The mechanism of dark bridge formed with very slow contact separating speed was empirically shown with some discussions from the experimental results which the authors have obtained by using the cantilever system for controlling contact gap. By analyzing the obtained results, we will assume some concept of formation mechanism of dark bridge. Since the thermal effect is inevitable in the bridge formation, this will be inferred from two points; one is the a-spot change at initial state of bridge forming, and another one is the thermal expansion due to bridge current that affects the shape of bridge. This paper will show from these two points the conceptual bridge formation mechanism in case of non-melting state of contact bridge which is called here dark bridge.

Phase performance in a temperature sensor using a conventional single-mode step-index fiber is studied. Two modes are excited in a so called single-mode fiber when the wavelength of a laser source is shorter than the one suggested by the specification of a fiber. The phase shift due to the temperature change of a step-index fiber is analyzed. The intensity fluctuation by the interference of two modes is observed in the experiment and estimated in the computer simulation.

A negative thermal expansion ceramic substrate and an athermal fiber Bragg grating component with the substrate were subjected to reliability tests. We confirmed that the component has adequate durability for use as optical filters in the WDM system, under test conditions of damp heat, low temperature, mechanical shock and vibration. (50 words)

Several packaging methods for athermalization of Fiber Bragg Grating (FBG), which is equipped with negative expansion substrates, have been proposed. However, those methods have some deficiency resulted from the substrates such as complex structure or poor thermal expansion characteristics. In order to provide a suitable substrate for the athermalization of FBG, the authors have developed a Negative Expansion Ceramic Substrate (NECS) which has simple structure and suitable thermal expansion characteristics. NECS consists of polycrystalline β-quartz solid solution (Li2O-Al2O3-nSiO2, n>2), and has thermal expansion coefficient of about -65 to -85 10-7/C, which is sufficient large enough for total compensation of the Bragg wavelength shift. No difference in the thermal expansion was observed between the specimen as prepared and the one on which an epoxy adhesive was applied. NECS is produced by means of a sintering method, which enables flexible design of the chemical composition. It was found that the hysteresis in thermal expansion of the NECS depends upon the chemical composition and crystalline structure. We decreased thermal expansion hysteresis by controlling the SiO2 ratio in the composition and the crystal grain size. We confirmed that the temperature dependence of the FBG mounted on the NECS with an epoxy adhesive was decreased to -2.3 10-3 nm/C from 10.0 10-3 nm/C, in good agreement with the calculated value of -2.6 10-3 nm/C. The hysteresis in Bragg wavelength shift was less than 0.03 nm, that is sufficiently small enough for practical use. It was confirmed that NECS has suitable thermal expansion characteristics for the athermalization of FBG.

Phase performance in a fiber optic temperature sensor using a mode-division multiplex is studied. The phase shift due to the temperature change of a multimode graded-index optical fiber is analyzed. The intensity fluctuation by the interference of two modes is estimated in computer simulation.