Ricardo Canute Kamikawachi

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.

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.

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.

Tailoring fiber grating sensors for assessment of highly refractive fuels

Three approaches that allow the tailoring of long period gratings based refractometric sensors for concentration measurement in fuel blends are employed to assess the fuel quality in biodiesel and biodiesel-petrodiesel blend. To allow the analysis of fuel samples with refractive index higher than fiber cladding one, the samples refractive indices were changed by thermo-optic effect and by dilution in a standard substance with low refractive index. The obtained results show the sensor can detect oil concentration in biodiesel samples with resolution as better as 0.07% and biodiesel concentration in biodiesel-petrodiesel samples with average resolution of 0.09%.

CR (III) and CR (VI) detection in water environment using an optical fiber grating sensor

This work shows the application of an optical sensor to detect the presence of Cr(III) and Cr(VI) oxidation states in water solutions. The sensing device is a long period grating produced in a standard telecommunication optical fiber by applying a suitable number of point-to-point electrical arc discharges from a fusion splicer. In order to analyze the Cr(III) and Cr(VI) solutions, the sensing device is kept into a recipient under constant longitudinal stress and the Chromium solution samples are added to it. The results show that the sensor can not only detect the presence, but also differentiate between the two-oxidation states. Samples with Chromium concentrations from 0.16 mg/l to 5.2 mg/l are analyzed, leading to maximum wavelength shifts (relative to the LPG in pure water) of 0.14 nm and 0.26 nm for Cr(III) and Cr(VI), respectively. A comparison between the results obtained for both solutions shows that is possible to identity any of the Chromium oxidation states present in the sample until a lower limit concentration of 2.6 mg/l for the employed set-up.

Petroleum hydrocarbon detection with long period gratings

We report the use of long period gratings (LPG) fabricated with electrical arc discharges from a splice machine for hydrocarbon detection in both fuel and air. In the fuel experiments, we used four different types of solvents. Wavelength shifts as longer as 10.3 nm, 2.6 nm, 50.6 nm, and 6.1 nm were obtained when different concentrations of Tozan/spl reg/, naphtha, thinner, and anhydrous alcohol, respectively, were added to commercial Brazilian gasoline. We also report a LPG application for butane detection in air.

Modeling and production of high-birefringence FOBG sensors

Fiber Optic Bragg Grating Sensors are modeled and studied in high birefringence fiber. Coupled mode theory and multilayer approach are used to predict the spectral characteristics of the structure. Results are compared with experimental characterization of devices produced by UV light using the external method.

Nonlinear Temperature Dependence of Etched Fiber Bragg Gratings

In this work, we experimentally study the thermal sensitivity of etched fiber Bragg gratings immersed in surrounding media with high refractive index (1.444). The fiber diameter was reduced at four different values 15, 10, 9, and 8 mum. A nonlinear behavior is observed, due two combined effects: the silica thermo-optic effect that causes a red shift and the surrounding thermo-optic effect that causes a blue shift.