Laser ultrasonic nondestructive evaluation (NDE) methods have been proposed to replace conventional in vivo dental clinical diagnosis tools that are either destructive or incapable of quantifying the elasticity of human dental enamel. In this work, a laser NDE system that can perform remote measurements on samples of small dimensions is presented. A focused laser line source is used to generate broadband surface acoustic wave impulses that are detected with a simplified optical fiber interferometer. The measured surface wave velocity dispersion spectrum is in turn used to characterize the elasticity of the specimen. The NDE system and the analysis technique are validated with measurements of different metal structures and then applied to evaluate human dental enamel. Artificial lesions are prepared on the samples to simulate different states of enamel elasticity. Measurement results for both sound and lesioned regions, as well as lesions of different severity, are clearly distinguishable from each other and fit well with physical expectations and theoretical value. This is the first time, to the best of our knowledge, that a laser-based surface wave velocity dispersion technique is successfully applied on human dental enamel, demonstrating the potential for noncontact, nondestructive in vivo detection of the development of carious lesions.
A key step towards the commercialization of microstructured polymer optical fibers is the ability to cleave and splice
them. The cleaving of polymer optical fiber (whether by cutting or fracture) depends upon the mechanical properties of
the material. These in turn depend on the conditions under which the fiber is drawn from the preform. The relationship
between fiber draw conditions, mechanical properties of the drawn fiber and the ability to cut the heated fiber with a hot
razor blade has been investigated for PMMA fibers of varying hole structure. Differential scanning calorimetry measurements
indicate that the type of PMMA used exhibits two 'relaxations' with inflexion points at 115±3oC and 80±2oC
respectively, independent of draw conditions. The first of these is in the range expected for the α-relaxation. The origin
of the second is unknown. Dynamic mechanical analysis of fiber samples indicates that the temperature dependence of
the elastic and loss moduli of the fiber vary significantly with draw conditions. The end-face produced by cutting with a
razor blade also varies with draw conditions. Fiber drawn under high tension splinters during cutting and fiber drawn
under low tension undergoes ductile deformation and fracture. However for intermediate draw conditions the fiber can
be cleanly cut with a razor blade at a temperature of 80±10oC.
The drawing of optical fiber is essentially a stabilization process with a 'slow’ feedback system used to control diameter drift. Such a system is capable of producing high precision fiber with a ±0.3 μm deviation around the mean diameter being standard even on a high-speed commercial tower. However an emerging demand for even greater precision in fiber diameter has focused attention on the neck-down process itself and the effect of furnace operating conditions on the stability of this crucial stage. In this paper we present the results of an extensive experimental study into the nature of cladding diameter variations along optical fibers based on real-time measurements made during fiber drawing. It was found that the statistical distribution of cladding diameter measurements is never Gaussian with multiple peaked distributions being quite common. Power spectra indicate that the change in diameter is generally random but with detectable maximum frequency components up to 5-10 Hz. An incorrectly set up furnace can produce strongly periodic variations. Results will be presented that quantify the impact of furnace temperature, furnace purge gas flow and iris aperture on the variation in fiber diameter. Some evidence of 'diameter modes’ -- that is, preferred diameters separated by gaps in the observed measurements -- has been found. The overarching conclusion is that better process control is not the route to reducing variation in fiber diameter. Rather the solution lies in moderating the extent to which short-time scale variations in the temperature and flow fields within the furnace interact with the preform neck-down region.
The utilization of scintillation light as a measure of radiation dose has many attractive features for medical applications. When high doses of ionizing radiation are being administered to cancer patients, precise and accurate dosimetry in terms of absolute dose and its location are essential. Fiber Optic Dosimeters [FOD] are unique in this pplication, since compared to other medical radiation dosimeters, they are smaller, more reliable and most significantly, they are human tissue equivalent. The principal limitation of the FOD is its signal to noise ratio, a feature that we discuss in terms of materials science and physical optics. The aim of this study is to outline a theoretical approach to dosimeter design based on geometrical optics that has the potential to increase the signal and decrease the noise.