Wavelength-tunable dispersion-imbalanced loop mirror based on dispersion-flattened high-nonlinearity photonic crystal fiber and its application in suppression of the incoherent interferometric crosstalk

Abstract

Wavelength-tunable nonlinear suppression of incoherent interferometric crosstalk has been experimentally demonstrated using the dispersion-imbalanced loop mirror based on a 64-m-long dispersion-flattened high-nonlinearity photonic crystal fiber. The signal quality improvement is achieved over a wavelength range of more than 25 nm (1545-1570 nm).

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005 1911
Wavelength-Tunable Dispersion-Imbalanced
Loop Mirror Based on Dispersion-Flattened
High-Nonlinearity Photonic Crystal Fiber and
Its Application in Suppression of the Incoherent
Interferometric Crosstalk
Zhaoxin Wang, Chinlon Lin, Kin-Kee Chow, Yuen-Ching Ku, and Anders Bjarklev
Abstract—Wavelength-tunable nonlinear suppression of in-
coherent interferometric crosstalk has been experimentally
demonstrated using the dispersion-imbalanced loop mirror based
on a 64-m-long dispersion-flattened high-nonlinearity photonic
crystal fiber. The signal quality improvement is achieved over a
wavelength range of more than 25 nm (1545–1570 nm).
IndexTerms—Dispersion-imbalancedloopmirror(DILM),non-
linear optical signal processing, photonic crystal fiber (PCF).
I. INTRODUCTION
I
arises from the beating between a main optical signal and an
interfering optical signal at a closely spaced frequency. When
the frequency difference falls within the detector bandwidth, a
beat signal will be generated, resulting in a severe degradation
of the signal. This degradation is much larger than those pre-
dicted from simple power-addition without field interference.
For such crosstalk, it is preferable to suppress it in the op-
tical domain instead of the electrical domain. One promising
technique to suppress the crosstalk is to use a dispersion-imbal-
anced loop mirror (DILM) [2]. In [3], dispersion-shifted fiber
(DSF) is used as the nonlinear element. However, the necessity
of selecting the signal wavelength within the zero-dispersion
wavelength region may limit the flexibility of the optical net-
work. Dispersion-flattened fiber is also used to increase the
wavelength tuning flexibility [4]. However, the relative low
nonlinear coefficient of the fiber makes it necessary to use
fiber several hundred meters long or even longer, which may
make the DILM sensitive to environmental disturbances. Con-
sequently, high-nonlinearity fiber, which leads to a shorter fiber
length, would be preferable for making the DILM a more stable
and compact device. In this letter, we experimentally demon-
strate the nonlinear suppression of incoherent interferometric
N A transparent optical network, one of the major impair-
ments is the incoherent interferometric crosstalk [1], which
Manuscript received March 1, 2005; revised April 18, 2005.
Z. Wang, C. Lin, K.-K. Chow, and Y.-C. Ku are with the Department of
Information Engineering and Electronic Engineering, The Chinese University
of Hong Kong, Shatin, N.T., Hong Kong (e-mail: zxwang3@ie.cuhk.edu.hk;
chinlon@ie.cuhk.edu.hk; kkchow@ee.cuhk.edu.hk; ycku2@ie.cuhk.edu.hk).
A. Bjarklev is with COM, Technical University of Denmark, DK-2800 Kgs.
Lyngby, Denmark (e-mail: ab@com.dtu.dk).
Digital Object Identifier 10.1109/LPT.2005.851936
Fig. 1.
filter. PC: Polarization controller. ATT: Attenuator. EDFA: Erbium-doped fiber
amplifier.
Experimental setup. MZM: Mach–Zehnder modulator. BPF: Bandpass
crosstalk in a DILM based on only 64-m-long dispersion-flat-
tened high-nonlinearity photonic crystal fiber (PCF) with low
normal dispersion. The advantage of using this kind of fiber lies
in the following four aspects: First, since the PCF has a very
small dispersion slope in the 1550-nm region, a widely tunable
wavelength range of more than 25 nm is obtained. Second, the
high nonlinearity of the PCF greatly shortens the loop length,
making the DILM more compact and stable. Third, since the
PCF has a normal dispersion over a wide wavelength range,
the modulation instability-induced amplitude noise [5] can be
suppressed and a clear eye diagram can be observed. This is a
real advantage over other conventional high-nonlinearity fiber
with very small dispersion slope, such as the fiber reported in
[6], since they will exhibit an anomalous dispersion in some
of the wavelength range that we concern. Last but not least,
the PCF we used is fully spliced to standard single-mode fiber
(SMF) instead of free space coupling, as reported in many
experimental works related with PCF, which makes it easier
for our DILM to achieve a stable operation.
II. EXPERIMENT
The experimental setup is shown in Fig. 1. The upper part
of Fig. 1 is used to generate the input optical signal with
adjustable incoherent interferometric crosstalk. First, the input
optical signal is a
return-to-zero pseudorandom binary
sequence signal at 10 Gb/s, which is obtained by external
modulation of a wavelength tunable actively mode-locked fiber
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1912 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005
laser with a pulsewidth of 2.5 ps. Next, the optical signal is
split into two parts via an 80/20 fiber coupler. The 20% arm
incorporates a decorrelating 2-km DSF [3], [4], a tunable
picosecond delay line, a polarization controller PC1, and a
variable attenuator. Thus, the noise in a signal channel induced
by homo-frequency incoherent crosstalk can be simulated and
investigated. The power level of the crosstalk with respect
to the signal channel can be varied by adjusting the variable
attenuator. The polarization and time delay can be aligned to
maximize the crosstalk effect in order to simulate the “worst
case”scenario.Aftercombiningthesignalchannelandcrosstalk
channel together with a 50/50 fiber coupler, the signal is then
amplified by an erbium-doped fiber amplifier with saturation
power of
23 dBm and launched into the DILM.
The lower part shows the constructed DILM. The DILM
consists of a 64-m-long dispersion-flattened high-nonlinearity
PCF [7], a 100-m dispersion-compensating fiber (DCF), and a
polarization controller PC3. The PCF used here was supplied
by Crystal Fiber A/S and has a three-fold symmetric hybrid
core region with a core diameter of 1.5
dispersion of this PCF is flat over a wide wavelength range
(less than
3 ps/km/nm over 1500–1600 nm) with a nonlinear
coefficient of 11.2 W km
function of wavelength is within 1 ps/km/nm in the wavelength
range from 1465 to 1655 nm. Also, the attenuation of the fiber
is less than 10 dB/km in the 1550-nm range and both ends
are spliced to SMF, yielding a total loss of 2.6 dB. Because
this PCF shows some birefringence, a polarization controller
(PC2) is used before DILM to adjust the input polarization
and overcome this birefringence. If this birefringence can be
eliminated in the fabrication process, the PC2 can be removed
from the scheme. Thus, this scheme will become polarization
insensitive and more attractive. The dispersion of the DCF
is
165 ps/nm/km at 1550 nm. When this kind of DCF is
combined with a suitable length of SMF for dispersion com-
pensation, the average dispersion is below
1520–1570 nm. The natural birefringence of the fiber can be
compensated by adjustment of PC3 in the loop. A 1-km-long
SMF is attached to the DILM output for chirp compensation.
Since the SMF is outside the loop, it will have a negligible
effect on the stability of the loop.
m [7]. The overall
. The dispersion variation as a
1 ps/km/nm over
III. RESULTS AND DISCUSSION
At first, we tune the signal wavelength to 1550 nm to study
thetransfercharacteristicsoftheDILM.Forpulseinputwithout
noise injection, the transmitted power as a function of input
power is measured and shown in Fig. 2(a). The results show
that the DILM can be used as a nonlinear intensity discrimi-
nator for signal restoration: The nonlinear filtering not only re-
jectsthelow-intensitycrosstalkcomponentsat“0”levelbutalso
clamps the amplitude fluctuation at “1” level [2]. Without noise
loading, the measured bit-error rate (BER) through the DILM
at
23 dBm of input power is shown in Fig. 2(b) as asterisks.
When compared to the BER without DILM (circles), one can
see thattheinsertion of DILMimprovesthesignal quality about
0.3 dB in receiver sensitivity by enhancing the extinction ratio
and shortening the pulsewidth [3], [4].
Then, the crosstalk noise is added to study the DILM perfor-
mance in suppressing the incoherent interferometric crosstalk.
(a)(b)
Fig. 2.
and without DILM in case of no noise loading.
(a) Transfer characteristics of DILM and (b) BER measurements with
(a)(b)
Fig. 3.
?10-dB crosstalk level at 1550 nm.
Eye diagram measured (a) before and (b) after the DILM in case of
Fig. 3 shows the measured eye diagrams with and without the
DILM corresponding to a crosstalk level of
really a severe crosstalk in the network. It is seen that the DILM
producescleareyediagram[Fig.3(b)]fromaseverelydegraded
input signal [Fig. 3(a)]. The power penalty improvements (at
BER
) with the DILM are measured at various crosstalk
levels with the results shown in Fig. 4(a). In case of no DILM,
the power penalty is small at the crosstalk level of less than
20 dB. However, the power penalty shows an exponential in-
crease as the crosstalk level is further increased, in agreement
with the results in [1]. In case of
induced power penalty is even larger than 20 dB and cannot be
measuredinourexperiment.WhentheDILMisadded,itgreatly
reduces the power penalty, resulting in a less than 2-dB power
penalty for a relative crosstalk level from
The penalty improvementis also dependent on the time delay
between the signal and the crosstalk. Fig. 4(b) shows the power
penalty improvements with DILM for various time delays from
10 to 10 ps. In all cases, the crosstalk level is fixed at
and the wavelength is chosen at 1550 nm. When there is no
DILM, the power penalty is about 2 dB when the absolute value
of the time delay is larger than 6 ps. When the signal and the
crosstalk come closer to each other, the power penalty greatly
increases until it cannot be measured. The application of DILM
can significantly reduce the power penalty to less than 2 dB for
all cases.
In order to investigate the wide wavelength tunable property
of the DILM, we tune the actively mode-locked fiber laser
for various wavelengths from 1545 to 1570 nm with 5-nm
wavelength spacing. In all cases, the crosstalk level is fixed at
10 dB, and the results for the power penalty improvement
with the DILM are shown in Fig. 4(c). The power penalty
without the DILM is so large that it cannot be measured in this
situation. The result shows that the signal quality enhancement
is achieved over a wide wavelength range of 25 nm. It is likely
10 dB, which is
10-dB crosstalk level, the
30 to 10 dB.
10 dB

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WANG et al.: WAVELENGTH-TUNABLE DILM BASED ON DISPERSION-FLATTENED HIGH-NONLINEARITY PCF 1913
Fig. 4.
1550 nm without time delay, (b) power penalty measurements at 1550 nm with
?10-dB crosstalk for various time delays, and (c) power penalty measurements
at ?10-dB crosstalk for various wavelengths without time delay.
(a) Power penalty measurements at various crosstalk levels for
that our setup can operate over a wider wavelength range but
the demonstration is limited by the tuning range of the actively
mode-locked fiber laser.
Inourexperiment,theinputpulsewidthisabout2.5ps.Toim-
prove the spectral efficiency in the transmission, a wider signal
pulsewidth will be preferred. When a larger duty ratio signal is
used as the input of the DILM, in order to accumulate enough
phase change in the PCF span, we can increase the input pulse
power or adopt a span of new PCF with higher nonlinearity or
longerlength.Ontheotherhand,aspanofDCFwithhigherdis-
persion value or longer length might be needed to broaden the
input pulse to a certain extent.
As an intensity discriminator, the DILM can also be used
as a reamplifying and reshaping (2R) regenerator for signal
restoration in presence of white noise, e.g., amplifier sponta-
neous emission noise, as shown in [8]. However, the receiver
sensitivity cannot be improved if the DILM is directly put in
front of the receiver. This is because the bit error is caused by
the overlap between noise distributions associated with marks
and spaces, but the DILM cannot differentiate that overlap.
Only when the DILM is put into the transmission line can
it become effective. Although it cannot improve the receiver
sensitivity, the reshaping of signal can greatly slow down the
signal deterioration in the subsequent transmission and thus
extending the transmission reach, as analyzed in [9].
Compared with other 2R regenerators, the DILM has the fol-
lowingadvantages:First,thenear-instantaneousresponseofthe
Kerr nonlinearity of the fiber makes the DILM very attractive
for ultrahigh-speed operation. Second, since the DILM only re-
lies on the self-phase modulation effect instead of cross-phase
modulation and four-wave mixing, there is no need to introduce
another beam of light, so the operation can be simplified. What
is more, the DILM is totally passive, thus increasing its relia-
bility. Last, its wavelength-shift-free property also facilitates its
operation in the transmission system and network.
IV. SUMMARY
Nonlinear suppression of the incoherent interferometric
crosstalk has been demonstrated using a DILM constructed
from a relatively short-length of dispersion-flattened high-non-
linearity PCF. As a nonlinear intensity discriminator, the DILM
not only rejects the low-intensity crosstalk components at
the “0” level but also clamps the amplitude fluctuation at the
“1” level. With the DILM, the crosstalk-noise induced power
penalty is greatly suppressed to less than 2 dB even when the
relative crosstalk level is up to
istics of the PCF ensure a broad-band (more than 25-nm range)
signal quality improvement and make the DILM more compact
and stable.
10 dB. The special character-
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