1320 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 8, AUGUST 1999

Theoretical and Experimental Study of 10 Gb/sTransmission Performance Using 1.55m

LiNbO3-Based Transmitters withAdjustable Extinction Ratio

and ChirpSung Kee Kim, O. Mizuhara, Y. K. Park, L. D. Tzeng, Y. S. Kim,

and Jichai Jeong,Senior Member, IEEE

Abstract—This paper has experimentally and theoreticallyinvestigated transmission performance depending on chirpingand extinction ratio for a 10 Gb/s transmission system withthe standard single-mode fiber. The transmission performancecan be dramatically degraded or improved by adjusting chirpand extinction ratio in a 1.55 �m LiNbO 3 modulator-basedtransmitter and erbium-doped fiber amplifier (EDFA)-pin diodereceiver configuration. To estimate the transmission performance,bit error rate (BER) characteristics rather than eye-openingpenalty (EOP) have been calculated by solving the nonlinearSchrodinger equation with including the model of chirping andextinction ratio for the transmitter, and noise and intersymbolinterference for the receiver. This simulation can predict themeasured BER characteristics well enough to see interplayingbetween chirping and extinction ratio.

Index Terms—Chirp modulation, optical communication, opti-cal fiber dispersion, optical transmitters.

I. INTRODUCTION

W ITH the advent of erbium-doped fiber amplifiers(EDFA’s) operating near 1.55m, fiber loss no longer

presents a fundamental limit to extend transmission distanceand increase date rate. Chromatic dispersion then becomesthe main limiting factor for increasing transmission speedand distance. So various techniques have been suggested toovercome the limitation using modulator-based low chirp ornegative chirp devices [1], and/or dispersion compensation[2]. Dispersion compensation by specially designed fiber withgroup velocity dispersion (GVD) of the opposite sign tothat of conventional fiber [3] is a simple technique, but hassmall freedom in changing the magnitude of compensation.

Manuscript received March 25, 1998; revised January 5, 1999. The work ofJ. Jeong was supported by KOSEF-OERC-98K3-0809-02-05-1 from KAIST.

S. K. Kim, Y. S. Kim, and J. Jeong are with the Department of RadioEngineering, Korea University, Seoul 136-701 Korea.

O. Mizuhara and L. D. Tzeng are with Lucent Technologies, Bell Labora-tories, Breinigsville, PA 18031 USA.

Y. K. Park is with Lucent Technologies, Bell Laboratories, Murray Hill,NJ 07904 USA.

Publisher Item Identifier S 0733-8724(99)06349-5.

It is difficult to cope with the time variation in the fiberand device characteristics [4]. On the other hand, chirp andextinction ratio have been exploited to extend transmissiondistance at multigigabit applications. MQW modulator-integrated DFB laser with negative chirp can be exploitedto extend transmission distance over 100 km at 10 Gb/s [5].Electroabsorption-modulated distributed feedback lasers withthe negative chirp and low-extinction ratio have been utilizedfor low-cost 10 Gb/s transmission through the standard single-mode fiber [6]. However, chirp and extinction ratio of thesemodulator-integrated lasers are not independent each other likedirectly modulated laser, while a LiNbO3 modulator-basedtransmitter can independently utilize these two parameters bychanging the driving voltage applied to the electrodes [7]. Inorder to exploit chirping and extinction ratio for improvingtransmission performance, it is important to understand thatchirp and extinction ratio can interplay together to achievelonger transmission distance.

In this paper, we experimentally and theoretically inves-tigate transmission performance depending on the chirpingand extinction ratio interplay adjusted from Mach–Zehndertype LiNbO3 modulator with double electrodes. Eye-openingpenalty (EOP) has been normally calculated for transmissionperformance [8]. However, BER characteristics rather thanEOP were calculated from the same parameter used in theexperiment to see how well simulation results predict closelyexperimental results.

In Section II, we describe the experimental results of 10Gb/s transmission depending on chirp and extinction ratiousing a LiNbO3 modulator-based transmitter and EDFA-pinreceiver configuration, and the standard single-mode fiber.Section III deals with the simulation results of 10 Gb/s trans-mission in terms of BER characteristics for the transmissionsystem with the same experimental condition such as-parameter and extinction ratio so that we can see how oursimulation predicts the experimental results and understandthe transmission performance with negative-parameter andlow-extinction ratio. In Section IV, we draw the conclu-sions.

0733–8724/99$10.00 1999 IEEE

KIM et al.: STUDY OF 10 Gb/s TRANSMISSION PERFORMANCE 1321

Fig. 1. Sensitivity at 10�9 BER measured LiNbO3 transmitter withlow-extinction ratio (solid line) and high-extinction ratio (dashed line).

II. TRANSMISSION EXPERIMENTS WITH

ADJUSTABLE CHIRP AND EXTINCTION RATIO

A 1.55- m 10 Gb/s Mach–Zehnder type Ti:LiNbO3 exter-nal modulator with double electrodes was driven with largeelectrical PRBS NRZ data of the word length. Thechirp and extinction ratio of the transmitter were variedby adjusting the voltage ratio and the voltage applied tothe two electrodes, respectively. At the transmitter, the-parameter related to chirp was accurately measured from thetime-resolved chirp measurement [9] and the extinction ratio

% % was measured from histogram of the opticaloutput pulse. Extinction ratio and-parameter will be used forthe simulation in the next section. The input power into thefiber was maintained below 3 dBm to exclude the nonlineareffect in fiber so that we can obtain transmission performancemainly depending on chirp and extinction ratio. Finally, theoptical signal was detected by a regenerator and then BER’swere measured with changing average received power andfiber length. The decision threshold voltage in the regeneratorwas optimized to get the best sensitivity at 109 BER foreach fiber length. Fig. 1 shows the sensitivity variations at10 9 BER as a function of the transmission distance forthe extinction ratio of 7 dB (solid line) and 12 dB (dashedline) with positive and negative -parameters. As expected,the sensitivity was significantly degraded for the positive-parameter with both low- and high-extinction ratio, comparedto the negative -parameter. These results are due to the pulsebroadening induced from the positive-parameter. Due to thepulse compression, the sensitivity was not degraded so muchfor the negative -parameter.

For the negative -parameter of 1.67, the sensitivity wasimproved over 80 km with the low-extinction ratio and thedispersion power penalty of 0.5 dB was obtained after the100 km transmission. Even after the 120 km transmission,the dispersion power penalty was measured to be less than 1dB. However, the dispersion power penalty of 2.2 dB wasobtained after the 100 km transmission with the negative

-parameter of 1.82 and the high-extinction ratio. Thesensitivity difference between high- and low-extinction ratio

TABLE IPARAMETERS USED IN THIS SIMULATION

with the lowest negative -parameter was 2.5, 2.2, 1.3, and1.0 dB after the 0, 40, 80, and 100 km transmissions, respec-tively. For the high-extinction ratio, we could not measurethe sensitivity at 120 km due to the error floor. These resultssuggest that the transmission performance over 100 km withthe low-extinction ratio and negative-parameter is muchbetter than that with the high-extinction ratio regardless of

-parameter. Also, these results are consistent with the resultsobtained from 10 Gb/s transmission with electroabsorptiondistributed feedback laser diodes [6]. In order to understandthis transmission performance with the negative chirp andlow-extinction ratio, we solved the nonlinear Schrodingerequation including model for the LiNbO3 modulator-basedtransmitter and pin diode-based receiver. In the next section,we’ll describe the simulation in detail.

III. SIMULATION RESULTS AND DISCUSSIONS

In order to estimate BER characteristics by simulationfor the 10 Gb/s optical fiber transmission system used inour experiment, we can divide the system into three parts:fiber, transmitter and receiver. Modeling of each part will bedescribed for the calculation of BER characteristics.

For the nonlinear, dispersive, and lossy single-mode fiber,the evolution of the slowly varying electric field pulse enve-lope can be obtained from the nonlinear Schr¨odingerequation [10]

(1)

where is the amplitude of the slowly varying envelopefunction, is the inverse group velocity, and arethe first- and second-order group velocity dispersion,is theabsorption coefficient, and is the nonlin-earity coefficient ( is the nonlinear coefficient and isthe effective core area). Nonlinear Schrodinger equation is anonlinear partial differential equation that does not generallylend itself to analytic solutions. This equation was solved bythe split-step Fourier method (SSFM) [10]. The parametersused for solving the nonlinear Schrodinger equation are listedin Table I.

1322 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 8, AUGUST 1999

(a)

(b)

Fig. 2. Optical pulse shape (solid line) and time resolved frequency chirping(dashed line). (a) Case of� = �1:82 and " = 12 dB. (b) Case of� = �1:672 and " = 6:6 dB.

For the transmitter using a Mach–Zehnder type Ti:LiNbO3

external modulator, the slowly varying electric field pulseenvelope at the output of the modulator can be modeled asfollowing [3]:

(2)

where is the normalized 128 pseudorandom bitpattern, is constant calculated from the average inputoptical power, and is phase. From the measured-parameter,the phase in the modulator can be calculated from thefollowing equation [11]:

(3)

where is the instantaneous modulator output power. The slowly varying electric field pulse envelope

will be used for the input pulse for the fiber. Thenormalized input optical pulse shape and their related chirpingcalculated using (2) and (3) are shown in Fig. 2(a) for theextinction ratio of 12 dB and the-parameter of , andFig. 2(b) for the extinction ratio of 6.6 dB and the-parameterof . For the low-extinction ratio [Fig. 2(b)], the cal-culated chirping ( spectral shift) are smaller and broaderdistribution over time due to the longer rising and fallingtimes, compared to the high-extinction ratio [Fig. 2(a)]. These

(a)

(b)

(c)

(d)

Fig. 3. Calculated optical eye diagrams of� = �1:672 and" = 6:6 dB.

different chirp characteristics significantly affect transmissionperformance depending on the low-extinction ratio and nega-tive chirping. The improved transmission performance will bediscussed later in detail. Fig. 3 shows the calculated opticaleye diagrams at 0, 40, 80, and 120 km using the input opticalpulse shown in Fig. 2(b) (the extinction ratio of 6.6 dB and the

-parameter of 1.67). These calculated optical eye diagramsare well matched to the measured ones, as shown in Fig. 4.

At the receiver, the transmitted optical pulses througheach fiber length were filtered using the measured frequencyresponse of the receiver from pin photodiode to automatic-gain-controlled amplifier. The filtered electrical eye diagramsare shown in Fig. 5. Using the calculated optical and electricaleye diagrams, BER characteristics can be calculated by thefollowing equation [12]:

BER

(4)

KIM et al.: STUDY OF 10 Gb/s TRANSMISSION PERFORMANCE 1323

(a)

(b)

(c)

(d)

Fig. 4. Measured optical eye diagrams of� = �1:672 and" = 6:6 dB.

where the is the time-averaged signal photo currents,and are the eye closures at mark (“1”) and space (“0”),respectively. and are the standard deviation of thetotal noise including shot noise, signal-spontaneous beat noise,spontaneous-spontaneous beat noise, and thermal noise formark and space, respectively. To obtain more actual BERcharacteristics representing the real receiver characteristics,the holding time of 15 ps and decision ambiguity level of5% have been included as the property of the decision circuit.Fig. 6 shows the calculated BER characteristics using (4) fromFigs. 3 and 5, in which the extinction ratio is 6.6 dB andthe -parameter is 1.67. After 40 km transmission (closedcircle in Fig. 6), we obtained the negative dispersion powerpenalty of 0.5 dB, compared to 0 km transmission (closed

(a)

(b)

(c)

(d)

Fig. 5. Calculated electrical eye diagrams of� = �1:672 and" = 6:6 dBusing measured frequency response of receiver.

square in Fig. 6). This agreed well with the experimentalresults. However, using the Butterworth filter or Bessel filter,we could not obtain such negative dispersion power penalty,as seen in our experiment. Also, the chirp model for theLiNbO3 modulator-based transmitter using (3) provides betterprediction for BER characteristics than that used the voltageratio applied to the two electrodes [9].

From repeating the above simulation for the-parameterfrom 1.82 to 1.06 and the extinction ratio of 7 or 12dB, receiver sensitivities as a function of fiber length canbe estimated at the 109 BER. Fig. 7 shows the simulatedreceiver sensitivities (high-extinction ratio: dashed line andlow-extinction ratio: solid line) for the extinction ratio and

-parameter measured from our experiments. As seen in ourexperiments, for the negative-parameter of 1.67 and low-extinction ratio of 6.6 dB, the sensitivity was decreased dueto the slightly stronger pulse compression over the pulsebroadening and then maintained as increasing the transmission

1324 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 8, AUGUST 1999

Fig. 6. Calculated BER characteristics of� = �1:672 and" = 6:6 dB.

Fig. 7. Sensitivity at 10�9 BER calculated LiNbO3 transmitter withlow-extinction ratio (solid line) and high-extinction ratio (dashed line).

distance up to 60 km. At 100 km transmission, the sensitivitiesare almost same regardless of the extinction ratio. Moreover,the sensitivity over 100 km becomes better for the low-extinction ratio. In order to see the difference in transmissionperformance between the low- and high-extinction ratio forthe negative -parameter, we measured the pulse pattern (bitsequence: 110011101010011110) for different extinction ratioand -parameter. Fig. 8 shows the measured and calculatedoptical pulse shapes after 60 km transmission with the-parameter of 1.67 and the extinction ratio of 6.6 dB. Themeasured and calculated pulses are similar to each other. Theoptical pulse shape was almost maintained except for the slightpeaking at the rising and falling edges. This indicates that pulsebroadening due to dispersion can be almost exactly canceledout by the pulse compression from the broadly-distributednegative chirping. Optical pulses with low-extinction ratioexperience relatively small dispersion due to narrower spectraldistribution inferred from the longer rising and falling times,shown in Fig. 2(b). However, in Fig. 9, the optical pulseshape was distorted for the extinction ratio of 12 dB andthe -parameter of 1.82; large peaking near the rising andfalling edges. In this case, pulse broadening due to the shorterrising and falling times cannot be canceled out by pulsecompression due to the negative-parameter for the wholetime region. This is due to the narrow distribution of chirping

(a)

(b)

Fig. 8. Measured and calculated pulse shapes after 60 km transmission. (a)Measured pulse shape (� = �1:672; " = 6:6 dB). (b) Calculated pulseshape (� = �1:672 and " = 6:6 dB).

(a)

(b)

Fig. 9. Measured and calculated pulse shapes after 60 km transmission. (a)Measured pulse shape (� = �1:82; " = 12 dB). (b) Calculated pulse shape(� = �1:82 and " = 12 dB).

and relatively large dispersion due to wide spectral distributioninferred from the shorter rising and falling times for high-extinction ratio, shown in Fig. 2(a). These results show that

KIM et al.: STUDY OF 10 Gb/s TRANSMISSION PERFORMANCE 1325

transmission performance can be significantly improved bytrading-off between extinction ratio and-parameter, and thecalculated BER characteristics are well enough to predict themeasured ones.

IV. CONCLUSION

We have been measured 10 Gb/s transmission performancethrough the standard single-mode fiber using LiNbO3

modulator-based transmitter and EDFA-pin receiver withvarying -parameter and extinction ratio. It is found thatgood transmission performance can be achieved for thenegative chirp parameter and low-extinction ratio becauseof dispersion cancellation by pulse compression induced frombroadly distributed negative chirping and relatively smalldispersion due to narrow spectral distribution of optical pulses.In order to understand this transmission performance withthe interplay of negative chirp and low-extinction ratio, wesolved the nonlinear Schr¨odinger equation including modelfor the LiNbO3 modulator-based transmitter, pin diode-basedreceiver, and noise and intersymbol interference for receiver.The results show that the simulation can predict the measuredBER characteristics well enough to see the chirp and extinctionratio interplay.

ACKNOWLEDGMENT

The authors would like to thank K. Ogawa and V. D.Mattera, Jr. for their support.

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[4] N. Suzuki and T. Ozeki, “Simultaneous compensation of laser chirp,kerr effet, and dispersion in 10 Gb/s long-haul transmission systems,”J. Lightwave Technol., vol. 9, pp. 1486–1494, 1993.

[5] K. Morito, R. Sahara, K. Sato, and Y. Kotaki, “Penalty free 10 Gb/sNRZ transmission over 100 km of standard fiber at 1.55�m with a bluechirp modulator integrated DFB laser,”IEEE Photon. Technol. Lett.,vol. 8, pp. 431–433, 1996.

[6] Y. K. Park, T. V. Nguyen, P. A. Morton, J. E. Johnson, O. Mizuhara,J. Jeong, L. D. Tzeng, P. D. Yeates, T. fullowan, P. F. Sciortino, A.M. Sergent, W. T. Tsang, and R. D. Yadvish, “Dispersion-penalty-freetransmission over 130 km standard fiber using a 1.55�m 10 Gb/sintegrated EA/DFB laser with low extinction ratio and negative chirp,”IEEE Photon. Technol. Lett., vol. 9, pp. 1255–1257, 1996.

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Lightwave Technol., vol. 6, pp. 87–92, 1988.[12] R. J. Nuyts, L. D. Tzeng, O. Mizuhara, and P. Gallion, “Effect of

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Sung Kee Kim was born in Seoul, Korea, in 1974.He received the B.S. and M.S. degrees in radioengineering from Korea University, Seoul, Korea, in1996 and 1998, respectively. He is currently work-ing towards the Ph.D. degree in radio engineeringat the same university.

He is interested in modeling of transmission per-formance of high-speed lightwave systems.

O. Mizuhara, photograph and biography not available at the time of pub-lication.

Y. K. Park , photograph and biography not available at the time of publication.

L. D. Tzeng, photograph and biography not available at the time of publi-cation.

Y. S. Kim, photograph and biography not available at the time of publication.

Jichai Jeong(SM’96) received the B.S. degree fromKorea University, Seoul, Korea, in 1980, the M.S.degree from Korea Advanced Institute of Scienceand Technology, Korea, in 1982, and the Ph.D. de-gree in electrical engineering from Carnegie MellonUniversity, Pittsburgh, PA, in 1988.

From 1982 to 1985, he worked as a Researcherat Korea Institute of Science and Technology. From1988 to 1993, he was Member of Technical Staffwith AT&T Bell Laboratories, Murray Hill, NJ,where he worked in optoelectronic integrated cir-

cuits and semiconductor lasers for optical communications. From 1993to 1995, he was the faculty of the Electrical Engineering Department ofPohang University of Science and Technology. In 1995, he joined thefaculty of the Radio Engineering Department at Korea University. His currentresearch interests include modeling and simulation of optical components andtransmission performance for fiber optic communication systems.