A self-pulsating laser diode is used to generate a multicarrier microwave optical signal for use in a hybrid radio/fiber system. The self-pulsation frequency of the laser is controlled by external light injection, and can be varied between 14-24 GHz. The hybrid radio/fiber system, employing the self-pulsation laser, is used to distribute two 155-Mb/s data signals on two radio frequency (RF) carriers (at 18.5 and 18.9 GHz). Experimental results show the overall system performance for both RF channels, and demonstrate that the performance is improved by around 17 dB compared with the case when the laser is used without external injection, and thus, does not self-pulsat

Anandarajah, Prince M.

Barry, Liam P.

Kaszubowska-Anandarajah, Aleksandra

English

DCU Online Research Access Service

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 11, NOVEMBER 2002 1599Multiple RF Carrier Distribution in a HybridRadio/Fiber System Employing a Self-PulsatingLaser Diode TransmitterA. Kaszubowska, L. P. Barry, and P. AnandarajahAbstract—A self-pulsating laser diode is used to generate a mul-ticarrier microwave optical signal for use in a hybrid radio/fibersystem. The self-pulsation frequency of the laser is controlled byexternal light injection, and can be varied between 14–24 GHz.The hybrid radio/fiber system, employing the self-pulsation laser,is used to distribute two 155–Mb/s data signals on two radio fre-quency (RF) carriers (at 18.5 and 18.9 GHz). Experimental resultsshow the overall system performance for both RF channels, anddemonstrate that the performance is improved by around 17 dBcompared with the case when the laser is used without external in-jection, and thus, does not self-pulsate.Index Terms—External light injection, microwave photonics,optical communications, optical systems, self-pulsation, semicon-ductor laser diode.I. INTRODUCTIONH IGH CAPACITY mobile networks of the future will prob-ably use high-frequency microwave signals as the accessmedium (15–60 GHz), as this offers a large bandwidth for datatransfer. These high-capacity microwave systems are likely toemploy an architecture in which signals are generated at a cen-tral location and then distributed to remote base stations usingoptical fiber, before being transmitted over small areas usingmicrowave antennas [1], [2]. Such an architecture should proveto be highly cost efficient, since it allows sharing the transmis-sion and processing equipment (remotely located in the centralcontrol station) between many base stations. It is also expectedthat these broad-band mobile networks will divide the availabletransmission bandwidth into a number of RF channels for broad-casting data “over the air.” This use of multiple carrier distri-bution is normally required in high-capacity multipath environ-ments in order to overcome multipath fading effects.On the transmission side of a radio-over-fiber distributionnetwork it is necessary to generate the microwave opticaldata signal using semiconductor laser diodes. The simplesttechnique available to generate optical microwave signalsinvolves direct modulation of the laser with the microwavedata carriers. However, the limited bandwidth of laser diodesmeans that we are normally unable to use high frequency RFcarriers GHz . One possible solution is to use activemode locking of a laser diode cleaved to an appropriate lengthsuch that resonant enhancement of the microwave signalsmay be achieved with direct modulation [3]. However, thisManuscript received May 23, 2002; revised July 17, 2002.The authors are with the School of Electronic Engineering, Dublin City Uni-versity, Dublin 9, Ireland (e-mail: liam.barry@dcu.ie).Digital Object Identifier 10.1109/LPT.2002.803318.technique requires specially designed devices that only operateat specific frequencies. Another technique to overcome thelimited bandwidth of laser diodes is to employ external opticalinjection into the laser, as this can greatly increase the intrinsicmodulation bandwidth of the diode [4], [5]. In addition, at highinjection levels, the laser can start to self-pulsate at frequenciesthat are suitable for RF transmission, thus making the deviceuseful for the generation of microwave optical signals in hybridradio/fiber networks [6], [7]. In a previous letter, we showedthat the performance of a single channel radio-over-fibersystem can be enhanced by using this external injectiontechnique [8]. In this letter, we characterize the self-pulsationin the laser under external injection locking conditions, anddemonstrate how the injection-locked commercial laser may beemployed in a hybrid radio/fiber system for the distribution ofmultiple-carrier RF data signals.II. SELF-PULSATING LASER CHARACTERISTICSThe laser diode used for the experiments is a standard mul-tiple-quantum-well distributed-feedback (DFB) device fromNEL. The laser has a threshold current of 26 mA, and an intrinsicmodulation bandwidth of around 8 GHz. By injecting lightfrom a wavelength tunable external cavity laser, at the samewavelength as the DFB emission wavelength, we significantlyalter the modulation response of the device, and can achieveexcellent response at frequencies from 14 to 25 GHz. Theenhanced response at these frequencies is caused by the externalinjection inducing instability in the laser diode. The instability, inturn, results in the output power from the laser undergoing strongoscillations due to beating between the optical field componentsin the laser cavity [6]. Fig. 1 displays the optical spectrum fromthe laser when it is biased at 60 mA and the externally injectedpower from the external cavity laser is 5 mW. We can clearly seethe modulation on the spectrum at a frequency of around 20 GHz.To further investigate the oscillation from the laser, the opticaloutput from the laser under external injection was detected witha 50–GHz photodiode and displayed on a 50-GHz oscilloscope.Triggering was achieved by splitting the electrical signal afterthe detector in two, and using one of the outputs as the trigger.Fig. 1(b) displays the detected signal. We can clearly see theoscillation at a frequency of around 20 GHz, and also the signif-icant level of noise and jitter on the signal. This noise and jitteron the oscillation from the laser is also evident in the detectedelectrical spectrum [Fig. 1(c)], and the broad linewidth is causedby the jitter between the DFB laser diode and the tunable cavity1041-1135/02$17.00 © 2002 IEEE1600 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 11, NOVEMBER 2002Fig. 1. (a) Optical spectrum. (b) Detected output power oscillation. (c)Electrical power spectrum of DFB laser biased at 60 mA with external injectionlevel of 5 mW.laser. The frequency of the oscillation from the injection-lockedlaser depends on the strength of the injected optical signal [6],and Fig. 2 displays how the oscillation frequency varies as afunction of the injected power level. In addition, by measuringthe peak-to-peak voltage of the oscillation on the oscilloscope,we have been able to determine that the optical output from thelaser is 100% modulated.III. MULTICARRIER RADIO-OVER-FIBER SETUPAs we have stated in the introduction, future broad-bandRF networks will require multicarrier microwave systems toovercome multipath fading. It will be necessary for opticallyfed microwave systems employing self-pulsing laser diodesto be capable of handling multiple RF carrier data signals. Todemonstrate this, we have used the experimental arrangementdescribed in Fig. 3. A 155-Mb/s nonreturn-to-zero (NRZ) datastream from an Anritsu pattern generator is initially passedthrough a 117-MHz low-pass filter to minimize the bandwidthof the electrical data signal, and then split in an RF coupler.The two signals are subsequently propagated through cableswith a length difference of 1.5 m in order to ensure that thereis no correlation between the two data channels. One of thetwo data streams is then mixed with an 18.5-GHz RF carrier,and the second data stream is mixed with an 18.9-GHz RFcarrier, resulting in two binary phase-shift keyed (BPSK) datasignals. The RF data signals are then combined together, andthe resulting multicarrier microwave data signal is used todirectly modulate the DFB laser diode described above. TheFig. 2. Self-pulsation frequency from externally injected DFB laser as afunction of injected power level.Fig. 3. Experimental set up for multicarrier hybrid radio/fiber system usingself-pulsating laser diode.data signal comprising two RF carriers can be applied either tothe free-running laser or to the laser diode into which light isinjected from the tunable external cavity laser. In both cases,the RF data signal is combined with a dc bias current of 60 mA.The resulting optical microwave data signal from the laser isthen passed through 5 km of dispersion-shifted fiber beforebeing detected with a 50-GHz p-i-n photodiode. In a completeradio/fiber system, the detector output would be transmittedthrough an RF antenna to the mobile stations, however, in ourexperiment we have concentrated on the optical part of thesystem. Hence, the down conversion of the two RF carrier datasignals takes place after the photodiode. To recover the twodata channels simultaneously, the detected signal is split in two.One of the signals is then down converted by mixing it withthe 18.5-GHz local oscillator, and the second signal is downconverted by mixing it with the 18.9-GHz local oscillator. Thetwo down-converted signals are subsequently passed throughlow-pass filters to ensure that only the required baseband signalis examined using the 50-GHz oscilloscope and error analyzer.Using the oscilloscope, we are able to characterize the receivedeye diagrams of the two 155-Mb/s data signals, and with theAnritsu error analyzer we can determine the bit-error rate(BER) of the received signals.Fig. 4(a) displays the received eye diagrams (displayed si-multaneously on the oscilloscope) of the two down-convertedKASZUBOWSKA et al.: MULTIPLE RF CARRIER DISTRIBUTION IN A HYBRID RADIO/FIBER SYSTEM 1601Fig. 4. Received eye diagrams of the two 155-Mb/s data signals from theoptically fed microwave system using (a) free running laser diode and (b) laserdiode with the external injection level of 4 mW. Received optical power (beforephotodiode) was  9 dBm in both cases.Fig. 5. BER versus received optical power for the two RF data channels usingdirectly modulated laser with and without external injection.155-Mb/s data signals after propagation through the optical mi-crowave link using the DFB laser with no external light injec-tion. The detected average optical power is 9 dBm. As wecan see, because the response of the laser is so poor at fre-quencies around 18 GHz, the received eye diagrams are com-pletely closed. We then proceeded to inject light into the DFBlaser from the wavelength tunable external cavity laser at thesame wavelength as the DFB emission wavelength. The injec-tion level is set to around 4 mW, as this is the power level thatoptimizes the modulation response of the laser at the frequen-cies of the microwave data signal. Fig. 4(b) displays the simul-taneously received eye diagrams with a detected optical powerof 9 dBm. The eyes are clearly open for both data channels,demonstrating good system performance. By varying the de-tected optical power level using an optical attenuator, and mea-suring the BER of the received signals, we can plot system per-formance against received optical power, both with and withoutthe external injection. Fig. 5 shows that the external injectionimproves the overall performance of the system by around 17 dBfor each RF data channel. The difference in performance for thetwo RF data channels is mainly due to the different synthesizersused for the 18.5 and 18.9-GHz carriers.The RF powers in the two data signals applied to the laserare around 12 dBm. At these low power levels, the laserdoes not tend to completely lock to either of the data signalsapplied. However, the small-signal modulation response of theself-pulsating laser does show greatly enhanced performanceover a significant bandwidth [similar to that shown in Fig. 1(c)],and it is this enhanced response that improves the performanceof the multicarrier system. On the issue of crosstalk, inter-channel crosstalk between the data signals is negligible, asthe 400-MHz channel spacing is sufficiently larger than the117-MHz bandwidth of the data signals. In addition, crosstalkdue to fiber nonlinearity, which is predominantly caused byfour-wave-mixing in subcarrier multiplexing (SCM) systemswith low data-rate per channel and narrow channel spacing [9],can be neglected thanks to the small number of channels andlow optical power launched in the fiber 4 dBm . However,as the channel spacing is further reduced to improve spectralefficiency, and the number of channels and launched power isfurther increased, it will be important to investigate degrada-tions in system performance due to crosstalk in more detail.IV. CONCLUSIONWe have demonstrated the use of a self-pulsating laser diode(achieved using external injection into a commercial DFB laser)for the distribution of multicarrier RF data signals in a hybridradio/fiber communication system. As the frequency of the self-pulsation is determined by the external injection level, we canvary the set up to operate in different frequency bands. Ourresults show that we can successfully modulate the self-pul-sating laser with two RF data signals simultaneously, giving usa 17-dB improvement in system performance, for each RF datachannel, above what would be achieved using the laser diodewithout external light injection. In future work, we intend to in-vestigate how intermodulation distortion will effect the perfor-mance of a multicarrier hybrid radio/fiber system using self-pul-sating laser transmitters, however, our initial work has demon-strated that such a scheme may be feasible for the developmentof broad-band microwave communication systems.REFERENCES[1] R. P. Braun, G. Grosskopf, H. Heidrich, C. von Helmolt, R. Kaiser, K.Kruger, U. Kruger, D. Rohde, F. Schmidt, R. Stenzel, and D. 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Hybrid radio/fiber distribution system based on a directly modulated laser transmitter with external injection,”

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Optical transmission of narrow-band millimeter-wave signals by resonant modulation of monolithic semiconductor laser,”

Subcarrier multiplexing for high-speed optical transmission,”

Multiple RF carrier distribution in a hybrid radio/fiber system employing a self-pulsating laser diode transmitter

Multiple RF carrier distribution in a hybrid radio/fiber system employing a self-pulsating laser diode transmitter

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