27th European Conference on Optical Communication(ECOC 2001), Sept. 30-Oct. 4, 2001, Amsterdam, theNetherlands, paper Th.F.2.1 (Invitede Talk).

Ultrahigh-speed all-optical data regeneration andwavelength conversion for OTDM systems
Y. Ueno, S. Nakamura, J. Sasaki, T. Shimoda, A. Furukawa,T. Tamanuki, T. Sasaki, and K. Tajima
Photonic and Wireless Devices Research Laboratories, NEC Corporation
34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan (y-ueno@cb.jp.nec.com)
Abstract: The latest results in ultrafast data regeneration and wavelength conversion with SMZ-type all-opticalsemiconductor switches are reviewed. In these operations, the SMZ-type switch is driven by random data pulses. The
fastest bit rate reported to date for such input-data-driven all-optical operation has reached 168 Gb/s. With these
all-optical devices, we may be able to build 160-Gb/s OTDM nodes for packet communications systems in the near
future.
O-TDM, Packet

WDM, Point-to-Point(Undersea)(Trunk, Inter-City, Metro)
NODE

LAN
LAN

LAN
NODE
NODE

≥ 160 Gb/s
10 ~ 200 km
NODE

1R

O TDM-WDM
conversion∼10,000 km

NODE

LAN

LAN

Figure 1: OTDM-based mesh networks
160-Gb/s packets
160-Gb/s packets
Paylo ad

In

Asyn.
ga te

HeaderHeaderDrop

Header Recog(XOR)

Buffer(2R∼3R)

MUX

Expand er(Demux)Packet Constr(40 GHz)

Recv(40 GHz)

Buffer(40 GHz)

Add
3R

Out

Clk

Node Control

Figure 2 shows an example of an OTDM-based node
system for packet communications that we may be able tobuild with the above-mentioned SMZ-type devices. An
SMZ all-optical switch inside a packet-expander unit [20]will demultiplex and expand 160-Gb/s OTDM packets topackets at ETDM bit rates. A switch of the same type isalso applicable to 3R regeneration and makes the system
more tolerant to GVD, PMD, XPM, and ASE noise
accumulation. The 3R regenerator will be a key componentof the optical buffer (regenerative buffer), too. The SMZ
switch also works as an asynchronous gate [21] because of
its jitter-tolerence. Furthermore, SMZ-type wavelength
convertors will cross-connect the OTDM signals between
WDM channels.
In this report, we review the latest results in 3R
regeneration, wavelength conversion with SMZ-type
switches, and some of the related topics in this field.
Data pulses (with jitter & noise)

R egenerated data pulses(no jitter, no noise)

Optical clock
to LANCT

from LAN
CL

Figure 2: Example of OTDM-based node system
1. Introduction : OTDM networks beyond 40 Gb/s/ch
Ultrafast optical-time-division-multiplexing (OTDM)
technology is very attractive in that it provides bandwidths
that are not limited by opto-electronic devices, leads tomore flexible TDM packet networks, and reduces thenumber of cross connections required in multi-Tb/s WDM
systems (Fig. 1). A number of essential ultrafast all-optical
functions, including demultiplexing [1-5], wavelength
conversion [6-13], 3R regeneration [13-18], and OTDMWDM conversion [19], have been demonstrated andstudied. All of the all-optical devices in the reports cited
above are based on interferometric semiconductor switchesof the Symmetric-Mach-Zehnder (SMZ) type [1, 2, 7].
These switches form rectangular-like switch windows, inprinciple, with ultrafast response times (rise and fall times).
The response time is not limited by the slow semiconductor
carrier relaxation times, because optical interference insideeach device masks the slow-relaxation-induced opticalcomponents. Some of the works cited above were done ator below 40 Gb/s but the bit rate in research experiments isshifting from 40 Gb/s to the 80-160 Gb/s range [4, 5, 10-13,
15, 18], with the aim of providing devices for use in the
OTDM-based network systems.

Local optical clock source
All-optical switchorUltrafast -locking clock extractor

Input Data Pulses (with jitter)
Switch Windows(opened by input data pulses)
Clock Pulses
Re-timed Data Pulses (no jitter)

Figure 3: All-optical 3R scheme (top) and re-timing
feature required for the switch (bottom)
2. The principle of 3R regeneration and the mechanismof the SMZ all-optical switch
Figure 3 shows how an ideal 3R regenerator (reamplification, re-shaping, and re-timing) functions. The alloptical switch receives data and clock pulses so that thedata pulses all-optically encode the stream of clock pulses.
Suppose the clock pulses have no timing jitter, while theinput data pulses have some jitter. As each data pulse formsa rectangular-like switch window with respect to the clock
pulses, the timing jitter in the input data pulses is removed

(i.e., re-timed). Furthermore, when the transmissivity of theswitch window does not depend on a specific range ofintensity fluctuation of the input data pulses, the intensity
fluctuation is also removed (re-shaped). These are the two
features that are required for all-optical switches used in 3R
regeneration.
84-Gb/s random data

84-Gb/sregenerated d ata

p hase shifter
po lSOA
filter(−∆t)-delayed∆t-delayed
84-GHz clock(a)random d ata
regenerated d ata
SOA p hase sh ifterSOA
filterclock∆t-delayed(b)10.5-GHz
divided clock
SOA p hase sh ifter

10.5-Gb/s
demultiplexed data

pair of matched birefringent crystals (or panda fibers),
polarizers, etc.
Figures 4(b) and 4(c) show the original SMZ switch[1], for comparison. With the input scheme in Fig. 4(b), theswitch works for the 3R regeneration [14]. With the input
scheme in Fig. 4(c), it works for demultiplexing [4]. The
switch window’s width in the SMZ switch is predetermined by the delay time (∆t) between the two
asymmetric input arms in Figs. 4(b) and 4(c).
The nonlinear refractive-index changes inside SMZtype switches for the 3R operation, wavelength conversion,and asynchronous gating must be repeated at the ultrafast
OTDM data rate and this is in contrast to the situation fordemultiplexing. Furthermore, SMZ switches for the 3R
operation, etc. are controlled by random data pulses while
those for demultiplexing are controlled by regular(continuous) clock pulses. Data-pattern-induced effects arethus a potential problem for the SMZ switches for 3R, etc.
The pattern-induced effects are suppressed by accelerating
the carrier relaxation with increasing the input clock-pulse
power [14].

SOA

(c)

0

3 0 00

Optical Power (a.u.)

Figure 4(a) shows a schematic view of one of the
SMZ-type
switches,apolarization-discriminating
Symmetric-Mach-Zehnder (PD-SMZ) all-optical switch [2],and its input scheme for 3R regeneration [15, 18]. The
switch consists of a semiconductor optical amplifier (SOA)
and a pair of polarization-dependent delay lines (PDD).
Each time a data pulse is amplified in the SOA, therefractive index of the SOA’s active layer increases andthen relaxes due to the band-filling effect induced by acarrier-density change. Here, the rise time of the refractiveindex change is as fast as the width of the data pulse while
its relaxation is relatively slow. Each successive clock
pulse is split into two orthogonally-polarized pulses in thefirst PDD. After one of the two-split pulses is given a delay
time ∆t with respect to the other pulse, the two-split pulses
are recombined at the PDD output. The two-split ∆t-spaced
orthogonally-polarized pulses are then amplified by the
SOA and enter the second PDD. In the second PDD, thedelay time between the two-split pulses is compensated. As
a result, the two-split pulses are recombined at the second
PDD’s output and then interfere with each at the 45°
polarizer output. Thus, the pair of PDDs work as a MZI(with respect to clock pulses). The optical phase of this
MZI is initially biased so that the two-split pulses either
destructively or constructively interfere with each other.
When a data pulse co-propagates between the two-split ∆tspaced pulses, the one pulse experiences the nonlinear
refractive-index change while the other pulse does not andconsequently the initial interference condition changes. The
data pulses thus all-optically encode the co-propagating
clock pulses via the PD-SMZ switch. Due to the interference between the two-split pulses, the switch effectively
masks the slow relaxation of the refractive-index and formsa rectangular-like switch window. The switch window’s
width is passively determined by the pulse-propagation
delay time (∆t) between the two delay lines of the PDD’s.
All of the PD-SMZ switches reported previously were notintegrated (on a chip) but built by experimentally
combining an SOA (or a semiconductor waveguide) with a(a)

4 0 00

10 20 30
Time (ps)

2 0 001 0 00

3.0

(b)

2.0
1.0
0.0

0
0

1 00

2 00

0

Time (ps)

20

40

60

80

1 00

Time (ps)Figure 5: Waveforms of 84-Gb/s regenerated data (a)and eye diagram after demultiplexing to 10.5-Gb/s (b)
10 -4

10

3R + Demux
Demux
baseline
10 -6
10

231-1

-8

10

-1 0

10

-1 2

-40

10 -6
10
10

(a)

-4

(b)
Bit Error Rate

Figure 4: (a) PD-SMZ switch for 3R, (b) SMZ switch
for 3R, and (c) SMZ switch for demultiplexing.

5 0 00Intensity (a.u.)∆t-delayed

Bit Error Rate

filter
1 68-Gb/s random d ata

-8

-10

10 -12

-35

-30

-25

Received Power (dBm)

-20

+0

+10

+20

+30

+40

+50

Halfwave-pl ate direct ion (deg)Figure 6: Measured bit-error rates
3. Experimental results in 3R regeneration
Figure 5(a) shows average waveforms, as observedwith a high-speed streak camera, of 84-Gb/s pseudorandom
data pulses regenerated by the PD-SMZ switch in Fig. 4(a)[13, 18]. The extinction ratio for the regenerated pulses was
17 dB, as indicated in the inset. Figure 5(b) shows a typicaleye diagram of regenerated data after demultiplexing from84 to 10.5 Gb/s with the hybrid-integrated SMZ switch inRef. 4. The input powers of the 84-Gb/s 1560-nm data andthe 84-GHz 1545-nm clock pulses coupled to the SOA
were –4 dBm and +2 dBm, respectively. The switchwindow width was set to half of the repetition time (∆t= 6.2
ps). The optical phase of the MZI inside the switch wasbiased so that an input ‘1’ pulse would be inverted to a ‘0’pulse at the output and vice versa (inversion operation).
The solid curve in Fig. 6(a) shows measured bit-error
rates of the renegerated and demultiplexed 10.5-Gb/s datapulses of Fig. 5(b). The length of the pseudorandom pattern
was set to 231-1. For comparison, the dashed curve showsthose of demultiplexed 10.5-Gb/s data pulses without 3R
regeneration, while the dotted curve indicates the 10.5-Gb/sbaseline of our detection system. Although the all-opticaldemultiplexer exhibited a slight power penalty, the penaltyof the 3R regeneration in itself (the difference between thesolid and dashed curves) was negligibly small.
-4

Bit Error Rat e

10

1 0-6

2.3 ps

1 0-8
1 0-101 0-12- 3.0 - 2.0 - 1.0 +0.0 +1.0 +2.0 +3.0

Delay Time (ps)Figure 7: Tolerence to input-timing jitter
NON-UNIFORM INP UT . M
S P , Ap r 42 001
D38. DAN , Nov 102 000

Intensity (a.u.)

3 00 0

UNIF OR M OUTP UT .SM P , Apr 4 2001
D4 1
. DAN ,N ov 10 2000

3 00 0(a)(b)

2 00 02 00 0

1 00 0

1 00 00

0

10 20 30
Time(ps)

non-uniformity was sucessfully removed from theregenerated pulses.
Although the intensity fluctuation in the input 1’s isabsent in Fig. 8(b), we found that our PD-SMZ switch only
partially re-shapes the 84-Gb/s data pulses at present: theintensity fluctuation in the input 0’s is not removed but istransferred to the output 1’s (in the inversion operation).
This is because the nonlinear phase shift of our 3R
regeneration at 84 Gb/s was compressed to below π, in
contrast to the result at 20 Gb/s in Ref. 14. In fact, thenonlinear phase shift in the 84-Gb/s 3R operation wasmeasured as 0.4-0.6π. We interpreted these results in thefollowing way: (a) even when the nonlinear phase shift isless than π, either 0’s or 1’s are re-shaped due to the MZI’ssinusoidal transfer function, as is schematically shown in
Fig. 9. (b) When the nonlinear phase shift reaches π, both
0’s and 1’s are re-shaped. According to these results, wesuggest that the re-shaping properties of any SMZ switch
should be investigated separately for 0’s and 1’s.
Measuring the nonlinear phase shift helps in physicallyunderstanding how each pulse is re-shaped.
The data and clock pulses in all of the above 3R
experiments were generated by using a pair of synchronized pulse sources. In the next section, we address
potential techniques for generating the optical clock in areal 3R system.

0
0

10 0

20 0

0

Time (ps)

10 0Time (ps)Transmissivity

Figure 8: Re-shaping capability of the PD-SMZ switch
sinusoidal transfer function

Output (uniform)

Phase
Input (non-uniform)

Figure 9: Mechanism of the re-shaping shown in Fig. 8
It should be noted here that the PD-SMZ switch inFig. 4(a) is, in principle, polarization-insensitive to the
input data pulses when a polarization-insensitive SOA isused. Our experiment proved this: the measured bit-error
rate depended very little on the polarization state of the
input data pulses [Fig. 6(b)]. Here, half-wave and quarterwave plates were used to rotate the polarization state of the
84-Gb/s input data pulses around the Poincare sphere.
Figure 7 indicates the re-timing capability of the PDSMZ switch, where the bit-error rate was measured as afunction of the relative delay time between the data andclock pulses. The bit error rates remained below 10-9 evenwhen the data pulses were delayed by 2.3 ps. This amountof jitter tolerence is reasonable with the switch window
width (∆t= 6 ps) and the data pulse width (2.0 ps), because
the rise and fall times of the ‘rectangular-like’ switch
window are close to the data-pulse width.
Figure 8 indicates the re-shaping capability of the PDSMZ switch. Here, 84-Gb/s pseudorandom input datapulses having non-uniform peak intensities were intentionally generated [Fig. 8(a)]. As indicated in Fig. 8(b), the

20 0

4. Clock pulse generation for 3R systems
To date, synchronization and clock-extraction
techniques of many types have been proposed anddemonstrated for use with ultrafast optical signals. Here,
we introduce a few of them, divided into two alternative
strategies.
When the packets are relatively short, we may be able
to use a local clock source and operate all of the nodefunctions asynchronously (Fig. 2). At the node input, an
SMZ switch having a specific jitter tolerence works as an
asynchronous gate. Via this asynchronous gate, the local
clock pulses are all-optically encoded by the asynchronous
input data packets. This works when the data frequency isclose enough to the clock frequency and the packet is shortenough [21]. Thus, the asynchronous gate converts the
input data packets from asynchronous to synchronous form
and then passes them to other units or to neighbour systems,
without having to extract clocks from the packets.
When the packets are relatively long, we may be able
to use an all-optical clock extractor to extract optical clock
pulses very quickly from ultrafast packets. One promisingtechnique for ultrafast clock extraction is the application ofself-pulsation lasers: locking times within some tens of bits
have recently been reported [22]. Other possible clock
extractors are those with SOA-based ring structures, where
an SOA works as a nonlinear element [23, 24]. The locking
time can be reduced dramatically by integrating each ring
structure into a small chip and thus reducing the pulse
circulation time. An SMZ-type switch is used in one of theabove reports [24], where the extracted clock-pulse width isstabilized by the pre-determined delay time ∆t of the switch.
5. Wavelength conversion
With SMZ-type switches, as implied above, thewavelength of the data pulses is converted to that of theclock pulses each time they are regenerated. Thus, one canconvert the wavelength at the 3R unit and the asynchronous
gate unit in designing the node in Fig. 2.

Separately, a very compact wavelength convertor, thedelayed-interference signal-wavelength convertor (DISC[7]) in Fig. 10, has been exhibiting ultrafast performance.
The mechanism of this wavelength convertor is similar tothat of the original SMZ switch. The optical interferencemasks the carrier-induced relaxation time of the SOA. The
width of the all-optically-formed switch window isdetermined by a delay time ∆t between the two arms of theasymmetric MZI. With a pseudorandom input, it slices the
co-propagating continuous-wave (CW) light and consequently generates wavelength-converted pseudorandom
data pulses.

6. Technical issues
The decrease in the nonlinear phase shift inside the
SOA with the switch-repetition rate is one of the issues inboth 3R regeneration and wavelength conversion, although
it has not been discussed enough in the literature. We
believe that we can improve the nonlinear phase shift at
160 Gb/s and at even higher rates by re-designing the SOA
for all-optical purposes. The pulse-width tunability,
wavelength tunability, and timing jitter of current shortpulse sources are other technical issues for research into
these ultrafast devices. Advances in these technical areas
will be a further drive for the research activity in this field.

168-Gb/s
random data

168-Gb/s random data

7. Conclusion

phase shifter
SOA

CW∆t-delayed

We have reported on the latest results in 3R andwavelength conversion. The successful 168-Gb/s datapulse-driven random operation of an SMZ-type all-opticalswitch (a wavelength convertor) has suggested that we areapproaching the possibility of building 160-Gb/s node
systems that contain 3R regenerators, asynchronous gates,
and wavelength convertors.
This work was performed partially under the
management of the Femtosecond Technology Research
Association (FESTA) supported by the New Energy andIndustrial Technology Development Organization (NEDO).

filterFigure 10: The DISC wavelength converter

(a)

6000
4000
2000

In te nsit y (m W)

Intensity (a.u.)

8000

(b)

3.0
2.0
1.0
0.0

0

0

0

10

20

25

50

75

T ime (ps )30Time (ps)Figure 11: Average waveform of the 168-Gb/s
wavelength-converted data (a) and the eye diagram
after demultiplexed to 10.5-Gb/s (b)-4PRBS 27-1

log(BER)-5
168-Gbps
λ-conversion

-6
-7
-8
-9 10. 5-Gbps-10 baseline
168-G bps
-11
DEMUX
-12
-40
-35
-30

-25

-20

Received Power (dBm)

Figure 12: Measured bit-error rates in the 168-Gb/s
wavelength-conversion process
Figure 11(a) shows an averaged waveform of the 168Gb/s pseudorandom data pulses after wavelength
conversion from 1564 to 1547 nm [13]. The input powersof the 168-Gb/s data pulses and the CW light were +7 dBm
and +13 dBm, respectively. The output pulses’ extinction
ratio was 13 dB. The input and output data-pulse widths
were measured with an autocorrelator as 1.8 and 2.0 ps,
respectively (the output pulse width matched well with ∆t).
Nevertheless, the nonlinear phase shift involved in thiswavelength conversion was less than that seen in the PDSMZ switch during 84-Gb/s 3R operation.
Figure 11(b) shows a clear eye opening in the
wavelength-converted and demultiplexed 10.5-Gb/s datapulses. Under these operating conditions, error-freewavelength conversion at 168 Gb/s was successfully
achieved (Fig. 12). To the authors’ knowledge, this was thefirst error-free operation of any all-optical switch driven byrandom data pulses at 160 Gb/s.

100

References
/1/
/2/
/3/

K. Tajima, Jpn. J. Appl. Phys. 32 (1993) L1746.
K. Tajima et al., Appl. Phys. Lett. 67 (1995) 3709.
S. Nakamura et al., IEEE Photonics Technol. Lett. 10(1998) 1575.
/4/ S. Nakamura et al., IEEE Photonics Technol. Lett. 12(2000) 425.
/5/ R. Ludwig et al., Electron. Lett. 36 (2000) 1405.
/6/ B. Mikkelsen et al., Electron. Lett. 33 (1997) 2137.
/7/ Y. Ueno et al., IEEE Photonics Technol. Lett. 10(1998) 346.
/8/ Y. Ueno et al., Opt. Lett. 23 (1998) 1846.
/9/ Y. Ueno et al., Jpn. J. Appl. Phys. 38 (1999) L1243.
/10/ Y. Ueno et al., Jpn. J. Appl. Phys. 39 (2000) L806.
/11/ J. Leuthold et al., Electron. Lett. 36 (2000) 1129.
/12/ Y. Ueno et al., ECOC 2000, Munich, pp. 13-15.
/13/ S. Nakamura et al., Optical Amplifiers and their
Applications, July 9-12, 2000, Quebec, PD4-1.
/14/ L. Billes et al., ECOC ’97, Edinburgh, Vol. 2, 269.
/15/ A.E. Kelly et al., Electron. Lett. 35 (1999) 1477.
/16/ D. Wolfson et al., IEEE Photonics Technol. Lett. 12(2000) 332.
/17/ St. Fisher et al., IEEE Photonics Technol. Lett. 12(2000) 335.
/18/ Y. Ueno et al., IEEE Photonics Technol. Lett. 13(2001) 469.
/19/ St. Fisher et al., IEEE Photonics Technol. Lett. 11(1999) 1262.
/20/ A. Hasegawa and H. Toda, IEICE Trans. Commun.
E81-B (1998) 1681.
/21/ D. Cotter and A. Ellis, J. Lightwave Technol. 16(1998) 2068.
/22/ B. Sartorius, OFC 2001, Anaheim, MB7.
/23/ K. Vlachos et al., OFC 2000, Baltimore, ThP2.
/24/ Y. Ueno et al., Jpn. J. Appl. Phys. 39 (2000) L803.