Dynamic strain measurement using two wavelength-matched fiber Bragg grating sensors interrogated by a cascaded long-period fiber grating

In this work, we describe a fiber Bragg grating (FBG) sensing system using two wavelength-matched FBG sensors for static and dynamic strain measurement. A cascaded long-period fiber grating (CLPG)-based demodulation technique has been used to interrogate the two wavelength-matched FBG sensors. Experimental results of static strain measurement show that the proposed system has a strain resolution of 1 με. This system has also been used for dynamic strain measurement. An eddy current displacement meter-based system has been used as a comparison measurement. Experimental results of dynamic strain measurement have proved that the FBG sensing system has a good performance in the measurement of dynamic strain. The results of static and dynamic strain measurement indicate that the sensing system using two wavelength-matched FBG sensors is superior to the single FBG sensor system.     

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by the sensor system. The experiment results agree with theoretical analyses.     

Fiber Bragg grating strain sensor based on fiber laser

A study on fiber Bragg grating (FBG) strain sensor, based on erbium-doped fiber (EDF) laser, is presented. A strain-sensing element, FBG, also acts as the lasing wavelength selecting component. When strain is applied on the FBG, the laser cavity loss changes, leading to a modification of the laser transient. Strain measurements are obtained in the time domain by simply measuring the EDF laser build-up time. Relative variation in the build-up time of up to 190%, for a strain range from 0 με to 2350 με, is achieved with a resolution corresponding to a strain of better than 2.35 με. This study demonstrates a novel fiber sensor concept and the technical feasibility to develop fiber strain measurement.     

A simple temperature-insensitive fiber Bragg grating displacement sensor

The study on fiber Bragg grating (FBG) displacement sensor based on monitoring the back-reflected power from an array of novelly embedded FBG sensors is presented. The sensor is a uniform FBG with three sections that are embedded in different layer in a composite lamina. Its bandwidth is displacement dependent and hence its reflected power varies almost linearly with displacement and it is insensitive to temperature variation. Thus, only low-cost photodetector (PD) is required to monitor displacement. This study demonstrates a novel fiber sensor, a method of fabricated the same, and a method to achieve simultaneous multi-sensor measurement.     

Photonic instantaneous measurement of microwave frequency using fiber Bragg grating

A photonic approach to realizing instantaneous measurement of microwave frequency based on optical monitoring using a fiber Bragg grating (FBG) is proposed and demonstrated. In the approach, a frequency-unknown microwave signal is modulated on an optical carrier in a Mach-Zehnder modulator biased at the minimum transmission point. After detecting the transmission and reflection optical powers at the output of the FBG, the microwave frequency can be determined according to the value of transmission-to-reflection power ratio, due to the fixed relationship between the microwave frequency and the power ratio. A proof-of-concept experiment has been performed, which demonstrates that a measurement resolution of ±0.08 GHz over a 10 GHz measurement bandwidth is achieved. The measurement performance in terms of resolution is better than previously reported results.     

Temperature-insensitive strain measurement using differential double Bragg grating technique

A novel differential double Bragg grating sensor for temperature-insensitive strain measurement is presented. The sensor consists of two identical weak measuring and reference gratings separated by some distance. The reference grating is placed inside the silica capillary that made it almost strain insensitive. The basic idea in differential double Bragg grating sensor is to measure the energy of the oscillating term in the reflection spectrum of double Bragg grating structure. The normalized energy of the interference term depends on the pitch difference of the two gratings and does not change when pitch variations of both gratings are equal with temperature variation. Therefore, the normalized energy of the interference term can be used for the temperature-insensitive strain measurement.     

A cheap and practical FBG temperature sensor utilizing a long-period grating in a photonic crystal fiber

A whole temperature sensor in one package utilizing a fiber Bragg grating (FBG) made in a conventional single-mode fiber and which uses a long-period grating (LPG) made in a photonic crystal fiber is proposed and experimentally demonstrated. The function of the interrogation is that the wavelength change of the FBG with environmental temperature is transferred to the intensity of the output via the LPG. Utilizing the temperature-insensitivity of the LPG in the PCF, the interrogation is stable and enables a cheap and practical temperature measurement system with a wide dynamic range.     

An optimized strain demodulation method based on dynamic double matched fiber Bragg grating filtering

An optimized strain demodulation method based on dynamic double matched fiber Bragg grating (FBG) filtering driven by a piezoelectric transducer (PZT) is proposed and experimentally demonstrated. Using an optimized triangular waveform to drive PZT, the nonlinear effect is eliminated, and extraction and processing of the peak signals are more convenient. Furthermore, taking advantage of double matched-FBGs and strain amplification structure to demodulate the Bragg wavelength, the wavelength scanning range can be extended to 6.3 nm, so that a large-scale and high-precision strain measurement can be achieved. The experimental results show that in the strain range of 0-3800με, the linearity of the relationship between strain and scanning time-difference is greater than 99.8%, and the stability of repeated measurements is up to 99.5%. This demodulation method can be widely applied in the FBG measurement system.     

Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber

A new type fiber bending sensor based on a tilted fiber Bragg grating (TFBG) interacting with a multimode fiber (MMF) is presented. The sensing head is formed by insertion of a small section of MMF between a single-mode fiber (SMF) and the TFBG. The average reflective power in the cladding modes decreases with the increase of curvature. The measurement range of the curvature from 0 to 2.5 m⁻¹ with a measurement sensitivity of −802.4 nW/m⁻¹ is achieved. The proposed sensor is also proved as temperature-independent from the experimental investigation.     

A planar lightwave circuit based micro interrogator and its applications to the interrogation of multiplexed optical fiber Bragg grating sensors

Optical fiber Bragg grating sensors have found potential applications in many fields, but the lack of a simple, field deployable and low cost interrogation system is hindering their deployment. To tackle this, we have developed a micro optical sensor interrogator using a monolithically integrated planar lightwave circuit based echelle diffractive grating demultiplexer and a detector array. The design and development of this device are presented in this paper. It has been found that the measurement range of this micro interrogator is more than 25 nm with better than 1 pm resolution. This paper also reports the applications of the micro interrogator developed to the monitoring of commercial optical fiber Bragg grating (FBG) temperature sensors and mechanical sensors. The results obtained are very satisfactory and in some cases, they are better than those obtained using commercial bench top lab equipment.     

Addressing fiber Bragg grating sensors with wavelength-swept pulse fiber laser and analog electrical switch

A pulse train with a wavelength dependent time sequence is generated in a fiber laser configuration, which contains a cascaded wavelength-division-multiplexing (WDM) fiber Bragg grating (FBG) array and a tunable F-P filter. By distributing pulses to corresponding channels with a 1 × N analog electrical switch, a novel FBG sensors interrogation technique with advantages of high signal-to-noise ratio (SNR) and high interrogation speed is experimentally demonstrated. Then, a FBG sensing system based on this interrogation technique and the mature unbalanced scanning Michelson interferometer (USMI) demodulation technique is realized. The system has shown a sensitivity of 1.610°/με, for the 1555 nm FBG, which agrees well with the theoretical value of 1.674°/με.     

Temperature sensitivity of long period fiber grating in SMF-28 fiber

In this paper we have presented long period fiber grating (LPFG) as temperature sensor. Temperature based sensors have found a number of applications in commercial and industrial fields. In LPFG based temperature sensors, they respond to shift in various peak resonant wavelengths corresponding to various attenuation bands of the transmission spectrum. Temperature effect on the various attenuation bands of a LPFG have been investigated to create a highly sensitive measurement device. The temperature sensitivities of various attenuation bands of a LPFG over the wavelength region of 1.1–1.7 μm, for a grating period of 280 μm period, are obtained by monitoring the wavelength shift of each peak resonant wavelength with temperature increment of 20 °C, ranging from 0 °C to 100 °C.     

Simultaneous measurement of curvature and temperature based on PCF-based interferometer and fiber Bragg grating

A sensor head consisting of a photonic crystal fiber (PCF)-based Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. The MZI fabricated by splicing a short length of PCF between two single-mode fibers with the air-hole structure that completely collapsed near the splicing points, is sensitive to fiber bending and surrounding temperature, while the FBG is only sensitive to the later. Simultaneous measurement of curvature and temperature is therefore obtained. Sensitivities of 4.06 nm/m^− 1 and 6.30 pm/°C are achieved experimentally for curvature and temperature, respectively. And the corresponding resolutions are 5.2 × 10^− 4 m^− 1 and 1.25 °C for curvature and temperature, respectively, based on the wavelength measurement resolution of 10 pm.     

Fiber torsion sensor demodulated by a high-birefringence fiber Bragg grating

A new high-birefringence (Hi-Bi) fiber torsion sensor demodulated by a Hi-Bi fiber Bragg grating (FBG) is proposed in this study. The twisted Hi-Bi fiber sensor characteristics are analyzed using the Jones matrix. The intensity ratio from two reflected Bragg wavelengths is associated with the twist angle of the measured Hi-Bi fiber. It is found that the twist angle resolution is estimated at around 0.3° under ±0.1 dB readout from an optical spectrum analyzer if the polarization state of the light source is stable. The advantages of this new torsion sensor are: (1) insensitivity to intensity variations from the light source, (2) insensitivity to the torsion gauge length, and (3) absolute measurement in the twist angle. However, the polarization state of light in the proposed method needs to be controlled, and any birefringence change in the twisted Hi-Bi fiber needs to be prevented.     

Fiber grating sensors for high-temperature measurement

Two fiber grating sensors for high-temperature measurements are proposed and experimentally demonstrated. The interrogation technologies of the sensor systems are all simple, low cost but effective. In the first sensor system, the sensor head is comprised of one fiber Bragg grating (FBG) and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured based on the wavelength-shifts of the FBG induced by the strain. In the second sensor system, a long-period fiber grating (LPG) is used as the high-temperature sensor head. The LPG is very-high-temperature stable CO₂-laser-induced grating and has a linear function of wavelength–temperature in the range of 0–800 °C. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by either the first or the second sensor system. The experimental results agree with theoretical analyses.     

Erbium-doped fiber ring laser sensor for high temperature measurements based on a regenerated fiber Bragg grating

This article presents an erbium-doped fiber ring laser for high temperature measurement with high accuracy. The proposed laser sensor employs a regenerated fiber Bragg grating (RFBG) as a sensor element. Through thermal treatments, the RFBG with enhanced thermal resistance was obtained. The laser emission optical spectrum presents good performance with a high optical signal-to-noise ratio of 58dB. Experimental results demonstrate a wavelength sensitivity to the temperature is 15.5 pm/ºC with the temperature range from 300ºC to 900ºC, and the correlation coefficient is 0.999. The results prove it is able to provide potential applications in high temperature measurement.     

Hetero-core fiber Bragg grating for simultaneous measurement of strain and bending based on cladding mode ratiometric detection

A simple but practical method using multimode-single mode fiber Bragg grating (FBG) for cladding mode based simultaneous measurement of strain and bending is proposed and investigated experimentally. Experimental results show that the intensity ratio between cladding mode and FBG resonance of hetero-core FBG show monotonous changes following with the increase of deflection, by which strain and bending discrimination can be achieved by measuring the wavelength shift of FBG resonance and ratiometrically detecting the intensity changes between cladding modes and FBG resonance.     

Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror

We present an all-fiber sensor for simultaneous measurement of temperature and strain. The sensing head is formed by introducing a fiber Bragg grating into a high-birefringence fiber loop mirror that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. A sensing resolution of ±1 °C in temperature and ±21 με in strain has been experimentally achieved over a temperature range of 60 °C and strain range of 600 με.     

Temperature-insensitive transverse load sensing with improved accuracy using stress induced birefringence effects of fiber Bragg grating

Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on direct spectral analysis, a new method exploiting the polarization properties of FBG for temperature-insensitive transverse load sensing is proposed. In this paper, the birefringence effects on FBG under transverse load are analyzed. The evolution of the polarization dependent loss (PDL) of FBG with wavelength for the transmitted signals with respect to the transverse load is also studied theoretically and experimentally. We proposed utilizing the PDL evolution of FBG to precisely recover the wavelength spacing between the two peaks of x-mode and y-mode for accurate transverse load measurement. Good agreements between experimental results and numerical simulations had been obtained.     

Elimination of environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating sensor array

We present a novel reference compensation method for eliminating environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating (FBG) sensors. By employing a shielded wavelength-division-multiplexed reference FBG in the system, the environmental noise is measured from the reference channel, and then subtracted from the demodulation result of each sensor channel. An approximate 40 dB reduction of the environmental noise has been experimentally achieved over a frequency range from 20 Hz to 2 kHz. This method is also suitable for the elimination of broadband environmental noise. The corresponding FBG sensor array system proposed in this paper has shown a wavelength resolution of .