Analytical Study of Highly Sensitive Mems Based Bragg Grating Pressure Sensor

The wavelength shift of single Bragg grating integrated into a waveguide in optical MEMS pressure sensors gives inaccurate result due to the cross sensitivity caused by multiple external factors including temperature. Novel dual waveguide Bragg gratings (WBGs)-based microelectromechanical system pressure sensors with silicon micromachined diaphragm are presented here. The sensor consists of a curved waveguide with two identical WBGs located on the diaphragm. When pressure is applied on the diaphragm, the pitch of the gratings changes, and hence, there is a shift in the corresponding Bragg wavelength. The effect of temperature is assumed to be the same for both the gratings. As temperature shifts the Bragg wavelength of the two gratings equally, the error due to the temperature change on pressure sensitivity is eliminated. This method of pressure measurement effectively cancels out the temperature sensitivity. Three different configurations of the sensor with circular, square, and rectangular diaphragms are designed and simulated using COMSOL Multiphysics. Pressure sensitivity for the circular diaphragm configuration is found to be 2.1 and 0.744 pm/Pa for the two gratings, which are higher than the square and rectangular diaphragm configuration. Hence, the circular diaphragm configuration is better suited for multiplexing. The temperature independent properties of the sensors as well as the adjustability of sensitivity and measurement range by means of altering the dimensions of the sensors are studied. The temperature sensitivity is the same for both the gratings and is found to be 11.6 ppm/°C. Using modulation techniques and suitable designs, these sensors find applications in remote distributed sensing.

We present a novel method for the synthesis of complex fiber Bragg gratings (FBGs) which is based on an impedance reconstruction layer aggregation technique. The method takes as a starting point the accurate approximation of the sinusoidal perturbation (uniform or nonuniform) of the refractive index by local step functions by means of an inverse Fourier series relationship. This reduces the otherwise continuous scattering to only two points per local period and thus, the maximum spatial resolution as limited by the Nyquist criterion is achieved and there is no need to lump various periods of the FBG into an equivalent mirror. The main advantage brought by the method is the possibility of synthesizing structures containing defects or discontinuities of the size of the local period, a feature which yields different results with priorly reported methods to which the one reported here can be considered as complementary. In addition, this enhanced spatial resolution allows the synthesis of very strong FBG devices providing convergent solutions. We illustrate this fact by demonstrating the synthesis of a flat-top filter with a maximum reflectivity of 99.999%. The method renders directly the refractive index profile n(z) as it does not rely on the coupled mode theory since the grating is reconstructed using a V-I matrix formulation. The method is simple and its complexity is equivalent to those previously reported in the literature. Examples are given which prove that perfect reconstruction is achieved in both simple and complex grating profiles. Although a layer aggregation is presented here, the layer peeling (i.e., moving time frame) version of this algorithm is straightforward.

A fiber Bragg grating (FBG) pressure sensor packaged by using a hard core in the membrane is presented. By utilizing the unique membrane‐based FBG packaging method, its pressure sensitivity has been effectively enhanced. The pressure sensitivity of the FBG reaches 5.75 × 10−3/MPa within the pressure range of 0–0.16 Mpa. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1279–1281, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24335

A high sensitive pressure sensor based on Fiber Bragg grating (FBG) integrated with a thin metal diaphragm was designed and demonstrated. To enhance the pressure sensitivity FBG is firmly glued across the diameter of the diaphragm. Under pressure, the diaphragm deforms and produces an induced strain along the length of the fiber causes shift in Bragg wavelength of the FBG. Pressure measurement is made by measuring the Bragg wavelength shift against change in pressure. The sensor was tested up to the maximum pressure of 140 psi and the corresponding pressure sensitivity was found to be 0.0204 nm/psi, which is approximately 970 times higher than that can be achieved with a bare FBG. The experimental results show good agreement with the theoretical results and possess good linearity and repeatability. This sensor can be used for the measurement of medium pressure, liquid level and depth of underwater.

Wavelength independent chemical sensing using etched thermally regenerated FBG

The monitoring of temperature during operation of lithium ion batteries is crucial for a stable and safe performance of a battery system. However, a major challenge is to obtain exact internal temperature progression upon charge and discharge processes. Though commonly used resistance temperature sensors can give considerably accurate temperature values, they are limited to the application outside of the cell due to the distortion of their signal through induction effects of the applied cell currents, if installed inside the pouch cell and therefore, they are not suitable for in situ measurements. This poses serious problems especially for big cell stacks, as core temperatures for those cells differ greatly from the outer parts and particularly the surface of the cell encasement. Fiber Bragg Grating (FBG) sensors on the other hand, provide data that are unaffected by induction effects as their operating principle is solely based on the reflection of light of certain wavelengths within a glass fiber. However, a specific difficulty is the distinction between temperature and pressure effects, as the reflected signal is sensitive to both. Often times additional tubing around the sensor is needed to eliminate pressure effects or an additional sensor is placed in close proximity, inside or outside, to the FBG-sensor as a reference. The former, however, can affect the cell stack physically acting as a defect spot with unwanted side effects.In our work, we show that a careful calibration of the FBG-sensor can be accomplished, so that both effects can be separated and the FBG-sensor can then be used to gather information not only about temperature but also about volume changes in an operating cell. Therefore, a temperature calibration took place, with the cell being held at a constant SoC. Subsequently, a calibration of the SoC - dependent pressure-induced mechanical deformation was performed at very low C-rate to minimize thermal effects. The so gathered calibration parameters can then be applied to all performed cycling tests, regardless of the C - rate. A calibration routine and mathematical solution to separate the influences of temperature and pressure to receive exact in situ temperature values for a 10 Ah lithium ion battery cell is presented. The calibration method is applied to cycling and rate tests at various C-rates.The results show, that wavelength shifts upon charging and discharging processes are mainly resulting from volume changes of the active cell components, meaning that the FBG – sensor has a very high sensitivity towards pressure changes. With the before mentioned calibration method the impact of mechanical deformation on the FBG - sensor can be subtracted from the raw data and allow for a direct temperature reading. The temperature data correlates with data gathered from commonly used sensors placed on the outside of the pouch cell. Tests with varying discharge rates confirm these results and show, that the method produces precise temperature data even at different C-rates, even though only slow C-rates were applied during calibration.In detailed comparison, the FBG - sensors show a faster response towards temperature changes compared to conventional external temperature sensors, which can be related to the insulating effect of the pouch foil. In case of high discharge rates, we were able to show that the measured temperatures inside of the pouch cell are higher in comparison to the outside ones. Furthermore, SoC - induced changes in pressure measured by two FBG-sensors, which were located at different positions on the cell stack, are almost identical. This indicates that pressure build-up within a pouch cell occurs evenly through the cell stack.

A temperature independent high sensitive pressure sensing system using fiber Bragg grating (FBG) and ‘C’ shaped Bourdon tube (CBT) is demonstrated. The sensor is configured by firmly fixing the FBG (FBG1) between free and fixed ends of the CBT. Additional FBG (FBG2) in line to the FBG1 is introduced which is shielded from the external pressure, tend to measure only the ambient temperature fluctuations. The CBT has an elliptical cross section where its free end is sealed and the fixed end is open for subjecting the liquid or gas pressure to be measured. With the application of pressure, the free end of CBT tends to straighten out results in an axial strain in FBG1 causes red shift in Bragg wavelength. The pressure can be determined by measuring the shift of the Bragg wavelength. The experimental pressure sensitivity is found to be 66.9 pm/psi over a range of 0 to 100 psi. The test results show that the Bragg wavelength shift is linear corresponds to change in applied pressure and well agreed with the simulated results. This simple and high sensitive design is capable of measuring static/dynamic pressure and temperature simultaneously which suits for industrial applications.

A testbed for differential pressure flow meter was built and tested. The testbed is targeted for intravascular pressure and flow monitoring applications. The testbed consists of a data acquisition system and two MEMS pressure sensors (SM5108) measuring 0.65×0.65×0.65mm. The MEMS pressure sensors were wire bonded to gold plated ceramic pads and placed inside vinyl tubing. The signals from the pressure sensors were amplified using two low-power high performance instrumentation amplifiers. These amplifiers are a part of the data acquisition system which is designed to acquire data from the pressure sensors and communicate with a user in real time. For simplicity our initial evaluation of the testbed was using air flow. Air was pumped across a simulated blockage (a valve) and pressure was built up to intravascular pressure ranges (35 to 230 mmHg (gauge)). The base fractional flow rate (FFR) equation was used to calculate flow, hinting that the SM5108 pressure sensors can become a part of an implantable differential pressure flow meter.

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.

This study proposes a novel optical fiber Bragg grating (FBG)-based load cell sensor using 3-D printing technology. It explores the impact of the infill density and pattern on the FBG’s temperature and mechanical strain sensitivity. Two FBGs were used, one for strain measurement, FBG L, and the other for temperature compensation, FBG T. Conducted measurements show that the strain-sensitive FBG L and strain-free FBG T exhibited a similar average temperature sensitivity value of around 10.5 pm/°C with a good linearity value of 99.92%. This will also allow the unstrained FBG T to effectively compensate for FBG L’s temperature changes during load measurement. Under varied load conditions, load cell sensors with lower infill density, such as 20%, displayed more significant average wavelength shift and increased strain sensitivities compared to those with higher infill density, such as 80%, due to having more voids and hollow sections within its actuator. This facilitates easier deformation, thus inducing more significant stretching on FBG L. Regarding the infill pattern, the gyroid pattern outperformed the tri-hexagon in both average wavelength shift and strain responses, showing its suitability for precise measurements under diverse loads. Therefore, this work demonstrates the critical role of the infill density and pattern in the performance of an efficient load cell sensor using FBGs.

This paper presents an innovative calorimetric method for measurements of AC losses of high-temperature superconducting (HTS) tapes by using effect of optical fiber Bragg grating (FBG) wavelength shift resulting from temperature change caused by AC losses, where it is adopted as temperature sensor. The HTS specimen on which the FBG are attached on are adiabatically processed before measuring. Comparing with conventional electric and calorimetric methods, FBG temperature sensor has advantage of rapid response and anti-electromagnetic interference due to its thin geometrical sizes and optical characteristics, it is thus very suitable to AC loss measurement of HTS applications in much more complicated electromagnetic circumstance. By measuring the effect of temperature on wavelength shift in low temperature range, it is indicated that FBG has good enough repeatability of dependence of wavelength shift on temperature measurement around liquid nitrogen temperature of 77 K. After calibration, AC loss of BSCCO HTS tape in self-field is measured by the FBG sensor, and the measured loss is also measured by conventional electric method using lock-in amplifier (LIA). It is shown that the measured AC loss is agreement with the results measured by electric method and those based on Norris theory, so that the presented calorimetric method is available for measuring the AC loss of superconductors and expected to be generalized to AC loss or thermal measurements of other bulk superconductors.

A means to calculate the multimodal spectral response of Bragg gratings in general non-circular multimode waveguides is proposed. To illustrate the power of the technique, the spectra of two Bragg temperature sensors are numerically calculated in which coupling between 100 modes is considered. It is shown how the Bragg wavelength in multimode Bragg grating waveguides is affected by the number of modes and energy distribution among them. Good matching of the simulated spectrum of a multimode Bragg grating on a planar inverted rib waveguide to the measured spectrum is seen.

This paper presents a 3D touch trigger probe based on Fiber Bragg Grating (FBG). The sensing principle is Bragg equation λ=2nΛ. Mutative strain and temperature outside alter both the refractive index (n) and grating pitches (Λ) of the fiber core, so the Bragg wavelength λ will change accordingly. The probe adopts FBG sensor system which has four FBGs provided with same parameter (three as sensor FBG and one as match FBG). Laser beam from broadband light source enter sensor FBGs through one coupler, the reflected light is imported to match FBG via another coupler, eventually captured by a high precision optoelectronic detector which monitors energy of the laser reflected by match FBG. The tip ball swings when it contact work pieces, and causes rotation of the plank by rigid connection, the displacement of the tip ball will be transferred to strain exerting on sensor FBGs. Consequently the strain results in Bragg wavelength shift of the reflected laser beam. The displacement of the probe leads to shift of Bragg wavelength of the sensor FBG, therefore, results in energy change of reflected light from the matching FBG. The probe based on FBG sensor brings an untouched branch of the application of Fiber grating sensors. It is also studied on key points of a touch trigger probe such as repeatability, trigger force and resolution.

In this work, we have demonstrated a dual fibre Bragg grating (FBG) sensor for a temperature-insensitive chemical concentration or refractive index sensor by etching the cladding of one of the FBGs. The dual FBG sensor was used to measure different solution concentrations of glycerol in water. The sensor showed sensitivities of 2.13×10-3 and 2.28×10-3 at 85% and 0% (pure water) glycerol solution concentrations, respectively. The dual FBG sensors demonstrate that it can be used to detect chemical or biological changes in the surrounding media by measuring the Bragg wavelength and reflectivity differences between the dual FBG’s reflection spectrum peaks.

Characterisation of Tactile Sensors based on Fibre Bragg gratings Towards Temperature Independent Pressure Sensing