[13a-C31-1~8] 3.7 Laser processing

[13a-C31-4] Interference of optical waveguide for an optical fiber sensor induced by using femtosecond laser internal processing

〇Naoki Chinen¹, Masahiko Shiraishi¹, Kenji Goya², Atsushi Seki¹, Kazuhiro Watanabe¹, Shouichi Kubodera¹ (1.Soka Univ., 2.AIST.)

Keywords:Femtosecond laser, Optical fiber sensor, Void

In this paper, a novel optical fiber sensor was demonstrated by using femtosecond laser internal processing. The proposed sensor is fabricated by using tightly focused ultra-short pulse laser beam. The sensor has unique structure inserted a number of cavity regions (voids) in line at the boundary between optical fiber core and cladding. Propagating light in the fiber optic waveguide was scattered because of hitting against the voids. The interference between core-modes and cladding-modes was occurred. Then, cladding modes were influenced by refractive index of outside situation near the sensing structure. The wavelength of induced interference spectrum was shifted due to refractive index (RI) with changing ambient fabricated sensor portion. Proposed sensor is robustness to being bent and easy to make because of just pulse laser processing in an optical fiber instead of deforming optical fiber without thermal processing and utilizing photonic crystal fiber. First, the sensing structure was fabricated. And then, light intensity spectra were obtained changing RI using by the sensor and optical equipment. In order to fabricate the voids, laser pulse beam was focused by objective lens (NA: 0.45) to 1.46 μm at the laser focus. Ti: Sapphire lasers (IFRIT Cyber Laser Inc.) system for internal optical fiber processing produced 18 mJ pulse energy with the duration of 210 fs at a wavelength of 800 nm, allowing optical fiber to be micro-machined only in its inside. To acquire light intensity spectra, we used white light source (AQ 4305, YOKOGAWA) and an optical spectrum analyzer (AQ-6315A, ANDO). External RI was changed by immersing sensor portion into pure water, ethanol, and glycerin solution (concentration: 90 %), these RI values are 1.333, 1.361, and 1.459 respectively. Consequently, we confirmed that interference spectrum whose amplification are more than 0.20 dB occurred. The sensitivity of proposed sensor to RI changes has been achieved 31.8 nm/RIU for a RI range of 1.000 - 1.333. The interference spectrum was ascertainable when the sensor portion was in air and immersed into pure water, but not gained when sensor portion was in relatively higher RIs. The reason why these phenomena occurred is the phase deviation of cladding-modes because of optical path length that cladding-modes propagates and the total reflection critical angle of the boundary between cladding and outside of optical fiber changed due to peripheral RI changes.