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.

Influence of mounting on the hysteresis of polymer fiber Bragg grating strain sensors

Fiber Bragg grating sensors recorded in poly(methyl methacrylate) fiber often exhibit hysteresis in the response of Bragg wavelength to strain, particularly when exposed to high levels of strain. We show that, when such a fiber grating sensor is bonded directly to a substrate, the hysteresis is reduced by more than 12 times, compared to the case where the sensor is suspended freely between two supports. © 2013 Optical Society of America.

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.

Highly sensitive temperature and strain sensors based on all-fiber 45°-TFG Lyot filter

We demonstrate highly sensitive temperature and strain sensors based on an all-fiber Lyot filter structure, which is formed by concatenating two 45°-TFGs (tilted fiber gratings) with a PM fiber cavity. The experiment results show the all-fiber 45°-TFG Lyot filter has very high sensitivity to strain and temperature. The 45°-TFG Lyot filters of two different cavity lengths (18cm and 40 cm) have been evaluated for temperature sensing by heating a section of the cavity from 10°C to 50°C. The experiment results have shown remarkably high temperature sensitivities of 0.616nm/°C for 18cm and 0.31nm/°C for 40cm long cavity filter, respectively. The 18cm long cavity filter has been subjected to strain variations up to around 550μ ε and the filter has exhibited strain sensitivities of 0.02499nm/μ ε and 0.012nm/μ ε for two straining situations, where its cavity middle section of 18cm and 9cm were stretched, respectively. © 2012 SPIE.

Demultiplexing of fibre Bragg grating temperature and strain sensors

We describe a demultiplexing scheme for fibre optic Bragg grating sensors in which signal recovery is achieved by locking each sensor grating to a corresponding receiver grating. As a demonstration, the technique is applied to strain and temperature sensing, achieving a resolution of 3.0 µe and 0.2°C, respectively.

Fibre Bragg grating strain sensors

The fabrication of in-fibre Bragg gratings (FBGs) and their application as sensors is reported. The strain and temperature characteristic results for a number of chirped and uniform gratings written into three different host fibres are presented. The static and dynamic temperature response of a commercially available temperature compensated grating is reported. A five sensor wavelength division multiplexed fibre Bragg grating strain measurement system with an interrogation rate of 25 Hz and resolution of 10 was constructed. The results from this system are presented. A novel chirped FBG interrogation method was implemented in both the 1.3 and 1.5 m telecommunication windows. Several single and dual strain sensor systems, employing this method, were constructed and the results obtained from each are reported and discussed. These systems are particularly suitable for the measurement of large strain. The results from a system measuring up to 12 m and with a potential measurement range of 30 m are reported. This technique is also shown to give an obtainable resolution of 20 over a measurement range of 5 000 for a dual sensor system. These systems are simple, robust, passive and easy to implement. They offer low cost, high speed and, in the case of multiple sensors, truly simultaneous interrogation. These advantages make this technique ideal for strain sensing in SMART structures. Systems based on this method have been installed in the masts of four superyachts. A system, based on this technique, is currently being developed for the measurement of acoustic waves in carbon composite panels. The results from an alternative method for interrogating uniform FBG sensors are also discussed. Interrogation of the gratings was facilitated by a specifically written asymmetric grating which had a 15 nm long linearly sloped spectral edge. This technique was employed to interrogate a single sensor over a measurement range of 6 m and two sensors over a range of 4.5 me. The results obtained indicated achievable resolutions of 47 and 38 respectively.

The application of fibre Bragg grating networks as strain sensors and as phased array antenna true time delay elements

The fabrication of in-fibre Bragg gratings, and the application of arrays of such gratings as strain sensors and as true time delay elements for the control of phased array antennas is reported. Chirped period Bragg gratings were produced using the fibre deformation fabrication technique, with chirps of between 2.9nm and 17.3nm achieved. Arrays of 5mm and 2mm long uniform period Bragg gratings were fabricated using the inscription method, for use as true time delay elements,dissimilar wavefronts and their spectral characteristics recorded. The uniform period grating arrays were used to create minimum time delays of 9.09ps, 19.02ps and 31ps; making them suitable for controlling phased array antennas operating at RF frequencies of up to 3GHz, with 10° phase resolution. Four 4mm long chirped gratings were produced using the dissimilar wavefronts fabrication method, having chirps of 7nm, 12nm, 20nm and 30nm, and were used to create time delays of between 0.3ps and 59ps. Hence they are suitable for controlling phased array antennas at RF frequencies of up to 48GHz. The application of in fibre Bragg gratings as strain sensors within smart structure materials was investigated, with their sensitivity to applied strain and compression measured for both embedded and surface mounted uniform period and fibre Fabry-Perot filter gratings. A fibre Bragg grating sensor demultiplexing scheme based on a liquid crystal filled Fabry-Perot etalon tuneable transmission filter was proposed, successfully constructed and fully characterised. Three characteristics of the LCFP etalon were found to pose operational limitations to its application in a Bragg grating sensor system; most significantly, the resonance peak wavelength was highly (-2,77nm/°C) temperature dependent. Several methods for minimising this temperature sensitivity were investigated, but enjoyed only limited success. It was therefore concluded that this type (E7 filled) of LCFP etalon is unsuitable for use as a Bragg grating sensor demultiplexing element.

Demountable connection for polymer optical fiber grating sensors

The authors fabricated a demountable Ferrule connector/Physical contact connection between silica fiber and a polymer optical fiber (POF) containing a fiber Bragg grating. The use of a connector for POF grating sensors eliminates the limitations of ultraviolet glued connections and increases the ease with which the devices can be applied to real-world measurement tasks.

Temperature-referenced high-sensitivity point-probe optical fiber chem-sensors based on cladding etched fiber Bragg gratings

Point-probe optical fiber chem-sensors have been implemented using cladding etched fiber Bragg gratings. The sensors possess refractive index sensing capability that can be utilized to measure chemical concentrations. The Bragg wavelength shift reaches 8 nm when the index of surrounding medium changes from 1.33 to 1.44, giving maximum sensitivity more than 10 times higher than that of previously reported devices. More importantly, the dual-grating configuration of the point-probe sensors offers a temperature reference function, permitting accurate measurement of refractive index encoded chemical concentrations.

Temperature-referenced high-sensitivity point-probe optical fiber chem-sensors based on cladding etched fiber Bragg gratings

Point-probe optical fiber chem-sensors have been implemented using cladding etched fiber Bragg gratings. The sensors possess refractive index sensing capability that can be utilized to measure chemical concentrations. The Bragg wavelength shift reaches 8 nm when the index of surrounding medium changes from 1.33 to 1.44, giving maximum sensitivity more than 10 times higher than that of previously reported devices. More importantly, the dual-grating configuration of the point-probe sensors offers a temperature reference function, permitting accurate measurement of refractive index encoded chemical concentrations.

Demultiplexing of fibre Bragg grating temperature and strain sensors

We describe a demultiplexing scheme for fibre optic Bragg grating sensors in which signal recovery is achieved by locking each sensor grating to a corresponding receiver grating. As a demonstration, the technique is applied to strain and temperature sensing, achieving a resolution of 3.0 µe and 0.2°C, respectively.

Effects of annealing, pre-tension and mounting on the hysteresis of polymer strain sensors

When exposed to high levels of strain, polymer optical fibre grating sensors recorded in poly(methyl methacrylate) based fibre often exhibit hysteresis in the response of their Bragg wavelength to strain. We demonstrate that the application of pre-tension and annealing of the polymer fibre can reduce this hysteresis when the fibre is suspended freely between two supports, but much better performance is obtained when the sensor is attached directly to a substrate. In this case, the hysteresis can be lessened by more than a factor of 12. © 2014 IOP Publishing Ltd.

Non-invasive respiratory monitoring using long-period fiber grating sensors

In non-invasive ventilation, continuous monitoring of respiratory volumes is essential. Here, we present a method for the measurement of respiratory volumes by a single fiber-grating sensor of bending and provide the proof-of-principle by applying a calibration-test measurement procedure on a set of 18 healthy volunteers. Results establish a linear correlation between a change in lung volume and the corresponding change in a local thorax curvature. They also show good sensor accuracy in measurements of tidal and minute respiratory volumes for different types of breathing. The proposed technique does not rely on the air flow through an oronasal mask or the observation of chest movement by a clinician, which distinguishes it from the current clinical practice. © 2014 Optical Society of America.

Demountable connection for polymer optical fiber grating sensors

The authors fabricated a demountable Ferrule connector/Physical contact connection between silica fiber and a polymer optical fiber (POF) containing a fiber Bragg grating. The use of a connector for POF grating sensors eliminates the limitations of ultraviolet glued connections and increases the ease with which the devices can be applied to real-world measurement tasks. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).