Radiation and taper tuning of Long Period Grating for high sensitivity strain measurement

Turn around point (TAP) in long period fiber grating (LPG) provides extremely high sensitivity to external environmental parameters like temperature, strain and refractive index. It also opens up new dimensions for development of photonic devices. Higher order cladding mode coupling in a LPG requires relatively lower grating period (<; 300 μm), which can be achieved by improving the spot size in CO₂ laser based grating inscription methods. But the inscription of LPG with resonance exactly at TAP within 950-1700 nm wavelength band requires a period precision better than 0.5 μm. Since, our fabrication system based on automated CO₂ laser has limited translational resolution, we have used exposure to gamma radiation and tapering of fiber as tools to tailor the spectral characteristics of our gratings. Total Gamma radiation exposure of about 6 kGy resulted in ± 35nm wavelength shift of dual resonant loss peaks in a 206 μm period near TAP-LPG, almost merging the two peaks. This exposed LPG was then tapered using CO₂ laser for exact TAP operation. Amplitude based strain sensitivity of 1.13dB/me was obtained with tuned TAP LPG.