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

Abstract

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 CO 2 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 CO 2 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 CO 2 laser for exact TAP operation. Amplitude based strain sensitivity of 1.13dB/mε was obtained with tuned TAP LPG.

Optical Fibre Long-Period Grating Sensors Operating at and around the Phase Matching Turning Point

“…Some of these include local exposure of the fibre to a UV laser [28,32,33,34], CO 2 laser [35,36], femtosecond laser [23,37] or by electrical arc discharge [30,38]. PMTPs have been written in conventional single mode and doped fibres [12,11,33,34,36,39], and have been theoretically investigated using photonic crystal fibres [23]. The length of an LPG tends to range from 30 to 50 mm and have refractive index changes of around 10 -4 [40].…”

“…A PMTP can also be tuned after an LPG has been fabricated, by tuning the mode coupling and effective index guiding via the means of tapering [12], UV exposure [43], with a thin film overlay [31,28], etching [44] and radiation exposure [12]. The LPG can also been enhanced by reducing the cladding via hydrofluoric (HF) acid [13,[45][46][47] and plasma [48] etching to tailor the coupling strength of the cladding mode at PMTP.…”

“…This process assists in the precise post processing of nano-coated fibres in hard and chemically resistant films, for example, diamond-like carbon [50]. Radiation exposure has also been demonstrated to alter the refractive index of B-Ge co-doped fibres, with an equivalent increase in core refractive index of around 1 × 10 -5 [12].…”

“…This type of LPG configuration has become increasingly popular due to its ultra-high sensitivity, a property usually desirable for a sensor. Approaches employed to improve the sensing capability of LPGs have included methods such as tapering [12] and etching [13]; however this can weaken the structure of the fibre and requires more delicate handling or complicated packaging. These sensors have been successfully used for measuring parameters such as temperature [14][15][16], strain [14][15][16] and refractive index (RI) [17][18][19][20].…”