ultrahighspeed otdm

4616 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006

Ultrahigh-Speed OTDM-Transmission TechnologyHans-Georg Weber, Reinhold Ludwig, Sebastian Ferber, Carsten Schmidt-Langhorst, Marcel Kroh,

Vincent Marembert, Christof Boerner, and Colja Schubert

Invited Paper

AbstractThis paper reviews ultrahigh-speed data transmis-sion in optical fibers based on optical time division multiplexing(OTDM) transmission technology. Optical signal processing in thetransmitter and receiver as well as the requirements on ultrahigh-speed data transmission over a fiber link are discussed. Finally,results of several OTDM-transmission experiments, including160-Gb/s transmission over 4320 km, 1.28-Tb/s transmission over240 km, and 2.56-Tb/s transmission over 160-km fiber link, aredescribed.

Index TermsOptical communication, optical signal process-ing, time division multiplexing (TDM), ultrafast optics.

I. INTRODUCTION

O PTICAL networks use a combination of wavelength divi-sion multiplexing (WDM) and time division multiplexing(TDM) to optimize the transmission capacity. TDM may berealized by electrical multiplexing (ETDM) or by optical multi-plexing (OTDM) to a high-speed data signal. Currently, the first40-Gb/s systems based on ETDM have been installed, and inlaboratories, the first 100-Gb/s ETDM experiments have beenperformed. On the other hand, at the same data rates, OTDM-transmission experiments were carried out about ten yearsearlier. For instance, the first 100-Gb/s OTDM-transmissionexperiment over a 36-km fiber link was already reported in 1993[1]. Since then, OTDM-transmission technologies have made alot of progress toward much higher bit rates and much longertransmission links, as has been described in several reviewarticles [2][4] and will be discussed in this paper with specialemphasis on the most recent developments. For example, wewill report on 160-Gb/s transmission over a record length of4320 km [5] and on 2.56-Tb/s transmission over 160 km [6].

OTDM-transmission technology is often considered to bean interim technique with which to study high-speed datatransmission in fiber and which will be replaced by ETDM assoon as electrical signal processing becomes available at therequired data rate. We expect that ETDM will replace OTDM

Manuscript received April 26, 2006; revised September 20, 2006. This workwas supported in part by the Bundesministerium fuer Bildung und Forschung(BMBF) of the Federal Republik of Germany in the National Program Multi-TeraNet and in part by the Companies Lucent Technologies in Nuremberg,Germany, and Fujitsu Laboratories Ltd. in Japan.

The authors are with the Fraunhofer Institute for Telecommunications,Heinrich-Hertz-Institut, Einsteinufer 37, 10587 Berlin, Germany (e-mail:[email protected]; [email protected]; [email protected];[email protected]; [email protected]; [email protected]; [email protected];[email protected]; [email protected]).

Digital Object Identifier 10.1109/JLT.2006.885784

at the TDM bit rate of 100 Gb/s within the next two yearsand probably at the TDM bit rate of 160 Gb/s in the future.From this viewpoint, the main task with regard to OTDMtechnology is to investigate the feasibility of ultrahigh-speeddata transmission. One has to study how the advantages ofhigh TDM bit rates are eventually eroded by an increase indetrimental effects. A higher TDM bit rate makes transmissionsystems more vulnerable to chromatic dispersion (CD) andpolarization-mode dispersion (PMD), as well as creating theneed for a higher optical signal-to-noise ratio (OSNR) in thewavelength channel. A higher OSNR is obtained by employinga higher signal power, and this will make the system moresensitive to fiber nonlinearity.

A different and more challenging view as regards OTDMtechnology is that optical networks will evolve into photonicnetworks, in which ultrafast optical signals of any bit rateand modulation format will be transmitted and processed fromend to end without opticalelectricaloptical conversion. Withthis as the target, OTDM technology presents us with thechallenge of investigating and developing high-speed opticalsignal processing and exploring the ultimate capacity for fibertransmission in a single wavelength channel. The photonicnetwork appears to be a task for the distant future, and ETDMtechnology will dominate commercial transmission systems inthe near future.

This paper is organized as follows: Section II-A gives ageneral description of an OTDM system, followed by a discus-sion of the OTDM transmitter in Section II-B, of the OTDMreceiver in Section II-C, and of the fiber link in Section II-D.Transmission experiments are described in Section III, start-ing with a review on 160-Gb/s transmission experiments inSection III-A and followed by a detailed description of two160-Gb/s transmission experiments, transmission with long-term stability in Section III-B, and transmission over a recordfiber length of 4320 km in Section III-C. In Section III-D,we report on transmission experiments at data rates beyond160 Gb/s, including a detailed description of a 1.28-Tb/s trans-mission over 240 km and on a 2.56-Tb/s transmission over160 km. Finally, Section IV summarizes our conclusions on thepresent state of OTDM technology.

II. OTDM-TRANSMISSION SYSTEM

A. General Description

Fig. 1 (upper part) is a schematic illustration of a 160-Gb/sOTDM-transmission system as an example. The essential

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Fig. 1. Schematic view of a 160 Gb/s OTDM transmission system (upper part)and of a simplified laboratory system (lower part of the figure).

component on the transmitter side is an optical-pulse source.The repetition frequency of a generated pulse train depends onthe base data rate (or on the symbol rate, see Section II-B)used. The system shown in Fig. 1 has a base data rate of40 Gb/s. The 40-GHz optical-pulse train is coupled into fouroptical branches, in which modulators (MOD) driven by40-Gb/s nonreturn-to-zero (NRZ) electrical data signals gen-erate 40-Gb/s optical return to zero (RZ) data signals. Themodulation formats include ONOFF keying (OOK), differen-tial phase shift keying (DPSK), differential quadrature phaseshift keying (DQPSK), etc. The four optical data signals(TDM channels) are bit-interleaved to generate a multiplexed160-Gb/s optical data signal. Multiplexing (MUX) can be suchthat all bits of the multiplexed data signal have the samepolarization (SP multiplexing, SP signal), or adjacent bitshave alternating (orthogonal) polarization (AP multiplexing,AP signal). On the receiver side, the essential component is anoptical demultiplexer (DEMUX), which separates the four baserate data signals (TDM channels) for subsequent detection andelectrical signal processing.

The DEMUX shown in Fig. 1 comprises two parts: anoptical gate and a clock-recovery device. The optical gate isa fast switch with a switching time that is shorter than the bitperiod (6.25 ps for 160 Gb/s) of the multiplexed data signal.The clock-recovery device provides the timing signal forthe optical gate. The transmission link requires, in general,compensation for CD and PMD, which both depend on thetype of single-mode fiber used in the transmission system. InSections II-BD, we discuss the OTDM transmitter, the OTDMreceiver, and the fiber transmission line in more detail.

Laboratory systems are frequently simplified as follows(Fig. 1, lower part): On the transmitter side, only one modulatoris used and combined with the pulse source for a 40-Gb/s opti-cal transmitter. The generated optical data signal is then multi-plexed by a fiber-delay-line multiplexer (MUX) to a 160-Gb/sdata signal using either SP or AP multiplexing. On the receiverside, only one 40-Gb/s TDM channel is selected and detectedby one 40-Gb/s optoelectronic receiver at a given time. In aproper experiment, all TDM channels are measured succes-sively in this way.

In this paper, we discuss point-to-point transmission links,as shown in Fig. 1. We do not consider network components

Fig. 2. Pulse spectrum of mode-locked laser diode, which was successfullyoperated in 160 Gb/s DPSK transmission experiments.

and subsystems such as adddrop MUXs (e.g., [7][10]), wave-length converters (e.g., [11][14]), modulation-format convert-ers (e.g., [15]), optical regenerators (e.g., [16]), and opticalsampling systems (e.g., [17], [18]). Also, OTDM/WDM trans-mission systems are not discussed in detail.

B. OTDM Transmitter

The pulse source is a key component in an OTDMtransmitter. The pulse source must provide the following: awell-controlled repetition frequency and wavelength, transformlimited pulses, a pulsewidth significantly shorter than the bitperiod of the multiplexed data signal, a timing jitter much lessthan the pulsewidth, low amplitude noise, and a high extinctionratio. Typical values for stable 160-Gb/s transmission are jitter< 300 fs, pulsewidth [full-width at half-maximum (FWHM)]< 2 ps for SP multiplexing and < 4 ps for AP multiplexing,extinction ratio > 27 dB, and amplitude noise < 3%. Moreover,if some sort of phase-modulation format such as DPSK orDQPSK is used, there are further pulse-source requirements,namely, the pulse source must be highly stable in terms ofcarrier phase and wavelength.

Pulse sources used for high bit-rate transmission experimentsinclude mode-locked laser diodes (MLLDs), either externalcavity devices (e.g., [19], [20]) or monolithically integrateddevices (e.g., [21], [22]), mode-locked fiber lasers (MLFLs)(e.g. [23], [24]), mode-locked solid-state lasers (MLSLs) [25],and externally modulated cw lasers [pulse carving, e.g., byan electro-absorption modulator (EAM)] (e.g., [26]). Thesepulse sources provide in general a 10-GHz or 40-GHz pulsetrain with a pulsewidth of a few picoseconds. If the pulsewidthis not sufficiently narrow for the considered bit rate, somesort of subsequent pulse compression and optical regeneration(e.g., [28][31]) is used. High-power pulses can generatea supercontinuum spectrum. Spectral slicing provides amultiwavelength pulse source, which is of particular interest forWDM/OTDM applications (supercontinuum pulse generation,SC-pulses) (e.g., [27]).

Fig. 2 shows the optical spectrum (mode comb spectrum) ofa 40-GHz pulse train generated by a monolithically integratedMLLD. A small linewidth and a large contrast ratio of the mode


combs indicate a pulse train with low phase and amplitudenoise. If phase-modulation formats such as DPSK or DQPSKare used, this spectrum needs to have a long-term stability inthe wavelength position appropriate for the phase demodulatorin the receiver (IIC). This is often a problem for the MLFL.This laser needs harmonic mode locking, and long-termwavelength stability is a critical issue.

In OTDM experiments, Lithium-Niobate (LiNbO3) modula-tors are most frequently used to modulate the generated pulsetrain. Only in few OTDM experiments, EAMs were applied.The modulation characteristics of LiNbO3 modulators are verybroadband (up to 80 GHz) and little dependent on wavelengthin the 1.3-to-1.5-m range. In addition, a nearly perfect phase shift is obtained in DPSK and DQPSK systems by useof a pushpull-operated LiNbO3 MachZehnder modulator,which is biased at zero transmission and driven by two timesthe -switching voltage [32], [33]. This modulator may showsome residual amplitude modulations. Those, however, are lessdetrimental than deviations from the desired phase shift [34].

In a DPSK system, each pulse is modulated in the phasewith n , n {0, 1}, whereas in a DQPSK system, eachpulse is modulated in the phase with n /2, n {0, 1, 2, 3}.Consequently, in a DQPSK-modulated signal, each symbol(optical pulse) carries one out of four logical states instead ofone out of two states as in an OOK or a DPSK system. ForOOK and DPSK, the data rate is equal to the symbol rate,whereas for DQPSK, the data rate is twice the symbol rate.

Most optical MUXs are of the kind as schematically depictedin the lower part of Fig. 1. These delay-line MUXs generatea high bit-rate test signal for laboratory experiments by com-bining several replicas of one data signal with different relativedelays. They are realized by using 2 2 optical couplers andoptical delay lines either as fiber devices or as planar lightwavecircuits. An important requirement for these test multiplexersis that there is no correlation between the adjacent bits of themultiplexed data signal. This is obtained by employing a delaytime, which is long compared with the bit period of the in-put signal.

Real OTDM MUXs would use different modulators (seeupper part of Fig. 1) to provide a multiplexed data signal. An ex-ample of a real MUX is reported in [35] and [36]. This MUXenables the multiplexing of eight different 20-Gb/s data signalsto one multiplexed 160-Gb/s data signal by using an integratedplanar lightwave circuit. Yet another real MUX is reported in[37]. It provides independent modulation of all TDM channelsand optical-phase alignment between adjacent bits.

In general, the delay-line MUX provides an arbitrary relativephase of adjacent pulses in the multiplexed data signal, becauseusually, no effort is made in these experiments to adjust andstabilize the delay-line MUX for a well-defined relative phaseof the adjacent pulses. The effect of a well-defined relativephase of adjacent data pulses in the multiplexed data signal isexpected to increase the tolerance of the transmission systemwith respect to CD and fiber nonlinearity, and it will increasethe spectral efficiency [38]. Therefore, the controllabilityof the optical-phase alignment between adjacent bits in themultiplexed data signal is an important feature of an amplitudemodulated system. Techniques for realizing optical-phase

alignment have been introduced [39], [40], and several OTDMand OTDM/WDM transmission experiments have been per-formed using such formats as carrier-suppressed RZ(CS-RZ), in which the optical pulses in adjacent bit slots havea relative phase shift of [37], [41][44]. On the other hand, theimprovement for the transmission system is not as significantas to justify the effort of adjustment and stabilization of atest-multiplexer, even not for phase-modulated data signals.In DPSK and DQPSK transmission systems, the well-definedphase of adjacent pulses is only required behind the DEMUX(i.e., within the TDM channel). The relative phase of adjacentpulses within a TDM channel is independent of the adjustmentof the delay-line MUX.

C. OTDM Receiver

Various optical gates have been used for demultiplexing.At data rates beyond 160 Gb/s, the optical gates are mostlyfiber-based using cross phase modulation (XPM) or four wavemixing (FWM) in fibers [2][4]. A well-known example is thenonlinear optical loop mirror (NOLM) [45], which was appliedas a DEMUX for data rates up to 640 Gb/s, which is thefastest DEMUX reported so far [6], [46][48]. Another classof optical gates is based on XPM and FWM in a semicon-ductor optical amplifier (SOA). Examples of XPM-based op-tical gates include the SOA in a MachZehnder interferometer(SOA-MZI) [49], [50], the SOA in a polarization discriminatingswitch (SOA-UNI) [51], [52], and the SOA in a Sagnac inter-ferometer [53]. In general, the SOA-based optical gates operatewell at data rates up to 160 Gb/s, although operation at datarates up to 320 Gb/s was also reported [49], [50].

SOA- and fiber-based optical gates use all-optical switching.An optical signal controls the gate, which switches an opticaldata signal. Thus, these optical gates require also an appropriateoptical-pulse source. Another optical gate used in many high-speed transmission experiments is the EAM. In this device,an electrical control signal controls the gate that switches theoptical data signal. This is an enormous simplification of theDEMUX. This switch has been used as DEMUX in manytransmission experiments, as described in Section III. Recently,an EAM was monolithically integrated with a photodiode, andan electrical signal from the photodiode drove the EAM directly[54], [55]. Demultiplexing up to a data rate of 500 Gb/s wasreported. However, this gate needs an optical control signal.

The DEMUXs described above are capable of selectingonly one TDM channel of the multiplexed data signal (single-channel output operation). Multiple-channel output operationcan be achieved by a serial-parallel configuration of severalof these switches. Examples for multichannel output operationusing only one device are reported in [3] and [4] and, morerecently, in [56].

At data rates of 160 Gb/s and beyond, prescaled opto-electronic clock recovery was successfully achieved by usingphase-locked-loop (PLL) configurations with either optical-phase comparators based on FWM or XPM in an SOA or withopto-electrical phase comparators based on EAMs. SOA-basedand EAM-based clock-recovery devices have been operatedat up to 400 [57] and 320 Gb/s [58], respectively. Many


Fig. 3. Schematic of demodulators for DPSK and DQPSK transmission at40 Gbaud base rate.

transmission experiments were also performed without recov-ering the clock signal from the multiplexed data signal, becausean appropriate clock-recovery device was unavailable. Twoalternative approaches were used. A clock signal was generatedat the transmitter and transmitted together with the data signalover the fiber at a separate wavelength [clock transmitted(e.g., [52])], or the MUX at the transmitter end was adjustedfor slightly different pulse amplitudes [clock modulation(e.g., [46])] such that a simple photodetector was able to detectthe clock signal at the receiver end.

In OTDM-transmission experiments, the output of theDEMUX is in general connected with the O/E receiver via anoptical amplifier and an optical filter. In DPSK and DQPSKtransmission experiments, a demodulator is additionally placedbetween DEMUX and O/E receiver. The demodulator con-verts the phase-modulated data signal into two complementaryamplitude-modulated data signals. In the DPSK experiment, thedemodulator is a MachZehnder interferometer, as shown inFig. 3, with a delay between both interferometer arms of onebit period at the base rate, for instance, 25 ps for a base rateof 40 Gb/s. In case of DPSK, adjacent bits with zero phasedifference, which carry the logical information equivalent toa space in OOK, interfere constructively at one port (e.g.,port 1 in Fig. 3) and destructively at the other port (port 2) of theinterferometer, whereas adjacent bits with phase difference,which carry the logical information equivalent to a mark inOOK, interfere constructively at port 2 and destructively atport 1. If the two complementary signals are detected by abalanced photodetector, an improvement of 3 dB, as comparedwith OOK, is obtained [34].

The demodulator for a DQPSK signal is also shown in Fig. 3.It comprises two MachZehnder interferometers, each with adifferential delay (in the interferometer arms) of one bit periodat the symbol base rate (i.e., 25 ps for 40 GBd or 80 Gb/s)plus an additional phase shift of +/4 or /4 to detect thein-phase or quadrature component, respectively. The DPSK(or DQPSK) demodulator needs to be actively matched to thetransmitter wavelength for proper operation. This requires also

a pulse source in the transmitter, which is highly stable in termsof the carrier wavelength.

D. Transmission Line

For application in commercial systems, a fiber-link lengthin the order of 1000 km is desirable. For these fiber lengths,compensation of CD and of PMD is essential for high-speeddata transmission. Already for a 160-Gb/s system, it is neces-sary to compensate for both the path-averaged CD (D = 0) atthe center wavelength of the pulse and for the path-averagedCD slope (dD/d = 0), because the dispersion slope producesoscillations near the trailing edge of the data pulse, even ifD() = 0 for the center wavelength of the pulse. Currently,the most mature dispersion-compensation technique is basedon dispersion compensating fiber (DCF), which compensatessimultaneously for both D and dD/d. Generally, the DCFis localized as module in the repeaters and does not con-tribute to the transmission length. On the other hand, DCFhas further evolved into inline dispersion-managed-fiber (DMF)transmission lines. The DMF represents a pair of transmissionfibers, which together compensate for the path-averaged D anddD/d over a wide wavelength range.

Various types of transmission fiber in combination with theirassociated DCF have been investigated for high-speed datatransmission at 1550 nm. Examples are standard single-modefiber (SMF, D 17 ps/nm/km), dispersion shifted fiber (DSF,D 0.1 ps/nm/km), and various types of nonzero DSF(NZDSF, D 48 ps/nm/km). Additionally, there are sev-eral types of DMF such as SMF/RDF (SMF/reverse disper-sion fiber) or SLA/IDF. The latter comprises Super LargeArea fiber (SLA, D 20 ps/nm/km) and Inverse DispersionFiber (IDF, D 40 ps/nm/km), which together compensatefor D and dD/d.

The tolerances with regard to residual dispersion or residualSMF or DCF length are particularly crucial for high-speedsystems. For instance, for a data rate of 160 Gb/s (pulsewidth1.3 ps), simulations yield a tolerance (eye-opening penalty of1 dB) of +/2.5 ps/nm, which corresponds to an SMF lengthof +/150 m [59]. However, in experiments, a fine tuning of+/50 m is appropriate. To maintain such tolerances over alarge environmental temperature range requires automatic dis-persion compensation in addition to the DCF. Various tunable-dispersion compensators have been proposed [60][63].

For 640 Gb/s and beyond, dispersion compensation usingDCF is insufficient for most fibers. Higher order dispersionterms (d2D/d2) have to be taken into account. Simultane-ous compensation of the dispersion slope (dD/d) and ofd2D/d2 is obtained by applying excess dispersion (D) anda phase modulation to the pulse [64], [65]. This dispersion-compensation technique was successfully realized in a1.28-Tb/s transmission experiment over a 70-km DMF linkcomprising SMF-RDF [48]. On the other hand, modern fiberslike the SLA/IDF fiber are more suitable for high-speed datatransmission. A 2.56-Tb/s DQPSK data signal, correspondingto a symbol rate of 1.28 TBd, was transmitted over 160-kmSLA/IDF fiber without application of an additional sophisti-cated compensation scheme [6].


PMD is also a severe limitation with respect to high bit-ratedata transmission. PMD is caused by a slight birefringence ofthe fiber and of other components in the transmission link. Forexample, a PMD value of less than 0.05 ps/

km is needed

to realize a low-penalty 160-Gb/s transmission over a 160-kmfiber link [66]. Modern fiber such as the SLA/IDF fiber hasa PMD value of less than 0.05 ps/

km. On the other hand,

older installed fiber generally has a larger PMD value. UnlikeCD, PMD is much more difficult to compensate for becauseit changes with time and wavelength in a nondeterministicway. Therefore, automatic and adaptive PMD compensation isrequired. Adaptive PMD compensation has been demonstratedfor data rates of up to 160 Gb/s [44], [67][70]. In mostultrahigh bit-rate transmission experiments, PMD (first order)was compensated for by manually adjusting the polarization ofthe data signal at the transmission link input (principal statetransmission).

Fiber nonlinearity is another cause for signal degradation.High-speed transmission experiments are commonly performedin the quasi-linear (pseudolinear) transmission regime, wherethe nonlinear length is much greater than the dispersion length(e.g., [40]). A high local dispersion is advantageous for thistransmission regime, provided that the path-averaged disper-sion and dispersion slope are close to zero. The short pulsesof the data signal disperse very quickly in the fiber, spreadinginto many adjacent timeslots before the original pulse sequenceis restored by dispersion compensation. Therefore, the peakpower of the pulses is low for most of the path along thefiber. Consequently, fibers with high dispersion D are favorablefor high-speed transmission. For example, with 160-Gb/s datatransmission, impressive results with transmission distancesof up to 2000 km have been obtained using NZDSF [71].However, 160-Gb/s data transmission over the SLA/IDF fiberwith its high local dispersion and low nonlinearity (large ef-fective area) achieved a transmission distance of more than4000 km [5].

III. TRANSMISSION EXPERIMENTS

A. 160-Gb/s Transmission Experiments: Summary

The first 160-Gb/s transmission experiment was performed inthe NTT laboratories in 1995 based on OOK soliton transmis-sion over 200-km DSF using AP multiplexing [72]. The pulsesource was an MLFL followed by an optical-pulse compressor,and the DEMUX comprised a polarizer and a NOLM. EDFAswere used as inline amplifiers. Using essentially fiber-basedoptical-signal-processing technology for the terminal equip-ment, the NTT group increased the OTDM bit rate in a numberof spectacular experiments [46][48], [73][76] within fiveyears up to 1.28 Tb/s (AP-multiplexing), as will be discussed inSection III-D. Here, we mention only the 3-Tb/s (160 Gb/s19 channel, SP multiplexing) transmission over 40-km DSFas a contribution of this group to the 160-Gb/s WDM/OTDMtechnology [76]. This experiment combined the technologies of160-Gb/s OTDM and ultrawideband WDM. The 19 channelswere generated by an SC pulse source. The DEMUX in theOTDM-receiver comprised an optical gate based on FWM infiber. The terminal equipment comprised fiber-based devices

except for the clock recovery, which was based on FWM inan SOA.

The employment of semiconductor devices for optical signalprocessing in the transmitter and receiver of the OTDM systemis an alternative to fiber-based optical signal processing. Inparticular, the SOA has long been considered a key devicefor optical signal processing, such as optical gates and opticalclock-recovery devices. There are many transmission exper-iments resting upon SOA-based optical signal processing inthe terminal equipment [9], [49][53], [77][79]. Examples ofsingle wavelength channel transmission experiments are thefirst 160-Gb/s field trial involving unrepeatered transmissionover 116-km field-installed SMF using an MLLD as pulsesource and an SOA-UNI as optical gate for demultiplexing(DEMUX) [52] and another 160-Gb/s field trial over variouslink lengths of installed fiber of up to 275-km SMF usinga DEMUX based on FWM in an SOA [9]. The latter fieldexperiment is of particular interest because it also includes a160-Gb/s adddrop node based on gain-transparent operationof an SOA. This is the first OTDM networking experimentusing deployed fiber. Examples of OTDM/WDM transmissionexperiments are 1.28 Tb/s (160 Gb/s 8 channel) unre-peatered transmission over 140-km SMF [79] and 3.2-Tb/s(320 Gb/s 10 channel) transmission over 40-km SMF[49], both with use of a hybrid-integrated SOA-MZI DEMUX.In these experiments, the spectral efficiencies were 0.4 and0.8 b/s/Hz, respectively. Also of particular importance isthe first 160-Gb/s OTDM-transmission experiment with all-channel independent modulation and all-channel simultaneousdemultiplexing achieved by using a MUX and a DEMUXbased on periodically poled Lithium Niobate and SOA hybridintegrated planar lightwave circuits [36].

For 160-Gb/s OTDM systems, the SOA-based optical gateis probably the better choice for demultiplexing applications ascompared to fiber-based optical gates because of its compact-ness and the possibility of integration with other components.On the other hand, another semiconductor device (the EAM)has been developed into a very effective component for opti-cal signal processing for data generation and demultiplexing[80], [81]. In 1999, the first 160-Gb/s transmission experi-ment was reported, which used the EAM as an optical gatein the DEMUX, as a basic element in the clock-recoverycircuit, and as a device for optical-pulse generation (CW +Mod) [82]. This investigation also stimulated similar experi-ments in many laboratories [83][91] because the EAM-based160-Gb/s OTDM technology does not require such sophis-ticated devices as interferometric optical gates or FWMconfigurations for demultiplexing applications. Moreover, theEAM-based DEMUX is polarization insensitive, which is diffi-cult to achieve with the fiber-based and SOA-based DEMUXs.Also, the EAM-based DEMUX does not need an optical-pulsesource to operate the optical gate. The simpler technology ofthe EAM-based OTDM-system stimulated also several N 160 Gb/s OTDM/WDM transmission experiments [41], [42],[44], [92], [93]. In particular, we mention the 170-Gb/s (160 +7% FEC) 8 channel, AP-multiplexing, CS-RZ, 430-kmfield trial experiment comprising an EAM as an optical gate inthe DEMUX, as a basic element in the clock-recovery circuit,


Fig. 4. Experimental set-up of 160 Gb/s DPSK transmission over either a 334 km SMF fiber link or a 320 km SLA/IDF (Ultrawave) fiber link.

and as a device for optical-pulse generation (CW + Mod) [44].Here FEC stands for forward error correction.

Currently, the EAM-based optical gate is the best choicefor demultiplexing applications and clock-recovery devices in160-Gb/s OTDM-transmission systems. On the other hand,the EAM is probably not the best choice for optical-pulsegeneration (CW + Mod), because it requires in general somesort of subsequent pulse compression. In the following, we de-scribe two 160-Gb/s OTDM-transmission experiments in detail,which use EAM-based terminal equipment in the receiver, anMLLD in the transmitter, and the modulation-format DPSK.

B. 160-Gb/s Transmission With Long-Term Stability

The system performance for 160-Gb/s DPSK transmissionin combination with balanced detection is at least 3 dB betterthan the system performance for OOK transmission [34], [89].The increased system margin was used to demonstrate long-term stability of the DPSK transmission system [90], [91].

Fig. 4 schematically shows the experimental setup. The160-Gb/s transmitter comprised a 40-GHz optical-pulse source,a pushpull-operated LiNbO3 MachZehnder DPSK phasemodulator driven by a pattern generator, and a fiber-delay-lineMUX providing an SP multiplexed 160-Gb/s PRBS data signal(sech2 pulses, FWHM 1.4 ps, jitter < 250 fs, almost transformlimited ft = 0.32) at the input of the fiber link. The optical-pulse source was a 10-GHz MLLD followed by a compactand temperature stabilized 10- to 40-GHz fiber-delay-line pulsemultiplier.

The 160-Gb/s receiver comprised an automatic polarizationcontroller (PC), the EAM-based 160- to 40-Gb/s DEMUXconsisting of the optical clock recovery [58] and the opticalgate, a 3-nm optical bandpass filter, a DPSK-demodulator (seeSection II-C, a balanced photodetector [94], and a 40-Gb/s BERanalyzer.

Two transmission experiments were performed, one exper-iment using a 334-km SMF fiber link (upper link in Fig. 4)and the other experiment using a 320-km SLA/IDF fiber link(lower link in Fig. 4). This DMF is also known as Ultrawavefiber. The PRBS word lengths were 27 1 and 231 1 bits inthe experiments. The results showed that the penalty due to thelonger pattern is negligible. Using PRBS sequences, a DPSKprecoder or decoder could be avoided. The dispersion wascarefully compensated for in both transmission links to providea pulsewidth below 1.7 ps at the receiver. The reamplificationwas realized by EDFAs, and the average power to the spans wasset to 10 dBm in both experiments.

The mean differential group delay (DGD) of the SLA/IDFfiber link (incl. EDFAs) was 0.7 ps, and PMD mitigation wasnot required. In the SMF experiment, the mean DGD was 0.8 ps

Fig. 5. BER-measurements versus decision threshold, back-to-back and after334 km SMF transmission, and corresponding eye diagrams.

for the SMF link and 1.4 ps for the two DCF modules includingthe EDFAs. Because of the high DGD of the DCF, a PMDmitigation scheme was used in the SMF experiment, as shownin Fig. 4. It consisted of an automatic PC before the DCF mod-ules and a polarimeter after the DCF modules. The polarizationwas controlled such that the degree of polarization (DOP) afterthe dispersion compensation was maximum. This is a feasibleconcept because the dispersion compensation was realized aspostcompensation and thus was located altogether in front ofthe receiver. The described scheme avoided an adjustment to theprincipal states of polarization. This scheme was dynamic butnot adaptive. In the worst case, it could not compensate for thePMD of the SMF. Hence, it was a PMD mitigation but not a fullPMD compensation scheme. A complete PMD compensationscheme is described in [70].

The BER measurements versus the received power Precresulted in a back-to-back sensitivity of 28.4 dBm, a penaltyof almost 3 dB for transmission using the SMF link, and apenalty below 1.5 dB for transmission using the SLA/IDF fiberlink. More details of these measurements are reported in [90]and [91]. For the SMF experiment, Fig. 5 shows the BERover the decision threshold of the receiver. The correspondingeye diagrams are shown as insets. A wide error-free rangewas obtained, even for transmission over the SMF link. Thecurves also confirm the good balance of the receiver as the op-timum threshold is at about 0 mV. Degradations narrow thecurve but do not change the decision threshold. This enabledstable operation without realignment or active control of thedecision threshold and is the main cause of the long-termstability of the DPSK transmission system in combination withthe increased system margin. A BER in the order < 1015 isexpected from these measurements and was confirmed in thelong-term measurements.


Fig. 6. Measurements of absolute number of errors versus time. The dottedline represents the evolution of the number of errors if a BER = 1012is assumed.

Fig. 7. Schematic view of set-up in 160 Gb/s DPSK long haul transmission.

Fig. 6 depicts the results of the long-term measurementswithout any manual readjustment for the transmission overboth fiber links. The polarization to the DCF were tracked andaligned by a computer control (only for the SMF experiment).The plot shows the absolute number of bit errors, which hap-pened during the time interval of 10 h. For a period of morethan 5 h, the system was running without any bit error forboth fiber links. This indicates a bit-error floor due to intrinsictransmission impairments in the order of 1015, which confirmsthe results in Fig. 5. Since the laboratory system did not includeany automatic feedback (in contrast to a commercial system), aslight drift of the system started after some time. However, eventhe steep increase still corresponds to a bit-error ratio of 1012(plotted as dotted line on the left-hand side). As the system canbe adjusted by using power monitors (mainly the in- and outputpowers of the EDFAs), the adjustment can be easily automated.These results suggest that this 160-Gb/s DPSK transmissionsystem can be used in deployed systems and can be operatederror-free for years if some feedback for compensating the slowdrifts and FEC is implemented.

C. 160-Gb/s Ultralong Haul Transmission

Fig. 7 shows schematically the experimental setup of a170-Gb/s (160 Gb/s + 7% FEC) DPSK transmission exper-iment using a recirculating loop arrangement [5]. Transmis-

sion was investigated using SP signals and AP signals. The170-Gb/s transmitter and receiver were similar to those ofthe long-term stability experiment in Section III-B. The loopcomprised six 80-km-DMF spans (480 km) of SLA/IDF(Ultrawave) fiber (see Section II-D). Using commercial Ramanpump modules (RPM1 to RPM6) with pump lasers operatingaround 1444 and 1456 nm, the span losses including Ramancouplers were fully compensated. The average span loss includ-ing couplers was 19.3 dB, and the average span noise figurewas 16.6 dB. The dispersion slope at the operating wavelengthof 1551.5 nm was only 0.1 ps/nm2, and the residual dispersionwas minimized by inserting short SMF fiber pieces in the loop(DC in Fig. 7). The 2-ps (FWHM) data pulses were broadenedto 2.6 ps after 4320-km propagation.

The average DGD of the 480-km setup was 0.9 ps. Trans-mission of 170-Gb/s data signals significantly beyond thisdistance requires PMD compensation or suitable polarizationadjustment in the loop. To mitigate PMD effects, the SP signalmust propagate in one of the principal states of polarizationof the transmission link for each round trip in the loop. Weobtained this by maximizing the DOP using the PCs PC1 andPC2 and a polarization analyzer with high temporal resolution(microsecond range). For the AP signal, PC2 was adjusted forminimum BER after the ninth roundtrip. In this case, the slowand fast DGD axes seem to interchange after each round trip.Otherwise, the accumulated DGD would cause a shift of therelative temporal position of adjacent orthogonally polarizedOTDM tributaries, yielding a degradation of the transmissionperformance, because no polarization demultiplexing was usedin the receiver.

The BER was measured for SP- and AP signals using a PRBS231 1 sequence. In both cases, no polarizing filter was used atthe receiver. For the SP signal, BER values of 5 109 and 1.6 104 were obtained after 960 and 2880 km, respectively. Forthe AP signal, the optimum launched power was consistently23 dB higher than for the SP signal due to smaller nonlineardegradations. This enabled longer transmission distances. Fora distance of 1920 km (four round trips), a BER of 2 109was obtained. At 4320-km transmission distance, a minimumBER value of 5 105 was achieved. Using advanced FEC, theBER values for transmission distances up to 4320 km can becorrected to values below 1012.

D. Beyond 160-Gb/s Transmission

OTDM data transmission beyond 160 Gb/s was first per-formed in the NTT laboratories. Examples of this work arethe following single wavelength channel, single polarization(SP) transmission experiments: 200 Gb/s over 100-km DSF[73], 400 Gb/s over 40-km SMF [74], 640 Gb/s over 60- and92-km SMF [46], [47], and the OTDM/WDM experiment:1.4 Tb/s (200 Gb/s 7 channel) over 50-km DSF [75]. Inthese experiments, OOK modulation format was used, and theterminal equipment mainly comprised fiber devices. The pulsesource in the transmitter was either an MLFL (see Section II-B)followed by an optical-pulse compressor or a pulse sourcebased on supercontinuum generation (SC-pulse source). TheDEMUX in the OTDM-receiver was an optical gate-based


Fig. 8. Schematic view of the experimental set-up for 1.28 Tb/s and 2.58 Tb/s DQPSK transmission over a 160 km or 240 km SLA/IDF (Ultrawave) fiber link.

either on a NOLM or on FWM in fiber. An exception was theoptical clock-recovery device, which comprised a PLL with aphase comparator based on FWM in a SOA. EDFAs were usedas inline amplifiers.

Of particular importance is the 1.28 Tb/s, AP multiplex-ing, single -channel transmission experiment over 70-kmSMF + RDF performed in 2000 [48]. This experiment in-cluded generation of pedestal-free femtosecond optical pulses,compensation of higher order CD up to the fourth order, andultrafast demultiplexing employing a NOLM with vanishingwalk off. The experiment combined all achievements in high-speed data transmission technology using the modulation-format OOK.

There are also a few transmission experiments at data ratesbeyond 160 Gb/s, which do not use fiber-based optical signalprocessing in the terminal equipment in particular for the op-tical gate in the DEMUX. Some 320-Gb/s transmission exper-iments using AP multiplexing and a polarizer either before orbehind the DEMUX require essentially 160-Gb/s optical signalprocessing in the terminal equipment [87], [88]. In anotherexperiment, the 160-Gb/s OTDM technology (MLLD, EAMfor DEMUX, and CLOCK) was used to realize 640 Gb/stransmission by applying the modulation-format DQPSK andAP multiplexing [95]. The 640-Gb/s DQPSK data signal wastransmitted over 480-km SLA/IDF (Ultrawave) fiber error-free(BER 109). In a WDM/OTDM experiment, 10 channels(spectral efficiency 0.8 b/s/Hz), each carrying a 320-Gb/s OOKSP signal, were transmitted over 40-km SMF [49]. In thispaper, the optical gate comprised a hybrid-integrated SOA-MZIDEMUX, and an SC-pulse source was used.

Recently, DQPSK transmission with AP multiplexing wasalso realized at the data rates of 1.28 and 2.56 Tb/s [6]. Fig. 8shows the experimental setup of these transmission experi-ments. In the transmitter, a 10-GHz MLSL (1550 nm, 2.1 ps,see Section II-B) and a pulse-compression unit (dispersion-decreasing fiber soliton compression followed by a dispersionimbalanced loop mirror) provided a 10-GHz optical-pulse trainwith a pulsewidth of 0.42 ps. An optical bandpass filter centeredat 1556 nm defined the final pulsewidth for transmission. Animportant requirement for the pulse-compression unit was topreserve the phase coherence of the pulse train, as requiredfor a DQPSK modulated data signal. Data generation based on10 GBd was used in order to improve the pulse compressionand data modulation. This pulse train was modulated andmultiplexed to generate an 80-Gb/s DQPSK signal (symbol rate40 GBd, see Section II-B). Finally, the 80-Gb/s DQPSK signalwas multiplexed by a delay-line MUX (OTDM-MUX) to an APsignal with the data rate 1.28 or 2.56 Tb/s.

The data signal was transmitted over two or three 80-km-DMF spans of SLA/IDF (Ultrawave) fiber (see Section II-D).

Fig. 9. BER measurements for 2.56 Tb/s and 1.28 Tb/s back-to-back (solidsymbols) and 160 km fiber transmission (hollow symbols) experiment.The dispersion was precisely matched by the insertion of shortSMF pieces, while the dispersion slope was compensated forby the SLA/IDF-combination and combining different spanswith opposite residual slope. The span input power was set to12 dBm. The average DGD of the link was below 0.7 psfor three spans. To mitigate the detrimental effects of PMD,the input polarization was adjusted to the principal states ofpolarization of the fiber.

In the receiver, a polarization DEMUX (polarization beamsplitter, PBS) was followed by a NOLM (300-m HNLF) asthe optical gate. An MLFL (40 GHz, 1.0 ps, see Section II-B)provided the control pulses for the NOLM. This laser wassynchronized to the data signal by a clock-recovery unit, whichoperated well up to a symbol rate of 320 GBd [58]. In the2.56-Tb/s experiment, this clock recovery could also be op-erated at the symbol rate of 640 GBd. We attributed this tosmall imperfections of the optically multiplexed data signal.At the output of the NOLM, the signal passed a 6-nm filter toseparate the data signal from the control pulses. The 80-Gb/sDQPSK receiver comprised a delay-line interferometer asphase demodulator, a balanced photodetector, a 40- to 10-Gb/sDEMUX, and a BER analyzer.

Fig. 9 shows the results of BER measurements of the2.56 Tb/s and the 1.28-Tb/s AP signals versus the receivedsignal power at the input of the receiver (as indicated in Fig. 8)after transmission over 160-km SLA/IDF (Ultrawave) fiber(two spans) and in back-to-back measurements. This figure alsoshows the results of back-to-back measurements of 80- and640-Gb/s SP signals for comparison. The results for 2.56 Tb/scover the dashed area in Fig. 9, because the overlapping


pulse tails caused coherence crosstalk resulting in variationsof the performance. The pulsewidth was 0.65 ps at the outputof the transmitter. The pulsewidth after 160-km transmissionwas 0.75 ps. For the 2.56-Tb/s transmission, shorter pulsesare desirable. However, the reduction of the pulsewidth inthe present setup was limited by the bandwidth of the cas-caded nongain-flattened EDFAs, fourth-order CD, and higherorder PMD.

For 2.56-Tb/s transmission, the system performed nearlyerror-free (BER 109) in the back-to-back configuration andrevealed BER values 105 after 160-km transmission. BERvalues of less than 104 result in an effective BER < 1012if standard FEC (assuming a 7% overhead) is used. This FECwould reduce the payload to 2.4 Tb/s. For 1.28 Tb/s, error-free(BER < 109) transmission was obtained. At this data rate,error-free transmission was also possible over a fiber link of240-km Ultrawave fiber (three spans) for all tributaries of the1.28-Tb/s data signal (not shown in Fig. 9).

IV. CONCLUSION

OTDM-transmission technology at 160 Gb/s is already a ma-ture technology. Combined with the modulation-format DPSKand balanced detection, it provides very stable operation, whichseems to be already appropriate for deployed systems if well-established techniques like forward-error correction and Ramanamplification are implemented to further increase the systemmargin.

There are several optical-signal-processing technologiesavailable for 160-Gb/s transmission. Currently, the optimumterminal equipment includes an optical gate and a clock-recovery device based on EAMs in the receiver and an MLLDas a pulse source in the transmitter. These components arecommercially available today.

We think that OTDM technology for 160 Gb/s will be aninterim technique, which will be replaced by ETDM as soonas electrical signal processing becomes available at 160 Gb/sand will be less expensive than optical signal processing. Onthe other hand, some OTDM components may find applica-tions in ETDM systems. This is probably true for monolithi-cally integrated MLLDs as transmitter pulse sources. OTDMDEMUXs perform better than ETDM receivers already at datarates of 80 Gb/s. We may expect communication networks tobe based on an appropriate combination of WDM, ETDM, andOTDM technologies. Also, OTDM components, such as pulsesources, optical gates, and clock-recovery devices are alreadybeing employed in optical sampling systems for waveform andsignal-quality monitoring.

The 160-Gb/s OTDM-transmission experiments also re-vealed some limitations for data transmission over deployedfiber links. Many old fiber links have very high PMD values,which make 160-Gb/s data transmission impractical even ifPMD-compensators are implemented. The investigation of thefeasibility of ultrahigh-speed data transmission is a main taskwith regard to OTDM technology.

This task is important in particular in the studies of ultrahigh-speed data transmission beyond 160 Gb/s. An OTDM data rateof 2.5 Tb/s is currently not applicable for installed systems.

However, it is a challenging task for OTDM technology toinvestigate the physical limits of high-speed fiber transmissionand to search for appropriate techniques for data generation,transmission, and demultiplexing to extend these limits.

ACKNOWLEDGMENT

The authors would like to thank S. Watanabe and F. Futamifrom Fujitsu Laboratories Ltd., Kawasaki, Japan, as well asS. Weisser from Lucent Technologies Nuremberg, Germany,for their cooperation in several common research activities inthe field of OTDM technology. The authors would also like tothank M. Nakazawa from the Research Institute of ElectricalCommunication of Tohoku University, Sendai, Japan, for manyfruitful discussions.

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[52] U. Feiste, R. Ludwig, C. Schubert, J. Berger, C. Schmidt, H. G. Weber,B. Schmauss, A. Munk, B. Buchold, D. Briggmann, F. Kueppers, andF. Rumpf, 160 Gb/s transmission over 116 km field-installed fibre using160 Gb/s OTDM and 40 Gb/s ETDM, Electron. Lett., vol. 37, no. 7,pp. 443445, Mar. 2001.

[53] K. Suzuki, K. Iwatsuki, S. Nishi, and M. Saruwatari, Error-freedemultiplexing of 160 Gb/s pulse signal using optical loop mirror in-cluding semiconductor laser amplifier, Electron. Lett., vol. 30, no. 18,pp. 15011503, Sep. 1994.

[54] S. Kodama, T. Yoshimatsu, and H. Ito, 320 Gb/s error-free demulti-plexing using ultrafast optical gate monolithically integrating a photo-diode and electroabsorption modulator, Electron. Lett., vol. 39, no. 17,pp. 12691270, Aug. 2003.

[55] , 500-Gb/s demultiplexing operation of monolithic PD-EAM gate,presented at the Eur. Conf. Optical Commun. (ECOC), Rimini, Italy,2003, Postdeadline Paper Th4.2.8.

[56] I. Shake, H. Takara, K. Uchiyama, I. Ogawa, T. Kitoh, T. Kitagawa,M. Okamoto, K. Magari, Y. Suzuki, T. Morioka, 160 Gb/s full opticaltime-division demultiplexing using FWM of SOA-array integrated onPLC, Electron. Lett., vol. 38, no. 1, pp. 3738, Jan. 2002.


[57] O. Kamatani and S. Kawanishi, Prescaled timing extraction from400 Gb/s optical signal using a phase lock loop based on four-wavemixing, IEEE Photon. Technol. Lett., vol. 8, no. 8, pp. 10941096,Aug. 1996.

[58] C. Boerner, V. Marembert, S. Ferber, C. Schubert, C. Schmidt-Langhorst,R. Ludwig, and H. G. Weber, 320 Gb/s clock recovery withelectro-optical PLL using a bidirectionally operated electroabsorptionmodulator as phase comparator, presented at the Optical Fiber Commun.Conf. (OFC), Anaheim, CA, 2005, OTuO3.

[59] S. Vorbeck and R. Leppla, Dispersion and dispersion slope tolerance of160-Gb/s systems, considering the temperature dependence of chromaticdispersion, IEEE Photon. Technol. Lett., vol. 15, no. 10, pp. 14701472,Oct. 2003.

[60] B. J. Eggleton, B. Mikkelsen, G. Raybon, A. Ahuja, J. A. Rogers,P. S. Westbrook, T. N. Nielsen, S. Stulz, and K. Dreyer, Tunable dis-persion compensation in a 160 Gb/s TDM system by voltage controlledchirped fiber Bragg grating, IEEE Photon. Technol. Lett., vol. 12, no. 8,pp. 10221024, Aug. 2000.

[61] C. K. Madsen, Integrated waveguide allpass filter tunable dispersioncompensator, presented at the Optical Fiber Commun. Conf. (OFC),Anaheim, CA, 2002, Paper TuT1.

[62] T. Inui, T. Komukai, M. Nakazawa, K. Suzuki, K. R. Tamura,K. Uchiyama, and T. Morioka, Adaptive dispersion slope equalizer us-ing a nonlinearly chirped fiber Bragg grating pair with a novel dis-persion detection technique, IEEE Photon. Technol. Lett., vol. 14, no. 4,pp. 549551, Apr. 2002.

[63] S. Wakabayashi, A. Baba, H. Moriya, X. Wang, T. Hasegawa, andA. Suzuki, Tunable dispersion and dispersion slope compensator basedon two twin chirped FBGs with temperature gradient for 160 Gb/s trans-mission, IEICE Trans. Electron., vol. E87-C, no. 7, pp. 11001105,Jul. 2004.

[64] M. D. Pelusi, X. Wang, F. Futami, K. Kikuchi, and A. Suzuki, Fourth-order dispersion compensation for 250-fs pulse transmission over 139-kmoptical fiber, IEEE Photon. Technol. Lett., vol. 12, no. 7, pp. 795798,Jul. 2000.

[65] T. Yamamoto and M. Nakazawa, Third- and fourth-order active disper-sion compensation with a phase modulator in a terabit-per-second opticaltime-division multiplexed transmission, Opt. Lett., vol. 26, no. 9, p. 647,May 2001.

[66] L. E. Nelson and R. M. Jopson, Introduction to polarization mode disper-sion in optical systems, in Polarization Mode Dispersion, A. Galtarossaand C. R. Menyuk, Eds. New York: Springer Science + Business Media,Inc., 2005.

[67] M. Westlund, H. Sunnerud, J. Li, J. Hansryd, M. Karlsson, P. O.Hedekvist, and P. A. Andrekson, Long-term automatic PMD compen-sation for 160 Gb/s RZ transmission, Electron. Lett., vol. 38, no. 17,pp. 982983, Aug. 2002.

[68] T. Miyazaki, M. Daikoku, I. Morita, T. Otani, Y. Nagao, M. Suzuki, andF. Kubota, Stable 160 Gb/s DPSK transmission using a simple PMDcompensator on the field photonic network test bed of JGN II, presentedat the Optoelectron. Commun. Conf. (OECC), Yokohama, Japan, 2004,PD-1-3.

[69] F. Buchali, W. Baumert, M. Schmidt, and H. Buelow, Dynamic distortioncompensation in a 160 Gb/s RZ OTDM system: Adaptive 2 stage PMDcompensation, in Proc. OFC, 2003, pp. 589590.

[70] S. Kieckbusch, S. Ferber, H. Rosenfeldt, R. Ludwig, A. Ehrhardt,E. Brinkmeyer, and H. G. Weber, Automatic PMD compensator ina single-channel 160 Gb/s transmission over deployed fiber usingRZ-DPSK modulation format, J. Lightw. Technol., vol. 23, no. 1,pp. 165171, Jan. 2005.

[71] A. H. Gnauck, G. Raybon, P. G. Bernasconi, J. Leuthold, C. R. Doerr, andL. W. Stulz, 1 Tb/s 6 170.6 Gb/s transmission over 2000 km NZDFusing OTDM and RZ-DPSK, IEEE Photon. Technol. Lett., vol. 15,no. 11, pp. 16181620, Nov. 2003.

[72] M. Nakazawa, K. Suzuki, E. Yoshida, E. Yamada, T. Kitoh, andM. Kawachi, 160 Gb/s soliton data transmission over 200 km, Electron.Lett., vol. 31, no. 7, pp. 565566, Mar. 1995.

[73] S. Kawanishi, H. Takara, T. Morioka, O. Kamatani, and M. Saruwatari,200 Gb/s, 100 km time-division-multiplexed optical transmission usingsupercontinuum pulses with prescaled PLL timing extraction and all-optical demultiplexing, Electron. Lett., vol. 31, no. 10, pp. 816817,May 1995.

[74] S. Kawanishi, H. Takara, T. Morioka, O. Kamatani, K. Takiguchi,T. Kitoh, and M. Saruwatari, Single channel 400 Gb/s time-division-multiplexed transmission of 0.98 ps pulses over 40 km employing disper-sion slope compensation, Electron. Lett., vol. 32, no. 10, pp. 916917,May 1996.

[75] S. Kawanishi, H. Takara, K. Uchiyama, I. Shake, O. Kamatani, andH. Takahashi, 1.4 Tb/s (200 Gb/s 7 ch) 50 km optical transmissionexperiment, Electron. Lett., vol. 33, no. 20, pp. 17161717, Sep. 1997.

[76] S. Kawanishi, H. Takara, K. Uchiyama, I. Shake, and K. Mori, 3 Tb/s(160 Gb/s 19 channel) optical TDM and WDM transmission experi-ment, Electron. Lett., vol. 35, no. 10, pp. 826827, May 1999.

[77] R. Ludwig, U. Feiste, S. Diez, C. Schubert, C. Schmidt, H. J. Ehrke,and H. G. Weber, Unrepeatered 160 Gb/s RZ single-channel transmis-sion over 160 km of standard fibre at 1.55 m with hybrid MZI opticalDEMUX, Electron. Lett., vol. 36, no. 16, pp. 14051406, Aug. 2000.

[78] T. Yamamoto, U. Feiste, J. Berger, C. Schubert, C. Schmidt, R. Ludwig,and H. G. Weber, 160 Gb/s demultiplexer with clock recovery usingSOA-based interferometric switches and its application to 120 km fibertransmission, presented at the Eur. Conf. Optical Commun. (ECOC),Amsterdam, The Netherlands, 2001, Tu.L.2.6.

[79] A. Suzuki, X. Wang, T. Hasegawa, Y. Ogawa, S. Arahira, K. Tajima, andS. Nakamura, 8 160 Gb/s (1.28 Tb/s) DWDM/OTDM unrepeateredtransmission over 140 km standard fiber by semiconductor-based de-vices, presented at the Eur. Conf. Optical Commun. (ECOC), Rimini,Italy, 2003, Paper Mo3.6.1.

[80] D. D. Marcenac, A. D. Ellis, and D. G. Moodie, 80 Gb/s OTDM usingelectroabsorption modulators, Electron. Lett., vol. 34, no. 1, pp. 101103,Jan. 1998.

[81] A. D. Ellis, J. K. Lucek, D. Pitcher, D. G. Moodie, and D. Cotter, Full10 10 Gb/s OTDM data generation and demultiplexing using elec-troabsorption modulators, Electron. Lett., vol. 34, no. 18, pp. 17661767,Sep. 1998.

[82] B. Mikkelsen, G. Raybon, R.-J. Essiambre, K. Dreyer, Y. Su, L. E. Nelson,J. E. Johnson, G. Shtengel, A. Bond, D. G. Moodie, and A. D. Ellis,160 Gb/s single-channel transmission over 300 km nonzero-dispersionfiber with semiconductor based transmitter and demultiplexer, presentedat the Eur. Conf. Optical Commun. (ECOC), Nice, France, 1999, Post-deadline Paper 2-3.

[83] J. Yu, K. Kojima, N. Chand, M. C. Fischer, R. Espindola, andT. G. B. Mason, 160 Gb/s single-channel unrepeatered transmission over200 km of non-zero dispersion shifted fiber, presented at the Eur. Conf.Optical Commun. (ECOC), Amsterdam, The Netherlands, 2001, PaperPD M.1.10.

[84] J.-L. Aug, M. Cavallari, M. Jones, P. Kean, D. Watley, andA. Hadjifotiou, Single channel 160 Gb/s OTDM propagation over480 km of standard fiber using a 40 GHz semiconductor mode-lockedlaser pulse source, presented at the Optical Fiber Commun. Conf. (OFC),Anaheim, CA, 2002, Paper TuA3.

[85] E. Lach, M. Schmidt, K. Schuh, B. Junginger, G. Veith, and P. Nouchi,Advanced 160 Gb/s OTDM system based on wavelength transparent4 40 Gb/s ETDM transmitters and receivers, presented at the OpticalFiber Commun. Conf. (OFC), Anaheim, CA, 2002, Paper TuA2.

[86] H. Murai, M. Kagawa, H. Tsuji, K. Fujii, Y. Hashimoto, andH. Yokoyama, Single channel 160 Gb/s (40 Gb/s 4) 300 kmTransmission using EA modulator basedOTDM module and 40 GHzexternalCavity mode-locked LD, presented at the Eur. Conf. OpticalCommun. (ECOC), Copenhagen, Denmark, 2002, Paper 2.1.4.

[87] M. Schmidt, E. Lach, K. Schuh, M. Schilling, P. Sillard, and G. Veith,Unrepeatered 320 Gb/s (8 40 Gb/s) OTDM transmission over 80 kmTeraLightTM-reverse TeraLightTM fiber link, presented at the Eur. Conf.Optical Commun. (ECOC), Rimini, Italy, 2003, We.4.P.117.

[88] B. Mikkelsen, G. Raybon, R.-J. Essiambre, A. J. Stentz, T. N. Nielsen,D. W. Peckham, L. Hsu, L. Gruner-Nielsen, K. Dreyer, andJ. E. Johnson, 320 Gb/s single-channel pseudolinear transmission over200 km nonzero-dispersion fiber, IEEE Photon. Technol. Lett., vol. 12,no. 10, pp. 14001402, Oct. 2000.

[89] S. Ferber, R. Ludwig, C. Boerner, A. Wietfeld, B. Schmauss, J. Berger,C. Schubert, G. Unterboersch, and H. G. Weber, Comparison of DPSKand OOK modulation format in 160 Gb/s transmission system, Electron.Lett., vol. 39, no. 20, pp. 14581459, Oct. 2003.

[90] S. Ferber, R. Ludwig, C. Boerner, C. Schubert, C. Schmidt-Langhorst,M. Kroh, V. Marembert, and H. G. Weber, 160 Gb/s DPSK transmissionover 320 km fibre link with high long-term stability, Electron. Lett.,vol. 41, no. 4, pp. 200202, Feb. 2005.

[91] S. Ferber, C. Schmidt-Langhorst, R. Ludwig, C. Boerner, C. Schubert,V. Marembert, M. Kroh, and H. G. Weber, 160 Gb/s OTDM long-haul transmission with long-term stability using RZ-DPSK modulationformat, IEICE Trans. Commun., vol. E88-B, no. 5, pp. 19471954,2005.

[92] B. Mikkelsen, G. Raybon, B. Zhu, R.-J. Essiambre, P. G. Bernasconi,K. Dreyer, L. W. Stulz, and S. N. Knudsen, High spectral efficiency(0.53 bit/s/Hz) WDM transmission of 160 Gb/s per wavelength over


400 km of fiber, presented at the Optical Fiber Commun. Conf. (OFC),Anaheim, CA, 2001, Paper ThF2-1.

[93] K. Schuh, M. Schmidt, E. Lach, B. Junginger, A. Klekamp, G. Veith,and P. Sillard, 4 160 Gb/s DWDM/OTDM transmission over3 80 km TeraLightTM-Reverse TeraLightTM-fibre, presented at theEur. Conf. Optical Commun. (ECOC), Copenhagen, Denmark, 2002,Paper 2.1.2.

[94] A. Beling, H. G. Bach, D. Schmidt, G. Mekonnen, R. Ludwig, S. Ferber,C. Schubert, C. Boerner, B. Schmauss, J. Berger, C. Schmidt,U. Troppenz, and H. G. Weber, Monolithically integrated balanced pho-todetector and its application in OTDM 160 Gb/s DPSK transmission,Electron. Lett., vol. 39, no. 16, pp. 12041205, Aug. 2003.

[95] S. Ferber, C. Schubert, R. Ludwig, C. Boerner, C. Schmidt-Langhorst,and H. G. Weber, 640 Gb/s DQPSK single-channel transmission over480 km fibre link, Electron. Lett., vol. 41, no. 22, pp. 12341235, 2005.

Hans-Georg Weber received the Dr. rer. nat. andDr. habil. degrees in physics from Marburg Uni-versity, Marburg, Germany, and from HeidelbergUniversity, Heidelberg, Germany, in 1971 and 1976,respectively.

From 1977 to 1978, he served as a Max-Kade-Fellow at Stanford University, Standford, CA, andfrom 1979 to 1984, he was a Heisenberg Fellow atthe University Heidelberg. He became Head of aresearch group at the Fraunhofer Institute for Tele-communications, Heinrich-Hertz-Institut, Berlin,

Germany, in 1985, and also in 1985, he became Professor of physics with theTechnical University of Berlin. Since 2006, he has been Retired Professorand has been serving as Consultant for the Heinrich-Hertz-Institut, Berlin.He is author and coauthor of more than 400 journal articles in the field ofoptical communications. He is coeditor of the Journal of Optical and FiberCommunications Reports.

Prof. Weber received the Philip Morris award in 1999, and he was nominatedfor the Innovation Award of the German Bundespraesident.

Reinhold Ludwig was born in Lahnstein, Germany,in 1952. He received the Ing. grad. degree fromthe Fachhochschule Koblenz, Koblenz, Germany, in1974 and the Dipl.-Ing. and Dr.-Ing. degrees from theTechnical University of Berlin, Berlin, Germany, in1985 and 1993.

He joined the Heinrich Hertz Institute (HHI),Berlin, in 1985, where he is involved in researchon photonic components and systems. He workedas a Visiting Scientist at Nippon Telephone andTelegraph Company (NTT), Japan, in 1991, and at

Bell Labs in 1993. Since 1985, he has authored and coauthored more than 300scientific papers. He is the holder of several patents. In 1996, he founded thefirst HHI spin-off company (LKF Advanced Optics GmbH) and served as CEOuntil the merger of LKF and u2t Innovative Optoelectronic Components GmbHin 2001.

Dr. Ludwig is a member of the Verband Der Elektrotechnik ElektronikInformationstechnik (VDE). In 1999, his group received the Philip MorrisResearch Award, and he was nominated for the Innovation Award of theGerman Bundespraesident.

Sebastian Ferber was born in 1975. He receivedthe degree in physics from Technische UniversittBerlin, Germany, in 2001. He is currently work-ing toward the Ph.D. degree at the Heinrich-Hertz-Institut, Berlin.

He joined the OTDM group at the Heinrich-Hertz-Institut, where he is currently finishing his Ph.D.thesis on the topic of advanced modulation formatsin high-speed transmission systems. He is author orcoauthor of more than 40 papers.

Mr. Ferber has received several awards.

Carsten Schmidt-Langhorst was born in Berlin,Germany, in 1972. He received the diploma (Dipl.-Phys.) and the Ph.D. degrees (Dr. rer. nat.) in physicsfrom the Technical University of Berlin, in 1997 and2004, respectively.

He has been with the Fraunhofer Institute forTelecommunications, Heinrich-Hertz-Institute, Ber-lin, since 1998 and has been engaged with all-opticaltransmission, processing, and detection of opticaldata signals at a picosecond time scale, and, in partic-ular, all-optical sampling techniques. He is currently

heading several projects in the field of ultrafast optical transmission technology.Dr. Schmidt-Langhorst is a member of the Deutsche Physikalische

Gesellschaft. He received the Philip Morris Research Award in 1999.

Marcel Kroh was born in Berlin, Germany, in 1972.He studied physics at the Technical University ofBerlin. From 1996 to 1997, he stayed at The Uni-versity of Manchester (UMIST), Manchester, U.K.,and received the B.Sc. degree in physics. Afterwards,he received the Dipl.-Phys. degree from the Tech-nical University of Berlin, in 2000. He is currentlyworking toward the Ph.D. degree in semiconductorlaser processing, pulse generation with mode-lockedlasers, and fiber-based pulse compression for opticalsignal processing at Technical University of Berlin.

He joined the Heinrich-Hertz-Institute, Berlin, in 1999 and worked on thetopics of semiconductor laser processing, pulse generation with mode-lockedlasers, and fiber-based pulse compression for optical signal processing.

Vincent Marembert was born in Bourges, France,in 1977. He received the Diploma degree from EcoleNationale Suprieure de Physique de Marseille,Marseille, France, in 2001. He was an exchange stu-dent at the Technische Universitt Berlin, Germany,from 2000 to 2001.

He worked as a Research Student with theFraunhofer Institute for Telecommunications,Heinrich-Hertz-Institut, Berlin. He is currently withthe Fraunhofer Institute for Telecommunications,Berlin, working on optical switches for optical-time-division-multiplexed (OTDM) networks.

Christof Boerner was born in Wuerzburg,Germany, in 1966. He studied physics at theTechnische Universitt Braunschweig and the FreieUniversitt Berlin, Germany, where he receivedthe Dipl.-Phys., in 1995. Currently, he is workingtoward the Ph.D. degree on the topic clock recoveryfor optical high bit-rate signals at the TechnicalUniversity of Berlin.

He worked with the Center for Public Understand-ing of Science, Spectrum, Berlin, and in a start-upcompany in the field of spectroscopy. In 2001, he

joined the Heinrich-Hertz-Institut, Berlin, working mainly on clock recoveryfor optical high bit-rate signals.

Colja Schubert was born in Berlin, Germany, in1973. He received the Dipl.-Phys. and Dr. rer. nat. de-gree in physics from Technische Universitaet Berlinin 1998 and 2004, respectively.

He was an exchange student at Strathclyde Uni-versity, Glasgow, U.K., from 1996 to 1997. Duringhis diploma thesis in 19971998, he worked with theMax-Born-Institute for Nonlinear Optics and ShortPulse Spectroscopy, Berlin. Since 2000, he has beena member of the scientific staff at the FraunhoferInstitute for Telecommunications, Heinrich-Hertz-

Institut, Berlin, doing research on high-speed transmission systems and all-optical signal processing.

Dr. Schubert is a member of the German Physical Society.

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