Example 5: Dispersion and attenuation

Plastic fiber-optic cables in cable carriers

Plastic fiber-optic cables are used for data transmission in industrial applications due to their excellent resistance against electro-magnetic fields, their small size and low weight. However, operating as a continuous-flex cable inside a cable carrier places high demands on these cables.

The most important properties of a fiber-optic cable relate to 'dispersion' and 'attenuation'. Dispersion is the spreading of the signal in time. In plastic fiber-optic cables this involves ‘modal dispersion’: the spreading of the signal in time resulting from the different propagation modes in the fiber. Dispersion determines important transmission properties, such as bandwidth, cut-off frequency or maximum bit rate. Significant changes in dispersion could not be determined in any of the tests carried out, so it can be concluded that the industrial application of igus® Chainflex® fiber-optic cables inside cable carriers is unproblematic in terms of dispersion changes.

Attenuation determines the maximum possible length of a transmission path. The attenuation of a plastic fiber, such as a glass fiber, is also strongly dependent on the wavelength of the light used. For this reason, all the tests were carried out with a wavelength of 666nm.

Depending on the output of the transmitter and the sensitivity of the receiver, the operator has an attenuation budget available for the complete transmission path - including all junction and transition regions. This attenuation budget (typically approx. 20dB) must not be exceeded if a secure data transmission is needed. To know whether and to what extent increases in attenuation are to be expected for a particular application is important so these can be taken into account for the compilation of the user's own attenuation budget.

In addition to continuous bending stress, further mechanical stresses that can occur during installation or operation must be taken into account. For example, relatively large tensile forces can occur when integrating the cable into a cable carrier. Fixing the cables at the ends of the cable carrier using cable clamps also leads to permanent transverse loads. To analyze the behavior under transverse loads, tests were carried out according to DIN VDE 0472, Part 223. Since the cable clamps only exercise pressure in an area covering a few inches, increases in attenuation are relatively low. Attenuation under tensile loads depends to a great extent on the composition of the cable. Cables with integrated copper conductors or strain relief elements do not reveal a noticeable increase in attenuation until much greater tensile forces are applied.