IEC TR 61292-6:2023
(Main)Optical amplifiers - Part 6: Distributed Raman amplification
Optical amplifiers - Part 6: Distributed Raman amplification
IEC TR 61292-6:2023 which is a Technical Report, relates to distributed Raman amplification (DRA). Its main purpose is to provide background material for future standards related to DRA, such as specifications, test methods and operating procedures. This document covers the following aspects:
- general overview of Raman amplification;
- applications of DRA;
- performance characteristics and test methods related to DRA;
- operational issues relating to the deployment of DRA.
As DRA is a relatively new technology, and still rapidly evolving, some of the material in this document can become obsolete or irrelevant in a fairly short period of time. This document will be updated frequently to minimize this possibility. This second edition cancels and replaces the first edition published in 2010. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
- correction of the formula for noise figure;
- correction of errors in Figure 10.
General Information
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Standards Content (Sample)
IEC TR 61292-6 ®
Edition 2.0 2023-01
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 6: Distributed Raman amplification
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IEC TR 61292-6 ®
Edition 2.0 2023-01
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 6: Distributed Raman amplification
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.10; 33.180.30 ISBN 978-2-8322-6340-2
– 2 – IEC TR 61292-6:2023 IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, and abbreviated terms . 7
3.1 Terms and definitions . 7
3.2 Abbreviated terms . 7
4 Background . 8
4.1 General . 8
4.2 Raman amplification process . 8
4.3 Distributed vs. lumped amplification . 10
4.4 Tailoring the Raman gain spectrum . 11
4.5 Forward and backward pumping configuration . 11
4.6 Typical performance of DRA . 13
5 Applications of distributed Raman amplification . 14
5.1 General . 14
5.2 All-Raman systems . 14
5.3 Hybrid EDFA Raman systems . 15
5.3.1 General . 15
5.3.2 Long repeaterless links . 15
5.3.3 Long span masking in multi-span links . 16
5.3.4 High capacity long haul and ultra-long-haul systems . 16
6 Performance characteristics and test methods . 16
6.1 General . 16
6.2 Performance of the Raman pump module. 16
6.2.1 Basic configuration . 16
6.2.2 Pump wavelengths. 17
6.2.3 Pump output power . 17
6.2.4 Pump degree-of-polarization (DOP) . 17
6.2.5 Pump relative intensity noise (RIN) . 18
6.2.6 Insertion loss . 18
6.2.7 Other passive characteristics . 19
6.3 System level performance . 19
6.3.1 General . 19
6.3.2 On-off signal gain . 19
6.3.3 Gain flatness . 20
6.3.4 Polarization dependant gain (PDG) . 21
6.3.5 Equivalent noise figure . 21
6.3.6 Multi-path interference (MPI) . 21
7 Operational issues . 22
7.1 General . 22
7.2 Dependence of Raman gain on transmission fibre . 22
7.3 Fibre line quality . 23
7.4 High pump power issues . 23
7.4.1 General . 23
7.4.2 Laser safety . 24
7.4.3 Damage to the fibre line. 24
8 Conclusions . 25
Bibliography . 26
Figure 1 – Stimulated Raman scattering process and Raman gain spectrum for silica
fibres . 9
Figure 2 – Distributed vs. lumped amplification . 10
Figure 3 – The use of multiple pump wavelengths to achieve flat broadband gain . 11
Figure 4 – Simulation results showing pump and signal propagation along an SMF
span . 12
Figure 5 – On-off gain and equivalent NF for SMF using a dual pump backward DRA
with pumps at 1 424 nm and 1 452 nm . 13
Figure 6 – Typical configuration of an amplification site in an all-Raman system . 15
Figure 7 – Typical configuration of a Raman pump module used for counter-
propagating DRA . 17
Figure 8 – Model for signal insertion loss (IL) of a Raman pump module used for
counter-propagating DRA. 18
Figure 9 – Typical configuration used to measure on-off gain of DRA . 20
Figure 10 – Variations of Raman on-off gain for different transmission fibres . 23
– 4 – IEC TR 61292-6:2023 IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 61292-6 has been prepared by subcommittee 86C: Fibre optic systems and active
devices, of IEC technical committee 86: Fibre optics. It is a Technical Report.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) correction of the formula for noise figure;
b) correction of errors in Figure 10.
The text of this Technical Report is based on the following documents:
Draft Report on voting
86C/1822/DTR 86C/1831/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts of the IEC 61292 series, published under the general title Optical amplifiers,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.
– 6 – IEC TR 61292-6:2023 IEC 2023
INTRODUCTION
Distributed Raman amplification (DRA) describes the process whereby Raman pump power is
introduced into the transmission fibre, leading to signal amplification within the transmission
fibre though stimulated Raman scattering. This technology has become increasingly widespread
in recent years due to many advantages that it offers to optical system designers, including
improved system optical signal-to-noise ratio (OSNR) and the ability to tailor the gain spectrum
to cover any or several transmission bands.
A fundamental difference between distributed Raman amplification and amplification using
discrete amplifiers, such as erbium-doped fibre amplifiers (EDFAs), is that the latter can be
described using a black box approach, while the former is an inherent part of the transmission
system in which it is deployed. Thus, a discrete amplifier is a unique and separate element with
well-defined input and output ports, allowing rigorous specifications of the amplifier
performance characteristics and the methods used to test these characteristics. On the other
hand, a distributed Raman amplifier is basically a pump module, with the actual amplification
process taking place along the transmission fibre. This means that many of the performance
characteristics of distributed Raman amplification are inherently coupled to the transmission
system in which a Raman amplifier is deployed.
This document provides an overview of DRA and its applications. It also provides a detailed
discussion of the various performance characteristics related to DRA, as well as some of the
methods that can be used to test these characteristics. Information is also provided on some of
the operational issues related to the distributed nature of the amplification process, such as the
sensitivity to transmission line quality and eye-safety.
The material provided is intended to provide a basis for future development of specifications
and test method standards related to DRA.
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
1 Scope
This part of IEC 61292, which is a Technical Report, relates to distributed Raman amplification
(DRA). Its main purpose is to provide background material for future standards related to DRA,
such as specifications, test methods and operating procedures. This document covers the
following aspects:
– general overview of Raman amplification;
– applications of DRA;
– performance characteristics and test methods related to DRA;
– operational issues relating to the deployment of DRA.
As DRA is a relatively new technology, and still rapidly evolving, some of the material in this
document can become obsolete or irrelevant in a fairly short period of time. This document will
be updated frequently to minimize this possibility.
2 Normative references
There are no normative references in this document.
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
APR automatic power reduction
DCF dispersion compensating fibre
DOP degree of polarization
DRA distributed Raman amplification
DRB double Rayleigh backscattering
DWDM dense wavelength division multiplexing
EDFA erbium-doped fibre amplifier
ESA electrical spectrum analyzer
FBG fibre Bragg grating
FWHM full width half maximum
– 8 – IEC TR 61292-6:2023 IEC 2023
GFF gain flattening filter
LRFA lumped Raman fibre amplifier
MPI multi-path interference
NZDSF non-zero dispersion shifted fibre
OA optical amplifier
OFA optical fibre amplifier
OSA optical spectrum analyzer
OSC optical supervisory channel
OSNR optical signal-to-noise ratio
PDG polarization dependent gain
PMD polarization mode dispersion
RIN relative intensity noise
ROADM reconfigurable optical add drop multiplexer
SMF single mode fibre
4 Background
4.1 General
Clause 4 provides a brief introduction to the main concepts of Raman amplification. Further
information can be found in IEC TR 61292-3 and ITU-T G.665 as well as in the Bibliography.
4.2 Raman amplification process
Raman scattering, first discovered by Sir Chandrasekhara Raman in 1928, describes an
inelastic scattering process whereby light is scattered by matter molecules and transferred to a
longer wavelength (lower energy). In this interaction between light and matter, a photon excites
the matter molecules to a high (virtual) energy state, which then relaxes back to the ground
state by emitting another photon as well as vibration (i.e., acoustic) energy. Due to the vibration
energy, the emitted photon has less energy than the incident photon, and therefore a longer
wavelength.
Stimulated Raman scattering describes a similar process whereby the presence of a longer
wavelength photon stimulates the scattering process, namely the absorption of the initial shorter
wavelength photon, resulting in the emission of a second longer wavelength photon, thus
providing amplification. This process is shown in Figure 1 a) for silica fibres, where a ~1 550 nm
signal is amplified through absorption of pump energy at ~1 450 nm. Unlike doped OFAs, such
as EDFAs, where the gain spectrum is constant and determined by the dopants, with Raman
amplification the gain spectrum depends on the pump wavelength, with maximum gain occurring
at a frequency of about 13 THz (for silica fibres) below that of the pump. This is shown in
Figure 1 b).
a) Stimulated Raman scattering process b) Raman gain spectrum for silica fibres
Figure 1 – Stimulated Raman scattering process and
Raman gain spectrum for silica fibres
In its most basic form, a Raman amplifier consists of a Raman pump laser, a fibre amplification
medium, and a means of coupling the Raman pump and input signal into the fibre. The main
performance parameter characterizing the Raman amplifier is the on-off gain, which is defined
as the ratio of the output signal (i.e., the signal at the fibre output) when the Raman pumps are
on to the output signal when the Raman pumps are off (the on-off gain will be further discussed
in 6.3.2). Neglecting pump power depletion (i.e., small input signal regime), the on-off gain of a
Raman amplifier can be approximated by
G= 4,34 C PL
R eff
where
G is the on-off gain (in dB);
C is the Raman efficiency between pump and signal;
R
P is the coupled pump power;
L is the effective length of the fibre with respect to the Raman process, defined as
eff
−α L
p
1e−
L ≡
eff
α
p
where
α is the fibre attenuation coefficient at the pump wavelength.
p
The Raman efficiency C depends on the separation between the pump and signal wavelengths,
R
as well as their relative polarization. If the pump and signal polarizations are orthogonal, then
C = 0, whereas if they have the same polarization, C is maximum. In many cases, the pump
R R
is depolarized, and then C is approximately half the maximum value. In other cases, the pump
R
and signal relative polarization changes continuously as they propagate along the fibre
amplification medium, so that C has the same average value as for the depolarized pump case.
R
However, in this case, C can have some residual dependence on signal polarization, resulting
R
in PDG.
– 10 – IEC TR 61292-6:2023 IEC 2023
Taking as an example conventional single-mode fibre (SMF) and a depolarized pump with
wavelength of 1 450 nm, then C for a signal located at 1 550 nm is approximately
R
−1
–1 –1
0,4 W km . In the limit of a long fibre, where L ≈≈α 17 km, a 500 mW pump provides
eff p
approximately 15 dB of on-off gain, illustrating the low gain efficiency of the Raman process.
The gain efficiency can be increased using highly non-linear fibre (such as DCF); however, a
relatively long length of fibre (approximately 10 km) is still needed to achieve reasonably high
gain.
4.3 Distributed vs. lumped amplification
Typically, OFAs are deployed as lumped (or discrete) amplifiers, meaning that the amplification
occurs within a closed amplifier module. These modules are placed at various points along the
optical link (discrete amplification sites at the end of each fibre span), so that the transmission
signal, which is attenuated along the fibre span, is amplified back to the desired power level at
the discrete site at the end of each span. This is illustrated graphically by the green curve in
Figure 2. Raman amplifiers can also be used as discrete amplifiers. However, as shown in 4.2,
this requires special highly non-linear fibres. Even then the application of such amplifiers is
limited due to multi-path interference (to be discussed in 6.3.6), and other issues, and in most
cases other lumped amplifiers, such as EDFAs, are preferable.
While most OFAs require a special doped fibre (such as Erbium doped fibre for EDFAs) to
provide amplification, Raman amplification can occur in any fibre, and within the transmission
fibre itself. This enables distributed Raman amplification (DRA), i.e., the process whereby the
transmission fibre itself is pumped to provide amplification for the signal as it travels along the
fibre. The blue curve in Figure 2 shows signal evolution for distributed Raman amplification in
counter-propagating ("backward") configuration, where the Raman pump power is introduced
at the end of each span and propagates counter to the signal. Since gain occurs along the
transmission fibre, DRA prevents the signal from being attenuated to low powers where noise
is significant, thus improving the optical signal-to-noise ratio (OSNR) of the transmitted signal.
The fact that the net attenuation of the signal along the span is reduced can also be utilized to
launch the signal into the transmission fibre with less power, which can be important in
applications where signal non-linear effects are an issue. DRA can also be used in a co-
propagating ("forward") configuration, where the Raman pump power is introduced at the span
input and propagates with the signal. The distinction between the two configurations is
discussed in more detail in 4.5.
Figure 2 – Distributed vs. lumped amplification
4.4 Tailoring the Raman gain spectrum
As mentioned earlier, the shape of the Raman gain spectrum depends on the pump wavelength,
with the maximum gain occurring at a wavelength approximately 100 nm higher than the pump
wavelength. This unique feature of Raman amplification enables amplification in any
wavelength band, just by using the appropriate pump wavelengths. Furthermore, multiple
pumps with different wavelengths can be used to achieve flat broadband gain over a large
spectral region, as illustrated in Figure 3.
Besides achieving flat broadband gain, multiple pump wavelengths also help to reduce the
polarization dependent gain (PDG) which can be significant when a single pump is used. This
will be discussed in more detail in 6.2.4 and 6.3.4. The PDG can be further reduced by using
two pumps with the same wavelength but with orthogonal polarization.
Figure 3 – The use of multiple pump wavelengths to achieve flat broadband gain
4.5 Forward and backward pumping configuration
DRA can be deployed in either forward (co-propagating) configuration, where the pump is
introduced together with the signal at the span input, or backward (counter-propagating)
configuration, where the pump is introduced at the end of the span and propagates counter to
the signal. These two pumping configurations are illustrated in Figure 4. Assuming a small input
signal and the same pumps, the on-off gain in both configurations is the same, with the
difference being the position along the span where the amplification takes place.
– 12 – IEC TR 61292-6:2023 IEC 2023
a) Forward pumping configuration b) Backward pumping configuration
NOTE Two pumps at different wavelengths provide a total of 500 mW, resulting in 10 dB on-off gain across the
C-band.
Figure 4 – Simulation results showing pump and signal propagation along an SMF span
The main advantage of the forward pumping configuration is that each dB of Raman gain is
equivalent to effectively increasing the signal launch power by one dB, thus achieving a dB of
OSNR system improvement. However, there are several issues that reduce the overall
effectiveness of the forward pumping configuration.
• Signal non-linear effects: Since the Raman gain occurs a few tens of km within the fibre, the
maximum signal power within the span is less than what would occur if a lumped amplifier
with equivalent gain were to be placed at the beginning of the span. While this reduces
signal non-linear effects, these can still become an issue when the effective launch power
per channel increases, thus placing a practical limit on the amount of forward Raman gain
that can be used.
• Pump relative intensity noise (RIN): Typical commercial semi-conductor Raman pump lasers
have RIN values in the order of –115 dB/Hz. In forward pumping configuration, there is a
long walk-off length between signal and pump, which results in significant transference of
the pump RIN to the signal, thus resulting in a system penalty which can accumulate along
many spans. This is discussed in more detail in 6.2.5.
• Pump depletion: As the composite signal input power increases, pump depletion occurs,
resulting in the reduction of Raman gain. For example, 650 mW of pump power configured
to provide 15 dB flat gain across the C-Band for SMF fibre in the small signal regime will
only provide about 8,5 dB of gain when the composite input signal is 20 dBm. Pump
depletion can also lead to large transient effects when the input signal changes abruptly
(e.g., due to channel add/drop). Unlike EDFAs, where transient effect can be suppressed
using electronic feed-back and feed-forward mechanisms, such effects cannot be fully
suppressed in forward DRA due to the fast response time of the Raman effect and the
distributed nature of the amplification.
While the backward pumping configuration does not suffer from the above disadvantages, the
OSNR improvement is typically more modest since the amplification occurs in the last few tens
of km of the fibre span. For example, 10 dB of Raman gain in the backward configuration will
typically result in about 5 dB OSNR improvement (relative to a lumped amplifier providing the
same gain at the end of the span), while increasing the Raman gain further will only result in
an additional 1 dB to 2 dB OSNR improvement. Further OSNR improvement (typically another
1 dB to 2 dB) can be achieved using complex multi-order Raman pumping schemes, which
involve boosting the Raman pump energy in the transmission fibre with additional pumps at
even shorter wavelengths. Thus, the Raman gain occurs deeper within the span, leading to
improved OSNR.
Overall, the backward pumping configuration usually provides better system performance for
the same amount of Raman pump power and is simpler to implement. Thus, in most systems,
backward pumped DRA is usually deployed first, and then forward pumped DRA only for those
spans where backward pump DRA alone cannot supply sufficient OSNR improvement.
4.6 Typical performance of DRA
As described in Clause 5, DRA is most often used to provide moderate (10 dB to 15 dB) flat
on-off gain in the C-Band, most often in the backward configuration, and less often in the
forward configuration.
Figure 5 shows the gain for SMF in the C-Band provided by a triple pump backward DRA with
pump wavelengths of 1 424 nm (two pumps) and 1 452 nm (one pump). In this example, a gain
of 10 dB is achieved with about 450 mW of composite pump power, and a gain of 14 dB with
650 mW pump power. Figure 5 also shows the equivalent noise figure (NF) of the backward
DRA for different gains, which is defined as the NF of an equivalent lumped amplifier
(generating the same gain and same amount of ASE) placed at the end of the span (see 6.3.5
for further detail). In a hybrid EDFA Raman system (see 5.3), backward DRA is used as a pre-
amplifier for a conventional EDFA, which provides the remaining gain to compensate the span
loss. Since the DRA has a very low effective NF, and since it acts as a pre-amplifier, it mainly
determines the NF of the combined EDFA Raman amplifier. Thus, assuming a typical NF of
about 5 dB for an EDFA, the combined EDFA/Raman amplifier can be shown to have a
composite NF of about 0 dB in the case of 10 dB on-off Raman gain, which results in a 5 dB
OSNR improvement compared to an equivalent EDFA-based system.
NOTE The various curves correspond to different composite pump powers.
Figure 5 – On-off gain and equivalent NF for SMF using a dual pump
backward DRA with pumps at 1 424 nm and 1 452 nm
– 14 – IEC TR 61292-6:2023 IEC 2023
5 Applications of distributed Raman amplification
5.1 General
DRA offers two unique advantages compared to conventional amplifiers like EDFAs. One
advantage is improved system OSNR and the second is the ability to provide flat gain for any
and multiple transmission bands. These advantages are offset by the high cost of DRA, due to
the high optical pump powers, as well as by operational issues which will be further discussed
in Clause 7. For this reason, DRA is usually only utilized in those applications where it offers a
significant advantage or there are no other viable alternatives. These applications will be
discussed in Clause 5.
5.2 All-Raman systems
All-Raman systems are systems which utilize only Raman amplification, both DRA and lumped
Raman amplifiers. By using only Raman amplification, such systems benefit from the inherent
OSNR improvement provided by DRA and can be operated in wavelength ranges for which it is
impossible or impractical to provide amplification with more common technologies such as
EDFAs. In particular, all-Raman systems can operate in the L-Band, for which EDFA technology
is much less efficient compared to the C-Band. Since L-Band systems allow for longer system
reach compared to C-Band systems when using non-zero dispersion shifted transmission fibre
(NZDSF), all-Raman L-Band systems are particularly well suited for ultra-long haul (> 1 500 km)
optical links.
A typical configuration of an all-Raman amplification site is shown in Figure 6. The configuration
comprises three Raman pump modules: one for backward DRA, one for forward DRA, and one
for providing lumped Raman amplification within the DCF fibre. In a typical system, such an
amplification site is placed every 80 km along the transmission fibre to provide approximately
20 dB of net gain. The 20 dB gain is achieved by using forward and backward DRA and by
pumping the DCF so that its net gain is zero (i.e., the on-off Raman gain exactly compensates
the DCF insertion loss, typically about 10 dB). Since DCF has a relatively high Raman efficiency
(due to its small effective area), a relatively small amount of pump power is sufficient to pump
the DCF.
Besides the relatively high cost of all-Raman systems, it is also difficult to upgrade them to
support reconfigurable optical add drop multiplexers (ROADMs), which are an integral part of
more modern optical networks. The reason for this is two-fold.
• Firstly, additional lumped amplification needs to be provided to compensate for the added
insertion loss of the ROADM modules. One option for providing the additional Raman gain
is to pump the DCF with higher pump power. However, this can lead to increased MPI due
to double Rayleigh backscattering (see 6.3.6). Another option is to use a separate lumped
Raman amplifier, which further adds to the overall cost of the system.
• Secondly, the transients resulting from system reconfiguration are difficult to suppress,
especially in the case of forward DRA.
For these reasons, the application of all-Raman systems is limited to ultra-long-haul point-to-
point (i.e., non-reconfigurable) optical links.
Figure 6 – Typical configuration of an amplification site in an all-Raman system
5.3 Hybrid EDFA Raman systems
5.3.1 General
EDFA based transmission systems are by far the most common optical communication systems
in deployment today. EDFA technology is mature and can provide a cost effective and efficient
solution for most common applications. However, there are some challenging applications for
which EDFA technology is not sufficient. In these cases, DRA, and particularly backward DRA,
can be employed to improve the end-to-end system OSNR.
The cost of adding DRA to EDFA based systems can be reduced by tightly integrating the
Raman pump module with the EDFA and optimizing the overall design. This is particularly useful
for long-haul and ultra-long-haul applications (see 5.3.4), where DRA is used in every span of
the link. Integration and optimizing of the design can include, for example, mounting the Raman
and EDFA pumps in the same physical package, thus reducing package costs and footprint.
Additionally, a combined gain flattening filter (GFF) can be designed to take into account the
Raman gain spectral shape as well as the EDFA gain spectral shape, thus reducing gain
flattening requirements for both the EDFA and the Raman amplifier (and reducing the number
of separate Raman pumps). Due to the pre-amplifier function of the DRA, the GFF can be placed
before the EDFA without significantly increasing the composite NF of the hybrid module, thus
reducing the EDFA pump power.
In 5.3.2 to 5.3.4, applications for hybrid EDFA Raman systems are discussed.
5.3.2 Long repeaterless links
Long repeaterless links (> 150 km) have many applications, such as connecting islands or oil
rigs, traversing hostile or inaccessible terrain, and links where repeater sites can pose a security
or logistic challenge.
By utilizing backward DRA, the system OSNR can typically be improved by 5 dB to 7 dB,
depending on the pump power. For example, using a 700 mW Raman pump module configured
to provide approximately 15 dB of on-off Raman gain across the C-Band, an OSNR
improvement of approximately 6 dB can be achieved, depending on the transmission fibre type,
thus allowing the link reach to be extended by approximately 30 km.
For even longer links, it is possible to use forward DRA in addition to backward DRA. Assuming
a system with a 20 dBm EDFA booster, for example, the addition of a 700 mW forward DRA
pump module will provide ~8,5 dB Raman on-off gain. This corresponds to about 7 dB OSNR
improvement (considering the insertion loss of the Raman pump module).
Thus, using forward and backward DRA with moderate pump power (e.g., up to 700 mW), the
system OSNR can be increased by up to 13 dB compared to corresponding EDFA only systems.
– 16 – IEC TR 61292-6:2023 IEC 2023
5.3.3 Long span masking in multi-span links
Most multi-span links are typically constructed in such a way that in-line EDFA repeaters are
placed after every 80 km to 100 km fibre span. However, geographical constraints can require
individual spans to be longer, or practical considerations can be an incentive to reduce the
number of spans and thus increase the length of one or more spans. In both cases, DRA can
provide the extra OSNR margins needed to support the longer spans. In addition, many systems
are designed such that the in-line EDFA can support a limited gain range while still maintaining
flat gain. In this case, besides providing improved OSNR, DRA allows longer spans to be
supported while still using standard EDFAs in the system, thus increasing system flexibility and
utility.
While repeaterless links, discussed in 5.3.2, tend to be static (i.e., non-reconfigurable) point-
to-point links, multi-span links are most often dynamic and provide ROADM functionality.
Therefore, by nature, such systems can generate transient events, which are problematic to
suppress when forward DRA is used. This is one reason why forward DRA is not often used in
such applications, and backward DRA is much more common.
5.3.4 High capacity long haul and ultra-long-haul systems
In high capacity systems (with high bit rate and/or dense channel spacing), OSNR quickly
becomes a critical issue as the number of spans increases. By utilizing backward DRA in every
span in the system, the OSNR can be increased significantly, thus allowing the system to
support more spans and/or higher capacity. For example, by providing 10 dB of backward DRA
in each span (approximately 500 mW pump power), the system OSNR can be improved by
about 5 dB compared to an equivalent EDFA-only system, allowing a 3-fold increase in the
reach of the system.
6 Performance characteristics and test methods
6.1 General
Clause 6 describes important performance parameters relevant to DRA and considers test
methods for these parameters. As discussed previously, a fundamental difference between DRA
and lumped amplifiers is that the performance of DRA depends on the transmission fibre, so
that a full characterization of the amplifier performance can only be performed on a system
level, rather than on a device level. However, there are some performance parameters that are
specific to the Raman pump module, which can be specified and measured independently of
the system in which the module is installed. Furthermore, those parameters that are system
dependent can be characterized on average for various types of transmission fibre, so that the
expected performance of a system can be predicted. In what follows, we first discuss these
device level characteristics and then proceed to system level performance.
6.2 Performance of the Raman pump module
6.2.1 Basic configuration
A Raman pump module typically consists of several Raman pump lasers together with passive
components designed to multiplex the output of these pump lasers with the signal. The module
can also contain detectors for monitoring pump power and signal power, as well as circuits and
software for controlling the amplifier. A possible construction of a Raman pump module used
for counter-propagating DRA is shown in Figure 7. In this example, the pump module contains
three pumps laser diodes, which are two polarization multiplexed diodes at wavelength λ , and
one laser diode at wavelength λ .
Figure 7 – Typical configuration of a Raman pump module used
for counter-propagating DRA
6.2.2 Pump wavelengths
The spectrum of the pump power exiting the pump out port of the Raman pump module is critical
in determining the on-off Raman gain spectrum of the signals propagating in the fibre span
connected to the module. The pump power spectrum typically consists of a number of discrete
wavelengths, each of which can originate from one or more pump sources (as for wavelength
λ in the example shown in Figure 7). The pump spectrum can be measured by connecting the
pump output port to an OSA (usually via an attenuator due to the high pump power), resulting
in a list of wavelengths corresponding to the peaks that comprise the spectrum. Another relevant
parameter that can be measured by the OSA is the width of each peak, measured for example
as FWHM. For most 14xx-nm FBG stabilized pump laser diodes on the market today, the FWHM
is of the order of 1 nm to 2 nm.
6.2.3 Pump output power
The pump power exiting the
...
IEC TR 61292-6 ®
Edition 2.0 2023-01
REDLINE VERSION
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Optical amplifiers –
Part 6: Distributed Raman amplification
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IEC TR 61292-6 ®
Edition 2.0 2023-01
REDLINE VERSION
TECHNICAL
REPORT
colour
inside
Optical amplifiers –
Part 6: Distributed Raman amplification
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.160.10; 33.180.30 ISBN 978-2-8322-6366-2
– 2 – IEC TR 61292-6:2023 RLV IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 2
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 8
4 Background . 8
4.1 General . 8
4.2 Raman amplification process . 9
4.3 Distributed vs. lumped amplification . 10
4.4 Tailoring the Raman gain spectrum . 11
4.5 Forward and backward pumping configuration . 11
4.6 Typical performance of DRA . 13
5 Applications of distributed Raman amplification . 14
5.1 General . 14
5.2 All-Raman systems . 14
5.3 Hybrid EDFA Raman systems . 15
5.3.1 General . 15
5.3.2 Long repeaterless links . 15
5.3.3 Long span masking in multi-span links . 16
5.3.4 High capacity long haul and ultra-long-haul systems . 16
6 Performance characteristics and test methods . 16
6.1 General . 16
6.2 Performance of the Raman pump module. 16
6.2.1 Basic configuration . 16
6.2.2 Pump wavelengths. 17
6.2.3 Pump output power . 17
6.2.4 Pump degree-of-polarization (DOP) . 17
6.2.5 Pump relative intensity noise (RIN) . 18
6.2.6 Insertion loss . 18
6.2.7 Other passive characteristics . 19
6.3 System level performance . 19
6.3.1 General . 19
6.3.2 On-off signal gain . 19
6.3.3 Gain flatness . 21
6.3.4 Polarization dependant gain (PDG) . 21
6.3.5 Equivalent noise figure . 21
6.3.6 Multi-path interference (MPI) . 22
7 Operational issues . 22
7.1 General . 22
7.2 Dependence of Raman gain on transmission fibre . 22
7.3 Fibre line quality . 23
7.4 High pump power issues . 24
7.4.1 General . 24
7.4.2 Laser safety . 24
7.4.3 Damage to the fibre line. 24
8 Conclusions . 25
Bibliography . 26
Figure 1 – Stimulated Raman scattering process and Raman gain spectrum for silica
fibres . 9
Figure 2 – Distributed vs. lumped amplification . 11
Figure 3 – The use of multiple pump wavelengths to achieve flat broadband gain . 11
Figure 4 – Simulation results showing pump and signal propagation along an SMF
span . 12
Figure 5 – On-off gain and equivalent NF for SMF using a dual pump backward DRA
with pumps at 1 424 nm and 1 452 nm . 13
Figure 6 – Typical configuration of an amplification site in an all-Raman system . 15
Figure 7 – Typical configuration of a Raman pump module used for counter-
propagating DRA . 17
Figure 8 – Model for signal insertion loss (IL) of a Raman pump module used for
counter-propagating DRA. 19
Figure 9 – Typical configuration used to measure on of gain (a)
for co-propagating DRA and (b) for counter-propagating DRA on-off gain of DRA . 20
Figure 10 – Variations of Raman on-off gain for different transmission fibres . 23
– 4 – IEC TR 61292-6:2023 RLV IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition IEC TR 61292-6:2010. A vertical bar appears in the margin
wherever a change has been made. Additions are in green text, deletions are in
strikethrough red text.
IEC TR 61292-6 has been prepared by subcommittee 86C: Fibre optic systems and active
devices, of IEC technical committee 86: Fibre optics. It is a Technical Report.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) correction of the formula for noise figure;
b) correction of errors in Figure 10.
The text of this Technical Report is based on the following documents:
Draft Report on voting
86C/1822/DTR 86C/1831/RVDTR
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts of the IEC 61292 series, published under the general title Optical amplifiers,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.
– 6 – IEC TR 61292-6:2023 RLV IEC 2023
INTRODUCTION
Distributed Raman amplification (DRA) describes the process whereby Raman pump power is
introduced into the transmission fibre, leading to signal amplification within the transmission
fibre though stimulated Raman scattering. This technology has become increasingly widespread
in recent years due to many advantages that it offers to optical system designers, including
improved system optical signal-to-noise ratio (OSNR) and the ability to tailor the gain spectrum
to cover any or several transmission bands.
A fundamental difference between distributed Raman amplification and amplification using
discrete amplifiers, such as erbium-doped fibre amplifiers (EDFAs), is that the latter can be
described using a black box approach, while the former is an inherent part of the transmission
system in which it is deployed. Thus, a discrete amplifier is a unique and separate element with
well-defined input and output ports, allowing rigorous specifications of the amplifier
performance characteristics and the methods used to test these characteristics. On the other
hand, a distributed Raman amplifier is basically a pump module, with the actual amplification
process taking place along the transmission fibre. This means that many of the performance
characteristics of distributed Raman amplification are inherently coupled to the transmission
system in which it a Raman amplifier is deployed.
This document provides an overview of DRA and its applications. It also provides a detailed
discussion of the various performance characteristics related to DRA, as well as some of the
methods that can be used to test these characteristics. Information is also provided on some of
the operational issues related to the distributed nature of the amplification process, such as the
sensitivity to transmission line quality and eye-safety.
The material provided is intended to provide a basis for future development of specifications
and test method standards related to DRA.
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
1 Scope
This part of IEC 61292, which is a Technical Report, relates to distributed Raman amplification
(DRA). Its main purpose is to provide background material for future standards related to DRA,
such as specifications, test methods and operating procedures. This document covers the
following aspects:
– general overview of Raman amplification;
– applications of DRA;
– performance characteristics and test methods related to DRA;
– operational issues relating to the deployment of DRA.
As DRA is a relatively new technology, and still rapidly evolving, some of the material in this
document may can become obsolete or irrelevant in a fairly short period of time. This document
will be updated frequently to minimize this possibility.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60825-2, Safety of laser products – Part 2: Safety of optical fibre communication systems
(OFCS)
IEC 61290-3, Optical amplifiers – Test methods – Part 3: Noise figure parameters
IEC 61290-3-1, Optical amplifiers – Test methods – Part 3-1: Noise figure parameters – Optical
spectrum analyzer method
IEC 61290-3-2, Optical amplifiers – Test methods – Part 3-2: Noise figure parameters –
Electrical spectrum analyzer method
IEC 61290-7-1, Optical amplifiers – Test methods – Part 7-1: Out-of-band insertion losses –
Filtered optical power meter method
IEC 61291-1, Optical amplifiers – Part 1: Generic specification
IEC/TR 61292-3, Optical amplifiers – Part 3: Classification, characteristics and applications
IEC/TR 61292-4, Optical amplifiers – Part 4: Maximum permissible optical power for the
damage-free and safe use of optical amplifiers, including Raman amplifiers
ITU-T G.664, Optical safety procedures and requirements for optical transport systems
ITU-T G.665, Generic characteristics of Raman amplifiers and Raman amplified subsystems
NOTE A list of informative references is given in the Bibliography.
– 8 – IEC TR 61292-6:2023 RLV IEC 2023
There are no normative references in this document.
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
APR automatic power reduction
DCF dispersion compensating fibre
DOP degree of polarization
DRA distributed Raman amplification
DRB double Rayleigh backscattering
DWDM dense wavelength division multiplexing
EDFA erbium-doped fibre amplifier
ESA electrical spectrum analyzer
FBG fibre Bragg grating
FWHM full width half maximum
GFF gain flattening filter
LRFA lumped Raman fibre amplifier
MPI multi-path interference
NZDSF non-zero dispersion shifted fibre
OA optical amplifier
OFA optical fibre amplifier
OSA optical spectrum analyzer
OSC optical supervisory channel
OSNR optical signal-to-noise ratio
PDG polarization dependent gain
PMD polarization mode dispersion
RIN relative intensity noise
ROADM reconfigurable optical add drop multiplexer
SMF single mode fibre
4 Background
4.1 General
Clause 4 provides a brief introduction to the main concepts of Raman amplification. Further
information can be found in IEC TR 61292-3 and ITU-T G.665 as well as in the Bibliography.
4.2 Raman amplification process
Raman scattering, first discovered by Sir Chandrasekhara Raman in 1928, describes an
inelastic scattering process whereby light is scattered by matter molecules and transferred to a
higher longer wavelength (lower energy). In this interaction between light and matter, a light
photon excites the matter molecules to a high (virtual) energy state, which then relaxes back to
the ground state by emitting another photon as well as vibration (i.e., acoustic) energy. Due to
the vibration energy, the emitted photon has less energy than the incident photon, and therefore
a higher longer wavelength.
Stimulated Raman scattering describes a similar process whereby the presence of a higher
longer wavelength photon stimulates the scattering process, namely the absorption of the initial
lower shorter wavelength photon, resulting in the emission of a second higher longer
wavelength photon, thus providing amplification. This process is shown in Figure 1 a) for silica
fibres, where a ~1 550 nm signal is amplified through absorption of pump energy at ~1 450 nm.
Unlike doped OFAs, such as EDFAs, where the gain spectrum is constant and determined by
the dopants, with Raman amplification the gain spectrum depends on the pump wavelength,
with maximum gain occurring at a frequency of about 13 THz (for silica fibres) below that of the
pump. This is shown in Figure 1 b).
a) Stimulated Raman scattering process b) Raman gain spectrum for silica fibres
Figure 1 – Stimulated Raman scattering process and
Raman gain spectrum for silica fibres
In its most basic form, a Raman amplifier consists of a Raman pump laser, a fibre amplification
medium, and a means of coupling the Raman pump and input signal into the fibre. The main
performance parameter characterizing the Raman amplifier is the on-off gain, which is defined
as the ratio of the output signal (i.e., the signal at the fibre output) when the Raman pumps are
on to the output signal when the Raman pumps are off (the on-off gain will be further discussed
in 6.3.2). Neglecting pump power depletion (i.e., small input signal regime), the on-off gain of a
Raman amplifier can be approximated by
G = 4,34 C PL
R eff
where
G is the on-off gain (in dB);
C is the Raman efficiency between pump and signal;
R
P is the coupled pump power;
L is the effective length of the fibre with respect to the Raman process, defined as
eff
– 10 – IEC TR 61292-6:2023 RLV IEC 2023
−α L
p
1e−
L ≡
eff
α
p
where
α is the fibre attenuation coefficient at the pump wavelength.
p
The Raman efficiency C depends on the separation between the pump and signal wavelengths,
R
as well as their relative polarization. If the pump and signal polarizations are orthogonal, then
C = 0, whereas if they have the same polarization, C is maximum. In many cases, the pump
R R
is depolarized, and then C is approximately half the maximum value. In other cases, the pump
R
and signal relative polarization changes continuously as they propagate along the fibre
has the same average value as for the depolarized pump case.
amplification medium, so that C
R
However, in this case, C may can have some residual dependence on signal polarization,
R
resulting in PDG.
Taking as an example conventional single-mode fibre (SMF) and a depolarized pump with
wavelength of 1 450 nm, then C for a signal located at 1 550 nm is approximately
R
−1
–1 –1
0,4 W km . In the limit of a long fibre, where km, a 500 mW pump provides
L ≈≈α 17
eff p
approximately 15 dB of on-off gain, illustrating the relatively low gain efficiency of the Raman
process. The gain efficiency can be increased using highly non-linear fibre (such as DCF);
however, a relatively long length of fibre (approximately 10 km) is still required needed to
achieve reasonable reasonably high gain.
4.3 Distributed vs. lumped amplification
Typically, OFAs are deployed as lumped (or discrete) amplifiers, meaning that the amplification
occurs within a closed amplifier module. These modules are placed at various points along the
optical link (discrete amplification sites at the end of each fibre span), so that the transmission
signal, which is attenuated along the fibre span, is amplified back to the required desired power
level at the discrete site at the end of each span. This is illustrated graphically by the green
curve in Figure 2. Raman amplifiers may can also be used as discrete amplifiers. However, as
shown in 4.2, this requires special highly non-linear fibres. Even then the application of such
amplifiers is limited due to multi-path interference (to be discussed in 6.3.6), and other issues,
and in most cases other lumped amplifiers, such as EDFAs, are preferable.
While most OFAs require a special doped fibre (such as Erbium doped fibre for EDFAs) to
provide amplification, Raman amplification can occur in any fibre, and in particular within the
transmission fibre itself. This enables distributed Raman amplification (DRA), i.e., the process
whereby the transmission fibre itself is pumped to provide amplification for the signal as it
travels along the fibre. The blue curve in Figure 2 shows signal evolution for distributed Raman
amplification in counter-propagating ("backward") configuration, where the Raman pump power
is introduced at the end of each span and propagates counter to the signal. Since gain occurs
along the transmission fibre, DRA prevents the signal from being attenuated to very low powers
where noise is significant, thus improving the optical signal-to-noise ratio (OSNR) of the
transmitted signal. The fact that the net attenuation of the signal along the span is reduced can
also be utilized to launch the signal into the transmission fibre with less power, which can be
important in applications where signal non-linear effects are an issue. DRA can also be used in
a co-propagating ("forward") configuration, where the Raman pump power is introduced at the
span input and propagates with the signal. The distinction between the two configurations is
discussed in more detail in 4.5.
Figure 2 – Distributed vs. lumped amplification
4.4 Tailoring the Raman gain spectrum
As mentioned earlier, the shape of the Raman gain spectrum depends on the pump wavelength,
with the maximum gain occurring at a wavelength approximately 100 nm higher than the pump
wavelength. This unique feature of Raman amplification enables amplification in any
wavelength band, just by using the appropriate pump wavelengths. Furthermore, multiple
pumps with different wavelengths can be used to achieve flat broadband gain over a large
spectral region, as illustrated in Figure 3.
Besides achieving flat broadband gain, multiple pump wavelengths also help to reduce the
polarization dependent gain (PDG) which can be significant when a single pump is used. This
will be discussed in more detail in 6.2.4 and 6.3.4. The PDG can be further reduced by using
two pumps with the same wavelength but with orthogonal polarization.
Figure 3 – The use of multiple pump wavelengths to achieve flat broadband gain
4.5 Forward and backward pumping configuration
DRA can be deployed in either forward (co-propagating) configuration, where the pump is
introduced together with the signal at the span input, or backward (counter-propagating)
configuration, where the pump is introduced at the end of the span and propagates counter to
the signal. These two pumping configurations are illustrated in Figure 4. Assuming a small input
signal and the same pumps, the on-off gain in both configurations is the same, with the
difference being the position along the span where the amplification takes place.
– 12 – IEC TR 61292-6:2023 RLV IEC 2023
a) Forward pumping configuration b) Backward pumping configuration
NOTE Two pumps at different wavelengths provide a total of 500 mW, resulting in 10 dB on-off gain across the
C-band.
Figure 4 – Simulation results showing pump and signal propagation along an SMF span
The main advantage of the forward pumping configuration is that each dB of Raman gain is
equivalent to effectively increasing the signal launch power by one dB, thus achieving a dB of
OSNR system improvement. However, there are several issues that reduce the overall
effectiveness of the forward pumping configuration.
• Signal non-linear effects: Since the Raman gain occurs a few tens of km within the fibre, the
maximum signal power within the span is less than what would occur if a lumped amplifier
with equivalent gain were to be placed at the beginning of the span. While this reduces
signal non-linear effects, these can still become an issue when the effective launch power
per channel increases, thus placing a practical limit on the amount of forward Raman gain
that can be used.
• Pump relative intensity noise (RIN): Typical commercial semi-conductor Raman pump lasers
have RIN values in the order of –115 dB/Hz. In forward pumping configuration, there is a
long walk-off length between signal and pump, which results in significant transference of
the pump RIN to the signal, thus resulting in a system penalty which can accumulate along
many spans. This is discussed in more detail in 6.2.5.
• Pump depletion: As the composite signal input power increases, pump depletion occurs,
resulting in the reduction of Raman gain. For example, 650 mW of pump power configured
to provide 15 dB flat gain across the C-Band for SMF fibre in the small signal regime will
only provide about 8,5 dB of gain when the composite input signal is 20 dBm. Pump
depletion can also lead to large transient effects when the input signal changes abruptly
(e.g., due to channel add/drop). Unlike EDFAs, where transient effect can be suppressed
using electronic feed-back and feed-forward mechanisms, such effects cannot be fully
suppressed in forward DRA due to the fast response time of the Raman effect and the
distributed nature of the amplification.
While the backward pumping configuration does not suffer from the above disadvantages, the
OSNR improvement is typically more modest since the amplification occurs in the last few tens
of km of the fibre span. For example, 10 dB of Raman gain in the backward configuration will
typically result in about 5 dB OSNR improvement (relative to a lumped amplifier providing the
same gain at the end of the span), while increasing the Raman gain further will only result in
an additional 1 dB to 2 dB OSNR improvement. Further OSNR improvement (typically another
1 dB to 2 dB) can be achieved using complex multi-order Raman pumping schemes, which
involve boosting the Raman pump energy in the transmission fibre with additional pumps at
even shorter wavelengths. Thus, the Raman gain occurs deeper within the span, leading to
improved OSNR.
Overall, the backward pumping configuration usually provides better system performance for
the same amount of Raman pump power and is simpler to implement. Thus, in most systems,
backward pumped DRA is usually deployed first, and then forward pumped DRA only for those
spans where backward pump DRA alone cannot supply sufficient OSNR improvement.
4.6 Typical performance of DRA
As described in Clause 5, DRA is most often used to provide moderate (10 dB to 15 dB) flat
on-off gain in the C-Band, most often in the backward configuration, and less often in the
forward configuration.
Figure 5 shows the gain for SMF in the C-Band provided by a triple pump backward DRA with
pump wavelengths of 1 424 nm (two pumps) and 1 452 nm (one pump). For 10 dB gain about
450 mW of composite pump power is required, whereas for 14 dB gain 650 mW pump power is
required. In this example, a gain of 10 dB is achieved with about 450 mW of composite pump
power, and a gain of 14 dB with 650 mW pump power. Figure 5 also shows the equivalent noise
figure (NF) of the backward DRA for different gains, which is defined as the NF of an equivalent
lumped amplifier (generating the same gain and same amount of ASE) placed at the end of the
span (see 6.3.5 for further detail). In a hybrid EDFA Raman system (see 5.3), backward DRA
is used as a pre-amplifier for a conventional EDFA, which provides the remaining gain required
to compensate the span loss. Since the DRA has a very low effective NF, and since it acts as
a pre-amplifier, it mainly determines the NF of the combined EDFA Raman amplifier. Thus,
assuming a typical NF of about 5 dB for an EDFA, the combined EDFA/Raman amplifier can be
shown to have a composite NF of about 0 dB in the case of 10 dB on-off Raman gain, which
results in a 5 dB OSNR improvement compared to an equivalent EDFA case-based system.
NOTE The various curves correspond to different composite pump powers.
Figure 5 – On-off gain and equivalent NF for SMF using a dual pump
backward DRA with pumps at 1 424 nm and 1 452 nm
– 14 – IEC TR 61292-6:2023 RLV IEC 2023
5 Applications of distributed Raman amplification
5.1 General
DRA offers two unique advantages compared to conventional amplifiers like EDFAs. One
advantage is improved system OSNR and the second is the ability to provide flat gain for any
and multiple transmission bands. These advantages are offset by the high cost of DRA, due to
the high optical pump powers required, as well as by operational issues which will be further
discussed in Clause 7. For this reason, DRA is usually only utilized in those applications where
it offers a significant advantage or there are no other viable alternatives. These applications will
be discussed in Clause 5.
5.2 All-Raman systems
All-Raman systems are systems which utilize only Raman amplification, both DRA and lumped
Raman amplifiers. By using only Raman amplification, such systems benefit from the inherent
OSNR improvement provided by DRA and can be operated in wavelength ranges for which it is
impossible or impractical to provide amplification with more common technologies such as
EDFAs. In particular, all-Raman systems can operate in the L-Band, for which EDFA technology
is much less efficient compared to the C-Band. Since L-Band systems allow for longer system
reach compared to C-Band systems when using non-zero dispersion shifted transmission fibre
(NZDSF), all-Raman L-Band systems are particularly well suited for ultra-long haul (> 1 500 km)
optical links.
A typical configuration of an all-Raman amplification site is shown in Figure 6. The configuration
comprises three Raman pump modules: one for backward DRA, one for forward DRA, and one
for providing lumped Raman amplification within the DCF fibre. In a typical system, such an
amplification site is placed every 80 km, and thus is required along the transmission fibre to
provide approximately 20 dB of net gain. The 20 dB gain is achieved by using forward and
backward DRA and by pumping the DCF so that its net gain is zero (i.e., the on-off Raman gain
exactly compensates the DCF insertion loss, typically about 10 dB). Since DCF has a relatively
high Raman efficiency (due to its small effective area), a relatively small amount of pump power
is required sufficient to pump the DCF.
Besides the relatively high cost of all-Raman systems, it is also difficult to upgrade them to
support reconfigurable optical add drop multiplexers (ROADMs), which are an integral part of
more modern optical networks. The reason for this is two-fold.
• Firstly, additional lumped amplification needs to be provided to compensate for the added
insertion loss of the ROADM modules. One option for providing the additional Raman gain
is to pump the DCF with higher pump power. However, this may can lead to increased MPI
due to double Rayleigh backscattering (see 6.3.6). Another option is to use a separate
lumped Raman amplifier, which further adds to the overall cost of the system.
• Secondly, the transients resulting from system reconfiguration are difficult to suppress,
especially in the case of forward DRA.
For these reasons, the application of all-Raman systems is mainly limited to ultra-long-haul
point-to-point (i.e., non-reconfigurable) optical links.
Figure 6 – Typical configuration of an amplification site in an all-Raman system
5.3 Hybrid EDFA Raman systems
5.3.1 General
EDFA based transmission systems are by far the most common optical communication systems
in deployment today. EDFA technology is mature and well developed, and can provide a cost
effective and efficient solution for most common applications. However, there are some more
challenging applications for which EDFA technology may is not be sufficient. In these cases,
DRA, and particularly backward DRA, is required can be employed to improve the end-to-end
system OSNR.
The cost of adding DRA to EDFA based systems may can be reduced by tightly integrating the
Raman pump module with the EDFA and optimizing the overall design. This is particularly useful
for long-haul and ultra-long-haul applications (see 5.3.4), where DRA is used in every span of
the link. Integration and optimizing of the design may can include, for example, mounting the
Raman and EDFA pumps in the same physical package, thus reducing package costs and
footprint. Additionally, a combined gain flattening filter (GFF) can be designed to take into
account the Raman gain spectral shape as well as the EDFA gain spectral shape, thus reducing
gain flattening requirements for both the EDFA and the Raman amplifier (and possibly reducing
the number of separate Raman pumps). Due to the pre-amplifier function of the Raman DRA,
the GFF can be placed before the EDFA without significantly increasing the composite NF of
the hybrid module, thus reducing the required EDFA pump power.
In 5.3.2 to 5.3.4, applications for hybrid EDFA Raman systems are discussed.
5.3.2 Long repeaterless links
Long repeaterless links (> 150 km) have many applications, such as connecting islands or oil
rigs, traversing hostile or inaccessible terrain, and links where repeater sites may can pose a
security or logistic challenge.
By utilizing backward DRA, the system OSNR can typically be improved by 5 dB to 7 dB,
depending on the pump power. For example, using a 700 mW Raman pump module configured
to provide approximately 15 dB of on-off Raman gain across the C-Band, an OSNR
improvement of approximately 6 dB may can be achieved, depending on the transmission fibre
type, thus allowing the link reach to be extended by approximately 30 km.
For even longer links, it is possible to use forward DRA in addition to backward DRA. Assuming
a system with a 20 dBm EDFA booster, for example, the addition of a 700 mW forward DRA
pump module will provide ~8,5 dB Raman on-off gain. This corresponds to about 7 dB OSNR
improvement (considering the insertion loss of the Raman pump module).
Thus, using forward and backward DRA with moderate pump power (e.g., up to 700 mW), the
system reach for repeaterless links OSNR can be increased by up to 13 dB compared to
corresponding EDFA only systems.
– 16 – IEC TR 61292-6:2023 RLV IEC 2023
5.3.3 Long span masking in multi-span links
Most multi-span links are typically constructed in such a way that in-line EDFA repeaters are
placed after every 80 km to 100 km fibre span. However, geographical limitation may constraints
can require individual spans to be longer, or practical considerations may provide can be an
incentive to reduce the number of spans and thus increase the length of one or more spans. In
both cases, DRA can provide the extra OSNR margins required needed to support the longer
spans. In addition, many systems are designed such that the in-line EDFA can support a limited
gain range while still maintaining flat gain. In this case, besides providing improved OSNR, DRA
allows longer spans to be supported while still using standard EDFAs in the system, thus
increasing system flexibility and utility.
While repeaterless links, discussed in 5.3.2, tend to be static (i.e., non-reconfigurable) point-
to-point links, multi-span links are most often dynamic and thus required to provide ROADM
functionality. Therefore, by nature, such systems may can generate transient events, which are
problematic to suppress when forward DRA is used. This is one reason why forward DRA is not
often used in such applications, and backward DRA is much more common.
5.3.4 High capacity long haul and ultra-long-haul systems
In high capacity systems (with high bit rate and/or dense channel spacing), OSNR quickly
becomes a critical issue as the number of spans increases. By utilizing backward DRA in every
span in the system, the OSNR can be increased significantly, thus allowing the system to
support more spans and/or higher capacity. For example, by providing 10 dB of backward DRA
in each span (approximately 500 mW pump power), the system OSNR can be improved by
about 5 dB compared to an equivalent EDFA-only system, allowing a 3-fold increase in the
reach of the system.
6 Performance characteristics and test methods
6.1 General
Clause 6 describes important performance parameters relevant to DRA and considers test
methods for these parameters. As discussed previously, a fundamental difference between DRA
and lumped amplifiers is that the performance of DRA depends on the transmission fibre, so
that a full characterization of the amplifier performance can only be performed on a system
level, rather than on a device level. However, there are some performance parameters that are
specific to the Raman pump module, which can be specified and measured independently of
the s
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