FprEN 4533-002
(Main)Aerospace series - Fibre optic systems - Handbook - Part 002: Test and measurement
Aerospace series - Fibre optic systems - Handbook - Part 002: Test and measurement
This document examines the requirements to enable accurate measurement of fibre optic links from start of life and during the life cycle of the system from installation and through-service. This document explains the issues associated with optical link measurement and provides techniques to address these issues. This document discusses the measurement of key parameters associated with the passive layer (i.e. transmission of light through an optical harness). This document does not discuss systems tests, e.g. bit error rates.
Luft- und Raumfahrt - Faseroptische Systeme - Handbuch - Teil 002: Prüfung und Messung
No Scope available
Série aérospatiale - Systèmes des fibres optiques - Manuel d’utilisation - Partie 002 : Essais et mesures
Le présent document examine les exigences nécessaires à la mesure précise des liaisons optiques en début de vie et tout au long du cycle de vie du système depuis son installation jusqu’à ses services. Cette Partie 002 explique les implications associées à la mesure des liaisons optiques et fournit des techniques pour gérer ces implications. Le présent document traite de la mesure des paramètres clés associés à la couche passive (c’est-à-dire la transmission de lumière à travers un harnais optique). Il ne traite pas des essais des systèmes (par exemple : les taux d’erreurs binaires).
Aeronavtika - Sistemi iz optičnih vlaken - Priročnik - 002. del: Preskušanje in merjenje
General Information
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Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 4533-002:2024
01-junij-2024
Aeronavtika - Sistemi iz optičnih vlaken - Priročnik - 002. del: Preskušanje in
merjenje
Aerospace series - Fibre optic systems - Handbook - Part 002: Test and measurement
Luft- und Raumfahrt - Faseroptische Systeme - Handbuch - Teil 002: Prüfung und
Messung
Série aérospatiale - Systèmes des fibres optiques - Manuel d’utilisation - Partie 002 :
Essais et mesures
Ta slovenski standard je istoveten z: prEN 4533-002
ICS:
33.180.01 Sistemi z optičnimi vlakni na Fibre optic systems in
splošno general
49.060 Letalska in vesoljska Aerospace electric
električna oprema in sistemi equipment and systems
oSIST prEN 4533-002:2024 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN 4533-002:2024
oSIST prEN 4533-002:2024
DRAFT
EUROPEAN STANDARD
prEN 4533-002
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2024
ICS 49.060 Will supersede EN 4533-002:2017
English Version
Aerospace series - Fibre optic systems - Handbook - Part
002: Test and measurement
Série aérospatiale - Systèmes des fibres optiques - Luft- und Raumfahrt - Faseroptische Systeme -
Manuel d'utilisation - Partie 002 : Essais et mesures Handbuch - Teil 002: Prüfung und Messung
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee ASD-
STAN.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 4533-002:2024 E
worldwide for CEN national Members.
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Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Fibre types . 5
5 Test and measurement: key parameters . 6
5.1 Insertion loss (I.L.) . 6
5.2 Return or reflection loss. 7
5.3 Optical power measurement . 8
5.4 Light distribution . 11
5.5 Temporal measurements . 11
6 Test and measurement in single-mode systems. 11
7 Test and measurement in multi-mode systems . 12
7.1 General. 12
7.2 Launch conditions. 12
7.3 Generating suitable launch conditions . 16
7.4 Examples of suitable launch conditions for aerospace testing . 17
8 Testing network paths: reflectometry and footprinting . 22
8.1 General. 22
8.2 OTDRs. 23
8.3 OFDRs . 26
8.4 Footprinting . 28
9 General considerations for test and measurement in fibre optic systems . 29
9.1 General. 29
9.2 Instrument-issues . 29
9.3 Test leads . 29
9.4 Adapters (uniters) . 30
9.5 Connectors . 30
9.6 Filters and test leads . 32
10 Practical testing techniques . 32
10.1 General. 32
10.2 Insertion loss . 33
10.3 Different insertion loss techniques . 33
10.4 Single connector insertion loss . 38
10.5 Mode conditioning in test leads . 38
10.6 Return loss schemes . 39
11 Reporting arrangements . 41
12 Techniques for system design . 41
12.1 General. 41
12.2 Interpretation of component data sheets . 41
12.3 Computer modelling . 42
13 Appendix: matrices . 44
Bibliography . 47
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European foreword
This document (prEN 4533-002:2024) has been prepared by ASD-STAN.
After enquiries and votes carried out in accordance with the rules of this Association, this document has
received the approval of the National Associations and the Official Services of the member countries of
ASD-STAN, prior to its presentation to CEN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 4533-002:2017.
The main changes with respect to the previous edition are as follows:
— update of the document to remove trademarks.
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Introduction
a) The handbook
This handbook aims to provide general guidance for experts and non-experts alike in the area of
designing, installing, and supporting fibre-optic systems on aircraft. Where appropriate more detailed
sources of information are referenced throughout the text.
It is arranged in 4 parts, which reflect key aspects of an optical harness life cycle, namely:
— Part 001: Termination methods and tools;
— Part 002: Test and measurement;
— Part 003: Looming and installation practices;
— Part 004: Repair, maintenance, cleaning and inspection.
b) Background
It is widely accepted in the aerospace industry that photonic technology significant advantages over
conventional electrical hardware. These include massive signal bandwidth capacity, electrical safety,
and immunity of passive fibre-optic components to the problems associated with electromagnetic
interference (EMI). Significant weight savings can also be realized in comparison to electrical harnesses
which may require heavy screening. To date, the EMI issue has been the critical driver for airborne
fibre-optic communications systems because of the growing use of non-metallic aerostructures.
However, future avionic requirements are driving bandwidth specifications from 10s of Mbits/s into the
multi-Gbits/s regime in some cases, i.e. beyond the limits of electrical interconnect technology. The
properties of photonic technology can potentially be exploited to advantage in many avionic
applications, such as video/sensor multiplexing, flight control signalling, electronic warfare, and
entertainment systems, as well as sensor for monitoring aerostructure.
The basic optical interconnect fabric or ‘optical harness’ is the key enabler for the successful
introduction of optical technology onto commercial and military aircraft. Compared to the mature
telecommunications applications, an aircraft fibre-optic system needs to operate in a hostile
environment (e.g. temperature extremes, humidity, vibration, and contamination) and accommodate
additional physical restrictions imposed by the airframe (e.g. harness attachments, tight bend radii
requirements, and bulkhead connections). Until recently, optical harnessing technology and associated
practices were insufficiently developed to be applied without large safety margins. In addition, the
international standards did not adequately cover many aspects of the life cycle. The lack of accepted
standards thus leads to airframe specific hardware and support. These factors collectively carried a
significant cost penalty (procurement and through-life costs), that often made an optical harness less
competitive than an electrical equivalent. This situation is changing with the adoption of more
standardized (telecoms type) fibre types in aerospace cables and the availability of more ruggedized
COTS components. These improved developments have been possible due to significant research
collaboration between component and equipment manufacturers as well as the end use airframers.
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1 Scope
This document examines the requirements to enable accurate measurement of fibre optic links from
start of life and during the life cycle of the system from installation and through-service. This document
explains the issues associated with optical link measurement and provides techniques to address these
issues. This document discusses the measurement of key parameters associated with the passive layer
(i.e. transmission of light through an optical harness). This document does not discuss systems tests, e.g.
bit error rates.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
4 Fibre types
This clause gives a brief summary of some of the different fibre types in use within the aerospace
industry. Historically, large core, step index multimode fibres were the first to be used on aircraft. At the
time of design, these fibres enabled sufficient data bandwidth and the large core enabled ease of
coupling (of light) into the fibre as well as ease of fibre alignment in connectors (also termed
interconnects). Therefore, in some current and legacy systems, fibre optic harnesses based on large
core fibres can be found. Common larger core fibres include 200 µm/280 µm, 200 µm/300 µm and
100 µm/140 µm (where the notation indicates the core/cladding size).
Improvements in bandwidth (mainly from reduced temporal dispersion), for multimode fibres is
possible by using graded index fibres. In simple terms, the graded refractive index profile allows
equalization of different optical paths through a multimode fibre to reduce any pulse spreading in time
(dispersion). These results in higher bandwidths compared to step index refractive index profiles.
Early graded index fibres for aerospace included 100 µm/140 µm sized fibres.
More recently, fibre sizes commonly used in the telecoms and datacomms fields have been utilized for
aerospace. Multimode fibres of size 62,5 µm/125 µm and 50 µm/125 µm and with graded index profile
are now being deployed for data transmission on both civil and military aircraft, fixed wind and rotary
craft. Fibres are available with different bandwidths. Multimode fibres are designated by the OM
identification (meaning ‘optical multimode’). OM1 describes 62,5 µm/125 µm fibre, OM2, OM3 and OM4
describe 50 µm/125 µm fibres of increasing bandwidth. Using these sizes of fibre (particularly with a
125 µm outer diameter enables the use of volume production parts (e.g. ceramic alignment ferrules)
from the telecoms industry.
As will be discussed in this document, the issue of test and measurement in multimode systems is
complicated by the light distribution in the fibre and also the relatively short length of installed fibre
which typically has several connector breaks in the harness path (e.g. connectors located at airframe
production breaks). The light distribution launched into the fibre to make measurements is critically
important for making consistent measurements in multimode systems.
Whilst most of the deployed fibre in aerospace is currently multimode, there is increasing interest in
using single-mode fibres. Single-mode fibres (sometimes called monomode fibres) are optical fibres
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designed to support only a single propagation mode per polarization direction for a given wavelength.
They usually have a relatively small core (with a diameter of only a few µm’s) and a small refractive
index difference between core and cladding. The mode radius is typically a few microns.
Single-mode fibres are often termed OS1 (for ‘optical single-mode’). There are also other types of single-
mode fibre as OS2 and A2. The small core enables many benefits to be realized (e.g. higher bandwidth
(minimal dispersion), wavelength multiplexing, novel sensor applications). However, the smaller core
makes the coupling and alignment more difficult at the source and at connectors (particularly in the
harsh aerospace environment with potential extremes of temperature and vibration).
The issue of test and measurement in single-mode fibres is not as complicated as for multimode
systems. This is principally because the light travels down the fibre in a predominant single mode or
path.
It should be remembered that the optical fibres discussed above will be packaged in rugged cable form
suitable for installation and performance on a harness. More detail of cable constructions can be found
in EN 4533-001. It is further noted that modern fibre optical cable designs are now utilizing bend
tolerant optical fibres and a number of aerospace designs exist. These exhibit lower losses when bent to
a small radius (A2 fibre).
Test and measurement in glass fibre multimode systems will generally use LEDs or multimode VSCSELs
as the test light source. Test and measurement in single-mode glass fibre systems will generally use
semiconductor lasers or newer single-mode VCSEL light sources. The common transmission
wavelengths used for glass multimode fibres are 850 nm (sometimes 1 300 nm). Glass single-mode
systems generally use 1 550 or 1 300 nm transmission wavelength.
For completeness, it is noted that plastic optical fibre (POF) is being considered for some applications in
aerospace. However, at the present time, the TRL of this technology is much lower than for glass fibre
(at least in an aerospace environment). POF is generally much larger than glass fibre, e.g. with size
980 µm/1 000 µm. This large core makes coupling and connector alignment much easier. However, POF
is much more lossy (higher attenuation) than glass fibre and works best with visible transmission
wavelengths (typically in the 520 nm to 650 nm region).
5 Test and measurement: key parameters
5.1 Insertion loss (I.L.)
5.1.1 General
Insertion loss is probably the most frequent measurement performed on a fibre optic component or
link/harness during its life cycle.
When an optical device, component or fibre section is inserted into an optical link, some of the optical
power will be lost in the device (e.g. a splitter) or at optical interfaces (e.g. at a connector or splice).
Fibre sections will also introduce a loss albeit small (the attenuation at the common transmission
wavelength for glass fibre is very low).
Some of the optical power will be lost due to non-perfect interfaces, e.g. reflective surfaces or scattering.
Another aggravating factor is misalignment in connectors. Such misalignment can be lateral, axial or
angular. Contamination will also impact on the insertion loss (dirty connectors will have higher loss
than clean connectors).
The insertion loss (or attenuation) is usually specified in decibels, calculated as 10 times the logarithm
of base 10 of the ratio of input power (in) to the output power (out). For fibre connectors, for example,
it is often of the order of 0,2 dB. High-quality fusion splices may reach values like 0,02 dB.
I.L. = 10 log (P /P )
10 in out
EXAMPLE For a transmission of 90 % (0,9), the insertion loss would be 0,46 dB.
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5.1.2 Importance of low-insertion loss
Clearly for efficient light transmission, a low-insertion loss is desired. This means that only a small
amount of light will be lost at the component or link under test. The system power budget will generally
dictate how much power needs to be transmitted through the link from source to receiver.
The difference between the source power and the minimum required receiver power will give a power
budget figure. The total insertion loss of the components shall not exceed this value. It is also useful to
have a safety ‘margin’ to allow for system degradation and ageing. A 3 dB ageing margin is typical for
aircraft links.
5.1.3 Measurement techniques
Various methods exist for measuring insertion loss depending on the type of component and whether it
is connectorized. These are detailed in EN 2591-601. In terms of equipment, insertion loss can be
measured with a fibre optic light source and power metre arrangement or more sophisticated
equipment such as an OTDR or OFBR (discussed later in subclause 7.1 and subclause 7.2) can also be
used.
An important point to emphasize particularly for multimode systems is that the insertion loss measured
for a component, fibre section or complete link will depend on the light distribution in the component
(and the launch light distribution from the source). This means that if two different light sources
(e.g. from different manufacturers) are used to test the insertion loss it is possible that different
insertion loss values will be obtained. This makes it difficult to design systems using test data on
components alone especially where the method used to make measurements is not specified.
Clause 6 of this document will discuss how launch conditions can be practically controlled to ensure
consistency in test and measurement in multimode systems.
Subclause 10.2 discusses some aspects of practical insertion loss measurement.
5.2 Return or reflection loss
5.2.1 General
The return loss R.L. (or reflection loss) of an optical device or link gives a measure of how much light is
reflected back to the light source compared with the amount of light sent into a system.
R.L. is defined by:
R.L. = 10 log (P P )
10 in/ refl
Usually, the return loss is specified in units of decibels. For example, if the return loss is 30 dB, the
returning light has only 1/1 000 of the power of the incident light. Note only directly returned light is
measured – and not light which is reflected into a different direction, e.g. at an angle-cleaved fibre end.
RL is a positive number, and a high RL means that only a small amount of power is reflected from the
link. The power values are generally measured in Watts or mW (noting that all powers should be in the
same units. For lower amounts of reflected light, the return loss therefore becomes a larger number
(e.g. 1/5 000 power reflection is 37 dB return loss). Developing this idea further a perfect device or
component with infinite return loss would reflect no light back towards the source. In reality, real
devices will have a finite return loss.
In avionic applications, due to the short link lengths employed, the back reflected power from the fibre
itself is very small with respect to the reflected power from optical connectors. That said mated
connectors such as UPC and APC (Ultra Physical Contact and Angled Physical Contact) can also have
extremely small return loss in the region of ~ 55 dB to 65 dB (values for single-mode fibre).
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5.2.2 Importance of high-return loss
Some systems (particularly using single-mode and single-frequency lasers) are sensitive to back-
reflected light. If too much light is reflected, this may destabilize the laser operation and cause excessive
laser noise and/or emission on multiple optical frequencies. In high-speed optical fibre
communications, back-reflected light may ultimately increase the bit error rate.
5.2.3 Measurement techniques
As for insertion loss, there are different methods available to measure the return loss. Methods are
detailed in EN 2591-605 (Return loss). A typical method uses a coupler device (e.g. a 50:50 splitter) to
introduce light into a system and also route the reflected light to a power metre. This document is
currently being reviewed and updated to include the latest techniques for measuring return loss with
return loss metres, OTDRs and OFDRs (optical time domain and optical frequency domain reflectometre
instruments respectively).
5.2.4 Return loss versus reflectance
It is important to include a note on terminology as different terms may be encountered in different
texts. Return loss is generally used to describe the amount of reflected light from an optical assembly.
This may be composed of discreet elements (e.g. connectors, fibres, couplers, splices, etc.). Another term
that may be seen is “reflectance”. The reflectance of a connector or of any other type of reflective event
in the fibre (e.g. a kink, damage, or discontinuity) is generally defined for a single discrete event. If we
denote by P the reflected power.
ref
Reflectance is defined by:
Reflectance = 10 log (P /P )
10 ref in
Reflectance is a negative number (the reflected power is less than the incoming power), and a lower
Reflectance means a better connector. In the case of a single reflecting event in an assembly, both RL
(of the whole assembly) and reflectance (of the event) represent the same parameter, with opposite
signs.
IMPORTANT Reflectance or RL of a single event represents the same parameter, with opposite signs,
reflectance being a negative number and RL being a positive number. Accordingly, a good connector
shall have low reflectance (negative value) or equivalently high RL (positive value).
Subclause 10.6 discusses some practical aspects of return loss measurement.
5.3 Optical power measurement
5.3.1 General
Optical power may need to be measured at various points in a fibre link. Power values are important in
assessing the output from a light source or data transmitter. Power at the end of a link (before a
detector) will determine the received power and this will determine the performance of the system.
Power measurements feed into the main calculations of insertion loss and return loss.
Power is normally measured in units of Watts although power levels in fibre systems are typically in the
mW region. It is also common to see power measured in dBm units. These are defined as follows:
−3
Power (dBm) = 10 log10 [Power (Watts)/10 (Watts)].
Thus, a power of 1 mW would equate to 0 dBm. Powers less than 1 mW would be negative
(e.g. 0,5 mW = −3 dBm). A power of 2 mW would be + 3 dBm.
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5.3.2 Measurement techniques
Optical power in fibre optic systems is routinely measured using power metres (see Figure 1).
These are portable instruments usually with a fibre optic interface (to allow connection of different
connector types to the metre). Most power metres will have the option to display the optical power in
units of Watts or dBm. Some instruments will also allow a power to be held as a reference power.
Following the referencing operation, the power measured after this is displayed as a dB loss (compared
to the reference power value). This can be useful for measuring insertion loss of components.
Figure 1 — Benchtop and portable optical power metres
Power metres may be used in conjunction with couplers to measure return loss. Return loss metres are
available that integrate the coupler and power metre components (and sometimes an optical source).
5.3.3 Photodetectors requirements
Power metres will have an active detection area. This should be large enough to collect all the light
emerging from the fibre under test (linked to the NA of the fibre). Thus, the distance to the active
detector element should not be too small so that some of the power is not captured (see Figure 2).
This may only be a problem in very large NA fibres or possibly in large core POF fibre. This problem can
be managed if the power metre uses an integrating sphere. In some cases, it may be important to only
collect light within a given distribution or angular spread. An example might be where a data receiver
has a smaller area or is fibre coupled (having a short section of fibre from a detector to the active
receiver element). In these cases, a test lead at the receiver end can be useful in restricting the detected
power. Measuring the power with a large area detector may over-estimate the useable system power at
the receiver.
Another other important aspect of power measurement is the accuracy of reading. Power metres should
be calibrated regularly to ensure that the power displayed is correct (traceable to national standards).
Most instruments will also have the facility to select a measurement wavelength (this will commonly be
one of the main glass fibre transmission wavelengths (e.g. 850 nm, 1 300 nm, 1 550 nm) although some
power metres (using different detector materials) can measure at other wavelengths (an example might
be 650 nm used for POF systems).
If the power metre readings are to be used to infer insertion loss or return loss values, then it is
important that the power metre has a linear response (the absolute power measurement accuracy is
then not critical as the calculation uses a ratio of power readings).
It is also worth noting that the power emitted says from an LED or laser source may not all be coupled
into a fibre. The fibre will have an acceptance angle (defined by the NA). In some cases, it may be
important to know the ‘fibre coupled’ power, i.e. how much useful light is guided by the fibre. This
measurement may therefore use a section of fibre connected to the light source with the power then
measured at the fibre end.
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Power metres detect light with an active photodetector usually based on semiconductor material.
Common types are Silicon, Germanium and Indium Gallium Arsenide. These materials have different
responsivities at different wavelengths. In the specific case of a photodetector, responsivity measures
the electrical output per optical input. The responsivity of a photodetector is usually expressed in units
of either amperes or volts per watt of incident radiant power. For most aerospace multimode systems
at 850 nm, Silicon PIN photodiodes will be used. These have very good spatial and angular uniformity.
The detector can have a filtering effect on the power distribution if its response varies across the area of
the detector or varies with the angle of incidence of the light. This may be worse at longer wavelengths,
but effects will be detailed in manufacturer data sheets.
Key
1 Area of projected beam from the fibre
2 Detector area
3 End of fibre
Figure 2 — To prevent the detector in the power metre from filtering the power distribution, the
detector has to be larger than the projected beam of light coming from the fibre of the test lead
It should be ensured that the power metre is capable of detecting power at the wavelength(s) in the
system under test. The maximum power level in the system should be detectable under test without
saturation. Also, the power metre should have enough dynamic range to enable correct measurements
to be made at both high and lower power levels. At very low power levels, noise levels may become
important to consider.
It should be remembered that power metres generally measure the average power that is detected
(e.g. if detecting a data modulated light source) although for much of the standard testing (e.g. insertion
loss, back reflection) a cw (continuous wave) source or light launch system will be used.
The refractive index of the materials that are used as detectors is generally high and this means that a
proportion of the light that strikes a detector is reflected. This light can be reflected back onto the
detector by the surroundings and the power reading will be anomalously high. Blacking the
surroundings of the detector will minimize their reflectivity and improve the accuracy of the power
reading. Fibre adapters used to hold the fibre connector in front of the detector are generally black for
this reason.
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5.4 Light distribution
Although not a routine measurement on installed optical harnesses, it may be necessary to measure the
light distribution, e.g. from a source or fibre. Light distributions are critically important especially for
test and measurement in multimode systems where the distribution of light can influence
measurements such as insertion loss. These effects are discussed in more detail in Clause 7. It can be
difficult to measure the near and far-field distributions of some sources like LEDs because the
packaging can obscure some of the emitted light. The distributions can only be measured after passing
through a short length of optical fibre.
Detector arrays (e.g. CCD) may be used along with an imaging system to detect the spatial profile of
light emerging from a system under test. Such systems may also use software to determine the 3D
distribution of light. Alternatively scanning the imaged system with a single detection system
(e.g. pinhole, slit or fibre linked to a power metre) may allow slices across the optical distribution to be
measured. Systems are also available to automatically measure the near field profile of fibres and fit
distributions to pass/fail temples.
5.5 Temporal measurements
For measurements that require temporal detection of the light system, e.g. measurement of data rates,
bandwidth, modulation effects, Bit Error Rate (BER), etc. the light detection system will then generally
be a fast optical detector (with bandwidth capable of detecting the highest data rates). It will usually be
used in conjunction with specialist measurement equipment (oscilloscopes, BER test equipment, signal
analysers for example) to derive the appropriate measurement parameters. Exact details are beyond
the scope of this document. Generally, however, the fast detector will have a front end featuring a fibre
optic interface to allow the system under test to be coupled.
6 Test and measurement in single-mode systems
For single-mode fibre systems, because the light travels down the fibre in a dominant single optical
mode, measurements are not generally dependent on the launch condition into the fibre. The transverse
intensity profile at the fibre output generally has a fixed shape, which is independent of the launch
conditions and the spatial properties of the injected light, assuming that no ‘cladding modes’ can carry
substantial power to the fibre end. The launch conditions only influence the efficiency with which light
can be coupled into the guided mode.
Single-mode fibres are characterized by a physically small core size (typically ~9 µm). They guide in a
single-mode regime within a certain wavelength range. These types of fibre have a single-mode ‘cut-off’
wavelength, beyond which the fibre supports multiple modes.
Although real applications of single-mode fibre are only just starting to be seen in aerospace systems,
single-mode fibres will generally use ‘standard’ telecoms wavelengths e.g. 1 300 nm, 1 550 nm.
Sources are typically lasers. Longer wavelength VCSELs are starting to emerge for single-mode
application. For test and measurement, small compact cw (continuous wave) lasers at these test
wavelengths are available. Note that many multimode aerospace systems use a shorter wavelength of
850 nm because reliable and rugged sources (LEDs and VCSELs) are available at that wavelength and
attenuation is still low at this wavelength (aerospace platforms are relatively short and do not require
ultra-low attenuation as might be required for long haul links).
Efficiently launching light into a single-mode fibre requires that the light beam is of high quality (ideally,
the M beam quality factor should be close to a value of 1) and that the light has a focus at the fibre
input end (for matching to the fibre mode) as well as being axially and angularly aligned to the fibre
core and axis. Any error in position shall be well below the beam radius, and the angular misalignment
shall be small compared with the beam divergence of the mode.
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7 Test and measurement in multi-mode systems
7.1 General
Test and measurement in multimode systems is more complicated than for single-mode. This is because
the measurements made depend on the light distribution within components and fibres.
7.2 Launch conditions
The input launch condition into a multimode fibre system is especially important. In simple terms,
different light sources can have different launch conditions, and this can lead to different measured
values, e.g. of insertion loss. Clearly this is undesirable. Further, the insertion loss of a particular
component can also depend on its position within the link if launch conditions are not controlled.
Key
1 Light launch system
2 Optical power metre
3 Insertion loss (dB)
4 Test equipment
Figure 3 — Variation in insertion loss measurements made on the same harness (round robin
test) using different test sources. This figure emphasizes the need to carefully control light
launch conditions in multimode aerospace links
The effect of launch conditions on the loss of fibre links is especially dramatic in short haul multi-mode
avionic systems that may feature short lengths of fibre as well as many connector breaks and possibly
other optical components. The light in such systems does not settle to an equilibrium modal
distribution as might happen in say a longer haul link.
It is possible to obtain a very large range of loss figures for a system by using different test sources.
For example, a source that injects most of its power into a very narrow angle inside the component will
measure a much smaller loss compared to a source that injects power over a larger range of angles.
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This is because light at the higher angles is more likely to be attenuated by imperfections in the
construction of the component than light at smaller angles.
This effect is clearly demonstrated in Figure 3 which shows the range of insertion loss values recorded
by different test houses on the same optical harness and using a range of light sources. The insertion
loss varied by some 12,1 dB. This Figure emphasizes the need to clearly define and control launch
conditions in multimode fibre links to provide repeatable and reproducible measurements on a
component or system with a low spread of values. An illustration of the advantage of mode conditioning
can be seen in Figure 4 below. Here two measurements were made on an aircraft harness using two
different, unconditioned light sources. The difference in the insertion loss measurements on the harness
was 0,27 dB. By conditioning the light distribution after the source (by using a simple cable mandrel
wrapping technique), the difference between the two power measurements was reduced to 0,02 dB.
Generally, a source will have two key distributions. Firstly, the variation of power across the aperture of
the source (the near field distribution). Secondly, the variation in power with angle or the far field
power distribution. Optical fibres have key parameters of core size (diameter = 2a) and also numerical
aperture (NA). The launch condition is effectively a comparison of the source power distribution to the
parameters of the fibre system under test. For example, a launch condition of 85:85 would describe a
source with a near field profile that fills 85 % of the core and has a far field angular distribution that fills
85 % of the fibre NA.
Key
1 Optical
Figure 4 — Variation in insertion loss measurements made on an aircraft same harness with and
without modal conditioning. This figure emphasizes the value of carefully controlling light
launch conditions in multimode aerospace links
If the launch condition of the source is 100:100, this is called a ‘fully filled’ launch. If either of the
percentages are greater than 100 %, the launch is ‘overfilled’ in either the near or far-field distribution.
If either of the percentages are less than 100 %, the launch is ‘underfilled’ in the corresponding
distribution.
In avionic harnesses, a distribution that fully fills the fibre core and the numerical aperture will lose
light launched close to the core boundary and the maximum angle allowed by the fibre’s numerical
aperture. The resulting distribution will slightly underfill the fibre. The same distribution will
eventually be obtained by a very ‘underfilled’ launch in a harness with a large number of components
because the manufacturing imperfections in the components will scatter light into higher angles and
core positions.
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The ideal launch condition distribution would lie somewhere between an overfilled launch condition
(that would give an unduly pessimistic value for the insertion loss) and a grossly underfilled launch
condition (that would give an optimistic value). In general, the optimum launch condition slightly
underfills both the core of the fibre and the numerical aperture. This removes that part of the source’s
light distribution that is most likely to be attenuated by the harness components due to manufacturing
imperfections. Studies have shown that the correct launch condition lies somewhere between an 80:80
and 90:90 launch.
Another consequence of using an ideal launch condition to make insertion loss measurements is that
the difference between the loss of a complete harness and the sum of the losses of the individual
harness components is minimized. It should be mentioned that the launch distribution of a source used
to transmit data down the fibre in a real system will also have a given power distribution. This may well
be different to the source used to characterize the fibre system although if the two are similar, then the
performance of the real system link can be anticipated from the loss data of the system components.
In summary, well defined launch conditions are required to:
— ensure reliable measurement of component loss (and therefore allow system loss to be calculated);
— enable reliable and repeatable characterization of an aerospace multi-mode fibre optic link, in
particular to be sensitive to extreme and undesirable effects suc
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