el%3A19940618

fonnance of the DSD light source and successful timeslot interchange between the four packets are well understood from Fig. 2.
Polarisation guiding in ultralong distance
soliton transmission
T. Widdowson, A. Lord and D.J. Malyon
Indexing terms: Opticalfibre polarirarion, Soliton transmission
~
Signal polarisation rotation due to polarisation dependent loss
has been observed for the first time in an optical communications
transmission system. In particular its effect as a limiting factor in
solitonic polarisation multiplexed systems is discussed. An
analytic expression for the degree of rotation is presented and
experimentally verified. Additionally, SGbit/s solitons have been
transmitted a distance of 1 I OOO km without transmission control.
-52
- 48
-40
-44
received optical power,dBm
Fig. 3 Bit eror rate curves of each packet
0 baseline
A packet A
0 packet B
U packet C
0 packet D
The measured bit error rate curves taken for each packet are
shown in Fig. 3. The bit-error free operations of each packet are
accomplished under switching. Power penalties for each packet
from the baseline are distributed in the range 4 6dB at a bit
error rate of lW9. The power penalties are mainly caused by the
imperfect extinction ratio performance of the 2x2 optical switches
in the DSD light source and the optical TSI fabric. The power
penalty variation of -2dB is due to the difference of the TSI
switching pattern and the transmission imbalance of the arrayedwaveguide demultiplexer.
In conclusion, it is experimentally revealed that the crossconnect
switching fabric examined here successfully operates under optical
switching in both the frequency and time domains. Bit-error free
operation is demonstrated for four channels of 3OGHz-spaced 622
Mbit/s data in a 53 byte packet.
~
Introduction: Polarisation division multiplexing (PDM) has been
proposed as a possible means of doubling the capacity of soliton
transmission systems [I]. Demultiplexing a PDM signal may he
achieved with polarisers provided that the two channels remain
largely orthogonal on propagation. The advantage of this technique is that it offers greater immunity to Gordon-Haus (GH) jitter and soliton-soliton interactions (SSI) than optical time division
multiplexing (OTDM). However, theoretical work to date [l] has
assumed that there is no polarisation dependent loss (PDL) in the
transmission path and, therefore, the two soliton data streams
remain orthogonal. When a signal propagates through a PDL element the vector component of the electric field in the low-loss
polarisation state experiences less attenuation than the component
orthogonal to it. This has the effect of rotating the field very
slightly towards the lower loss polarisation state. Thus, in a PDM
system both channels will he ‘guided’ towards the same state of
polarisation (SOP). Consequently, orthogonality is lost and
demultiplexing with simple polarisers may not be possible. Indeed,
in the limit an OTDM demultiplexer would be required, therefore
losing the advatages offered by PDM.
90 I\
I
I
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0 IEE 1994
25 March 1994
Electronics Letters Online No: 19940563
S. Kuwano, 0 . lshida and N. Shibata (NTT Transmission Systems
Laboratories. 1-2356 Take, Yokosuka. Kanagawa, 238-03 Japan)
H. Ishu and T. Kitoh (NTT Opto-Electronics Laboratories, 1-2356
Take. Yokosuka, Kanagawa. 238-03 Japan)
o
50
References
100
150
200
(3300)
(4950)
(6600)
section number
m
(distance, k m )
Fig. 1 Systematic variation of f3 assuming all low loss states aligned at
45” to launch fwld vectors ( P D L = 0.165dB). and 60 variation of p
allowing PDL elements to be randomly orientated (PDL = 0.165dB)
(1650)
1
KOBRINSKI, H.,
VECCHI, M.P.,
GOODMAN, M S.,
GOLDSTEIN, E L . ,
c , and MENOCAL, s 0.:
‘Fast wavelength-switching of laser transmitters and amplifiers’,
IEEE J. Se]. Areas Commun., 1990, SAC-8, (6), pp. 1190-1201
2 GLANCE, B , KOREN, U., BURRUS, C.A., and EVANKOW, I.D.: ‘Discretelytuned N-frequency laser for packet switching applications based on
WDM’, Electron. Lett., 1991, 27, (15), pp. 1381-1383
3 SHIBATA, N., ISHIDA, o., TADA, Y , KUWANO, s., TOHMORI, Y., and
SUZUKI, s.: ‘Performance of four-channel FDM crossconnect
switching without bit loss’, Electron. Lett., 1993, 29, (9), pp. 800CHAPLJRAN, T E., COOPER, I.M., TUR, M., ZAH,
801
s., TADA, Y., and SHIBATA, N.: ‘IO0 ps frequency switching
without bit loss for a lOGb/s ASK modulated signal’, IEEE
Photonics Technol. Lent., 1993, 5, (3), pp. 35&356
5 HUNTER, D K., ANDONOVIC, I , CULSHAW, E., and BARNSLEY, P.E.:
‘Experimental test-bed for optical time-domain switching fabrics’.
Dig. Conf. Optical Fiber Communicationht. Conf. on Integrated
Optics and Optical Fiber Communication, 1993 Tech. Dig. Series,
1993,4, Tu02, pp. 71-72
6 ISHIDA, o., TADA,Y., SHIBATA,N., and ISHII,H.: ‘Fast and stable
frequency switching employing a delayed self-duples (DSD) light
sonrce’, to be published in IEEE Photonics Technol. Lett.
7 TAKAHASHI. H., NISHI, I., and HIBINO, Y.: ’IOGHZ spacing optical
frequency division multiplexer based on arrayed-waveguide
grating’, Electron. Lett., 1992, 28, (4), pp. 380-381
4
KUWANO.
ELECTRONICS LE77ERS
26th May 1994
Vol. 30
(i) variation of t3 assuming low loss stat= aligned at 45”
(ii) 60 variation of f3
Theory: The direction of an electrical field is rotated by a PDL
element. Two initially orthogonal fields are rotated towards each
other causing a reduction in the angle p between them. Assuming
all i PDL elements are aligned with the low loss axis bisecting the
two fields, then by summing the rotations of the fields caused by
every element it can be shown that
Fig. 1 shows this curve for P D L = 0.165dB to allow comparison
with experiment. Fig. 2 shows the variation of total power of the
two fields measured both parallel and orthogonal to the low loss
state. In a real system the PDL elements will be randomly oriented
to the incoming signal polarisation. Because there is no preferred
No. 11
879
direction, the mean relative rotation of the fields will be zero.
However, there will be a significant standard deviation about this
mean caused by a random walk effect. The second curve of Fig. 1
shows the 60 condition for a system with elements having a mean
PDL of 0.165dB. Clearly random alignment cannot be relied on to
eliminate the effect.
L
.
s
0 ,-
23
42
32
564
loss
state
IOW
distance.2000 km/division
a
b
Fig. 4 Evolution of power in the SGHz spectral component
a Low loss state
b High loss state
high 1055
state
-1 5
I
\,
I
I
50
200
(1650)
(6600)
100
150
(3300)
(4950)
section number
(distance , km 1
Fig. 2 Theoretical power in high loss and low loss states
0
33 km
33 km
33 km
DSF
DSF
DSF
with the theoretical plot given in Fig. 2 in which timing jitter is
not modelled. Fig. 5a shows the pulse pattern detected through the
polariser after lOOOkm of transmission with the polarisation at the
receiver adjusted such that maximum signal power was obtained
from one of the channels (channel 1). It can be seen that channel 2
time .ZOO ps/division
time, 500 ps/division
a
b
rn
Fig. 5 Pulse pattern ai IDODkrn and pulse pattern at 156Wkm
a Pattern at IOOOkm (bandlimited to 32GHz)
b Pattern at 15600km (bandlimited to 1.76GHz)
Fig. 3 Recirculating loop confipuration
Experiment: The experimental configuration is illustrated in Fig. 3.
Details of the soliton source and recirculating loop transmission
path can he found in [2]. The ring laser pulse stream was modulated with a 2’-1 PPBS pattern and hit interleaved to form a 2 x
2.5Gbitk PDM/OTDM transmitted signal. At the receiver the signal was optically time division demultiplexed to 2.5Gbith via a
20GHz LiNbO, modulator, providing a 170ps acceptance window
to soliton jitter. A polariser with 30dB extinction was situated in
front of the demultiplexer to ensure that solitons from only one
polarisation state were detected. The mean transmission fibre dispersion at the operating wavelength was 0.5psinmlkm and the
mean PDL per section was 0.165dB. Polarisation controllers
within the loop allowed accurate control of the signal SOPS. In
particular, the eigenstate that corresponds to the signal expenencing the same polarisation state on every recirculation could be
found. The transmitter data stream was then configured such that
both signal SOPs were at 45” to the low loss state of the loop.
Results and discussion: Fig. 4 details the evolution of power in the
5GHz spectral component of the pulses in both the low loss and
high loss eigenstates of the loop. With the polariser aligned to the
low loss state (Fig. 4a) an increase in power of 3dB is observed as
the two channels rotate from initial orthogonality to being coincident in the low loss state at 3000km. With further transmission
the spectral component diminishes due to the accumulation of timing jitter. Power in the high loss state (Fig. 46) falls rapidly as the
signal rotates into the low loss SOP. Fig. 4 is in excellent agreement
880
is no longer orthogonal, 66% of its power is in the same polarisation state as channel 1. To ascertain the validity of the data, BER
measurements were performed on both channels after the signal
had passed through the polariser. Error free operation was possiBER was measured
ble to a distance of IlOOOkm where a
this compares well with other work in which significantly stronger
wavelength guiding was used [3]. Further transmission resulted in
errors due to soliton timing jitter. This was due to the combined
effects of GH and SSI. Because transmission control is not used,
GH builds rapidly over the first few thousands of kilometres and
results in the possibility of adjacent solitons being jittered much
closer together. Also over this distance the two channels rotate
into the same polarisation state. Consequently, on subsequent
transmission, far stronger interaction forces are experienced than
would be expected for unperturbed solitons. Fig. 56 shows the
received pulse pattern at 15600 km revealing SSI on adjacent ‘1’s
with isolated ‘1’s appearing to be unaffected. To reduce the interaction forces one of the channels was disconnected at the transmitter and BER measurements performed on the other through the
OTDM demultiplexer. Error free operation was possible to a distance of 15600km which is in agreement with analytic theory as
being the G H limit for this system. Thus, SSI was limiting the
maximum transmission distance in the two channel experiment.
0 IEE 1994
Electronics Letters Online No: 19940618
8 April 1994
T. Widdowson, A. Lord and D. J. Malyon (ET Laboratories.
Martlesham Heath. Ipswich, Suffolk IPS 7RE. United Kingdom)
References
1 EVANGELIDES, S G.,
MOLLENAUER, L.F.,
GORDON, I.P.,
and
BERGANO, N.s.: ‘Polarisation multiplexing with solitons’, J.
2
3
Lightwave Technol., 1992, LT-10, (I), pp. 28-35
WIDDOWSON, T., MALYON. D.I., SHAN, x., and WATKINSON, P.J.:
‘Soliton propagation without transmission control using a phase
locked erbium fibre ring laser’, Electron. Lett., 1994, 30, (8), pp.
661463
SUZUKI, M., TAGA, H , EDAGAWA, N., TANAKA, H I YAMAMOTO, S., and
AKIBA, A.: ‘Experimental investigation of Gordon-Haus limit on
soliton transmission by using optical short pulses generated by an
InGaAsP electroabsorption modulator’, Electron. Lett., 1993, 29,
(18), pp. 1643-1644
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