A directly modulated laser diode with external light injection is used to generate microwave optical signals for a hybrid radio/fiber system. The external light injection greatly enhances the frequency response of the laser, and thus, significantly improves the overall performance of the hybrid system. Experimental results show a 14-dB improvement in system performance for the externally injected laser in a hybrid radio/fiber communication link used for distributing 155-Mb/s data signal

Anandarajah, Prince M.

Barry, Liam P.

Kaszubowska-Anandarajah, Aleksandra

English

DCU Online Research Access Service

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 2, FEBRUARY 2002 233Improved Performance of a Hybrid Radio/FiberSystem Using a Directly Modulated LaserTransmitter With External InjectionA. Kaszubowska, Student Member, IEEE, P. Anandarajah, Student Member, IEEE, and L. P. Barry, Member, IEEEAbstract—A directly modulated laser diode with external lightinjection is used to generate microwave optical signals for a hybridradio/fiber system. The external light injection greatly enhancesthe frequency response of the laser, and thus, significantly improvesthe overall performance of the hybrid system. Experimental re-sults show a 14-dB improvement in system performance for the ex-ternally injected laser in a hybrid radio/fiber communication linkused for distributing 155-Mb/s data signals.Index Terms—External light injection, microwave photonics,modulation bandwidth, optical communications, optical systems,semiconductor laser diode.I. INTRODUCTIONAS THE DEMAND for broadband mobile services suchas video-on-demand and mobile computing increases, sodoes the need to develop high-capacity mobile communicationnetworks, which are capable of delivering broadband signals toremote areas “over the air.” High-capacity mobile networks ofthe future will probably use high frequency microwave signalsas the access medium (15 – 60 GHz), as this offers a large band-width for data transfer. These high-capacity microwave accessnetworks are likely to employ an architecture in which signalsare generated at a central location and then distributed to remotebase stations using optical fiber, before being transmitted oversmall areas using microwave antennas [1], [2]. Such an archi-tecture should prove to be highly cost efficient, since it allowssharing the transmission and processing equipment (remotelylocated in the central control station) between many base sta-tions.To develop an optically fed microwave wireless network ofthis type, requires the amalgamation of many different technolo-gies. On the transmission side, the main technical challenge isto generate the microwave optical data signals using semicon-ductor laser diodes. The simplest technique available to gen-erate optical microwave signals, involves direct modulation ofthe laser with the microwave data carrier. However, the limitedbandwidth of laser diodes means that we are normally unable touse high frequency RF carriers ( 10 GHz), which could be em-ployed to transmit very broadband data signals. This difficultywith direct modulation has resulted in the use of more com-plicated techniques for optical microwave signal generation inhybrid radio/fiber systems [3], [4], such as employing externalManuscript received June 20, 2001; revised October 26, 2001.The authors are with the School of Electronic Engineering, Dublin City Uni-versity, Dublin 9, Ireland (e-mail: liam.barry@dcu.ie).Publisher Item Identifier S 1041-1135(02)00491-3.Fig. 1. Experimental setup for hybrid radio/fiber system using directlymodulated laser with external injection.modulators or dual mode laser diodes. However, it has recentlybeen shown [5], [6], that by using external injection into a laserdiode, its modulation response can be significantly enhanced. Inthis letter, we demonstrate that this enhancement of the modu-lation response allows us to greatly improve the performance ofan optically fed microwave system based on direct modulationof laser diodes. A 14-dB improvement in system performance isobtained with a hybrid radio/fiber system used for distributing155-Mb/s data signals on an 18-GHz RF carrier.II. EXPERIMENTAL SETUPThe experimental setup to examine the use of a directlymodulated laser diode with external injection in a hybridradio/fiber system is presented in Fig. 1. An Anritsu patterngenerator produces a 155-Mb/s nonreturn-to-zero (NRZ) datastream, which is initially mixed with an 18-GHz RF carrier togenerate the binary phase-shift keying (BPSK) data signal. Theresulting microwave data signal is then used to directly modu-late a single-mode laser diode from NTT Electronics. The laserused is a distributed feedback (DFB) multiple-quantum-welldevice with a threshold current of 26 mA, a central emissionwavelength of 1543.2 nm, and a 3-dB electrical bandwidth of8 GHz. The 18-GHz RF data signal can be applied either tothe free-running laser, or it can be applied to the laser diodeinto which light is injected from a tunable external cavity laser(ECL). In both cases, the RF data signal is combined with a dcbias current of 55 mA. The resulting optical microwave data1041–1135/02$17.00 © 2002 IEEE234 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 2, FEBRUARY 2002Fig. 2. Modulation response of single-mode laser measured under free runningconditions, and with external injection levels of  3 dBm (dotted line), 0 dBm(solid line), and 3 dBm (dashed line) from ECL. The dc bias current was 55 mAin all cases.signal from the laser is then passed through 5 km of disper-sion-shifted fiber (to minimize chromatic dispersion effectsthat may cause severe RF power degradations in the system[8]) before being detected with a 50-GHz pin photodiode. Ina complete system, the output signal of the detector wouldbe transmitted through an RF antenna to the mobile networkstations, where the data is received by down converting theincoming signal using a local oscillator. However, in our exper-iment, we have concentrated on the optical part of the system,hence, the down conversion takes place after the photodiode,by mixing the data signal with an 18-GHz local oscillator. Theresulting 155-Mb/s data signal may then be displayed on theoscilloscope, or fed into the Anritsu error analyzer to determinethe bit-error rate (BER) of the received signal.III. EXPERIMENTAL RESULTSThe improvement in the modulation bandwidth of a laserdiode that can be obtained by external injection has recentlybeen demonstrated both theoretically and experimentally [5],[6]. In our setup, we use external injection from an ECL into asingle-mode laser to greatly enhance the bandwidth of the laserdiode, the complete experimental set-up for the external injec-tion is similar to that described in [7]. The wavelength of theinjected signal must be accurately tuned to ensure that the op-timal enhancement in modulation response is obtained. Fig. 2displays the modulation response of the device under free run-ning conditions, and also when the external injection level fromthe ECL is set to 3, 0, and 3 dBm. The wavelength of the in-jected light from the ECL was set to 1543.24 nm. By varying theinjected power level, we can optimize the lasers response at anyspecific frequency (up to approximately 22 GHz). For this ex-periment, we set the injection power level to 0 dBm, as this gaveus maximum response at 18 GHz, which is the frequency we areusing in our hybrid fiber/radio system demonstration. Propaga-tion of the resulting optical microwave data signal over 5 kmFig. 3. Received eye diagrams of 155-Mb/s data signal from the optically fedmicrowave system using (a) free running laser diode. (b) Laser diode with theexternal injection level of 0 dBm. Received optical power (before photodiode)was  12 dBm in both cases.of dispersion-shifted fiber does not greatly effect the systemperformance, as the total dispersion in the transmission fiber isnegligible at the operating wavelength ( 9 ps/nm). For trans-mission over longer distances, or over standard single-modefiber, chromatic dispersion may cause severe RF power degra-dations in the system [8], due to the double side-band natureof the signal from the directly modulated laser diode. However,in a real system, we expect that this problem may be overcomeby employing an optical filter to select out the optical carrierand one signal side-band, before the optical signal is detected.Clearly, the transmission characteristic of this filter will needto be as close to square shaped as possible, with a bandwidthslightly larger than the RF carrier frequency. Custom designedfiber Bragg grating type filters should be suitable for this appli-cation.Fig. 3 displays the received eye diagrams for the caseswhen the directly modulated laser was 1) free running, and 2)had an external injection level of 0 dBm from the ECL. Thereceived optical power (before the photodiode) in both caseswas 12 dBm. The major improvement in system performancewhen external injection is employed can be clearly seen. Wethen placed a variable attenuator before the photodiode andmeasured the received BER as a function of received opticalpower, with and without external injection. These results arepresented in Fig. 4, which shows that there is a 14-dB improve-ment in system performance, for a received BER of 10 , whenexternal injection is applied to the directly modulated laser.This improvement in system performance is less than wouldbe expected (16 dB) from the enhanced electrical frequencyresponse shown in Fig. 3. This is attributed to the fact that theexternal injection not only improves the laser bandwidth, butalso reduces the laser threshold, thus, increasing the averageoptical output power from the device.KASZUBOWSKA et al.: IMPROVED PERFORMANCE OF HYBRID RADIO/FIBRE SYSTEM 235Fig. 4. BER versus received optical power using directly modulated laser with(circles) and without (squares) external injection.IV. CONCLUSIONThe future development of hybrid fiber-radio systems ishighly dependent on the use of simple and efficient technolo-gies to generate the optical microwave signals at the centralstation for distribution over fiber to remote base stations. Directmodulation of a laser diode with the RF data signal is thesimplest technique available, but the limited bandwidth of laserdiodes is a major constraint with this technique. In this letter,we have shown that by using external injection into the directlymodulated laser, we can enhance the modulation response ofthe laser, such that the system performance of a radio overfiber distribution network is significantly improved. By varyingthe power level of the external injection, we can also varythe microwave frequency at which the modulation responseis enhanced. We have demonstrated an overall improvementin system performance of 14 dB when using this technique todistribute a 155-Mb/s data signal on an 18-GHz RF carrier inan optically fed microwave system.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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Meng, T. Chau, and M. C. Wu, “Experimental demonstration ofmodulation bandwidth enhancement in distributed feedback lasers withexternal light injection,” Electron. Lett., vol. 34, pp. 2031–2032, 1998.[7] L. P. Barry, P. Anandarajah, and A. Kaszubowska, “Optical pulse gen-eration at frequencies up to 20 GHz using external-injection seeding ofa gain-switched commercial Fabry-Perot laser,” IEEE Photon. Technol.Lett., vol. 13, pp. 1014–1016, Sept. 2001.[8] G. H. Smith, D. Novak, and Z. Ahmed, “Overcoming chromatic – dis-persion effects in fiber-wireless systems incorporating external modu-lators,” IEEE Trans. Microwave Theory Tech., vol. 45, pp. 1410–1415,Aug. 1997.

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Fiber-optic millimeter-wave downlink system using 60 GHZ-band external modulation,”

Optical microwave generation and transmission experiments in the 12- and 60-GHz region for wireless communications,”

Optical millimeter-wave generation by dual-mode operation of semiconductor modelocked lasers,”

Optical pulse generation at frequencies up to 20 GHz using external-injection seeding of a gain-switched commercial Fabry-Perot laser,”

Overcoming chromatic – dispersion effects in fiber-wireless systems incorporating external modulators,”

Improved performance of a hybrid radio/fiber system using a directly modulated laser transmitter with external injection

Improved performance of a hybrid radio/fiber system using a directly modulated laser transmitter with external injection

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