Temperature-compensated high strain measurement based on a Fabry-Perot interferometer with the virtual-assisted Vernier effect.

A highly sensitive optical fiber Fabry-Perot interferometer (FPI) for strain measurement with temperature compensation is proposed. Instead of using another actual reference interferometer, a virtual FPI is constructed to superpose with the sensing FPI to form the Vernier effect. The fundamental and the first-order harmonic Vernier effect are generated to increase the sensitivity by adjusting the parameter of the virtual FPI. In order to separate the strain from the environment temperature, an FBG is cascaded to distinguish the applied temperature. Experimental results demonstrate that, with the help of the fundamental Vernier effect, the sensitivity and temperature of the FPI increases from 1.05 pm/°C to 10.63 pm/°C in the temperature range of 40-120°C, and the sensitivity of strain increases from 2.635 pm/µÎµ to 33.11 pm/µÎµ in the strain range of 0-400 µÎµ. In order to access the tracking points more easily and further enhance the sensitivities, the first-order harmonic Vernier effect is generated by modifying the virtual FPI. Results show that the temperature and strain sensitivities are 21.25 pm/°C and 62.25 pm/µÎµ, respectively. In addition, with the help of the FBG, the strain can be separated from the temperature by solving the cross-sensitivity matrix.

High-precision temperature-compensated fiber Bragg grating axial strain sensing system based on a dual-loop optoelectronic oscillator with the enhanced Vernier effect.

A temperature-compensated fiber Bragg grating (FBG) axial strain sensor based on a two-dual-loop optoelectronic oscillator (OEO) with the enhanced Vernier effect is proposed and experimentally demonstrated. The sensing head consists of two cascaded FBGs, one of which acts as a sensing FBG to measure both the axial strain and temperature and the other as a reference FBG to detect temperature. Acting as the optical carrier, the reflected optical signal of the sensing head is divided into two paths with opposite dispersion coefficients and slightly different lengths to achieve an enhanced Vernier effect. After being divided by a wavelength division multiplexer, the optical signal launches into two electrical paths with different electrical bandpass filters (EBPFs) for frequency division multiplexing. The EBPF I selects the microwave signal generated by the sensing FBG, while the EBPF II selects the microwave signal generated by the reference FBG. Therefore, the axial strain and temperature can be recovered by recording the microwave frequency within EBPF I, and the temperature can be interrogated by tracking the microwave frequency within EBPF II. The axial strain applied on the sensing FBG can be distinguished by solving the cross-sensitivity matrix. The results show that the sensitivity of the dual-loop OEO is much greater than that of the single-loop OEO. The maximum measurement error for the axial strain is 0.112µÎµ, and the maximum temperature compensation error is as low as 0.024°C in the dual-loop OEO, which is far less than that in the single-loop OEO. The enhanced Vernier effect not only improves the sensitivity, but also reduces the temperature compensation error.

Distributed strain interrogation with a linearly chirped fiber Bragg grating based on an optoelectronic oscillator.

A novel distributed strain sensor method, to the best of our knowledge, based on a linearly chirped fiber Bragg grating (LCFBG), which can simultaneously determine the strain value and ascertain the position, is proposed and experimentally demonstrated. Different from the traditional distributed grating interrogation system via analyzing the optical spectrum of an LCFBG, the system is mainly based on the frequency domain measurement by using an optoelectronic oscillator (OEO) structure, which has the characteristics of fast response and high resolution. Based on this structure, when the distributed strain is applied to the LCFBG, the frequency response of the OEO for a reflective point of a certain wavelength will change. The strain value can be obtained by detecting the frequency shift of the OEO. Combined with the one-to-one correspondence between the wavelength and the spatial position of the LCFBG, the exact position of the strain point can be determined. In a proof-of-concept experiment, interrogation of fully distributed grating sensors with nonuniform strain distributions is demonstrated experimentally. A spatial resolution of ∼3µm over a gauge length of 53 mm and a strain resolution of <1µÎµ have been achieved.

Temperature sensor based on fiber Bragg grating combined with a microwave photonic-assisted fiber loop ring down.

A temperature sensor based on fiber Bragg grating (FBG) combined with a microwave photonic-assisted fiber loop ring down (FLRD) is proposed and experimentally investigated. An optical edge filter (OEF) is inserted in the FLRD to provide a wavelength dependent loss; thanks to the linear response of the OEF, the wavelength shift of the FBG caused by the applied temperature is linearly converted to the additional loss of the FLRD. The frequency response of the FLRD is measured by a vector network analyzer (VNA), the time domain ring-down curves are calculated by applying invert faster Fourier transform (IFFT) to the frequency response. Subsequently, the relationship between the ring-down time and the temperature applied to the FBG is obtained. Results show a good linearity between the ring-down time and the temperature. Limited by the VNA used in our experiment, the sensitivity of the proposed sensor is 6.30 ns/°C in the temperature range of 40-45 °C with a resolution of ±0.14 °C.