Resonance-temperature-insensitive phase-shifted long-period fiber gratings induced by surface deformation with anomalous strain characteristics

Abstract

Received February 18, 2005 We report the fabrication of phase-shifted long-period fiber gratings by use of a CO2 laser with a surface deformation technique and a point-by-point method. Due to periodicities that are structurally induced by a heat source and refractive-index modulation caused by a perturbational photoelastic effect, the results of thermal testing show a grating temperature coefficient of 28 pm/ degrees C at a range from 22 to 180 degrees C, indicating that the band rejections of the grating that is formed remain unchanged in their resonant strengths even at temperatures up to the fictive point of fiberglass. It is found for what is believed to be the first time that such a grating possesses an anomalous strain behavior of resonance with -1.52 x 10(-4) dB/muepsilon for the right notch and 7.05 x 10(-5) dB/muepsilon for the left notch.

Review of long period fiber gratings written by CO2 laser

“…͑3͒ Microtaper-based LPFGs were also fabricated by means of tapering periodically a conventional glass fiber with a focused CO 2 laser beam. 19,[68][69][70] Negative temperature coefficient of the resonant wavelength was observed in the microtaper-based LPFGs. 68 Apart from conventional single mode fibers ͑SMFs͒, there have been a number of studies on CO 2 -laser writing of LPFGs in boron-doped fibers.…”

“…[71][72][73][74][75] The grating writing efficiency can be enhanced with a fiber annealed at a sufficiently high temperature. 29,71,72 Compared with normal UV-laser exposure technique, the CO 2 laser irradiation technique is easily used to write special gratings such as phase-shifted LPFGs, 28,69 chirped LPFGs, 76 complicated apodized LPFGs, 77 grating pairs, 62 and ultralong period fiber gratings with a period of up to several millimeters. [78][79][80] LPFGs have also been successfully achieved with a CO 2 laser during the fiber-drawing process.…”

“…Possible mechanisms for refractive index modulation in the CO 2 -laser-induced LPFGs could be attributed to residual stress relaxation, [73][74][75]99 glass densification, 71,72,100,101 and/or physical deformation, 19,20,[23][24][25][26][27][68][69][70] depending on the types of the employed fibers and on the practical fabrication techniques. A few methods have been demonstrated to measure the refractive index modulation in the CO 2 -laser-induced LPFGs.…”

“…Such physical deformation induces a change in effective refractive index in the fiber, and thus LPFGs are created in the periodically taped optical fibers. 19,[68][69][70] Moreover, the CO 2 -laser-induced high temperature in the fiber causes, not only the fiber elongation and the diameter decrease but also the ablation of glass on the fiber surface and the partial or complete collapses of air holes in the PCFs. As a result, different types of LPFGs are written in conventional glass fibers in which periodic grooves/microbends are created on the fiber surface, 19,23,[68][69][70] and in microstructured optical fibers, such as solid-core PCFs ͑Refs.…”

“…The CO 2 -laser-induced LPFGs with physical deformations exhibit unique optical properties while a tensile strain is applied, 23,24,63,69,70,131 thus being excellent strain sensing elements. As shown in Fig.…”