Jharna Mandal

Bragg grating-based fiber-optic laser probe for temperature sensing

A novel Bragg grating-based fiber-optic laser probe for temperature sensing using erbium-doped fiber as the active gain medium is reported. The combination of a chirped grating and a normal grating was used to form the laser cavity to achieve temperature-tunable laser action over a wide measurement range. The laser probe used a metal sheath to enhance its mechanical strength and contain the normal grating at the sensing point. The temperature dependence of the wavelength of the laser probe gives a sensitivity of 12.01 pm//spl deg/C and a repeatability of /spl plusmn/1.7/spl deg/C from room temperature to 300/spl deg/C.

Characteristics of potential fibre Bragg grating sensor-based devices at elevated temperatures

Fibre Bragg gratings (FBGs) of type I and IIA were fabricated in Ge-doped and B–Ge co-doped fibres using a 248 nm excimer laser and their performance characteristics were tested and compared with those of a chemical composition grating (CCG), written in a fluorine–germanium doped fibre, over a wide range of temperatures. Long-term testing (more than 600 h) involving a series of step-wise incremental temperature changes shows for the first time the potential of FBGs for high temperature measurement applications (up to and beyond 1100 °C), this depending on the type of FBG involved and the material and composition of the substrate fibre (the CCG was observed to be the most durable at very high temperatures). These gratings are likely to be useful for the simultaneous measurement of strain and temperature over these higher temperature ranges.

Analysis of thermal decay and prediction of operational lifetime for a type I boron-germanium codoped Fiber Bragg grating

The thermal decay of a type I fiber Bragg grating written at 248 nm in boron-germanium codoped silica fiber was examined in terms of its reflectivity and Bragg wavelength change. In addition to the decay in reflectivity, which was observed, a shift in Bragg wavelength over the temperature range considered was seen. A mechanism for the decay in the reflectivity was developed and modeled according to a power law, and the results were compared with those from the aging curve approach. The wavelength shift was simulated by modification of the power law, which was also found to fit well to the experimental data. Temperature-induced reversible and irreversible change in the grating characteristic were observed and considered to be a means to predict the working lifetime of the grating at comparatively low temperatures. Accelerated aging was also reviewed and compared in terms of reflectivity and Bragg wavelength shift. It was shown that the temperature-induced irreversible shift in the Bragg wavelengths could not be predicted by use of the isothermal decay of the refractive-index modulation. The results were discussed within the framework of the current theoretical approaches for predicting the stability of gratings of this type.

Simultaneous measurement of strain (to 2000 /spl mu//spl epsiv/) and temperature (to 600/spl deg/C) using a combined Sb-Er-Ge-codoped fiber-fluorescence and grating-based technique

An optical fiber-based sensing scheme for the simultaneous measurement of strain and temperature over a wider range has been demonstrated by writing a grating with very high reflectivity in a 10-cm-long specially fabricated antimony-erbium-germanium (Sb-Er-Ge)-doped silica fiber. The scheme exploits the grating sensitivity to both strain and temperature in association with the temperature-dependent peak power ratio of the two fluorescence peaks around 1535 and 1552 nm of the amplified spontaneous emission due to /sup 4/I/sub 13/2//spl rarr//sup 4/I/sub 15/2/ transition arising from the presence of erbium ions in the fiber core, using a 980-nm laser diode as a pumping source. The sensor created using this fiber can be used for the simultaneous measurement of strain and temperature over the wide ranges of 0-2000 /spl mu//spl epsiv/ and 20/spl deg/C-600/spl deg/C, with root-mean-square errors of 36 /spl mu//spl epsiv/ and 2.8/spl deg/C, respectively.

A wide temperature tunable fibre laser using a chirped grating and a type IIA fibre Bragg grating

A fibre laser sensor has been developed to operate over a wide temperature range from room temperature to 440 °C, where the laser cavity has been formed using a combination of a chirped grating and a type IIA fibre Bragg grating (FBG), enclosing a length of erbium doped fibre as the active gain medium. A FBG stabilized 1480 nm laser diode was employed as the pump source to achieve laser oscillation associated with the type IIA Bragg grating wavelength on the wavelength band around 1550 nm. The broadband-chirped grating was used as one of the cavity end reflectors to achieve temperature-tunable laser action for sensing applications over a wide range. The sensitivity of the sensor was found to be 13.94 pm °C−1 with a root mean square error of 8.85 °C. The primary advantage of this laser-based sensor over passive optical fibre sensors is the significant improvement in both the signal-to-noise ratio and the narrow linewidth of the laser signal, making it especially well suited to multiplexing situations.

Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Er-doped photosensitive fibers

Three different types of Er doped photosensitive fibers, germanium/erbium (Ge/Er) fiber, tin/germanium/erbium fiber (Sn/Er) and antimony/germanium/erbium fiber (Sb/Er) have been manufactured and studied for use in optical sensor systems. Their characteristics of photosensitivity, the temperature sustainability of fiber Bragg gratings (FBGs) written into these fibers and the fluorescence emission from the Er dopant were investigated and compared. It has been shown in this work that these fibers all show a satisfactory degree of photosensitivity to enable the fabrication of FBGs and a significant level of fluorescence emission within the 1550 nm band for sensor use. The high temperature sustainability of the FBGs written into these fibers was investigated and seen to be quite significant at temperatures as high as 850 °C, in particular for the Sn/Er and Sb/Er fibers. A fiber laser using the Sb/Er fiber as the gain medium was demonstrated, giving evidence of the strong fluorescence emission from the Er dopant. These fibers are all suitable for use in a variety of sensing applications for the simultaneous measurement of temperature and strain by means of monitoring both the fluorescence characteristics and the peak wavelength shift of the FBGs formed in fiber laser sensor application.

A Parallel Multiplexed Temperature Sensor System Using Bragg-Grating-Based Fiber Lasers

A parallel multiplexed temperature sensor scheme using a Bragg grating-based fiber laser approach has been developed and evaluated. Multiple laser cavities were formed as the active gain media of the system using a common broadband chirped fiber Bragg grating (CFBG) and several normal FBGs, which were used as optical feedback elements, in conjunction with different lengths of erbium-doped fibers (EDFs). These gain media were externally pumped by light from a 1480-nm laser diode (LD) through a 1480-nm 1 times 4 splitter. Normal FBGs were used as the wavelength-selective and sensing elements of the laser system. Simultaneous laser action at three different wavelengths corresponding to channels 1, 3, and 4, respectively, was obtained using this scheme. The temperature was measured over the range from room temperature (27 degC) to a maximum of 540 degC, which shows the potential of the scheme for quasi-distributed sensor applications

Strain-imposed fiber optic laser-based system for wide range temperature measurement applications

This paper focuses on optical fiber laser-based wide range temperature measurement applications, under circumstances where different strain values were applied to the chirped fiber Bragg grating at a fixed temperature and the uniform type IIA fiber Bragg grating was used temperature-tuned. This grating forms the end reflector in the laser cavity feedback configuration. Erbium doped fiber was used as the laser gain medium and was pumped by using light from a 1480 nm laser diode. The response of the sensor was achieved over the tunable-temperature range from 70°C to 500°C when a fixed strain value of 1000 με was applied to the chirped Bragg grating, with a root mean square error of 10°C, using linear fitting over the above measurement range.

Combined fluorescence and grating-based technique for wider range strain-temperature simultaneous measurement using Sb-Er-doped fibre

A fiber-optic fluorescence-based sensing scheme for simultaneous measurement of a wide range of both strain (0 - 2000)με and temperature (20 - 600)°C has been demonstrated by using a grating with high reflectivity in an Sb-Er-Ge codoped silica fiber.

Wide range of temperature and strain measurement using Bragg-grating-based fibre laser approach

A fiber laser-based sensor system has been developed to measure wide range of temperatures (22 - 500°C) and strain (0 - 1200με) using a normal and a chirped grating as optical feedback elements and Er3+-doped fiber as gain medium.