We demonstrate that temperature-and strain-insensitive long-period gratings can be fabricated in conventional optical fibers. The former is employed to measure strain with resolution of 20 ⑀ under thermal fluctuations in the surroundings, while the latter is used to detect temperature variations as small as 0.8°C in the presence of axial strain. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)00407-8]

Long-period gratings that couple the fundamental guided mode to discrete cladding modes were first proposed by Vengsarkar et al. ' and have since been demonstrated as amplified spontaneous emission (ASE) filters,' erbium-doped fiber amplifier (EDFA) gain equalizers2 and sens o r~,~ A grating with period A results in resonance bands at wavelengths, where, (Sn)m is the differential effective index between the guided mode and an HE,, cladding mode. Gratings written in standard fibers typically have periods in hundreds of micrometers such that the characteristic A versus A curve has a positive slope (dhldh > 0). The temperatureinduced shifts3 vary from 5-15 nml100"C and hence fibers with special profiles4 need to be designed to overcome this limitation. We show that gratings fabricated in standard fibers with dAldA < 0 can have an order of magnitude smaller temperature sensitivity than conventional gratings (dhldh > 0). We also demonstrate that such gratings with reduced temperature coefficient retain their capability to detect strain and ambient index variations.We selected Corning 1060-nm Flexcor fiber for our analysis and using Eq. (1) found that resonance bands with dX/dA < 0 are typically available for A < 100 pm. Several gratings with A = 40 pm were fabricated in hydrogen-loaded fibers using an appropriate amplitude mask and UV exposure from a cw laser. The annealed gratings were tested for their temperature, strain, and refractive-index sensitivity. Figure 1 reveals that a 100°C increase in temperature results in a -0.18-nm shift in the grating resonance band. Other gratings were found to have temperature shifts in the range -0.15 to -0.45 nm for 100°C change in temperature. The temperature sensitivity of a grating is given by?where the first term on the right-hand side is the material contribution and the second represents the waveguide contribution. The condition dhldh < 0 generally involves coupling to cladding modes with large order m. Changes in 6n due to temperature variation can be shown to produce small shifts in the corresponding resonance hands for these higher-order modes. The operation with dhldh < 0 produces a negative waveguide contribution to the overall temperature sensitivity. Figure 2 depicts the response of the grating to axial strain. The coefficient of -2 1.44 nm/%E results from the large negative waveguide contribution to the strain-induced shift and its magnitude is comparable to gratings3 with dA/dA > 0. The spectral displacement due to changes in 0 -1 Y 5 -2

Long-period gratings have recently gained popularity as versatile optical fiber sensing elements that are simple and economical to fabricate and demodulate. Long-period gratings are periodic photoinduced structures in fiber cores that couple light from guided to cladding modes. We discuss the applications of these devices to strain measurements in high-performance materials and structures. Experimental results from a preliminary loading test carried out on a reinforcingbar commonly used in civil structures are presented. The temperature cross-sensitivity of longperiod grating-based strain sensors is analyzed and two methods to overcome this limitation are presented. We also demonstrate that strain-insensitive long-period gratings can be fabricated in standard optical fibers. The application of such gratings to temperature measurements in the presence of actively varying axial strain is discussed. Preliminary results indicate that longperiod gratings hold tremendous potential for health monitoring of advanced materials and structures.

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