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5 OFC 2024 © Optica Publishing Group 2024
122.6 Tb/s S+C+L Band Unrepeated
Transmission over 223 km link with Optimised
Bidirectional Raman Amplification
Jiaqian Yang1,* , Romulo Aparecido1 , Henrique Buglia1 , Pratim Hazarika2 ,
Eric Sillekens1 , Ronit Sohanpal1 , Mingming Tan2 , Dini Pratiwi2 , Ruben S. Luis3 ,
Benjamin J. Puttnam3 , Yuta Wakayama4 , Wladek Forysiak2 , Polina Bayvel1 , and
Robert I. Killey1
1
Optical Networks Group, UCL (University College London), London, UK
2 Aston Institute of Photonic Technologies, Aston University, Birmingham, UK
3 National Institute of Information and Communications Technology, Koganei, Japan
4 KDDI Research, Fujimino, Japan
* jiaqian.yang.18@ucl.ac.uk
Abstract: A 223 km unrepeated transmission link is experimentally demonstrated us-
ing 121 nm optical bandwidth. Optimised bidirectional Raman amplification a s w ell as
Thulium- and Erbium-doped fibre amplifiers enable a record throughput of 122.62 Tb/s.
©202
4TheAut
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(s)
1. Introduction
Optical communication systems have been the backbone of global data transmission over the last 30 years [1].
The achievable distance and capacity, enabled by the exceptionally low-attenuation spectral region spanning tens
of terahertz, make optical fibre an attractive transmission medium. However, the growth in data demand, driven
by cloud computing and data streaming, necessitates expansion beyond the currently deployed C- and L-bands,
with the use of the S-band proposed to extend the transmission bandwidth. In configurations where installation of
in-line amplifiers is challenging or prohibitive, unrepeated systems are a suitable solution, allowing distances in
excess of 200 km to be bridged without repeaters. The capacity and reach of such systems have been extended
through the use of novel amplification technologies, such as wideband semiconductor optical amplifiers [2], and
the optimisation of distributed Raman amplification (DRA) schemes [3]. These have enabled transmission band-
width up to 100 nm, including extension of the conventional C- and L-bands to include the long S-band [2].
Figure 1(a) summarises the throughput and distance of recent unrepeated transmission experiments.
In this work, we demonstrate unrepeated 223 km transmission of a 121 nm signal including S-, C- and L-bands,
supported by Thulium- and Erbium-doped fiber amplifiers (TDFAs and EDFAs, respectively) along with bidi-
rectional DRA. Forward and backward propagating Raman pumps were optimised using a differential evolution
(DE) algorithm and inter-channel stimulated Raman scattering (ISRS) was managed by tilting the signal power
spectrum. Overall, we achieve a throughput of 122.62 Tb/s with 490×32 GBaud channels, which corresponds to
a record capacity-distance product exceeding 27.3 Pb/s·km, as shown in Fig. 1(a). This result demonstrates the
potential of S+C+L band transmission to significantly extend the capacity of unrepeated transmission systems.
2. Experimental Demonstration
Figure 1(b) shows the experimental setup of the S+C+L band unrepeated transmission link consisting of
490 × 32 GBaud wavelength-division multiplexing (WDM) channels. Three 32.5 GHz-spaced carriers with
<100 kHz linewidth were generated and amplified by either a TDFA or EDFA. After polarisation alignment,
the centre channel and the neighboring channels were connected to two dual polarisation in-phase quadrature
(DP-IQ) modulators (35 GHz electrical bandwidth) driven by 92 GSa/s 8-bit digital-to-analogue converters. After
amplification, the 3 channels were modulated by digitally pre-distorted [13] 32 GBaud root-raised cosine shaped
signals with 1% roll-off. The three transmitted channels were then shifted across the S-, C-, and L-bands. The
performance of the centre channel was used to assess the system throughput. Different geometrically-shaped (GS)
constellations were used in each optical band according to the channel signal-to-noise ratio (SNR). Co-propagating
channels were emulated using wideband spectrally-shaped amplified spontaneous emission (SS-ASE) [14] be-
tween 1470 nm to 1620 nm. The SS-ASE was shaped using wavelength selective switches (WSSs) to produce
a flat or tilted spectrum. A notch was also carved by the WSSs to position the three transmitted channels. The
symbol rate of 32 GBaud was selected because the transceiver SNR was measured to be approximately 27 dB,
requiring 31 dB optical signal-to-noise ratio (OSNR), which is just below the maximum notch depth obtainable
from the WSSs. Based on the SS-ASE bandwidth and the wavelength of the tunable lasers, the channel-under-test
could cover the wavelength range from 1470 nm to 1526 nm in the S-band, from 1531 nm to 1565 nm in the
C-band, and from 1572 nm to 1608 nm in the L-band, resulting in 228×, 132×, and 130× channels respectively.
The transmission link consisted of 152 km Corning® SMF-28® ULL fibre and 71 km low water peak G.652.D
single mode fibre (SMF) with an average total loss of 39 dB across the S-, C-, L- bands. The forward and backward
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S C L band ASE generation (a)
120
S/C/L WDM splitter
S/C/L WDM splitter
S+C+L ASE source
This work
1470 – 1620 nm
WSS WSS WSS
L
L L
Throughput (Tb/s)
VOA
95 [2]
C 90%
C
C [3]
VOA
90:10 Coupler
70
S
S S
45 [9] [8]
TL DP-IQ
[7] [5]
TL Mod [10]
[12] [6] [4]
DP-IQ 10% 20 [11]
TL VOA
Mod
200 250 300 350 400
Transmitter: Distance (km)
test channels DAC 92 GSa/s
(b)
S/C/L WDM splitter
S/C/L WDM splitter
L
LO
Fiber transmission &
Raman amplification 1365 1385 1405 1425
Receiver
C
Raman pumps Pump Combiner
BPF VOA
WDM WDM
ISO S
ISO Receiver
152km SMF-28 ULL 71km SMF
WSS Wavelength selective switch TL Tunable laser
Polarisation controller DP-IQ modulator
WDM Wavelength division multiplexer Amplifier for channel under test ISO Isolator ULL Ultra-low loss fibre
SMF Single mode fibre BPF Band pass filter VOA Variable optical attenuator LO Local oscillator
Fig. 1: (a) Throughput and transmission distance of recent unrepeated experiments. (b) Experimental setup for S-, C-, L-band unrepeated
transmission over 223km with bidirectional distributed Raman amplification.
Raman pump wavelengths λ and maximum powers Pmax are listed in Table 1, without considering the WDM
insertion loss of about 1dB. The forward Raman pumps improve the signal OSNR and the backward Raman
pumps mainly compensate for the power loss in the S-band due to the ISRS power transfer from the S- to L-
band. In-line lumped amplifiers were positioned at the end of the fibre link, and the received channel under
Table 1: Raman pumps configuration.
Forward Pumps Backward Pumps
λ (nm) 1410 1417 1423 1430 1437 1445 1451 1458 1365 1385 1405 1425
Pmax (dBm) 24.27 23.93 21.58 21.09 21.15 21.30 21.03 20.48 27.02 27.02 27.13 27.23
Popt (dBm) 24.27 23.93 21.28 21.09 21.15 18.67 16.90 20.48 27.02 27.02 26.58 23.65
test was filtered out using an optical band pass filter with tunable centre wavelength and a fixed bandwidth of
35 GHz. The channel power was boosted by a pre-amplifier in the corresponding band, and the 70-GHz coherent
receiver detected the DP-IQ signal. A 10 bit 256 GSa/s real-time oscilloscope was used to digitise and capture the
waveform, and details of the pilot-based DSP can be found in [15]. System performance was quantified in terms
of information rates, and the SNR and the post-forward error correction achievable information rate (AIR) after
deducting the pilot overhead were estimated for each channel. DVB-S2X low-density parity check codes were
used to decode the received signal [16].
Before transmitting data, a coarse OSNR estimation was carried out by carving multiple notches in each band,
transmitting the SS-ASE through the fibre link, and measuring the received notch depth (i.e., the received OSNR).
The received OSNR was then interpolated to estimate the values for all channels and mapped to SNR and capacity
with the Shannon capacity equation. Using the throughput as a metric, optimisation of the launch power tilt and
bidirectional pump powers was carried out using the DE algorithm. For each launch power tilt (from 0 dB to
25 dB, with 5 dB step size), the powers of the 8 forward pumps and 4 backward pumps were optimised to find the
highest achievable throughput. The best performance was seen with 15 dB launch power tilt, and the optimised
pump powers are listed as Popt in Table 1. The total powers of the forward and backward Raman pumps were
1.14 W and 1.69 W, respectively.
3. Result and Discussion
Figure 2a shows the measured tilted spectrum at the fibre input with the inset showing the 3 × 32GHz channels
which were placed within the sliding notch. The launch powers were 19.6 dBm, 12.2 dBm, 8.6 dBm for S-, C-
and L-band, respectively, and the signal power was attenuated to match the SS-ASE power spectral density. After
223 km single mode fibre transmission, as shown in Fig. 2b, the orange line shows the spectrum at the fibre output
without any amplification, indicating the effect of wavelength-dependent fibre loss and ISRS. The red line shows
the received spectrum with bidirectional Raman amplification and lumped amplification. Here, the received signal
power is recovered to values similar to the launch powers at most of the wavelengths except the lower half of the
S-band, which is due to ISRS and the gain profile of the TDFAs. The maximum Raman on/off gain of more than
30 dB was observed in the S-band with backward Raman pumps only, and more than 40 dB maximum gain with
bidirectional Raman pumps, as shown in Fig. 2c. Even though the forward Raman pump wavelengths reach up
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to 1458 nm, which presents the power transfer peak around mid-C-band, we can observe a gain of about 10 dB
throughout the L-band. A small reduction in gain when the forward pumps were added can be seen in the lower
wavelengths of the S-band due to stronger ISRS power transfer from S- to L-band. The trade-off between the
S- and L-band power spectral density justifies the need to optimise the launch power tilt and the Raman pump
powers. Figure 2d shows the received OSNR measured from the notch for different wavelengths per band, and
for both the maximum and the optimised pump powers. The average OSNR improved from 16 dB to 16.3 dB,
while the approximated capacity increased from 122.8 Tb/s to 124.2 Tb/s. The improvement is mainly in the S-
band, allowing a higher throughput by reducing pump power and lowering ISRS, at the cost of slightly poorer
performance in L-band. The figure also shows the geometrically-shaped modulation formats transmitted for each
channel, which was optimised for the channel SNR [17].
Power Density (dBm/nm)
50
On/off Gain (dB)
−20 15 dB 40 BW Raman only
−40 30 FW & BW Raman
−15
−60 20
−25
−80 1505.6 1506.1 1506.6
10
0
1470 1540 1610 1470 1540 1610
Wavelength (nm) Wavelength (nm)
(a) (c)
Power Density (dBm/nm)
25
0
20
−20 OSNR (dB)
−40 15
GS-64QAM 12dB
−60 10 GS-256QAM 16dB
Max pump
−80 Launch 5 Optimised pump
Fibre out 0
1470 DFA out 1540 1610 1470 1540 1610
Wavelength (nm) Wavelength (nm)
(b) (d)
Fig. 2: (a) Shaped launch spectrum with 15dB tilt. Inset: 3 × 32 GHz channels being placed within the notch. (b) Optical spectra at fibre input,
fibre output and doped fibre amplifier (DFA) output. (c) Raman amplification on/off gain. FW: forward, BW: backward. (d) Received OSNR
in the optimisation process and the choice of different constellation shaping methods.
Figure 3 shows the received measurement, with both SNR and AIR values, after the transmission over 223 km.
At the lowest S-band wavelengths, the OSNR is too low to transmit any data and the total optical bandwidth used
is 121 nm. 49.31 Tb/s, 45.66 Tb/s, and 27.65 Tb/s were achieved for S-, C-, and L-band, respectively, totalling
122.62 Tb/s. The gap to the estimated capacity of 124.2 Tb/s is mainly attributed to the pilot overhead.
20 20
AIR (bit/4-D symbol)
15 15
SNR (dB)
10 10
5 5
S - 49.31 Tb/s C - 45.66 Tb/s L - 27.65 Tb/s
0 0
1480 1500 1520 1540 1560 1580 1600
Wavelength (nm)
Fig. 3: Transmission SNR and AIR measurements after 223 km for 228×S-, 132×C- and 130×L-band channels.
4. Conclusion
We have experimentally demonstrated an ultra-wideband (121 nm) unrepeated transmission link of 223 km with
a total throughput of 122.62 Tb/s, using bidirectional Raman amplification and doped fibre amplifiers. The launch
power profile and Raman pump powers were optimised to overcome the inter-channel stimulated Raman scatte-
ring. To the best of our knowledge, this work achieved the highest capacity distance product, 27.34 Pb/s · km, for
an unrepeated single mode fibre link.
Acknowledgements: This work was supported by EPSRC grants EP/R035342/1 TRANSNET, EP/W015714/1
EWOC, and EP/V000969/1 ARGON. We thank Dr Sergejs Makovejs (Corning Inc.) for loan of the SMF-28 ULL
fibre.
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