Marcia Muller

Application of a long-period fibre grating-based transducer in the fuel industry

This work shows prospects of long-period fibre grating applications as transducers for fuel conformity analysis. The proposed long-period grating transducer was employed to assess the gasoline conformity in commercial gas stations. Grating responses were used to train and validate a radial base function topology of an artificial neural network. The obtained results show that fibre optic sensors supervised by artificial neural networks can integrate systems for smart sensing with high applicability in the petrochemical field. The radial base function had reached a correct classification probability of approximately 94%. The device applicability in the analysis of hydrated ethanol fuel was also investigated by measuring the concentration of ethanol in ethanol?water mixtures. The results showed that the developed transducer can be used to infer the ethanol?water concentration with a resolution of up to 0.23%.

Metrological Evaluation of Optical Fiber Grating-Based Sensors: An Approach Towards the Standardization

This paper discuss a set of tools for metrological evaluation of optical fiber grating sensors, including uncertainty analysis of the measurements. Background and definitions about basic specifications of the optical fiber grating sensors, such as response curve, calibration curve, sensitivity, resolution, repeatability, reproducibility and conformity are presented. Besides, the paper proposes the procedures to identify, quantify, and express uncertainties in optical fiber grating sensor measurements based on the International Standard Organizations Guide to the Expression of Uncertainty in Measurement. In order to illustrate the application of the approach, two fiber grating measuring systems were evaluated under the light of the methodology. The proposed route is a pragmatic step towards the standardization of optical fiber sensors.

Salinity measurement in water environment with a long period grating based interferometer

In this work, a comparative study of the behaviour of an in-fibre Mach–Zehnder interferometer for salinity measurement in a water solution is presented. The fibre transducer is composed of two nearly identical long period gratings forming an in-series 7.38 cm long device written in the same fibre optic. Two inorganic and one organic salts (NaCl, KCl, NaCOOH) were characterized within the concentration range from 0 to 150 g L−1. For the long period grating interferometer, the average obtained sensitivities were −6.61, −5.58 and −3.83 pm/(g L−1) for the above salts, respectively, or equivalently −40.8, −46.5 and −39.1 nm RIU−1. Salinity measured by means of fibre refractometry is compared with measurements obtained using an Abbe refractometer as well as via electrical conductivity. For the long period grating refractometer, the best resolutions attained were 1.30, 1.54 and 2.03 g of salt per litre for NaCl, KCl and NaCOOH, respectively, about two times better than the resolutions obtained by the Abbe refractometer. An average thermal sensitivity of 53 pm °C−1 was measured for the grating transducer immersed in water, indicating the need for the thermal correction of the sensor. Resolutions for the same ionic constituent in different salts are also analysed.

Alternative technique for biodiesel quality control using an optical fiber long-period grating sensor

We report the use of an optical fiber sensor to measure the soybean oil concentration in samples obtained from the mixture of pure biodiesel and commercial soybean oil. The operation of the device is based on the long-period grating sensitivity to the surrounding medium refractive index, which leads to measurable modifications in the grating transmission spectrum. The proposed analysis method results in errors in the oil concentration of 0.4% and 2.6% for pure biodiesel and commercial soybean oil, respectively. Techniques of total glycerol, dynamic viscosity, density, and hydrogen nuclear magnetic resonance spectroscopy were also employed to validate the proposed method.

Influence of surrounding media refractive index on the thermal and strain sensitivities of long-period gratings

A detailed study of the thermal and strain sensitivities of a long-period grating when the device is immersed in different external media is presented. The range of refractive indices analyzed are within 1.000 to 1.447, corresponding to samples of air, water, ethanol, naphtha, thinner, turpentine, and kerosene. Within the same range of refractive indices, the strain sensitivity is between (-0.24 +/- 0.03) and (-0.94 +/- 0.11) pm/microepsilon. For the grating immersed in these fluids, the refractive index sensitivity ranges from -3 to -1035.6 nm per refractive index units. The coupling thermo-optic coefficients and the strain-optic coefficients are also measured, resulting in the range from (2.45 +/- 0.04)x10(-5) to (15.89 +/- 0.82)x10(-5) deg C(-1) and (-1.15 +/- 0.04) to (-1.61 +/- 0.04) microepsilon(-1), respectively. A noticeable nonlinear behavior of the thermal sensitivity is found for external media with refractive indices higher than 1.430.

Bragg gratings in standard nonhydrogenated fibers for high-temperature sensing

Fiber Bragg gratings engraved in standard telecommunications-grade single-mode fibers without previous hydrogen loading show enhanced thermal stability for high-temperature measurements up to 800 °C. The reflectivity decay at that temperature is adequate for industrial applications with a weekly change of sensing heads.

Control of the long period grating spectrum through low frequency flexural acoustic waves

We have shown experimental results of the excitation of long period fiber gratings by means of flexural acoustic waves with a wavelength larger than the grating period, validated by numerical simulations. The effect of the acoustic wave on the grating is modeled with the method of assumed modes, which delivers the strain field inside the grating, then used as the input to the transfer matrix method, needed for calculating the grating spectrum. The experimental and numerical results are found to be in good agreement, even though only the strain-optic effects are taken into account.

Influence of the surrounding refractive index on the thermal and strain sensitivities of a cascaded long period grating

This work shows the refractive index sensitivity of a cascaded long period grating and the influence of surrounding media with a different refractive index on the devices thermal and strain sensitivities. The refractive index sensitivity of the double-grating device experiences a maximum increase of about 2.6 times when compared with the sensitivity of a single-grating device, mainly attributed to the cavity between the gratings. The maximum obtained sensitivity of −1554.1 nm per refractive index unit for the cascaded long period grating relates to a sensor resolution of about 6 × 10−6 for refractive index measurements, if an optical spectrum analyser of ±5 pm of wavelength stability is used in the experiments, whereas this resolution for the long period grating alone is 1.6 × 10−5. The strain and thermal sensitivities show an increase when the surrounding refractive index increases. These dependences rely on the influence of the surrounding refractive index on the effective index of the cladding mode. A noticeable nonlinear behaviour of the thermal response is found when the surrounding refractive index increases towards the fibre cladding index. This behaviour is attributed to the combined effects of the devices response to the temperature and refractive index.

Refractometric optical fiber sensor for measurement of ethanol concentration in ethanol-gasoline blend

In this paper, an optical fiber device based on long period grating is applied as refractive index transducer to measure the ethanol concentration in ethanol-gasoline blends. The device metrological characteristics - response curve, sensitivity, resolution, conformity, repeatability, combined uncertainty and expanded uncertainty - were determined and compared with characteristics associated with an Abbe refractometer. For ethanol concentration ranging from 20% to 40%, the LPG combined uncertainty was 0.70% and the expanded uncertainty was 1.70 %.

Long-term stability decay of standard and regenerated Bragg gratings tailored for high temperature operation

Thermal stability of both standard and regenerated Bragg gratings written in normal and photosensitive optical fibers was accessed. An apparent spectral wavelength stabilization of common gratings with no thermal hysteresis was reached after thermal treatments. However, after a time interval of 5 months, gratings exhibited a shift in the resonance Bragg wavelength at room temperature, as well as important changes in the thermal sensitivity above 200 oC. Regenerated gratings proved to be stable only at temperatures below the critical regeneration temperature, with significant loss of reflectivity above that critical value.