SIST EN 4533-001:2020

SLOVENSKI STANDARD
01-maj-2020
Nadomešča:
SIST EN 4533-001:2009
Aeronavtika - Sistemi iz optičnih vlaken - Priročnik - 001. del: Metode določanja in
orodja
Aerospace series - Fibre optic systems - Handbook - Part 001: Termination methods and
tools
Luft- und Raumfahrt - Faseroptische Systemtechnik - Handbuch - Teil 001:
Verarbeitungsmethoden und Werkzeuge
Série aérospatiale - Systèmes des fibres optiques - Manuel d'utilisation - Partie 001 :
Méthodes des terminaisons et des outils
Ta slovenski standard je istoveten z: EN 4533-001:2020
ICS:
33.180.10 (Optična) vlakna in kabli Fibres and cables
49.060 Letalska in vesoljska Aerospace electric
električna oprema in sistemi equipment and systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 4533-001
EUROPEAN STANDARD
NORME EUROPÉENNE
February 2020
EUROPÄISCHE NORM
ICS 49.090 Supersedes EN 4533-001:2006
English Version
Aerospace series - Fibre optic systems - Handbook - Part
001: Termination methods and tools
Série aérospatiale - Systèmes des fibres optiques - Luft- und Raumfahrt - Faseroptische Systemtechnik -
Manuel d'utilisation - Partie 001 : Méthodes des Handbuch - Teil 001: Verarbeitungsmethoden und
terminaisons et des outils Werkzeuge
This European Standard was approved by CEN on 2 March 2018.

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. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists 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, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 4533-001:2020 E
worldwide for CEN national Members.

Contents Page
European foreword . 5
Introduction . 6
a) The Handbook . 6
b) Background . 6
1 Scope . 7
1.1 General . 7
1.2 Need to high integrity terminations . 8
2 Normative references . 8
3 Component Selection . 8
3.1 Elements . 8
3.2 Fibre optic cables . 9
3.2.1 General . 9
3.2.2 Cable construction . 9
3.2.3 Fibre choice . 10
3.2.4 Cladding materials . 12
3.3 Primary buffer materials. 13
3.3.1 Function . 13
3.3.2 Acrylate . 13
3.3.3 Polyimide . 13
3.3.4 Silicone . 14
3.3.5 Strength Members . 14
3.4 Outer jacket . 14
3.5 Fibre optic interconnects (connectors) . 15
3.5.1 Introduction . 15
3.5.2 The Optical interface . 15
3.5.3 Single-way Interconnects/Connectors . 23
3.5.4 Multi-way Interconnects/Connectors . 23
3.5.5 Choice of tooling . 24
4 Health and safety aspects . 25
4.1 General . 25
4.2 Chemicals . 25
4.3 Sharps . 26
5 Termination process . 26
5.1 Objective . 26
5.2 Cable preparation . 26
5.2.1 General . 26
5.2.2 Cutting to length . 26
5.2.3 Removal of outer jacket . 28
5.2.4 Cable Handling tools (gripping the cable) . 33
5.2.5 Strength member trimming/ removal . 34
5.3 Removal of secondary coating(s) . 35
5.4 Removal of primary coatings . 36
5.4.1 General . 36
5.4.2 Mechanical techniques for primary coating removal . 36
5.4.3 Alternative techniques . 42
5.4.4 Troublesome coatings – Polyimide and Silicone . 43
5.4.5 Evidence of strength reduction when stripping primary buffer coatings . 45
5.4.6 To clean or not to clean . 46
5.5 Adhesives . 47
5.5.1 General . 47
5.5.2 Adhesive types . 47
5.5.3 The importance of glass transition temperature (T ) . 49
g
5.5.4 Epoxy cure schedule . 51
5.5.5 Usability. 53
5.5.6 Qualification . 57
5.6 Connector preparation . 57
5.6.1 Dry fitting . 57
5.7 Attachment of fibre to the terminus . 59
5.7.1 Application of adhesive . 59
5.7.2 Inserting Fibre ‘Best-Practice’ . 62
5.8 Adhesive cure . 66
5.8.1 General . 66
5.8.2 Orientation . 66
5.8.3 Curing equipment . 67
5.9 Excess Fibre removal . 71
5.9.1 General . 71
5.9.2 Post-cure rough cleaving . 71
5.9.3 Pre-cleave . 73
5.9.4 Safety . 73
5.9.5 Cleaving tools . 73
5.9.6 Sprung blade hand tools . 74
5.9.7 Cleaving fibres in Multi-fibre Ferrules . 75
5.10 Polishing . 75
5.10.1 Rationale . 75
5.10.2 Performance metrics . 75
5.10.3 End face geometries . 75
5.10.4 End-face geometry parameters . 76
5.10.5 Polishing stages . 86
5.10.6 Methods for controlling end-face geometry . 100
6 Beginning of life Inspection . 106
6.1 Optical or Visual Inspection . 106
6.2 Interferometric Inspection . 109
6.2.1 Inspection and Pass/Fail Criteria . 110
Bibliography . 113

European foreword
This document (EN 4533-001:2020) has been prepared by the Aerospace and Defence Industries
Association of Europe — Standardization (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, prior to its presentation to CEN.
This document shall be given the status of a national standard, either by publication of an identical text
or by endorsement, at the latest by August 2020, and conflicting national standards shall be withdrawn
at the latest by August 2020.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 4533-001:2006.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: 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, Turkey and the United
Kingdom.
Introduction
a) The Handbook
The purpose of EN 4533 is to provide information on the use of fibre optic components on aerospace
platforms. The documents also include best practice methods for the through-life support of the
installations. Where appropriate more detailed sources of information are referenced throughout
the text.
The handbook is arranged into 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 offers 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 aero structures. However,
future avionics requirements are driving bandwidth specifications from 10’s 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, vibrations, 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 users air framers.
1 Scope
1.1 General
Part 001 of EN 4533 examines the termination of optical fibre cables used in aerospace applications.
Termination is the act of installing an optical terminus onto the end of a buffered fibre or fibre optic
cable. It encompasses several sequential procedures or practices. Although termini will have specific
termination procedures, many share common elements and these are discussed in this document.
Termination is required to form an optical link between any two network or system components or to
join fibre optic links together.
The fibre optic terminus features a precision ferrule with a tight tolerance central bore hole to
accommodate the optical fibre (suitably bonded in place and highly polished). Accurate alignment with
another (mating) terminus will be provided within the interconnect (or connector) alignment
mechanism. As well as single fibre ferrules, it is noted that multi-fibre ferrules exist (e.g. the MT ferrule)
and these will also be discussed in this part of the handbook.
Another technology used to connect 2 fibres is the expanded beam. 2 ball lenses are used to expand,
collimate and then refocus the light from and to fibres. Contacts are not mated together. It helps reducing
the wear between 2 contacts and allows more mating cycles. This technology is less sensitive to
misalignments and dust. Losses are remaining more stable than butt joint contact even if the nominal
loss is higher.
NOTE Current terminology in the aerospace fibre optics community refers to an optical terminus or termini.
The term optical contact may be seen in some documents and has a similar meaning. However, the term contact is
now generally reserved for electrical interconnection pins. The optical terminus (or termini) is housed within an
interconnect (connector is an equivalent term). Interconnects can be single-way or multi-way. The interconnect or
connector will generally house the alignment mechanism for the optical termini (usually a precision split-C sleeve
made of ceramic or metal). The reader should be aware of these different terms.
An optical link can be classified as a length of fibre optic cable terminated at both ends with fibre optic
termini. The optical link provides the transmission line between any two components via the optical
termini which are typically housed within an interconnecting device (typically a connector) with tight
tolerancing within the alignment mechanisms to ensure a low loss light transmission.
Part 001 will explain the need for high integrity terminations, provide an insight into component
selection issues and suggests best practice when terminating fibres into termini for high integrity
applications. A detailed review of the termination process can be found in section 4 of this part and is
organised in line with the sequence of a typical termination procedure.
The vast number of cable constructions and connectors available make defining a single termination
instruction that is applicable to all combinations very difficult. Therefore, this handbook concentrates on
the common features of most termination practices and defining best practice for current to near future
applications of fibre optics on aircraft. This has limited the studies within this part to currently available
‘avionic’ silica fibre cables and adhesive filled butt-coupled type connectors. Many of the principles
described however would still be applicable for other termination techniques. Other types of termination
are considered further in the repair part of this handbook.
It is noted that the adhesive based pot-and-polish process is applicable to the majority of single-way fibre
optic interconnects connectors and termini for multi-way interconnects and connectors. They share this
commonality.
1.2 Need to high integrity terminations
In order to implement a fibre optic based system on an aircraft it is vital to ensure that all the constituent
elements of the system will continue to operate, to specification, over the life of the system. An important
aspect of this requirement is the need for reliable interconnection components. Interconnects are a key
component in any fibre optic system or network. Digital communications links, sensor systems,
entertainment systems etc. all require interconnects both at equipment interfaces and for linking cables
and harness sections together over the airframe.
Interconnects need to be robust to mating and demating operations, environmental changes and also the
effects of contamination. They need to be amenable to inspection and cleaning for through life support.
The choice of technology used in optical links and connections is mainly dependant of the environment.
In service performance is a pillar in the component selection. Cable to connector interface needs to be
assessed to prove the effectiveness of the solution.
High integrity terminations are required to ensure reliable, low loss light transmission through the
interconnection. High integrity terminations are produced by observing best practice and using the
correct materials, tools and procedures with appropriate controls.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
All interconnection technologies are taken in account in the context of the EN 4533-001.
EN 4533-002, Aerospace series — Fibre optic systems — Handbook — Test and measurement
EN 4533-003, Aerospace series — Fibre optic systems — Handbook — Looming and installation practices
EN 4533-004, Aerospace series — Fibre optic systems — Handbook — Repair, maintenance, cleaning and
inspection
3 Component Selection
3.1 Elements
It is important to recognise that a fibre optic termination, while appearing straightforward, is in fact a
complex interaction of the constituent elements such as: fibre, ferrule, fibre coatings, connector design,
cable strength member anchorage method, adhesive type and cure regime (where used), material
properties and so on. Each of these elements will have an impact on the termination, in terms of reliability,
integrity and process complexity.
The following sections discuss the key elements to the termination.
3.2 Fibre optic cables
3.2.1 General
There are many types of fibre optic cable on the market today. Cables are essentially assemblies that
contain and protect the optical light guide (used to carry the system light signal). The central light guide
is usually made from silica glass although other materials can be used. Glass is inherently strong
although it must be protected from external damage and other factors that could cause weakening
(generally moisture and fluid contamination in the presence of any defects and stress). The cable
provides the protective layers to the glass and generally also incorporates a strength member (this
element is important in the termination for providing strain relief) and a protective outer jacket.
For aerospace applications, most encountered cables will carry a single, central optical fibre (suitably
protected as discussed in the following sections). There can be variation in single fibre cable designs.
Some may be of tight jacket construction, some of loose jacket construction. Cables are also being
developed with many fibres contained within a protective tube construction. It is noted that many of the
cable designs used in terrestrial telecommunications and data communications will not be suitable for
aerospace use. This is generally due to environmental capability limitations often due to environmental
characteristics.
3.2.2 Cable construction
As mentioned in the introduction, the cable construction provides the protection to the central
lightguide(s).
Although the design of fibre optic cable for use on aircraft is fairly similar from one manufacturer to
another there are important differences between cables. The two main areas of difference are fibre
coatings and the cable strength member materials. Each has its own positive and negative attributes in
the context of termination procedures. Avionic fibre optic simplex cables are typically constructed as in
Figure 1.
Another distinction between cable designs is whether all the coatings are “tight” or “semi loose” onto the
underlying layers. This will also impact the operation of the terminated cable, (referring to full pull
proofness achievable with loose structure cables)
A tight cable is a cable which shows no movements between all layers.
A semi-loose cable is a cable which shows limited movements between layers. It could be a movement
between the fibre and the buffer (case of 900 µm cables) or between the buffer and the above layers
(case of simplex 1,8 mm cables)
A tight construction is generally easier to terminate but can be more sensitive to environmental changes if
materials are not well chosen. Some cable designs have a semi-loose construction where the central fibre
has some mobility within one of the cable layers (usually an inner sheath). This design is generally more
difficult to terminate but can have superior environmental performance (because the fibre is isolated from
the other layers).
The behaviour of the connector is different whether the cable is tight or semi loose. Generally on tight
construction fibre contact is interrupted when pulling. The semi loose construction permits a pull safe
termination.
Key
1 Core
2 Cladding
3 Primary buffer
4 Secondary buffer
5 Strength member
6 Outer jacket
Figure 1 — Typical avionic fibre optic cable construction
NOTE The glass fibre lightguide comprises the core and cladding regions.
The figure highlights the key elements of an aerospace fibre optic cable. These elements are now
discussed in more detail.
Figure 2 — Examples of EU standardised cables
3.2.3 Fibre choice
The central lightguide is defined by the core/cladding region. This is the fibre that needs to be suitably
protected by the cable. It is noted that both the core and the cladding are generally formed from glass.
The glass in the core is of higher refractive index than the cladding and this allows light guiding along the
fibre via total internal reflection. Whilst most aerospace fibres are made from glass it is recognised that
other fibre constructions exist including plastic optical fibre (POF), plastic clad silica (PCS). Very novel
fibres such as photonic crystal fibres (PCF) or polarisation maintaining fibre (PM) may also find some
specialised aerospace applications in the future.
One of the primary distinctions between cables is whether the cable carries a singlemode or a multimode
optical fibre lightguide. The choice of lightguide will be dictated by the system or network. Most current
data communication systems on aircraft use multimode based cables. The relatively short lengths
encountered on aircraft mean that multimode fibres can currently provide sufficient bandwidth (up to
~10 Gbps) and their relatively large cores are easier to interconnect (compared to singlemode). Sensor
systems will generally require singlemode based cables. Future bandwidth requirements or the need for
data multiplexing down common fibres may drive the need for more singlemode fibre cables in
aerospace although it must be recognised that singlemode fibres (~ 9 µm core size) are harder to align
and keep free from contamination.
Multimode fibres can be either Step Index (SI) or Graded Index types. Graded index fibres have a graded
profile to the refractive index of the fibre. In essence this increases the bandwidth of the fibre by
equalising the various possible light paths within the core region (thus reducing any dispersion or data
pulse spreading that can occur). Higher data rates are possible with graded index fibres. Step index fibres
may be seen particularly on legacy systems. As its name suggests, the refractive index profile shows a
step change in value defining the change from core to cladding material.
Historically, avionic fibre sizes have tended to be larger than the standard high volume fibres such as
those used in the data communication and telecommunication market and have therefore had an
associated cost and availability penalty (associated components required for termination have also been
non-standard and therefore more expensive). Examples of larger fibre sizes are 200/280 µm,
100/140 µm (where the convention denotes the core/cladding dimension). The data communications
and telecommunications industries typically use fibres of size 62,5/125 µm, 50/125 µm (multimode) and
9/125 µm (singlemode). The last fibres are now being specified for new systems on aircraft with these
fibre sizes, which is becoming the standard configuration.
Importantly for termination, these fibres have a common outer cladding diameter of 125 µm. This means
that the ferrules used in fibre optic termini can be lower cost (these components are mass produced for
the telecommunications market). A number of companies are now packaging these data communication
and telecommunication standard fibres in an aerospace cable meaning that higher bandwidth cables are
now available to the aircraft industry.
Other factors worth mentioning in the choice of fibre are
 Bandwidth:
 Multimode fibres (within the cable) are designated by the OM identification (meaning ‘optical
multimode’). OM1 describes 62,5/125 µm fibre, OM2, OM3 and OM4 describe 50/125 µm fibres
of increasing bandwidth.
 Radiation resistance ( radiation hard):
 These may be specified on some military programs.
 Bend resistance:
 Cables with bend tolerant or bend resistant fibres are now also becoming more widely
manufactured. These cables exhibit lower losses when bent compared to the ones which are
based on bend sensitive fibres. However, as noted elsewhere in EN 4533, fibres should not be
bent beyond their recommended minimum bend radius. They are no stronger than conventional
fibres
The below table is summarising the basics feature of a fibre. Fibres have been categorised according to
ITU rules.
Table 1 — Basics feature of a fibre according to ITU rules
Minimum modal
Bandwidth
Ø core
Mono / multi-mode Category
(µm)
(MHz.km)
850nm / 1 310nm
Mono 9 n/a G652
Mono 9 n/a G657
Multi 62,5 200 / 500 OM1
Multi 50 500 OM2
Multi 50 1500 / 2000 OM3
50 3500 / 4700 OM4
From the perspective of termination there is little difference between small and larger core optical fibres.
The main fibre issues that impact upon the termination process relate to cladding size and primary
coating materials.
The emerging use of multifibre array connectors (e.g. those based on the MT ferrule discussed later) in
some aerospace applications means that cables with multiple fibres are required. A typical construction
is shown below. Early multifibre cables designs were of a flat ‘ribbon’ type. However more recent designs
have been of a round profile cable with loose fibres (suitably protected) within. The cables typically also
include a strength member. This technology is not yet standardised.

Figure 3 — Example of multi-way cable
3.2.4 Cladding materials
3.2.4.1 Coatings and Buffers – A note on terminology
The central lightguide is protected in the cable by various layers of material. The reader should be aware
that different texts will refer to these layers in different ways. Common to most texts however is the
designation of the order of layers. Thus primary layers exist immediately next to the lightguide (usually
applied onto the cladding layer of the fibre). Secondary layers will be applied above the primary layer
and so on.
Where there is sometimes confusion is the inconsistent use of terms such as coatings, buffers and sheaths.
For instance it is common for the terms primary buffer and primary coating to be seen in different texts.
Terms such as secondary coating and secondary buffer would also refer to a coating lying above the first
(primary) layer of protection. Secondary layers can sometimes be hybrid, composed of different materials
(sometimes difficult to separate). Finally a boundary sheath layer may exist in the cable. The term
boundary sheath implies a tube type construction that allows the coated fibre to move within the cable
(semi loose).
3.3 Primary buffer materials
3.3.1 Function
Immediately above the optical fibre is a primary buffer layer. The major function of the primary buffer is
to protect the fibre from abrasive and environmental damage. It also limits micro-bending losses in the
fibre. Generally this coating is applied at the time of fibre manufacture. It provides the first layer of
protection to the glass. It must provide protection but also be easily removable when performing a
termination.
Most fibres use an acrylate type material for the primary buffer, other materials can be encountered
however, such as silicone, polyimide, proprietary polymers and even metal, such as gold or aluminium
(although these are somewhat specialised and will not be considered here). These alternative buffer
materials can extend the operating temperature of the fibre. Carbon is sometimes applied to special
fibres to hermetically seal the fibre surface and prevent moisture reaching the glass surface (typically
used on space applications). For a detailed review of materials see below sections
It should be emphasised that the temperature capability of a glass fibre is not limited over the
operational envelope of an aircraft. Glass will survive (and indeed is used in other applications) at very
high and very low temperatures. It is the temperature range of the protective layers (which are essential
in preventing damage to the fibre) that limit the temperature performance of the cable. In comparison,
other types of fibre (e.g. POF and PCS) may be fundamentally limited by the operating temperature of the
fibre material itself.
In aerospace applications, the most widely used primary coating materials are, acrylate, polyimide and
silicone. A brief description of each material is placed below.
3.3.2 Acrylate
This is perhaps the most common of all the optical fibre primary buffer materials and is relatively easy to
remove with hand tools. The buffer is usually a UV cured acrylate that is translucent and is typically the
same thickness as the fibre. Standard acrylates have a limited temperature performance of up to
approximately 90 °C to 100 °C (above this temperature they can break down and become discoloured
and brittle) however in recent years higher temperature acrylate (HTA) has become a standard buffer
material and is now being packaged in aerospace cables. HTAs extend the operation to the region of
150 °C and up to 180 °C. Low temperature limits are in the region of − 60 °C. Acrylate is subject to
degases when used in unpressurised environments. Some manufacturers have operated these buffers
down to − 65 °C with no degradation.
3.3.3 Polyimide
This buffer has a higher temperature range than UV cured acrylates and can be used in temperatures up
to 300 °C and up to 400 °C short term. Although useful for high temperature applications, polyimide
buffers are difficult to remove using common mechanical tools. Fibres employing this material are
designed to be installed into connector ferrules without the need to remove the primary buffer. This is
only possible because the core/cladding/primary buffer concentricity and outer diameter tolerances are
tightly controlled. This would appear to be an ideal design solution because the fibre surface does not
need to be touched. However the enlarged polyimide diameter is not compatible with standard
connector ferrule bore dimensions, thus non-standard ferrules need to be used with an associated cost
and availability penalty. Removal of polyimide buffers is discussed later in this document (see 5.4.4).
Polyimide is not degasing when used in unpressurised environments.
3.3.4 Silicone
The main benefits of silicone as a primary coating are the reduction of fibre micro-bend effects due to the
“cushioning” effect of the soft primary coating layer, its high temperature capability (up to 200 °C), its
resilience to water penetration and its low flammability. However, as with acrylate, this material needs
to be stripped prior to inserting optical fibres into fibre optic connectors. This is by no means easy (see
later section on removing troublesome primary coatings) and can leave a residue which could
compromise fibre/ferrule bonding. Again removal of silicone is also discussed in 5.4.4 some ‘soft silicone’
coatings may allow lower temperature operation of fibres e.g. down to − 100 °C.
3.3.5 Strength Members
Almost all fibre optic cables employ some form of strength member layer. Its function is to isolate cable
external loads from the fibre within and provide excellent longitudinal strength; it is usually in the form
of stranded fibres running along the fibre axis or woven in a braid. The most common material used for
this purpose is a very tough, strong material known as aramid yarn. However, it is by no means the only
material used for this purpose – fibreglass being one of the main alternatives. New designs of aerospace
fibre optic cable are now implementing a fibreglass / aramid yarn mix to provide a strength member
with lower smoke emissions than that of a pure aramid yarn strength member.
Fibreglass is better matched to the optical fibre’s thermal coefficient of expansion compared to aramid
yarn and has been used where high temperature (> 135 °C) dimensional stability is required of a cable.
This aspect must be considered if a cable is to be subjected to prolonged exposure of rapid thermal

cycling stresses over a wide temperature range. However, aramid yarnappears to meet most current
avionic temperature requirements (− 65 °C to 150 °C).
These two materials need to be treated in quite different ways in order to achieve effective optical fibre
load isolation during the termination process. Aramid yarn and other similar materials can be crimped
directly onto a connector or termini; fibreglass can
...