Fabrication of Tilted Fiber Bragg Grating as a Sensor of Refractive Index of Liquids

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Abstract

We have developed a fabrication technique of tilted fiber Bragg gratings (TFBG) for refractive index sensor of liquids. We demonstrated that a simple technique using a combination of 266-nm laser and a phase mask with a period of 1.065 μm was quite effective for the fabrication of the TFBG. Using fabricated TFBGs which had the tilted angles of 3.3˚, 6.7˚, 7.3˚, 8.0˚, and 9.9˚, we tried to measure the refractive index of liquids which have different indices. Water, ethanol and glycerine solutions (12%, 24%, 35%, 46%, 66%, and 84%) were used as samples. For the measurement, a 10-mm long TFBGs were covered with a sample liquid droplet. The transmission spectra in the cladding mode and core mode were observed by an optical spectrum analyzer. We have directed our attention to the fact that wavelength of cladding mode shifts to be longer with the increase of refractive index of sample liquids. Utilizing this wavelength shift, we proposed a new measurement method. As a result, we could successfully measured the refractive index of liquids within a range from 1.00 to 1.41 with a maximum resolution of 3.0_x10^-3. In addition, we have found that a contact length of only 2.4 mm is necessary to obtain 90% of signal intensity of 10-mm long TFBG.

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