IEC 60728-6:2001

Cabled distribution systems for television and sound signals - Part 6: Optical equipment

Lays down the measuring methods, performance requirements and data publication requirements of optical equipment of cable networks for for television signals, sound signals and interactive services.

General Information

• Status: Published
• Publication Date: 29-Jan-2001
• ICS: 33.060.40 - Cabled distribution systems
• ICS: 33.180.10 - Fibres and cables
• Technical Committee: TC 100 - Audio, video and multimedia systems and equipment
• Current Stage: DELPUB - Deleted Publication
• Start Date: 31-Jul-2003
• Completion Date: 26-Oct-2025

Relations

• Revised: IEC 60728-6:2003 - Cable networks for television signals, sound signals and interactive services - Part 6: Optical equipment
• Effective Date: 05-Sep-2023

Overview

IEC 60728-6:2001 - Cabled distribution systems for television and sound signals – Part 6: Optical equipment - defines the measurement methods, performance requirements and data publication requirements for optical equipment used in cable networks carrying television, sound and interactive services. It applies to optical transmitters, receivers, amplifiers, splitters, directional couplers, isolators, multiplexers, connectors and splices and covers typical wideband frequency ranges (example given 5 MHz to 3 000 MHz). The standard ensures reproducible testing, minimum guaranteed performance and clear published data for optical components used in CATV/MATV/SMATV and related headend systems.

Key topics and technical requirements

IEC 60728-6 specifies methods and parameters to assess optical equipment performance, including:

• Methods of measurement for optical power, loss, isolation, directivity, return loss and saturation.
• Modulation and spectral parameters: central wavelength, spectral width under modulation, linewidth and chirp (single‑mode lasers), extinction ratio and optical modulation index (OMI).
• Receiver characteristics: voltage responsivity, frequency range and flatness, receiver intermodulation.
• Nonlinear and distortion metrics: composite second-order (CSO), composite triple beat (CTB), composite cross‑modulation.
• Noise and quality metrics: carrier‑to‑noise ratio (C/N), relative intensity noise (RIN), noise figure of optical amplifiers and bit error rate (BER).
• Influence factors: polarization effects, dispersion and environmental/EMC/safety considerations.
• Passive component tests: connector and splice loss, splitter/coupler isolation and directivity.
• Data publication: required reporting of guaranteed performance values for use by network designers and purchasers.

Applications

• Designing and qualifying optical front-end equipment for cable TV headends and distribution networks.
• Factory acceptance testing and compliance verification of optical transmitters, receivers and amplifiers.
• Specifying components (splitters, couplers, isolators) for hybrid optical/coaxial distribution systems.
• Troubleshooting and performance benchmarking in operator networks for video, audio and interactive services.

Who uses this standard

• Cable network operators (CATV, MATV, SMATV)
• Optical and RF equipment manufacturers
• Test laboratories and certification bodies
• System integrators and headend engineers
• Procurement teams specifying guaranteed performance data

Related Standards

• IEC 60728-1, -2, -3, -5 (other parts of the cabled distribution systems series)
• IEC 60825-1 (safety of laser products)
• IEC 60068 (environmental testing) and relevant EMC/safety standards

Keywords: IEC 60728-6, optical equipment standard, cabled distribution systems, optical transmitter, optical receiver, optical amplifier, CATV, measurement methods, return loss, CSO, CTB, noise figure, BER.

Standards Content (Sample)

INTERNATIONAL IEC
STANDARD
60728-6
First edition
2001-01
Cabled distribution systems
for television and sound signals –
Part 6:
Optical equipment
Reference number
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.

Consolidated editions
The IEC is now publishing consolidated versions of its publications. For example,

edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the
base publication incorporating amendment 1 and the base publication incorporating
amendments 1 and 2.
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The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology. Information relating to
this publication, including its validity, is available in the IEC Catalogue of
publications (see below) in addition to new editions, amendments and corrigenda.
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INTERNATIONAL IEC
STANDARD
60728-6
First edition
2001-01
Cabled distribution systems
for television and sound signals –
Part 6:
Optical equipment
 IEC 2001  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: [phone] e-mail: [e-mail] IEC web site http://www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
X
International Electrotechnical Commission
For price, see current catalogue

– 2 – 60728-6  IEC:2001(E)
CONTENTS
Page
FOREWORD . 4

INTRODUCTION .5

Clause
1 Scope . 6

2 Normative references . 6
3 Terms, definitions, symbols and abbreviations . 7
3.1 Terms and definitions. 7
3.2 Symbols. 11
3.3 Abbreviations . 13
4 Methods of measurement . 14
4.1 General measurement requirements. 14
4.2 Optical power. 14
4.3 Loss, isolation, directivity and coupling ratio . 15
4.4 Return loss . 16
4.5 Saturation of output power of an optical amplifier. 18
4.6 Influence of polarization . 19
4.7 Central wavelength and spectral width under modulation . 20
4.8 Linewidth and chirp of transmitters with single-mode lasers . 21
4.9 Extinction ratio . 23
4.10 Optical modulation index . 25
4.11 Voltage responsivity of an optical receiver . 26
4.12 Frequency range and flatness . 27
4.13 Composite second-order distortion (CSO) of optical transmitters . 28
4.14 Composite triple beats (CTB) of optical transmitters . 29
4.15 Composite cross-modulation of optical transmitters . 30
4.16 Receiver intermodulation. 33
4.17 CSO of optical amplifiers. 36
4.18 CTB of optical amplifiers . 36
4.19 Carrier-to-noise ratio . 36
4.20 Method for combined measurement of relative intensity noise (RIN), optical

modulation index and equivalent input noise current . 40
4.21 Noise figure of optical amplifiers. 42
4.22 Bit error rate (BER) . 44
4.23 Influence of dispersion . 45
5 Universal performance requirements and recommendations . 46
5.1 Safety . 46
5.2 Electromagnetic compatibility (EMC) . 46
5.3 Environmental. 46
5.4 Marking. 47
6 Active equipment. 47
6.1 Optical transmitters . 47
6.2 Optical receivers . 48
6.3 Optical amplifiers . 49

60728-6  IEC:2001(E) – 3 –
Clause Page
7 Passive equipment . 50

7.1 Connectors and splices . 50

7.2 Multiplexers, splitters, directional couplers and isolators . 50

Figure 1 – Measurement of optical power. 15

Figure 2 – Measurement of optical loss, directivity and isolation . 16

Figure 3 – Measurement of the optical return loss . 17

Figure 4 – Measurement of the saturation of the optical output power. 18
Figure 5 – Saturated optical output power . 19
Figure 6 – Measurement of the polarization stability . 20
Figure 7 – Measurement of central wavelength and spectral width under modulation . 21
Figure 8 – Measurement of the chirp and the linewidth of transmitters. 22
Figure 9 – Measurement of the extinction ratio. 24
Figure 10 – Measurement of the optical modulation index . 25
Figure 11 – Measurement of the voltage responsivity of an optical receiver . 27
Figure 12 – Measurement of the frequency range and flatness . 28
Figure 13 – Device under test for measuring CSO of optical transmitters. 29
Figure 14 – Device under test for measuring CTB of optical transmitters . 30
Figure 15 – Arrangement for measuring composite cross-modulation
of optical transmitters . 32
Figure 16 – Arrangement of test equipment for measuring receiver intermodulation. 35
Figure 17 – System with internal noise sources . 36
Figure 18 – PIN diode receiver. 37
Figure 19 – Optical transmission system under test. 38
Figure 20 – Arrangement of test equipment for carrier-to-noise measurement . 39
Figure 21 – Measurement set-up for determination of the noise parameters
and the optical modulation index. 41
Figure 22 – Measurement of the noise figure of an optical amplifier. 43
Figure 23 – Measurement of the bit error rate . 45

– 4 – 60728-6  IEC:2001(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________
CABLED DISTRIBUTION SYSTEMS FOR TELEVISION

AND SOUND SIGNALS –
Part 6: Optical equipment
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60728-6 has been prepared by subcommittee 100D: Cabled
distribution systems, of IEC technical committee 100: Audio, video and multimedia systems and
equipment.
The text of this standard is based on the following documents:
FDIS Report on voting
100/169/FDIS 100/198/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.
The committee has decided that the contents of this publication will remain unchanged until
2002. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition; or
• amended.
A bilingual version of this standard may be issued at a later date.

60728-6  IEC:2001(E) – 5 –
INTRODUCTION
Standards of the IEC 60728 series deal with cable networks for television signals, sound

signals and interactive services including equipment, systems and installations

– for headend-reception, processing and distribution of sound and television signals and their

associated data signals, and
– for processing, interfacing and transmitting all kinds of interactive multimedia signals using

all applicable transmission media.

They cover all kinds of networks such as

– CATV-networks,
– MATV-networks and SMATV-networks,
– individual receiving networks,
and all kinds of equipment, systems and installations installed in such networks.
The scope of these standards extends from antennas and special signal source inputs to
headend or other interface points, to networks as a whole up through system outlets, or
terminal inputs where no system outlet exists.
The standardization of any user terminals (i.e. tuners, receivers, decoders, multimedia
terminals, etc.) is excluded.
– 6 – 60728-6  IEC:2001(E)
CABLED DISTRIBUTION SYSTEMS FOR TELEVISION

AND SOUND SIGNALS –
Part 6: Optical equipment
1 Scope
This part of IEC 60728 lays down the measuring methods, performance requirements and data
publication requirements of optical equipment of cable networks for television signals, sound
signals and interactive services.
This standard
– applies to all optical transmitters, receivers, amplifiers, splitters, directional couplers,
isolators, multiplexers, connectors and splices used in cable networks;
– covers the frequency range 5 MHz to 3 000 MHz;
NOTE The upper limit of 3 000 MHz is an example, but not a strict value. The frequency range or ranges, over
which the equipment is specified, shall be published.
– identifies guaranteed performance requirements for certain parameters;
– lays down data publication requirements with guaranteed performance;
– describes methods of measurement for compliance testing.
All requirements and published data relate to minimum performance levels within the specified
frequency range and in well-matched conditions as might be applicable to cable networks for
television signals, sound signals and interactive services.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 60728. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 60728 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.

IEC 60068-2 (all parts), Environmental testing – Part 2: Tests
IEC 60416, General principles for the formulation of graphical symbols
IEC 60417-1, Graphical symbols for use on equipment – Part 1: Overview and application
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60617 (all parts), Graphical symbols for diagrams
IEC 60728-1:1986, Cabled distribution systems – Part 1: Systems primarily intended for sound
and television signals operating between 30 MHz and 1 GHz

60728-6  IEC:2001(E) – 7 –
IEC 60728-2, Cabled distribution systems for television and sound signals – Part 2:

1)
Electromagnetic compatibility of equipment

IEC 60728-3: 1997, Cabled distribution systems for television and sound signals – Part 3:

Active coaxial wideband distribution equipment

IEC 60728-5, Cabled distribution systems for television and sound signals – Part 5: Headend

1)
equipment
IEC 60825-1, Safety of laser products – Part 1: Equipment classification, requirements and

1)
user's guide
3 Terms, definitions, symbols and abbreviations
3.1 Terms and definitions
For the purposes of this part of IEC 60728, the following definitions apply.
3.1.1
optical transmitter
device for converting electrical signals into optical signals. It consists of a light source (for
example, Iaser) and its associated components as well as all components between the coaxial
input and optical output connectors
3.1.2
optical receiver
device for converting optical signals into electrical signals. It consists of a detector (for
example, PlN-diode) and its associated components as well as all the components between the
optical input and coaxial output connectors
3.1.3
optical amplifier
device for amplifying optical signals direct. It consists of an active medium (and its associated
components), which amplifies the optical signal without demodulation or regeneration
3.1.4
optical isolator
device which transports optical power in one direction only
3.1.5
optical fibre splice
permanent joint of two fibre ends
3.1.6
splitter
device in which the signal power at the (input) port is divided equally or unequally between two
or more (output) ports
NOTE Some forms of this device may be used in the reverse direction for combining signal energy.
3.1.7
directional coupler
splitter in which the attenuation between any two output ports exceeds the sum of the
attenuations between the input port and each of those output ports
___________
1)
To be published
– 8 – 60728-6  IEC:2001(E)
3.1.8
multiplexer
device in which the signal energy covering a frequency band at one input port is divided

between two or more output ports each of which covers a part of that frequency band

NOTE 1 For example, a diplexer is a two-port multiplexer.

NOTE 2 Some forms of this device may be used in the reverse direction for combining.

3.1.9
extinction ratio
ratio of the high-level φ φ
optical power to the low-level optical power of a modulated optical
h l
transmitter:
φ
h
e =
(1)
φ
l
This term is mainly used for digital systems
3.1.10
optical modulation index
the optical modulation index is defined as:
φ − φ
hl
m =
(2)
φ + φ
hl
where φ is the highest and φ is the lowest instantaneous optical power of the intensity
h l
modulated optical signal. This term is mainly used for analogue systems
3.1.11
noise figure/factor
figures of merit describing the internally generated noise of an active device. The noise factor
NF is the ratio of the carrier-to-noise ratio at the input to the carrier-to-noise ratio at the output
of an active device, assuming the incoming carrier is noise-free:
/
C N
1 1
NF =
(3)
/
C N
2 2
where
C is the signal power at the input;
C is the signal power at the output;
N is the noise power at the input
(ideal thermal noise for electrical devices; quantum noise for optical devices);

N is the noise power at the output.
In other words, the noise factor is the ratio of noise power at the output of an active device to
the noise power at the same point if the device had been ideal and added no noise:
N
2,actual
NF =
(4)
N
2,ideal
The noise factor is dimensionless and is often expressed as noise figure F in dB:
F = 10 lg NF
(5)
60728-6  IEC:2001(E) – 9 –
3.1.12
relative intensity noise (RIN)

ratio of the mean square of the intensity fluctuations in the optical power of a light source to the

square of the mean of the optical output power

NOTE The value for the RIN can be calculated from the results of a carrier-to-noise measurement for the system

(see 4.19).
3.1.13
noise equivalent power (NEP)
notional optical power which, when applied to the input of a noiseless optical receiver, would
give rise to an electrical output noise power density equal to that observed at the output of an

actual receiver under consideration
NOTE The NEP can be calculated from the carrier-to-noise ratio C/N (see 4.19) of a receiver using:
− C/N
mP
(6)
NEP = 10
2B
In this equation, m is the optical modulation index, P is the received optical power and B is the bandwidth. The NEP
shall be expressed in units of W/√Hz.
3.1.14
equivalent input noise current density
notional input noise current density which, when applied to the input of an ideal noiseless
device, would produce an output noise current density equal in value to that observed at the
output of the actual device under consideration
NOTE It can be calculated from the carrier-to-noise ratio C/N (see 4.19) of a device or system using:
C
=
I r
(7)
C/N
Z
In this equation, C is the amplitude of the carrier at the input of the device or system and Z is its input impedance.
The equivalent input noise current density shall be expressed in units of A/√Hz.
3.1.15
bit error rate (BER)
number of erroneous bits at the output of a system divided by the total number of received bits.
This term is used in digital transmission systems
3.1.16
responsivity
ratio of the output current of a photodiode to the incident optical power
I
r = (static responsivity) (8)
s
P
dI
r = (dynamic responsivity) (9)
d
dP
For practical purposes, static and dynamic responsivities can be assumed to be equal.
3.1.17
voltage responsivity of an optical receiver
ratio of the change of output voltage to the change of the incident optical power
dU
=
r (10)
V
dP
– 10 – 60728-6  IEC:2001(E)
3.1.18
chromatic dispersion
minus the change of group travel time per unit length of fibre per change of wavelength

NOTE The velocity at which an optical pulse travels on a fibre depends on its wavelength.

3.1.19
wavelength
the wavelength λ of light in vacuum is given by

c
λ = (11)
f
where
cis 2,99793 × 10 m/s (speed of light in vacuum);
f is the optical frequency.
Although the wavelength in dielectric material such as fibres is shorter than in a vacuum, only
the wavelength of light in a vacuum is used
3.1.20
chirp
incidental frequency modulation caused by the intensity modulation of a laser diode
NOTE Chirping effectively broadens the laser spectral bandwidth. Due to the chromatic dispersion of the fibre,
parts of the spectrum travel at different speeds, resulting in harmonic distortion of the transferred signal.
3.1.21
polarization
projection of the electric vector on a plane perpendicular to the direction of transmission of the
polarized light wave
3.1.22
linewidth
spectral bandwidth of an individual mode of a laser, defined as the difference between those
optical frequencies at which the amplitude reaches or first falls to half of the maximum
amplitude
3.1.23
coherence time and coherence length
coherence time is the time which light needs to travel the coherence length; coherence length
is the reciprocal of 2π times the linewidth. Both values are used to describe the phase stability
of a light source
3.1.24
well-cleaved
well-cleaved end of a fibre has a clean plane front perpendicular to the axis of the fibre
3.1.25
amplified spontaneous emission (ASE)
part of an optical amplifier's output power caused by photons emitted from excited ions whose
lifetime was over before their energy was used for amplification
3.1.26
directivity
attenuation between the output port and interface port minus the attenuation between input and
interface port, of any equipment or system

60728-6  IEC:2001(E) – 11 –
3.1.27
central wavelength
average of those wavelengths at which the amplitude of a light source reaches or last falls to

half of the maximum amplitude
3.1.28
spectral width
difference of those wavelengths at which the amplitude of a light source reaches or last falls to

half of the maximum amplitude
3.2 Symbols
The following graphical symbols are used in the figures of this standard. These symbols are
either listed in IEC 60617 or based on symbols defined in IEC 60617.
E
Optical transmitter [10-14-01]
O
O
Optical receiver [10-14-01]
E
Optical amplifier [02-09-01, 10-15-01]
Optical fibre [10-23-1]
Isolator [10-08-20]
Coupler [02-01-01, 10-09-04]
Directional coupler [02-01-01, 10-09-09]
Delay line [10-16-23]
τ
Polarization control device
– 12 – 60728-6  IEC:2001(E)
Low-pass filter [10-16-5]
Bandpass filter [10-16-6]
Variable attenuator [10-16-02]
A
G
Pulse generator [10-13-04]
G
Sine-wave generator [10-13-02]
G
Bit pattern generator
Voltmeter [02-01-01, 08-02-01]
V
Current meter [02-01-01, 08-01-01]
A
Power meter [02-01-01, 08-01-01]
P
Bit error detector [02-01-01, 08-01-01]
BER
Oscilloscope [02-01-01, 08-02-10]
Selective voltmeter [02-01-02, 02-03-01, 08-01-01, 10-16-06]
V
60728-6  IEC:2001(E) – 13 –
P(f) Spectrum analyser [02-01-01, 08-01-01]

RF choke [04-03-01]
Ground [02-15-04]
Resistor [04-01-01]
Capacitor [04-02-01]
DC power supply [02-16-03]
Amplifier [10-15-01]
Photodiode with fibre pigtail [10-24-02]
3.3 Abbreviations
The following abbreviations are used in this standard:
AC alternate current
AGC automatic gain control
ALC automatic level control
ASE amplified spontaneous emission
BER bit error rate
CATV community antenna television (network)
C/N carrier-to-noise ratio
CSO composite second order
CTB composite triple beat
CW continuous wave
dB decibel
DC direct current
EMC electromagnetic compatibility
IF intermediate frequency
MATV master antenna television (network)
MTBF mean time between failure

– 14 – 60728-6  IEC:2001(E)
NEP noise equivalent power
NF noise figure
PS polarization stability
PRBS pseudo random bit sequence

RF radio frequency
RIN relative intensity noise
SMATV satellite master antenna television (network)

XM composite cross-modulation
4 Methods of measurement
4.1 General measurement requirements
For all methods of measurements described in this section, the following requirements shall be
considered.
4.1.1 Input specification
The following conditions shall be obtained from the device specification:
– supply voltage(s);
– control signal(s), if any, with correct impedance, level and frequency.
4.1.2 Measurement conditions
Unless otherwise specified, all measurement shall be carried out under the following
conditions:
– the ambient or reference point temperature shall be (25 ± 5) °C;
– the ambient humidity shall be in the range 40 % to 70 %;
– sufficient care shall be taken to ensure that optical reflection does not impair the accuracy
of the measurement;
– during measurement any control input signal(s) shall be held constant.
4.2 Optical power
4.2.1 Purpose
The purpose of this test method is to measure the average optical power emanating from the
end of a test fibre. The test fibre and the coupling means shall be as specified by the manu-
facturer. The optical power shall be expressed in dBm.
4.2.2 Equipment required
4.2.2.1 An optical power meter with a range suitable for the expected power. The detector
system of the power meter shall have a sufficiently large area to collect all the radiation from
the test fibre and a spectral sensitivity compatible with the light source. A minimum accuracy of
±10 % is recommended.
4.2.2.2 A length of fibre for connecting the light source to the power meter.

60728-6  IEC:2001(E) – 15 –
4.2.2.3 A cladding mode stripper if the fibre has no cladding mode stripping coating.

4.2.2.4 Test signal generator(s).

4.2.3 General measurement requirements

4.2.3.1 A digital transmitter shall be modulated with a pseudo random bit sequence (PRBS)

having a sequence length of at least 2 – 1, with the specified pulse repetition frequency and

pulse width at the specified extinction ratio. Analogue transmitters shall be modulated with at

least one modulation carrier at the specified optical modulation index.

4.2.3.2 Cladding modes shall be stripped from the fibre by means of suitable cladding mode
stripping techniques.
4.2.4 Procedure
4.2.4.1 Set the supply voltage(s) and any control input signal(s) to the specified value(s).
4.2.4.2 Connect the equipment as shown in figure 1.
Test fibre
E
G
P
O
Cladding
mode stripper
IEC  2640/2000
Figure 1 – Measurement of optical power
4.2.4.3 Connect the optical output of the device under test to the detector (a power meter)
through the test fibre and the specified coupling means.
4.2.4.4 Measure and record the output power using the power meter.
4.2.5 Potential sources of error
4.2.5.1 The inaccuracy of the power meter, for example, if its dark current is not sufficiently
low.
4.2.5.2 The attenuation of the test fibre and the specified coupling means.

4.3 Loss, isolation, directivity and coupling ratio
The measurement of the following parameters is based on the measurement of optical power,
and therefore no special methods of measurement are given for these items:
– loss of fibres, connectors, multiplexers, and optical isolators;
– gain of optical amplifiers;
– directivity of optical couplers;
– isolation of optical isolators, multiplexers and optical couplers.

– 16 – 60728-6  IEC:2001(E)
4.3.1 General measurement requirements

4.3.1.1 Optical couplers, multiplexers and isolators shall be tested with a light source suitable

for the specified wavelength range.

4.3.1.2 All optical inputs or outputs not involved during the measurement shall be terminated

to make sure that no reflected light can impair the accuracy of the measurement.

4.3.2 Principle of measurement

4.3.2.1 Connect the light source to the power meter and measure the optical output power P
of the light source (see 4.2).

4.3.2.2 Connect the device under test to the light source and the optical power meter as
shown in figure 2 and measure the power P .
Terminator
E
Device
Terminator
under
test
O
P
IEC  2641/2000
Figure 2 – Measurement of optical loss, directivity and isolation
4.3.2.3 The loss, gain, directivity or isolation is calculated by
P
a = 10 lg
(12)
P
4.4 Return loss
4.4.1 Purpose
Reflection of the transmitted light occurs at the boundary of two different dielectric media. The

return loss is the reflectance which is the ratio of the incident optical power P to the reflected
in
optical power P , expressed in dB. The purpose of this test is to measure the return loss of
back
an optical equipment.
4.4.2 Equipment required
4.4.2.1 A fused fibre coupler with a directivity higher than the return loss to be measured.
4.4.2.2 A continuous light source.
4.4.2.3 An optical power meter with a dynamic range higher than the return loss to be
measured.
4.4.2.4 Lengths of fibre for connecting the optical equipment.
4.4.2.5 Two optical terminators with reflection ideally 20 dB better than the return loss to be
measured.
60728-6  IEC:2001(E) – 17 –
4.4.2.6 A well-cleaved end of a fibre, which provides 14,5 dB return loss.

4.4.3 General measurement requirements

The length of the fibre for connecting the light source to the coupler shall be longer than the
coherence length of the light source.

4.4.4 Procedure
4.4.4.1 Set the supply voltage(s) and any control input signal(s) to the specified value(s).

4.4.4.2 Connect the equipment as shown in figure 3.

Terminator
E
O
Fibre end
A
Terminator
P
Device
under
test
IEC  2642/2000
Figure 3 – Measurement of the optical return loss
4.4.4.3 Connect the well-cleaved fibre end with 14,5 dB return loss to port A of the coupler and
note the reading of the power meter.
P
4.4.4.4 Connect the second terminator to port A of the coupler and note the reading of the
P
power meter.
4.4.4.5 Connect the device under test to port A of the coupler and note the reading P of the
power meter. If the device under test has more than one optical port, the other ports shall be
terminated with low reflection.
4.4.4.6 The return loss of the device shall be calculated from:

−
PP12
=+14,l5 10 g  (dB)
a (13)
r
−
PP32
4.4.5 Potential sources of error
4.4.5.1 The return loss of the connection at port A shall be at least as high as the return loss
of the device under test. Otherwise, the dynamic range of the measurement will suffer.
4.4.5.2 If the impedance matching of the terminators produces a reflection which is not much
less than the reflection of the device under test, the accuracy will suffer.
4.4.5.3 The instability of the light source.
4.4.5.4 The inaccuracy of the power meter.

– 18 – 60728-6  IEC:2001(E)
4.5 Saturation of output power of an optical amplifier

4.5.1 Purpose
The purpose of this test method is to measure the mean optical output power of a test fibre

whose far end is connected to the optical output port of a saturated optical amplifier. The

saturated optical output power shall be expressed in dBm.

4.5.2 Equipment required
4.5.2.1 A light source of suitable wavelength and output power for driving the optical

amplifier.
4.5.2.2 A calibrated variable optical attenuator for adjusting the optical power fed to the
optical amplifier.
4.5.2.3 Three lengths of fibre for connecting the equipment.
4.5.2.4 An optical power meter with a range suitable for the expected power. The detector
system of the power meter shall have a sufficiently large area to collect all the radiation from
the fibre and a spectral sensitivity compatible with the light source. A minimum accuracy of
±10 % is recommended.
4.5.2.5 A cladding mode stripper if the fibre has no cladding mode stripping coating.
4.5.2.6 An optical isolator and connecting length of fibre for use when the optical amplifier to
be tested does not incorporate one.
4.5.3 General measurement requirements
Cladding modes shall be stripped from the fibre by means of suitable cladding mode stripping
techniques.
4.5.4 Procedure
4.5.4.1 Set the supply voltage(s) and any control input signal(s) to the specified value(s).
4.5.4.2 Connect the equipment as shown in figure 4.
4.5.4.3 Connect the optical output of the device under test to the detector (a power meter)
through the specified coupling means.
4.5.4.4 Starting with low input power levels, decrease the attenuation of the variable optical
attenuator in suitable steps. During this procedure, measure and record the output power using

the power meter.
E
A
P
O Cladding
mode stripper
Optical isolator
(if not embodied
in optical amplifier)
IEC  2643/2000
Figure 4 – Measurement of the saturation of the optical output power

60728-6  IEC:2001(E) – 19 –
4.5.4.5 A typical result of this measurement is shown in figure 5. At low input levels the output

power increases linearly. At higher input levels the gain decreases. The saturated output power

is reached when the output power lags 3 dB behind the extrapolated linear value.

P
out
3 dB
P
sat
P
in
IEC  2644/2000
Figure 5 – Saturated optical output power
4.5.5 Potential sources of error
4.5.5.1 The attenuation of the fibres and the coupling means.
4.5.5.2 The inaccuracy of the optical power meter and the optical attenuator.
4.5.5.3 The amplitude and wavelength instability of the light source.
4.5.5.4 The amplified spontaneous emission (ASE) at the output port of the amplifier.
4.6 Influence of polarization
4.6.1 Purpose
The purpose of this test method is to measure the effect of polarization changes on loss or
gain under specified conditions. The polarization stability shall be expressed as the logarithmic
ratio, in dB, of the maximum and minimum amplitude at the output of a device when the
polarization at the input changes between 0° and 360°.
4.6.2 Equipment required
4.6.2.1 A light source with a wavelength suitable for the device under test. The polarization of
the emanating light shall be constant.

4.6.2.2 A polarization control device capable of changing the polarization of the test signal
by 360°.
4.6.2.3 An optical power meter with a range suitable for the expected power at the output of
the device under test. The detector system of the power meter shall have a sufficiently large
area to collect all the radiation from the fibre and a spectral sensitivity compatible with the light
source. A minimum accuracy of ±10 % is recommended.
4.6.2.4 Lengths of fibre for connecting the optical devices. These shall be short enough,
straight, unstressed and not birefringent to ensure that the polarization is not changed by them.
4.6.2.5 A cladding mode stripper if the fibre has no cladding mode stripping coating.
4.6.3 Procedure
4.6.3.1 Set the supply voltage(s) and any control input signal(s) to the specified value(s).

– 20 – 60728-6  IEC:2001(E)
4.6.3.2 Connect the equipment as shown in figure 6.

E
Device
P
under
test
O Cladding
mode stripper
Polarization
control device
IEC  2645/2000
Figure 6 – Measurement of the polarization stability
4.6.3.3 Vary the polarization of the light fed to the device under test by 360° (in not less than
1 s in the case of optical amplifiers). Record the minimum (P ) and the maximum power
min
(P ) at the output.
max
4.6.3.4 The polarization stability PS is derived as follows:
P
max
10 lg
PS =
(14)
P
min
4.6.4 Potential sources of error
4.6.4.1 The inaccuracy of the power meter and the polarization control device.
4.6.4.2 The amplitude and wavelength instability of the light source.
4.7 Central wavelength and spectral width under modulation
4.7.1 Purpose
The purpose of this test method is to measure the central wavelength λ of the spectrum and
the spectral width Δλ of a transmitter under modulation. The central wavelength and the
spectral width shall be expressed in nm. The method described is not suitable for light sources
and transmitters with very narrow spectral width (single-mode laser) or for measuring the chirp
of transmitters (see 4.8).
4.7.2 Equipment required
4.7.2.1 An optical spectrum analyser with a wavelength range suitable for the transmitter to

be tested.
4.7.2.2 A length of fibre for connecting the transmitter to the optical spectrum analyser.
4.7.2.3 A test signal generator for modulating the transmitter.
4.7.3 General measurement requirements
A digital transmitter shall be modulated with a pseudo random bit sequence (PRBS) having a
sequence length of at least 2 – 1, with the specified pulse repetition frequency and pulse
width at the specified extinction ratio. Analogue transmitters shall be modulated with at least
one modulation carrier at the specified optical modulation index.

60728-6  IEC:2001(E) – 21 –
4.7.4 Procedure
4.7.4.1 Connect the equipment as shown in figure 7.

E
Optical
G
P(λ) spectrum
analyser
O
Generator(s)
IEC  2646/2000
Figure 7 – Measurement of central wavelength and spectral width under modulation
4.7.4.2 Measure the power level of the highest power spectrum using the optical spectrum
analyser.
4.7.4.3 Set the optical spectrum analyser to a short wavelength and then adjust it to a
λ
progressively longer wavelength. Record the wavelength , at which half of the maximum
peak reading is obtained or exceeded for the first time.
4.7.4.4 Set the optical spectrum analyser to a long wavelength and then adjust it to a
progressively shorter wavelength. Record the wavelength λ , at which half of the maximum
peak reading is obtained or exceeded for the first time.
4.7.4.5 The central wavelength is calculated from
+
λ λ
1 2
(15)
=
λ
4.7.4.6 The spectral width is calculated from
(16)
Δλ = λ − λ
2 1
4.7.5 Potential sources of error
4.7.5.1 The inaccuracy of the optical spectrum analyser.
4.7.5.2 Mode partition noise and instability of the transmitter. This can be avoided by
averaging a suitable number of measurements.
4.7.5.3 Using connectors with angled front can lead to wrong wavelength readings if the input

of the optical spectrum analyser is not a fibre.
4.7.5.4 Any temperature instability of the device.
4.8 Linewidth and chirp of transmitters with single-mode lasers
4.8.1 Purpose
The purpose of this test method is to measure the linewidth and the frequency modulation, or
chirp, of a transmitter with single-mode laser. The linewidth shall be expressed in MHz. The
chirp shall be expressed in MHz/mA.

– 22 – 60728-6  IEC:2001(E)
4.8.2 Equipment required
4.8.2.1 An RF signal generator which can be gated on and off with a 50 % duty cycle so that

the transmitter is operating unmodulated for a time, τ, and then modulated for an identical time.

The magnitude and the waveform of the generated signal shall be suitable for the transmitter to

be tested. The signal frequency shall be lower than the linewidth of the transmitter to be tested.

4.8.2.2 A fibre-optic Mach-Zehnder interferometer with a delay line producing a delay

difference τ between the 2 arms and with a polarization controller in one of the arms.

4.8.2.3 An optical receiver with a 1 dB bandwidth higher than the expected frequency

deviation of the optical output signal of the transmitter to be tested.

4.8.2.4 An electrical spectrum analyser with a bandwidth greater than the expected
frequency deviation of the optical output signal of the transmitter to be tested.
4.8.2.5 Lengths of fibre for connecting the optical equipment.
4.8.2.6 An optical isolator, if not embodied in the transmitter, to prevent reflected light from
perturbing the lineshape of the transmitter.
4.8.2.7 An RF voltmeter with the same input impedance as the optical transmitter to be
measured.
4.8.3 General measurement requirements
The delay time τ (identical to the gating time τ of the signal generator) shall be at least three to
five times the coherence time of the transmitter to make sure that the combining signals from
the two arms of the interferometer are uncorrelated.
4.8.4 Procedure
4.8.4.1 Connect the equipment as shown in figure 8.
G G
Mach-Zehnder interferometer
URF voltmeter
Optical Optical
τ
coupler coupler
Delay
E O
P(
...