Fiberoptic Cable Testing Methods

Transcription

1 Fiberoptic Cable Testing Methods Per IEC Synergy Cables Quality Assurance Department Written by Dr. Hanan Yinnon July 2007 Page 1

2 1. Tensile performance Per IEC Method E Object This measuring method applies to optical fiber cables, which are tested at a particular tensile strength in order to examine the behavior of the attenuation and the fiber elongation strain as a function of the load on a cable, which may occur during installation and/or operation Sample The cable length under test is 150 meters. Additional cable length is needed to connect the fibers to be tested Apparatus The apparatus consists of: An attenuation measuring apparatus, typically an OTDR. A fiber elongation strain measuring apparatus based on dispersion testing equipment. A specially designed tensile test machine capable of tensioning 150 meters of optical cable in six legs of 25 meters each. The machine is equipped with a motor for controlled tensioning and a load cell for measuring the actual tension applied on the cable. Figure 1 shows a typical machine. Figure 1: Typical Tensile Load Machine 1.4. Procedure The cable is wound in the machine over appropriately sized sheaves. The cable ends extend to reach the measuring instruments. A pre-determined number of fibers within the cable are concatenated by fusion splicing. Typically, two sets of fibers are used, one will serve to measure attenuation change and the other serves for elongation monitoring. After all initial measurements and calibration are carried out, the cable is pulled at a specified rate until a pre-determined tension is applied. The cable is laid to rest under tension as per detail specifications, and then the attenuation and fiber length are measured. This process may involve several tensile levels in order to characterize the entire tensile behavior of the cable, or be carried out only at the defined maximum allowed tension level. Page 2

3 1.5. Pass/Fail criteria Under load, the fiber attenuation is not increased more than a predetermined value, typically 0.05 db over the fiber length measured. Under load, the fiber does elongate by more than a pre-determined value over its initial length. The allowed elongation under installation load is typically 0.25%. 2. Crush (Compression) Per IEC Method E Object The purpose of this test is to determine the ability of an optical fiber cable to withstand compression Sample The sample is a representative cable length sufficient to carry out the specified test Apparatus The apparatus allows a sample of cable to be crushed between a flat steel base plate and a movable 100 mm long steel plate. The edges of the movable plate are rounded with a radius of about 5 mm. A typical apparatus is shown in Figure 2. Figure 2: Typical Compressive Load Machine 2.4. Procedure The cable sample is mounted between the plates so that lateral movement is prevented, and a pre-determined force is applied gradually. The maximum applied force is typically maintained for 10 minutes. Two types of measurements may be defined: The fiber attenuation is measured at the end of the 10-minute period while the cable is still under pressure. The fiber attenuation is measured 5 minutes after pressure release. This requirement is usually carried out at higher compressive loads than the previously described measurement Pass/Fail criteria The attenuation must not change by more than a pre-determined value, typically 0.05 db. In all cases, the cable elements should not fracture or crack. Signs of compression are not considered as damage to the cable elements. Page 3

4 3. Impact Per IEC Method E Object The purpose of this test is to determine the ability of an optical fiber cable to withstand impact Sample The sample is a representative cable length sufficient to carry out the specified test Apparatus The apparatus allows a hammer with a 25 mm rounded edge to drop vertically on a cable sample fixed on a flat steel plate. The apparatus may allow a single or multiple repeated impacts to be imparted on the cable sample. The energy of the impact is determined by the drop height and by the weight of the hammer. A typical impact test set-up is shown in Figure 3. Figure 3: Typical Impact Testing Machine 3.4. Procedure Two procedures are commonly used: A repeated impact test whereby the hammer drops on the same location in the cable sample 25 times. The hammer is allowed to drop on 3 different locations in the cable, typically separated by 50 cm from each other. The number of impacts in each location is limited (1 to 3). Page 4

5 3.5. Pass/Fail criteria The fiber attenuation does not increase by more than a predetermined value, typically 0.05 db. In some case, the cable passes the test if there are no fiber breaks. This requirement is usually applied when very high impact energy is used. There are no breaks or cracks in the cable elements. Signs of the impact are considered normal. 4. Repeated bending (Cyclic Flexing) per IEC Method E Object The purpose of this test is to determine the ability of an optical fiber cable to withstand repeated bending Sample The sample is a representative cable length sufficient to carry out the specified test Apparatus The apparatus allows a cable sample to be bent backwards and forwards through at an angle of 180, the two extreme positions making an angle of 90 on both sides of the vertical, while the sample is subjected to a tensile load. A typical apparatus is shown in Figure 4. The bending arm is designed to permit holding the cable securely during the entire test, without crushing the optical fibers or inducing optical loss. The bending radius is controlled by replaceable cushioning reels chosen to match the cable bending radius. The apparatus is capable of cycling at a rate of 30 cycles per minute. Figure 4: Typical Cyclic Flexing Machine 4.4. Procedure A cable sample is secured to the bending arm while it is in an upright position. A predetermined weight is attached to the bottom of the cable sample. The motor is turned on and the arm oscillates between the two extreme positions flexing the cable. The number of flexing cycles depends on the cable design. Page 5

6 4.5. Pass/Fail criteria The fiber attenuation does not increase by more than a predetermined value, typically 0.05 db. 5. Torsion per IEC Method E Object This test method is intended to establish the ability of a fiber optic cable to withstand mechanical twisting Sample The length of the specimen under torsion should be sufficient to permit the appropriate clamping and twisting, and long enough to sustain the applied torsion. A typical value for small cables is 1 m, whereas larger cable samples are usually defined in terms of the cable diameter. A common length value is 125 times the cable diameter. Armored cables may require a longer cable sample or a smaller number of torsion cycles. The actual sample length is longer than the part under torsion to allow connecting the fibers to be tested Apparatus The twisting apparatus consists of two cable gripping devices or clamps, one fixed and one that can rotate. The distance between the clamps defines the cable length under twist and is adjustable. The rotating clamp is connected to suitable turning equipment. A typical apparatus is illustrated in Figure 5. The clamps are designed to prevent crushing force on the cable and to allow the cable end to exit from both sides to allow optical measurements Procedure Figure 5: Typical Torsion Testing Machine The cable is installed in the test fixture. A pre-determined weight is fixed to one of the clamps to provide tension to the cable sample. The fiber ends are connected to suitable attenuation or optical power measurement equipment. The rotating clamp is then rotated as follows: 180 in one direction back to the starting position 180 in the opposite direction Back to the starting position. These four steps constitute a cycle. The cable sample is subjected to a predetermined number of such cycles, typically 10. Page 6

7 5.5. Pass/Fail criteria The attenuation must not change by more than a pre-determined value, typically 0.05 db. The cable should not show any indications of mechanical failure, such as jacket cracks, armor opening etc. 6. Bend per IEC Method E Object The purpose of this test is to determine the ability of an optical fiber cable to withstand bending around a test mandrel. 6.2 Sample The sample is a representative cable length sufficient to carry out the specified test. 6.3 Apparatus The apparatus is a simple mandrel of the specified diameter on which a cable sample can be wrapped tangentially in a close helix. A simple apparatus is shown in Figure 6. Figure 6: A mandrel and bent cable sample 6.4 Procedure The sample is wrapped in a close helix around the mandrel at a uniform rate. Sufficient tension is applied to ensure that the sample contours the mandrel. The sample is then unwrapped. A cycle consists of one wrapping and one unwrapping. This test is often performed at low temperature to verify the cable ability to be installed at such temperatures without being damaged. 6.5 Pass/Fail criteria The attenuation must not change by more than a pre-determined value, typically 0.05 db. The cable should not show any indications of mechanical failure, such as jacket cracks. Page 7

8 7. Temperature Cycling per IEC Object This measuring method is designed to determine the stability of the fiber attenuation in cables submitted to extreme temperature conditions. 7.2 Sample The sample is a standard cable of a length sufficient to make accurate measurement of the attenuation change. Single mode fiber cables should be at least 500 meters long, and preferably 1000 m long. Multi-mode fiber cables may be shorter. A sufficient number of fibers distributed over the cable structure are tested, at least one fiber per tube in Loose Tube cables. It is important to test a different fiber in each tube to receive a statistically meaningful estimate of the cable performance under different temperature conditions. In order to gain reproducible values, the cable sample is placed in the climate chamber as a loose coil or on a reel. In most cases the tested fibers (from different tubes) are fusion spliced in series and measured together using OTDR. The fusion splices are located outside the climate chamber. 7.3 Apparatus A climate chamber is used of a suitable size to accommodate the sample. The chamber temperature can be controlled to within ±3 C of the specified testing temperature. Such a chamber is illustrated in Figure 7. The fiber attenuation may be monitored using a stabilized light source / power meter combination or, preferably, an OTDR. Figure 7: A Climate Chamber and tested cables Page 8

9 7.4 Procedure A typical temperature cycle is depicted in Figure 8. A reference value for attenuation is determined on the cable sample already installed in the climate chamber and ready for the temperature cycling. The temperature in the chamber is then lowered to the appropriate low temperature TA at the appropriate rate of cooling. After temperature stability in the chamber has been reached, the sample is exposed to the low temperature conditions for the appropriate period t 1. The temperature in the chamber is then raised to the appropriate high temperature TB at the appropriate rate of heating. After temperature stability in the chamber has been reached, the sample is exposed to the high temperature conditions for the appropriate period t 1. The temperature in the chamber is lowered to ambient temperature at the appropriate rate of cooling. This procedure constitutes one cycle. Typically, cables are exposed to 2 such cycles. 7.5 Pass / Fail Criteria Figure 8: A Typical Temperature Cycle The attenuation of the fibers in the cable should not increase by more than a pre-determined value. 8. Water penetration per IEC Method F5 8.1 Object The purpose of this test is to determine the ability of a cable to block water migration along a specified length. 8.2 Sample A 1 to 3 meter long representative cable sample is used. A watertight seal is applied to one end of the sample to allow a 1 m height of water to be applied. Page 9

10 8.3 Apparatus The apparatus consists of a vertical pipe containing water at a height of 1 meter. The pipe is connected at its bottom to a flexible tube that allows watertight connection to the cable under test. See Figure 9 for a basic representation of such a set-up. The cable sample is laid flat and a suitable arrangement is made in order to determine if water leaks from its exposed end. Figure 9: Water penetration test set-up 8.4 Procedure The sample is supported horizontally and a 1 m height of water is applied for 24 h. A water-soluble fluorescent dye or other suitable coloring agent may be used to aid in the detection of water seepage. 8.5 Pass / Fail Criteria No water leaks are detected from the exposed cable end. Page 10