Simultaneous measurement of axial strain, temperature, and transverse load by a superstructure fiber grating

Abstract

A novel and simple fiber-optic sensor based on a superstructure fiber grating for simultaneous measurement of temperature, axial strain, and transverse load is proposed and demonstrated. By measurement of the shift and split of broadband and narrow-band loss peaks, one can determine the temperature, axial strain, and transverse load simultaneously over the ranges 0 -140 Recently, long-period gratings (LPGs) have been demonstrated as individual sensors for temperature, strain, and transverse-load measurement, with higher sensitivities than those of FBGs. 3,4The superstructure f iber grating (SFG) is a special type of FBG that uses periodically modulated exposure over the length of a phase mask. 5Similarly to a FBG, the SFG couples the forward-propagating LP 01 mode to the reverse-propagating LP 01 mode at a series of narrow-band loss peaks in the transmission spectrum (as shown in Fig. 1). The SFG also functions as a LPG that couples the forward-propagating LP 01 mode to the forward-propagating cladding modes and introduces broadband loss peaks in the transmission spectrum. For multiparameter measurement, SFG has been used successfully to determine temperature and axial strain simultaneously. 6 In this Letter, we demonstrate a novel application of SFG to measurement of temperature, axial strain, and transverse load simultaneously.The wavelengths of the broadband and the narrowband loss peaks are sensitive to temperature and axial strain, with different sensitivities. In addition, the broadband peak will split as a result of birefringence induced under a transverse load. 4 In our experiment, we select one broadband and one narrow-band loss peak to demonstrate the sensing principle. Figure 2 shows the original and the changed spectra of a SFG subject to simultaneous variation in temperature, strain, and load, where only one narrow-band loss peak is shown and the wavelength shift and the mode split are exaggerated. In previous work, it was shown that the peak wavelengths of LPGs and FBGs change linearly with temperature and axial strain, and the mode split of the LPG loss peak and the transverse load also have a linear relation. 3,4 Our experiment confirms these linear relationships. In Fig. 2, l Bo and l No are the original wavelengths of the broadband and the narrow-band loss peak, respectively; l B and l N are the wavelengths shifted as a result of temperature and strain change; l Bx and l By are the wavelengths of the broadband x-and y-polarizing modes as a result of transverse-load-induced birefringence. The relationship among temperature T , axial strain e, and transverse load L can be expressed as

Optical Fiber Sensor Based on Second-Order Superstructure Fiber Bragg Grating

“…The sensors have the potential to be used to measure pressure and deformations in numerous environments, such as smart structures or composite materials in bridges or dams. (5,6) For these applications, different types of fiber gratings can be designed and fabricated according to the index modulation distribution and grating period design. (2) In general, a single channel fiber grating is more often used for sensing only one physical parameter by measuring the Bragg wavelength shift.…”

“…(3) On the basis of the codirectionality and the counterdirectional coupling of an SFBG, a fiber sensor with a single sensing component has been used to measure multiple parameters simultaneously, such as axial strain, pressure, temperature, and curvature. (4)(5)(6) In this study, a second-order SFBG was successfully fabricated for the first time. Then, we experimentally demonstrated that the refractive index (RI) sensitivity of this sensor is larger than that of the first-order SFBG.…”

A new method for sampled fiber Bragg grating fabrication by use of both femtosecond laser and CO_2 laser

“…Such a modulation can be achieved by periodically triggering on and off the light exposure along the fiber [7,8], which needs an accurate control of the light exposure time, power and location for each subgrating, and the apolization is obtained by multiple exposures with different power levels for different subgrating sections [9]. Alternatively, a specially designed amplitude mask can be employed together with the phase mask during the grating inscription process however, owing to the fixed modulation envelope, different amplitude masks have to be used to produce different modulation profiles [5,10].…”

Response of fiber Bragg grating transmission dips at twice the Bragg wavelength to transverse strain

“…For FBGs inscribed in birefringent fiber, the pair of closely spaced Bragg wavelengths separates further when transverse strain is applied 13 ; use of two FBGs with different periodicities produces 4 separate Bragg peaks, enabling the measurement of all three strain directions and temperature, simultaneously. Transverse strain measurement has also been demonstrated using long period gratings 14 , π -phase-shifted gratings 15 and superstructure gratings 16 . Previously we had reported on the effect of transverse strain on features in FBG spectra at 2/3 of the Bragg wavelength 17 , which are now understood 11 to be the 3 rd harmonic of a grating with Λ = Λ pm , in which the response depends upon the fiber orientation 18 .…”