BS/EN 2591-100-2018 pdf download.Aerospace series – Elements of electrical and optical connection – Test methods Part 100: General.
Generally a source will have two key distributions. Firstly, the variation of power across the aperture of the source (the near field distribution). Secondly, the variation In power with angle or the far field power distribution. Optical fibres have key parameters of core size (diameter = 2a) and also numerical aperture (NA). The launch condition is effectively a comparison of the source power distribution to the parameters of the fibre system under test. For example, a launch condition of 85:85 would describe a source with a near field profile that fills 85 % of the core and has a far field angular distribution that fills 85% of the fibre NA.
The ideal launch condition distribution would lie somewhere between an overfilled launch condition (that would give an unduly pessimistic value for the insertion loss) and a grossly underfilled launch condition (that would give an optimistic value). In general, the optimum launch condition slightly underfills both the core of the fibre and the numerical aperture. This removes that part of the source’s light distribution that Is most likely to be attenuated by the harness components due to manufacturing imperfections. Studies have shown that the correct launch condition lies somewhere between an 80:80 launch and 90:90 launch.
Another consequence of using an ideal launch condition to make insertion loss medurements Is that the difference between the loss of a complete harness and the sum of the losses of the individual harness components Is minimised. It should be mentioned that the launch distribution of a source used to transmit data down the fibre in a real system will also have a given power distribution. This may well be different to the source used to characterise the fibre system although if the two are similar, then the performance of the real system link can be anticipated from the loss data of the system components.
In summary, well defined launch conditions are required to:
• Ensure reliable measurement of component loss (and therefore allow system loss to be calculated).
• Enable reliable and repeatable characterisation of an aerospace multi-mode fibre optic link, In particular to he sensitive to extreme and undesirable effects such as tight bends or incorrect installation practices.
• Ensure manufacturers and operators make the ‘same’ standardised measurement.
In order to avoid any differences between various suppliers’ sources, the whole launch condition profile shall ideally be defined. The specification defines 3 points on near field and far field scans. For graded-Index fibre, note that the far field requirements are not mandatory because It Is mathematically linked to the near field.
5.2.2 Generating the correct launch conditions
To create a specified launch condition it is possible to manipulate the light inside a fibre optic lead. This may involve scrambling (mixing up) the modes within a multi-mode fibre (to more fully fill the fibre) and then subsequently filtering the light (e.g. mode stripping through bends in the fibre), to produce a desired distribution. Another alternative is to use a different fibre type to launch into a fibre (different core and NA) to give a particular launch.
Previous versions of this standard specified a general optical system based on bulk optics (lenses, beam splitters) and diaphragms, capable of generating required launch conditions. Despite its versatility, the system was a large assemblage of laboratory type components and was rather unwieldy.
Nowadays test sources are available with controlled launches or an in-built modal conditioning stage (before the output fibre connector). Alternatively, stand-alone mode conditioners are available that can take a range of Input light distributions and produce the required distribution or launch condition at the output connector. They are essentially ‘black box’ devices. The operator simply connects their light source to the Input and the output will have the correct launch condition to test the given fibre system. These types of source are considered to be easier to use and more readily available to the industry. Such equipment should be verified (calibrated) to ensure the launch condition output is as specified. This evidence would be provided In supporting documentation for the measurements made.
BS EN 2591-100:2018
EN 2591-100:2018 (E)
Note launch condition issues are not a problem for single mode systems where the fibre usually supports one propagating mode.
In the following sections. launch conditions are defined for the two most common standard multi- mode fibre types, namely 62.5/125 pm and 50/125 pm (core/cladding size). Whilst It Is recognised that other fibres are in use In the aerospace industry (e.g. 100/140 pm. 200/ 280 pm). these are considered legacy fibres.
The avionics launch conditions are specified as cross sectional teniplates of the near and far field power distributions. It is noted that the telecommunication industry may specify launch conditions in terms of encircled flux (e.g. ISO/IEC 61280-4-1). This Is an integrated measure of the power within regions/bands of the fibre cross section. It Is useful for measuring coherent sources where speckle Interference may be observed between modes. The reader should be aware of these different approaches.
5.2.3 Launch conditions specification for 62,5/125 pm fibres and cables (NA = 0.275)
This launch condition is considered suitable for testing general 62,5/125 pm fibres and cables with a NA (numerical aperture) oIO,275.BS/EN 2591-100-2018 pdf download.