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Aerospace series — Fibre optic systems — Handbook —

Part 002: Test and measurement

The European Standard EN 4533-002:2006 has the status of a British Standard

ICS 49.060

This British Standard was

published under the authority

of the Standards Policy and

Subcommittee ACE/6/-/10, Aerospace — Fibre optic systems and equipment

A list of organizations represented on ACE/6/-/10 can be obtained on request to its secretary

This publication does not purport to include all the necessary provisions of a contract Users are responsible for its correct application

Compliance with a British Standard cannot confer immunity from legal obligations.

Amendments issued since publication

NORME EUROPÉENNE

ICS 49.060

English Version

Aerospace series - Fibre optic systems - Handbook - Part 002:

Test and measurement

Série aérospatiale - Systèmes des fibres optiques - Manuel

d'utilisation - Partie 002 : Essais et mesures

Luft und Raumfahrt Faseroptische Systemtechnik Handbuch - Teil 002: Tests und Messungen

-This European Standard was approved by CEN on 28 April 2006.

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 Central Secretariat 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 Central Secretariat has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION

C O M I T É E U R O P É E N D E N O R M A L I S A T I O N

E U R O P Ä I S C H E S K O M I T E E F Ü R N O R M U N G

Management Centre: rue de Stassart, 36 B-1050 Brussels

© 2006 CEN All rights of exploitation in any form and by any means reserved

worldwide for CEN national Members.

Ref No EN 4533-002:2006: E

Contents

Foreword 3

Introduction 4

1 Scope 5

2 Normative references 5

3 Problem areas and limitations 5

3.1 The problem of testing avionic, multi-mode fibre installations 5

3.2 Limitations of current insertion loss prediction and measurement techniques 6

3.2.1 General 6

3.2.2 Harness design 6

3.2.3 Insertion loss measurements 6

3.2.4 Optical time domain reflectometry 7

3.3 The way forward 7

4 Techniques for system design 8

4.1 General 8

4.2 Interpretation of component data sheets 8

4.3 Computer modelling 9

4.4 Matrices 10

5 Practical testing techniques 13

5.1 General 13

5.2 Launch conditioning of test sources 13

5.2.1 Distributions 13

5.2.2 How are power distributions defined? 13

5.2.3 What is the launch condition of a source? 13

5.2.4 Why do we need to condition the test source? 14

5.2.5 Why do we need to condition the light entering the power meter? 15

5.2.6 Optimum launch conditions 15

5.2.7 How can we condition the test source? 17

5.2.8 Usable power 19

5.3 Test configurations 20

5.3.1 Preferred methods 20

5.3.2 Similar connectors 20

5.3.3 Dissimilar connectors 21

5.3.4 Accuracies and resolutions 24

5.3.5 Calibration 24

5.4 Detector characteristics 25

5.5 Testing networks 26

5.6 Network testing – OTDRs 27

6 Reporting arrangements 29

Bibliography 30

This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by January 2007, and conflicting national standards shall be withdrawn at the latest by January 2007

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom

Introduction

a) The handbook

The handbook draws on the work of the Fibre-Optic Harness Study, part sponsored by the United Kingdom’s Department of Trade and Industry, plus other relevant sources It aims to provide general guidance for experts and non-experts alike in the area of designing, installing, and supporting multi-mode 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



b) Background

It is widely accepted in the aerospace industry that photonic technology offers a number of 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) To date, the latter 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 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 in sensing many of the physical phenomena on-board aircraft

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 lead 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

c) The fibre-optic harness study

The Fibre-Optic Harness Study concentrated on developing techniques, guidelines, and standards associated with the through-life support of current generation fibre-optic harnesses applied in civil and military airframes (fixed and rotary wing) Some aspects of optical system design were also investigated This programme has been largely successful Guidelines and standards based primarily on harness study work are beginning to emerge through a number of standards bodies Because of the aspects covered in the handbook, European prime contractors are in a much better position to utilise and support available fibre optic technology

1 Scope

Insertion loss is the most frequent measurement performed on a fibre optic link The avionic system designer will want to know or predict the insertion loss of a link to determine its performance Aircraft manufacturers will want to measure the insertion loss of harness components during assembly and before it is delivered to the customer to highlight faults and to provide a record of the performance of the harness at the beginning of its lifetime (footprinting) The insertion loss will be measured at intervals during the lifetime of the aircraft to discover or identify faults and any gradual degradation in performance of the harness

There is, however, one problem It is difficult to collect reliable and consistent measurements of the insertion loss on any multi-mode fibre optic harness where the distance between components is relatively small (less than 100 metres) The reason is that the insertion loss of a component or a harness depends on the power distribution of the light injected into it This leads to very large differences in the measured value of the insertion loss [1] depending on the power distribution of the source used to make the measurement

This Part of EN 4533 will explain the measurement problem and the techniques used to overcome them in greater detail

The following referenced documents are indispensable for the application of this document For dated references, only the edition cited applies For undated references, the latest edition of the referenced document (including any amendments) applies

EN 4533-004, Aerospace series – Fibre optic systems – Handbook – Part 004: Repair, maintenance and

inspection

ARP5061, Guidelines for Testing and Support of Aerospace, Fiber Optic, Inter-Connect Systems 1)

3 Problem areas and limitations

3.1 The problem of testing avionic, multi-mode fibre installations

The insertion loss of a fibre optic harness can be divided into two contributions The first is the intrinsic loss of the harness caused by the properties of the materials such as the absorption of the silica of the fibre core In the case of multi-mode fibres, this would include the variation in the loss of the component caused by changes

in the power distribution When discussing insertion loss, the term ‘power distribution’ will be used to describe the spatial and angular variation of the power across the fibre’s core rather than the temporal variation of the power along the length of the fibre The second contribution is the additional loss that is introduced into the harness from extrinsic losses such as misalignment errors in connectors and contamination

The insertion loss of components used in any fibre optic link depends on the power distribution of the light that passes through them However, in some types of fibre harness, the shape of the power distribution does not change as the light propagates through it and the component insertion loss is independent of its position within the harness For example, the power distribution in single-mode fibre harnesses is fixed by the fibre parameters and the source’s wavelength Long-haul (a few kilometres between components), multi-mode fibre harnesses also effectively have a fixed power distribution because the distance between components is sufficient for the power distribution to reach an equilibrium state that depends only on the fibre parameters The insertion loss of a long-haul multi-mode harness component is defined by this equilibrium power distribution



1) Published by: Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, PA 15096-0001

In contrast, short-haul multi-mode fibre harnesses (less than 200 metres), like those found in avionic systems,

do not have sufficient distance between components to allow the equilibrium power distribution to be attained The power distribution entering a component within such a harness will now depend on the original power distribution from the source and changes to this distribution caused by the preceding components of the harness There are two consequences of this power distribution dependent insertion loss:

a) The insertion loss will depend on the source used to make the measurement

b) The insertion loss of a particular component will depend on its position within the harness

A way of reducing the variation in the measured insertion loss is to use modal filters on the source and power meter that alter the source’s power distribution to a known or standard distribution If all measurements are made with the same power distribution, the insertion loss value will be much more repeatable Clause 5 will describe some of these standard distributions and the various techniques that can be used to measure the insertion loss of harnesses and their components

3.2 Limitations of current insertion loss prediction and measurement techniques

3.2.1 General

This section outlines some of the limitations of the current design and measurement techniques that are used

to determine the insertion loss of multi-mode fibre optic harnesses (See Figure 1)

3.2.2 Harness design

It is difficult for fibre optic harness designers to predict with accuracy the harness performance It is unlikely that insertion loss values of commercially available components will have been measured with a specific power distribution based on the parameters of any avionic fibre The designer will therefore usually apply a pessimistic estimate for the insertion loss and obtain a much poorer prediction of the harness performance than may necessarily be the case This may not be important in simple point-to-point communication links between transmitter and receiver where there can be a very large power budget However, it will be significant

in more complex networks, where there are many connector breaks, or in networks that contain components such as passive star-couplers where the power budget is likely to be much tighter

3.2.3 Insertion loss measurements

Many measurements that are made on short-haul fibre optic harnesses use no form of filtering on the output

of the source to modify its power distribution This may not be very important if the measurements are for comparison with measurements made with exactly the same source However, they cannot give reliable measurements of insertion loss that can be reproduced by another manufacturer’s set of test equipment These measurements cannot be used to guarantee the performance of the harness to a customer because it

is unlikely that the customer will be able to check the loss measurements without using the supplier’s test equipment

The filtering used to modify the power distribution could also be inappropriate for an avionic application For example, a power distribution more appropriate for telecommunication links may be used which will underestimate the loss of components in an avionic harness Additionally, the power distribution used to make the measurement should be changed for each fibre type that is used because the fibre parameters will be different

a) A large area detector collects light from the fibre that is outside of the defined limits for the source

b) The removal of an interface by not using a test lead between the lead under test and the power meter

Figure 1 — Causes of unrepresentative power measurements

3.2.4 Optical time domain reflectometry

Optical time domain reflectometry (OTDR) is a single ended diagnostic/measurement technique that relies on the backscatter of light from ‘imperfections’ and discontinuities in a fibre-optical system It is used extensively

in the telecommunication industry for optical system commissioning and testing OTDR technology potentially enables a reference insertion loss footprint to be generated requiring only a single measurement to characterise

an entire harness under test Furthermore, comparison between current and previous traces can be performed automatically by standard OTDR software largely de-skilling measurement and diagnostic operations However, current OTDRs struggle to meet the specific requirements of airborne optical harnesses, in particular spatial resolution (dead zone) performance

3.3 The way forward

The following three options would enable a more reliable means of predicting and testing the performance of multi-mode fibre optic harnesses:

1) A method of predicting the power distribution throughout the harness Those power distributions can

be used to find an exact value for the insertion loss of each component The insertion loss will then

be specific to that particular component at that particular position within the harness

2) A fixed power distribution that is used to measure the insertion loss of all the harness components If the same distribution is always used, the value of the insertion loss should be consistent and independent of the test equipment used

3) A means of measuring the power distribution throughout the harness by interrogating the harness at

a single access point (probably a connector) This would be realised by optical time domain reflectometry

The first option is more applicable to the system designer and two methods are mentioned here and explained

in more detail in Clause 3 The first method is a validated computer design package that can predict the power distributions and calculate the expected insertion losses of the individual components for any source (see 5.2) The second method is the use of matrices that represent the harness components rather than the more common insertion loss value They represent how the power distribution is altered by the component and can

be multiplied together to find the overall system loss (see 5.3)

The second option is applicable to the practical measurement of the insertion loss There has to be a well defined power distribution that has to be used when making insertion loss measurements The distribution should be ‘appropriate’ for short-haul avionic harnesses and is likely to vary for different fibre types and parameters (see 5.2) The measurements should also be made with well defined procedures that minimise the systematic errors (see 5.3)

The third option is again applicable to the practical measurement of the insertion loss However, the use of an OTDR (as opposed to a power meter) potentially enables the entire optical system to be measured from a single connector break or purpose built test port Exactly the same criteria apply with regard to the need for a well defined power distribution from the OTDR’s source that is representative of short-haul avionic harnesses Although attractive, this solution is limited by the performance of current OTDR instruments (see 5.6)

4 Techniques for system design

4.1 General

The introduction outlined the problems of making consistent measurements of component and system loss in short-haul, multi-mode fibre optic harnesses typical of aircraft installations A system designer needs to be able to predict the optical power distribution at any point in the harness before he can accurately predict the loss of the individual components and the overall system Alternatively, the designer can use the worst case loss for all of the harness components and simply add them together However, this will produce a pessimistic estimate of the transmission of the harness and could put unnecessary constraints on the harness design For example, it may restrict the number of connectors that can be included in the harness and therefore reduce the maintainability of the harness The following will describe the problem of using insertion loss values from component data sheets and two methods available to the designer that can help to more accurately predict the loss of components and harnesses

4.2 Interpretation of component data sheets

During the system design phase the designer or engineer responsible for the physical interconnect has to derive how an optimum physical implementation can be achieved whilst keeping within the system power budget Many decisions have to be considered when deciding on the optimum physical implementation, such as:

a) How many connections there should be and where they should they be placed to ease maintenance and repair while maximising reliability?

b) If the system is not point-to-point then what type of component is going to be used to provide the required connectivity?

c) Are specific components such as cable and connectors already mandated for the system?

When choosing components to meet the system requirements, the designer would ideally like to have all of the relevant performance data on the components that could meet his requirements The designer can then make an informed decision as to which components best meet his needs Determining the path loss should simply be a case of extracting the insertion loss figures from the data sheets of components that lie between the source and receiver and adding them to obtain the total path loss However, these values cannot presently

be treated in this way because, as already discussed in Clause 3, the insertion loss of short haul systems is critically dependent on the power distribution of the light launched into it

The test method may be relevant when used on another application, e.g long haul telecommunications, but it

is unlikely that it will be appropriate for short haul avionic installations Many of the test procedures that components are qualified against use a launch condition that is not required to fill the fibre to the same extent

as those recommended as a result of work in the Harness Study2) This underfilled launch results in an optimistic estimate of the insertion loss

Ideally, component manufacturers would not specify a loss of a component specifically but define how the input launch is changed by the component before launching into the next component by means of a component matrix This matrix method will be described in 5.4 Using this approach, a representative loss of the component can be achieved that is accurate for all launch conditions The problem with this method is that, at present, there is no internationally accepted practical test for component manufacturers to determine the matrix elements

A more practical approach is for the component manufacturer to specify an optical performance figure based

on a test that is representative for short haul applications These test conditions have been defined using raytracing software developed within the Fibre Optic Harness Study and these may require different test sources for the specific fibre types being used

If the structure and typical tolerances of a commercial component are known, it should be possible to use the raytracing model that will be described in the next section to convert the insertion loss attained with the manufacturer’s launch condition into the loss for a standard, avionic launch condition

4.3 Computer modelling

A computer can be used to predict the way in which the optical power distribution in an optical system changes as it passes through components There are many optical design packages in the marketplace which can be roughly divided into two types The first are lens design packages that use raytracing to predict the power distributions in lens systems such as those found in photographic equipment The second are waveguide design packages that calculate the strength of the light’s electric field to find the power distributions

in integrated optic components Raytracing is a valid technique where it can be assumed that the apertures of the components are large compared to the wavelength of the light passing through them Multi-mode fibres have core diameters that normally fulfil this criterion and it is therefore valid to use raytracing to predict the power distributions in multi-mode harnesses

During the Fibre Optic Harness Study, a multi-mode fibre optic system design package was written that uses raytracing to predict the power distributions This model was validated against experimental data Figure 2 is a view of the model screen showing the various components that can be connected together to construct a complete harness The model can be used to predict the power distribution at any point in the harness and can calculate the loss of individual components Amongst other facilities, typical component errors can be introduced, as can representations of contamination It is also capable of calculating the component matrices that will be described in the following section



2) For graded index fibre the best correlation between the two estimates of system loss was found for a useable power definition of 95 % of the core diameter and 95 % of the maximum acceptance angle In step index fibre the best definition

of useable power was found to be 100 % of the core diameter and 90 % of the maximum acceptance angle

Figure 2 — The user interface for the raytracing model showing the various components

that can be included

4.4 Matrices

In a fibre system in which the power distribution does not change, it is possible to represent the component losses by a single number, the insertion loss As discussed in Clause 3, this applies to single-mode fibre systems and multi-mode harnesses where the component separation is large, e.g telecommunication links In shorter links, a single insertion loss value will say nothing about the way in which the component’s loss is influenced by the power distribution into it or how the component itself alters the power distribution Representing the components as matrices can overcome these problems [2][3][4]

The matrices are generated by dividing some parameter of the light distribution into a number of regions and recording how the light is attenuated and how it moves between the regions as it passes through a harness component The parameter used will depend on the type of fibre used in the harness For example, a suitable parameter for a component in a step-index fibre harness is the angle of the light Figure 3 shows the division

of the angular power into three regions before and after such a component A column of three numbers can represent the power in the regions at the input and output The power in the three output regions, A’, B’ and C’, can be calculated by multiplying the power in the input regions, A, B and C, by the component matrix:

C m B m A m C

C m B m A m B

C m B m A m A

C m B m A m

C m B m A m

C m B m A m

C B A m m m

m m m

m m m C

B A

33 32

31

23 22

21

13 12

11

33 32

31

23 22

21

13 12

11

33 32 31

23 22 21

13 12 11

' ' '

' ' '

+ +

=

+ +

=

+ +

+ +

+ +

23 22 21

13 12 11

33 32 31

23 22 21

13 12 11

and

n n n

n n n

n n n N m

m m

m m m

m m m M

then the resulting matrix, R, would be:

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

=

=

33 33 23 32 13 31 32 33 22 32 12 31 31 33 21 32 11 31

33 23 23 22 13 21 32 23 22 22 12 21 31 23 21 22 11 21

33 13 23 12 13 11 32 13 22 12 12 11 31 13 21 12 11 11

.

n m n m n m n m n m n m n m n m n m

n m n m n m n m n m n m n m n m n m

n m n m n m n m n m n m n m n m n m N M R

The order in which the components are multiplied together is crucial If a system is comprised of components with matrices, A B C and D, and they appeared in this order going from the source to the receiver, the matrix for the complete system, R, will be the multiple of the matrices in the reverse order:

A B C D

R =

This is further illustrated in Figure 4

Key

1 Source

2 Receiver

Figure 4 — The order in which component matrices are multiplied to find

the matrix of the system

In this way, a system designer can use these matrices to construct a single matrix that describes how the system will alter and attenuate the light passing through it By multiplying this matrix by the column vector that represents the input power distribution, the designer can predict the insertion loss of a system and the output power distribution

The number of elements in the matrix is rather arbitrary The larger the number, the greater the accuracy of representing the power distribution In the past, 2 by 2 matrices (4 elements) have been used but the accuracy has not been very good [5] A better performance has been obtained with 3 by 3 matrices [6] The raytracing model can generate matrices with up to 25 elements (5 by 5 matrix)

The raytracing model can generate matrices by tracking the variation of the parameters of rays as they pass through the harness components However, it is much more difficult to extract the matrices from practical measurements of the power distribution This is particularly true of harness components used with graded-index fibre because the parameter used to generate the matrix is not directly related to either the spatial (near-field) or angular (far-field) power distributions [7] Matrices for graded-index fibre harnesses have been successfully generated from near-field power distributions [8] Compared to graded-index fibre components, matrices for components in a step-index fibre harness are relatively easy to generate from the far-field power distribution The technique can be illustrated by Figure 3 An optical system launches light at angles within region A into the component A power meter measures the power within output regions A’, B’ and C’ The values of the matrix elements m11, m12 and m13 are:

A

C m

A

B m

A

A m

in power '

in power

in power '

in power

in power '

in power

B

B m

B

A m

in power '

in power

in power '

in power

in power '

in power

23 22 21

C

B m

C

A m

in power '

in power

in power '

in power

in power '

in power

33 32 31

5 Practical testing techniques

5.1 General

This clause will describe how to make repeatable and reliable measurements of the insertion loss of a fibre optic harness It is assumed that the source is a light emitting diode (LED) rather than a laser diode The radiation from laser diodes is coherent and the power distribution includes interference fringes or ‘speckles’ that can make the measured power reading vary with time and reduces the reliability of the measurement Subclause 5.2 describes the modifications needed to the power distribution of the test source to make reliable measurements Subclause 5.3 describes the measurement techniques that are available and their associated errors Subclause 5.4 looks at the influence of the detector in the power meter on the insertion loss measurement Subclause 5.5 outlines some of the problems that may be encountered when making measurements on optical networks Subclause 5.6 describes the use of OTDR to make measurements on a network

5.2 Launch conditioning of test sources

5.2.1 Distributions

Subclause 5.4 states that fixed power distributions are required to make reliable measurements of the insertion loss of multi-mode fibre optic components The following sections will provide a more detailed description of how these power distributions are defined for test sources and how they can be created A source can be any component that is capable of injecting light into the component under test and includes light from a piece of test equipment, an LED or a test lead

5.2.2 How are power distributions defined?

Generally, the power distribution of a source is defined by two separate distributions: the variation of the power across the aperture of the source and the variation of power with the angle of the light coming from the source These two distributions are known respectively as the near-field and the far-field power distributions Typical near and far-field power distributions for step-index and graded-index fibre are shown in Figure 5 These power distributions were gathered by apparatus built on the Fibre Optic Harness Study It is also possible to buy commercial equipment that can record near and far-field power distributions [9] 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

5.2.3 What is the launch condition of a source?

The launch condition of a source is a comparison of the source’s near and far-field power distribution to the parameters of the fibre used in the harness The near-field is compared to the core radius (or diameter) of the fibre and the far-field is compared to the numerical aperture (NA) of the fibre

An additional parameter that affects the launch condition is whether the fibre core has a step or graded refractive index profile This is very important because the shape of the near and far-field distributions is strongly dependent on the index profile, as illustrated in Figure 5 ) and b) are the near and far-field power distributions from a graded-index fibre illuminated by an LED source The LED source provides a nearly fully-filled launch condition c) and d) are the near and far-field power distributions of a step-index fibre that is illuminated by a white light source The source provides an overfilled launch condition

a) 0.26 NA graded-index fibre with an

LED source near-field b) 0.26 NA graded-index fibre with an LED source far-field

c) 0.2 NA step index fibre with white

light source near-field d) 0.2 NA step index fibre with white light source far-field

Figure 5

The launch condition is expressed as a percentage of the core radius and numerical aperture of the fibre in the harness Say, for example, that a harness uses a fibre with a core radius of 50 µm and a numerical aperture of 0,29 If the near-field profile of the source corresponds to a fibre with a radius of 40 µm and the far-field to a numerical aperture of 0,25, the launch condition into the fibre will be 80:86 (80 % fill of the core radius and an 86 % fill of the numerical aperture) 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 ‘under filled’ in the corresponding distribution

One of the important implications of the launch condition being defined by the fibre parameters is that the launch condition of a source will change depending on the fibre type and parameters being used in the harness Thus two different sources will be needed to produce identical launch conditions in two fibres with different parameters

5.2.4 Why do we need to condition the test source?

As discussed in the introduction, making measurements of the insertion loss of components used in mode optical fibre harnesses is difficult It is complicated by the fact that the loss depends on the power distribution of the source that is used to make the measurement 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 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

multi-Position (microns)

Position (µm)

200 10

20 30 40 50

-10 0.2

0.4 0.6

A particular power distribution needs to be chosen to enable more repeatable measurements of the insertion loss to be made This 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

5.2.5 Why do we need to condition the light entering the power meter?

If there was no conditioning of the light that falls on the detector of the power meter, the measured power would depend on the size of the detector To obtain consistent measurement of the power two conditions have

to be satisfied:

1) The light falling on the detector has to be conditioned so that only light within the defined standard power distribution falls on it

2) The detector dimensions have to be larger than the area of the patch of light that falls on it (see 5.3)

5.2.6 Optimum launch conditions

One solution to this measurement problem is for everyone to agree to always use a standard power distribution But which distribution is the correct one? The answer is the distribution that is most characteristic

of the harness system being measured For example, telecommunication harnesses have long lengths of fibre between components and in these distances it is possible for the light power distribution to reach an equilibrium condition This means that the power distribution no longer changes shape as it propagates down the fibre but simply reduces in power For telecommunication fibre, this equilibrium power distribution is obviously the correct power distribution to use Unfortunately, it is more difficult to define the power distribution for short-haul avionic harnesses

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 and is the distribution that should be used to make insertion loss measurements 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 This has been developed in the United States as a practical technique to find the optimum distribution for an avionic harness [10] A test harness with typical components

is constructed and the optimum distribution found by making measurements of the output power distribution

as the input distribution is altered This has resulted in limits being set to the launch condition of the Spectran 'Flightguide' [11] The limits for this fibre have been set at three percentage levels (75 %, 15 % and 5 %) within the normalized near and far-field power distributions These limits are shown in Figure 6, together with the curve of ‘ideal’ near and far-field power distributions for a fibre with 'Flightguide' fibre parameters The ‘ideal’ distributions fall quadratically from the maximum intensity to the value of the core radius or numerical aperture Test equipment with these launch conditions is available as ‘off-the-shelf’ items [12] Similar limits have also been derived for fibre with a 62,5 µm core diameter [11] Upper and lower limits to the near and far-field power distributions as defined in standard ARP5061 The limits are compared to curves that represent ‘ideal’ 100 % and 80 % launch conditions for a fibre with a 100 µm core diameter and a NA of 0.29