Optical amplifiers - Part 6: Distributed Raman amplification

IEC/TR 61292-6:2010(E) deals with distributed Raman amplification (DRA). The main purpose of the report is to provide background material for future standards (specifications, test methods and operating procedures) relating to DRA.

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IEC/TR 61292-6
Edition 1.0 2010-02
TECHNICAL
REPORT
Optical amplifiers –
Part 6: Distributed Raman amplification
IEC/TR 61292-6:2010(E)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC/TR 61292-6
Edition 1.0 2010-02
TECHNICAL
REPORT
Optical amplifiers –
Part 6: Distributed Raman amplification
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
ICS 33.160.10; 33.180.30 ISBN 978-2-88910-482-6
® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – TR 61292-6 © IEC:2010(E)
CONTENTS

FOREWORD...........................................................................................................................4

INTRODUCTION.....................................................................................................................6

1 Scope...............................................................................................................................7

2 Normative references........................................................................................................7

3 Abbreviated terms.............................................................................................................8

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........................................................................10

4.5 Forward and backward pumping configuration........................................................11

4.6 Typical performance of DRA ..................................................................................12

5 Applications of distributed Raman amplification...............................................................13

5.1 General .................................................................................................................13

5.2 All-Raman systems................................................................................................13

5.3 Hybrid EDFA Raman systems ................................................................................14

5.3.1 Long repeaterless links ..............................................................................14

5.3.2 Long span masking in multi-span links .......................................................15

5.3.3 High capacity long haul and ultra-long haul systems ...................................15

6 Performance characteristics and test methods ................................................................15

6.1 General .................................................................................................................15

6.2 Performance of the Raman pump module ..............................................................16

6.2.1 Pump wavelengths.....................................................................................16

6.2.2 Pump output power ....................................................................................16

6.2.3 Pump degree-of-polarization (DOP)............................................................17

6.2.4 Pump relative intensity noise (RIN) ............................................................17

6.2.5 Insertion loss .............................................................................................17

6.2.6 Other passive characteristics .....................................................................18

6.3 System level performance......................................................................................18

6.3.1 On-off signal gain ......................................................................................18

6.3.2 Gain flatness .............................................................................................19

6.3.3 Polarization dependant gain (PDG) ............................................................20

6.3.4 Equivalent noise figure...............................................................................20

6.3.5 Multi-path interference (MPI)......................................................................20

7 Operational issues ..........................................................................................................21

7.1 General .................................................................................................................21

7.2 Dependence of Raman gain on transmission fibre..................................................21

7.3 Fibre line quality ....................................................................................................22

7.4 High pump power issues........................................................................................22

7.4.1 Laser safety...............................................................................................23

7.4.2 Damage to the fibre line.............................................................................23

8 Conclusions....................................................................................................................24

Bibliography ..........................................................................................................................25

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TR 61292-6 © IEC:2010(E) – 3 –

Figure 1 – Stimulated Raman scattering process (left) and Raman gain spectrum for

silica fibres (right) ...................................................................................................................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

in forward (right plot) and backward (left plot) pumping configurations ...................................11

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 ....................14

Figure 7 – Typical configuration of a Raman pump module used for counter-propagating

DRA......................................................................................................................................16

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 of gain (a) for co-propagating DRA

and (b) for counter-propagating DRA .....................................................................................19

Figure 10 – Variations of Raman on-off gain for different transmission fibres .........................22

---------------------- Page: 5 ----------------------
– 4 – TR 61292-6 © IEC:2010(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

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with the International Organization for Standardization (ISO) in accordance with conditions determined by

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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC

Publications.

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.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC 61292-6, which is a technical report, has been prepared by subcommittee 86C: Fibre optic

systems and active devices, of IEC technical committee 86: Fibre optics.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
86C/910/DTR 86C/936/RVC

Full information on the voting for the approval of this technical report can be found in the report

on voting indicated in the above table.
---------------------- Page: 6 ----------------------
TR 61292-6 © IEC:2010(E) – 5 –

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

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 amendment and the base publication will

remain unchanged until the stability date indicated on the IEC web site under

"http://webstore.iec.ch" in the data related to the specific publication. At this date, the

publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.

---------------------- Page: 7 ----------------------
– 6 – TR 61292-6 © IEC:2010(E)
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 the many advantage that it offers 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 system in

which it is deployed. Thus, a discrete amplifier is a unique and separate element with a well

defined input and output ports, allowing rigorous specifications of the amplifiers 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 system in which

it is deployed.

This technical report provides an overview of DRA and its applications. It also provides a

detailed discussion of the various performance characteristics related to DRA, some of the

methods that can be used to test these characteristics, and 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.
---------------------- Page: 8 ----------------------
TR 61292-6 © IEC:2010(E) – 7 –
OPTICAL AMPLIFIERS –
Part 6: Distributed Raman amplification
1 Scope

This part of IEC 61292, which is a technical report, deals with distributed Raman amplification

(DRA). The main purpose of the report is to provide background material for future standards

(specifications, test methods and operating procedures) relating to DRA. The report 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 young technology, and still rapidly evolving, some of the material in this

report may become obsolete or irrelevant in a relatively short period. This technical report will

be frequently updated in order 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

---------------------- Page: 9 ----------------------
– 8 – TR 61292-6 © IEC:2010(E)
NOTE A list of informative references is given in the Bibliography.
3 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

This clause provides a brief introduction to the main concepts of Raman amplification. Further

information can be found IEC/TR 61292-3, 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 from matter molecules to a higher

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

wavelength.

Stimulated Raman scattering describes a similar process whereby the presence of a higher

wavelength photon stimulates the scattering process, i.e. the absorption of the initial lower

---------------------- Page: 10 ----------------------
TR 61292-6 © IEC:2010(E) – 9 –

wavelength photon, resulting in the emission of a second higher wavelength photon, thus

providing amplification. This process is shown in Figure 1 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 on the right side of Figure 1.
Raman coefficient in silica fibers
Virtual state
100 nm100 nm
∼1 450 nm pump
∼1 550 nm amplification
Photon relaxation 4
Vibrational states
Ground state
0 100 200 300 400 500 600
Pump - Signal wavelength difference (1/cm)
IEC 418/10 IEC 419/10
Figure 1 – Stimulated Raman scattering process (left)
and Raman gain spectrum for silica fibres (right)

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 6.2.1), Neglecting pump power depletion (i.e. small input signal regime), the on-off gain of a

Raman amplifier can be approximated by
G = 4,34C PL
R eff

where G is the on-off gain (in dB), C the Raman efficiency between pump and signal, P the

coupled pump power, and L the effective length of the fibre with respect to the Raman

eff
process, defined as
−α L
1− e
L ≡
eff
where α is the fibre attenuation coefficient at the pump WL.
The Raman efficiency C depends on the separation between the pump and signal

wavelengths, 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 is depolarized, and then C is approximately half the maximum value. In

other cases, the pump 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. However, in this case, C may have some residual dependence on

signal polarization, resulting in PDG.
Raman coefficient
X1e-14 (m/W)
---------------------- Page: 11 ----------------------
– 10 – TR 61292-6 © IEC:2010(E)

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
–1 –1

0,4 W km . In the limit of a long fibre, where L ≈ α ≈ 17km, a 500 mW pump provides

eff p

approximately 15 dB of on-off gain, illustrating the relatively low gain efficiency of the Raman

process. The gain efficiency can be increase using highly non-linear fibre (such as DCF),

however, a relatively long length of fibre (approximately 10 km) is still required to achieve

reasonable 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 power level at

the discrete site at the end of each span. This is shown graphically by the green curve in

Figure 2. Raman amplifiers may also be used as discrete amplifiers, however, as shown in 4.2,

this requires special highly non-linear fibre. Even then the application of such amplifiers is

limited due to multi-path interference (to be discussed in 6.3.5, and other issues, and in most

cases other lumped amplifiers, such as EDFA’s, are preferable.

While most OFAs require a special doped fibre (such as Erbium doped fibre for EDFA’s) to

provide amplification, Raman amplification can occur in any fibre, and in particular within the

transmission fibre itself. This enable distributed Raman amplification (DRA), i.e. the process

whereby the transmission fibre itself is pumped in order 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 co-propagating (“forward”) configuration, where the Raman pump power is

introduced at input to the span and propagates with the signal. The distinction between the two

configurations will be discussed in more detail in 4.5.
EDFAs High nonlinearities
Distance (km)
Raman amplification
High noise
EDFA amplification
IEC 420/10
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

Signal power (dB)
---------------------- Page: 12 ----------------------
TR 61292-6 © IEC:2010(E) – 11 –

pumps with different wavelengths can be used in order 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 6.2.3 and 6.3.3. The PDG can be further reduced by using

two pumps with the same wavelength but orthogonal polarization.
Resulting gain profile
P1 P3
P2 P4
Wavelength (nm)
IEC 421/10
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 input to the span, 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.

Fibre span Fibre span
Signal Signal
Pump Pump
Pump Pump
unit unit
IEC 422/10 IEC 423/10

NOTE Two pumps at different wavelength 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

in forward (right plot) and backward (left plot) pumping configurations
Gain (dB)
---------------------- Page: 13 ----------------------
– 12 – TR 61292-6 © IEC:2010(E)

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 a number of issues that reduce the overall

effectiveness of the
...

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