IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 5, MAY 2001

469

Penalty-Free Error-Free All-Optical Data
Pulse Regeneration at 84 Gb/s by Using a
Symmetric-Mach–Zehnder-Type Semiconductor
Regenerator
Yoshiyasu Ueno, Shigeru Nakamura, and Kazuhito Tajima

Abstract—Penalty-free data-pulse regeneration at 84 Gb/s wasachieved down to an error rate level of 1 10 11 by using a data
pattern length of 231 1. A symmetric-Mach–Zehnder-type all-optical polarization-insensitive semiconductor regenerator was used.
Index Terms—3R, all-optical, interferometric,
Mach–Zehnder, regeneration, semiconductor, ultrafast.

jitter,I. INTRODUCTIONULTRAFAST optical time-division multiplexing (OTDM)
technology, whose bandwidth is not limited by thatof electronic devices, is attractive because it leads to more
flexible packet networks. Several types of all-optical devices, such as demultiplexers [1]–[5], wavelength convertors[6]–[10], and 3R regenerators [10]–[12], have been proposedand demonstrated. All of these devices in these reports arebased on the symmetric-Mach–Zehnder (SMZ)-type interferometric semiconductor switch structure and its mechanism [1],which enables these devices to form a rectangular-like switch
window with an ultrafast response time (rise and fall times).
The response time is not limited by the slow semiconductor
carrier lifetime because the interference cancels out carrier-recovery-induced components. Error-free demultiplexing from
168 to 10 Gb/s has been successfully demonstrated [5].
For 3R-renegeration or wavelength-conversion of ultrahigh
bit-rate OTDM data pulses at over 40 Gb/s, not only the impulseresponse but also the device’s repetition rate must be ultrafast.
In addition, the devices must follow input data patterns. To date,the SMZ mechanism has been proved to work at repetition rates
up to 168 Gb/s [8]–[10], where an ultranarrow switch window
width of 1.5 ps was still observed. The input-data pattern-induced effect is suppressed either by the input clock pulses [10],[11] or by the input continuous-wave light [6], [9].
The 3R regeneration (reamplifying, reshaping, and retiming)is an important function for realizing OTDM systems because of
their shorter time frame and their larger pulse-quality degradaManuscript received October 25, 2000; revised February 8, 2001. This work
was supported by the New Energy and Industrial Technology Development Organization, under the management of the Femtosecond Technology ResearchAssociation.
The authors are with the Photonic and Wireless Devices Research Laboratories, NEC Corporation, Tsukuba, Ibaraki 305-8501, Japan (e-mail:y.ueno@ieee.org).
Publisher Item Identifier S 1041-1135(01)03745-4.

Fig. 1. SMZ-type all-optical device structures: (a) 3R regenerator schemewith SMZ structure, (b) demultiplexer scheme with SMZ structure, and (c) 3R
regenerator scheme with PD-SMZ structure.

tion due to the nonlinear effects in silica fibers. Recently, all-optical 3R regeneration at up to 84 Gb/s, where the regenerators
exhibit power penalties of 3–4 dB, has been demonstrated [10],[12]. In this work, we reduced the penalty at 84 Gb/s to almost
with a data pattern length of 2.
zero at an error rate of 10II. DEVICE STRUCTURE
Fig. 1(a) shows a schematic structure of the SMZ-type 3Rregenerator [11]. Only the input scheme is different, but thestructure is exactly the same as the SMZ demultiplexer [seeFig. 1(b)] used in [1], [4], and [5]. Each time a data pulse triggers the regenerator, the regenerator all-optically forms a rectangular switch window and passes a copropagating clock pulsethat coincides with the window (Fig. 2). Thus, input data pulses
are replaced with retimed clean data pulses.
In this work, we used an optional SMZ structure, that is,a polarization-discrimination SMZ (PD-SMZ) structure [seeFig. 1(c)] that was originally proposed in [2] and has been usedas both demultiplexers [2], [3] and regenerators [10], [12].
This structure needs only one semiconductor optical amplifier(SOA) and its interference is inherently stable (because of thecoaxial pulse propagation). The SMZ mechanism works forthis device as well [2]. Each data pulse induces a nonlinear

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 5, MAY 2001

Fig. 2.

All-optical 3R regeneration.

Fig. 3.

Experimental setup.

refractive-index change in the SOA. Each clock pulse is split
into two components in the first MZ interferometer (MZI)
and passes through the SOA. When the two components arerecombined at the second MZI output, they interfered either
destructively or constructively depending on if the clock pulse
arrives with a data pulse or not. In this work, the two MZIs
were built with calcite crystals and other classical opticalcomponents [3], [10] and lens-coupled to the pigtailed fibersof a polarization-insensitive SOA module. The birefringencyof the two calcite crystals (6.2 ps) determines the width of therectangular switch window. A Babinet–Soleil phase shifter
for adjusting the interference phase bias was inserted into thesecond MZI.
III. EXPERIMENTAL SETUP
Fig. 3 shows our experimental setup. The 84-Gb/s 1560-nm
pseudorandom data pulses were generated with a mode-locked
fiber ring laser (Pritel Inc.) synchronized to a 10.5-GHz external RF clock, a 10-GHz lithium niobate modulator, and threestages of fiber multiplexers. The base frequency of 10.5 GHz
was chosen so that our 82-MHz streak camera was subharmonically synchronized. The 84-GHz 1545-nm clock pulses weregenerated with another synchronously mode-locked fiber laser
and multiplexers.
,The pattern length of the 10.5-Gb/s data was set to 2
where each pattern includes a 30-consecutive-“0” pattern. Whenthe delay times in multiplexing the 10.5-Gb/s data to 84 Gb/sdata are too short (as in [10]) to separate the consecutive-“0”patterns from each other, the multiplexed 84-Gb/s data includes
even 200-consecutive-“0” patterns. Such patterns can induce toolarge pattern-induced effects for the clock pulses to suppress,
with temporarily increasing the average carrier density in theSOA. For separating (decorrelating) the 30-consecutive-“0” patterns at 84 Gb/s, we used delay times much longer than thoseFig. 4. 84-Gb/s data pulses at the input: (a) averaged image with a streak
camera and (b) autocorrelation trace.

in the previous reports. The delay times (fiber lengths) for thethree multiplexer stages were chosen to be 25, 50, and 100 ns (5,10, and 20 m). The minimum delay time was still much shorter
than the 10.5-GHz pattern length (0.2 s) but was long enough toseparate the 2.9-ns-long 30-consecutive-“0” patterns from eachother.
Despite the long delay fibers inside the multiplexers, we
obtained stable 84-Gb/s data pulses with uniform peak heights,
uniform spacings, and a short pulsewidth (see Fig. 4). The
center peak in Fig. 4(b) is two times higher than the otherpeaks because of the mark ratio of 50% of the random pulses.
The pulsewidths of the data and clock pulses measured atthe SOA input with an autocorrelator were 2.1 ps and 2.8 ps,respectively.
The average powers of the data and clock pulses coupled tothe SOA chip inside the regenerator were 4 dBm (9 fJ/pulse)and 2 dBm, respectively. For measuring the error rate of theregenerated 84-Gb/s pulses, a hybrid-integrated SMZ all-opticaldemultiplexer in Fig. 1(b) [5] and a commercial 10-GHz receiver system were used.
IV. RESULTS AND DISCUSSION
Fig. 5(a) shows the regenerated 84-Gb/s waveforms observedwith a streak camera. As seen in the inset, the typical extinction
ratio of the regenerated pulses was larger than 17 dB. As shownin Fig. 5(b), 30-GHz sampling after the all-optical demultiplication indicates a clear eye opening.
Fig. 6(a) shows the error rate of the regenerated-and-demultiplexed 10.5-Gb/s data pulses as a function of the delay time forthe 84-Gb/s data pulses with respect to the 84-GHz clock pulses.
A timing jitter of 1 ps ( 8% of the 84-GHz time frame) increases the error rate by a factor of only 10. This jitter-tolerence
will be improved by optimizing the switch window shape.

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471

data pulses without regeneration, as a reference (the dottedcurve shows the baseline of our 10-GHz detection system).
As shown, the power penalty of the 3R regeneration alone is
negligibly small down to an error rate of 10 . We attribute
this improvement in the power penalty to the above-mentioned data multiplexer. These results indicate that the opticalsignal-to-noise ratio of the regenerated 84-Gb/s data pulses
is as high as that of the input
with a pattern length of 2clock pulses.
V. SUMMARY
We have achieved penalty-free error-free data-pulse regeneration at 84 Gb/s, for the first time, with a data pattern length of, where an all-optical inherently polarization-insensitive2
3R regenerator with a PD-SMZ structure was used. Tolerence
to the input timing jitter was also discussed and clearly demonstrated.
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Fig. 5. Regenerated data pulses: (a) streak camera images of the regenerated
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Fig. 6.

Error rates of the regenerated pulses.

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