Accurate determination of the thermal variation of the aperture of step-index optical fibers.
Thermal variations of the aperture of step-index optical fibers have been studied by measuring the transmitted power when a laser beam is launched with a variable incidence. For not too-long fibers only the rays launched with the higher incidence are affected. The variations of their transmittance have been accurately related to the refractive-index ones. Silicone clad silica fibers are sensitive to temperature because the silicone index varies much more the the silica index. On the contrary, due to a compensation phenomenon between index variations of core and cladding materials, PMMA core fibers have apertures that are nearly insensitive to temperature.
Accurate characterization of the transmittivity of largediameter multimode optical fibers.
We describe an original technique that permits a specially accurate measurement of the variations of the transmittance of optical fibers vs the incidence of launched light. For large-diameter large-aperture fibers, which prefer light power transmission, the transmittance may be modeled using geometric optics. Taking into account the reflection losses at both ends, the core attenuation and a very weak lack of reflectivity at the core-cladding interface, good agreement is obtained between calculated variations and experimental results for the three kinds of fiber tested. The remaining difference has to be attributed to the mode coupling process which may be thus evaluated. The too high attenuation coefficient, which is attributed to the cladding to account for the losses at the core-cladding interface, characterizes in fact a thin perturbed layer where the materials contact. Thus, this method is a good means to evaluate separately the various processes contributing to the attenuation of open beams along the fibers and to determine which process should be worked on to improve the transmittance.
Fatigue effects on the coordinative pattern during cycling: kinetics and kinematics evaluation.
The aim of the present study was to analyze the net joint moment distribution, joint forces and kinematics during cycling to exhaustion. Right pedal forces and lower limb kinematics of ten cyclists were measured throughout a fatigue cycling test at 100% of PO(MAX). The absolute net joint moments, resultant force and kinematics were calculated for the hip, knee and ankle joint through inverse dynamics. The contribution of each joint to the total net joint moments was computed. Decreased pedaling cadence was observed followed by a decreased ankle moment contribution to the total joint moments in the end of the test. The total absolute joint moment, and the hip and knee moments has also increased with fatigue. Resultant force was increased, while kinematics has changed in the end of the test for hip, knee and ankle joints. Reduced ankle contribution to the total absolute joint moment combined with higher ankle force and changes in kinematics has indicated a different mechanical function for this joint. Kinetics and kinematics changes observed at hip and knee joint was expected due to their function as power sources. Kinematics changes would be explained as an attempt to overcome decreased contractile properties of muscles during fatigue.