Materials Being Used: Fluoropolymer materials such as Teflon® (TFE, FEP, PFA) and crosslinked Tefzel® (ETFE) successfully applied in electrical cabling, have been similarly applied in optical cable designs as buffers and jackets. These materials are rated for high and low temperatures required for space flight application and meet standard outgassing requirements. In most mulitmode cable the buffer (loose or tight) and outer jacket are both fluoropolymers however Hytrel and Kynar can also be used. Regardless of the tradename, fluoropolymer materials have shrinkage problems. Kynar, a somewhat stiffer alternative to Tefzel, also shrinks. Vendors claim that the method of extrusion used in buffer and jacket manufacture can leave stresses in the material that only relax at elevated temperatures. There are also other extrusion process techniques which can eliminate some of the tendency to shrink. It is important to note that this shrinkage behavior is not driven by the material’s CTE.

Effect of Shrinking Cable Components: This shrinkage problem has been reported by flight cable users including McDonnell Douglas (for Space Station), NASA Goddard, and Lockheed-Martin. This property of fluoropolymers has no effect on electrical cable performance but will directly affect signal attenuation in an optical cable. For the cable to accommodate the shrunken ETFE jacket, the fiber is forced to bend repeatedly inside the cable. These microbend cause the attenuation increase. In single mode applications that use tight buffers, the shrinkage problem could have a much greater impact on optical performance in the form of higher losses or a shift in polarization state if the application demands birefringent fiber. Jacket shrink data taken at GSFC showed that all of the cable components (the buffer, strength members and the jacket) shrank together and the coated fiber remained its original length. It has not been shown whether the cable components all shrank equally or whether the shrinking of the jacket pulled the other cable components back with it. The optical effect is the same in either case.

Collected Data: Evaluations by the users named above have shown that thermal cycling with temperature excursions between -45°C and 85°C (and wider ranges) will cause an ETFE jacket to shrink between 0.4% and 1.5% for cable lengths 10 feet and above. Loss was found to increase by a factor of 2 and 3 for shrinks of 0.6% and 3.6% respectively. No additional effect was found due to exposure to vacuum conditions. The shrinking tended to flatten out after about 50 cycles.

Preconditioning Procedure: Thermal conditioning procedures are now being used by McDonnell Douglas and Lockheed-Martin to force as much shrinking as possible before the cable is terminated. The following procedure has been found to be effective for "empty" cable: a 6 cycle exposure to temperature extremes of 140°C and minus 30° with heat transfer transient times of 50 and 40 minutes respectively. Meaning there should be 40 to 50 minutes to allow the fixtures to reach the temperature of the thermal chamber. Dwell times (at temperature extremes) need be no longer than 1 minute since the length of time spent at the steady state temperature extremes does not affect the shrinkage mechanism of the cable materials. Although the ramp rate does not affect the shrinkage, what has been used was 5°C/minute due to constraints of the thermal chamber. However, the goal is to precondition and not temperature shock the materials. As a reference in the EIA-455-3 (test procedure to measure temperature cycling effects on optical fiber, optical cable, and other passive fiber optic components) it is specified for cable containing optical fiber that the ramp rate be at maximum 40°C/hour. The preconditioning procedure described here is for cable without the fiber inserted.

Kynar has a lower temperature rating than that of Teflon which may make this procedure less effective for cables with a Kynar buffer. Kynar’s melting point is around 182°C while Teflon’s limit is 343°C. A procedure has not been identified for cable which includes the fiber and it should be noted that the coatings being recommended for flight grade fiber will not withstand exposure to 140°C. An evaluation is being designed at GSFC which will include more investigation into cable component shrinkage and suitable preconditioning procedures. The best solution remains having the manufacturer precondition the materials before the fiber is included in the cable.

---

Table of Contents

Next Section: Strength Member Issues and Concerns