Long-period fiber gratings were inscribed in a commercial silica fiber by a point-by-point arc discharge technique with different discharge conditions. The refractive index (RI) profile change induced by arc discharge was measured using the quantitative phase microscopy for the first time to our knowledge. The causes of the transmission variations induced by different arc discharges and the mechanisms of the RI profile change were investigated based on the measured phase profiles. The RI in the core and the cladding has clearly changed due to arc discharge. The central dip in the core profile diminished very much, and the index gradient became gradual. The resonance wavelengths have fluctuated by discharge current and time owing to variations of the reduction of the core-cladding RI difference and the extent of the RI change region.

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Spectral response of acoustically induced microbending through thin optical fiber is discussed from mode-coupling of core and cladding modes. The thin fiber is analyzed in three-layered structure (core-cladding-air) to gain insights into acousto-optic modulation. We explained the dependence of core and/or cladding diameters on acoustic source parameters from numerical calculations. According to the calculations, we successfully fabricated all-optical tunable filter using this thin fiber that yields an efficient mode-coupling at flexural wave frequencies less than 1 MHz. To increase the acousto-optic effect, we used a specially designed thin optical fiber (80 µm of cladding diameter) in the section where flexural wave is produced, and spliced both ends of the thin fiber to the tapered 125 µm fibers. The frequency and voltage tuning of fabricated filter is also confirmed by changing the driven frequency and applied voltage of the PZT, respectively. This result suggests a possibility of fiber-optic device application as all-optical tunable filter at 1.55 µm.