Example 5: Dispersion and attenuation

Plastic fiber-optic cables in Energy Chains®

Plastic fiber-optic cables have been introduced for data transmission in industrial applications due to their excellent interference-proof properties against electro-magnetic fields and further advantages such as the possibility of reducing dimensions and weights. The application as flexible link lines particularly in Energy supply chains places high demands on plastic fiber-optic cables. The most important characteristic values of a fiber-optic cable are dispersion and attenuation. Dispersion is the term used to describe the scattering of the travel time of the signal in the fiber-optic cable. In plastic fiber-optic cables this is essentially caused by the mode dispersion, which arises from the different travel times of individual light beams. Dispersion determines important transmission properties such as bandwidth, cut-off frequency or maximum bit rate. Significant changes in dispersion could not be ascertained in any of the investigations carried out. The industrial application of igus Chainflex®-lines with plastic fiber-optic cables in supply chains for example is therefore unproblematic with regard to changes in dispersion. The second important characteristic property, attenuation, determines the maximum possible length of a transmission path.
The attenuation of a plastic fiber, like that of the glass fiber, is also strongly dependent on the wavelength of the light used. For this reason all the investigations were carried out with a wavelength of 666nm.
Depending on the output of the transmitter and the sensitivity of the receiver the operator has a certain "attenuation budget" available for the complete transmission path including all junction and transition regions. This attenuation budget (typical value approx. 20dB) must not be exceeded if a secure transmission of the data is to be guaranteed.
For this reason it is of great interest to the user to know whether and to what extent increases in attenuation are to be expected for his particular application so that these can be taken into account in the compilation of his own attenuation budget.
In addition to continuous bending stress, which is typical for operation in an Energy chain, further mechanical stresses that can occur during installation or operation must be taken into account. Thus, for example, relatively large tensile forces can occur when integrating the line into an Energy chain. The fixing of the lines at the ends of the energy chain using cable clamps leads to permanent transverse loads. The test of the behaviour under transverse load is carried out following DIN VDE 0472, Part 223. Since the cable clamps only exercise pressure in an area covering a few centimetres, increases in attenuation are relatively low. Attenuation under tensile load depends to a great extent of course on the composition of the line. Lines with integrated copper conductors or strain relief elements do not reveal a noticeable increase in attenuation until very much greater tensile forces are applied than is the case with pure fiber-optic cables.