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Letter Vol. 49,No. 6 /15 March 2024 / Optics Letters 1429
Experimental comparison of E-band BDFA and
Raman amplifier performance over 50 km G.652.D
fiber using 30 GBaud DP-16-QAM and DP-64-QAM
signals
Aleksandr Donodin,∗Pratim Hazarika, Mingming Tan, Dini Pratiwi,
Shabnam Noor, Ian Phillips, Paul Harper, AND Wladek Forysiak
Aston Institute of Photonic Technologies, Aston University, Birmingham, UK
*a.donodin@aston.ac.uk
Received 8 December 2023; revised 22 January 2024; accepted 6 February 2024; posted 6 February 2024; published 4 March 2024
We compare the performance of three optical amplifiers in
the E-band: a bismuth-doped fiber amplifier (BDFA), a dis-
tributed Raman amplifier, and a discrete Raman amplifier
(RA). Data transmission performance of 30 GBaud DP-16-
QAM and DP-64-QAM signals transmitted over 50 km of
G.652.D fiber is compared in terms of achieved signal-to-
noise (SNR). In this specific case of relatively short distance,
single-span transmission, the BDFA outperforms the dis-
tributed and discrete Raman amplifiers due to the impact of
fiber nonlinear penalties at high input signal powers.
Published by Optica Publishing Group under the terms of the Cre-
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work must maintain attribution to the author(s) and the published arti-
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https://doi.org/10.1364/OL.515331
Introduction. To tackle the ever-increasing demand for data
capacity and an over-burdened C-band, non-conventional tech-
nologies for increasing the capacity of wavelength-division
multiplexing systems such as ultra-wideband transmission have
been identified as an important, alternative, short-to-medium
term approach to the long-term solution of space-division mul-
tiplexing (SDM) [1–3]. A key technology requirement to open
up the spectrum of the optical fiber, and so make full use of the
low loss window, is the optical amplifier for unconventional (non
C+L-band) signal band amplification. To this end, the E-band
can be used, especially due to the lower impact of the inter-band
stimulated Raman scattering on C- and L-bands from the E-band
when an appropriate guard band between them is created [4].
The E-band provides an impressive 100 nm bandwidth (15.1
THz) to work with, potentially providing capacity for more data
traffic than the C- and L-bands combined. This is possible due to
the availability of low water peak fibers and a diverse selection
of amplification techniques [1,5].
The general benefit of all Raman amplifiers (RAs) is their
ability to achieve gain in any required band given the availabil-
ity of the appropriate pump wavelengths. In distributed RAs,
the pump power is extended into the transmission fiber, which
acts as the Raman gain fiber. Compared to discrete, or lumped,
amplifiers in which gain is provided after the transmission fiber
in a discrete location, the distributed RA compensates the fiber
attenuation along the transmission fiber. Distributed RAs have
advantages such as improved noise performance and higher
signal-to-noise ratio (SNR), leading to better transmission per-
formance [6,7]. Discrete RAs require the use of nonlinear fiber
as their gain medium. RAs have no energy storage, with the
conversion of pump photons to signal photons occurring on
sub-picosecond timelines. This, in turn, leads to a number of
noise sources that are unique to RAs [8]. The length of the
gain medium of RAs is in the order of kilometers. On the other
hand, there are two doped fiber amplifier techniques that provide
gain in the E-band: bismuth-doped fiber amplifiers (BDFAs)
[9], and neodymium-doped fiber amplifiers (NDFAs) [10]. Even
though NDFAs provide good noise figure (NF), challenges with
competitive electronic transitions require wavelength selective
micro-structured fibers to enable efficient E-band amplification
[10]. BDFAs exhibit greater gain and good NF [11,12], however,
until recently the substantial gain per meter to realize relatively
short amplifiers was not considered possible [12].
Our recent work has demonstrated optically amplified, E-band
coherent data transmission using a distributed RA [13], a dis-
crete RA [14], and a bismuth-doped fiber amplifier (BDFA) [15].
However, the transmission conditions (fiber length and channel
count) of these experiments were quite different, and so direct
comparison of the E-band amplifier performance was not pos-
sible. In this Letter, we further explore the performance of these
optical amplifiers in the E-band. In the following investigation,
the experimental performance of the amplifiers is compared at
1430, 1445, and 1460 nm in terms of gain, NF, effective NF, and
achieved bit error ratio (BER) based on the transmission of 30
GBaud dual polarization DP-16-QAM and DP-64-QAM signals
through 50 km of G.652.D fiber with no water peak.
Experimental setup. The schematics and characteristics of
the three amplifiers compared in this work are presented in
Figs. 1(a)–1(c). The gain and NF of the amplifiers under test
is presented in Fig. 1(d). The NF and effective NF for 2 dBm
launch power are shown in Fig. 1(e).
0146-9592/24/061429-04 Journal ©2024 Optica Publishing Group
1430 Vol. 49, No. 6 /15 March 2024 /Optics Letters Letter
Fig. 1. (a) Schematic of the developed BDFA, (b) the distributed RA, (c) and the discrete RA; comparison of the amplifiers’ characteristics:
(d) gain, (e) NF.
The BDFA consists of two signal isolators and two thin-film-
filter wavelength-division multiplexers (TFF-WDMs). The input
filter is used to block the unabsorbed pump light. The 173-m-
long active bismuth-doped fiber has a 6 µm core diameter and
125 µmcladding diameter. The refractive index difference (∆n)
is around 0.007. The fiber core consists of 95 mol% SiO2, 5
mol% GeO2, and <0.01 at%+of bismuth. The cutoff wave-
length (λc) of the fiber is measured to be around 1000 nm. This
amplifier design has been described previously [16], but here,
we use a single pump laser diode with 460 mW at 1320 nm
for backward pumping. The NF and gain of the BDFA for 2
dBm launch power (approximately −12 dBm input power) is
shown in Figs. 1(d) and 1(e). The amplifier features an aver-
age NF of 6.5 dB and an average gain of 28 dB. It should be
noted that the BDFA used here was designed for loss com-
pensation of longer fiber spans; thus, its gain is significantly
higher than those of the RAs. However, as the input to the
receive amplifier is fixed to −20 dBm throughout the experi-
ment, the higher gain of the BDFA under test does not impact the
comparison.
The distributed RA utilizes the 50-km-long G.652.D fiber
simultaneously as a transmission and amplification medium.
The distributed RA comprises three pump lasers emitting radi-
ation at 1325, 1345, and 1365 nm that are combined with a
pump combiner and then a WDM filter [Fig. 1(b)]. The polar-
ization diversity is achieved by combining two orthogonally
polarized pump diodes via a polarization beam splitter (PBC)
at each wavelength. The pump powers are indicated in Table 1.
All the pumps are counter-propagated to the signal to minimize
the effect of relative intensity noise (RIN) and nonlinear impair-
ments in the amplifier. The amplifier pump powers are optimized
to achieve flattop gain with an average of 14 dB. The average
effective NF is −1.5 dB. The distributed RA exhibits minimum
Table 1. Pumping Wavelength and Power of Discrete
and Distributed RAs
Wavelength Distributed Discrete
1325 nm 213 mW 261 mW
1345 nm 276 mW 111 mW
1365 nm 148 mW 267 mW
Fig. 2. Experimental setup of the transmission over 50-km-long
G.652.D fiber.
noise accumulation in comparison to its other counterparts due
to the negative value of the effective NF.
The discrete RA comprises 7.5 km of inverse dispersion fiber
(IDF) as a gain medium [17], backward-pumped using the same
set of diode pump laser wavelengths, with pump powers as
indicated in Table 1. The NF and gain of the discrete RA for 2
dBm launch signal power (approximately −12 dBm input power
to the discrete RA) is shown in Fig. 1. An average NF of 7.5 dB
and an average gain of 13 dB are achieved.
The setup of the E-band data transmission experiment is
presented in Fig. 2. The data carrier signal is generated by a
transmitter (Tx) comprising a tuneable laser (TL) operating
from 1430 to 1460 nm and a DP-IQ modulator driven by a
digital-to-analog converter (DAC) to generate a 30 GBaud DP-
16-QAM signal. After the modulator, the signal is amplified by
a second in-house BDFA designed for E- and S-band operation
[9], followed by a variable optical attenuator (VOA) to control
the input power to the transmission line. When transmission is
performed, the signal is directed to a 50-km-long G.652.D fiber
(13 dB total loss with 0.26 dB/km attenuation at 1445 nm) and
then amplified by one of the three in-line amplifiers under test.
The signal is then directed to an optical bandpass filter (OBPF)
with 5 dB internal loss, where the data carrier is filtered from
the amplified spontaneous emission (ASE).
In all transmission experiments, after filtering by the OBPF,
the signal is attenuated to a fixed input power of −20 dBm with
a VOA having 1 dB minimal internal loss and then amplified by
the receive BDFA. The receive BDFA is based on the doped fiber
Letter Vol. 49,No. 6 / 15 March 2024/ Optics Letters 1431
Fig. 3. Comparison of SNR for different launch powers for amplifiers under test for (a) 1430 nm, (b) 1445 nm, and (c) 1460 nm for 30
GBaud 16-QAM and for (d) 1430 nm, (e) 1445 nm, and (f) 1460 nm for 30 GBaud 64-QAM.
reported previously [16] and has a similar design to the booster
amplifier. The input power to the coherent receiver is controlled
by another VOA to 8 dBm. A second TL operating from 1430
to 1460 nm is used as the local oscillator (LO) for the coherent
detection. Channel reception is completed by a standard set of
balanced receivers and 80 GSa/s analog-to-digital converters
(ADCs) and a digital signal processing (DSP) chain described
previously [18].
Results. The 30 GBaud DP-16-QAM data transmission
results are shown in Figs. 3(a)–3(c). Each data point is the mean
signal-to-noise ratio (SNR) for the two polarizations, averaged
over ten captured traces of 200,000 samples. The wavelength
dependence of the SNR is recorded by tuning the wavelength
of the TLs (signal and local oscillator) to 1430, 1445, and 1460
nm. This limited wavelength range was selected to avoid any
impact of polarization imbalance at wavelengths below 1430
nm due to the non-optimized transceiver, as previously reported
[19]. Moreover, the operation bandwidth and Raman pump
wavelengths lie beyond the water peak of legacy G.652 fiber
(especially pronounced for 1370–1400 nm), to which, there-
fore, the results might also apply. The bit error ratio (BER)
measurement is conducted with each amplifier and at each
wavelength for launch powers ranging from −10 to 8 dBm
for 30 GBaud DP-16-QAM signals and from −6 to 8 dBm
for 30 GBaud DP-64-QAM signals. The BER is then con-
verted to the SNR using Eq. (1) from [20] and thus includes
the impact of linear and nonlinear noise contributions to the
SNR. We note that the underlying assumption of independent
Gaussian noise distributions is limited in accuracy for single-
span transmission, but nevertheless the calculated SNR is a
widely adopted and useful performance metric enabling reason-
able estimation and easy visualization of transmission penalties
[19]. The input power to the receiver amplifier (−20 dBm) and
the coherent receiver (8 dBm) remained constant throughout the
experiment.
To begin with, the performance of the amplifiers is compared
using a 30 GBaud 16-QAM signal [Figs. 3(a)–3(c)]. As shown
in Fig. 3(a), at 1430 nm, the BDFA gives the best transmission
performance overall with the optimum launch power at 2 dBm.
In the linear regime, the distributed RA gives better performance
from −10 to −6 dBm, due to a significantly lower NF, compared
with the other two amplifiers. In the nonlinear regime, the BDFA
shows the best SNR performance, whereas the distributed and
discrete RAs show a similar trend. By comparison, the dis-
tributed RA suffers from higher fiber nonlinearity due to higher
averaged signal power along the fiber, while the discrete RA suf-
fers from high nonlinearity in the 7.5 km of IDF (Raman gain
fiber). Balancing the impact of linear noise and nonlinearity, the
distributed RA shows the lowest optimum launch power of −2
dBm, while the discrete RA and BDFA have similar optimum
launch power.
The performance of the BDFA is best at 1445 nm [Fig. 3(b)].
At this wavelength, the linear regime of the BDFA extends up
to a launch power of 2 dBm where the SNR is 20.6 dB. As at
1430 nm, the distributed RA again outperforms the BDFA and
discrete RA in the linear regime (between −10 and −6 dBm).
The optimal launch power of the distributed RA is again around
−2 dBm. As at 1430 nm, the BDFA has the slowest decrease
in the nonlinear regime among the three amplifiers, with the
discrete and distributed RAs exhibiting similar, relatively rapidly
decaying nonlinear regimes.
Finally, the results of the 30 GBaud 16-QAM transmis-
sion at 1460 nm are presented in Fig. 3(c). The performance
trends at this wavelength are similar to those at 1430 and
1445 nm. However, the best performance of the distributed
RA is almost identical to that of BDFA and reaches 20.4 dB
SNR at the optimal launch power of 0 dBm. The BDFA still
has a slightly higher SNR of 20.5 dB at the optimal launch
power of 5 dBm and a flatter nonlinear regime compared to
1445 and 1430 nm. The overall performance of the discrete
RA is worse than both the BDFA and the distributed RA.
The performance in the nonlinear regime of both the dis-
crete and distributed RA is better than that at 1445 and 1430
nm. The dynamic of the nonlinear regimes with wavelength is
determined by the induced nonlinearity in the SMF and is dis-
cussed in detail below. Note that the wavelength dependence
of the maximum SNR achieved is not only determined by the
in-line amplifiers but also by the transmitter and receiver wave-
length dependence through the booster and receive amplifiers
(also BDFAs), the operation of the modulator, and the spectral
response of the receiver photodiodes and the optical hybrid. In
particular, the most limiting factor of the achieved SNR is the
receive BDFA, due to low input signal power. However, this
limitation is the same for all three cases with different in-line
amplifiers.
1432 Vol. 49, No. 6 /15 March 2024 /Optics Letters Letter
Fig. 4. Parameters of the fibers utilized in the experiment: (a)
nonlinear coefficient and (b) group velocity dispersion.
The 30 GBaud DP-64-QAM results are presented in
Figs. 3(d)–3(f). It should be noted that the range of the input sig-
nal power for DP-64-QAM is limited from −6 to 8 dBm. Overall,
the performance trends are similar to those of DP-16-QAM. The
distributed RA has the best performance at the beginning of the
linear regime, which then saturates. This results in the BDFA
having the best peak performance among the three amplifiers at
all wavelengths. The discrete RA exhibits the worst performance,
probably because the signal experiences the highest induced
nonlinearity of the three cases in the IDF.
Discussion. It is to be expected that the distributed RA will
outperform both the discrete RA and BDFA in terms of achiev-
able SNR, if only NF is taken into account [Fig. 1(e)]. The
slightly better performance of a distributed RA over an EDFA
has been previously demonstrated in the C-band for a relatively
short (75.6 km) single-span link [21]. However, here in the
E-band, in terms of the achieved optimal SNR, the BDFA out-
performs the distributed RA at all three tested wavelengths. This
is due to the induced nonlinearity in the SMF (especially at lower
wavelengths), as observed by the steep degradation of the per-
formance of the distributed RA in the nonlinear regime (Fig. 3).
The details of power distribution along the transmission fiber
leading to differences in accumulated nonlinearity for different
amplification schemes is important here and has been reported
in [21].
In addition, we note that the difference between the opti-
mal SNR for the BDFA and the distributed RA decreases with
wavelength. This is because in SMF, the nonlinear interfer-
ence (determined by the nonlinear coefficient and inversely
proportional to the absolute value of dispersion) decreases with
wavelength. The spectral variations of the nonlinear coefficient
and group velocity dispersion for IDF and SMF are presented
in Fig. 4. As seen from the figures, the impact of nonlinearity
is higher at shorter wavelengths in SMF. In the discrete RA, a
similar behavior can be observed and is attributable to the sig-
nal propagation in the 7.5-km-long IDF. IDF has a nonlinear
coefficient 2.7 times higher than the G.652.D fiber [presented in
Fig. 4(b)].
In contrast to the two RAs, in the case of the BDFA, the
nonlinear penalties are predominantly caused by the signal trans-
mission through the G.652.D fiber where the signal power is
always less than in the distributed RA. In the case of the dis-
crete RA, nonlinear penalties in the G.652.D fiber are identical to
those in the BDFA case, and the additional nonlinearity arises in
the 7.5-km-long IDF. When the WDM scenario is considered,
the performance is expected to be worse for all three scenar-
ios (especially in the nonlinear regime) and with a potentially
greater penalty in the RA cases.
Thus, our study shows that the NF and effective NF do not
provide the full picture of the potential signal performance
degradation and should be compared with care, particularly in
short, single-span systems operating at high power. In summary,
out of the three E-band amplifiers, the BDFA shows the high-
est peak performance for 50 km span transmission in the range
of 1430–1460 nm, followed by the distributed RA, and, finally,
the discrete RA. However, the NF advantage plays a bigger role
in the long-haul transmission scenario, with the distributed RA
expected to outperform both the discrete RA and BDFA.
Conclusion. We have presented an experimental comparison
of a bismuth-doped fiber amplifier, a discrete Raman amplifier,
and a distributed Raman amplifier, in terms of signal-to-noise
ratio of the received 30 GBaud DP-16-QAM and DP-64-QAM
signals transmitted over 50 km of G.652.D fiber. In this specific
case of relatively short distance transmission, both the G.652.D
fiber in the case of the distributed Raman amplifier and the
IDF in the case of the discrete Raman amplifier introduce addi-
tional nonlinear penalties, which result in the bismuth-doped
fiber amplifier achieving the highest peak performance among
these three amplifiers.
Funding. Engineering and Physical Sciences Research Council
(EP/R035342/1, EP/V000969/1).
Disclosures. The authors declare no conflicts of interest.
Data availability. Data underlying the results presented in this paper are
available upon reasonable request.
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