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Different fiber-based sensing structures have been developed for torsion detection, such as long-period fiber gratings (LPFGs) based on screw-type distortion in the bare optical fiber , the engraving of angle-chirped LPFGs , helicoidal long-period fiber gratings (H-LPFGs) , twist tapers, and special fibers like the polarization-maintaining fibers (PANDA, Bow Tie) for their high linear birefringence .
In this work, the response of long-period fiber gratings (LPFGs) manufactured by C[O.sub.2] laser radiation to torsion in an asymmetrical fiber exposure configuration is discussed.
In addition, a smaller grating length can facilitate the creation of more complex structures, like multiplexed or coated LPFGs. However, engraving shorter-length gratings has specific practical challenges since typical procedures cannot be followed .
LPFGs are periodic structures engraved in the optical fiber along its longitudinal axis, with the purpose of inducing a slight change in the refractive index of the fiber core.
As demonstrated in the previous work , the coupled-mode theory and 3D finite element modeling for predicting the refractive index variation produced by the laser irradiation are adequate to model and understand the mechanisms enabling the creation of ultrashort LPFGs. In fact, both theoretically and experimentally, it was demonstrated that the amount of strain applied to the fiber plays an important role in the quality of the sensors and has a direct relation to the required lengths of the gratings.
The sensitivity of LPFGs to torsion arises from the connection between changes in the grating's birefringent structure due to axial twists (caused by the photoelastic effect) and the state of polarization (SOP) of the guided light [10, 16, 17], producing a shift of the resonance wavelength and/or an increment or reduction of the intensity of the transmitted light.
The response of the fiber is linear and sensitivities are within 0.06-0.08 nm/[degrees]C in the temperature range of 25[degrees]C to 75[degrees]C, for the LPFG lengths considered in our study.
(i) for the temperature, sensitivities in the interval 0.06-0.08 nm/[degrees]C in the measuring range of 25 to 75[degrees]C and for the overall LPFG lengths being studied;
Rao, "CO2-laser induced LPFG torsion characteristics depending on length of twisted fibre," Electronics Letters, vol.
Rebordao, "Automation methodology for the development of LPFG using CO2 laser radiation," in Proceedings of the 8th Iberoamerican Optics Meeting, RIAO 2013 and 11th Latin American Meeting on Optics, Lasers, and Applications, OPTILAS 2013, prt, July 2013.
Caption: FIGURE 3: Theoretical attenuation at the resonance peak of a 600 [micro]m period LPFG when considering (a) a length of 4.8 mm and different weights and (b) 5 g (0.049 N) and (c) 80 g (0.78 N) weights and different length.
Caption: FIGURE 4: Micrograph showing the tapering effect in the irradiated zones in a 2.4 mm LPFG with 600 [micro]m period written on an SMF-28 optical fiber (P [approximately equal to] 4.5 W; [t.sub.on] = 0.6 s; F [approximately equal to] 0.78 N).
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