Emerging Subsea Networks
Cable and Splice Performance of 153µm2 Ultra Large Area Fiber for
Coherent Submarine Links
Ole Levring, David Peckham, Peter Borel, Kenneth Carlson, Alan Klein, Alan
McCurdy, Kasyapa Balemarthy, Bill Hatton, Christian Larsen, Andrew Oliviero,
and Robert Lingle, Jr.
Email: [email protected]
OFS Fitel Denmark, Priorparken 680, DK-2605 Brøndby, Denmark
Abstract: Early in the development of coherent transport technology, it was recognized that
– in addition to low attenuation – an optical fiber optimized for the new paradigm would have
a large effective mode area (Aeff) and large chromatic dispersion (CD). Under the
assumptions of the Gaussian Noise model, the noise power due to non-linearity adds to the
ASE noise power from the amplifier to limit the effective OSNR. A large Aeff allows higher
launch power without increasing the non-linear noise. A large CD suppresses non-linear
cross-talk between channels through walk-off effects. Here we show that fibers with effective
areas of 153 µm2 have excellent loss and splice performance in submarine cables.
1. Introduction
With the introduction of advanced
modulation formats, coherent detection,
and digital signal processors for electronic
dispersion compensation, the optical fiber
path has changed dramatically. Fibers for
On-Off Keying were designed to balance
highly controlled chromatic dispersion
characteristics with influence from nonlinear impairments and attenuation, and
much attention was made to ensure low
polarization mode dispersion. Fibers
optimized for coherent detection are
mainly characterized by their effective area
and optical attenuation. The positive effect
on OSNR by reducing the optical
attenuation is self-evident, while the
importance of increasing the effective area
of the fibers has been viewed with
suspicion due to concerns over increased
bend sensitivity and splicing performance
[1].
2. New Fiber Designs
Large effective area fibers began with the
positive dispersion, large effective area
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Ultrawave® SLA fiber, which was paired
with the UltraWave IDF negative
dispersion fiber to form a dispersion and
slope-matched pair. A new generation of
fibers with even larger effective areas,
ranging from 120 µm2 to 153 µm2 at 1550
nm, have been developed in response to
the change to coherent detection schemes.
Increased effective area of a standard
single mode ITU-G.652 fiber can be
achieved by increasing core size and
lowering core index to assure single mode
transmission at 1550 nm, but such a
simplified approach leads to very poor
bending performance, both with respect to
micro bending and macro bending, due to
poor light confinement. Increased effective
area could then lead to increases in both
micro and macro bending sensitivity which
may result in increasing cable loss.
Increased macro bending sensitivity could
show as increased loss from coiling in joint
boxes and increased sensitivity to excess
fiber length (EFL) in cables.
New fiber designs where a part of the
cladding has a depression i.e. an area of
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Emerging Subsea Networks
low index (lower than both the core index
and outside cladding index) or a trench, a
more narrow and deeper depression in the
inner cladding, have been developed. Both
depressed cladding designs and trench
assisted designs have much better bending
performance than simple step index fiber
profile designs. The depressed cladding
fiber designs have been known and widely
used almost since the earliest single mode
fibers, whereas trench assisted designs
have come into widespread use in the
terrestrial market for bend optimized
fibers.
3. Designing the Ultra Large Area
Fiber
Developing a consistent model for fiber
attenuation from doping levels in the fiber
and cable loss from calculated bending
performance parameters, makes it possible
to find an optimal fiber design by varying
fiber index profile parameters, at a given
effective area bend performance and
optical attenuation. The crucial parameter
here is to establish the correlation between
calculated micro bending sensitivity and
actual bending performance in fiber and in
the cable.
Fig 1. Modeled micro bending sensitivity of tens
of thousands of fiber designs. Each point is an
individual fiber design. For any given effective
area there is a minimum micro bending
sensitivity. The red curve represents the
optimum solution.
Copyright © SubOptic2016
Based on the experience from modeled
sensitivity and cable performance, an
effective area of 153 µm2 was chosen for
the TeraWaveTM ULA design.
Fig 2. Figure-of-Merit for different combinations
of attenuation and effective area at 1550 nm.
Span length is 80 km. The reference level (0 dB)
is an UltraWave SLA fiber, with Aeff 110 µm2
and span loss of 0.19 dB/km, both at 1550nm
wavelength. The diamond indicates the
TeraWave ULA fiber parameters yielding a
FOM improvement of 2.3 dB compared to
Ultrawave SLA.
The importance of ultra high effective area
and low loss, can be shown in a simplified
model for comparing different fibers. A
figure-of-merit FOM has been proposed
[2], that compares fibers with different
properties in coherent systems. The FOM
that was suggested here was derived for
erbium doped fiber amplified systems.
Here α and D are
attenuation and
dispersion of the fiber respectively, L is the
span length, while Leff is the non-linear
interaction length of the fiber [3], which
depends on the fiber attenuation and span
length.
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Emerging Subsea Networks
Non-linear Cross talk due to cross-phase
modulation between channels depends on
the local intensity of light in the fiber, and
the temporal overlap. A large chromatic
dispersion limits the interaction length and
the large effective area limits the local
power intensity, both minimizing the cross
talk. The TeraWave ULA fiber has a
chromatic dispersion coefficient of 21.2
ps/nm/km at 1550 nm compared to a
SSMF G.652 with approximately 16.5
ps/nm/km at 1550 nm.
The FOM values that are shown in Fig. 2
used a fixed dispersion. The actual
dispersion for a given effective area will
depend on the fiber design and even if the
dispersion for most large area fibers is
dominated by the material dispersion,
differences are noticeable. TeraWave ULA
with the nominal dispersion coefficient of
21.2 ps/nm/km at 1550 nm, adding 0.04
dB, when comparing with pure silica core
fibers that typically have dispersion value
of 20.8 ps/nm/km.
Fig 3 . Measured average spectral attenuation of
tens of thousands of kilometers of TeraWave
ULA. Average attenuation at 1550 nm is 0.175
dB/km.
With a germanium doped core, it is
possible to assure high productivity in the
fiber manufacturing, and keeping material
costs down with the use of un-doped silica
cladding, and still obtain low attenuation.
Hydrogen and radiation sensitivity thus are
as for other standard submarine fibers.
Copyright © SubOptic2016
Large core area, trench assisted fibers also
have the characteristic that the micro bend
induced losses are fairly wavelength
independent which is a common
characteristic for most cut-off shifted
designs. This implies that if the µ- bending
induced loss is acceptable at 1550 nm, the
micro bending losses will also be
acceptable throughout the L-band.
Fig 4. Typical measured microbend added loss
sensitivity using the test method IEC TR 6221
Method C (drum test). The added loss is fairly
independent of wavelength through the C+L
band. On this scale a value of 2 dB corresponds
to the micro bending sensitivity of a large area
NZDF submarine fiber (TW-XL) at 1550 nm.
The modeled bending sensitivity is verified
through testing of macro bending and
micro bending, and as indicated above it is
very important to know how the results of
such testing relates to cable loss, since
different cable designs may have different
sensitivities to micro and macro bending (
EFL).
The change toward coherent detection and
large area fibers has benefited from the
development of new coating systems with
offering better micro bend performance.
The TeraWave ULA fiber utilizes a highly
protective, highly stable acrylate dualcoating system, which is also standard on
terrestrial fibers in millions of kilometers.
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Emerging Subsea Networks
and we show the averaged spectral loss of
ULA fibers.
Fig 5. Macro bend added loss for a single turn
on a 30 mm diameter coil. Data at both 1550 nm
and 1625 nm.
Fig 6. Low Mode-Coupling Polarisation Mode
Dispersion of TeraWave ULA
The TeraWave ULA fibers have extremely
low polarization mode dispersion, PMD.
In Fig. 6 we show low mode coupling
sample PMD of fibers representing more
than 100,000 km of fiber. Low mode
coupling samples represents the actual
PMD in the final cables with great
accuracy [4]. The very low PMD of the
fibers diminishes the need for mitigation of
PMD in the digital signal processors.
4. Cable & Splice Performance
In Fig. 7 we show the measured
attenuation of OFS fibers deployed in
finished sub marine cable. The attenuation
is measured on the final lightweight cable,
Copyright © SubOptic2016
Fig 7. Attenuation data of OFS TeraWave ULA
fibers in a submarine lightweight cable
construction.
The TeraWave ULA has excellent splicing
performance. Using standard splice
equipment typical splice losses can be 0.05
dB per splice at both 1550 nm and 1625
nm. The strength splices are typically >
400 kpsi. Splices can be proof tested with
235 kpsi with high yield. Further, splicing
a large mode field diameter ULA fiber to a
small mode field diameter SSMF can be
done with a typical splice loss of 0.15 dB
Fig.8 Splice data for a long inter-regional
system. Data of ULA to ULA fibers. Fibers are
proof tested at 235 kpsi after splicing. At 1625
nm the typical average splice loss is 0.03 dB
5. Is 153 µm2 Effective Area Too Large?
It has been discussed in the literature
whether an effective area of 153 µm2 might
be so large as to actually reduce system
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Emerging Subsea Networks
Much confusion in the literature arises
from the misuse of butt-couple joint loss
formulae to estimate splice loss between
ULA fiber and the SSMF pigtails on the
EDFAs.
The conceptual difference
between a butt-couple found in a connector
and a splice is illustrated in the cartoon of
Fig. 9. The use of a formula for the
coupling loss due to mode field or effective
area mismatch corresponding to Fig. 9a
will overestimate the splice loss.
Splicing ULA to Fibers with Very Different MFD
Loss by Mode-Field Mismatch (dB)
performance [1]. One line of reasoning is
whether there is sufficient EDFA output
power available to reach the optimum
launch power. Another has been whether
the splice losses to the repeater pigtails
with smaller effective areas would negate
the benefits of the larger effective area. It
will be shown here that both concerns are
unwarranted.
10
Theoretical Butt Couple Loss (dB)
Optimized Production Splice Loss (dB)
1
6
7
8
9
10
11
12
13
14
IDF-ULA
0.1
DSF-ULA
SSMF-ULA
ULA-ULA
0.01
MFD of Other Fiber (µm)
Fig 10. Comparison of overly-conservative
estimated splice loss using butt couple formula
against experimental splice loss data, plotted
against the difference between mode field
diameter of the ULA and another fiber. IDF
refers to a slope-matched, negative dispersion
fiber, DSF to a dispersion shifted fiber, and
SSMF to a standard single mode fiber.
The following equation can be used to
analyze both cases cartooned in Fig. 9,
where the discontinuity of case 9a is
approximated by N=1, while the taper of
case 9b is approximated by N=5. In this
formula, there are N+1 fiber segments, N
splices, and Ak is the effective area of the
kth fiber segment.
Eqn.1
An analysis, based on a butt couple
geometry of a splice between a ULA fiber
and other fibers of various MFDs will give
the solid blue line shown in Fig. 10.
However actual splice losses are shown by
the red points in Fig. 10. In all cases,
experimental splice loss values are below
those predicted by the butt couple formula.
The greater the predicted loss toward the
left side of the graph, the greater the
difference between the butt couple
prediction and the actual splices.
Copyright © SubOptic2016
Analyzing the splice loss between a ULA
fiber and the repeater pigtail using an N=1
estimate of splice loss leads to the
pessimistic contour plot of the dependence
of repeater spacing on effective area shown
in Fig. 11 top, matching the result of Ref.
[5]. A more accurate analysis using the
N=5 case is shown in Fig. 11 bottom. In
this case it is clear that repeater spacing
continues to increase as the effective area
is increased beyond 160µm2. Note that the
maximum available launch power from the
EDFA was limited to 18 dBm in Fig. 11;
submarine amplifiers are available today
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Emerging Subsea Networks
which exceed this value, further improving
performance of the largest area fibers in
bottom panel.
Fig.11 Estimated repeater spacing as a function
of fiber loss and effective area using the
Gaussian Noise model with representative
assumptions for a 6,000km coherent link. The
orange star corresponds to the ULA fiber. Top
panel uses the overly-conservative splice loss
calculation based on Eqn 1 with N=1; Bottom
panel uses more accurate N=5 for a tapered
splice, showing that effective areas of 153 µm2
and even larger remain beneficial to system
performance.
7. References
[1] M. Hirano et al., “Optimal fiber
proposal for digital subsea transmission
considering EDFA output”. SubOptic 2013
paper Tu1C-2.
[2] A. Carena et al., “Novel figure of merit
to compare fibers in coherent detection
systems with uncompensated links”.
Optics Express 20(1), 2012.
[3] E.P. Ippen, “Laser applications to
optics and spectroscopy” Vol 2. (AddisonWesley, Reading, MA 1975, Chap. 6.
[4] O. Levring et al., “Optical Fiber
Polarization Mode Dispersion for 40 Gb/s
Trans-Oceanic Transmission Systems”
Proceedings of SubOptic 2010, paper THU
3A 05; (2010).
[5] M. Hirano et al., “Analytical OSNR
Formulation Validated with 100G-WDM
Experiments and Optimal Subsea Fiber
Proposal,” OFC 2013, paper Tu2B.6.
6. Summary
Here we have shown results on cabling of
TeraWaveTM ULA fibers with what we
believe is the largest nominal effective area
in the industry. The fibers have excellent
loss in cable in the C and L band, as well
as good splice performance.
The TeraWaveTM ultra large area fibers
have been, and are being, deployed in
inter-regional and transoceanic routes in
volumes exceeding one quarter of a million
kilometres.
Copyright © SubOptic2016
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