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SSMF compensated by DCF gives more bandwidth in normal
average dispersion, and results in an equalisation of the channel
power in WDM systems compared to symmetric systems and systems using dispersion-shifted fibres.
0 IEE 1998
Electronics Letters Online No: 19981406
7 August 1998
A. Berntson and D. Anderson (Institute of Electromagnetics, Chalmers
University of Technology, 412 96 Goteborg, Sweden)
N.J. Doran, W. Forysiak and J.H.B. Nijhof (Department of Electronic
Engineering and Computer Science, Aston University, Aston Triangle,
Birmingham, B4 7ET, United Kingdom)
DORAN, N.J., KNOX, F.M., and FORYSIAK, w.: ‘Energyscaling characteristics of solitons in strongly dispersion-managed
fibres’, Opt. Lett., 1996, 21, pp. 1981-1983
NIJHOF, J.H.B., DORAN, N.J., FORYSIAK, w., and KNOX, F.M.: ‘Stable
soliton-like propagation in dispersion managed systems with net
anomalous, zero and normal dispersion’, Electron. Lett., 1991, 33,
pp. 1126-1121
GRIGORYAN, v.s., and MENYUK, c.R.: ‘Dispersion-managed solitons
at normal average dispersion’, Opt. Lett., 1998, 23, pp. 609-611
TURITSYN, s.K., and SHAPIRO, E.G.: ‘Dispersion-managed solitons in
optical amplifier transmission systems with zero average
dispersion’, Opt. Lett., 1998, 23, pp. 682-684
KUTZ, J.N., and EVANGELIDES, s.G.: ‘Dispersion-managed breathers
with average normal dispersion’, Opt. Lett., 1998, 23, pp. 685-681
dependence of dispersion-managed solitons at anomalous, zero,
and normal path-average dispersion’, Opt. Lett., 1998, 23, pp. 900902
where d(Z) represents the variable group velocity dispersion and
u*((z>represents the exponential variation of the power as a result
of nonadiabatic losses and amplifiers inserted periodically. For
strong dispersion management, the stationary pulse has a periodically varying [email protected] We thus assume the solution of eqn. 1 to have
the form
q(T,2 ) = A f i v ( 7 , Z )exp[iCT2/2 iQ]
+ 18% + ~ A ~ I u-l ~-7K u2 v- -0
az,, 2 a 7 2
where 2”= j$ p(Z’)dZ’, 2’ = 1; a2 (Z) dZ, and K is a positive
-i av
constant. We note that v(z, Z ) can be interpreted as the wave
function confined by the self-trapping potential and linear quadratic potential. Assuming V ( T , Z) = j(T)exp(im, we obtain a localised stationary solution of AT),the waveform of which has a
profile which lies somewhere between hyperbolic-secant and Gaussian. The dispersion profile is given by
d ( 2 ) = cl(0)a2(Z)[cosh(bZ’)
T. Hirooka and A. Hasegawa
By using optical phase conjugation,the quasi-soliton is shown to
support ultra-high speed single-channel optical communication
over lo2 Tb,km/s.
Optical phase conjugation (OPC), which compensates for both
fibre chromatic dispersion [l] and nonlinear effects [2], is an
attractive scheme for transmission control in fibre optic communication systems. OPC is an effective technique also for optical
soliton transmission [3]. Although solitons take advantage of selfphase modulation to cancel out the dispersion-induced chup, OPC
is still effective at suppressing nonlinear interactions between signal and noise (Gordon-Haus effect) andor among adjacent pulses,
as well as the self-induced Raman effect, which are major limitations to the transmission capacity. No matter which format is
employed, ultra-high bit rate transmission (2 100GbitMchannel)
requires the dense packing of short pulses. M s e breathing
observed in dispersion-managed densely packed pulses brings
about the problem where each bit may not be properly detected in
the middle of a transmission line, which is serious especially in
network applications. Although the pulsewidth of an ideal soliton
does not vary during transmission, interaction forces between
adjacent pulses present a problem when two solitons are packed
In this Letter, we consider the use of OPC for quasi-soliton
transmission [4] to overcome these difficulties. The quasi-soliton is
a nonlinear stationary pulse which is characterised by reduced
nonlinearity as a result of the periodic variation of the chirp. With
the help of pulse confinement by means of a properly programmed
dispersion map, the quasi-soliton is shown to have less variation
of pulsewidth and suffer less interaction between adjacent pulses
than a dispersion-managed soliton [5]. The outline of the quasisoliton propagation is summarised as follows: the propagation of
nonlinear optical pulses in dispersion-managed fibres can be
described, in a normalised unit, by the modified nonlinear
Schrodinger equation
where 6 = d(lc;d(0)2 + Cad(0)Z).The quasi-soliton recovers its
pulsewidth at Z’l such that d(Z’,) = p(0) (this means that C(T1)=
2; = - 1
I n ( 6-Cod(0)
Quasi-soliton propa ation with periodic
optical phase conjugation
p T (2)
We fxst program the dispersion so that it is proportional to az((z)
to remove the inhomogeneity owing to the losses and amplifiers.
In addition, we further assume parameterp to be given by p = l/d.
eqn. 2 then becomes
If we reverse the sign of the chirp at every Z,, we regain the initial
In previous work, the chirp has been reversed by the use of fibre
gratings [4, .6] or dispersion compensating fibres [5]. These
schemes, however, produce linear potential with opposite sign, i.e.
detrapping, when averaged over one period, owing to residual
nonlinearity. This fails to maintain the localised stationary solution 161. On the other hand, if we employ the OPC, this problem is
avoided and the linear potential always contributes to pulse confinement.
We have conducted numerical simulations to show the validity
of the proposed scheme. For practical feasibility, we approximate
the profile by the combination of three fibres with different constant dispersions. Fig. 1 shows the comparison between the dispersion profile given by eqn. 4 and the stepwise profile. System
parameters are as follows: wavelength = 1.55pn, loss = 0.2dB/km,
dispersion slope = 0.02ps/nm%n, nonlinear refractive index = 3.2
x 1W6cm2/W,core effective area = 40pn2, Raman characteristic
time = 3fs, pulsewidth = 2.2ps, bit rate = SOGbitk, input peak
power = 12mW, the amplifier noise figure = 5dB, ampllfier spacing = 30km, and power penalty at the phase conjugator = 15dB.
amplifier and
phase conjugator
dispersion managed fibre
distance, km
Pig. t Dispersion profile
stepwise approximation
- _ _ _ eqn. 4
15th October 1998
Vol. 34
No. 2 1
In-line fifth order Bessel-Thomson fdters with 36OGHz bandwidth
are inserted at each amplifier. Fig. 2 shows the received eye diagram after 2000km propagation. Despite the lack of dispersion
slope compensation and insufficient phase conjugation efficiency,
successful transmission is achieved over thousands of kilometres.
Since OPC does not compensate for third-order dispersion, thirdorder dispersion becomes the major limiting factor to the transmission capacity. In fact, we have confirmed in simulations that if
the dispersion slope is made zero, 16OGbiVs data can be transmitted beyond a distance of 4000km using this scheme.
and MARUTA, A.: 'Chirped nonlinear
propagation in a dispersion-compensated system', Opt. Lett., 1997,
22, pp. 1689-1691
Reduction of OH 'absorption loss by
deuteration in Na20-AI2O3-SiO2
glass fibre
K. Tsujikawa and M. Ohashi
The authors investigated the effect of deuteration on reducing OH
absorption loss for Na20-A1203-SiOzVAS) glass, which is a
candidate for use in fabricating ultra-low-loss fibre. A deuterated
NAS glass fibre was fabricated using the double crucible method
2 0.6
and a loss reduction of 800dBikm was achieved at a wavelength
of 1.55pn. Further, the authors found that a loss increase above
1 . 5 in~ the NAS glass fibre was caused by the absorption of
hydrogen-bonded OH groups.
time, ps
Fig. 2 Received eye diagramformed from 32bit data detected by electricalfilter with 42GHz bandwidth
We note that the system is significantly resistant to fluctuations
in the dispersion profile or peak pulse power owing to the symmetric property of a phase-conjugated system. Without third-order
dispersion and polarisation mode dispersion, the initial conditions
are completely recovered every two stages, even under mismatched
initial conditions. Since OPC can compensate for phase distortion
due to group-velocity dispersion and Kerr as well as Raman nonlinearities, the present scheme would also be useful for transmission using the NRZ format. However, since NRZ pulses are
nonstationary and are less tolerant to third-order dispersion due
to the interplay with self-phase modulation, the resultant transmission capacity is found to be more limited than that using quasisolitons.
In conclusion, it is shown that quasi-solitons supported by optical phase conjugation can provide ultrafast single-channel transmission. Combined with WDM technology, it may enablle us to
construct terabit wide-area networks over thousands of kilometres.
Acknowledgments: This research is supported by JSPS Research
Fellowships for Young Scientists and by the STAR project of Japanese Ministry of Posts and Telecommunications.
0 IEE 1998
14 August 1998
Electronics Letters Online No: 19981378
T. Hirooka (Graduate School of Engineering, Osaka University, 2-1,
Yamada-oka, Suita, Osaka, 5650871, Japan)
E-mail: [email protected]
A. Hasegawa (Kochi University of Technology and N T T Science and
Core Technology Laboratory Group, A T R Bldg., 2-2 tlikaridai,
Seikacho, Soraku-gun, Kyoto 6190288, Japan)
Experimental procedure: We obtained deuterated bulk NAS glass
samples with different concentration ratios of OH to OD by melting, as previously reported [5]. We also applied this technique to
the fabrication of NAS glass fibre by the conventional double-crucible method. First, powder reagents Na,C03, A&03and SiO, were
consolidated into porous glass and decompressed to extract the
OH impurities at -700°C. Next, the OH groups in the porous
glass were replaced with OD groups at 200°C in an autoclave.
The NAS glass fibre was then drawn by the double-crucible
method in an atmosphere of N, and D,O.
The absorption spectra and Rayleigh scattering of the bulk
glass samples were measured using, respectively, a Fourier transform infrared spectrometer FT-IR (Nicolet) and a dynamic light
scattering spectrometer DLS700 (Otsuka Electronics). The optical
loss of the NAS glass fibres was measured using the conventional
cut-back method.
t ' ' ' . ' ' ' ' ' ' . ' ' ' ' . ' ' ~
and PEPPER, D.M.: 'Compensation for channel
dispersion by nonlinear optical phase conjugation', opt. Lett.,
1979, 4, pp. 52-54
2 FISCHER, R.A., SUYDAM, B.R., and YEVICK, D.: 'Optical phase
conjugation for time-domain undoing of dispersive self-phasemodulation effects', Opt. Lett., 1983, 8, pp. 611-613
3 FORYSIAK, w., and DORAN, N.J.: 'Conjugate solitons in ;amplified
optical fibre transmission systems', Electron. Lett., 1994, 30, pp.
Introduction: Low-loss optical fibres are important media for longdistance transmission systems. Among the various kinds of glass
materials, NAS (Na,O-Al,O,-SiO,) is known to be a candidate for
use in the fabrication of ultra-low-loss fibre because of its low
Rayleigh scattering [l 31. NAS is compatible with present optical
transmission systems because it has a theoretical mini"
loss of
O.O5dB/km [2] and a zero material dispersion wavelength at
-1 . 5 ~ However,
ultra-low-loss multi-component glass fibre has
not yet been reported. This is mainly because the fabrication of
extremely pure multi-component glasses is very difficult. In particular, a technique for extracting OH groups which increase optical
losses at - 1 . 5 ~is required
In this Letter, we apply the deuteration technique [4, 51 to the
fabrication of NAS glass fibre and investigate the effect of deuteration on reducing the loss due to OH impurity absorption.
3.0 3.5
2L , , ,
h w
Fig. 1 Loss spectra of bulk NAS glasses and NAS glass fibres
a IR absorption loss spectra of NAS glasses
b Optical loss spectra of NAS glass fibres
and HASEGAWA, A.: 'Quasi-soliton propagation in
dispersion-managed optical fibers', Opt. Lett., 199'7, 22, pp. 372374
5 K U M A R , ~ . , WALD, M., LEDERER, F., and HASEGAWA, A.: 'Soliton
interaction in strongly dispersion-managed optical fibers', Opt.
Lett., 1998, 23,pp. 1019-1021
Experimental results: Fig. 1 shows absorption loss spectra of bulk
NAS glasses and the optical loss spectra of NAS glass fibres. The
reference glass and fibre were prepared by the usual melting
method, and not deuterated. The composition of the bulk NAS
glass was 3ONa,O-10A120,-60Si0, (in mol%) which was almost the
same as that of the core glass of the fibres. The OH absorption
bands at 2.8 and 3 . 6 were
~ considerably suppressed and an OD
absorption band was observed at 3 . 8 instead
in the deuterated
No. 21
15th October 1998
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