Fault analysis for optical cables

(1)

Fault analysis for optical cables

The inner structures of an optical cable which has been

used in a particularly harsh environment are revealed in

a microfocus CT scan

(2)

FibreOptics\Beratung\FO6\Part1 FO 6 Part 1 LWLK1 Slide2

The first investigations were carried out on cut-out cable length, using the back scatter method in the 1625 nm wavelength range

First the results were evaluated with special software, then the cable was divided into15 sections, based on sudden attenuation losses or inhomogeneities.

(3)

The second stage in the investigation was to determine actual cable excess length in

relation to design type, with the aid of the MTS Kimmich measuring technology service.

The results of this measurement for 2 cable samples showed that there was excessive

scatter distribution in both. This could cause inadmissible attenuation and PMD

changes in individual fibres in the case of temperature fluctuation.

1480 m

rel. fibre excess length Delta L in rel to MW (°/°°) Cable partially armoured, Lm ess=39,20 m

-1,50 -1,00 -0,50 0,00 0,50 1,00 1,50 2,00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Fibre No. De lt a L /M W L ( °/° °) Überl.

rel. fibre excess length Delta L bez. auf MW (°/°°)

Cable 1, length 1663,5-1535,3=128,20 m -1,00 -0,50 0,00 0,50 1,00 1,50 2,00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Fibre No. D e lt a L /M W L ( °/ °°) Überl.

(4)

FibreOptics\Beratung\FO6\Part1 FO 6 Part 1 LWLK1 Slide4 LWL 1 1625nm -0,500 -0,300 -0,100 0,100 0,300 0,500 0,700 A B C D E F G H I J [Zyklus] [d B ] 2pt Loss [ dB] Ins. Loss [dB] Sollwert +0,05 [ dB] Sollwert -0,05 [ dB] J I H G F E D C B A Cycle / 105,2 [m] Length under stress 1750 1750 1750 200 2000 200 1450 1000 200 [N] Pull. strength(2)  4 cycles shut open 13,3 13,3 [m] Length under stress 20 2100 2250 2000 20 2500 20 2000 1500 20 [N] Pull. strength(1)

Torsion under tension FO-Test No. 1471 Cable tensile strength

FO-Test No. 1461 Test

For the next stage, the testing sample with the highest attenuation jump was chosen for the verification test for

attenuation change and fibre elongation, "Cable tensile strength" in acc. with. EN 60794-1-2 Verf. E1 and "Torsion resistance under tension" sim. to Bellcore GR20 (R6-61). The

unarmoured cable was then tested with reference to the guideline value for „Pulling and stretching of the armoured cable„.

Diagram:

Attenuation change (overview)

after "Long-term tensile stress" and "Torsion under tensile stress" carried out on Fibre 1 (average excess length) Specified value +/- 0,1 dB under tensile stress

(5)

8 L2 (F2) 5 KN (F1) 20 KN 8 L1 A A 1539 1658 Twisted section 3 m Twisted length 13 m _E

Test set-up

Cable tensile strength in acc. with IEC EN 60794-1-2 Method E1

Torsion under tension, Bellcore GR 20

L1 Load end (20 KN load cell Cable elongation 1)

Inner metre measuring tape 1658,0 m; Counting direction = continual stranding twist to the left.

Length under tensile stress & torsion: 13 m L2 Holding end (5 KN load cell

Cable elongation 2)

Inner metre measuring tape 1539,5 m; Length under load 105 m

A Length under torsion / twisted length 8 Fixed deflecting device 320 mm

(6)

After testing the tensile strength of the cable, a red light source was used and a CT scan carried out on the sample length (inner sheath removed).

This made it possible to make the attenuation

jump in the cable length visible as fibre compression. The picture shows testing section A (L1).

Picture:

Fault detected "Fibre compression due to inhomogeneities of the core filling material" Specimen1 with metre number 1651,30

Fibre compression can be seen at the fault points marked

1 Fault identification marking (visible under red light)

2 Counting tube, marked as No. 6 3 Fibre compression in the "red"

counting tube (source of fault) 4 Central GFK pulling & supporting

(7)

Picture:

Fibre compression D1 red (3) in comparison with adjacent fibres and tubes D2 and D6 (2)

(8)

 Picture:

1 Optical fibre compression in the red counting tube

2 Optical fibre in tubes 2 or 6

 Picture :

Density variation in the tube filling material (side view)

 Picture :

Density variation in the tube filling material (seen from below)

(9)

 Picture:

Air trapped in tube D2  Picture:

D2 Density variation Cause: trapped air D6 Counting direction

(10)

dead zone fibre

Determination of the fibre radius of curvature at 1625 nm at the point of compression (attenuation jump) a mandrel (4) of varying dimensions is positioned between two identical cable samples (2) taken from the test cable (2) and the optical fibre (2) looped around it through 360° (5) in the measuring direction of the OTDR-Meßrichtung (6), which positions it in front of the fusion splicer (3.2).

1 Radius 20,0 mm

2 Radius 12,5 mm = 1,115 dB 3 Radius 15,0 mm = 0,347 dB

4 Radius 0,0 mm

(11)

Result of testing:

Fibre compression is pinpointed

The faulty section is removed from the tube sheath :

TGA analysis "6,9% residue" resulting concentration of SiO₂ This led to malfunctioning under operating conditions, due to the influence of segregation

processes, heat, vibration etc. on the compressed fibre overlength.

(12)

A comparison of the FT-IR spectra of tube filling materials (plastic tube and metal tube)

Plastic tube

produced in1998,

filled with 3 optical fibres

(aged under extreme conditions)

Metal tube

produced in 2007/2008,

filled with 30 optical fibres (not aged)

The FT-IR spektra images of the filling materials for plastic and metal tubes have been superimposed. Both exhibit the typical absorption bands of saturated hydrocarbons.

The plastic tube filling material has two additional bands at 1106 und 812cm -1_{, which indicate the}

presence of silicium dioxide (SiO₂): these are not present in the spectrum of the metal tube filling material. The metal tube filling material has an additional band at 699 cm-1, which is not present in the spectrum of the plastic tube filling material.

Metal tube