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21 8 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 1, NO. 8, AUGUST 1989

5 Gbit/s Direct Optical DPSK Modulation of a1530-nm DFB LaserR. S . VODHANEL

Abstract-5 Gbit/s direct optical differential-phase-shift-keying @PSK)modulation of a 1530-nm distributed feedback laser bas been demon-strated using injection current modul ation with a bipolar signal forma t.There was no degradation of the optical DPSK signal due to thermalfrequency modulation of the laser. This direct DPSK modulationtechnique avoids the insertion loss and systems complexity of externalDPSK modulators.

Receiveriber

Fig. 1. Method for direct optical DPSK modulation of a DFB laser and delaydemodulation using an interferometer.INTRODUCTIONIRECT frequency modulation of distributed feedback DIRECT PTICALPSK MODULATIONN D DELAY(DFB) lasers is interesting for optical com munications at DEMODULATIONdata rates in the gigabit/second range because of the high- Direct DPSK modulation of a DFB laser is achieved usingspeed frequency modulation (FM) capability of DFB lasers [ l] injection current modulation with a differentially encodedand since the additiona l loss and syste m complexity of external bipolar signal, as shown schematically in Fig. 1. Delaymodulators can be avoided. There ha ve been recent reports of

delay time Tequal to the duration of one bit [3]. The input andlaser at 5 Gbit/s 121 and direc t phase modulation a t 4 Gbit/s [3]using a related technique of frequency m odulation with a differentially encoded NRZ signals are shown in Fig. 2(a) and

Dfrequency-shift-keying (FSK) modulation of a 153 O- m DFB is performed using an interferometer with a

return-to-zero signal format. For both direct detection andcoherent detection systems, direct frequency or phase modula- shown in Fig* 2(c) isr eswt ive ly . The current signal,

the time derivative of the NR Ztion of DFB lasers are attractive alternati ves to on-off in tensity signal Of Fig. 2@). An impo mt advantage Of themodulation, since a smallmodulation current of 1 ~ - 2 ~ignal format is that modulation of the laser temperature is

as the negligible V I since the bipolar pulse width is very shortminimize the intersymbol interferencedu e to fiber chromatic and since the time between successive current pulses, whichaccompanies direct on-off intensity m odulation considerably thermal response time. Since thermal FM is suppressed, thebroadens the optical spectrum relative to the information modulated optical frequency has the same bipolar waveform a s

zero-dispersion wavelength [4], [51. in Fig. 2(d), +(t ) is a replica of the differentially encode d NRZA problem with injection current modulation using unipolar signal of Fig. 2@).An optical phase shift of ?r radians can beobtained if Af A t = 0.5, where Af s the optical frequencysignals such as nonreturn-to-zero (NRZ) or return-to-zero(RZ) formats is the accompanying temperature modulation in deviation and A t is the bipolar pulse width. The demodulated

during long strings of 66zeros99r '60nes99 esulting in Severe the input intensity (we have ignored the residual amplitudedistortions in the demodulated In this letter, we report modulation, which is small when the laser is biased at high

FM , using injectioncurrent modulation with a signals from the tw o branches of the interferometer, and +o isthe interferometer phase offset. Fig. 2(e) shows A+ (t) whenthe interferometer delay equals a bit period, as shown in Fig.

produces a whose spectral width is asinformation bandwidth. A narrow spec tral width is desired to 'Ompared to the laser" thermal response time Of about ps ,dispersion [3]. In contrast, the wavelength chirping which are Of Opposite Polarity, is also much shorter than the

bandwidth, potentially resulting in large systems degradations theunless the operating w avelength closely matches the fiber's the time integral Of the

current. The Optical phase + ( t ) isOptical frequenc y. As show n

semiconductor asers [6]. Temperature modulation of the laser signal at the Output Of the interferometer, is given bycan produce large drifts in the optic- frequency or phase I ( t ) = (1012) [1 + COS (27th' T + 60 A+(t))], where Io isdirect optical differential-phase-shift-keyingDPSK) modula- Output powe r), f0 is the average Optical frequency, A +( t , is thetion of a 153 O- m DFB laser at 5 Gbits/s without degradations value Of the phase difference between the Opticaldu e tobipolar signal format [7].

Manuscript received April 10, 1989; revised May 31, 1989.The author is with Bellcore, Red Bank, NJ 07701.IEEE Log Number 8929809.2(d). By a djusting the opti-cal frequen cy or ph ase offset suchthat 27rf0*T + +o = 2?rm,where m s an integer, the outputintensity is I( t) = IOcos' [A+( t)/ 2] and the input NRZ signal

1041-1 135/89/08OO-0218$01.OO 0 98 9 IEEE

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219ODHANEL: OPTICAL DPSK MODULATION OF DF B LASER

(a) NRZ(c ) (t),f(t)(d )O(t)( e)AO( t ) 8( f ) I(t) 0

(b ) d-NRZ

TimeFig. 2. Signal waveforms: (a) NRZ input, (b) differentially encod ed NRZ,(c) bipolar modulationcurrent i(t) and optical FM signalflt), (d) modulatedoptical phase &( t )- he dashed curve is the same as the solid, but delayedone bit-period, (e) A&([), and (f) delay demodulated signal I( t) .

of Fig. 2(a) is recovered from the demodulated signal Z ( t ) , asshown in Fig. 2(f). The spikes, which occur betweenconsecutive zeros, are artifacts of delay demodulation ofdifferentially encoded bit sequences where the phase differ-ence between successive bits changes from + T o - r or - rto +T with a finite transition time. If this phase transition timeis greater than a significant fraction of the bit period due toinsufficient transmitter bandw idth, then som e eye closure mayresult. For the case of bipolar modulation where the drivepulse width equals T/2 , we estimate about a 1 dB eye closurewhen the transmitter bandwidth is to 0.7 times the bit ratecompared to when the transmitter bandwidth is twice the bitrate.

,

EXPERIMENTA 1530-nm DFB laser with a 12 GHz FM bandwidth [l ] isdirectly modulated with a 5 Gbit/s bipolar signal having apulse width of about 100 ps. NRZ-to-bipolar conversion isachieved with a transversal filter with a delay of about 100 ps.The bipolar signal current is am plified to a 18 mA, to producean optical frequency deviation of about k 5 GHz. T he laseroutput power is 8 mW, producing a linewidth of 15 MHz.Delay demodulation is accomplished using a birefringent fiberinterferometer [11 with a delay of about 200 ps, followed by ap-i-n photodetector with flat response out to 15 GHz. Nodifferential encoder is needed since the differentially encoded

pseudorandom pattern is the same as the original pseudoran-dom pattern, except shifted by several bits.RESULTSN D DISCUSSION

Fig. 3(a) and (b) shows the DPSK and AM waveforms,respectively, for 5 Gbit/s bipolar modulation of the DFB laserusing a 215 - 1 length pseudorandom pattern. Thesewaveforms correspond to a portion of the pseudorandompattern which contains a sequence of 15 zeros followed by14 ones. The 15 successive zeros in the DPSK signal areseparated by the spikes discussed above and shown in Fig.2(f). The following sequence of 14 ones represents theworst case for thermal FM for this pseudorandom pattern,since this sequence spans the longest time between successivebipolar pulses. There is no drift due to thermal FM in the 2.8ns long string of demodu lated ones, which is expected sincethis time interval is about three order of magnitude smallerthan the DFB lasers thermal response time. The suppressionof thermal FM is important since otherwise a high-speedmethod for tracking it would be necessary to maintain theproper phase adjustment for delay demodulation with an

4 &l300psD(a )

(C)Fig. 3 . Detected signals at 5 Gbits/s: (a) delay demodulated DPSK, (b)residual A M , and (c) DPSK eye pattern (50 pddiv).

interferometer in the case of direct detection, or for controllingthe intermediate frequency in the case of coherent detection.The AM signal shown in Fig. 3(b) follows the bipolarmodulation signal with an amplitude modulation of a 2 0percent. The small amplitude modulation is due to the lasershigh-output power level and the small modulation current. Incontrast, a large on-off ratio of 9 : l was obtained for theDPSK signal. Fig. 3(c) shows the open eye pattern which wasobtained for 5 Gbit/s DPSK modulation with a pseudorandompattern.The required I F linewidth for coherent DPSK detection at5 Gbits/s is about 15 MHz [8], which can be met withconventional DFB lasers. The DFB laser used in this work,which has a linewidth of 15 MHz, could be used in a coherentDPSK system if a narrow linewidth external cavity localoscillator laser is used in the coherent DPSK receiver. Furtherwork is required to establish heterodyne system bit-error-rateperformance as a function of laser linewidth using this directDPSK modulation technique.

CONCLUSION5 Gbit/s direct DPSK modulation of a 1530-nm DFB laserhas been demonstrated using injection current modulation with

a bipolar signal format. There was no degradation of thedemodulated DPSK signal due to thermal frequency m odula-tion.ACKNOWLEDGMENT

The author is grateful to R. E. Wagner for stimulating andinformative discussions and for his encouragement, and to D r.

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220N . Chinone of Hitachi Central Research Laboratory forproviding the DFB laser.

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R. S . Vodhanel and S . Tsuji, 12 GHz FM-bandwidth for a 1530-nmDFB laser, IEE Electron. Lett. , vol. 24, pp. 1359-1361, 1988.R . S . Vodhanel, T. P. Lee, and S . Tsuji, 5 Gbit/s optical FSKmodulation of a 1530-nm DFB laser, in Proc. 14th Euro. Conf. Opt.Commun., part 1, Brighton, 1988, pp. 171-174.M. Shirasaki, H. Nishimoto, T. Okiyama, and T. Touge, Fibretransmission proper ties of optical pulses produced through direct phasemodulation of DFB laser diode, IEE Electron. Lett., vol. 24, pp.486488, 1988.R. Heidemann, U. Scholz, and B. Wedding, 5 Gbit/s transmission

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