Int. Conf. on Solid State Devices & Materials (SSDM 2002),Nagoya, Sept. 17-19, 2002, paper G-4-1, pp. 342-343, Invited Talk.

Ultrafast SOA-based SMZ-type All-Optical Regenerators
and Wavelength Convertors
Yoshiyasu Ueno1, 2, Shigeru Nakamura1, and Kazuhito Tajima1
1

Networking Laboratories, NEC Corporation
34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan2
Univ. of Electro-Communications, Dept. of Electronic Eng.
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
Phone: +81-424-43-5807, Fax: +81-424-43-5210, E-mail: y.ueno@ieee.org
1. SMZ-type all-optical switches
Several types of picosecond-to-femtosecond-classall-optical switchings at repetition frequencies from 10
GHz to over 100 GHz have been successfully achieved in
the last 4-5 years with using semiconductor waveguidedevices, in spite of the relatively long carrier relaxation
times (50 ps to 1 ns) of conventional bulk-semiconductormaterials. Such ultrahigh-speed switchings are possible
with specific switch structures, i.e. Symmetric-MachZehnder (SMZ)-type switch structures [1-3], because theslow carrier-relaxation-induced decay components can becompletely cancelled out at an optical interference(between two-split mutually-delayed signal components)
inside these switches. Both the rise and fall times of theirswitching windows can basically be shortened with thewidth of optical input control pulses.
Divided clock
SOA phase shifter

recently been achieved with a divided-clock frequency of
10.5 GHz [4]. In case of regeneration (b) in contrast, eachof the high-frequency data pulses controls the switch,
forms a switching window, and allows only a corresponding clock pulse to go through. Consequently, theclean clock pulses are encoded by the data pulses, i.e.,the data pulses are ‘regenerated.’ In case of wavelength
conversion (c), each of the input data pulses controls theswitch, forms a corresponding switching window, andallows only the CW component within each window togo through. When the window width is optimized, datapulses at the CW wavelength are newly generated.
Holes
Optical pulses

Amplified pulses

Demultiplexed datatSOAfilter
Input data∆t-delayed(a)Input dataSOALocal clock∆t-delayedphase shifter
polfilter(−∆t)-delayed

Regenerated data(b)

Wavelength-converted data

Input dataSOACW

phase shifter∆t-delayed

filter(c)Fig. 1 Three configurations of SMZ-type all-optical switches. (a)original SMZ switch [1] for demultiplexing, (b) PD-SMZ switch[2] for regeneration, and (c) SMZ-DISC [3] for wavelengthconversion.

Figure 1 shows three configurations of SMZ-type
all-optical switches where semiconductor optical amplifiers (SOA’s) are used for significantly reducing theoptical control pulse energy. In case of the all-optical demultiplexing (a), divided-clock pulses control the switch,
form switching windows at the divided-clock frequency,
and extract one TDM channel out of the high-frequency
data pulses. Error-free demultiplexing of 336 Gb/s has

Electrons

Fig. 2 Nonlinear refractive-index changes are induced bystimulated amplification in the SOA’s.

The driving force of these all-optical switchings is thenonlinear refractive-index change inside the SOA’s (Fig.
2). Each time a control pulse is amplified in an SOA, thecarrier density drops almost instantaneously, whichcauses the refractive index to jump up due to thebandfilling effect. As a result of this ultrafast refractiveindex change and an optical interference, the switch
forms a window whose width is determined by a specificdelay time ∆t that is indicated respectively in Fig. 1 [1-3].
2. All-optical regeneration (Optical 3R)
We have achieved penalty-free error-free inputpolarization-insensitive regeneration at 84 Gb/s [5],where each of the 84-Gb/s input data pulses controlled
the SOA inside the PD-SMZ switch and consequentlyencoded each of the synchronized 84-GHz clock pulses.
To date, we have demonstrated two types of regenerative
properties. The one is regeneration in time, due to therectangular-like shape of the switching window [1]; thebit-error-rate tolerance against the input timing jitter hasbeen measured to be approximately 2.3 ps [5]. The otherproperty is regeneration in amplitude due to thesinusoidal transfer function of the interferometer; theintentionally-incorporated amplitude noise in the input[Fig. 3(b)] was removed in the output [Fig. 3(c)]. Themagnitude of the nonlinear phase shift was measured tobe slightly less than π (0.6π). This measurement wasdone by systematically observing the output waveforms
as a function of the interferometer phase bias [Fig. 3(d)].
STREAK 001110.SMP, Nov 13 2000
D38.DAN, Nov 10 2000

30000

STREAK 001110.SMP, Nov 13 2000
D35.DAN, Nov 10 2000

30000

Counts(b)20000Counts(a)
20000

10000

10000

0001000200

Time (ps)100200Time (ps)STREAK 001110.SMP, Nov 13 2000
D41.DAN, Nov 10 2000

30000

(d)

(c)
Counts20000(b), the output eye diagram after demultiplexing to 10.5 Gb/s (c),and the XPM spectrum (temporarily with all-one input pulses).

4. Nonlinear phase shifts at ultrahigh frequencies
One of the central issues in the ultrahigh-frequency
switchings is the decrease in the nonlinear phase shift
with the repetition frequency. We analyzed fundamental
properties of the nonlinear phase shift in the 40-to160-GHz range, in a pump-probe regime and under not
random but regular (periodical) switching conditions in
the first place [7]. We have found that the nonlinear
phase shift decreases almost linearly with the frequency
and also that the phase shift is recovered linearly with theinjection current to the SOA (Fig. 5). Because
conventionally designed SOA’s have been used in theabove-mentioned experiments, the results in Fig. 5 havesuggested that new SOA designs and new materials will
enable us to explore higher switching frequencies.

ΦB= 010000100

ΦB= +0.3π200Time (ps)Fig. 3 All-optical regeneration at 84 Gb/s. (a) Input random
pulses, (b) local clock pulses, (c) output random pulses, and (d)waveforms as a function of the interferometer phase bias.

1.00
0.50(a)0.10

0.6

Signal (a.u.)

0.4
0.3
0.2

0.6

Signal (a.u.)(a)0.5(b)

0.4
0.2

0.50(b)0.10

0.1

3. Wavelength conversion
Error-free wavelength conversion has been achieved
at up to 168 Gb/s [6], which is the highest pseudorandom-switching frequency ever reported to the authors’knowledge. The output pulse width was measured to be
2.0 ps with using an autocorrelator [Fig. 4(b)], whichmatched well with our ∆t design. The nonlinear phaseshift involved in the conversion was measured to beapproximately 0.2π from the output XPM spectrum [Fig.
4(d)] and also from a method similar to that in Fig. 3(d).

Phase shift, ∆Φ/π0Phase shift, ∆Φ/π

1.00

0

1.0

10.0

100.0

Input pulse energy (fJ)

1050

100

Injection current (mA)

Fig. 5 Measured nonlinear phase shifts as functions of the input
pulse energy and the injection current to the SOA. The input pulse
frequencies were 168 GHz (circles), 84 GHz (triangles), and 42
GHz (crosses).

5. Conclusion
We have shown our recent results of the all-opticalregeneration and the wavelength conversion. The SOAinduced nonlinear phase shifts at ultrahigh repetition
frequencies were discussed in details.
Acknowledgments
Part of this work was performed under the management of FESTA supported by NEDO.

0.1
0.0

0.0-500-50+50+0(c)

2.0
1.0
0.0
0204060

Time (ps)+50+10Intensity (dBm)

Optical Power (mW)

3.00

Delay (ps)Delay (ps)80100(d)-10-20-30-40-50
1542

1544

1546

1548

1550

Wavelength (nm)Fig. 4 Wavelength conversion at 168 Gb/s. Autocorrelation trace
for the input random pulses (a), that of the output random pulses

References[1] K. Tajima, Jpn. J. Appl. Phys. 32, L1746 (1993).
[2] K. Tajima et al., Appl. Phys. Lett. 67, 3709 (1995).
[3] Y. Ueno et al., IEEE Photon. Technol. Lett. 10, 346 (1998).
[4] S. Nakamura et al., Optical Fiber Comm. Conf. (OFC 2002),
Anaheim, Postdeadline paper FD3.
[5] Y. Ueno et al., IEEE Photon. Technol. Lett. 13, 469 (2001).
[6] S. Nakamura et al., IEEE Photon. Technol. Lett. 13, 1091
(2001).
[7] Y. Ueno et al., J. Opt. Soc. Am. B, Nov. 2002 issue, in press.

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