Measurement of CO2-laser-irradiation-induced refractive index modulation in single-mode fiber toward long-period fiber grating design and fabrication

Analysis of Long Period Gratings Inscribed by CO2 Laser Irradiation and Estimation of the Refractive Index Modulation

Long period gratings (LPGs) inscribed in single mode fibers (SMFs) using CO₂ laser irradiation were modelled numerically using the coupled mode method. The model considers the specifications of the inscription technique, such as the shape of the refractive index modulation that mimics the circularly symmetric point-to-point laser irradiation profile. A simple expression for predicting the resonant wavelength was obtained assuming a two-mode coupling model. However, to explain the spectra of the experimental LPGs, it was necessary to assume a reasonably high refractive index change and a multimode coupling model. Furthermore, using the developed model and a genetic algorithm to fit experimental resonances to simulated ones, we were able to estimate the maximum refractive index change, obtaining a value of 2.2 × 10^-3, confirming the high refractive index change. The proposed model also predicts a second order resonance for this high value of refractive index change that was confirmed experimentally. Hence, with this model, we found some significant differences in the LPGs behavior when compared with conventional ones, namely, the emergence of coupling between different cladding modes and the competition of first and second order resonances which change the LPG transmission spectrum.

Multi-Parameter Sensing Device to Detect Liquid Layers Using Long-Period Fiber Gratings

Insoluble liquids show layers such as water and oil. The detection of the exact interface locations and the level changes for layered liquids are of paramount importance for chemistry purifications, liquid storage in reservoirs, oil transportation, and chemical engineering. However, accurately measuring liquid layers is challenging. This paper introduces a multi-parameter sensing device based on a long-period fiber grating (LPFG) sensor simultaneously detecting boundary and level changes of layered liquids. Laboratory experiments demonstrated that the sensor device would respond to the liquid interface change as a sharp and sudden resonant wavelength change, while it would show a gradual and steady resonant wavelength change to the level changes of layered liquids. The lab experiments also showed that the sensor device has a higher sensitivity when a higher LPFG cladding mode is used.

All-fiber second-order optical vortex generation based on strong modulated long-period grating in a four-mode fiber

We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO₂-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP₀₁) to the high-order LP core mode (LP₂₁) with efficiency of 99.7% and then transform the LP₂₁ mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

A Fe-C coated long-period fiber grating sensor for corrosion-induced mass loss measurement

This Letter reports a Fe-C coated long period fiber gratings sensor with a grating period of 387±0.1  μm for corrosion monitoring of low carbon steel in a 3.5 wt. % NaCl solution. An LPFG sensor was first deposited with a 0.8 μm thick layer of silver (Ag) and then electroplated with a 20 μm thick Fe-C coating. The chemical composition of the Fe-C coating was designed to include the main elements of low carbon steel. The resonant wavelength of the coated sensor was correlated with the mass loss of steel over time. Test results indicated a corrosion sensitivity of 0.0423 nm per 1% mass loss up to 80% Fe-C mass loss and 0.576 nm per 1% mass loss between 80% and 95% Fe-C mass loss. The corrosion sensitivity of such a Fe-C coated LPFG sensor was a trade-off for the service life of the sensor, both depending on thicknesses of the inner silver layer and the outer Fe-C coating.

Long period fiber grating in two-core hollow eccentric fiber

Long period fiber gratings (LPGs) in a two-core hollow eccentric fiber (TCHF) have been demonstrated experimentally. Two LPGs have been fabricated into the respective core of the TCHF by a high frequency CO2 laser. The coupling characteristics in the TCHF-LPG have been studied using the coupling mode theory (CMT). The resonant peak is mainly caused by the coupling between the core mode LP01 and cladding mode LP81. The experimental results agree well with the simulation results. Furthermore, the sensing properties of the TCHF-LPG have been investigated with respect to bending, temperature and axial strain. Compared with the LPG in the single mode fiber (SMF), the experimental results indicate that the sensitivity of the TCHF-LPGs to bending curvature is low and even very small at some bending directions. In addition, TCHF-LPGs are insensitive to the axial strain while sensitive to the temperature. Therefore, the proposed TCHF-LPGs can efficiently sense the changing temperature that is independent of the strain. Moreover, the TCHF-LPGs can also be applied to two-channel filters without signal crosstalk between two cores.

Fabrication of fiber optic long period gratings operating at the phase matching turning point using an ultraviolet laser

It is known that optical fiber long period gratings (LPGs) exhibit their highest sensitivity to environmental perturbation when the period is such that the phase matching condition is satisfied at its turning point. The reproducible fabrication of LPGs with parameters satisfying this condition requires high resolution control over the properties of the grating. The performance of an LPG fabrication system based on the point-by-point UV exposure approach is analyzed in this paper, and the control of factors influencing reproducibility, including period, duty cycle, and the environment in which the device is fabricated, is explored.

Sensitivity enhancement of Faraday effect based heterodyning fiber laser magnetic field sensor by lowering linear birefringence

In this paper, we demonstrate that the sensitivity of Faraday effect based heterodyning fiber laser sensors for magnetic field can be effectively enhanced by lowering the intrinsic linear birefringence inside the fiber laser cavity. Well explained by theoretical analysis and confirmed by birefringence tuning through transversal force, it shows that the sensitivity to magnetic field intensity is inversely proportional to the linear birefringence. A CO(2)-laser treatment is therefore proposed to tune the intra-cavity linear birefringence. With CO(2)-laser treatment, the intra-cavity linear birefringence can be lowered permanently to effectively enhance the sensitivity of a heterodyning fiber laser sensor to magnetic field.

3D finite element model for writing long-period fiber gratings by CO2 laser radiation

In the last years, mid-infrared radiation emitted by CO₂ lasers has become increasing popular as a tool in the development of long-period fiber gratings. However, although the development and characterization of the resulting sensing devices have progressed quickly, further research is still necessary to consolidate functional models, especially regarding the interaction between laser radiation and the fiber's material. In this paper, a 3D finite element model is presented to simulate the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Dependence with temperature of the main parameters of the optical fiber materials (with special focus on the absorption of incident laser radiation) is considered, as well as convection and radiation losses. Thermal and residual stress analyses are made for a standard single mode fiber, and experimental results are presented.

Residual-stress relaxation and densification in CO2-laser-induced long-period fiber gratings

The first concurrent measurements of the three-dimensional refractive-index and residual-stress distributions in a CO(2)-laser-irradiated fiber are presented. A Corning SMF-28 fiber was exposed from one side to focused pulses with durations of 100-500 ms. The cross-sectional form of the index modulation is asymmetric with changes concentrated on the side of the fiber facing the exposure. The longitudinal form is Gaussian-like with a wide top and extends approximately 100  μm from the center of the exposure. Relaxation of frozen-in viscoelasticity results in a maximum index modulation of 5×10(-4) on the side of the fiber facing the exposure with mechanical stress relaxation contributing changes of less than 1×10(-4).