Fiber Bragg Grating Reflection Spectrum Research Articles

Demodulation of the overlapping reflection spectrum of serial FBGs based on a weighted differential evolution algorithm.

This study addresses the wavelength demodulation problem of the overlapping reflection spectrum of serial fiber Bragg gratings (FBGs) with nearly-identical wavelength. Specifically, a novel demodulation model for the overlapping reflection spectrum was presented based on spectrum similarity, and this model encodes FBGs through reflectivity. Subsequently, a weighted differential evolution algorithm was employed to calculate the FBG wavelengths. And the factors affecting the demodulation accuracy of the proposed method were simulated and analyzed. Finally, the proposed method was applied to demodulate the overlapping reflection spectra of serial FBGs. The experiment results indicate that the proposed method is suitable for completely overlapping, partially overlapping, and non-overlapping reflection spectra of serial FBGs. The wavelength demodulation accuracy demonstrated here in fully overlapping situations for two, three, and four FBGs was only 4.5, 14.9, and 24.6 pm, respectively.

Spectrum collapse in a 7-core Yb-doped fiber laser with an array of fs-inscribed fiber Bragg gratings.

Femtosecond inscription of fiber Bragg gratings (FBGs) in each core of a cladding-pumped seven-core Yb-doped fiber enables efficient (≈70%) 1064-nm lasing in a robust all-fiber scheme with ≈33 W power, nearly the same for uncoupled and coupled cores. However, the output spectrum is quite different: without coupling, seven individual lines corresponding to the in-core FBG reflection spectra sum up into a broad (0.22 nm) total spectrum, whereas the multiline spectrum collapses into a single narrow line at strong coupling. The developed model shows that the coupled-core laser generates coherent superposition of supermodes at the wavelength corresponding to the geometric mean of the individual FBG spectra, whereas the generated laser line broadens, with a power (0.04-0.12 nm) like the single-core mode of a seven-times larger effective area.

Spectral Splitting Sensing Using Optical Fiber Bragg Grating for Spacecraft Lateral Stress Health Monitoring

Optical fiber sensing is a promising detection method for spacecraft health monitoring, since optical fiber sensors are lightweight, small in size, easy to integrate and immune to electromagnetic interference. As a significant optical sensor, fiber Bragg gratings (FBG) are widely used for force sensing because of their axial strain characteristics. However, it is necessary to detect not only one-dimensional strain but also plane strain and its deformation in order to comprehensively evaluate the condition of the structure. Therefore, it is very important to analyze the reflection spectrum of FBG under lateral stress. When FBG are subjected to lateral stress, the refractive index of the waveguide in the x and y directions changes, resulting in a birefringence phenomenon. This result causes the reflection spectrum of FBG to split into two peaks. In this paper, a transverse stress detection method based on spectral split sensing for the fiber Bragg grating is proposed, intended for monitoring spacecraft–small particle collisions. The FBG local lateral stress detection system is designed and verified by experiments. The wavelength pressure correlation is established in the experiment by adjusting the number of weights to change the lateral pressure on the FBG. The loading range of FBG lateral pressure is 4.0–7.0 N, the step size is 0.5 N, and round-trip measurement is carried out four times. The wavelengths of the peak and split point of the FBG reflection spectrum are recorded. The experimental results show that FBG’s split point and right peak pressure sensitivities are 16.57 pm/N and 45.14 pm/N, respectively. The spectral splitting phenomenon can be applied in spacecraft structure health monitoring systems and has certain reference value for the simplification of sensor systems.

High-speed wavelength interrogation system based on crossed HiBi-FLMs for array FBGs

A novel ultrafast wavelength interrogation system for array fiber Bragg gratings (FBGs) has been proposed and experimentally demonstrated, the operation principle is based on pulsed optical source (POS) combined with edge-filtering method and time-division multiplexing (TDM) technique. Thanks to the Gaussian-shape nature and numerous passband channels of the HiBi-FLM transmission, the simultaneous interrogation of array wavelength-division multiplexing (WDM) FBGs can be realized by employing crossed HiBi-FLMs scheme. The HiBi-FLM can provide an individual edge filter for each WDM-FBG and map the FBG reflection spectra into corresponding optical intensity, thus realizing the simultaneous interrogation of multiple WDM-FBGs. The interrogation speed of the proposed system is as high as 200 kHz, which can be further improved to hundreds of megahertz (MHz) by increasing the repetition rate of the POS, the response speed of the photodetector (PD), and the sampling rate of field programmable gate array (FPGA), as well as reducing the pulse width and the lengths of the delay lines. The proposed wavelength interrogation system features high speed, outstanding multiplexing capability, large dynamic range and simple configuration, thus providing a promising solution for various applications, such as quasi-distributed sensing, high-speed communication, medical treatment and the monitoring of ultrafast dynamic phenomena (e.g., molecular dynamics sensing and aircraft engine diagnostics).

Single Peak Fiber Bragg Grating Sensors in Tapered Multimode Polymer Optical Fibers

This research demonstrates how low loss commercially available multi-mode (MM), perfluorinated polymer optical fibers (POFs) can be used for robust and reliable fiber Bragg grating (FBG) based sensing. A single peak FBG reflection spectrum is achieved by tapering the fiber to a diameter just small enough to make the fiber effectively single-moded and then inscribing the FBG in the waist section of the taper. Here we use plane-by-plane, direct writing inscription with a femtosecond laser. In particular we used the GigaPOF-50SR MM fiber and tapered it by a factor of 0.24, which was found to be the critical ratio that provided a single grating reflection peak. The GigaPOF-50SR fiber was chosen because it allows operation at 1550 nm and has a suitable small 50 μm core diameter, which minimizes the required taper ratio and thus the insertion loss of the taper. The FBG sensor was found to have a strain and relative humidity sensitivity of 20 nm/%strain and ∼ 6.7 pm/%RH, respectively. The grating proved to be largely insensitive to temperature. This approach to single-mode POF sensor fabrication will enable multi-point strain sensing at 1550 nm over 50 m long fiber. It combines the main advantages of single mode POFs (robust FBG sensing) and MM POFs (low transmission loss) and demonstrates that tapered MM POFs constitute an efficient solution for the development of robust and reliable long distance POF FBG sensor networks.

Cascaded Generation in Multimode Diode-Pumped Graded-Index Fiber Raman Lasers

We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M2~1.3) in both linear and half-open cavities, whereas the line is narrower (<0.2 nm) and more stable in the case of the linear cavity with two FBGs. However, an optimization of the FBG reflection spectrum used in the half-open cavity allows this linewidth value to be approached. The measured beam profiles show the dip formation in the output pump beam profile, whereas the first and second Stokes beams are Gaussian-shaped and almost unchanged with increasing power. A qualitative explanation of such behavior in connection with the power evolution for the transmitted pump and generated first, second and third Stokes beams is given. The potential for wavelength tuning of the cascaded Raman lasers based on LD-pumped multimode fibers is discussed.

Sensing characteristics of grating assisted counterpropagating cladding modes in optical fibers.

In this paper, using full vector modal field analysis, we present physical parameter sensing, e.g., temperature, axial strain, bulk refractive index, and affinity sensing characteristics of counterpropagating cladding modes in optical fibers. Counterpropagating cladding modes (HEmn) are considered to be excited by resonant power coupling from the fundamental core mode using a suitably designed fiber Bragg grating (FBG). We show that for such couplings, the bandwidths of reflection spectra are much smaller than those obtained for conventional core mode reflecting FBGs. Next, we also show that the reflection bandwidth increases linearly with increasing grating strength, whereas it decreases exponentially with increasing grating length. The reflection spectra of such FBGs, as a function of different ambient physical perturbation parameters, are obtained, and then the grating sensitivity corresponding to those parameters is established. Interestingly, we have noticed that despite having the same evanescent field in the analyte region, unlike the long-period grating assisted co-propagating cladding modes (HEmn), the same cladding modes while propagating counter to the core mode are weakly sensitive to changes in the ambient refractive index. The reason behind this observation is discussed.

High temperature in-situ sensing system based on fiber Bragg grating for reciprocating compressor

cncnA contactless high temperature in-situ sensing system based on fiber Bragg grating(FBG) is proposed to implement accurate temperature measurements of bearing bushing, which is the core component of reciprocating compressor(RC). The system consists of a sensing probe of a FBG putting in a metal capillary to eliminate the cross-sensitivity problem and a pair of optical fiber collimators to send the original optical signal and collect the reflected sensing modulation signal. Considering the intermittent variation of FBG reflection spectrum transmitted by optical fiber collimators with reciprocating relative motion, a demodulation algorithm of wireless FBG signal is proposed and implemented. The experimental results show that the in-situ FBG sensing system has good temperature measurement performance in the range of 10–350 °C, the sensitivity is 9.49pm/°C, and the linearity is more than 99.93%

Design of fiber bragg grating (FBG) temperature sensor based on optical frequency domain reflectometer (OFDR)

In this paper, the simulation of Fiber Bragg Grating (FBG) as a temperature sensor is conducted. The FBG temperature sensor is designed based on Optical Frequency Domain Reflectometer (OFDR) concept. A continuous wave (CW) laser is used as the optical source and it is transmitted to two FBGs. The two FBGs reflection spectra will produce a beat frequency that can be detected using a Radio Frequency (RF) Spectrum Analyzer. Any temperature change will shift Bragg wavelength, thus produce a shift for the beat frequency. In this work, an FBG with temperature sensitivity 10 pm/˚C is employed. It is found that by using this technique, a high-resolution temperature sensor can be designed with temperature resolution of 0.1˚C.

The Location Monitoring of Fatigue Crack Damage by Using the Spectral Area Extracted from FBG Spectra.

In this paper, a new damage feature, spectral area, was extracted to effectively detect crack location by studying the deformation mechanism of fiber Bragg grating (FBG) reflection spectra. In order to verify the robustness and reliability of spectral area to detect crack location, the following work was carried out: Firstly, the strain information was extracted by extended finite element method (XFEM) with fatigue crack propagation. The transmission matrix method (TMM) was used to simulate FBG reflection spectra using numerical results. Secondly, the fatigue crack growth monitoring experiment based on FBG sensors was carried out, and the digital image correlation (DIC) method was used to measure the strain values at the placement of FBG sensors with crack propagation. The temperature characteristic test of FBG was carried out to investigate the influence of temperature variation on the spectral area. The results presented that the spectral area was insensitive to temperature variation and experimental noise, and was greatly sensitive to the complex non-uniform strain field cause by crack damage. Moreover, compared with the 5 mm FBG sensor, the 10 mm FBG sensor showed a larger critical detection range for crack damage. Therefore, the spectral area can be used as a reliable damage feature to detect the crack location quantitatively based on the simulated and experimental results.

Fast peak-tracking method for FBG reflection spectrum and nonlinear error compensation

A universal method is proposed for fast tracking the peak wavelength of a fiber Bragg grating (FBG) reflection spectrum, which is the essential procedure of most FBG interrogators. Assuming the FBG reflection spectrum is sampled uniformly, we decompose the Moore–Penrose of the coefficient matrix into a formula that only contains a division and a dozen additions and multiplications; thereby, the cost of computation can be greatly reduced. Based on the analysis of the characteristics of the error introduced by the spectrum’s nonlinearity, we found and defined an error associated with the distance between the sampled peak and actual peak, and the compensating method for the error is also given. After compensation, the proposed method can speed up the tracking process 10 times or more without sacrificing the accuracy. An economical FBG interrogator with a commercial field programmable gate array (FPGA) and a microspectrometer was built up to verify the method, which was able to achieve a measurement frequency of 17 kHz.

All-fiber frequency shifter consisting of a fiber Bragg grating modulated via an acoustic flexural wave for optical heterodyne measurement.

We present an all-fiber frequency shifter (AFFS) consisting of a fiber Bragg grating (FBG) modulated via an acoustic flexural wave for optical heterodyne measurement. The AFFS can efficiently generate the frequency-shifted signal due to the resonance peak with a high-reflection efficiency and being completely separated from the reflection spectrum of the original FBG, simultaneously. The experimental result shows that the minimal measurable vibration amplitude and the resolution of the all-fiber optical heterodyne measurement setup constructed with the AFFS are 0.06nm and 30pm in the region of tens to hundreds of kilohertz, respectively.

Highly Sensitive FBG Strain Sensor With Enhanced Measurement Range Based On Higher Order FWM

To meet the simultaneous requirement of high sensitivity and high measurement range is an uphill task in the development of fiber Bragg grating (FBG) strain sensor. A unique scheme of highly sensitive FBG strain sensor is proposed based on nonlinear four wave mixing (FWM) with a high measuring range. In this paper, both the edges of FBG reflection spectrum are used without increasing the number of sources. The FWM products generated are tuned to the reflection edges of the FBG. The signals reflected from the edges of the FBG are then utilized in the subsequent FWM process to enhance the sensitivity of the system. The proposed scheme can provide a strain measurement range of 2310 με with strain sensitivity of 91.21 dBm/nm.

Fabrication of dual-parameter fiber-optic sensor by cascading FBG with FPI for simultaneous measurement of temperature and gas pressure

A dual-parameter fiber-optic sensor based on fiber Bragg grating (FBG) cascaded with Fabry–Pérot interferometer (FPI) structure is described and demonstrated to measure temperature and gas pressure simultaneously. The proposed sensing structure consists of an extrinsic FPI in the form of hemispherical ultraviolet (UV) curing glue capped on a FBG end face. The spectral interference peak produced by the UV glue FPI is sensitive to ambient temperature and gas pressure variations, whereas the reflection spectrum of FBG only depends on temperature and insensitive to pressure change, therefore, the temperature and gas pressure can be simultaneously measured by this cascaded sensor. Experimental results show that the sensor has higher temperature and gas pressure sensitivities of 223.4 pm/° C and 24.99 nm/MPa in a range from 30 ∘C to 110 ∘C and from 0.1 MPa to 0.7 MPa. Utilizing the temperature-dependent sensitivity of FBG, a constructed sensitivity matrix is used to distinguish measurement of temperature and gas pressure, and the measurement resolutions are determined to be 0.086 ∘C and 0.730 KPa. This sensing architecture is not only an effective method for single-point multi-parameter measurement, but also has merits of high sensitivity, compactness, ease of fabrication and low cost, which make it valuable for biosensors and medical applications.

The Analysis of FBG Central Wavelength Variation with Crack Propagation Based on a Self-Adaptive Multi-Peak Detection Algorithm.

We propose a peak seeking algorithm to extract the damage characteristic-variation of central wavelength to monitor the crack damage status in aluminum alloy plates using surface bonded fiber Bragg grating (FBG) sensors. The FBG sensors are sensitive to the uniform and non-uniform strain distribution along their longitudinal direction, and the effect appears in the power spectrum of the reflected light from the gauge section. In this paper, we propose a fast-self-adaptive multi-peak seeking algorithm to detect the central wavelength shifting of the FBG reflection spectrum with the crack propagation. The proposed peak searching algorithm results point to a significant improvement compared to other conventional methods. Then the central wavelength shifting is applied to explain the crack propagation behavior of the aluminum plates under quasi-static tensile test conditions. The different damages feature changing intervals which are associated with the crack position and the FBGs location, demonstrating that central wavelength shifting performs as an indicator to detect structural crack damage.

Dynamic Interaction Process Analysis Between Failure-Detecting Light Pulse and 2D Optical Encoder in Link Health Detection System for PON

We explore the dynamic interaction process between the failure-detecting light pulse and fiber Bragg grating (FBG) based two-dimensional (2-D) optical encoder in a link health detection system (LHDS) for a passive optical network (PON). The dynamic interaction model for PON-LHDS with a 2-D optical encoder supporting 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m-1</sup> users is derived by discussing the overlap between the failure-detecting light pulse spectrum and the FBG reflection spectrum, where m is the number of the failure-detecting light sources. A series of received light pulses for PON-LHDS with six wavelength channels involving 32 users are obtained based on the model, considering the variation of the FBG parameters. The simulation results show that the central wavelength difference between the failure-detecting light pulse and the coding FBG is the key factor affecting a system performance that 0.1 nm central wavelength difference will reduce the equivalent reflection coefficient to 0.89 and result in a 6.56-dB peak power difference of the coded signal. The PON-LHDS with a 32-user 2-D optical encoder is a setup, and the experiment results indicate that the peak power of the coded signal between two channels has a 2.8-dB difference due to the central wavelength difference, consistent with the calculation results of the dynamic interaction model.

Simultaneous Measurement of Axial Strain, Bending and Torsion With a Single Fiber Bragg Grating in CYTOP Fiber

This paper presents the development of a three-dimensional (3-D) displacement sensor based on one fiber Bragg grating (FBG). In order to obtain higher sensitivity and dynamic range, the FBG is inscribed in low-loss, multimode, cyclic transparent amorphous fluoropolymers using the direct-write, plane-by-plane femtosecond laser inscription method. The proposed sensor is based on the influence of each displacement condition, namely, axial strain, torsion, and bending on the FBG reflection spectrum. Such influence is analyzed with respect to the FBG wavelength shift, reflectivity, and full width half maximum. The operation principle and theoretical background of the proposed approach is numerically analyzed by means of finite element analysis for the strain along the grating length and coupled-mode theory with a modified transfer matrix formulation for the FBG spectrum. The sensor is experimentally characterized and validated in which the results show good agreement between the applied axial strain, bending, and torsion with relative errors below 5.5%. Thus, the proposed sensor is an interesting alternative for measuring displacements in 3-D applications, such as movement analysis and the instrumentation of novel soft robots.

On crack propagation monitoring by using reflection spectra of AFBG and UFBG sensors

Due to their light weight, small size and immunity to electromagnetic interference, Fiber Bragg Gratings (FBG) have shown great potential in structural health monitoring (SHM) area. Ordinarily, FBGs are used as the quasi-distributed sensors to detect structural damage by central wavelengths shifting, which has robust problem in real engineering application. It is found that the FBG reflection spectra can also be distorted with the non-uniform strain field caused by crack. Multiple indices can be extracted from the reflection spectra which can provides more information on the crack propagation. However, till now no researches have been investigated regarding the behaviors of different kinds of FBGs for crack propagation monitoring based on the FBG reflection spectra. In this paper, behaviors of the apodized FBG (AFBG) and the uniform FBG (UFBG) sensors for structural crack propagation monitoring are investigated. The reflective spectral behaviors of both UFBG and AFBG sensors under crack-generated non-uniform strain are illustrated by simulation and experiment. Three spectra indices, namely the central wavelength shift, the ratio of side lobe to main lobe and the information entropy, are proposed to indicate the propagation of the crack. The results show the UFBG sensors have better performance than the AFBG sensors which are more suitable for crack propagation monitoring.

High-precision FBG demodulation using amplitude ratio curve with sharp peak

Abstract For the fiber Bragg grating (FBG) sensor based on the reflection peak position searching demodulation algorithms, a key problem is that the demodulation precision of measurement signal is seriously limited by the full width half maximum (FWHM) of the FBG reflection spectrum. In this paper, in order to overcome this problem and enhance the measurement precision, we proposed a high-precision demodulation method based on amplitude ratio curve to instead of the FBG reflection spectrum, which can increase the sensing precision significantly. In this method, the Bragg wavelength of FBG is modulated periodically with a sinusoidal function to obtain a series of reflection spectra. Then at each wavelength the amplitude ratio between second harmonic frequency component and fundamental frequency component is calculated to create the amplitude ratio curve. Here, we present a comprehensive theoretical analysis and experimental demonstration to verify the advantages of this method. Comparing with the demodulation method based on the FBG reflection spectrum, the average maximum measure error (MME) of Bragg wavelength is reduced from ±1.54 pm to ±0.15 pm, which improved by 10.3 times. In addition, the influence of the modulation amplitude and sampling frequency on demodulation accuracy are also studied in this paper.

Distortion-tolerated high-speed FBG demodulation method using temporal response of high-gain photodetector

Abstract A distortion-tolerated fiber Bragg grating (FBG) demodulation method for high-gain or high-speed sensing is proposed. The temporal response of photodetector (PD) on FBG reflection spectrum for different electric gains and wavelength scanning frequencies is investigated theoretically and experimentally. Simulation results show that high electric gains and high wavelength scanning frequencies will induce a tailed distortion on the right side of the FBG reflection peak output by the PD. A temporal response function of PD is presented to fit the peak with distortion and determine the peak position on FBG reflection spectrum. Experiments with varying degrees of FBG acquisition distortion are carried out. Compared with the traditional Gaussian fitting method, the measurement error of the temporal-response based method is limited within ±1.777 pm and the standard deviation is less than 0.940 pm, which are respectively improved by 6.87 times and 4.38 times at least.