Multipoint temperature-independent fiber-Bragg-grating strain-sensing system employing an optical-power-detection scheme

Abstract

A temperature-independent fiber-Bragg-grating strains-sensing system, based on a novel optical-power-detection scheme, is developed and analyzed. In this system a pair of fiber Bragg gratings with reflection spectra either partially or substantially overlapping is placed side by side to form a temperature-independent strain-sensor unit. Conventional wavelength-interrogation techniques are not used here, and instead an optical-power-detection scheme is proposed to directly calibrate the measurand, i.e., the strain. Unlike the conventional approach in a multiplexed sensing system, the presented power-detection-based interrogation method does not need the fiber-Bragg-grating sensors to be spectrally separate. The only requirement is that the spectra of the two fiber Bragg gratings of each sensor unit in a multiplexed system be identical or slightly separate (slightly overlapping spectra would also work in the sensing scheme) and the source's optical power be sufficient for sensitive measurement. Based on a three-sensor-unit system, we demonstrate simple strain measurements of high linearity (+/- 0.4%), good sensitivity [2 microstrains (microS)], high thermal stability (+/- 0.8%), and zero cross talk. The effects of light source spectral flatness and fiber bending loss on measurement accuracy are also discussed.

Multi-Functional Measurement Using a Single FBG Sensor

“…As α and β are not changed during the experiments, the Bragg wavelength with the strain loading conditions, 1 Bsen_1 , 1 Bsen 1_1 , 1 Bsen 2_1 (subscript of _1 indicates the Bragg wavelength during the experiments) can be estimated by using t sen_1 , t ref 1_1 and t ref2_1 and equations (4)(5)(6). The shift in the Bragg wavelength Δ1 B is expressed in equation (7) by the change of strain Δε and the temperature ΔT [5].…”

“…Since the Bragg wavelength is changed by both strain and temperature variations, temperature compensation is required for accurate strain measurements. Several methods have been proposed to compensate for this temperature effect [5][6][7]. Xu et al [5] prepared special superimposed fibre gratings having different Bragg wavelengths to measure strain and temperature separately.…”

“…By monitoring the shift in Bragg wavelength of two different FBGs experiencing the same strain and temperature conditions, and using the difference of the strain-optic and thermo-optic coefficients, they successfully measured strain and temperature separately. Chiang et al [6] used two FBG sensors with identical, or similar, Bragg wavelengths as a sensor, putting them under the same temperature conditions but with one FBG kept free from strain. They measured the change of the reflected spectrum intensity and estimated strains without the temperature effect.…”

New structure for multipoint temperature-independent strain sensing system employing optical power detection scheme

“…For the past few years, many novel structures have been proposed to simultaneously measure the strain and temperature using dual fiber gratings [8][9][10][11]. Temperatureindependent strain measuring schemes were also proposed and demonstrated [12][13][14]. In the previous work [14], the effect of temperature was eliminated by using two spectrally-overlapped FBGs in an optical power detection scheme for multipoint strain measurement.…”

“…Temperatureindependent strain measuring schemes were also proposed and demonstrated [12][13][14]. In the previous work [14], the effect of temperature was eliminated by using two spectrally-overlapped FBGs in an optical power detection scheme for multipoint strain measurement. That power detection scheme is believed to be more cost-effective than any others that use expensive instruments (e.g., optical spectrum analyzer) for Bragg wavelength interrogation.…”