`The ldquocurvature gaugerdquo sensor monitors deflection of structures under mechanical loading in applications in which strain gauges have traditionally been used. Structural deflection-curvature is measured rather than material strain. The sensitive zone of the curvature gage is precision machined into the plastic optical fiber on grinding or milling machines. The cutout produced removes a part of the fiber core and introduces light loss that is related to the bend-radius of the fiber. This modulation mechanism is described analytically in this paper. Results relate the relative light loss to the fiber curvature for different parameters of the sensitive zone (depth, length, number of cuts, bend radius, and pitch of cuts). This allows a quantitative optimization of the gauge without having to produce thousands of sensors with slightly different combination of parameters in order to accomplish a similar objective experimentally.

Using the power flow equation and experimental measurements, investigated in this paper is the state of mode coupling in low NA (0.3) step-index (SI) plastic optical fibers under varied temperature. Numerical results obtained using the power flow equation agree well with experimental measurements. It is found that elevated temperatures of low-NA SI plastic optical fibers strengthened mode coupling. These properties remained after a year, with the fiber being subjected to environmental temperature variations of >35 K. These thermally induced changes of the fiber properties are attributed to the increased intrinsic perturbation effects in the PMMA material at higher temperatures. This results in an increase of the measured bandwidth with increasing fiber temperature as well as earlier the bandwidth switch from the functional dependence of 1/z to 1/z ^1/2 (slower bandwidth decrease).

Solving the time-dependent power flow equation (PFE) provides a useful method to study the transmission bandwidth of step-index silica photonic crystal fibers (SI SPCFs). The transmission bandwidth of these kinds of fibers is determined for different air-hole structures (different numerical apertures (NAs)) and different distribution widths of the Gaussian launch beam. The results indicate that the lower the NA of SI SPCFs, the higher the bandwidth (for example, for a lower NA of SI SPCFs, a bandwidth that is eight times larger is obtained at a fiber length of 3500 m). The narrower launch beam at short fiber lengths results in a wider bandwidth. The longer the fiber (>300 m), the much less the effect of the launch beam width on the bandwidth. The bandwidth becomes independent of the width of the launch beam distribution at the fiber length at which a steady-state distribution (SSD) is reached. These results are useful for some potential applications, such as high capacity transmission optical fiber systems.

A means of calculating optical power distribution in bent multimode optical fibers is proposed. It employs the power-flow equation approximated by the Fokker–Planck equation that is solved by the explicit finite-difference method. Conceptually important steps of this procedure include (i) dividing the full length of the bent optical fiber into a finite number of short, straight segments; (ii) solving the power equation for each segment sequentially to find its output distribution; and (iii) expressing that output distribution in rotated coordinates of the subsequent segment along the curved fiber to determine the input distribution for that subsequent segment and thus enable the calculation of the power flow and output distribution for it. The segment length and bend-induced perturbation of output angles are determined by geometric optics. The resulting power distributions are given at different cross sections along the curved fiber axis. They vary with the radius of fiber curvature and launch conditions. Results are compared to those for straight fiber. Bending loss is calculated as well.