Fibre optic cable loop

[Nortube]

Probably not signalling specific, so this may not be the right topic. Probably a question for Railtechnician!

Some years ago when the orange fibre optic cable was installed, in many places there was a loop of the cable, possibly about four feet(?) square diameter in the cable run. This was clearly seen in the open at many Underground stations wher it was mounted on the perforated vertical cable fixing. I was told that this was just "spare". Was that true, or does the loop serve a purpose? - not that I can think of one.

[Zippy]

My guess is that the termination is a potential weak point with fibre optics and the loop was to allow the cable to be re-terminated if it developed a fault.

[railtechnician]

The loops are known as 'remake loops' and they exist not only trackside as loops of cable but also as loops of individual cores where they are terminated in trays at the transmission racks. In the early days of fibre optic cabling the overall thickness of the glass in each core (generally the cables are anything from 4 core to 64 cores or more) could not be guaranteed not to vary in the length. When jointing fibre optic cores it is necessary as far as is possible to joint cores of like size otherwise transmitted light (the data signals) may be lost at the joint attenuating the signals or losing them altogether.
The idea of the loops was thus to enable the jointer to strip back each cable and match the cores for thickness. Modern fibre optics and better jointing methods mean that this is no longer an issue but it is good practice to leave slack in the cables in case they have to be diverted or the joint remade at some future time.
The reason for the trays is to ensure that the minimum bending radius is maintained in the slack, bending such a cable could severely attenuate the signal. Light of course likes to travel in straight lines and the cable extruded glass cores allow it to follow curves but if the curves are too tight, i.e. bends, the light waves are destroyed.

Fibre optic jointing technology has moved on somewhat since I was trained to joint such cables back in 1990. Originally the glass cores were laid into a vee block butt jointed with a glass cover to hold them in place in 1974 when LT installed its first 4 core fibre optic cable between the old Earls Court and Acton Town telephone exchanges to provide a 30 channel PCM transmission link. I was trained to use a jointing machine which fused cores together after carefully cleaning the ends with HV electrical impulses, each individual joint being sleeved with a steel reinforcing rod about 2" long to give mechanical support to the electrically fused butt joint. AFAIK the jointing method is the same as I learnt but the machines are much more sophisticated and easier to use in terms of core alignment and making the joint.

[Deleted]

And there's me thinking it was a radiated cable loop!

[railtechnician]

No the radiating cables which are used for train radio are not orange, depending upon location and type they have brown, blue, (the original Storno radio system cables) black or grey sheathing.
Orange sheathed cables are not necessarily fibre optic, they could be pyrotenax elsewhere than trackside and in the world beyond LU orange sheathed cables are generally used for street traffic light circuitry nowadays.

[Nortube]

Interesting
Whilst I've had lots of experience of cables, like most people, my only experience with fibre optices is either with fibre optic lamps or plugging the opto cable in to the amplifier! Fibre optic is the way to go these days, especially for communications, and is replacing many of the traditional cables.

[railtechnician]

Mar 12, 2013 10:27:54 GMT Nortube said:

Interesting
Whilst I've had lots of experience of cables, like most people, my only experience with fibre optices is either with fibre optic lamps or plugging the opto cable in to the amplifier! Fibre optic is the way to go these days, especially for communications, and is replacing many of the traditional cables.

When I retired LU were installing 64 fibre cables as standard to carry telephone, data, UTS, CCTV, PA and other channels. When I worked on Comms in the 1980s and 1990s the standard for main telephone transmission was 34Mb and 140Mb but on the Central line resignalling 565 Mb was the norm and when the Victoria line comms were upgraded in the 1990s the PCM transmission systems would've been at least 565 Mb. 
34 Mb is sixteen 2 Mb systems muliplexed together, each 2 Mb system being 30 telephone (voice) channels or 128 k bandwidth. 140 Mb is four 34Mb systems multiplexed together and 565 Mb is four 140 Mb systems multiplexed together or 7680 voice channels on a single fibre !

2 Mb is also known as an E1 system, 8 Mb is E2 (simply an intermediate system used to create 34 Mb), 34 Mb is E3, 140 Mb is E4 and 565 Mb is E5, these being European Telecoms standard designations.