Optical fiber sensors have many advantages for structural health monitoring and are often used to monitor the strain distribution of structures. However, using Fiber Bragg Grating (FBG) sensors with a general interrogation system, e.g. Wavelength Division Multiplexing (WDM), permits only the measurement of average strain within the gauge length and moreover, a large number of FBG sensors may be necessary to monitor overall stress concentrations. A distributed strain sensor with a higher spatial resolution and longer sensing length is therefore desired for the accurate and effective monitoring of stress concentration. To meet this need, we present a new strain measurement system with a long gauge FBG sensor based on the principle of Optical Frequency Domain Reflectometry (OFDR) which enables us to measure fully distributed strain at high spatial resolution. In this paper, we describe the principle and optical simulation model of our proposed measurement system and show the results of numerical calculations.

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As a method to overcome the moding problem by injection locking while the number of devices is increased when multiple device power combining is carried out with a cylindrical cavity, a power combiner of a double‐cavity multiple‐device configuration was proposed in which the oversized cavity with the desired mode of TMOnO (n ⩾ 2) is connected by a coaxial line to a fundamental mode cavity with the same frequency of the TMOnO Mode. First, by means of the modal analysis using the equivalent circuit model of the oscillator, the condition for sustaining the desired mode, the condition for suppression of the undesired mode, and the condition for existence of the double mode are obtained so that the range of stable growth of each mode is found. It was shown that the suppression of the undesired mode can be accomplished and the stable oscillation of the desired mode becomes easier by the present configuration. Further, as the oversized cavity, the TM020‐mode octuple‐device cavity and the TM030‐mode 15‐device cavity were used in the power‐combining experiments. The power‐combining efficiency of 91 and 95.8 percent was accomplished. With respect to the growth range of each mode, the theory and the experiment agreed qualitatively.

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