E. Ciaramella (1), M. Presi (1), L. Giorgi (1), A. O'Errico (1), S. Herbst (2), J.-P. Elbers (2). 1 : Scuola Superiore Sant'Anna, Via Giuseppe Moruzzi 1 , Pisa, Italy, ...
ECOC 2005 Proceedings - Vol.4 Paper Th 3.5.5
A Simple Scheme to Suppress Transient-Induced Degradations in Transparent Optical Networks E. Ciaramella (1), M. Presi (1), L. Gi orgi (1), A. O'Errico (1), S. Herbst (2), J.-P. Elbers (2)
1 : Scuola Superiore Sant'Anna, Via Giuseppe Moruzzi 1 , Pi sa, I ta ly, email: marco.presi@cniUt 2 : Marconi ONDATA, Backnang, Germany Abstract In transparent networks, fibre cuts can generate severe penalties affecting even disjoint lightpaths. We experimentally demonstrate that a simpfe and cost effective scheme is abfe to fuffy suppress them in
a
realistic
network environment.
l in e) ;
Introduction
dashed
Transparent optical networks can offer significant cost
dotted line) is added and routed to in D es ti na tio n 1. As
in node 2,
ot h er traffic (C channels,
benefits by eliminating unnecessary O/E/O converters
the fibre from Source 1 is Gut, A-channels are lost (may
[1].
be recovered by SDH/optical protection); in addition B
In these networks,
however,
failures such as
accidental fibre cuts can cause severe performance
channels suffer from transient effects,
degradations due to sudden dynamical changes of the
higher power and nonlinear impairments. We outline real networks,
because of
even e channels could be
optical signals. Indeed sudden variations of channels
that in
can lead to transient power variations and can also
impaired: being multiplexed together with B-channels,
change the spectral behaviour of the signals due to
their OSNR would decrease as B channels experience
Raman effect and Spectral Hole Burning (SHB) [2].
sudden gain increase.
Due to the optical transparency, these effects can
This scenario has been emulated by means of the
actually be transferred to other lightpaths, either joint or disjoint, and can introduce unacceptably long bit error bursts or even system outages [3], [4]. Proposed
EDFA-control
tec hniq u es
[5] (e.g.
high
speed gain control of optical amplifiers or link-control lasers [6]) are not free from limitations [5]. Moreover, oxc
although they could suppress optical power transients (typically in a limited input power range), they can
never address
the
spectral
changes due
to
the
Here
we
present
a simple
Fig. 1 In a axe (feft) as fibre cut is detected, an opticaf switch (right) replaces the missing channels, using the
changed Raman tilt and SHB [2]. scheme for
transient
symmetric, counter-propagating traffic.
suppression that can be cost-effectively implemented in
the
optical
cross-connects
(OXCs),
Transit Node 1
Transit Node 2
Destination 1
where
impairments can be induced to other lightpaths (see Fig. 1). The scheme prevents that power variations on
one link can degrade the performance of the other links. On each oxe in put port a system detects the
Source 1 (A channels) (C-channels. to dastinaban 1)
Destination 2
fibre cut, e.g. by monitoring the input power or using the OCh information. If a fault is detected, an optical circuit (right) is triggered to replace the input channels
Fig. 2 Network scenario emufated in the experiment.
using wavelength channels already present in the OXC. The scheme exploits the fact that the network has a bidirectional symmetry (as in SOH, for any WDM comb in any fibre there is a similar counter-propagating comb in the same cable). If replacement
channels
are
taken from the output port having the same wavelength allocation, each missing channel is exactly replaced and no spectral reconfiguration occurs. Experiments and Results The proposed scheme was tested by means of an experimental set-up emulating the network scenario shown in Fig. 2. Under normal operation, in a fibre between Node 1 and Node 2 traffic flows from source 1 (A channels)
977
and source 2 (B channels,
see the
Fig 3 Experimentaf set-up. The two transit nodes are indicated by the squares in fight grey.
ECOC 2005 Proceedings - VolA Paper Th 3.5.5
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are
steady-state
method we Simultaneously measured the BER on B
t
and C-channels. Results are shown in Fig. 5, where data refer to Q penalty for the sake of clarity. As a fibre
•
cut is simulated, significant penalty is found for all
J
surviving channels (dots): namely, channel B (here: ch. 5) suffers from higher power and hence nonlinear
effects,
C-channels
suffer
decreased
from
parameters, so that the impact on the various channel is different. Two channels (2 and 5) were found out of service, so that the indication of 2 dB penalty is just an (optimistic) bond. As expected, when the replacement circuit is enabled, all these penalties are completely
experimental set-up shown in Fig. 3. We used eight 10.66 Gb/s transponders connected in a daisy-loop configuration so that a single pattern generator fed all TXs with SDH-type traffic. In each RX a FEC chip measures the bit error rate (BER). The eight channels were used as follows: one was the traffic from source 2 (B), while seven channels were emulating traffic from source 3 (C). The missing A-channels were
whilst
OSNR. The degradation depends on several network
replacement (dots and diamonds, respectively).
CW
lightwaves, as they do not need any sophisticated system diagnostics. For the sake of simplicity, the first OXC was simulated by using two Arrayed Waveguide Gratings AWG (i.e. with no optical switching matrix but only the AWG filtering effect) . OXC2 was emulated by a filter to isolate the B-channel and an AWG to multiplex it with the C-channels. Between the two emulated OXCs, we had a transmission line with four spans of G.655 fibre (D=3 ps/nmfkm) and dispersion compensation fibre (DCF) . The output power level from all EDFAs was around 13 dBm. by an Acousto-Optic
Modulator (AOM) and in OXC1 we implemented the replacement scheme using low-cost electronics and Here,
transients any
To completely assess the validity of our stabilisation
Fig. 5 System penalty for all channels without/with
another AOM.
the also
power variation is eliminated. This is a clear indication
4 5 Charrel runber
The fibre cut was emulated
comb,
that all surviving channels are preserved.
Fig. 4 Transient suppression at the RX for the
signal
completely suppressed and
surviving B (a) and one of the C- channels (b).
0
WDM
similar
Tim. {1 00 ""d�1
(b)
0
outline that the lightpath of C channels is disjoint from the cut path). As can be seen, when the replacement circuit is used and missing channels are replaced by a
0.
(a)
0·:1
the C channel suffers from decreased power (we
the replacement circuit was
triggered by a photodiode monitoring the power at the input of the first EDFA and was able to provide fast replacement «0.5 J.ls), on a time scale much lower than the EDFA carrier lifetime. We first measured power transients when a fibre cut is simulated. In Fig. 4 we show the oscilloscope traces taken measuring the transient of the B (a) channel and of one of C channels (b), without and with the transient suppression enabled. As can be seen, in the first case the B-channel shows the well known transient gain peak and then reaches a steady state, where the power at the RX has ",,7 dB increase (see Fig. 4), whilst
recovered (diamonds) and are lower than 0.05 dB. Finally, we tested the robustness of the scheme. We first changed the settings of the first VOA in Fig. 3, and observed that the high performance was maintained even when we had a quite large power mismatch (around
±2.5
dB)
between
the
missing
and
the
replacement channels. A similar behaviour was also found when the replacement channels had a spectral shape (e.g. due to spectral tilt and ASE noise) quite different from the original one.
Conclusions We proposed and successfully demonstrated a simple scheme to maintain all surviving channels unaffected in case of a
II can be cost-effectively
fibre cut.
implemented in OXCs,with
