Download b. optical wave breaking in the pcf

The topics: Optical Communications. 作者联系方式：徐永钊, [email protected]
supercontinuum generation in photonic crystal fiber with all-normal group
velocity dispersion
Yongzhao Xu1, Xia Zhang2 , Shanglin Hou2 , Hongcheng Wang1, Dongxiong Ling1
1 School of Electronic Engineering, Dongguan University of Technology, Dongguan, Guangdong 523808, China
2 Key Laboratory of Optical Communication and Lightwave Technologies of Ministry of Education, Beijing University of Posts and
Telecommunications, Beijing 100876,China
E-mail: [email protected]
Abstract— We demonstrate supercontinuum generation in
the telecom C-band in a photonic crystal fiber with allnormal group velocity dispersion. Pumping this fiber with
picosecond pulses from a mode-locked semiconductor laser
generates a 90 nm bandwidth continuum with typical selfphase-modulation characteristics. Over 1000 optical carriers
with uniform precise channel spacing of 10 GHz are
generated. In the experiment, the appearance of optical
wave breaking is observed, and we provide a analysis of
optical wave breaking of picosecond pulses in the photonic
crystal fiber.
Keywords — Supercontinuum, Photonic crystal fiber, Selfphase modulation
I.
INTRODUCTION
Supercontinuum (SC) generation using photonic
crystal fibers (PCF) or microstructured fibers (MF) has been
an area of active research over the past decade [1-4], and
has resulted in the commercialization of broadband SC
sources targeting various spectral ranges. Broadband light
sources based on SC generation at telecom-band have
important application in the wavelength division
multiplexing (WDM) optical transmission systems or a
radio over fiber (ROF) system [5, 6].
SC spectra are usually generated by pumping highly
nonlinear PCF or MF with femtosecond or picosecond
pulses in the anomalous group velocity dispersion (GVD)
regime of the fiber, close to the zero-dispersion wavelength.
In this case, spectral broadening is dominated by soliton
effects. This process produces a spectrum with a fine and
complex structure, which is very sensitive to pump pulse
fluctuations. For communication use, the SC lightwave
should have sufficient quality, and low noise and high
stability are the essential characteristics of the SC. In order
to reduce the noise and increase the stability, it can be done
by generating the SC spectrum in the normal GVD regime
in which spectral broadening is mainly through self phase
modulation[7].
In this paper, we present experimental results of SC
generation in PCF with an all-normal GVD profile. SC
spectrum with a spectral width of 90 nm is generated in an
80m-long PCF. In the experiments, the phenomenon of
optical wave breaking is observed. We nimerically solve
the generalized nonlinear Schrodinger equation and provide
a analysis of spectral and temporal transformations of
picosecond pulses in the PCF.
II.
EXPERIMENTAL SETUP
The experimental setup is shown in Fig.1. The fiber
used in this experiment is an 80m-long PCF supplied by
Crystal Fiber A/S. Its cross section is shown in Fig.2(a). The
fiber has a three-fold symmetric hybrid core region with a
core diameter of 2.1um, and achieves the nonlinear
coefficient of ~11 W-1km-1. Fig.2(b) represents the fiber
dispersion curve. It indicates that the PCF was designed
with all-normal GVD, which is low and flat at 1550 nm to
allow for maximum spectral broadening and to reduce the
effects of third-order dispersion. The dispersion is about 0.58 psnm-1km-1 at 1550 nm and the dispersion variation is
less than 1.5 psnm-1km-1 between 1500 and 1650 nm.
The optical pulse source we used was a mode-locked
semiconductor laser and operated at 1546 nm. It produces
1.6 ps (FWHM) hyperbolic secant pulse train at a repetition
rate of 10 GHz with an average power of 0.26 mW. In order
to avoid any retro-reflections toward the laser, an optical
isolator was placed just after the output of the laser. The
pulses were amplified with an erbium-doped fibre amplifier
(EDFA) and coupled into the PCF. The pulses coming out
from the PCF were split into two parts by a coupler with 5%
to 95% power splitting ratio, and then they were
respectively fed into an optical spectrum analyzer and a
power meter for analyses.
III.
RESULTS AND DISCUSSIONS
A. SC GENERATION IN THE PCF
After amplifying and passing through the 80m-long
PCF, the spectrum of the pump pulse was strongly
broadened when the fiber-input power was increased. In
Fig.3, we show the typical spectra produced from the PCF
with a wavelength resolution of 0.5nm. The average
coupled powers into the PCF were set at 0.44, 0.83 and 2.14
W, which correspond to the peak powers of 24, 46 and 118
W, respectively.
From Fig.3, we can see that a higher input power
brings forth a broader spectrum. In a normal dispersion
fiber, the spectral broadening is proportional to soliton order
N which is defined as N=(LD/LNL)1/2, where LD is the
dispersion length, and LNL is the nonlinear length. The
dispersion length is defined as LD=T02/β2, where β2 is the
GVD, and T0 is the half-width at 1/e-intensity point and it
related to TFWHM=1.763 T0 for hyperbolic-secant pulses. The
nonlinear length can be written as, LNL=1/γP0, where γ is
nonlinear coefficient of the fibre, and P 0, the peak power of
the pump pulse. Thus, N can be written as (γP 0T02/β2)1/2.
Therefore, for a given input pulse width and a given fiber,
we can increase the spectral broadening by increasing the
input power. Because higher-order dispersion and higherorder nonlinear effects come into play, the pulse spectrum
becomes asymmetrical.
For an input peak power of 118W (N≈40, Fig.3(c)),
spectrum is generated from 1495 nm to 1595 nm, with the
6-dB bandwidth of 90nm, which was measured at -6 dB
level from the spectral peak.. We note that the spectra
display the typical oscillatory structrure associated with
SPM, which is created by spectral interference of identical
spectral components being present at different temporal
positions within the pulse. It suggests that self phase
modulation (SPM) dominates broadening in the all-normal
dispersion fiber supercontinuum. Fig.4(a) and (b) shows the
same measured SC (correspond to Fig.3(c), input peak
power=118W) where the wavelength resolution is 0.01nm.
As we magnify the horizontal axis of Fig.4(a), the
microstructure of the spectrum become clearly, as shown in
Fig.4(b). We note that the optical comb has the uniform
precise channel spacing of 10 GHz, where the spacing of the
individual modes exactly equals the repetition rate of pulsegenerating lasers. Therefore, a 90 nm bandwidth spectrum
can provide more than 1000 optical carriers with a wellcontrolled channel spacing.
B. OPTICAL WAVE BREAKING IN THE PCF
Compared with the spectra in Fig.3 (a) and (b), the
most notable feature of the spectrum in Fig.3(c) is that the
spectrum has spectral sidelobes. The physical origin of
spectral sidelobes is related to optical wave breaking [7].
The phenomenon of optical wave breaking is a
prominent and dramatic feature in the propagation of high
intensity pulse in optical fibers in cases involving a
interaction between nonlinear SPM and linear dispersive
effects. As the high intensity pulse travels down the fiber,
both GVD and SPM impose frequency chirp on the pulse.
Although the GVD-induced chirp is linear in time, the SPMinduced chirp is far from being linear across the entire pulse.
The SPM-induced chirp add to the GVD-induced chirp, as a
result, the composite chirp is still nonlinear. Because of the
nonlinear composite chirp, different parts of the pulse
propagate at different speeds. Since the fiber exhibits
normal dispersions at all wavelengths, the faster tail
eventually overtakes the slower bule-shifted intermediate
section and therefore optical wave breaking sets on.
To further understand the characteristic of optical wave
breaking, we nimerically solve the generalized nonlinear
Schrodinger equation and study the evolution of pulses in
the PCF. We simulate the propagation of 1.6 ps, 10 GHz,
1546nm, sech2 pulses in the PCF.
For an input peak power of 118W, the simulated output
spctrum from the 80m-long PCF is showed in Fig.5(a),
which much closely match the corresponding experiment
result shown in Fig.3 (c). Fig.5(b) is the simulated pulse
shape from the PCF. A noteworthy feature of the pulse
shape is that the rapid oscillations appear near the pulse
edges, which are always accompanied by the sidelobes in
the spectrum. The temporal overlap of two pulse
components with different instantaneous frequencies leads
to interference beats in the temporal pulse shape.
Oscillations near the pulse edges in Fig.5(b) are a result of
such inerference. The optical wave breaking can also be
understood as a four-wave mixing (FWM) process.
Nonlinear mixing of two different frequencies ω1 and ω2 in
the pulse tails generation of new frequency components at
ωFWM=2ω1- ω2 and ωFWM=2ω2 –ω1. Both temporal beats and
nonlinear frequency generation are evident in Fig.5.
We also studied the evolution of picosecond pulses
with different peak power passing through different fiber
length.Our simulations results indicate that the broadening
of the pulse spectrum in the initial stage is dominated by the
SPM. Afterwards, SPM become weaker while FWM is
enhanced due to energy being transferred from the central
frequency to the spectral wings by dispersion. When optical
wave breaking occurs, as the pulse propagates further on
along the fiber, energy is further redistributed from the inner
spectrum region to the outside wings until smooth temporal
and spectral profile are generated. In such case, no
inerference structures appear in neither temporal nor
spectral profile as the optical wave breaking process assigns
each frequency to a unique temporal position within the
pulse.
Optical wave breaking only occurs in the case of
normal GVD. When the effects of SPM are much larger
than that of GVD, optical wave breaking occurs, and the
distance at which optical wave breaking occurs depends on
the pump condictions and GVD.
IV.
CONCLUSIONS
In conclusion, a PCF with all-normal dispersion has
been used to generate supercontinuum in telecom-band,
spanning from 1495 to 1595 nm. More than 1000 optical
carriers with uniform channel spacing of 10 GHz were
generated. Furthermore, the evidence of optical wave
breaking was seen in the experiment, and the characteristic
of optical wave breaking is studied. The research results
show that SC spectrum generation in the PCF can be
divided into two stage. In the initial stage, the spectrum
broadening is dominated by the SPM. Afterwards, optical
wave breaking occurs, SPM together with FWM produce
spectral broadening. With a further increase in propagation
distance, SPM decreases and the FWM and GVD play a
more important role in the pulse evolution. SC spectrum
keeps shaping, and energy is further redistributed from the
inner spectrum region to the outside wings until smooth
temporal and spectral profile are generated.
ACKNOWLEDGMENT
This work are supported by the Guangdong Natural Science
Fund (No.10451170003004948), the Dongguan Science and
Technology Program (No.2008108101002), and the Science
Research Program of Dongguan University of Technology
(No.2010ZQ02).
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H.Takara,T.Yamamoto,
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[7] G. P. Agrawal, Nonlinear Fiber Optics, 4th Edition
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PCF
Isolator
EDFA
PM
Laser
OSA
Fig.1. Experimental setup. OSA: Optical spectrum analyzer, PM: power meter.
(a)
(b)
Fig.2. The PCF with all-normal dispersion used in the experiment.
(a) microstructure region of the fiber, (b) dispersion characteristics of the fiber
Intensity(Linear Scale)
0.16
(a)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1450
1500
1550
Wavelength(nm)
1600
1650
Intensity(Linear Scale)
0.16
(b)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1450
1500
1550
1600
1650
Wavelength(nm)
0.16
(c)
Intensity(Linear Scale)
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1450
1500
1550
1600
1650
Wavelength(nm)
Fig.3. Spectra generated from the PCF with all-normal dispersion for average input powers of (a) 0.44W, (b) 0.83W and (c)
2.14W.
Normalized Intensity(Linear Scale)
1.0
(a)
0.8
0.6
0.4
0.2
0.0
1480
1500
1520
1540
1560
1580
1600
Normalized Intensity(Linear Scale)
Wavelength(nm)
1.0
(b)
0.8
0.6
0.4
0.2
0.0
1549.0
1549.5
1550.0
1550.5
1551.0
Wavelength(nm)
Fig.4. Measured SC spectrum output from the PCF with a wavelength resolution of 0.01nm.(a) the whole spectrum; (b) the
magnified image of the spectrum in the wavelength range of 1549-1551nm
Normalized Intensity(Linear Scale)
1.0
(a)
0.8
0.6
0.4
0.2
0.0
1450
1500
1550
1600
1650
Wavelength(nm)
0.30
(b)
Normalized Intensity
0.25
0.20
0.15
0.10
0.05
0.00
-6
-4
-2
0
2
4
6
Time(ps)
Fig.5. Simulated spctral and temporal intensity profiles of the pulse from the 80m PCF, which correspond to the experiment
result in Fig.3(c). (a) output SC spectrum; (b) pulse shape.