HARSH ENVIRONMENT CABLES

It is not possible to assembly a good quality, special application fibre optic cable harness without using a proper optical fibre cable. The cables used for telecom applications and cables used for special, harsh environment applications represent totally different worlds. That is why their design and construction is completely unlike.
Special application optical cables are commonly used to build temporary networks like the following:

• military field deployable communication network,
• life transmissions from the TV (HDTV) cameras to the transmission vehicle (sport, cultural events, concerts etc.),
• underground mining equipment signal transmission,
• connection between the ship and the board,
• audio signal transmission during musical events and concerts.

The most critical issue for all above listed applications is the flexibility of the cable. It has to have a feature to be reeled and unreeled many times and besides maintaining its transmission parameters, it has to lay loose and flat on the surface when deployed, with no shape-memory effect. For the fact that the cables need to be operated in a broad range of temperatures, it is also very important that the cable retain their flexibility within the whole operating temperature range.

The cable that are used for telecommunication networks, gel-filled with the loose tube are the worst possible solution to be used for such applications. Their “tube” construction results in a lack of flexibility. Parallel mounting of the tubes with the central mechanical stress carrying element makes the cable very difficult to terminate on the connector. Besides, the fibres are located far from the centre of the cable, which makes them exposed to mechanical threads and higher tensions during bending the cable. The phenomenon of axial migration cased by gel moving down inside the cable makes these kind of cable to be disqualified from being used where the cables need to be operated on the slope – mining applications, board-to-ship, mountains, etc.

Tight buffer (no gel-filled) are by far the best solution for harsh environment application. Their tight structure is by definition designed to carry high axial stress (compression and tensile strength) and radial stress (crushing), while maintaining small diameter and the ideal moisture intake protection. The cables keep high flexibility and durability after many bending cycles, so they are able to be reeled and unreeled many times without loosing their transmission parameters. The tight buffer cables are ideal to be terminated with the high performance, harsh environment fibre optic connectors.

Structure of the special application cables

Distribution cables (mini-breakout) are made of few separate optical fibres of 125 µm diameter, each of them coated with acrylic and protected by the tight buffer of 900 µm diameter. The protection of buffered fibres are aramid (Kevlar) yarns which are also stress carrying element. The outer jacket, mostly of polyurethane, completes the construction. The advantages of this solution are: small outer diameter, small bend radius and low weight. The central location of the fibres prevents the fibres from the exposure to the stresses applied to the outer section of the cable when being bent.

	Distribution structure military optical fibre.		Breakout structure military optical fibre.

	Cross section of the military distribution cable, break-out type.		Cross section of the military distribution cable.

Breakout Cable

	Helical and parallel layout of the fibres in cable.		Cable with helically laid fibres.

One of the main differences between the standard telecom cable and the military or special application cables is helical turn of the fibres or the sub-cables. The fibres or sub-cables are pressed against each other on pushed into the centre of the cable, which caused the following phenomenon:

• the tensions are evenly distributed amongst the fibres or modules,
• the symmetric geometry and the outer diameter of the cable is always maintained,
• the flexibility of the cable is higher and of isotropic character (no matter which direction it bents).

Another important difference between the standard telecom cable and the special application cables is the high-pressure applied outer jacket (Core-Locked™). The matrail applied under the high pressure fills all the physical gaps between the fibres or modules.

	A comparison between the high-pressure applied outer jacket and the standard commercial telecom outer jacket.

That features:

• no matter of the tensions and strengths applied onto the cable, the fibres or modules remain always in the same position and maintain their mechanical parameters
• due to the lack of possibility of fibres or modules migrating inside the structure of the cable, the cable can be consider as a one, solid mechanical structure. It allows for example, the possibility of simple hanging the cables with the self-locking Kellems® grips, mounted directly to the outer jacked of the cable; the cable can be hanged vertically this way up to 1km, without thread to be broken with its own weight,
• protection against the exceeding bending of the cable,
• limitation of the water penetration section.