We fabricated an elliptical hollow-core photonic bandgap fiber (EC-PBGF) by controlling lateral tension in the hollow core region during the fiber drawing process. The absolute value of group modal birefringence becomes relatively high near the bandgap boundaries. We also experimentally measured the strain and temperature sensitivities of the fabricated EC-PBGF-based Sagnac loop interferometer. The strain and temperature sensitivities were very much dependent upon the wavelength. Moreover this PBGF-based interferometer can be a good sensor of physical parameters such as strain and temperature.

A photonic crystal fiber with two large air holes outside the holey cladding region is fabricated to induce an effective long periodic grating (LPG) in the core by an electric arc discharge. We believe that the two large air holes lead to the asymmetric perturbation in the core under the electric arc discharge, thereby introducing the coupling to the first higher-order mode. The transmission characteristics of the PCF with the LPG for the external perturbation such as strain, curvature, and temperature are also investigated. It was found that the shift of resonance peak in the transmission spectrum depends on the bending direction. The curvature of 8.55 m -1 results in the center wavelength shifts of 1.8, 4.3, and 11 nm for a vertical, diagonal, and horizontal direction of the curvature to the large air-hole alignment, respectively.

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We describe the fabrication of elliptical hollow-core photonic bandgap fibers (EC-PBGFs). It was shown that the aspect ratio of the hollow core can be controlled by tuning the negative pressure in the space between the intermediate preform cane and outer jacketing tube, and by placing this preform assembly off-center in the furnace, resulting in lateral tension during the final draw. Modal birefringences of fabricated PBGFs with different aspect ratio were measured using a Sagnac loop interferometer. For the elliptical hollow core PBGF with aspect ratio of 2.34, the modal birefringence was measured to be about 4.6x10(-2) at 1,550 nm.

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We fabricate highly birefringent photonic crystal fiber with new structure using a stack and draw method. Fabricated fiber has two big air holes, one at each side of the outside air cladding region, leading to core ellipticity during the drawing process. Birefringence of the fabricated Hi-Bi PCF is measured to be 2.29 × 10-4 (at 1550 nm

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