Fiber Bragg Grating Structure Research Articles

Immune to temperature interference sensor of carbon dioxide gas concentration based on a single modified fiber Bragg grating.

In this study, a novel method that can detect carbon dioxide (CO2) concentration and realize temperature immunity based on only one fiber Bragg grating (FBG) is proposed. The outstanding contribution lies in solving the temperature crosstalk issue of FBG and ensuring the accuracy of detection results under the condition of anti-temperature interference. To achieve immunity to temperature interference without changing the initial structure of FBG, the optical fiber cladding of FBG and adjacent optical fiber cladding at both ends of FBG are modified by a polymer coating. Moreover, a universal immune temperature demodulation algorithm is derived. The experimental results demonstrate that the temperature response sensitivity of the improved FBG is controlled within the range of 0.00407 nm/°C. Compared with the initial FBG (the temperature sensitivity of the initial FBG is 0.04 nm/°C), it decreases by nearly 10 times. Besides, the gas response sensitivity of FBG reaches 1.6 pm/ppm and has overwhelmingly ideal linearity. The detection error results manifest that the gas concentration error in 20 groups of data does not exceed 3.16 ppm. The final reproducibility research shows that the difference in detection sensitivity between the two sensors is 0.08 pm/ppm, and the relative error of linearity is 1.07%. In a word, the proposed method can accurately detect the concentration of CO2 gas and is efficiently immune to temperature interference. The sensor we proposed has the advantages of a simple production process, low cost, and satisfactory reproducibility. It also has the prospect of mass production.

Performance enhancement of optical communication system with cascaded FBGs of varying lengths

In this paper, an optical communication system constructed with cascaded fiber Bragg gratings or fiber Bragg gratings (FBGs) of varying lengths capable of operating in the C-band is presented. Here, initially, a passive optical device formed with four cascaded FBGs of varying lengths is proposed and analyzed. Subsequently, the proposed device is kept in the optical communication system to reduce the spectral width of the source, thereby enhancing the system’s performance. Analytical formulation based on the transfer matrix method of an optical device is incorporated. The effect of such a device on the system performance at various operating distances is discussed. Further, the system performance is studied with the apodized FBGs in the passive optical device. At the end, we investigated the effect of incorporating the cascaded FBG structure in the system utilizing four-level pulse amplitude modulation or PAM-4 modulation . As per the simulation results, the proposed device has given a maximum reflectivity of 98.39 and a minimum FWHM of 0.175 nm for uniform FBGs. But with the apodized FBGs, an FWHM of 0.07 nm with a reflectivity of 59.81 has been achieved. Simulation results reveal that the system formed with the proposed device has given better performance up to a distance of 105 km compared to the absence of the device. At the maximum operating distance of the system with apodized FBGs, the Q-factor and bit error rate (BER) are recorded as 6.712 and 9.5321×10−12, respectively. Apart from the Q-factor and BER estimation, eye height is also used to estimate the system performance at various operating distances.

Temperature sensing of the brain enabled by directly inscribed Bragg gratings in CYTOP polymer optical fiber implants

Brain temperature is a vital physiological parameter that has a great effect on metabolic processes, enzymatic activity, neurotransmitter function, blood flow regulation, neuroprotection, and cognitive performance. In this framework, the development of accurate and reliable brain temperature measurement tools is crucial in brain-related treatment and research, such as neurosurgery, therapeutic hypothermia, and the understanding of brain function and pathologies. Here, we developed the first large-core flexible low optical loss CYTOP polymer optical fiber (POF)-based brain temperature probe operating in the telecommunication spectral range. The temperature measurements were achieved by detecting the reflected spectrum from a fiber Bragg grating (FBG) directly inscribed at the tip of the POF using femtosecond pulses. A fluorinated ethylene propylene (FEP) tube was thermally drawn and used as a sleeve around the FBG structure to eliminate the cross-sensitivity with humidity and microstrain perturbations. The assembled POF implant has a sensitivity of 14.3 pm/°C. The local temperature of the cerebral cortex, corpus callosum, and striatum in a rat brain was measured in vivo. The results indicate that the deep brain regions have higher temperature than the top cortical ones with a relatively linear relationship between the brain structure depth and its temperature. In addition, the rectal body core temperature was detected in parallel with the brain temperature measurement to further validate the developed device. We believe that the presented CYTOP POF-based implantable temperature probe opens the way towards the development of flexible and stable tools for accurate brain temperature recording where large-core fiber-based neural devices are required.

Research on lateral pressure sensing based on Sagnac ring cascaded FBG

In the field of engineering, the deformation and lateral stress in structural health detection have attracted extensive attention to obtain the structural health information of building objects in time. Based on the birefringence effect of polarization maintaining fiber (PMF), a transverse pressure sensor based on Sagnac cascade fiber bragg grating (FBG) was proposed and prepared. The performance of the sensor is analyzed by the combination of the finite element method and experiment. The simulation results show that the cross section of PMF is deformed under the pressure along the fast axis and the slow axis and then modulates the refractive index. The experimental results show that by adding FBG structure, the lateral pressure sensitivity of the sensor is 408.7 pm/N, and the linear fit is 98.3%. The lateral pressure sensitivity of the structure without FBG is 281.7 pm/N, and the linear fit is 99.3%. That is to say, the FBG structure increases the sensitivity of the sensor by 45.1%. The experimental results are in full agreement with the theoretical derivation. The optical fiber lateral pressure sensor proposed in this paper has the characteristics of simple fabrication, low cost, and high repeatability. It plays an important role in the field of structural stress monitoring.

Measurement of magnetic field and temperature using single bending-ring cascaded with FBG structure Mach-Zehnder interferometer

This paper proposes and implements a macro-bending fiber M-Z interference sensor for simultaneously measuring magnetic field and temperature. The sensor is fabricated by fiber Bragg grating (FBG) cascaded single bending-ring fiber (SBF) immersed in a magnetic fluid (MF). Different dips are generated by the SBF and FBG, which respond differently to external magnetic fields and temperature changes. Experimental analysis shows that the proposed sensor possesses magnetic field sensitivity of 103.33 pm/mT and temperature sensitivity of 13.08 pm/°C. The suggested sensor's advantages include a small overall footprint, simple manufacture, high sensitivity, and low price.

Enhanced higher-order modulational instability in a parity–time-symmetric fiber Bragg grating system with modified saturable nonlinearity

With the use of cubic, quintic, and septic nonlinearities, we demonstrate the influence of modified nonlinear saturation on modulational instability (MI) in a nonlinear complex parity–time (PT)-symmetric fiber Bragg grating (FBG) structure. Using a modified coupled nonlinear Schrodinger equation and linear stability analysis, we derive a dispersion relation for instability gain spectra in a complicated PT-symmetric system. Our main aim is to examine the MI in non-Kerr nonlinearities with nonlinear saturation in three PT-symmetric regimes: below threshold point, at threshold point (breaking point), and above threshold point. The occurrence of MI is known to be problematic at the PT-symmetry threshold point in a standard FBG structure (A.K. Sharma, 2014). At the same time, MI can exist in the normal group velocity dispersion domain when the modified nonlinear saturation effect is used. With the help of a modified form of saturable nonlinearity, we discovered that MI could exist in all three regimes in a complex PT-symmetric FBG structure. In anomalous group velocity dispersion alone, we found bistability behavior in a PT-symmetric FBG structure with higher-order saturable nonlinearity. In the presence of a modified nonlinear saturation effect and higher-order non-Kerr nonlinearities, we found a novel type of dynamics in the PT-symmetric FBG structure. All alterations in the photonic device bandgap directly result from changes in the refractive index of the medium caused by the interaction of PT-symmetric potential with the cubic–quintic–septic and modified form of nonlinear saturation. As a result, we provide approaches for generating and managing the MI in a complex PT-symmetric FBG structure under the influence of the modified nonlinear saturation effect.

Sensitivity Enhancement of Fiber Bragg Grating Accelerometer Based on Short Grating

We propose and develop a high-sensitivity fiber Bragg grating (FBG) accelerometer based on a flexible hinge for vibration sensing. The proposed method employs a 1-mm FBG that is inscribed pointwise using a femtosecond laser such that the resultant stress concentration at the FBG is improved significantly by reducing the distance between the two bonding points of the FBG structure. Through theoretical calculations and finite element simulations, the relationship between sensitivity and distance was found to be inversely proportional. Using this structure, the sensitivity was enhanced to 804 pm/g at 15 and 30 Hz. The resonant frequency of the proposed accelerometer is 102 Hz, and the working frequency bandwidth is from 10 to 50 Hz. The performance of the accelerometer allows the detection of vibrations in seismic explorations with high reliability.

New Method of Temperature and Strain Decoupling Based on Directivity of Fiber Bragg Grating Sensing

Fiber Bragg Grating (FBG) has been widely used in temperature and strain measurement. Its center wavelength drift is affected by both temperature and strain. The influence of temperature and strain on center wavelength should be decoupled when measuring. In this paper, the sensing characteristics of FBG which pasted at different angles were simulated and analyzed, and it was found that FBG sensing for strain has strong directivity. A dual FBG composite construction based on the directivity of FBG sensing was proposed. Two FBGs were at an Angle of 62°. One FBG was sensitive to both temperature and strain, and the other was only sensitive to temperature. The structure can realize the decoupling of temperature and strain, and it doesn’t depend on feature of cantilever beam. It was verified by experimental analysis that the decoupling result was good by utilizing the combined FBG structure, and decoupling was realized easily.

Wavelength Dependent Odd Frequency Multiplication Based on a Superstructure FBG

In this letter, a photonic technique with wavelength tunable multiplication factor is proposed to generate odd multiple of modulating frequency. It is based on the cascading of two Mach-Zehnder modulators, super structured fiber Bragg grating, and a power combiner involving coherent heterodyne technique. Frequency tripling and quintupling of 10 GHz modulating signal have been demonstrated by tuning the wavelength of laser source and RF power control.The proposed scheme provides a simple method for photonic generation of widely tunable RF frequencies, and it has ability to change the multiplication factor by adjusting the wavelength of the laser source instead of tuning the passband frequency of the optical filter. The even multiplication capability of the proposed system can also be explored by changing the wavelength of the laser source.

Fiber Bragg Grating with Enhanced Cladding Modes Inscribed by Femtosecond Laser and a Phase Mask.

In this paper, we demonstrate a fiber Bragg grating (FBG) with a wide range and a comb with continuous cladding mode resonances inscribed in non-photosensitive single mode fibers using a femtosecond laser and a phase mask. The FBG is inscribed in the core and cladding, exciting a series of cladding modes in transmission. The birefringence induced by this FBG structure offers significant polarization-dependence for cladding modes, thus allowing the vector fiber twist to be perceived. By measuring the peak-to-peak differential intensity of orthogonally polarized cladding mode resonances, the proposed sensor presents totally opposite intensity response in the anticlockwise direction for the torsion angle ranging from −45° to 45°. The cladding mode comb approximately covers wavelengths over the O-, E-, S-, and C-bands in transmission. The cutoff cladding mode of air can be observed in the spectrum. Thus, the sensible refractive index range is estimated to be from 1.00 to 1.44. Temperature responsivity of the grating is also characterized. The proposed device potentially provides new solutions to the various challenges of physical vector and bio-chemical parameters sensing.

Hydrogen sensing performance investigations with optical heating and sensing element surface modification

Fiber optic hydrogen gas sensors with low detection limit and high precision are highly demanding nowadays. Self-referenced fiber Bragg grating (FBG) hydrogen gas sensor offers promising advantages due to its improved systemic stability and compactness. Nevertheless, the low sensitivity and the low stability of hydrogen sensitive materials still greatly limit its detection capability in extremely low concentration range. Here we demonstrate that an improved self-referenced FBG hydrogen gas sensor design can overcome this limitation. It is based on an integrated architecture which combines a FBG structure coated with polyaniline modified WO3–PdPt composite film and an optical heating device using a high-power laser source. Due to the higher working temperature and the protective coating structure of the PdPt catalytic layer, the sensor shows faster response and higher measurement stability, which together results in a significantly low detection limit down to 4.5 ppm in air. These greatly improved sensing hydrogen gas performances highlights its promising applications in future.

Study on the Sensitivity of Bare Fiber Bragg Grating for Ultrasonic Frequencies Response Under Various Temperature

This work presents a study on the sensitivity of bare fiber Bragg grating (FBG) to detect ultrasonic frequencies under various temperature. Two infrared (IR) laser with excitation wavelength, λ=1310nm and λ=1550nm were employed. Various types of FBG with operating wavelength of 1546nm, 1550nm and 1554nm were used to identify the optimum design of sensor in detecting range of ultrasonic frequencies between 5kHz until 30kHz under various surrounding temperature from 20°C until 30°C. The principle of FBG vibration detection lies in the fact that spectral shift would occur due to the acoustic-induced variations in the medium. In this study, the ultrasonic signal had been investigated by monitoring the amplitude of optical output power. At 30°C, the bare FBG with operating wavelength of 1554nm using 1310nm light source exhibits the optimum performance in detecting ultrasonic vibration frequency, in which its sensitivity was obtained as ∆P=0.10dBm. We believe that the sensitivity of the proposed sensor can be enhanced by introduce nanomaterials onto the FBG or by altering the physical structure of FBG.

Artificial neural network for the reduction of birefringence-induced errors in fiber shape sensors based on cladding waveguides gratings.

Cladding waveguide fiber Bragg gratings (FBGs) provide a compact and simple solution for fiber shape sensing. The shape sensing accuracy is limited by birefringence, which is induced by bending and the non-isotropic FBG structure (written by femtosecond laser point-by-point technique). An algorithm based on an artificial neural network for fiber shape sensing is demonstrated, which enables increased accuracy, better robustness, and less time latency. This algorithm shows great potential in the application of high-accuracy real-time fiber shape measurements.

Temperature and refractive index measurement using an optical fiber sensor featuring PCF-FP and FBG inscribed by femtosecond laser

The temperature and refractive index measurement using an optical fiber sensor featuring a Fabry-Perot interferometer (FPI) and fiber Bragg grating (FBG) has been proposed and realized both theoretically and experimentally. The FPI is formed by sequentially cascading a section of single-mode fiber, photonic crystal fiber (PCF), and multi-mode fiber. The FBG is inscribed using the direct point-by-point method of femtosecond laser. The reflection spectrum of the proposed sensor incorporates with both the PCF-FP and FBG characteristics, making it suitable for dual-parameter measurement. FBG peak at 1532.10 nm and PCF-FP interference dip at 1593.25 nm are chosen for following analysis. The temperature experimental results show that in the range of 30˜120℃, the temperature sensitivities are 11.46 pm/℃ for FBG peak and 8.62 pm/℃ for PCF-FP dip, respectively. The refractive index (RI) experimental results show that in the range of 1.3315˜1.3708, the RI sensitivities are 0 for FBG peak and 9.14 dB/RIU for PCF-FP dip, respectively. By analyzing wavelength shifts and parameter responses of the characterized wavelengths, the corresponding sensitivities can be determined. Such combination also avoids the cross-sensitivity problem by utilizing components with different sensing mechanisms. Therefore, the proposed sensor featuring PCF-FP and FBG structure can be efficiently used for temperature and RI measurement with promising application prospect and great research reference value.

2nd-order random lasing in a multimode diode-pumped graded-index fiber

Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950–954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.

Optical fiber sensor by cascading long period fiber grating with FBG for double parameters measurement

An optical fiber sensor for strain and temperature measurement based on long period fiber grating (LPFG) cascaded with fiber Bragg grating (FBG) structure has been proposed and realized both theoretically and experimentally. Theoretical analysis shows that two microstructures with similar sensitivities cannot be used for double parameters measurement. The LPFG is micromachined by the CO2 laser, and the FBG is micromachined by the excimer laser. For the validation and comparison, two FBGs and one LPFG are cascaded with three transmission valleys, namely FBG1 valley at 1 536.3 nm, LPFG valley at 1 551.2 nm, and FBG2 valley at 1 577.3 nm. The temperature and strain characteristics of the proposed sensor are measured at 45—70 °C and 250—500 μe, respectively. The sensitivity matrix is determined by analyzing wavelength shifts and parameter response characterization of three different dips. The proposed optical fiber sensor based on LPFG cascaded with FBG structure can be efficiently used for double parameters measurement with promising application prospect and great research reference value.

Temperature-compensated refractive index sensor based on bent-fiber interference

In this letter, we present a hybrid single-mode fiber (SMF) structure for refractive index (RI) measurement with the temperature compensation capability. The interferometer has simple structure, consisting of a bare fiber semicircular bending region (SBR) and a fiber Bragg grating (FBG). The section of SBR serves as a refractometer with a temperature compensator using a simple FBG. The sensing mechanism of this device has been investigated and experimentally demonstrated. The FBG structure is insensitive towards the change in the ambient RI but is sensitive towards the changes in the ambient temperature whereas the SBR structure is sensitive towards both of RI and temperature. In the RI range of 1.3288–1.3730, the corresponding RI sensitivity for the bare fiber bending region is 160 nm/RIU (refractive index unit) and the RI resolution is about 8.5 × 10−5 RIU.

Pure Directional Bending Measurement With a Fiber Bragg Grating at the Connection Joint of Eccentric-Core and Single-Mode Fibers

A novel fiber Bragg grating (FBG) structure based on an eccentric core fiber (ECF) and a single-mode fiber (SMF) was proposed and experimentally demonstrated for distinguishing the bending effect from the axial strain effect and measuring the pure directional bending. The structure is fabricated by writing a FBG on the fusion splice junction between an ECF and a SMF. The FBG formed on the ECF-SMF junction has two different resonant peaks because of the different refractive indices of the two fibers. The peak relating to the ECF is sensitive to both directional bending and axial strain, while the peak corresponding to the SMF is only sensitive to strain. By using the unique FBG structure, one can directly know whether a bending is accompanied by an axial strain, and can obtain both the direction and curvature of the bending. Experimental results show that the bending sensitivities of the FBG in the ECF part are 49.3 and −50.3 ${\rm{pm}}/{{\rm{m}}^{ - 1}}$ at directions of 0° and 180°, respectively, and the strain sensitivities of the FBG in the ECF and SMF are 0.74 and 0.70 pm/μϵ, respectively.

Dynamic Error Analysis Method for Vibration Shape Reconstruction of Smart FBG Plate Structure

Shape reconstruction of aerospace plate structure is an important issue for safe operation of aerospace vehicles. One way to achieve such reconstruction is by constructing smart fiber Bragg grating (FBG) plate structure with discrete distributed FBG sensor arrays using reconstruction algorithms in which error analysis of reconstruction algorithm is a key link. Considering that traditional error analysis methods can only deal with static data, a new dynamic data error analysis method are proposed based on LMS algorithm for shape reconstruction of smart FBG plate structure. Firstly, smart FBG structure and orthogonal curved network based reconstruction method is introduced. Then, a dynamic error analysis model is proposed for dynamic reconstruction error analysis. Thirdly, the parameter identification is done for the proposed dynamic error analysis model based on least mean square (LMS) algorithm. Finally, an experimental verification platform is constructed and experimental dynamic reconstruction analysis is done. Experimental results show that the dynamic characteristics of the reconstruction performance for plate structure can be obtained accurately based on the proposed dynamic error analysis method. The proposed method can also be used for other data acquisition systems and data processing systems as a general error analysis method.

Simultaneous Measurement of Refractive Index and Temperature Using a Cascaded FBG/Droplet-Like Fiber Structure

A simple and compact sensor based on a cascaded fiber Bragg grating (FBG)/droplet-like fiber structure is proposed for simultaneous measurement of refractive index (RI) and temperature. The sensor can be easily constructed by mechanically bending a section of coating-stripped single-mode fiber at an FBG’s pigtail. In the transmission spectrum, two kinds of dips induced by FBG and droplet-like structure are observed. The central wavelength of the dip caused by droplet-like structure is susceptible to both of surrounding RI and temperature, while that caused by FBG is only dependent on temperature. A sensor with a 9.5-mm diameter of droplet-like structure is fabricated and experimentally studied. The experimental results demonstrate the feasibility of the proposed sensor and show that sensor possesses a high RI sensitivity of 157.8891 nm/RIU for the RI range from 1.3330 to 1.3785 and a temperature sensitivity of 10.3 pm/°C.