P. Vengal Rao

Temperature compensated liquid level sensor using FBGs and a bourdon tube

A temperature compensated liquid level sensor using FBGs and a bourdon tube that works on hydrostatic pressure is presented. An FBG (FBG1) is fixed between free end and a fixed end of the bourdon tube. When hydrostatic pressure applied to the bourdon tube FBG1 experience an axial strain due to the movement of free end. Experimental result shows, a good linearity in shift in Bragg wavelength with the applied pressure. The performance of this arrangement is tested for 21metre water column pressure. Another FBG (FBG2) is included for temperature compensation. The design of the sensor head is simple and easy mountable external to any tank for liquid level measurements.

High sensitive FBG pressure sensor using metal bellows

A fiber-optic sensor scheme, capable of the simultaneous measurement of pressure and temperature using two in-line Fiber Bragg Gratings (FBGs) is reported. Sensor head is configured by embedding the two FBGs with metal bellows, such that FBG1 is sensitive to both pressure and temperature, whereas FBG2 is only sensitive to temperature. High pressure sensitivity is achieved because of the lower spring rate in longitudinal direction to that of the large elastic modulus in transverse direction of the metal bellows. Pressure and temperature measurement is made by monitoring the shift of Bragg wavelengths of the FBGs corresponds to variation in pressure and temperature. From the test results, the obtained pressure and temperature sensitivities are 86 pm/psi and 9.17 pm/°C, over a dynamic range of 0-40 psi pressure, and 25-110°C temperature measurements respectively. The experimental results well agreed with the theoretical results and show good linearity. This simple design, economical and all fiber optic sensors can be used for liquid and gas pressure measurements, and under-water applications.

Diaphragm based high sensitive FBG pressure sensor

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.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.

FBG-based novel sensor for high-temperature measurement and its low-cost interrogation

A novel sensor for high-temperature measurement using Fiber Bragg grating (FBG) along with its low-cost interrogation system has been designed and tested. The sensor works based on measurement of the shift in Bragg wavelength that corresponds to the temperature induced strain by making use of a mechanical transducer. The transducing element provides temperature dependent strain on FBG by means of differential linear thermal expansion of two different materials, stainless steel and mild steel. The shift in Bragg wavelength of FBG due to this temperature induced strain is measured by using optical spectrum analyser (OSA). Further the bulk and expensive OSA is replaced by a low cost interrogation system that employed an LPG, a photodiode, a transimpedance amplifier, and a digital multimeter. The LPG converts wavelength information of FBG into its equivalent intensity modulated signal which is captured by a simple photodiode and then converted into voltage signal using the transimpedance amplifier. The designed sensor measures the temperature from 20°C to 1000°C with a resolution of 2°C.

FBG sensor for temperature-independent high sensitive pressure measurement with aid of a Bourdon tube

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.

An FBG sensor for strain and temperature discrimination at cryogenic regime

A simple technique to discriminate the Strain and Temperature with a single Fiber Bragg Grating (FBG) at cryogenic regime is presented in this paper. An uniform FBG is divided into two parts, one half is without coating (FBG1) and other half is coated with Cyno-Acrylic Adhesive (FBG2). The measured temperature and strain sensitivities of the FBG1 are 4.05×10⁻⁶/K and 2.13×10⁻⁶/µε and FBG2 are 1.39×10⁻⁵/K and 1.72×10⁻⁶/µε respectively.

Multimode Fused Coupler Fiber Optic Pressure Sensor

The response of a pressure cell with copper diaphragm of different dimensions for the measurement of pressure using a fused multimode fiber optic coupler is presented. As the pressure varies, the deflection of the diaphragm varies and it modulates the intensity of the reflected light entering into the multimode fused coupler. The sensor is operated in back slope of its characteristic displacement curve and it enabled to sense larger range of pressures. The effect of variation of diameter and thickness of the diaphragm is also studied. The experimental results show that the sensitivity, linearity of the sensor is good and it may finds application in monitoring pressure in industries.

Design and development of a high sensitive low cost fiber optic refractometer

A simple and low-cost fiber optic refractometer has been designed and demonstrated. The sensor is configured by etching a short region of a standard single-mode circularly bent fiber. Circularly bent three fiber loops are etched in equal lengths and arranged parallel to each other. The working principle of the sensor is by detecting the evanescent field modulation with respect to the change in ambient refractive index. In the present study glycerin solution with various concentrations, possessing different refractive indices is chosen as the ambient. Bent and etched optical fiber sensors have increased sensing capabilities but they are very much prone to fracture and breakage. The proposed sensor is stable showing very less tendency towards any breakage or fracture. The sensitivity of the sensor is tested for glycerin with different concentrations. It is evident from the experimental results that with the increase of glycerin concentration the output intensity of the sensor is decreased linearly. The proposed sensor may find applications in detecting various chemical species and biochemical applications.A simple and low-cost fiber optic refractometer has been designed and demonstrated. The sensor is configured by etching a short region of a standard single-mode circularly bent fiber. Circularly bent three fiber loops are etched in equal lengths and arranged parallel to each other. The working principle of the sensor is by detecting the evanescent field modulation with respect to the change in ambient refractive index. In the present study glycerin solution with various concentrations, possessing different refractive indices is chosen as the ambient. Bent and etched optical fiber sensors have increased sensing capabilities but they are very much prone to fracture and breakage. The proposed sensor is stable showing very less tendency towards any breakage or fracture. The sensitivity of the sensor is tested for glycerin with different concentrations. It is evident from the experimental results that with the increase of glycerin concentration the output intensity of the sensor is decreased linearly. The proposed sensor may find applications in detecting various chemical species and biochemical applications.

Low-cost and high-resolution interrogation scheme for LPG-based temperature sensor

A low-cost and high-resolution interrogation scheme for a long-period fiber grating (LPG) temperature sensor with adjustable temperature range has been designed, developed and tested. In general LPGs are widely used as optical sensors and can be used as optical edge filters to interrogate the wavelength encoded signal from sensors such as fiber Bragg grating (FBG) by converting it into intensity modulated signal. But the interrogation of LPG sensors using FBG is a bit novel and it is to be studied experimentally. The sensor works based on measurement of shift in attenuation band of LPG corresponding to the applied temperature. The wavelength shift of LPG attenuation band is monitored using an optical spectrum analyser (OSA). Further the bulk and expensive OSA is replaced with a low-cost interrogation system that employ an FBG, photodiode and a transimpedance amplifier (TIA). The designed interrogation scheme makes the system low-cost, fast in response, and also enhances its resolution up to 0.1°C. The measurable temperature range using the proposed scheme is limited to 120 °C. However this range can be shifted within 15-450 °C by means of adjusting the Bragg wavelength of FBG.A low-cost and high-resolution interrogation scheme for a long-period fiber grating (LPG) temperature sensor with adjustable temperature range has been designed, developed and tested. In general LPGs are widely used as optical sensors and can be used as optical edge filters to interrogate the wavelength encoded signal from sensors such as fiber Bragg grating (FBG) by converting it into intensity modulated signal. But the interrogation of LPG sensors using FBG is a bit novel and it is to be studied experimentally. The sensor works based on measurement of shift in attenuation band of LPG corresponding to the applied temperature. The wavelength shift of LPG attenuation band is monitored using an optical spectrum analyser (OSA). Further the bulk and expensive OSA is replaced with a low-cost interrogation system that employ an FBG, photodiode and a transimpedance amplifier (TIA). The designed interrogation scheme makes the system low-cost, fast in response, and also enhances its resolution up to 0.1°C. The measurable temperature range using the proposed scheme is limited to 120 °C. However this range can be shifted within 15-450 °C by means of adjusting the Bragg wavelength of FBG.

Analyzing characteristics of Sagnac loop interferometric stress sensor

Effect of transverse induced stress on fiber optic Sagnac interferometer has been studied. The Sagnac loop is designed with conventional single mode fiber. Applying transverse stress on the fiber loop, resulting interference patterns are captured by a CCD and hence analyzed to obtain the angular shift of deformations by image processing technique using MATLAB. Data are taken from the single and double fiber optic loops of the Sagnac interferometric configurations and hence a comparative study is performed to achieve better sensitivity.