EN 4533-004:2018

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.3RSUDYLODLuft- und Raumfahrt - Faseroptische Systemtechnik - Handbuch - Teil 004: Reparatur, Wartung und InspektionSérie aérospatiale - Systèmes des fibres optiques - Manuel d'utilisation - Partie 004 : Réparation, maintenance, nettoyage et contrôleAerospace series - Fibre optic systems - Handbook - Part 004: Repair, maintenance, cleaning and inspection49.060Aerospace electric equipment and systems33.180.01VSORãQRFibre optic systems in generalICS:Ta slovenski standard je istoveten z:EN 4533-004:2018SIST EN 4533-004:2018en,fr,de01-marec-2018SIST EN 4533-004:2018SLOVENSKI
STANDARDSIST EN 4533-004:20091DGRPHãþD

EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 4533-004
January
t r s z ICS
v {ä r x r Supersedes EN
v w u uæ r r vã t r r xEnglish Version
Aerospace series æ Fibre optic systems æ Handbook æ Part
r r vã Repairá maintenanceá cleaning and inspection Série aérospatiale æ Systèmes des fibres optiques æ Manuel d 5utilisation æ Partie
r r v ã Réparationá maintenanceá nettoyage et contrôle
Luftæ und Raumfahrt æ Faseroptische Systemtechnik æ Handbuch æ Teil
r r vã Reparaturá Wartung und Inspektion This European Standard was approved by CEN on
t u July
t r s yä
egulations which stipulate the conditions for giving this European Standard the status of a national standard without any alterationä Upætoædate lists and bibliographical references concerning such national standards may be obtained on application to the CENæCENELEC Management Centre or to any CEN memberä
translation under the responsibility of a CEN member into its own language and notified to the CENæCENELEC Management Centre has the same status as the official versionsä
CEN members are the national standards bodies of Austriaá Belgiumá Bulgariaá Croatiaá Cyprusá Czech Republicá Denmarká Estoniaá Finlandá Former Yugoslav Republic of Macedoniaá Franceá Germanyá Greeceá Hungaryá Icelandá Irelandá Italyá Latviaá Lithuaniaá Luxembourgá Maltaá Netherlandsá Norwayá Polandá Portugalá Romaniaá Serbiaá Slovakiaá Sloveniaá Spainá Swedená Switzerlandá Turkey and United Kingdomä
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CEN-CENELEC Management Centre:
Avenue Marnix 17,
B-1000 Brussels
t r s z CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Membersä Refä Noä EN
v w u uæ r r vã t r s z ESIST EN 4533-004:2018

Contents Page European foreword . 3 Introduction . 4 1 Scope . 5 2 Normative references . 5 3 Fault analysis . 6 3.1 Fault notification . 6 3.2 Symptoms . 6 3.3 Potential faults . 7 3.4 Fault identification and location . 10 4 Repair techniques . 13 4.1 General . 13 4.2 Splice . 13 4.3 Structural repair . 14 4.4 Terminus recovery . 14 5 Inspection and cleaning . 16 5.1 End face inspection . 16 5.2 Cleaning . 20 6 Scheduled maintenance and inspection . 24 6.1 When to maintain/Inspect? . 24 6.2 Maintenance/Inspection of system . 25 6.3 Maintenance/Inspection of components . 26 Annex A (normative)
Termini end face contamination . 27 Annex B (normative)
Cleaning Methods . 32 B.1 Method 1 . 32 B.2 Method 2 . 34
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 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, 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 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. 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. SIST EN 4533-004:2018

3 Fault analysis 3.1 Fault notification A fault notification will arise from one or more of three sources; scheduled maintenance, Built-In-Test (BIT), or failure of equipment. Ideally, scheduled maintenance should highlight all latent faults i.e. those which initially have no effect on the system performance but may lead to a problem sometime later during aircraft operation. It should also highlight faults of the gradual degradation type i.e. those which gradually deteriorate the system performance but have yet to cause a failure and any other faults that slipped through the BIT net. BIT is the ability of the aircraft’s systems to diagnose themselves. It should identify all faults that occur in the time between scheduled maintenance and, with the exception of sudden catastrophic faults, before a failure occurs. It should also be able to provide some help in locating the fault. Failure is the worst case and should only be the result of a fault occurring which cannot be prepared for. 3.2 Symptoms This is where differences between fibre optic and electrical installations become apparent. The most common symptom in a fibre optic link is complete or partial loss of optical power. This occurs when light breaks its confinement from the fibre core and can be the result of damage to the fibre or interconnect. It can also be the result of contamination of the fibre optic terminus end face, excessive pressure, crushing or severe bending on the fibre optic cable. Depending on the magnitude of the loss, the result may be a fault that is above or below the link threshold – a fault below the link threshold is a failure. Severe damage, such as an optical fibre break may induce a complete loss of optical power. Intermittent optical signals are possible and may be the result of fibre movement e.g. vibration or bending of a fibre. An increase in optical power is also possible although this is more likely to be due to stability of the light source rather than the link itself. Gradual degradation of optical power is an important symptom to be able to detect as it could indicate the onset of a failure. Increasing contamination or proliferation of damage to the fibre optic terminus end face could be responsible. Outside of the harness it could be due to degradation of an optical source. Back reflection is a phenomenon that occurs at any interface with different refractive index, e.g. glass/air. Back reflection is of particular concern where active devices utilise laser-based systems where reflected light can affect the transmitting capabilities of the optical source. This can result in degradation of the transmitted signal and potentially damage the optical source.
There are also issues of fibre grow in/out (when the adhesion between fibre and ferrule fails) and fibre misalignment. Connectors also have the potential to be carrying latent faults such as over-tightening of the connection mechanism and inadequate strain relief. 3.3.4 Backshell Apart from physical damage to the backshell there is also potential for fibres to be crushed, bent or strangled if the routing within the backshell is not correct. 3.3.5 Conduit Breaking, kinking or crushing of conduit could have an effect on the optical fibre but experience with electrical harnesses suggests that damage to a conduit is likely to be a latent fault which is found during scheduled maintenance before it affects harness performance. 3.3.6 Pigtailed components A break or crack of a pigtailed fibre from a component would give rise to a total or partial optical power loss symptom. 3.3.7 Splices Splice faults that could have a direct effect on the optical signal include fibre separation. Additionally, mechanical splices may be degraded by contamination, fluids ingress, migration of index matching means. Latent faults are similar to those for connectors; inadequate strain relief and support. 3.3.8 Others The faults listed above are limited in scope to the harness. Faults at the hardware interface level e.g. transceivers and controlling electronics would result in a selection of the symptoms detailed in 3.2. Potential faults are summarised in Table 1.
EN 4533-004:2017 (E) 9 Table 1 — Possible symptoms for optical component failure
Complete loss of optical power Drop in optical power or intermittent loss of power or gradual degradation Increase in back reflection Decrease in optical power Latent fault symptom Fibre Fibre break Fibre break Fibre crack Micro-bending Fibre break Fibre crack Fibre localized stress — Cable — Tight Bend Cable Crushed — — Cable abrasion Cable crushing Cable split Terminus Severe contamination Fibre grow in/out Fibre break inside of terminus — Fibre grow in/out Fibre misalignment — Inadequate strain relief Connector Contamination Contamination Fluid ingress — Fluid ingress (index matching) Inadequate strain relief Backshell — Fibres crushed/bent/strangled — — Physical damage Conduit — Conduit kink or crush — — Conduit break, kink or crush Coupler Pigtail break Mixer rod crack Pigtail break/Crack Mixer rod crack Pigtail break — — Splice Fibre grow in/out Packaging failure Fibre grow in/out Packaging failure
Fluid ingress Poor end face preparation Poor end face alignment — Inadequate strain relief Other Transceiver fault Transceiver fault — Transceiver fault — SIST EN 4533-004:2018

3.4 Fault identification and location 3.4.1 General Fault finding techniques and strategies will play a key role in restoring and maintaining the integrity of aircraft fibre-optic systems. Unless appropriate solutions are available the aircraft operator could incur significant down time, cost, and inconvenience whilst the fault is being located. The problem is exasperated by the fact that the fibre-optic networks in question could be relatively complex, incorporating fan-out connection paths (enabled by passive couplers or active switches, for example) and may be harnessed into relatively inaccessible areas of the airframe. Criteria considered when assessing potential fault finding techniques included:  effectiveness of the technique for likely fault scenarios,  cost of equipment,  skill level and time required to perform the technique,  size, weight, power requirements, and robustness of equipment,  safety issues. The first factor that will influence the choice of fault location technique is the type of harness – inaccessible, embedded or open. Several of the techniques described below cannot be used on an embedded or inaccessible harness. The first step of fault identification is to determine the failed part: cable, connector or system. The following fault location methods are presented from the simplest to the most complex. 3.4.2 Good practices during maintenance/Inspection The following good practices are recommended to be included as part of any overall scheduled maintenance philosophy:  Whenever test equipment requires de-mating of a connector the appropriate cleaning procedure should be followed to ensure no contamination is introduced.  End protection must be used at all times. When de-mating is required the de-mated connectors should be protected. Dust caps should be kept clean. Disposable items are preferred.  Correct fibre handling procedures should be followed to avoid damage. Minimum bend radii should be observed. Exposed fibre should be treated as sharps.  Eye safety issues should be highlighted. Test equipment should all be eye safe and extra care needs to be taken if the system transceivers pose potential eye safety problems.  Correct sources and filters should be used for all footprinting, including O.T.D.R’s. Failure to do so will invalidate data collected. SIST EN 4533-004:2018

3.4.3 Inspection This is the simplest fault location procedure and falls into two categories – inspection of the end faces of the fibre optic termini and external inspection of cables and connector housings (which requires no de-mating). A clean, undamaged end face is essential for optimum performance. Inspection of multi-way connectors can be complicated, especially if the termini are recessed. Visual inspection of a fibre optic bundle is the same as for existing electrical harnesses. Inspection of fibre optic bundle components (if accessible) is the only viable way to control latent faults. 3.4.4 Continuity check A continuity test performed with a simple laser source is an easy method to show a fibre or connector break. This method needs to have access to both ends of the cable. It consists in lighting one end with the laser and to check the presence of the light on the other side. Eye protections must be used during this test. 3.4.5 Power measurement Provided appropriate launch and detection conditioning is applied (see EN 4533-002), optical power measurement is the recommended technique for determining attenuation in ‘useable power’ through an avionic fibre-optic harness or harness component. If compared to a previous equivalent measurement of the system, a measurement of a control sample, or a theoretical prediction, the presence of a localised fault or distributed degradation (due to ageing, for example) in the harness can be deduced. Optical power measurement can also be used for fault location given prior knowledge that a fault has been detected, e.g. from BIT. If the symptom is ‘signal below threshold’ then the use of appropriate conditioning is still recommended. For ‘no signal’ or ‘intermittent signal’ type faults this is much less critical. In fact, a tailored overfilled or under filled launch may be advantageous in certain cases, e.g. a significant under fill may minimise non-critical loss mechanisms that would otherwise confuse/distract the operator. In some senses this case is analogous to visual fault finding (especially when used for continuity checking) except that an optical power meter is used in place of the eye. Optical power measurement is intrinsically a double ended test. Thus, whatever the symptom, in order to localise the fault or loss mechanism further, access to in-line connectors within the harness is required for the test source and/or the detector. The technique is therefore far less applicable to ‘embedded’ harnesses than to ‘open’ harnesses. By accessing in-line connectors in open harnesses and then taking appropriate optical power measurements, a fault can be localised to a particular “connectorised” section of the harness. This can then be repaired, cleaned, or replaced depending on the repair policy adopted. Having taken appropriate repair action, optical power measurement also has a role to play in confirming the level of functionality of the harness.
To summarise, power measurement is easy to perform and interpret but is probably more time consuming than some of the others described in this report. It is the best technique for locating gradual degradation faults and drop in optical power as the data is simpler than that presented by an OTDR. However, power measurement cannot find the position of a fault on a cable so for a long run between connectors e.g. on a point-to-point link its usefulness is reduced. 3.4.6 Insertion loss measurement Insertion loss measurement is a variation of power measurement as it consists in determining the attenuation on the link. Contrary to power measurement where the measure is performed with the functional system connected to the link, insertion loss measurement needs to connect fibre ends to light launch system on one end and light detector system on the other end. It enables to quantify the attenuation of the link. The result must be compared to a reference measurement or to the theoretical insertion loss prediction of the link to identify a fault but it won’t localise the defect. The measurement method is described in EN 2591-601 standard. 3.4.7 Optical time domain reflectometry The use of an optical time domain reflectometry is a recognised method of accurately measuring the length of an optical link and identifying ‘events’ within the link. Designed for long haul, single mode optical fibres there have been doubts over their suitability for use on very short aerospace fibre optic links. New high resolution OTDR’s have been developed and are able to measure ‘events’ with centimetre resolution in multimode fibres. An event can be described as being identified losses within the link, such as; Insertion Loss (IL) (interconnects), attenuation (tight bends, fibre breaks), etc. This new capability allows operators to measure and map events in short optical links of less than 5 metres and up to 100’s of metres. An initial end to end measurement of an optical link can be taken and stored, future measurements can then be carried out from one end of the link. This allows full measurement of a link with minimal disturbance of the system. In addition, the equipment is also capable of providing IL and RL measurements across events, video inspection and visual fault location providing multi-function capability. Full details of OTDR measurement of short optical links is defined in EN 4533-002, Test and Measurement. 3.4.8 BIT information As well as signalling a fault, BIT can also play a part in fault location. In particular, some of the other techniques covered here, such as power measurement and OTDR analysis, ideally require information previously acquired from the BIT information to rationalise the number of fault possibilities.
4 Repair techniques 4.1 General Once the fault has been isolated a repair can be initiated. Ideally the repair strategy for a specific aircraft will be such that given the fault and scenario, a single repair technique will be appropriate. 4.2 Splice If a fibre optic link is damaged away it can be repaired with a splice. Generally 2 kinds of splices are recognised: Fusion splice and mechanical splice. 4.2.1 Fusion splice Fusion splice is performed routinely in the telecommunications field as a mean of installation and repair. Here fusion splicing is often used to give a permanent junction which can then be protected with a splice protection sleeve and housed in a splice tray. This technique fuses the fibres together using an electric arc. In recent years this technic has been successfully introduced to permanently repair the avionic harness, when it is mounted outside of the explosion hazard zone in the aircraft. To perform a mechanical splice, the two fibre lengths to be joined are prepared in a similar way as for a termination. The jacket, strength members and buffers are stripped away to pre-defined lengths and the fibre is exposed. After careful cleaning of the outer surface in order to remove all possible remains of buffer materials, the fibres are cleaved and positioned onto the alignment grooves of the fusion splice machine. Dedicated jigs might be in place to support this operation, when performed in the field. The fusion splice is protected in a suitable, space saving body which includes the necessary strain retention. This process is valuable for the replacement of damaged terminus or fibre interfaces in a connector, too.
Figure 1 — Fusion splice on 62,5/125 fibre optic cable
4.2.2 Mechanical splice To perform a mechanical splice, the two fibre lengths to be joined are prepared in a similar way as for a termination. The jacket, strength members and buffers are stripped away to pre-defined lengths and the fibre is exposed. The fibres are then cleaved and brought together in an alignment sleeve arrangement e.g. a glass capillary or fitted into termini which can be snapped into a ferrule housing. Depending on the design, adhesive and index matching gel (often pre-loaded) may be required at this stage. The whole sub-assembly is then placed in the outer body of the splice and the strain relief is attached to give a permanent, protected splice. There are many different commercially available mechanical splice techniques, they have been developed telecommunications market, and up to now no product has been qualified for aerospace i.e. are able to meet environmental demands and provide a permanent repair to last throughout the service lifetime of the cable. Mechanical splice might be a solution for temporary repair. To summarise, fusion splicing is the ideal solution where the repair can be performed outside of an explosion hazard zone Splices are designed to last the lifetime of the cable so the repair could be applied as a permanent repair. 4.3 Structural repair Where damage is to the cabling rather than to the fibre itself it may be possible to use a structural repair e.g. taping (as there are no issues concerning EMI shielding compromise when working with optical fibre). The decision to use a structural repair will depend on the depth of the damage on the cable – for damage extending deeper than the outer jacket of the cable, a more substantial repair technique should be used. 4.4 Terminus recovery 4.4.1 Repolish If cleaning fails to eliminate identified defects it may be possible to recover the terminus end face quality and geometry by re-polishing the terminus. Re-polishing may also avoid having to re-terminate an optical link however, it is important not to over polish the terminus as this could permanently damage the terminus. It is also important to be careful not to remove the chamfer of the ferrule. Therefore, it is recommended to use portable handheld polishing equipment with appropriate films and pads to achieve the required end face geometry and quality. Following re-polishing, the end face should be inspected and where required the end face geometry should be checked using 3D interferometry. It is recommended not to polish a terminus more than twice during its lifetime.
4.4.2 Re-terminate Re-termination is defined to mean the attachment of a connector (or termini within a connector) to the cable 'on aircraft'. The technique can be used to replace a damaged contact by 're-terminating' although it is equally applicable to repair mid-loom where two ‘re-terminations’ can be performed and mated – a procedure that could be used as an alternative to splicing. An ideal re-terminating solution would;  Be able to be performed in very confined areas.  Be able to restore the mechanical strength of the cable.  Be able to perform satisfactorily in the environment over the life of the airframe.  Require minimal time and skill to perform termination. Four generic techniques for re-termination have been identified, these are;  Pot and polish – attachment of fibre to connector is by some form of adhesive – similar to a standard termination.  Crimp and cleave – developed for plastic fibre – obtaining a good quality cleave with glass fibre is difficult.  Crimp, cleave and polish – as above with an extra polishing step suitable for glass fibre.  Pre-Inserted fibre – depends on a good cleave and requires adhesive. All of these techniques follow the procedure of an initial preparation of the cable, attachment of the connector to the cable, end face preparation and a final assembly. The main failure mechanism seen on existing installations of fibres in aircraft have been failures very close to a connection, induced mainly by maintenance actions. There are going to be many occurrences where the time and cost implication of stripping out a loom and replacing it are not a satisfactory solution. This is why the avionics industry needs a re-termination capability. The findings of studies suggest that the best approach, currently, is to re-terminate to the same standard of initial manufacture, where possible. This would draw upon the developments into miniaturisation of tools such as ovens and polishing fixtures. A drawback is that to allow for a re-termination extra fibre needs to be incorporated into the loom. Techniques for achieving this are discussed in the Looming and Installation Practices section of this handbook. If all of these techniques fail, it will be necessary to replace the link.
5 Inspection and cleaning 5.1 End face inspection
NOTE End face inspection should never be carried out with the application operating. Care must be taken to ensure that the application is not emitting light energy that could cause injury to the operator or others. Therefore, prior to any end face inspection being carried out the operator must make sure that any power supplies are removed from the links being inspected. Visual inspection is important in order to identify and assess for contamination and damage to the optical fibre core and surrounding contact material prior to mating. The process covers the detailed inspection of the end face of the fibre optic termini using magnification equipment. Currently the two main methods for inspection of fibre optic termini end-faces are the handheld microscope and video inspection. A handheld microscope requires the operator to directly view the end face of the termini through a focussing microscope whereas video inspection equipment comprises of a video probe assembly and remote visual image monitor or laptop/notebook screen. Video inspection equipment can also incorporate software that measures the events on the image of the end-face provides the operator with detailed information of event and particle size plus a 'Pass/Fail’ measurement based on a pre-set standard. Typically this information can also be downloaded and presented in a report format for recording. Video inspection microscope should be preferred to hand held microscope for end face inspection in order to be compliant with eye safety recommendations (see IEC 60825-1). 5.1.1 Inspection standards Inspection standards are applied at the completion of termination and assembly and a manufacturer would normally specify that the termini end faces are to be free from all contaminants and damage. This requirement can be described as ‘Start of Life’. However, once shipped and installed the link should be defined as being ‘in-service’ and an ‘acceptable’ standard defined to prevent un-necessary rejection of acceptable termini end face quality. An in-service standard should define inspection criteria for acceptable and unacceptable contamination and damage based on system performance and fibre size of a particular system or platform. SAE-AS5675, IEC 61300-3-35 and ARINC 805 are the main standards used in aerospace. 5.1.2 Inspection zones To aid visual inspection a fibre optic terminus end face can be separated into defined areas, or ‘zones’. Figure 2 shows the agreed zones and Table 1 provides zone definitions for Multimode and Single mode fibre. Specific requirements can be detailed for each of the zones to give an ‘inspection criteria’, based on the size of optical fibre core and allowable events, such as scratches and debris. It should also be recognised that contamination can also accumulate on the side of the ferrule or within the alignment sleeve and can be transferred to the end face, therefore inspection is not just limited to the fibre core/cladding and surrounding ferrule end face.
Key 1 Zone A – Core/Mode field 2 Zone C – Epoxy adhesive/Coating 3 Zone B – Cladding 4 Zone D – Contact/Ferrule Figure 2 — Inspection zones Table 2 — Typical zone definitions for multimode and single mode fibre Zone Name Multimode Zone Dimensions Single mode Zone Dimensions A Core/Mode field Core diameter multiplied by 1.1 (defined as Core diameter + 10 %) 9 µm B Cladding From the Core OD of Zone A to the OD of the cladding minus 5 µm From the mode field OD of Zone A to the OD of the cladding minus 5 µm C Adhesive/Coating (i.e. polyimide) From the OD of Zone B or coating to the ID of the ferrule plus 10 µm From the OD of Zone B to the ID of the ferrule plus 10 µm D Contact/Ferrule From the OD of Zone C to 2 x the cladding (not to exceed 450 µm) From the OD of Zone C to 2 times the cladding (not to exceed 450 µm)
5.1.3 Inspection criteria When cleaned and inspected a fibre optic terminus end face may show anomalies which can be defined as defects. Defects are events that are not removable by normal cleaning and can be caused by a number of factors and comprise of; pits chips, scratches, embedded debris, loose debris, cracks etc. These end face anomalies can be grouped into the following categories: a) Scratches – Linear surface defects that cannot be removed through cleaning. b) Defects – All other visible features not removed by the cleaning process. c) Cracks/Structural Defects – Cracks and structural defects are not just surface defects, but extend into the optical fibre.
The end face zones can be categorised for acceptable defects. Figure 3 shows the zones with the respective ‘In service’ acceptable defects for multimode fibres.
Key 1 Zone A – Core zone – Crack: none – Scratchesã no more than
u
·
u µm 2 Zone C – Epoxy zone – Cracks: no limit on size or number – Scratches: no limit on size number – Debris: maximum of
w pieces of debris
¶
s r µm in diameter 3 Zone D – Contact zone – Cracks: none – Scratches: no limit on size number – Debris: maximum of 5 pieces of debris
¶
s r µm in diameter 4 Zone B – Cladding zone – Crack: none – Scratchesã no more than
·
x
Ám width no limit on number – Debris: none > 3 µm Figure 3 — Typical acceptable in service inspection zone criteria for multimode fibres NOTE A 1 µm defect on a 200/280 µm core fibre is equivalent to 0,5 % of the area of the core but is 1,6 % of the core on a 62,5/125 µm fibre and 11 % of a 9/125 µm fibre. Therefore, any in-service inspection standard will also vary between fibre sizes. In addition, inspection standards will also have to take into account the effects of defects when using the same size fibre due to the system design, available power budget, components and architecture used. 5.1.4 Inspection magnification 5.1.4.1 Handheld direct viewing inspection microscopes For handheld direct viewing microscopes two inspection levels of magnification are typically used for aerospace fibre optic applications and are recommended for the following optical fibre types:  Multimode: 200 x for contact zone and core zone.  Mono mode: 400 x for contact zone and core zone.
5.1.4.2 Video inspection microscopes Video inspection microscopes differ in that there is no direct vision of the terminus end face. This allows the operator to access termini that are installed in areas that would not be accessible to handheld direct vision inspection microscopes. Video inspection microscopes can also be used on multimode and single mode applications without the need to change magnification. The image being displayed on a portable monitor screen or laptop/notebook. Interchangeable probe adaptors are available allowing inspection of interconnects with exposed and recessed termini within simplex and multiway interconnects. Figure 4 show inspection without sleeve holder on the left picture and inspection through sleeve holder on the right picture.
Figure 4 — Inspection without sleeve holder and inspection through sleeve holder on EN 4645 connector Another benefit of video inspection microscopes is the availability of software that evaluates the image, based on inspection criteria for the size of terminus end face and size of optical fibre being inspected being loaded into the software. The software will produce a pass/Fail result for each inspection allowing the operator to quickly ascertain the suitability of the terminus end face. It also removes any operator conjecture on whether an end face is acceptable. The software can be installed on a laptop/notebook or integral on some microscope models, and can be saved on the devices or downloaded and printed as hardcopy reports which can be used for storage or as a quality audit record. Fibre optic interconnects provide a means of accurately aligning optical fibres to achieve optimum light transmission at production breaks. To maintain optimum performance the end faces of optical fibre termini need to be free from contamination and particles that will affect the performance of the transmitted light. This performance can be identified as a loss in optical power and increases in bit error rate (BER). In addition, fibre optic termini are often spring loaded to maintain a mating force, should these springs become contaminated with debris there may be a detrimental effect on the mating forces being applied, leading to poor optical performance.
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