December 15, 2003 / Vol. 28, No. 24 / OPTICS LETTERS 2461

Generation and detection of 80-Gbit���s return-to-zerodifferential phase-shift keying signals

Lothar Möller, Yikai Su, Chongjin Xie, Xiang Liu, and Juerg Leuthold

Bell Labs, Lucent Technologies, Holmdel, New Jersey 07733

Douglas Gill and Xing Wei

Bell Labs, Lucent Technologies, Murray Hill, New Jersey 07974

Received July 18, 2003

Nonlinear polarization rotation between a pump and a probe signal in a highly nonlinear fiber is used asa modulation process to generate 80-Gbit�s return-to-zero differential phase-shift keying signals. Its per-formance is analyzed and compared with a conventional on–off keying modulated signal. © 2003 OpticalSociety of America

OCIS codes: 060.2330, 190.4370.

In conventional optical modulation methods the maxi-mum single-channel data rate is limited to �40 Gbit�sbecause of the bottleneck caused by the electrical band-width of the data multiplexer, the modulator driver,and the modulator itself. To overcome this limitation,optical time division multiplexing (OTDM) schemescombined with polarization multiplexing have beenapplied at data rates as high as 1.28 Tbit�s.1 Twomajor differences between conventional modulationschemes, in which the light of a cw running laser isexternally modulated, and the OTDM approaches arethe maximum achievable data rate and the basic sig-nal characteristics. While conventional modulationschemes can generate phase-correlated signals, OTDMapproaches have the drawback that they produce ran-dom optical phase jumps between the interleavedreplicas. Typically in 160-Gbit�s OTDM experimentsfour 40-Gbit�s data patterns are generated from thesame source, delayed with respect to one another inthe time domain, and finally interleaved. The delaytimes must be one half of the pseudorandom binarysequence (PRBS) pattern length if a pseudorandomcharacteristic is desired in the new sequence, whichmakes precise optical phase adjustment almost impos-sible. These random phase jumps between adjacentpulses prevent OTDM schemes for generating differ-ential phase-shift keying (DPSK) signals.2 Note thatOTDM schemes can be used to generate interleavedDPSK signals in which the information is not encodedbetween adjacent pulses but rather between the pulsesof one interleaved pattern.

In this Letter we report a method for generatingultra-high-speed phase-correlated data signals withnonlinear polarization rotation (NLPR) between apump and a probe light (cw), which results in apolarization–phase conversion. As an example, wedemonstrate, for the first time to our knowledge thegeneration of an 80-Gbit�s DPSK data signal.

Our method consists of two major steps. In thefirst step a polarization-state-modulated pump signalis generated. In the second step this pump signalimparts its polarization state information onto aprobe signal by producing a phase modulation throughnonlinear interaction.

0146-9592/03/242461-03$15.00/0

In the f irst step an 80-Gbit�s polarization-state-modulated pump signal is generated by op-tical phase modulation of the output of a mode-lockedlaser (MLL) producing short pulses with a 10-GHzrepetition rate. This phase-modulated 10-Gbit�ssignal is converted to an 80-Gbit�s signal by meansof a conventional OTDM scheme. Two replicas ofthe 80-Gbit�s signal are obtained by splitting, andone replica is delayed by 100 ps with respect to theother. Then the two signals are recombined withorthogonal states of polarization (SOPs) by use of apolarization beam combiner. Let us assume that thetwo 80-Gbit�s replicas have SOPs given by the Stokesvectors STE � S1 � �1, 0, 0� and STM �2S � �21, 0, 0�.After the replicas are recombined a common SOPfor each return-to-zero (RZ) pulse is built up. TheseSOPs reside in the S2 S3 plane of the Poincarésphere. We further assume that the polarization-multiplexed replicas have either a phase offset of 0or p. Then the corresponding common SOP of a RZpulse is given by S45 � �0, 1, 0� and S245 � �0, 21, 0�.Thus a polarization-modulated signal is obtained.Note that, to perform this phase–polarization-stateconversion, the MLL must produce a coherent outputstate, meaning that the adjacent pulses emittedfrom the laser at 10 GHz must have a strong phasecorrelation with each other. To illustrate the secondprocessing step, an additional polarization statetransformation is helpful. By means of a linearpolarization controller the two pump SOPs are rotatedaround the S1 axis by 90± so that they f inally resideon the north and south poles of the Poincaré sphere.

The second processing step includes mainly NLPRbetween two signals at different wavelengths, whichis generated by polarization-dependent cross-phasemodulation. We follow the same simplif ied picturethat was given previously.2 Two waves (index Pand S) with suff iciently high powers and differentwavelengths inf luence each other’s SOP throughNLPR.3 With Stokes space (Poincaré sphere) andassuming for the sake of simplicity that both waveshave the same power, one obtains the equationsthat describe the NLPR-induced SOP changes perpropagation length unit,

© 2003 Optical Society of America

2462 OPTICS LETTERS / Vol. 28, No. 24 / December 15, 2003

≠zSS � gSS 3 SP , (1)

≠zSP � gSP 3 SS , (2)

where SS and SP stand for the corresponding Stokesvectors and g takes into account the interactionmagnitude that depends on the power, fiber Kerrnonlinearity, absorption, etc.

Although Eqs. (1) and (2) do not exactly hold forpulsed waves with different powers, we use them toexplain the key idea behind our method. We assumethat at the input of a highly nonlinear fiber (HNLF)there is a strong pump wave with left-handed circu-lar SOP and a cw probe light with a SOP equal to S1.Then the SOP �SS� of the weak probe signal is rotatedalong the equator of the Poincaré sphere during prop-agation through the f iber, as sketched in Fig. 1 (dot-ted curve). In the case in which the pump wave hasan orthogonal SOP (right-handed circular), SS rotatesin the opposite direction (Fig. 1, dashed curves). Letus further assume that the f iber has a length suchthat the wave propagation stops when the probe sig-nal SOP crosses the S2 (or 2S2) coordinate. Then theprobe signal’s SOP at the fiber output can be writtenin Jones space with the equivalent vectors for the S2coordinate, as shown in Fig. 1. The elements insidethe Jones vector stand for the f ield contributions withSTE and STM polarization. Thus, by isolation of theprobe wave’s STM component with a polarizer alignedwith the 2S2 axis, a phase-modulated wave can beextracted. This wave has either 0 or p phase shiftcorresponding to the sign of the STM component. Insummary, a polarization-modulated pump signal canbe used to phase modulate a probe signal. Since theprobe signal stems from a coherent source, its pulsesare phase correlated within the coherence length of thelaser. Our simplif ied picture does not include the SOPchange of the pump caused by the probe signal andlinear fiber birefringence. For computer simulationthe nonlinear coupled Schrödinger equations must beapplied.4

A hybrid electrically mode-locked laser (10-GHzrepetition rate) produces pulses with a 1.7-ps FWHMthat are externally 0 or p phase modulated with a27 2 1 PRBS at a 10-Gbit�s data rate (Fig. 2) by use ofa Mach–Zehnder modulator. Commercially availableOTDM techniques, which are designed for a 27 2 1pattern length, convert the signal to an 80-Gbit�sPRBS. This single-polarized signal is superposedwith a delayed replica of itself that has equal powerbut orthogonal polarization in the polarization mul-tiplexer unit with exactly 100 ps of delay. Thus apolarization-state-modulated signal with an 80-Gbit�sdata rate is obtained. The pump signal (1546 nm) islaunched together with a cw (1556 nm) in a HNLF of2.5-km length, a Kerr nonlinearity of 12 W21 km21,low polarization mode dispersion, a zero-dispersionwavelength at 1551 nm, and a dispersion slope of0.02 ps nm22 km21. Typical average power levels forthe pump and probe signal are �20 and �16 dBm, re-spectively. Suppression of Brillouin scattering avoidsbackscattering of the probe signal. At the output ofthe HNLF a polarization controller connected to a

polarizer is adjusted such that, when the pump signalis switched off, the probe signal is blocked. Pump andprobe signals are separated after the polarizer with a3-nm-wide bandpass f ilter. The 80-Gbit�s RZ DPSKsignal is decoded and detected with a Mach–Zehnder(MZ) interferometer with a delay of 12.5 ps in one arm.Its single-ended output is launched into an opticallypreamplified OTDM receiver (RX) similar to that inRef. 5, consisting mainly of several erbium-doped fiberamplifiers for preamplification and loss compensation,bandpass filters for suppression of amplified sponta-neous emission, and two cascaded electroabsorptionmodulators. The two electroabsorption modulators,driven at 40 and 10 GHz, generate a switching win-dow of 3.5-ps width and 10-GHz repetition rate. Biterror rate (BER) measurements are performed afterdownconversion at 10 Gbits�s. Monitoring the probesignal spectrum behind the polarizer by means of anoptical spectrum analyzer (OSA) helps adjust the SOPand power of the launched pump signal at the input ofthe HNLF (symmetrical shape, maximum power).

The back-to-back performance of the eight tribu-taries of the 80-Gbit�s RZ DPSK signal differs by only�0.5 dB, where a typical tributary has a sensitivity of223.2 dBm at a BER of 1 3 1029 (Fig. 3). For compar-ison we use on–off keying (OOK) to modulate the MLLoutput (extinction ratio of .20 dB) and multiplex thesignal to 80 Gbits�s with the same OTDM equipment.The corresponding BER curves (Fig. 3) for 27 2 1 anda quasi 231 2 1 PRBS pattern, detected with the samereceiver, show only a small pattern-dependent penalty,

Fig. 1. Poincaré sphere and Jones space representationsof the SOP evolution within the sketched setup. LHC,left-handed circular; RHC, right-handed circular; BPF,bandpass f ilter.

Fig. 2. Setup topology. Erbium-doped f iber amplifiers,polarization controllers, etc. are left out for the sake of sim-plicity. BERT, bit error rate test.

December 15, 2003 / Vol. 28, No. 24 / OPTICS LETTERS 2463

Fig. 3. Measured BER curves, the corresponding eye dia-grams, and the spectra at the Mach–Zehnder interferome-ter output.

Fig. 4. Spectrum of the pump signal at the OTDM outputand pump and probe signal spectra after the polarizer.Note the smaller 3-dB bandwidth of the pump spectrumafter the polarizer. To avoid overlapping of both spectrafor visibility purposes, a convenient intensity offset wasadded.

indicating that long DPSK PRBSs would not substan-tially differ from the measured short PRBS (only smallpattern-dependent receiver characteristics). The non-linear conversion process should not be pattern lengthdependent, since the pump (probe) signal pulses prop-agate without overlapping through the HNLF. Thepenalty between on–off keying and DPSK modulationcould stem from the imperfect Mach–Zehnder interfer-ometer, slightly different receiver noise f igures at thepump and probe wavelength, and probe pulse broad-

ening. We determine the FWHM of the pump andDPSK signal with an optical autocorrelator and withthe assumption of sech2 pulse shapes. The FWHMof the pump signal pulses at the input of the HNLFand after the polarizer are 1.7 and 3.1 ps, respectively.The DPSK signal pulse widths before and after theMach–Zehnder interferometer were 2.4 ps.

Typical spectra of the pump and probe signals afterthe polarizer and of the pump after the OTDM revealthe probe–pump interaction (Fig. 4). Although thepump signal intrinsically has a smooth envelopebecause of the DPSK modulation, the interaction withthe probe signal and itself (self-phase modulation)causes a certain pattern over its bandwidth.

At a high bit rate �160 Gbits�s�, pulse jitter canbecome a limiting factor while causing an error f loor.The two eye diagrams of a pump pulse and thedemultiplexed DPSK signal (Fig. 3) show a smalljitter increase during the conversion process from617 to 640 fs, indicating that the conversion processitself causes a negligible amount of jitter. The eyediagrams were taken with a scope triggered by ahigh-precision time base (Agilent 86107) with anintrinsic jitter of �170 fs.

We have demonstrated a novel method for generat-ing optical phase-correlated ultra-high-speed data sig-nals based on nonlinear polarization rotation betweena pump and a probe signal. As an application, wehave shown for the first time to our knowledge the gen-eration of an 80-Gbit�s RZ DPSK data signal. Thistechnique should also be capable of generating othermodulation formats such as the carrier-suppressed RZsignals described in Ref. 2 and duobinary signals, evenat higher bit rates �160 Gbits�s�.

The authors thank u2t Photonics for providingthe mode-locked laser and General Photonics forproviding the polarization mode dispersion emulator.L. Möller’s e-mail address is lmoeller@lucent.com.

References

1. M. Nakazawa, T. Yamamoto, and K. R. Tamura, Elec-tron. Lett. 36, 2027 (2000).

2. L. Möller, Y. Su, X. Liu, J. Leuthold, and C. Xie, “Ul-tra-high-speed optical phase correlated data signals,”IEEE Photon. Technol. Lett. (to be published).

3. G. P. Agrawal, Nonlinear Fiber Optics (Academic,San Diego, Calif., 2001), Chap. 6.

4. D. Wang and C. R. Menyuk, J. Lightwave Technol. 17,2520 (1999).

5. B. Mikkelsen, G. Raybon, R. J. Essiambre, K. Dreyer,Y. Su, L. E. Nelsen, J. E. Johnson, G. Shtengel, A. Bond,D. G. Moodie, and A. D. Ellis, in Proceedings of the 25thEuropean Conference on Optical Communication (ECOC1999), (Institute of Electrical and Electronics Engineers,Piscataway, N.J., 1999), paper PD 2–3.