T. Venugopalan

LPG-Based PVA Coated Sensor for Relative Humidity Measurement

A long period grating (LPG) coated with polyvinyl alcohol has been developed and evaluated through exposure to a range of relative humidity (RH) levels (from 33%RH to 97%RH). The conditions for the creation of the sensor probes are described and the response of the device in terms of changes in the resonance loss of several of the spectral loss bands has been monitored. In order to make comparative measurements, a noncoated LPG was exposed to various known refractive index (RI) conditions and the grating response was monitored, analyzed and the results evaluated. This is then related to the change in RH when the sensor is exposed to various RH levels, as the coating RI changes, leading to the change in resonance loss. The PVA coated LPG sensor is showing a relatively fast response time of ~ 80 s to reach 97%RH from 33%RH and inducing ~ 2 dB change in transmission of resonance loss band.

The microbial habitability of weathered volcanic glass inferred from continuous sensing techniques.

Basaltic glasses (hyaloclastite) are a widespread habitat for life in volcanic environments, yet their interior physical conditions are poorly characterized. We investigated the characteristics of exposed weathered basaltic glass from a surface outcrop in Iceland, using microprobes capable of continuous sensing, to determine whether the physical conditions in the rock interior are hospitable to microbial life. The material provided thermal protection from freeze-thaw and rapid temperature fluctuations, similar to data reported for other rock types. Water activity experiments showed that at moisture contents less than 13% wet weight, the glass and its weathering product, palagonite, had a water activity below levels suitable for bacterial growth. In pore spaces, however, these higher moisture conditions might be maintained for many days after a precipitation event. Gas exchange between the rock interior and exterior was rapid (< 10 min) when the rocks were dry, but when saturated with water, equilibration took many hours. During this period, we demonstrated the potential for low oxygen conditions within the rock caused by respiratory stimulation of the heterotrophic community within. These conditions might exist within subglacial environments during the formation of the rocks or in micro-environments in the interior of exposed rocks. The experiments showed that microbial communities at the site studied here could potentially be active for 39% of the year, if the depth of the community within the outcrop maintains a balance between access to liquid water and adequate protection from freezing. In the absence of precipitation, the interior of weathered basaltic glass is an extreme and life-limiting environment for microorganisms on Earth and other planets.

Development of intrinsic optical fiber pH sensors for industrial applications

The development of intrinsic optical fiber pH sensors based on fluorescence from novel coumarin dyes which are covalently immobilized onto the distal end of the fiber is described. The sensors provide response in the pH range of 0.5–6 with good stability over 24 h. The response time of the sensors is approximately 25 s. The sensors show no sensitivity to ionic strength (IS) and have excellent photostability.

Evaluation and calibration of FBG-based relative humidity sensor designed for structural health monitoring

In this paper, the evaluation and testing of a fibre optic relative humidity sensor specifically designed for structural health monitoring is discussed in detail. The sensor is based on a polymer coated Fibre Bragg Grating (FBG) where with the increase of the moisture level, the polymer coating swells and thus applies strain to the FBG, causing a Bragg wavelength shift. The device is designed so that the temperature effect on the humidity sensor can be compensated by the inclusion of a second un-coated FBG packaged inside the same sensor probe. The sensor thus produced has been tested extensively and calibrated using both a humidity chamber and standard salt solutions. The probe has demonstrated a very good performance for relative humidity measurements.

Intrinsic Fiber Optic pH Sensor for Measurement of pH Values in the Range of 0.5–6

The development, design, and evaluation of an intrinsic optical fiber pH sensor for low pH values (in the range of 0.5-6) based on fluorescence from a novel coumarin dye, which is covalently immobilized onto the end surface of an optical fiber is described. The sensor provides a rapid response (of approximately 25 s) over this pH range of 0.5-6.0, showing with very good stability over a period of several months. The sensor has also demonstrated insensitivity to ionic strength and excellent photostability through a series of laboratory tests.

LPG based PVA coated sensor for relative humidity measurement

A long period grating (LPG) coated with polyvinyl alcohol has been developed and evaluated through exposure to a range of relative humidity (RH) levels (from 33%RH to 97%RH). The conditions for the creation of the sensor probes are described and the response of the device in terms of changes in the resonance loss of several of the spectral loss bands has been monitored. In order to make comparative measurements, a noncoated LPG was exposed to various known refractive index (RI) conditions and the grating response was monitored, analyzed and the results evaluated. This is then related to the change in RH when the sensor is exposed to various RH levels, as the coating RI changes, leading to the change in resonance loss. The PVA coated LPG sensor is showing a relatively fast response time of ~ 80 s to reach 97%RH from 33%RH and inducing ~ 2 dB change in transmission of resonance loss band.

Fibre optic chloride sensor based on fluorescence quenching of an acridinium dye

In this paper, research into the fabrication and characterization of a new fibre optic sensor for chloride ion detection is reported. This sensor is based on the fluorescence quenching of an acridinium dye, which is covalently bound to polymer substrates, in presence of chloride ions. The active material is formed as a powder and packed in a tablet form. Calibration data shows that this fibre optic sensor shows a sensitivity to chloride at concentrations larger than 0.1M.

Structural Health Monitoring Potential using a Long Period Grating-Based Humidity Sensor

In this research, a series of innovative optically-based sensors, which were designed, fabricated and characterized were created for potential evaluation for applications in determining moisture ingress in a range of concrete materials subjected to various environmental conditions. The approach taken to the creation of these novel humidity sensors is using long period grating (LPG) technology in an optical fiber. Several sensor configurations are fabricated by coating LPGs and then characterizing and cross-comparing and evaluating the resulting sensor performance. The thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level when coated onto a LPG written into an optical fiber, provides a means to change the optical propagation in the fiber and thus to induce the a wavelength shift in the attenuation bands of its transmission spectrum, which then is calibrated against the measured, humidity. When compared to the more familiar optical fiber-based humidity sensors, using Fiber Bragg gratings (FBGs), the LPG-based devices show a much higher measurement sensitivity, with more relaxed requirements for coating thickness and uniformity.

Structural concrete condition monitoring using a long period grating-based humidity sensor

An optically based sensor has been designed, fabricated and characterized in research to determine moisture ingress in a range of structural concrete samples subjected to a variety of changing environmental conditions. The approach taken to the creation of these novel humidity sensors is using long period grating (LPG) technology in an optical fibre. The sensor is configured by coating LPGs with a thin layer of polymer, following which it was characterized and its performance cross-compared with that of other sensors. The thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level, provides a means to change the optical propagation in the fibre and thus to induce the a wavelength shift in the attenuation bands of its transmission spectrum.