IEC TR 63309:2025

Active fibres – Characteristics and measurement methods – Guidance

IEC TR 63309: 2025, which is a Technical Report, provides an introduction of active fibres describing key characteristics and measurement methods. For the purpose of this document, an active fibre is a silica-based optical fibre doped in the core with rare-earth ions to allow optical gain, named rare-earth doped fibre. Other fibres enabling optical gain by means of different effects (e.g. Raman effect) are not included in the scope of this document.

General Information

• Status: Published
• Publication Date: 04-Jun-2025
• ICS: 33.180.10 - Fibres and cables
• Technical Committee: SC 86A - Fibres and cables
• Drafting Committee: WG 1 - TC 86/SC 86A/WG 1
• Current Stage: PRVDTR - Preparation of RVDTR
• Start Date: 13-Dec-2024
• Completion Date: 13-Dec-2024

Overview

IEC TR 63309:2025 - Active fibres – Characteristics and measurement methods – Guidance - is a Technical Report from the IEC that introduces active fibres, defined here as silica-based optical fibres whose cores are doped with rare-earth ions (rare‑earth doped fibre) to provide optical gain. Edition 1.0 (2025‑06) summarizes key characteristics, classification, and practical measurement methods used for characterizing active fibres for use in fibre lasers and fibre amplifiers. The scope explicitly excludes fibres that rely on other gain mechanisms (e.g., Raman-effect fibres).

Key topics

• Background & classification
  • Definition of active fibres and rare‑earth elements (Er, Yb, Nd, Tm, etc.)
  • Cladding and core geometries affecting pump coupling and gain
• Key characteristics analyzed
  • Fibre geometry (core/inner-cladding cross sections)
  • Optical characteristics: attenuation, numerical aperture (NA)
  • Laser transmission characteristics: absorption coefficient, slope efficiency, high‑power scaling, and photodarkening
  • Nonlinear and limiting effects: mode instability (MI), stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS)
• Measurement method guidance
  • General measurement principles and recommended apparatus layouts (pump calibration, oscillator/amplifier test setups)
  • Detailed guidance sections for slope efficiency, absorption coefficient, optical‑to‑optical conversion efficiency, photodarkening, and high‑power scaling
  • Practical notes on sample preparation, test procedures and data interpretation (figures and schematic diagrams support setup design)
• Informative annex
  • Annex A provides typical applications (e.g., Er‑doped fibre amplifiers) and contextual examples

Applications and practical value

IEC TR 63309 is practical guidance for:

• Manufacturers of rare‑earth doped fibres validating product performance (attenuation, slope efficiency, photodarkening resistance)
• Test laboratories developing reproducible measurement procedures and test rigs for fibre characterization
• Laser and amplifier designers selecting active fibre types and assessing high power scaling and nonlinear limits (SRS/SBS, MI)
• Component suppliers and system integrators optimizing pump coupling, numerical aperture and cladding shapes for improved pumping efficiency

The report helps translate laboratory metrics into system‑level expectations for fibre lasers, high‑power amplifiers, and precision optical amplifiers used in industrial, telecom and scientific applications.

Related standards

• IEC TR 61931 - Fibre optic terminology
• IEC TS 62627‑09 - Vocabulary for passive optical devices
• References also note IEC TR 61282‑4 regarding SRS/SBS phenomena

Keywords: IEC TR 63309, active fibres, rare‑earth doped fibre, fibre laser, measurement methods, slope efficiency, photodarkening, absorption coefficient, numerical aperture.

Standards Content (Sample)

IEC TR 63309 ®
Edition 1.0 2025-06
TECHNICAL
REPORT
Active fibres – Characteristics and measurement methods – Guidance

ICS 33.180.10  ISBN 978-2-8327-0453-0

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– 2 – IEC TR 63309:2025 © IEC 2025
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references. 6
3 Terms and definitions . 6
4 Abbreviated terms . 7
5 Background of active fibres . 8
6 Classification . 12
6.1 Cladding structure . 12
6.2 Rare-earth elements . 12
7 Analysis of key characteristics . 12
7.1 Fibre geometry . 12
7.2 Optical characteristics . 13
7.2.1 Attenuation . 13
7.2.2 Numerical aperture . 14
7.3 Laser transmission characteristics . 15
7.3.1 Absorption coefficient . 15
7.3.2 High power scaling . 16
7.3.3 Slope efficiency . 16
7.3.4 Photodarkening . 16
8 Measurement method guidance of key characteristics . 17
8.1 General . 17
8.2 Guidance for slope efficiency measurement . 19
8.2.1 Object . 19
8.2.2 Apparatus . 19
8.2.3 Sample preparation . 21
8.2.4 Procedure . 22
8.3 Guidance for absorption coefficient measurement . 28
8.4 Guidance for optical-optical conversion efficiency measurement . 28
8.5 Guidance for photodarkening measurement . 28
8.6 Guidance for high power scaling measurement . 28
Annex A (informative) Application of active fibres . 29
Bibliography . 30

3+
Figure 1 – Yb 's energy level diagram in a germane-silicate host [1] . 8
3+
Figure 2 – Typical absorption and emission spectra of Yb ions in germanosilicate
host [2] . 9
Figure 3 – Working principle diagram of active fibre in a fibre lasers . 10
3+
Figure 4 – Er 's energy level diagram of stimulated emission [3] . 11
3+ 3+
Figure 5 – Er /Pr co-doped energy level diagram of stimulated emission [4] . 11
Figure 6 – Schematic diagram of the geometric cross-sections of the core and inner
cladding used in the simulation . 13
Figure 7 – Pumping efficiency of different cladding shapes [5] . 13
Figure 8 – Light absorption along the longitudinal axis . 15
Figure 9 – Definition of slope efficiency . 16
Figure 10 – Photodarkening example [8] . 17

Figure 11 – Typical schematic diagram for apparatus to calibrate pump light power . 22
Figure 12 – Typical schematic diagram for apparatus to laser power test via oscillator
system with beam splitter mirror . 23
Figure 13 – Typical schematic diagram for apparatus to laser power test via oscillator
system without beam splitter mirror . 24
Figure 14 – Typical schematic diagram for apparatus to laser power test via amplifier
system with beam splitter mirror . 25
Figure 15 – Typical schematic diagram for apparatus to laser power test via amplifier
system without beam splitter mirror . 26
Figure 16 – Representation of slope efficiency graph . 28
3+
Figure A.1 – Typical structure of Er -doped fibre amplifiers . 29

Table 1 – Key characteristics and relevant standards . 18
3+
Table 2 – Characteristic wavelength of RE in silica host . 19

– 4 – IEC TR 63309:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ACTIVE FIBRES – CHARACTERISTICS AND
MEASUREMENT METHODS – GUIDANCE

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 international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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shall not be held responsible for identifying any or all such patent rights.
IEC TR 63309 has been prepared by subcommittee 86A: Fibres and cables, of IEC technical
committee 86: Fibre optic. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
86A/2515/DTR 86A/2586/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/publications.
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, or
• revised.
– 6 – IEC TR 63309:2025 © IEC 2025
ACTIVE FIBRES – CHARACTERISTICS AND
MEASUREMENT METHODS – GUIDANCE

1 Scope
This document provides an introduction of active fibres describing key characteristics and
measurement methods. For the purpose of this document, an active fibre is a silica-based
optical fibre doped in the core with rare-earth ions to allow optical gain, named rare-earth doped
fibre. Other fibres enabling optical gain by means of different effects (e.g. Raman effect) are
not included in the scope of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 TR 61931, Fibre optic – Terminology
IEC TS 62627-09, Fibre optic interconnecting devices and passive components – Vocabulary
for passive optical devices
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TR 61931, IEC TR
62627-09, and the following apply.
ISO and IEC maintain terminology 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
3.1
active fibre
optical fibre whose core includes an active medium capable of stimulated emission
3.2
rare-earth element
RE element
“ensemble of the chemical elements with atomic numbers between 58 and 71 such as erbium,
ytterbium, neodymium etc (see 6.2)
3.3
absorption coefficient
how far incident light of a certain wavelength penetrates a material before being absorbed
3.4
slope efficiency
slope of the linear (linearized) dependence of the output laser power on the pumping power
when the pump power is higher than threshold power

3.5
photodarkening
phenomenon that the optical power losses in a medium can grow when the medium is irradiated
with light at certain wavelengths
3.6
mode instability
MI
beam quality of the output suddenly becomes quite poor when pump power exceeds a certain
threshold value
Note 1 to entry: It is also called transverse mode instability (TMI).
3.7
stimulated Raman scattering
SRS
non-linear phenomenon of light scattering produced by the phonon interaction of light and
molecular vibrations
Note 1 to entry: The interaction is between a signal pump photon and with a transverse (optical) phonon.
Note 2 to entry: As described in IEC TR 61282-4.
3.8
stimulated Brillouin scattering
SBS
non-linear phenomenon of light scattering produced by light and sound wave variations in the
refractive index
Note 1 to entry: As described in IEC TR 61282-4.
3.9
fibre Bragg grating
FBG
fibre type passive optical device (component) which has modulated refractive index profile in
the core of the optical fibre
[SOURCE: IEC TR 62627-09:2016]
3.10
wavelength division multiplexing
WDM
non-linear phenomenon that separate wavelengths are allotted to several independent signals
(optical channels) for transmission over a common optical transmission medium
4 Abbreviated terms
CT charge transfer
FWM four-wave mixing
LD laser diode
MOPA master oscillator power amplifier
NA numerical aperture
ODC oxygen deficient centre
PD positive displacement
YAG yttrium aluminum garnet
– 8 – IEC TR 63309:2025 © IEC 2025
5 Background of active fibres
Active fibres are usually used in a fibre laser or a fibre amplifier, as shown in Annex A. The
typical technology of active fibres is rare-earth doped fibre. By pumping the rare-earth elements
with a certain wavelength pump light, the fibre can emit longer and sometimes shorter
wavelength fluorescence.
Fibre lasers usually use active fibre as gain medium. Pumped by a certain wavelength pump
light into a fibre's core or cladding (it is not applicable for single cladding fibre to be injected
pump light from cladding), power density in an active fibre could be high enough to obtain a
population inversion，and then generate laser oscillation within a resonant cavity. With high
conversion efficiency, low threshold, high gain and high beam quality, fibre lasers are becoming
the choice for most major production laser applications as well as converting traditional welding
and cutting processes to fibre laser technologies.
Rare-earth elements in fibre lasers could be neodymium (Nd), erbium (Er), ytterbium (Yb),
3+ 3+
thulium (Tm) etc. Figure 1 shows the absorption and emission spectrum of Yb ion. Yb
energy level structure consists of two manifolds, the ground manifold F (with four Stark
7/2
levels, labeled L to L ), and a well separated excited manifold F (with three Stark levels,
0 3 5/2
labeled U to U ). Approximated energies in wavenumbers above ground energy are indicated.
0 2
Absorption and emission cross sections for a germane-silicate host are shown with a blue solid
3+
line and red dash-dot lines in Figure 2, and its inset is the energy levels structure of Yb ions
in silica which has been introduced before. The absorption or fluorescence peak at 975 nm
represents the zero–line transition between the lowest energy levels of the ground state L and
the excited state U in the manifold. The absorption peak at shorter wavelength (B) corresponds
to 860 nm and 909 nm. In a similar way, other peaks of the two curves correspond to different
electronics jumping.
-1
11 630 cm
11 000
10 260
1 490
1 060
Reproduced with the permission of IntechOpen Limited.
3+ 1
Figure 1 – Yb 's energy level diagram in a germane-silicate host [1]
___________
The numbers in square brackets are shown in Bibliography.
860 nm
909 nm
975 nm
907 nm
962 nm
1 035 nm
975 nm
1 035 nm
1 087 nm
1 140 nm
909 nm
962 nm
1 006 nm
1 052 nm
Reproduced with the permission of Scientific Research Publishing.
(A): zero–line transition between the lowest energy levels of the ground state L and the excited
state U in the manifold, 975 nm band; (B): absorption peak at shorter wavelength, corresponds
to 860 nm and 909 nm; (C): 1 035 nm band absorption; (D): around 1 035 nm band emission;
(E): 909 nm band emission.
3+
Figure 2 – Typical absorption and emission spectra of Yb ions
in germanosilicate host [2]
Doping-ion and doping concentration can directly affect the pump excitation efficiency, then
influence the output power and photodarkening effect. Figure 3(a) shows an example of working
principle diagram of double cladding Yb-doped fibre, and Figure 3(b) shows a ring laser with a
single cladding Er-doped fibre as active fibre.

– 10 – IEC TR 63309:2025 © IEC 2025

a) a FBG laser configuration with a double cladding Yb-doped fibre as active fibre

b) a ring laser configuration
c) a single cladding Er-doped fibre as active fibre

Figure 3 – Working principle diagram of active fibre in a fibre lasers
Incident signal can be gained with a population inversion caused by doped ions under pump
light. As repeater, pre-amplifiers or power amplifiers in optical communication networks, doping
fibre amplifiers have many excellent characteristics, such as high gain, high bandwidth, high
output power, high pump efficiency, low insertion loss, and polarization insensitivity.

Figure 4 shows the Er's energy level diagram of stimulated emission. E1 is ground state energy
level which is the lower level of laser, E2 is metastable energy level which is the upper level of
laser, E3 is pump high energy level whose non-radiative decay probability is high. N1, N2 and
N3 is population at E1, E2 and E3 separately. Electrons are pumped from E1 to E3, and N3
would be larger than N1, which is population inversion mentioned previously. Then they decay
from E3 to E2 without any radiation. When they jump from E2 to E1, photons will be released.
The wavelength of the light is generally distributed within some specific range, such as from
1 520 nm to 1 570 nm.
N3 E3
Fast Decay
Pump
N2 E2
980 nm
1 520nm
Stimulated emission
to 1 570 nm
Pump
1 480 nm
1 550nm
Signal input
Signal output
1 550nm
N1 E1
Key
E1 ground state energy level, the lower level of laser
E2 metastable energy level, the upper level of laser
E3 pump high energy level
N1,N2,N3 population at E1, E2 and E3 separately
Reproduced with the permission of IJMCR.
3+
Figure 4 – Er 's energy level diagram of stimulated emission [3]
Figure 5 shows the energy level diagram of stimulated emission of Er/Pr co-doped fibre. It is
well known that pump ESA and cooperative up-conversion affect the lifetimes of the I and
11/2
4 3+ 3+
I levels of Er ions. Up-conversion luminescence from Er was also found at 530 nm
13/2
2 4 4 4 4 4 4 4
H → I ), 550 nm ( S → I ), 670 nm ( F → I ), and 550 nm ( F → I ) from
(
11/2 15/2 3/2 15/2 9/2 15/2 7/2 13/2
3+ 3+ 3+ 4
the Pr /Er co-doped fibre when the 986 nm pump source is used. Lifetimes of the Er : I
13/2
3+
level increases from 3,84 ms to 4,29 ms (±0,05 ms) by increasing the Er concentration. This
3+
is due to the energy migration and reabsorption processes among Er ions.

Reproduced with the permission of AIP Publishing.
3+ 3+
Figure 5 – Er /Pr co-doped energy level diagram of stimulated emission [4]

– 12 – IEC TR 63309:2025 © IEC 2025
6 Classification
6.1 Cladding structure
Active fibre families, according to the cladding structure, consist of the following types:
a) Single cladding active fibre;
b) Double cladding active fibre.
6.2 Rare-earth elements
Active fibre families, according to the type of rare-earth elements, consist of the following types:
a) erbium (Er) doped fibres;
b) ytterbium (Yb) doped fibres;
c) neodymium (Nd) doped fibres;
d) thulium (Tm) doped fibres;
e) holmium (Ho) doped fibres;
f) praseodymium (Pr) doped fibres;
g) other types not listed above, for instance combinations of rare-earth elements.
7 Analysis of key characteristics
7.1 Fibre geometry
Even though active fibre are doped in the core with rare-earth elements, they are essentially
fibres. This means that there is a need to determine their geometric dimensions: core diameter,
cladding diameter, core-cladding concentricity error, circularities, and coating diameter etc.
For active fibre fibres, core diameter is important since larger effective core area can guide
higher power levels without suffering nonlinearities, so core diameter has a significant effect on
the characteristics of fibre lasers and has undoubtedly become the first concerned geometry
index of all users.
As for cladding diameter, double cladding active fibre can have special characteristics.
Optical fibres typically possess an axially symmetric circular shape, however, it causes a major
issue when a cladding pumping scheme is employed. Some of the pump rays launched to the
first cladding does not cross the core due to the very high degree of symmetry in the fibre
structure, and as a result, the pump light is not efficiently absorbed. It is called a skew ray [5].
Figure 6 (a) shows the circular cladding and skew ray.
This skew ray can be mitigated by breaking circular symmetry in the fibre structure, as
schematically shown in Figure 6(b) to 6(d). Typical examples are offset core [6], D-shaped
cladding, or octagonal (or other polygonal) cladding [7]. Most fibres used in cladding pumping
scheme employs such a "mode-mixing" means for efficient pump absorption. Figure 7 shows
pumping efficiency of several cladding structures [5].

a) b) c) d)
Reproduced with the permission of Optica Publishing Group.
(a) Circular, (b) Rectangular, (c) D-shaped, and (d) Hexagonal inner cladding shape [5]
Figure 6 – Schematic diagram of the geometric cross-sections
of the core and inner cladding used in the simulation

Reproduced with the permission of Optica Publishing Group.
Figure 7 – Pumping efficiency of different cladding shapes [5]
It is very important to evaluate the inner cladding geometry, as it influences the cross-sectional
area of inner cladding and the matching with other active fibre and passive fibre. Because of
the irregular shape of the inner cladding of double cladding active fibre, the key issue is how to
define and measure an inner cladding diameter. A model can be used to fit a given cladding
shape to an intended shape, such as a quasi-octagonal, and dimension parameters will be
measured or calculated based on the fitting result then. The detailed definition and
measurement method of inner cladding diameter will be specified in a separate chapter or
specification.
7.2 Optical characteristics
7.2.1 Attenuation
Attenuation, as one of the most important characteristic parameters, determines the distance
of optical fibre communication. There are many reasons for the attenuation, mainly including
absorption attenuation (impurity absorption, intrinsic absorption etc), scattering attenuation
(linear scattering, nonlinear scattering etc) and other attenuation (micro bending attenuation).

– 14 – IEC TR 63309:2025 © IEC 2025
Generally, users of active fibre pay attention to attenuation at different wavelengths. When it
comes to double cladding active fibre, much attention has been paid to both core and cladding
attenuations.
Core attenuation of active fibre is sometimes referred to as background loss. Because core
attenuation characterizes the transmission performance due to OH group, impurities, or other
factors at 1 200 nm or 1 300 nm, which have less absorption or emission for doped rare-earth
elements, the term “background loss” is also used instead of core attenuation, but not cladding
attenuation. Some active fibre suppliers use the term “background loss” in the product
introductions, but mostly the term “core attenuation” is used.
Unlike absorption coefficient, core attenuation tends to avoid influence of pump absorption.
Under the condition of high power，if the core attenuation is high, the heat of the optical fibre
will be significant, and the temperature will rise quickly. On one hand, the probability of burnout
of optical fibres or devices will increase; on the other hand, high temperature will also reduce
the emission cross sections of rare-earth ions，and then reduce the efficiency of fibres. In
addition, high temperatures can also lead to thermal mode instability (TMI), including induced
photon darkening, which leads to a decrease in laser power stability. Therefore, it's necessary
to decrease the core attenuation with the best efforts to achieve high performance of the optical
fibre on laser power stability.
As for testing methods of active fibre's attenuation, both core attenuation and cladding
attenuation are measured according to method A of IEC 60793-1-40, cut-back. For core
attenuation testing, stripping cladding light is necessary, while for cladding attenuation testing,
it is not necessary to strip cladding light.
In order to measure core attenuation accurately, cladding light is completely stripped to make
sure that only signal light in the core and no residual cladding light launched into the power
meter or spectrum analyser. So, it is advisable to use a single cladding fibre as pigtail with
matched core diameter and core NA to launch signal light into the active fibre's core, to avoid
launching light into the inner cladding. If the launch signal cannot be restricted to the core using
a single cladding fibre, apply stripping treatment near the output end of the active fibre to
eliminate residual cladding light. Usually, more than 20 cm of coating (both lower refractive
index resin and outer coating resin) is stripped, and high refractive index resin is recoated. The
stripped length can be optimised by monitoring the output power when coating stripped length
is from short too long. If power no longer decreases, the stripping length is enough.
Different winding radius of optical fibre will also lead to different measurement values. It is good
practice to make sure the winding diameter is larger than 10 cm or 500 times the fibre's inner
cladding diameter.
It is advisable to select wavelengths with as little pump absorption as possible to test
attenuation. For example, for Yb doped fibres, as the absorption band of Yb ion is from 975 nm
to 1 080 nm or even longer, 1 200 nm or 1 300 nm is generally selected to test core at
...