Optics Communications 282 (2009) 1728–1733

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Optics Communications
journal homepage: www.elsevier.com/locate/optcom

200-Gb/s wavelength conversion using a delayed-interference all-opticalsemiconductor gate assisted by nonlinear polarization rotation
Jun Sakaguchi 1, Takehiro Nishida, Yoshiyasu Ueno *University of Electro-Communications, Department of Electronic Engineering, 1-5-1, Chofugaoka, Chofu, Tokyo 182-8585, Japana r t i c l e

i n f oArticle history:
Received 14 August 2008
Received in revised form 6 January 2009
Accepted 6 January 2009

Keywords:
All-optical signal processing
Wavelength conversion
Delayed-interference signal-wavelength
converter (DISC)
Semiconductor optical ampliﬁer (SOA)
Nonlinear polarization rotation
Pattern-induced intensity ﬂuctuationa b s t r a c t
The pattern-induced intensity ﬂuctuation (PIF) of output signals with a bit-rate above 160 Gb/s has been
one of the major issues regarding all-optical semiconductor gates. We have demonstrated that the nonlinear polarization rotation (NPR) in the semiconductor optical ampliﬁer (SOA) plays a signiﬁcant role in
the high-frequency operation of a delayed-interference signal-wavelength converter (DISC). We did thisusing a cross-correlation system whose temporal resolution is 1.5 ps which was developed to monitor
our 200-Gb/s, 4992-bit-long binary-patterned waveforms. When we experimentally optimized the NPR
effect inside our DISC specially for our 200-Gb/s wavelength conversion, the PIF was signiﬁcantly
improved (from 5.0 to 1.5, for example). Our systematically measured dependence of the PIF on the polarization settings was qualitatively explained with the new gate model that we developed earlier in thiswork.
Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction
Ultrafast all-optical signal processors based on cross-phasemodulation in semiconductor optical ampliﬁers (SOAs) continue
to draw many researchers’ interest as promising key elements infuture network systems [1,2]. In particular, signal-wavelength converters are expected to enable ultrafast and low-power-consumption optical cross-connects for use in future optical time divisionmultiplexing-wavelength division multiplexing (OTDM-WDM)
networks. Some research groups have demonstrated that errorfree wavelength conversion in the frequency range of 160–320
Gb/s is possible using delayed-interference signal-wavelength converters (DISCs) [3–5]. Consequently, a fundamental understandingof the high-frequency gate operation is important.
One major issue concerning SOA-based optical gates including
DISCs is pattern-induced intensity ﬂuctuation (PIF). It is widely
recognized that the output signal of an SOA gate often suffers from
PIF, because the carrier recovery rate in the SOA is relatively slow
compared to the operation frequency. So far only two methods tosuppress PIF have been reported. One method is to inject a strong
holding beam into the SOA [6], which accelerates the carrier recovery through stimulated recombinations; this method contributed* Corresponding author. Tel./fax: +81 424435807.
E-mail address: ueno@ee.uec.ac.jp (Y. Ueno).
1
Present address: Nara Institute of Science and Technology, Ikoma 630-0192,
Japan.
0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2009.01.028

to the ﬁrst demonstration of 160-Gb/s operation [3]. The othermethod is to detune the bandpass ﬁlter (BPF) after the SOA [7–9]
and utilize ultrafast chirp dynamics in SOAs; this method was usedin the subsequent 160—320-Gb/s experiments [4,5].
Actually, those two are not the only methods to reduce the PIF
signiﬁcantly. One method we report in this paper is to use nonlinear polarization rotation (NPR) in the SOA. NPR has been characterized since 1999 [10–12] and is often used for very low-frequency(2:5—10 Gb/s) all-optical gates in combination with polarizers[13–18]. Researchers have performed 40-Gb/s gating using NPR
and a polarizer combined with a detuned ﬁlter [19] or a special
high-speed SOA [20]. However, possible contributions from the
NPR combined with the well known DISC structure have not beenstudied, to the best of our knowledge. Neither has any experimental study with ultra-high bit-rate signals above 100 Gb/s beenreported.
In this paper, we demonstrate that the NPR in the SOA plays asigniﬁcant role in high-frequency DISC operation, and then propose
a design option for suppressing the PIF in such all-optical semiconductor gates. We have performed 200-Gb/s wavelength conversion
using a DISC, and investigated how the polarization states of thecontinuous-wave (cw) light before and after the SOA affect the output PIF through cross-correlation measurements. Systematic results show that we can exploit the NPR to suppress PIF. We
speculate that this effect would have possibly contributed to theprevious experiments using 160- and 320-Gb/s signals in combination with the other methods.

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J. Sakaguchi et al. / Optics Communications 282 (2009) 1728–1733
TE

2. Model of the DISC operation and the NPR effects

dnr
Dnc ðtÞ þ UTE
eq
dn
TM
dn
UTM ðtÞ ¼ k0 CL r Dnc ðtÞ þ UTM
eq ;dn

UTE ðtÞ ¼ k0 CL

In this section, we ﬁrst describe our model of the DISC operation
including NPR effect. Then we show through a numerical simulation that the NPR can be used to suppress PIF.
The original scheme of the DISC gate is shown in Fig. 1a. It consists of one SOA, one Mach–Zehnder interferometer (MZI), and aband-pass ﬁlter. The MZI in a practical DISC gate often consistsof a set of polarization-control devices. Fig. 1b is a schematic of
the DISC gate used in this study, which contains the polarizationbased MZI and polarization controllers Q 1 and H1 placed
before it. A rate equation for the carrier density in the SOA active
region,dg IOP nc ðtÞ
nc ðtÞ ¼ T 
dt
qV
sc
"
#
2
 jE
1
jECW j2
pulse ðtÞjGðnc ðtÞÞ  1;þV
hmpulsehmcw

ð4Þ

respectively. Here, k0 ¼ 2pmcw =c is the wave number of the light in
TE=TM=dn represents the nonlinear index changes for
vacuum and dnr
represents the steady-state anisotropic
TE/TM components. UTE=TMeq
phases for TE/TM components. Since the relative phase of the TE
and TM components after passing through the SOA changes dynamically with t, the polarization state of the combined light rotatesaccordingly. This polarization rotation results in a dynamic change
in the transmittance of the polarizer P1 , and can be used to compensate for the patterning in GðtÞ and UðtÞ if we adjust the angles of theTable 1
Parameter values used in the model calculation.

ð1Þ

has been used for accurate analysis of the DISC output waveforms[2,21]. Here, nc ðtÞ is the average nc ðz; tÞ over the active region
length, nc ðz; tÞ is the carrier density which contributes to the SOA
gain, IOP is the injection current, gT is the quantum conversion
efﬁciency, sc is the carrier lifetime, V is the active region volume,
and Epulse ðtÞ and ECW are the amplitudes of the control pulse andthe cw optical input, respectively. The ﬁrst, second, and third termson the right-hand side of Eq. (1), respectively, represent the carrier
injection, carrier relaxation, and stimulated recombinations. The
chip gain of the SOA is expressed asdgGðnc ðtÞÞ ¼ exp CLeffnc ðtÞ :dn

ð2Þ

Here, C is the conﬁnement factor, Leff is the effective length of theactive region, and dg=dn is the differential gain of the active region.
The injection of the control pulse Epulse ðtÞ instantaneously decreases
the carrier density by Dnc , induces an ultrafast refractive index
change of the active region, and causes cross-phase modulation to
the cw light.
We then extended our previous DISC model to take the NPR into
account. The NPR in the SOA has been primarily attributed to theanisotropy of the refractive index [10–12]. To express it in a simple
way, we assumed that the TE and TM components of the input cw
light were affected by anisotropic phase shifts:

SOA parameter

Description

Value

Iop
Leff
V

Injection current
Active region length (effective)
Active region volume
Conﬁnement factor
SOA carrier lifetime
SOA differential gain
Quantum conversion efﬁciency

200 mA
1100 lm
5:2  1011 (cm 3)
0.2
75 ps
6:36  1016 ðcm2 Þ
0.34

Refractive index change (TE)

4:0  1020 ðcm3 Þ

Refractive index change (TM)

3:0  1020 ðcm3 Þ

Gate parameter

Description

Value

f
T FWHM
EpulseP cw  jEcw j2
Dt

Operation frequency
Input pulse width (FWHM)
Input pulse energy
Input cw power
Delay time in the MZIFrequency of cw light
Frequency of control pulses
PRBS pattern length
MZI bias

200 GHz1.0 ps
30 fJ
5 mW
1.0 ps
193.7 THz
192.8 THz
29  1
1:0p

C

sC
dg=dn

gT
TE

dnr
dn

TM
dnr

dn

mcw
mpulseDUB
Derived parameter

Description

Value

G0

Unsaturated SOA gain
Effective recovery time

37 dB
10 ps

seff

1.0
TE polarization
TM polarization

cw input

ΔΦB

λ2

SOA

control pulse
λ1

Δt

MZI
Δt
MZI (ΔΦB )bcw input

BPF

Wavelengthconverted
outputλ2

inputpolarization

θ in

BPF

Q1 H1 P1 C1 Q2 P2
SOA

control pulse
λ1

Nonlinear phase shift (ΔΦ/π)

a

λ2

ð3Þ

0.1

cw input: 2.3 mW,
λ2=1540 nm
200-GHz pulse :
λ1=1560 nm
SOA injection current
IOP = 300 mA

0.01

Outputθ Q1 θ H1

λ2

Fig. 1. Schematics of the DISC gate. (a) original DISC scheme and (b) polarizationbased DISC scheme in this paper. Q: quarter-waveplate, H: half-waveplate, C:
calcite crystal, P: polarizer, BPF: band-pass ﬁlter.

0.001
0.01

0.1

1

10

Pulse energy into chip (fJ)
Fig. 2. Measured dependence of the nonlinear phase shift in our SOA for TE/TMpolarized cw light on the input pulse energy.

1730

J. Sakaguchi et al. / Optics Communications 282 (2009) 1728–1733

Intensity (arb. unit)

a

b
4

0.04

2

0.02

0.00

0
1

2

3

1

4

2

Time (ps)c34Time (ps)

30

10

PIF

θ in

0o

30o
45o
60o
90o

3

1
0

15

30

45

60

75

90

Waveplate rotation angle, Δθ H1 (deg)Fig. 3. Calculated DISC output eye diagrams and PIF dependence on the waveplate angle detuning. (a) hin ¼ 0 , DhH1 ¼ 0 ; (b) hin ¼ 60 , DhH1 ¼ 40 ; (c) PIF dependences.

quarter waveplate Q 1 and the half-waveplate H1 adequately. Using
Jones vector analysis, we deduced the following expressions for the
DISC output power Pout ðtÞ:Pout ðtÞ ¼ jEout ðtÞj2 ;ð5ÞTM
Eout ðtÞ  ETEout ðtÞ cosðhin ÞA þ Eout ðtÞ sinðhin ÞB;

ð6Þ
hpﬃﬃﬃﬃﬃﬃﬃﬃﬃ
n
o
TE=TM
ðt Þ þ DUB
GðtÞ exp i U
2noi
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
þ Gðt  DtÞ exp iUTE=TM ðt  DtÞ ;
ð7Þ


 
A
cosðp=4Þ
;ð8Þ
 M R ðhQ 1 ÞM Q M R ðhQ 1  hH1 ÞMH MR ðhH1 Þ
Bsinðp=4Þ






1 0
1 0
cosðhÞ  sinðhÞ; MQ ¼; MH ¼:
M R ðhÞ 
0 i
0 1sinðhÞ cosðhÞ

Ecw
ðtÞ 
ETE=TMoutð9Þ
Here, hin is the input polarization angle of the cw light into the SOA,hQ 1 is the rotation angle of Q 1 , and hH1 is that of H1 . Dt and DUB are
the delay time and phase bias of the MZI, respectively. The expresTE=TM
ðtÞ are similar to the consions of the amplitude waveforms Eout
ventional expression of the DISC output amplitude.
Using these equations and pseudorandom data inputs, we calculated the DISC output waveforms and their PIFs for several hin
values. Parameter values are summarized in Table 1. To use arealistic value for the refractive index anisotropy, we carefully
characterized polarization-dependent nonlinear phase shifts for
our SOA sample using a method from [2]. Fig. 2 shows one seriesof systematically-measured result on the polarization-dependent
nonlinear phase shifts. After several measurements, we estimatedthe refractive index anisotropy for our SOA to be 15—25%, and assumed 25% anisotropy in the model calculation. For each hin value,
we determined the starting values of hQ 1 and hH1 using only cw in2
TM
Thus UTE
eq and Ueq affect the starting values of hQ 1 and hH 1 only. They are notessential in the following PIF-suppression scheme.

put light as follows: we adjusted hQ 1 to eliminate the ellipticity of.2the cw light, which was caused by the steady-state phases UTE=TMeq
hH1 was chosen so that the polarization axis of the cw light would
coincide with the axis of the polarizer P1 . Then we injected data inputs and calculated the output waveform. Fig. 3a shows an exampleof the calculated eye diagram in such a case (hin ¼ 0 ); a large PIF
was observed due to the long effective carrier recovery time
(seff ¼ 10 ps). In this section, we evaluated the PIF using the intensity
ratio of the largest and smallest output pulses, and it was 7.3 for thecase of Fig. 3a. After that, we systematically detuned hH1 from thestarting value. hQ 1 was kept unchanged, so that the transmittanceof P1 shifts from maximum to minimum as the detuning DhH1 approaches 45°. Then the PIF was suppressed in some cases, as shownin Fig. 3b. Fig. 3c shows the calculated PIF dependence on DhH1 . Whenhin was set to 0° (=TE) or 90° (=TM), the PIF was equal to the standard
value (=7.3). In contrast, the PIF values for other hin settings showedvarious dependences on DhH1 . The dependences had a notable featurein common: the PIF values were highly suppressed when DhH1 wasslightly below 45°. These particular examples show the possibilityof an NPR-based PIF suppression method.
3. Waveform-monitoring setup
To demonstrate the signiﬁcance of NPR in high-frequency DISC
operation, we performed a 200-Gb/s wavelength conversion experiment and systematically investigated how the polarization settings before and after the SOA affect the output PIF. We
developed a high-resolution, long bit pattern waveform-monitoring system for this purpose because we wanted to evaluate the
PIF for a data pattern with certain reality. Fig. 4a gives an overview
of the whole setup; 12.48 GHz, 1.6-ps soliton pulses from a modelocked ﬁber laser (Pritel Inc., UOC-3) were adiabatically compressed in a dispersion decreasing ﬁber to 300 fs, and received
intensity modulation with a 312-bit data pattern. This pattern
was initially determined through a Monte Carlo method. After

1731

J. Sakaguchi et al. / Optics Communications 282 (2009) 1728–1733abit-pattern signal
MLFL

12.48-GHz
synthesizer

DDF

Synch. RF

PPG

OTDM
MUX
LN modλ2=1540 nm

DISC gate
PC 9 nm

BPF

DFBLD
sampling pulse
generatorb

200 Gb/s, 800 fs
λ1=1558 nm

1:16

1.6 ps40 MHzpulses, 700 fs

Power
meter

Power
meter

input polarization

cw+200G
input

9 nm

PC

cross
correlator

MZI

θ in

BPF

Q1 H 1 P 1 C 1 Q2 P 2
PBS

Power
meter

SOA
chip
PM-DSFθ Q1 θ H1

Δt= 1 ps 9 nm
ΔΦB = 1.0 π

Fig. 4. Experimental setup. (a) Overview, (b) DISC gate based on an SOA chip. PPG: programmable pattern generator, MLFL: mode-locked ﬁber laser, DDF: dispersion
decreasing ﬁber. PC: polarization controller, PBS: polarization beam splitter, PM-DSF: polarization-maintaining dispersion-shifted ﬁber.

the modulation, the pulses were multiplexed up to 199.7 Gb/s
through four-stage MUXs, received a dispersion compensation,
and were used for the gate input signal (width = 800 fs, patternlength = 4992 bits, k1 ¼ 1558 nm). In parallel to that, we generated
an ultrafast (700 fs), low-frequency (40 MHz), and high extinction
ratio (P27 dB) pulse train for the cross-correlator (Femtochrome
Research Inc., FR-103XR) reference input. This pulse train was synchronized to the 200-Gb/s, 4992-bit data input with a low timing
jitter, and consequently enabled cross-correlation measurementsof the patterned waveform with a time resolution of about
1.5 ps. Details of this pulse generator are explained in [22]. We
constructed the DISC gate using a custom-ordered SOA chip, asshown in Fig. 4b. The structure of this SOA chip was designed tobe the same as that of the commercially available SOA series. To
accurately control the polarization state of the cw light at the
SOA input facet, we used a polarization-maintaining dispersionshifted lensed ﬁber for the signal input. Monitoring of the ASE
intensities on the input and output sides of the SOA enabled on-

Table 2
Summary of experimental conditions.
Gate parameters

Description

Value

f
T FWHM
EpulseP cw
Dtkcw
kpulseDUB

Operation frequency
Input pulse width (FWHM)
Input pulse energy (chip)
Input cw power (chip)
Delay time in the MZIWavelength of cw light
Wavelength of control pulses
MZI bias199.7 GHz
0.8 ps
7 fJ
2.5 mW
1.0 ps1540 nm1558 nm
1.0p

SOA characteristics

Description

Value

Iop
P op
G0

Injection current
DC power consumption
Unsaturated SOA gain
SOA carrier lifetime
Saturation energy for pulse

300 mA580 mW
33.6 dB
42 ps
400 fJsC
Esatline coupling adjustments. The MZI was composed of cascadedpolarization controllers (Optoquest Inc.), as explained in [21]. The
operation parameters are summarized in Table 2. The nonlinear
phase shift induced by each control pulse was estimated to be
0:25p for TE component and 0:20p for TM component, respectively, from the separately-measured result in Fig. 2.
4. PIF measurement results
We monitored the input and output waveforms of the DISC gateusing the cross-correlation method. Since the time span of thecross-correlator was limited to about 180 ps, we could monitor
only a short segment of the 4992-bit-long waveform with one
scan. Fig. 5a shows two particular segments of the input waveform.
We could monitor another segment in another scan, by shifting the
12.48-Gb/s encoding pattern properly. The two examples in theﬁgure show a ‘0-dominant’ part (‘0000100000000001’) and ‘1dominant’ part (‘11101011111111’), respectively. As shown in thisﬁgure, the longest interval between two ‘1’ bits and that for two ‘0’
bits was 55 and 45 ps, respectively. Note that the 27  1-bit pseudorandom data pattern used in the past 160—320 Gb/s demonstrations contained only 6—7 successive ‘0’ and ‘1’. Considering thatthe typical effective recovery time of our SOA was within 40–
15 ps (for holding-beam power of 0.001–12 mW), we assert that
our measurement provided more stringent PIF evaluation.
Fig. 5b shows the corresponding segments of the DISC output
waveform when there was no NPR in the SOA ðhin ¼ 0 Þ. In thiscase, we see that the intensity of the output signal after the successive ‘0’ bits is large compared to that at the end of the successive ‘1’
bits. We evaluated the PIF to be 5.0, using the ratio of the largest
and smallest pulse intensities. We systematically rotated the H1angle, but no signiﬁcant PIF improvement was observed. The situation was almost the same when we changed the input polarization state from TE to TM ðhin ¼ 90 Þ.
The situation changed when we aligned the input polarization
to hin ¼ 30 , which lay between the TE and TM axes. When we
maximized the transmittance of P 1 by adjusting hQ 1 and hH1 , the1732

J. Sakaguchi et al. / Optics Communications 282 (2009) 1728–1733

0

0.20

d

a
-20
0.10
-40

-60

0.00

1550

Intensity (dBm/nm)

Intensity (arb.unit)

b
0.10

0.00

0.08

1570

1540

1550

1540

1550

0

0.20

0.10

1560

e
-20

-40

-60
1530
0

c

f

-20

0.06
0.04

-40
0.02
0.00
-50

0

+50

-60
1530

Time (ps)Wavelength (nm)Fig. 5. Measured 200-Gb/s digital signal waveforms and spectra. (a) Input waveforms, (b) output waveforms (hin ¼ 0 , DhH1 ¼ 0 ), (c) output waveforms (hin ¼ 30 ,DhH1 ¼ 42 ), (d)–(f) corresponding spectra. The solid traces and dashed traces in (a)–(c) show ‘0’-dominant parts and ‘1’-dominant parts of the 4992-bit patterned waveforms,
respectively.

PIF value was almost the same as before. As we detuned hH1 after
that, the output PIF was drastically reduced (=1.5) as shown in
Fig. 5c. Furthermore, no signiﬁcant signal degradation due to subpulse generation was observed [21]. It was potentially due to unexpected contribution from carrier heating in the SOA, but we do nothave deﬁnite answer at the moment. The spectra of the signals corresponding to Fig. 5a–c are shown in Fig. 5d–f, respectively.
We systematically measured the dependence of the PIF on DhH1
at several input polarization states to the SOA. The results areshown in Fig. 6a. When hin was 30°, the PIF was suppressed aroundDhH1 ¼ 30—42 . A similar reduction of the PIF below DhH1 < 45
was obtained for hin ¼ 45 , though within a narrower DhH1 range.
From the similarity of the measured PIF-suppression proﬁles andthe calculated ones in Section 2, we consider that we obtained convincing proof of the NPR-based PIF suppression.
One drawback of this NPR-based PIF-suppression method is thefairly large decrease in the average output power of the DISC.
Fig. 6b shows the dependence of the average output power on
DhH1 . As we approach the region where large PIF suppression occurs, the average output power tends to decrease because of the reduced transmittance of P1 . When we obtained the waveformshown in Fig. 5c, the average output power decreased from
15.3 to 26.4 dBm. Nonetheless, Fig. 5b and c indicate that the
weak pulses in the patterned output signals were not seriously degraded after the PIF suppression. Fig. 6c shows the dependence ofthe extinction ratio, which we deﬁned by the intensity ratio of thesmallest pulse and the background noise, on DhH1 . The extinction

ratio for Fig. 5c was 3.2, and not decreased much from that for
Fig. 5b (=3.8).3 We consider that this method in proper combination with other PIF-suppression methods (holding-beam injection
and BPF detuning) will reduce the drawback on the output power,
and enable more efﬁcient operation of DISCs and also other kinds
of SOA-based all-optical gates.
5. Conclusion
We systematically measured the pattern-induced intensity ﬂuctuation (PIF) of a wavelength-converted signal from a DISC at
200 Gb/s, and investigated how the nonlinear polarization rotation(NPR) in the SOA affected the PIF. By detuning the polarization angle of the cw light at the SOA input facet from its TE axis (hin ¼ 30 ,for example) and rotating the half-waveplate before the MZI by acertain degree ðDhH1 ¼ 30—42 Þ, we could suppress the PIF from
5.0 to 1.5 without using a strong holding beam or band-pass ﬁlter
detuning. Our DISC model including the NPR effect qualitatively
explained the observed PIF-suppression characteristics below
DhH1 < 45 . Thus, we conclude that the NPR in the SOA plays a signiﬁcant role in the high-frequency operation of the DISC gate, andthat it can be used for PIF suppression. We speculate that this effect
3
These values were fairly small, mostly because of large ASE noise from the EDFA
just before the cross-correlator input (Fig. 4a). We expect that the extinction ratio of
the DISC output signal before ampliﬁcation was much higher, but we could notevaluate it because of our experimental limitation.

J. Sakaguchi et al. / Optics Communications 282 (2009) 1728–1733

1733

would have considerably contributed to the results in past
160—320-Gb/s wavelength conversion experiments [3–5]. We expect this NPR-assisted PIF-suppression method to provide new design options not only for DISCs but also for various other kinds of
SOA gate.

a
10

PIF

Acknowledgements

4

θ in

o
0o (=TE)30o45o
90 (=TM)

2

1
0

15

30

45

60

75

90

Waveplate rotation angle, Δθ H1 (deg)Average output power (dBm)

b -10

θ in

o
0o (=TE)30o45o
90 (=TM)-15

References

-20
-25
-30
-35
0

15

30

45

60

75

90

Waveplate rotation angle, Δθ H1(deg)Extinction ratio

c

10

3

1

θ in

o
0o (=TE)30o45o
90 (=TM)

0.3

0.1
0

+15

+30

+45

+60

We thank Dr. Kohsuke Nishimura of KDDI Research Laboratories for the design and fabrication of the custom-ordered SOA
chips. We also thank Mr. Takeru Yamaji and Mr. Hiroyuki Takeuchi
for the supplemental characterization of the anisotropic nonlinear
phase shift in the SOA. This research was supported by the research
project grant, ‘‘Ultra-low-power and ultrafast optical memory/
switching device, of the New Energy and Industrial Technology
Development Organization (NEDO), and the 21st-Century University’s Center of Excellence (COE) program, ‘‘Innovation in coherent
optical science, of the University of Electro-Communications,Japan.

+75+90

Waveplate rotation angle, Δθ H1 (deg)Fig. 6. Measured dependences of (a) PIF, (b) output power, and (c) extinction ratioon the waveplate angle detuning.

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