The Chalcogenide glasses offers a range of infrared transmitting materials to the optical fiber technology and this review attempts to bring together the currently available data on the mechanical properties of these fibers. Information is presented on glass composition and mechanical qualities, mechanical characterization techniques, and coating technology. Different studies have shown that changes in composition have little influence on mechanical properties of Chalcogenide glasses, and so other techniques will have to be employed in order to improve the mechanical performance. Among those techniques are compressive surface stress and multi-coating polymers.
The fiber to the home technology is already in use and more fibers are necessary to carry all the demanded information. Ribbon optical cable, present in the market since the seventy’s, is the right technology to support those needs. Ribboned fiber mechanical strength is currently characterized using tensile tests, where the required handling affects the results. In this study, we demonstrate the use of two-point bending tests to characterize ribbon strength. By this method, handling was minimized and samples aged in harsh environments could be tested. Normal and Weibull Probability Distributions were used to characterize and compare the strength of two ribbons, and of the fibers used to construct them. Both distributions were efficient in describing the strength during an aging process done in harsh environment.
Keywords: Ribbon Cable, Two-Point Bending, Dynamic Fatigue, Weibull, Normal.
The strength reliability of spliced optical fibers is normally characterized using the tensile tests. When the fiber is under two- point bending, the glass surface area under maximum stress is very small and sharply localized, and it would be necessary exactly to situate the splice in that area, because that two-point bending is not used to study the strength of the splices. But an important question is related with the strength of spliced fibers: the strength degradation of the extremities of the fibers stripped during the splice operation. In those small lengths of fiber, around four centimeters, it is possible to use the two- point bending technique. In this work the two-point bending technique was used to measure the extremities strength of spliced fibers and a study of the flaw population presented in that vicinity is made comparing with tensile tests. The splices were prepared using an automatic method to strip, cleave and clean the fiber extremities.
In the Power Law Crack Growth Theory the strength reliability of optical fibers is based on some mechanical fiber parameters. The n parameter, or strength susceptibility parameter, is the most important because it is a two-order exponent number used to calculate the survival lifetime. To measure this parameter different mechanical tests can be performed, among them the dynamic fatigue in tensile and the static fatigue in two-point bending. Static fatigue in the two-point bending technique using glass tubes to accommodate the samples does not require a lot of space and it can easily be set in different harsh environments. But still some problems can affect the results of this technique: the internal diameter irregularities of the tubes, the fiber damage introduced to the samples in the lower diameter tubes, and the uncontrolled stress rate during the loading process for all tube diameters. Using a two-point bending apparatus, and faceplate supports, precise static fatigue bending tests were performed measuring the n error calculated with the tubes. This apparatus allowed us to hit the maximum stress under very well controlled stress rate and no sample manipulation during the entire process.
The Strip Force test is widely used in the fiber optic telecommunications industry. This force is connected with the mechanical properties of the coatings, and in some extent with the adhesion between the coating and the glass. If the polymer coating fails by cracks or delamination with water penetration, it can cause fiber strength degradation, because of the glass humidity susceptibility. Until now there has been no correlation found between strength degradation, strip force and coating damages. In this study the strip force and the strength were measured for different fibers aged in harsh environment for long periods of time. Short and long samples were used. Short samples are like coating damaged fibers opened to the humid environment, and the long samples simulated long length of fibers inside cable structures. The results of the tests point out the everlasting life of the strip force and coating quality, opposing to the strength degradation of fibers under long term aging.
The strength measurements of optical fibers is performed usually in tensile and in two-point bending. In tensile the weak flaw that will fracture the fiber has its effective length perpendicular to the length of the fiber. In two-point bending the maximum stress in the fiber will fracture the worst flaw which effective length is perpendicular to the length of the fiber. But the optical fiber manufacture process occurs in the length line and most defects originated during the process is aligned in length, and is not fully tested using two-point bending or tensile. Another important practical aspect not investigated is related with the low torsion stress present during long time in installed cables, that can cause fracture by fatigue. In torsion the maximum tensile stress is on planes forty-five degrees oriented with respect to the length of the fiber and it can better detect those defects. In this study dynamic fatigue torsion tests were performed and compared with other mechanical tests. The results show a good agreement between fracture distributions for fibers under torsion and under two-point bending.
Using Two-Point Bending and Tensile tests in commercial optical fibers it was found that strength results are connected. Because of that connection many fractures cannot be explained assuming isolated defects. Other explanations are proposed in terms of a family of defects spread in length and superficial stress areas originated during manufacture.
Polymeric coatings are the most important strength reliability factor of fiber optics. Diverse accelerated aging processes have been used in the last years with the objective to estimate the lifetime of fibers in real field situations. Because the humidity present at the glass- polymer interface determines the strength, a dry technique in fiber optics is used to understand the properties of coating permeability after aging process. Dry technique parameters can be connected with the strength evolution after months in hot water aging. To compare the efficiency of several coatings under this technique different fibers from different manufacturers were used. The strength measurements were made using the Two-Point Bending apparatus.
The two-point bending technique has been used to measure the strength of both polymer coated and bare fibers in liquid nitrogen after the fibers were first aged in an aggressive environment followed by a drying process. The results show that some strength recovery occurs upon drying of polymer coated samples while continuing degradation was seen when drying bare samples. The healing process observed for coated fiber is thought to be caused by condensation of the hydrated surface layer formed during aging.
Reliability of Optical Fiber Components, Devices, Systems, and Networks III
3 April 2006 | Strasbourg, France
SC319: Optical Fiber Characterization and Specifications for Optical Transmission
This course covers the precise nature of the manufacturing process of fiber optics. Because of the glass fragility, it is necessary for an immediate protection using coatings based in ultra-violet polymers. Because of those factors, the optical and mechanical characterization of optical fibers must be accurately reliable. Actual commercial fibers comply with International Standardization and guarantee a full operation lifetime to the consumer. This course presents optical fibers, and their characteristics.