryinga 2-Gb/s pseudo-random binary sequence (PRBS) data aresuccessfully delivered through a bidirectional fiber link. Forthe downlink, the error vector magnitude (EVM) is 2.2%when the received optical power is −4 dBm, and the receiversensitivity for the uplink at a bit error rate (BER) of 1 × 10−9

is about −24 dBm.

II. PRINCIPLE

Fig. 1 shows the schematic diagram of the proposedfull-duplex RoF link. In the CU, an optical carrier froma laser diode (LD) is coupled into a DPol-MZM with itspolarization state adjusted by a PC (PC1). The DPol-MZMis an integrated device consists of a PBS, two sub-MZMs anda PBC. By properly adjusting PC1, the optical carrier appliedto the DPol-MZM is equally split into two branches alongthe two orthogonal polarization directions (i.e., X and Y axes)by the PBS. The optical carrier along X axis is modulatedby the downstream RF signal via MZM1, and the opticalcarrier along Y axis is unmodulated. The modulated opticalmicrowave signal from the upper branch and the unmodulatedoptical carrier from the lower branch is combined by the PBC.Then, the polarization-multiplexed signal at the output of theDPol-MZM is passed through an optical circulator (OC, OC1)and transmitted to the RAU via a length of optical fiber. In theRAU, the incoming optical signal is passed through anotherOC (OC2), and polarization de-multiplexed into two portionsalong the X and Y polarization directions by using anotherPC (PC2) and another PBS (PBS2). Since the downstream RFsignal is only modulated on the X-polarized optical carrierin the CU, the downstream data can be recovered from theX-polarized portion by a photodetector (PD, PD1). Besides,the unmodulated Y-polarized portion is remodulated by the

upstream signal through another MZM (MZM3) and fed backto the fiber link after OC2. In the CU, the upstream opticalsignal is detected by another PD (PD2). The obtained upstreamsignal is downconverted and evaluated by a bit error ratetester (BERT).

III. EXPERIMENT AND RESULTS

A proof-of-concept experiment based on the configurationshown in Fig. 1 is carried out. A 1552.5 nm CW light wavegenerated by a LD (Agilent N7714A) is sent to a DPol-MZMvia PC1. The DPol-MZM (Fujitsu FTM7980) has a 3-dBbandwidth of 30 GHz and a half-wave voltage of 3.5 V @21.5 GHz for each sub-MZM in the two arms. An 18-GHzRF signal carrying a 50-Mbaud 16-QAM data is generatedby a vector signal generator (Agilent E8267D) and appliedto MZM1. The output optical signal of the DPol-MZM istransmitted to the RAU through a length of single modefiber (SMF). In the RAU, the X-polarized branch of PBS2is sent to a PD (U2t XPDV2150) with a 3-dB bandwidthof 50 GHz and a responsivity of 0.65 A/W. Meanwhile, theY-polarized optical signal is sent to a 40-Gb/s MZM (MZM3,Fujitsu FTM7938) and intensity modulated by an 18-GHzsignal carrying a 2-Gb/s PRBS data. In the CU, PD2(U2t XPDV2020R) with a bandwidth of 50 GHz and aresponsivity of 0.65 A/W is used for square-law detection.The obtained electrical signal is downconverted by an 18-GHzmicrowave mixer and then analyzed by a BERT(Anritsu MP1764C). The electrical spectrum and opticalspectrum are measured by an electrical spectrum analyzer(Agilent N9030A) and an optical spectrum analyzer(YOKOGAWA AQ6370C), respectively.

After transmission through a 150-m fiber link, the opticalspectra of the two orthogonal output signals of PBS2 in theRAU are shown in Fig. 1(a) and Fig. 1(b), respectively. Severalsidebands can be observed in Fig. 1(a), indicating that thedownstream RF signal is modulated on the X-polarized optical

854 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 8, APRIL 15, 2016

Fig. 2. (a) Electrical spectrum of the downstream RF signal obtained in theRAU (span 140 MHz), and (b) the corresponding constellation diagram of thedemodulated 16-QAM signal.

Fig. 3. EVMs versus the received optical power for the 150-m and1-km downstream links with or without upstream signal (Inset: constellationdiagrams of the demodulated 16-QAM signal).

carrier in the CU. But in Fig. 1(b), a very strong opticalcarrier is obtained, which means that the Y-polarized opticalcarrier is almost unmodulated. The sideband suppression ratiois larger than 50 dB, which is much better than that in mostof the previously reported wavelength reuse method based onoptical filters (typically ∼30 dB) [12]–[14]. Thanks to the highsideband suppression ratio, the unmodulated optical carrier canbe reused as the high quality optical carrier for the upstreamlink. Fig. 1(c) shows the remodulated optical spectrum at theoutput of MZM3. As can be seen, after being remodulated bythe 18-GHz upstream signal, large sidebands can be observedagain.

For the downstream link, Fig. 2(a) shows the electricalspectrum of the downstream RF signal with a frequency spanof 140 MHz. The corresponding constellation diagram of thedemodulated 16-QAM signal is also shown in Fig. 2(b). TheEVM evaluated by 1000 symbols is about 2.2 %, which showsa good transmission performance. Furthermore, the EVMversus the received optical power is evaluated and presented inFig. 3. As can be seen, as the received optical power increasesfrom −32 dBm to −4 dBm, the EVM is decreased from 11%to 2.2%, since a large optical power corresponds to a highersignal to noise ratio. In addition, it can be seen from Fig. 2(b)that the EVM is nearly unchanged when bidirectional fibertransmission is enabled, showing that the upstream signal hasnegligible effect to the downstream service. Fig. 3 also showsthe results when the fiber length is extended to 1 km. As canbe seen, since chirp is existed in the modulated signal afterMZM1, a better transmission performance is obtained due tothe existance of fiber dispersion.

For the upstream link, when the fiber lengthis 150 m, the electrical spectrum and eye diagram ofthe downconverted baseband signal in the CU are shown

Fig. 4. (a) The spectrum of the down-converted signal and (b) thecorresponding eye diagram.

Fig. 5. BER values versus the received optical power for the upstreamlink with/without optical fiber transmission when the fiber lengths are150 m and 1 km. (Inset: eye diagram of the downconverted baseband signal.).

Fig. 6. (a) The EVMs of the downstream signal and (b) BER valuesof the upstream signal in a period of one hour when the fiber lengths are150 m and 1 km.

in Fig. 4(a) and 4(b), respectively. As can be seen, the eyediagram is widely open, showing that error-free transmissionis achieved. Fig. 5 shows the BER value for the upstream linkversus the received optical power with or without downstreamsignal. For both conditions, the receiver sensitivity at BERof 1 × 10−9 is about −24 dBm, showing that the existence ofdownstream signal has almost no effect to the upstream signalthanks to the high purity of the reused optical carrier. TheBER performance when the fiber length is increased to 1 kmis also shown in Fig. 5. As can be seen, the BER performanceis improved by the introduction of fiber dispersion.

Since the polarization de-multiplexing is required in theRAU, the drift of polarization state due to the turbulence ofthe environment would possibly affect the performance of theproposed system. This problem can be solved by replacingPC2 with a commercial-available polarization tracker in theRAU, or using a polarization-maintaining fiber to replace theSMF. In practice, for the short-range applications, the effectof polarization drift could be very small, since the opticalfiber is quite short. In the experiment, the stability of theproposed RoF link is investigated to evaluate the influenceof the polarization drift. Fig. 6(a) shows the EVM variationsof the downstream signal in the lab environment within one

TANG AND PAN: FULL-DUPLEX RoF LINK BASED ON A DPol-MZM 855

hour when the fiber lengths are 150 m and 1 km, if the EVMvalues are both set to be about 2.3%. Only very small changescan be observed. The BER variation for the upstream link isalso measured for a period of one hour. Similarly, as shownin Fig. 6(b), the BER values are maintained at the 10−9 levelfor both the 150 m and 1 km cases.

IV. CONCLUSION

In conclusion, we have proposed a novel short-range full-duplex RoF link with a centralized optical source based ona DPol-MZM-based carrier reuse scheme. As compared withthe existing wavelength-reuse methods, the proposed approachis simple and potentially wideband, because no wavelength-related optical device is required in the RAU, which sig-nificantly increases the flexibility and simplifies the designof the RAU. In addition, since the downstream signal isonly modulated on the optical carrier along one polarizationdirection, a very pure optical carrier is easily obtained alongthe orthogonal direction, which ensures a good transmissionperformance for the bidirectional fiber link.

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