Andrei A. Stolov

Thermal Stability of Specialty Optical Fibers

Application of silica optical fibers at elevated temperatures is limited by the thermal stability of their polymer coatings, as thermal degradation in most polymer materials occurs at much lower temperatures than silica, which can result in degradation in the fiber performance. In this paper, we attempt to clarify the concept of thermal stability of specialty polymer-coated silica optical fibers and demonstrate a quantitative measurement method using thermogravimetric analysis (TGA) in combination with methods for optical and strength measurement of optical fiber. Based on the results, long-term use temperatures for various specialty optical fibers are estimated.

Effects of sterilization methods on key properties of specialty optical fibers used in medical devices

Optical fibers with different types of polymer coatings were exposed to three sterilization conditions: multiple autoclaving, treatment with ethylene oxide and treatment with gamma rays. Effects of different sterilization techniques on key optical and mechanical properties of the fibers are reported. The primary attention is given to behavior of the coatings in harsh sterilization environments. The following four coating/buffer types were investigated: (i) dual acrylate, (ii) polyimide, (iii) silicone/PEEK and (iv) fluoroacrylate hard cladding/ETFE.

Optical fibers with polyimide coatings for medical applications

Key properties of polyimide-coated optical fibers, unaged and exposed to various harsh environments, were investigated. The main intent was to model extreme conditions that can be encountered in medical applications of the fibers. A fiber designed by OFS showed good strength and was able to withstand exposure to extreme heat and humidity, multiple autoclave cycles, extended water soak and immersion in organic solvents. Similar fibers offered by other suppliers displayed shortcomings in some of the tested properties.

Thermal stability of specialty optical fiber coatings

Thermal stability of coatings used on specialty optical fibers is assessed via thermogravimetry (TG). The coating lifetimes are evaluated by analyzing dynamic TG curves and assuming a particular level of mass loss as a failure criterion. The studied coating systems include single and dual acrylate, fluorinated acrylate, epoxy, epoxy acrylate, silicone, silicone acrylate, silsesquioxane acrylate and polyimide materials. Processing the data by Arrhenius and Eyring equations indicates that a kinetic compensation effect (KCE) is observed for the decomposition processes, with distinct parameters for different classes of the coatings. Two aspects of the KCE are considered: (1) decomposition within several series of similar compounds in air and (2) decomposition of the same classes in different environments. An attempt is made to use the KCE to predict the lifetimes of the coatings at elevated temperatures.

Micro Attenuated Total Reflection Spectra of Bulk Silica Glass: Effects of Experimental Conditions and Glass Thermal History on Appearance of a Surface Polariton in the Si—O Stretching Region

Micro attenuated total reflection (micro-ATR) spectra of bulk silica glass were investigated for a variety of samples, including fused quartz slides, an optical fiber preform, and a series of optical fiber claddings. The experiments were performed at varied distances between the internal reflection element (IRE) and the sample. At certain conditions, a surface polariton peak is observed in the region 1100–1160 cm−1. The position of this peak is affected by the type of IRE (Ge, Si, ZnSe, or diamond), IRE–sample distance, and the material used as an interlayer between the IRE and the sample (air or Nujol). From the experimental data, the dielectric constant of silica is determined in the region between 1100 and 1160 cm−1. The polariton peak is also observed when glass is coated with a thin (40 nm) layer of carbon. It has also been found that the polariton peak position is affected by the thermal history of the glass, and an attempt is made to correlate the observed changes with the glass fictive temperature.

Bragg gratings in carbon coated optical fibers and their potential sensor applications in harsh environment

We have demonstrated that fiber Bragg gratings can be written through the carbon layer of carbon-coated optical fibers having different coating thicknesses. Specifically, grating index modulation amplitudes of ~2.5x10-5 and 0.52x10-5 were obtained in optical fibers having carbon layers 29 nm and 56 nm thick, respectively, without any extra photosensitization of the fibers. Subsequent experimental results showed that the carbon coatings in the grating areas didn’t change their hermetic properties. Finally, we describe the advantages of these gratings and their potential applications in fiber optic sensing.

Fictive Temperature of Larger Diameter Silica Optical Fibers

Fictive temperature (Tf) of pure silica cores was determined for a series of polymer-clad optical fibers via reflectance FTIR spectroscopy. The core diameters of the studied fibers were in the range 125-1500 μm. The fictive temperature was found to decrease gradually with increasing core diameter, which correlates with the anticipated fiber cooling rate. For a 1500 μm fiber, the radial distribution of the fictive temperature was examined. No dependence of Tf was observed upon the distance from the fiber axis, indicating that the heat conduction inside the silica fiber was much faster than the heat convection away from its surface, even for fibers with diameters greater than 125 μm.

Application of Micro-Attenuated Total Reflectance Infrared Spectroscopy to Quantitative Analysis of Optical Fiber Coatings: Effects of Optical Contact

Micro-attenuated total reflectance (ATR) infrared spectroscopy is one of the few methods applicable for an in situ analysis of polymer coatings. In this method, the information is collected using an internal reflection element (IRE), which is brought into contact with the coated substrate. Perfect optical contact is hardly achievable for non-flat substrates and/or for those samples that cannot be well aligned with respect to the IRE. Consequently, the infrared peak intensities, their ratios, and all quantities calculated from the spectra become dependent upon the optical contact quality. In this work, we suggest a model that describes the peak intensities in terms of the optical contact. As an illustration, we apply this model to polymer-coated optical fibers. Relatively small diameters of tested fibers and their cylindrical shape result in imperfect optical contact between the sample and the IRE. Spectroscopic approaches of determining the degree of cure of polymer coatings are analyzed in view of the obtained results. Ways of minimizing the error induced by imperfect optical contact are suggested.

Mechanical properties of polyimide coated optical fibers at elevated temperatures

High temperature mechanical strength and reliability of optical fibers have become important subjects as optical fibers are increasingly used for harsher environments. Theories and models of fiber mechanical properties established for traditional telecommunications applications may need to be validated for applications at elevated temperatures. In this paper, we describe the test setup for high temperature tensile strength of fiber and report initial results of dynamic tensile strength of polyimide coated optical fiber at 300 and 350ºC for different heating time intervals. The results are compared with room temperature strength data, data available in the literature, and our earlier work on thermogravimetric analysis (TGA) weight loss of the polyimide coating and the observations on surface morphology at elevated temperatures. Interesting observations are discussed and possible explanations are proposed.

Effects of low temperature and hot steam on reliability of specialty optical fibers designed for avionics applications

Miscellaneous avionics applications of optical fibers require their use at extreme temperatures and harsh environments. It is important that fiber properties, including optical attenuation and mechanical strength are immune to the environment conditions. Exposing specialty optical fibers to extreme conditions and testing their properties during and after the exposures helps in optimizing the fiber designs for specific applications. In this work, optical fibers with different refractive index profiles and different coatings were immersed in liquid nitrogen (LN2) and hot steam and their main properties were analyzed.