Development and Testing of a Fiber Bragg Grating Strain Sensor for Uniaxial Compression of Rock Specimens

Los sensores de fibra óptica son una tecnología que ha madurado en los últimos años, sin embargo, se requiere un mayor desarrollo de aplicaciones para materiales naturales como las rocas, que por ser agregados complejos pueden contener partículas minerales y fracturas de tamaño mucho mayor que las galgas eléctricas usadas tradicionalmente para medir deformaciones en las pruebas de laboratorio, ocasionando que los resultados obtenidos puedan ser no representativos. En este trabajo fueron diseñados, fabricados y probados sensores de deformación de gran área y forma curvada, usando redes de Bragg en fibra óptica (FBG) con el objetivo de obtener registros representativos en rocas que contienen minerales y estructuras de diversas composiciones, tamaños y direcciones. Se presenta el proceso de elaboración del transductor, su caracterización mecánica, su calibración y su evaluación en pruebas de compresión uniaxial en muestras de roca. Para verificar la eficiencia en la transmisión de la deformación de la roca al sensor una vez pegado, también fue realizado el análisis de la transferencia incluyendo los efectos del adhesivo, de la muestra y del transductor. Los resultados experimentales indican que el sensor desarrollado permite registro y transferencia de la deformación fiables, avance necesario para uso en rocas y otros materiales heterogénos, señalando una interesante perspectiva para aplicaciones sobre superficies irregulares, pues permite aumentar a voluntad el tamaño y forma del área de registro, posibilita también obtener mayor fiabilidad de resultados en muestras de pequeño tamaño y sugiere su conveniencia en obras, en las cuales los sistemas eléctricos tradicionales tienen limitaciones. ABSTRACT Optical fiber sensors are a technology that has matured in recent years, however, further development for rock applications is needed. Rocks contain mineral particles and features larger than electrical strain gauges traditionally used in laboratory tests, causing the results to be unrepresentative. In this work were designed, manufactured, and tested large area and curved shape strain gages, using fiber Bragg gratings in optical fiber (FBG) in order to obtain representative measurement on surface rocks samples containing minerals and structures of different compositions, sizes and directions. This reports presents the processes of manufacturing, mechanical characterization, calibration and evaluation under uniaxial compression tests on rock samples. To verify the efficiency of rock deformation transmitted to attached sensor, it was also performed the analysis of the strain transfer including the effects of the bonding, the sample and the transducer. The experimental results indicate that the developed sensor enables reliable measurements of the strain and its transmission from rock to sensor, appropriate for use in heterogeneous materials, pointing an interesting perspective for applications on irregular surfaces, allowing increasing at will the size and shape of the measurement area. This research suggests suitability of the optical strain gauge for real scale, where traditional electrical systems have demonstrated some limitations.

Microchanneled chirped fiber Bragg grating formed by femto-second laser aided chemical etching for refractive index and temperature measurements

In this work, a microchanneled chirped fiber Bragg grating (MCFBG) is proposed and fabricated through the femtosecond laser-assisted chemical etching. The microchannel (~550 µm) gives access to the external index liquid, thus inducing refractive index (RI) sensitivity to the structure. In the experiment, the transmission bands induced by the reduced effective index in the microchannel region were used to sense the surrounding RI and temperature changes. The experimental results show good agreement with the theoretical analysis. The proposed MCFBG offers enhanced RI sensitivity without degrading the robustness of the device showing good application potential as bio-chemical sensors.

Implementation of microwave photonic filtering using a profiled superstructured fiber Bragg grating and dispersive fiber

The microwave photonic response of a superstructured fiber Bragg grating is investigated. A bandpass response is achieved with the optical taps controlled by the combination of superstructured fiber grating characteristics and fiber dispersion. A rejection level of >45 dB is demonstrated.

Fiber Bragg grating structures inscribed using 800nm femtosecond laser in single- and multi-core mid-IR glass fibers with their spectral, thermal and strain properties

Single- and multi-core passive and active germanate and tellurite glass fibers represent a new class of fiber host for in-fiber photonics devices and applications in mid-IR wavelength range, which are in increasing demand. Fiber Bragg grating (FBG) structures have been proven as one of the most functional in-fiber devices and have been mass-produced in silicate fibers by UV-inscription for almost countless laser and sensor applications. However, because of the strong UV absorption in germanate and tellurite fibers, FBG structures cannot be produced by UVinscription. In recent years femtosecond (fs) lasers have been developed for laser machining and microstructuring in a variety of glass fibers and planar substrates. A number of papers have been reported on fabrication of FBGs and long-period gratings in optical fibers and also on the photosensitivity mechanism using 800nm fs lasers. In this paper, we demonstrate for the first time the fabrication of FBG structures created in passive and active single- and three-core germanate and tellurite glass fibers by using 800nm fs-inscription and phase mask technique. With a fs peak power intensity in the order of 1011W/cm2, the FBG spectra with 2nd and 3rd order resonances at 1540nm and 1033nm in a single-core germanate glass fiber and 2nd order resonances between ~1694nm and ~1677nm with strengths up to 14dB in all three cores of three-core passive and active tellurite fibers were observed. Thermal and strain properties of the FBGs made in these mid-IR glass fibers were characterized, showing an average temperature responsivity of ~20pm/°C and a strain sensitivity of 1.219±0.003pm/µe.

Simultaneous interrogation of fiber Bragg grating sensors using an acoustooptic tunable filter

We describe a novel technique to provide demultiplexing of fiber Bragg grating sensors, interrogated using interferometric wavelength shift detection. Amplitude modulation of multiple radio frequency driving signals allows an acoustooptic tunable filter to provide wavelength demultiplexing. We demonstrated a noise limited strain resolution of 150 nanostrain/v(Hz) and a crosstalk better than -50 dB.

Fiber Bragg grating sensor interrogation system using a CCD side detection method with Superimposed blazed gratings

We present, for the first time to our knowledge, experimental evidence showing that superimposed blazed fiber Bragg gratings may be fabricated and used to extend the dynamic range of a grating-based spectrometer. Blazed gratings of 4° and 8° were superimposed in germanosilicate fiber by ultraviolet inscription and used in conjunction with a coated charged-coupled device array to interrogate a wavelength-division-multiplexing sensor array. We show that the system can be used to monitor strain and temperature sensors simultaneously with an employable bandwidth which is extendable to 70 nm.

Optical in-fiber Bragg grating sensor systems for medical applications

Two in-fiber Bragg grating (FBG) temperature sensor systems for medical applications are demonstrated: (1) an FBG flow-directed thermodilution catheter based on interferometric detection of wavelength shift that is used for cardiac monitoring; and (2) an FBG sensor system with a tunable Fabry-Perot filter for in vivo temperature profiling in nuclear magnetic resonance (NMR) machines. Preliminary results show that the FBG sensor is in good agreement with electrical sensors that are widely used in practice. A field test shows that the FBG sensor system is suitable for in situ temperature profiling in NMR machines for medical applications.

Broadband fiber Bragg grating with channelized dispersion

We present here a new class of multi-channel Fiber Bragg grating (FBG), which provides the characteristics of channelized dispersion but does so with only a single reflection band. An FBG of this type can provide pure phase control of the spectral waveform of optical pulses without introducing any deleterious insertion-loss-variation. We anticipate that this new class of FBG will find some applications in wavelength-division- multiplexing systems.

Point-by-point inscription of first-order fiber Bragg grating for C-band applications

The influence of the fiber geometry on the point-by-point inscription of fiber Bragg gratings using a femtosecond laser is highlighted. Fiber Bragg gratings with high spectral quality and strong first-order Bragg resonances within the C-band are achieved by optimizing the inscription process. Large birefringence (1.2×10-4) and high degree of polarizationdependent index modulation are observed in these gratings. Potential applications of these gratings in resonators are further illustrated.

A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing

A liquid core waveguide as a refractometer is proposed. Microtunnels were created in standard optical fiber using tightly focused femtoscond laser inscription and chemical etching. A 1.2(h)x125(d) x500(l) µm micro-slot engraved along a fiber Bragg grating (FBG) was used to construct liquid core waveguide by filling the slot with index matching oils. The device was used to measure refractive index and sensitivity up to 10- 6/pm was obtained.

High frequency sensing interrogation system using fiber Bragg grating based microwave photonic filtering

A high frequency sensing interrogation system by using fiber Bragg grating based microwave photonic filtering is proposed, in which the wavelength measurement sensitivity is proportional to the RF modulation frequency applied to the optical signal.

High-frequency fiber Bragg grating sensing interrogation system using Sagnac-loop-based microwave photonic filtering

A novel high-frequency fiber Bragg grating (FBG) sensing interrogation system by using fiber Sagnac-loop-based microwave photonic filtering is proposed and experimentally demonstrated. By adopting the microwave photonic filtering, the wavelength shift of sensing FBG can be converted into amplitude variation of the modulated electronic radio-frequency (RF) signal. In the experiment, the strain applied onto the sensing FBG has been demodulated by measuring the intensity of the recovered RF signal, and by modulating the RF signal with different frequencies, different interrogation sensitivities can be achieved.

800 nm WDM interrogation system for strain, temperature, and refractive index sensing based on tilted fiber Bragg grating

A low-cost high-resolution wavelength-division-multiplexing (WDM) interrogation system operating around 800 nm region with operational bandwidth up to 60 nm and resolution of 12.7 pm utilizing a tilted fiber Bragg grating (TFBG) and a CCDarray detector has been implemented. The system has been evaluated for interrogating fiber Bragg grating based strain, temperature sensors, giving sensitivities of 0.59 pm/µe and 5.6 pm/°C, which are in good agreement with previously reported values. Furthermore, the system has been utilized to detect the refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10¯5. In addition, the vectorial expression of phase match condition and fabrication of TFBG have been discussed.

Self-mixing-based demodulation technique for dynamic fiber Bragg grating strain sensors

We report a novel demodulation scheme for the detection of small Bragg wavelength shifts in a fiber Bragg grating strain sensor by exploiting the optical feedback reflected from the grating structure back into a 1310 nm laser diode integrating a photodiode. The dynamic strain generated by a mechanical vibrator is applied transversely to the fiber Bragg grating and the desired longitudinal strain values inferred from the detected sawtooth-like optical feedback signals. Preliminary results demonstrate the feasibility of this demodulation technique for strain measurement which could be further extended to fiber Bragg grating-based sensors for the detection of different measurands in general.

Refractometer based on fiber Bragg grating Fabry-Pérot cavity embedded with a narrow microchannel

We report on inscription of microchannels of different widths in optical fiber using femtosecond (fs) laser inscription assisted chemical etching and the narrowest channel has been created with a width down to only 1.2µm. Microchannels with 5µm and 35µm widths were fabricated together with Fabry-Pérot (FP) cavities formed by UV laser written fiber Bragg gratings (FBGs), creating high function and linear response refractometers. The device with a 5µm microchannel has exhibited a refractive index (RI) detection range up to 1.7, significantly higher than all fiber grating RI sensors. In addition, the microchannel FBG FP structures have been theoretically simulated showing excellent agreement with experimental measured characteristics.