We demonstrate an analog photonic link with a high multioctave spurious-free dynamic range (SFDR) using a push-pull modulation technique of laser diodes combined with a balanced detection scheme. SFDR enhancements ranging from 5 dB to 18 dB, relative to the case of a single arm link, have been obtained in a frequency range of 2.5 GHz to 3.2 GHz

Etten, W. van

Marpaung, D.A.I.

Roeloffzen, C.G.H.

English

van Etten, Wim

University of Twente Research Information

Enhancement of Multioctave Dynamic Range in aPush-Pull Modulated Analog Photonic LinkD.A.I. Marpaung, C.G.H. Roeloffzen and W. van EttenUniversity of Twente, Faculty of Electrical Engineering, Mathematics and ComputerScience, Telecommunication Engineering group, P.O.Box 217, 7500 AE Enschede, TheNetherlandsWe demonstrate an analog photonic link with a high multioctave spurious-free dynamicrange (SFDR) using a push-pull modulation technique of laser diodes combined with abalanced detection scheme. SFDR enhancements ranging from 5 dB to 18 dB, relative tothe case of a single arm link, have been obtained in a frequency range of 2.5 GHz to 3.2GHz.IntroductionThese past few years, there have been numerous research efforts concentrated on the spu-rious free dynamic range (SFDR) enhancement of analog photonic links (APLs). Theseefforts are driven by various applications, such as military r dars [1] and phased-array an-tennas for radio astronomy [3], that impose stringent SFDR requirements to the APLs. Todate, most efforts to increase the SFDR have been directed toexternally-modulated APLsrather than directly modulated ones [2]. This, to some extent, is attributed to the factthat external modulation offers chirp-free operation and ahigher modulation bandwidthcompared to direct modulation. However, for applications iwhich a large number ofAPLs are required, employing external modulators might become too costly. In this case,directly modulated lasers (DMLs) are still very much preferred due to their low cost andsimple operation. These DMLs, nevertheless, should provide sufficiently large SFDR.One of the main limitations of DMLs is the high second-order intermodulation distortion(IMD2) [2]. This prevents them to be implemented in broadband systems in which thesignal has a bandwidth of more than one octave (i.e. multioctave). A solution to thisproblem is to suppress the IMD2 using a technique that combines push-pull modulationof DMLs with a balanced detection scheme [1, 3]. Using this technique, we recentlyreported one of the highest SFDR ever achieved with DMLs, shown at the frequency of2.5 GHz [3]. In this paper, we optimize our system to operate in an extended frequencyrange beyond 2.5 GHz.Push-Pull Modulated APLIdeally, the push-pull modulated APL consists of a 180o hybrid coupler, a pair of DMLsand a balanced photodetector (BPD) [3]. The 180o hybrid coupler supplies antiphase RFsignals to the DMLs such that they are modulated in a push-pull manner. This results ina pair of optical signals with a 180o (RF modulation) phase difference. Providing that theoptical fibers lengths to the BPD inputs are matched, differential detection of these an-tiphase optical signals will yield to additive photocurrents at the BPD output. SupposingHybridDML 2DML 1BPDRF att.180oVODLCombinerMENUSSTATUSMSG01 2 34 5 67 8 9FrequencyAmplitudeModePresetINPUT EXT INPUT 1EXT INPUT 2LF OUTPUTRF OUTPUT12.500 000 000 00 GHz 3.00 dBm FREQUENCY  AMPLITUDEADJUSTRANGE MODLASER DISPLAYTEC DISPLAYLASER TECC mAPOWER TEC MODE LASER MODEPARAMETERGPIB500 mA200 mA EXTERNALLOCALILX Lightwave LDC-3724 LASER DIODE CONTROLLERMENUSSTATUSMSG01 2 34 5 67 8 9FrequencyAmplitudeModePresetINPUT EXT INPUT 1EXT INPUT 2LF OUTPUTRF OUTPUT12.500 000 000 00 GHz 3.00 dBm FREQUENCY  AMPLITUDEADJUSTRANGE MODLASER DISPLAYTEC DISPLAYLASER TECC mAPOWER TEC MODE LASER MODEPARAMETERGPIB500 mA200 mA EXTERNALLOCALILX Lightwave LDC-3724 LASER DIODE CONTROLLERANALYZER SETUP MARKER UTILITYLXIMXAFrequencyAuto TuneCenter FreqStart FreqStop FreqCF StepCenter Freq   5.00000000 GHz Avg Type : Log-PwrAtten : 10 dBInput : RF1 2 3 4Res BW  30 Hz VBW  30 Hz Span 10 kHzRef -30.00 dBm-120-110-100-90-80-70-60STATUSMSGEnter01 2 34 5 67 8 9RF InputSignal Generator Signal Generator Laser ControllerLaser ControllerRF Spectrum AnalyzerFigure 1: Schematic of the realized push-pull modulated APL. DML: directly modulated laser,VODL: variable optical delay line, BPD: balanced photodetector.that the DMLs are identical, perfect amplitude matching of the antiphase optical signalscan be achieved. In this case, the output RF signal of the push-ll APL will be 6 dBhigher compared to the case of a single arm APL (which can be obtained by means ofdisconnecting one of the optical fibers to the BPD) and the IMD2at the BPD output willbe completely suppressed. This suppression will, in turn, increase the link multioctave(i.e. broadband) SFDR.Measurement SetupThe realized push-pull APL is shown in Figure 1. A two-tone measurement was carriedout to characterize the distortion (and subsequently the SFDR) of this APL. We use anetwork analyzer (Agilent N5230A ) and a vector signal generator (Agilent E4438C) tosupply a pair of RF tones to the DMLs via a 2:1 combiner and a 180o hybrid coupler. Theinsertion loss of the combiner and the hybrid coupler amounts to 10 dB. The tones are 10MHz apart and their center frequency,fc was varied from 1 GHz to 4 GHz with a step of100 MHz. The DMLs are 1310 nm DFB lasers from Fitel with 20 mW maximum outputoptical power and 4 GHz modulation bandwidth. The DMLs bias currents are adjustedwith a pair of laser diode controllers (ILX Lightwave LDC 3722 and LDC 3724).Since DML1 shows higher second-order nonlinearity relative o DML2 [3], an RF atten-uator is placed in the lower arm of the APL (Figure 1) to attenuate the RF power suppliedto this laser and subsequently to match the IMD2 amplitudes at the upper and the lowerarms. Because the IMD2 suppression is highly sensitive to phase matching, a variableoptical delay line (VODL) is used to correct the phase imbalances of the optical signalsdue to the length mismatch. The fundamental, IMD2 and the third order intermodulationdistortion (IMD3) powers are measured at the output of the BPD(Discovery Semicon-ductor DSC-710) with an RF spectrum analyzer (Agilent MXA N9020A) at frequenciesof fc +5 MHz , 2fc and fc +15 MHz, respectively.Measurement ResultsTo determine the optimum operating currents of the DMLs, theinjection current to eachlaser is varied from 40 mA to 85 mA and, for every bias point, the IMD3 power of theindividual laser is measured. For this characterization, the input RF power supplied to the40 45 50 55 60 65 70 75 80 85-90-85-80-75-70-65-60-55-501 GHz2 GHz3 GHz4 GHzIMD3 power (dBm)Bias Current (mA)Selected bias current= 55 mA(a)40 45 50 55 60 65 70 75 80 85-95-90-85-80-75-70-65-604 GHz3 GHz2 GHzIMD3 power (dBm)Bias current (mA)1 GHzSelected bias current= 73 mA(b)Figure 2:IMD3 power as a function of laser injection current for (a) DML1 and (b) DML2.1.0 1.5 2.0 2.5 3.0 3.5 4.0-39-36-33-30-27Single-arm APLsDML1DML2S 21 (dB)Modulating frequency (GHz)Push-pull APL(a)1.0 1.5 2.0 2.5 3.0 3.5 4.0-95-90-85-80-75-70-65Single-armAPLsDML2IMD2 Power (dBm)Modulating frequency (GHz)DML1Push-pull APL(b)Figure 3:(a) Fundamental signal and (b) IMD2 powers as functions of the modulation frequency.combiner is set at 9 dBm per tone. The characterization results for DML1 and DML2 areshown in Figures 2(a) and 2(b), respectively. Here, we have set the modulating frequencyas a parameter. Our aim is to determine the bias currents where t IMD3 powers are lowin a wide range of modulation frequency, from 1 GHz to 4 GHz. Asevident from Figure2, these optimum bias currents are 55 mA and 73 mA for DML1 and DML2, respectively.Having selected the DMLs bias currents, we optimize the system by adjusting the RFattenuator and the VODL with the objective of obtaining maximum IMD2 suppressionsfor a wide range of modulating frequencies. The optimum RF attenuation is found to be4 dB and the maximum IMD2 suppresion related to this attenuation is limited to 25 dB.Higher suppressions can be obtained by using an attenuator with a better step resolution.Ideally, the way to correct the phase imbalances in the system is to observe - in realtime - the IMD2 powers as the VODL is adjusted to obtain maximum IMD2 suppress-sions. However, this type of measurement requires a synchroized frequency sweepingfor modulation and detection at two different frequency ranges. For example, if the themodulation frequency isfc, the detection should be done at 2fc. Due to the limitation inour measurement setup, this type of measurement was unavailable. Thus, we optimize thesystem by means of adjusting the VODL while observing the fundamental signal powerinstead of the IMD2 power.2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2859095100105110Single-arm APLsDML1DML2IMD2-SFDR (dB.Hz1/2)Modulating frequency (GHz)Push-pull APL18 dBFigure 4:The measured multioctave SFDR ofthe push-pull and the single-arm APLs.The optimum setting is thus determined bythe widest range of modulation frequencyin which maximum signal enhancement isachieved. This is illustrated in Figure 3(a).A maximum signal enhancement of 6 dB,relative to the case of single-arm APLs, isobtained at a modulation frequency range of2 GHz to 3.5 GHz. With this arrangement,the IMD2 power in the push-pull configu-ration is measured. The result is shown inFigure 3(b). The suppression is achieved inthe modulation frequency range of 2.5 GHzto 3.2 GHz, shown as a grey area in Figure3(b). A maximum suppression of 25 dB isobtained at the frequency of 2.81 GHz. Theoverall suppression can be increased by using an attenuatorwith finer attenuation steps.In this frequency range, the SFDR of the push-pull and the single-arm APLs is charac-terized. Here, we use the definition of SFDR to be the signal-to-noise ratio at the inputRF power where the intermodulation distortion power (IMD) isequal to the noise power[2]. As evident from Figure 4, the push-pull APL shows improved IMD2-SFDR, overa considerably wide frequency range, compared to the single-arm APLs. A maximumimprovement of 18 dB is achieved at the modulation frequencyof 2.81 GHz. At this fre-quency, the IMD2-SFDR and the IMD3-SFDR of the push-pull APLare 107 dB.Hz1/2and 118 dB.Hz2/3, respectively. In contrast to the single arm APLs(IMD2-SFDR =90 dB.Hz1/2, IMD3-SFDR = 117 dB.Hz2/3), the push-pull APL provides more compara-ble sub-octave and multioctave SFDR values. This makes the impl mentation of this typeof APL in broadband applications very attractive.ConclusionsMeasurement results of a push-pull modulated APL have been presented. The link showsimproved IMD2-SFDR characteristic relative to the conventio al single-arm APLs. Mul-tioctave performance has been achieved at the frequency range of 2.5 GHz to 3.2 GHz.AcknowledgementsThis work is supported by the Dutch Ministry of Economic Affairs through the PACMANproject, SenterNovem project number TSIT 3049.References[1] S. Pappert, C. Sun, R. Orazi, and T. Weiner, “Microwave fiber optic links for shipboard antennaapplications”,Proc. 2000 IEEE Int. Conf. Phased Array Systems and Technology, pp. 345-348, 2000.[2] C. H. Cox, E. I. Ackerman, G. E. Betts and J. L. Prince “Limits and performance of RF-over-fiberlinks and their impact on device design ,”IEEE Trans. Microwave Theory & Tech., vol. 54, no. 2, pp.906-920, February 2006.[3] D. A. I. Marpaung, C. G. H. Roeloffzen and W. van. Etten, “ABroadband High Dynamic RangeAnalog Photonic Link using Push-Pull Directly Modulated Semiconductor Lasers,”IEEE/MTT-SInternational Microwave Symposium, pp. 507–510, 15–20 June 2008.

Enhancement of multioctave dynamic range in a push-pull modulated analog photonic link

A Broadband High Dynamic Range Analog Photonic Link using Push-Pull Directly Modulated Semiconductor Lasers,”

Microwave ﬁber optic links for shipboard antenna applications”,

Prince “Limits and performance of RF-over-ﬁber links and their impact on device design ,”

http://purl.utwente.nl/publications/65365

Enhancement of multioctave dynamic range in a push-pull modulated analog photonic link

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