Pair Of Fiber Bragg Gratings Research Articles

An optical fiber sensor for the simultaneous measurement of pressure and position based on a pair of fiber Bragg gratings

In this study, an optical fiber sensor for the simultaneous measurement of pressure and position based on a pair of fiber Bragg gratings (FBG) in a clamped beam is proposed. The FBG pair are pasted on a one-meter-long beam, and the pasted position is at 20 cm and 80 cm. The pressure and the position information applied at any point of the beam could be obtained through the wavelength shift of the FBG pair. The experimental results show that in the pressure range of 0 to 40 N, the maximum pressure sensitivity of the sensor is 54 pm/N. In the position range of 20 to 80 cm, the minimum error of position detection is 0.38 cm. The proposed sensor has the advantages of good stability, high sensitivity, simple manufacturing process, and low cost.

High dynamic range distributed acoustic sensing based on dual-wavelength fiber Bragg grating pairs.

A high dynamic range distributed acoustic sensing (DAS) system is demonstrated based on a dual-wavelength (2λ) interferometric acoustic sensor array, constructed using 2λ fiber Bragg grating pairs (FBGPs). Each FBGP is composed of two weak FBGs presenting -30-dB reflectivity and center wavelengths of 1535 nm and 1565 nm. With the phase demodulation results from the two wavelengths, 2λ linear regression phase unwrapping is implemented, which significantly enhances the dynamic range of the DAS system. By examining the four time-division-multiplexed sensors, an average background noise level of -105.2dBrad/Hz at 1 kHz and a cross talk level lower than -70dB at 1 kHz are achieved.

Analysis and experiment on simultaneous measurement of strain and temperature by etched and un-etched FBG pair

This paper presents analysis and experiment on simultaneous measurement of strain and temperature using pair of etched and un-etched fiber Bragg grating (FBG) fabricated in a single mode glass optical fiber. The intrinsic sensitivity for strain and temperature of FBG explicitly depends on the clad diameter. The mechanical properties of glass optical fiber such as Young’s modulus and Poisson’s ratio change with cladding diameter. This in turn will change the strain optic coefficient and thermal expansion coefficient of the optical fiber. Analytically, it is shown that the change in refractive index, Young’s modulus, Poisson’s ratio and thermal expansion coefficient was 0.04, 58.2 GPa, 0.29 and 0.99 × 10−6/°C respectively as the cladding diameter was decreased from 125 µm to 11 µm. The results obtained for strain and temperature sensitivity from the analysis are verified with experimental results. Experimentally, as the fiber clad diameter is reduced from 125 µm to 25 µm via hydrofluoric (HF) etching, the average strain sensitivity increases from 1.06 pm/µɛ to 1.20 pm/µɛ for the maximum applied strain of around 5500 µɛ and also the average temperature sensitivity increases from 11.48 pm/°C to 11.80 pm/°C for the maximum applied temperature of 150 °C. The temperature and strain can be measured simultaneously with high accuracy by using these sensitivity coefficients for two gratings.

Co-located angularly offset fiber Bragg grating pair for temperature-compensated unambiguous 3D shape sensing.

A 10 mm-long three-dimensional shape sensor in a single-mode fiber is described and demonstrated experimentally. The sensor is based on a pair of fiber Bragg gratings inscribed at the same location along the fiber axis but offset along different radial directions away from the fiber center. Each offset grating generates cladding mode resonances over a ${\sim}{20}\;{\rm{nm}}$-wide spectral bandwidth, and the two gratings are also offset in period so that their transmission spectra are separated by 40 nm, and thus non-overlapping and fully distinguishable. Directional bending sensitivity results from the differential amplitude response of the cladding mode resonances from the two gratings, depending on the relative orientation of the bend with the azimuthal direction of the grating offsets. It is further demonstrated that both axial deformation and temperature have no influence on the shape measurement as they both only cause a global wavelength shift of the spectra without amplitude change. The experimental results demonstrate that the shape orientation of an object can be unambiguously determined for bend directions covering the full 360° range around the fiber axis with sensitivities of the order of ${{1}}\;{\rm{dB/}}{{\rm{m}}^{- 1}}$ and small curvatures between 0 and ${{1}}\;{{\rm{m}}^{- 1}}$.

Fiber Bragg gratings fabricated by femtosecond lasers for narrow-linewidth fiber laser

A narrow-linewidth erbium-doped fiber laser based on the fiber Bragg gratings (FBGs) fabricated was proposed and demonstrated experimentally. A pair of FBGs with reflectivity of 92 % and 10 %, respectively, at the center wavelength of 1580 nm were inscribed in standard SMF-28 fibers without hydrogen loading by femtosecond laser point-by-point directly writing method, which was used as mirrors for wavelength selection of fiber laser, gratings full width at half maximum (FWHM) was less than 0.15 nm. A simple linear-cavity fiber laser was realized based on gain medium, pump source and FBGs. When gain medium of Er-doped fiber is selected 8 m, a stable single wavelength of 1579 nm laser with a 3 dB linewidth of 0.06 nm was obtained. Its output power is 24.1 mW when the maximum pump power is 600 mA, and its single wavelength of 1579 nm is consistent with the central wavelength of the fabricated FBGs. This type of laser source shows real promise for applications in coherent optical frequency domain reflectometry and high resolution fiber sensor systems.

A fiber Bragg gratings pair embedded in a polyurethane diaphragm: Towards a temperature-insensitive pressure sensor

We report the development of a temperature-insensitive pressure sensor based on a pair of fiber Bragg gratings (FBGs) embedded in a polyurethane diaphragm. The pressure is transversely applied, causing a bending on the diaphragm. In such condition, the FBGs are positioned in opposite directions considering the diaphragm bending’s neutral line as a reference. Thus, when the pressure is applied on the diaphragm, the wavelength shift occurs in opposite directions, i.e., one FBG has a blue shift and the other, red shift. The wavelength shift difference between both FBGs is analyzed, leading to a system with higher sensitivity when compared with the individual responses of each FBG. In addition, if the temperature sensitivities of each FBG are compensated, there is no difference in the wavelength shift (considering the difference between both FBGs) when the system is subjected only to temperature variations. However, if the system suffers simultaneous variation of temperature and pressure, there is a variation in the pressure sensitivity due to changes in the elastic modulus of the diaphragm caused by the temperature variations. For this reason, a compensation approach is proposed using the wavelength shift of each FBG as well as their difference. The sensor was tested for temperature, pressure and moisture absorption, where the maximum wavelength shift for the moisture absorption was 4 pm. For temperature and pressure responses, the sensor presented high sensitivity and linearity for all analyzed cases, leading to a pressure resolution of 1.75 Pa for constant temperature conditions. In pressure cycles under different temperatures, the compensation technique showed the feasibility of applying the proposed sensor in practical applications, where temperature variations as high as 30 °C were tested. The sensor presented a temperature cross-sensitivity of 0.33 Pa/°C.

High sensitivity optical fiber pressure sensor based on thin-walled oval cylinder

In this study, an optical fiber pressure sensor based on a thin-walled oval cylinder is proposed. To sense the degree of deformation, a pair of fiber Bragg gratings (FBGs) are fixed on the outer wall of the oval cylinder. A finite element analysis is performed to analyze the sensing characteristic of the sensor. The temperature cross-sensitivity is determined using an elimination method. The experiment results show that the pressure sensitivity is 1.198 nm/MPa within a range of 0–1 MPa, with a precision of 0.011 MPa. The proposed sensor combines the advantages of a thin-walled cylinder and a bourdon tube, which realizes pressure measurement with high sensitivity and low crosstalk. The sensor can be applied to specific urban pipeline pressure monitoring.

High-energy all-fiber gain-switched thulium-doped fiber laser for volumetric photoacoustic imaging of lipids

We demonstrate a high-energy all-fiber short wavelength gain-switched thulium-doped fiber laser for volumetric photoacoustic (PA) imaging of lipids. The laser cavity is constructed by embedding a short piece of gain fiber between a pair of fiber Bragg gratings (FBGs). Through using three pairs of FBGs with operation wavelengths at 1700, 1725, and 1750 nm, three similar lasers are realized with a cavity length of around 25 cm. Under a maximum pump energy of 300 μJ at 1560 nm, laser pulse energies of 58.2, 66.8, and 75.3 μJ are, respectively, achieved with a minimum pulse width of < 16.7 ns at a repetition rate of 10 kHz. Volumetric imaging of lipids is validated through scanning a fat beef slice with a PA microscopy system incorporated with the newly developed source, and a lateral resolution of 18.8 μm and an axial resolution of 172.9 μm are achieved. Moreover, the higher shooting speed of the developed source can potentially allow for increasing at twice the frame rate of current intravascular PA imaging.

530 W all-fiber continuous-wave Tm-doped fiber laser

In the past decades, thulium-doped fiber lasers (TDFLs) operating in an eye-safe range have attracted considerable attention, for they have extensive applications such as LIDAR, free space communication, medical diagnostics and pumping source for holmium-doped fiber laser or optical parameter oscillator. In this paper, we report a high power all-fiberized TDFL based on main-oscillator power-amplifier (MOPA) configuration. The preform is fabricated by the modified chemical vapor deposition method combined with solution doping technique and drawn into a core/clad size of 25/400 μm. The numerical aperture of the TDF is 0.1. The concentration of Tm<sub>2</sub>O<sub>3</sub> and Al<sub>2</sub>O<sub>3</sub> are 2.6 wt% and 1.01 wt%, respectively, measured by an electron probe micro-analyzer. The cladding absorption is 3 dB/m at 793 nm measured by cut-back method. The oscillator consists of 8 m 25/400 TDF mentioned above and a pair of fiber Bragg gratings. The oscillator yields maximum power of 91 W with pump power of 202 W and a 3 dB spectral bandwidth as narrow as 75 pm. In the amplifier stage, the bi-directional pumping scheme is employed. The narrow linewidth seed with output power of 57 W is scaled to 530 W through one-stage amplification, corresponding to a slope efficiency of 50%. The central wavelength of the Tm-doped MOPA is 1980.89 nm and the linewidth is broadened to 0.11 nm at 530 W. The measured <i>M</i> <sup>2</sup> factor at 100 W is less than 1.3. Neither obvious amplified spontaneous emission nor non-linear effect is observed, and the output power is only limited by pump power. To the best of our knowledge, this is the highest output power of TDF at present.

Symmetric Step-Apodized Distributed Feedback Fiber Laser With Improved Efficiency

We propose and demonstrate a symmetric step-apodized distributed feedback (DFB) fiber laser, which has an improved efficiency and a narrow linewidth. This special DFB laser cavity is based on a unique symmetric step-apodized π-phase-shifted fiber Bragg grating (FBG) directly inscribed in a heavily erbium-doped fiber. The symmetric step-apodized phase-shifted grating is equivalent to a weak uniform phase-shifted FBG embedded in a pair of strong FBGs, and this design could increase the effective cavity length of a DFB fiber laser. Moreover, the symmetric step-apodized DFB laser cavity proposed in this work has a phase shift located in the cavity center. It could simplify the design process significantly due to the symmetry in the laser cavity. Experimental results show that the slope efficiency of a symmetric step-apodized DFB fiber laser could be increased to 1.06%, which is significantly higher than that of a uniform DFB fiber laser (i.e., 0.45%) under the same cavity length and pump conditions. In addition, the proposed DFB fiber laser exhibits a high stability. The fluctuations in lasing wavelength and output power were less than 12 pm (corresponding to a frequency shift of 1.49 GHz) and 0.13 dB within 24 h, respectively. Moreover, the full-width at half-maximum linewidth of the symmetric step-apodized DFB fiber laser was ∼2.6 kHz, measured by the delayed self-heterodyne method with a 50-km fiber delayed line. As such, the proposed symmetric step-apodized DFB fiber laser could potentially be used as high-performance light source for fiber-optic sensors or coherent optical communication systems.

Switchable Dual-Wavelength Mode-Locked Fiber Laser Source for In-PCF Parametric Frequency Conversion Applied to CARS Microscopy

A self-started, switchable dual-wavelength all-polarization-maintaining mode-locked (ML) fiber laser for optical parametric generation (OPG) in photonic crystal fiber (PCF) applied to coherent anti-stokes Raman scattering (CARS) microscopy is reported in this paper. The complete laser setup is built in an all-fiber structure composed of commercial elements. It is based on a linear cavity with an active medium—namely, Ytterbium-doped fiber (YDF), a pair of fiber Bragg gratings as switchable partial mirrors between 1031.5 and 1049.7 nm, and a semiconductor saturable absorber mirror functioning also as the mode-locking device. The seed laser is amplified to 650 mW average power by two YDF amplifiers. The source delivers a pulse-train output of 30.9 ps at 1031.5 ps and 31.6 ps at 1049.7, a 3.1-MHz repetition rate, and over 200 nJ pulse energy and 6 kW of peak power at fundamental ML operation. The properties of this switchable dual-wavelength source ensures the correct OPG by degenerated four-wave mixing in PCF fulfilling the phase matching condition which corresponds to CARS resonance of 2850 $ {\mathbf{c}}{ {\mathbf{m}}^{ - 1}}$ (CH3-O molecular bonds) at 1031.5 nm and 1475 $ {\mathbf{c}}{ {\mathbf{m}}^{ - 1}}$ (CH2 δ molecular bonds) at 1049.7 nm.

An Efficient 4-kW Level Random Fiber Laser Based on a Tandem-Pumping Scheme

In this letter, we presented a monolithic high-power random fiber laser (RFL) with a tandem-pumping scheme. The RFL seed centered at 1064 nm was pumped by a 1018-nm fiber laser and reached an output power of 8.66 W. After two amplifiers, the seed was amplified to 4020 W, and the 3-dB width of laser spectrum was broadened from 0.78 nm to 0.99 nm. To increase the efficiency of the 4-kW laser, a specially designed high absorption coefficient (0.6 dB/m @ 1018 nm) 30/250- $\mu \text{m}$ Yb-doped fiber and high-brightness 1018-nm pump laser were adopted. The optical-to-optical efficiency could reach as high as 88.5%. No obvious mode instability was found during the experiment. Furthermore, with the same amplifier stage, we have also built another 4-kW level fiber laser seeded by an oscillator consisting of a pair of fiber Bragg gratings. The comparison of the experimental results showed that the high-power laser based on the RFL seed would have a better control on spectral broadening.

Analysis of mode characteristics of fiber Bragg gratings in large-mode-area double-clad fibers

Fiber Bragg gratings (FBGs) in large-mode-area double-clad fibers (LMA-DCFBGs) are the key components for high-power all-fiber lasers. Mode characteristics analysis of LMA-DCFBGs is of great significance for mode selection and performance optimization for high-power fiber lasers. In this paper, the spectral characteristics of LMA-DCFBGs were studied theoretically and experimentally, and confirmed that there is a power conversion between the LP01 mode and the LP11 mode in LMA-DCFBGs. Then, the growth curves of coupling coefficients of LMA-DCFBG with an increasing irradiation dose of ultraviolet (UV) light were measured. The results demonstrated that coupling efficiency between the LP01 mode and the LP11 mode is independent of the polarization state of UV lasers. In high-power fiber lasers, according to the spectral characteristics of LMA-DCFBGs, it can be considered that mode selection of fiber lasers could be realized by a pair of FBGs inscribed in fibers with different structural parameters, and that the appropriate wavelength matching condition could effectively avoid the laser power conversion from the LP01 mode to the LP11 mode, which could improve the beam quality of the output laser.

Fiber anemometer for simultaneously measuring wind speed and wind direction

AbstractIn this paper, we proposed a novel anemometer for simultaneously and conveniently monitoring wind speed and wind direction by using a pair of fiber Bragg gratings (FBGs). The anemometer comprises a plastic ball, a stainless steel rod, and two FBGs. The gratings are glued to the surface of the stainless steel rod with a location separation of 90°. In the operation mechanism, the bend in the stainless‐steel‐rod caused by the wind force drives the FBGs, thereby leading to a grating wavelength shift for the measurement both of wind speed and wind direction This fiber anemometer is experimentally demonstrated in real time with a sensitivity of 0.02 nm/(m/s) and a resolution of 0.49 m/s for wind speed measurement, with a wind speed error of less than 2.38%, and a wind direction angle error of approximately 0.69%.

Micro-Cavity Array With High Accuracy for Fully Distributed Optical Fiber Sensing

A micro-cavity array (MCA) fiber based on a dense ultra-short fiber Bragg grating (FBG) array is proposed for high-accuracy fully distributed sensing. In the MCA fiber, each pair of adjacent FBGs forms a micro-cavity, which acts as the sensing element. The high finesse interference spectra of the micro-cavities facilitate high demodulation accuracy. Large-scale multiplexing of 8557 FBGs results in 8556 micro-cavities in a 7.18-m MCA fiber. Combined with optical frequency-domain reflectometry, fully distributed temperature sensing with a spatial resolution less than 1 mm and measurement precision of 0.157 °C is demonstrated.

Acceleration of Carrier Lifetime in Gain-Clamped Semiconductor Optical Amplifiers

A new method to accelerate the carrier lifetime in semiconductor optical amplifiers (SOAs) is experimentally demonstrated. The four-wave mixing (FWM) effect is induced without recourse to external optical light sources. A pair of fiber Bragg gratings is utilized to generate pre-spectral sliced seed light channels as FWM inputs, by reflecting the backward amplified spontaneous emission from the SOA in a gain-clamped configuration. The theory predicts a three times shorter carrier lifetime, compared to coherent inputs in a travelling-wave arrangement. Furthermore, this method can be a more accurate measure of the nonlinear parameters of SOAs rather than conventional techniques, owing to the polarization-independent nature of the corresponding incoherent products.

A Submerged Optical Fiber Ultrasonic Sensor Using Matched Fiber Bragg Gratings.

A novel kind of fiber optic ultrasonic sensor based on matching fiber Bragg gratings (FBGs) is proposed and demonstrated. The sensors consist of a pair of matching FBGs fixed to a special bracket. The bracket plays a role in stretching and squeezing the FBGs, with the push–pull effect efficiently coupling the ultrasonic signal to the sensor, thus, improving the sensor’s sensitivity. Side-band filtering technology-based intensity interrogation was used to detect ultrasounds in water. With the synergic effect of the matching FBGs, the sensor performed with a high signal-to-noise ratio (56.9 dB at 300 KHz, 53 dB at 1 MHz and 31.8 dB at 5 MHz) and the observed ultrasonic sinusoidal signals were undistorted and distinguishable in the time domain.

Investigation on extreme frequency shift in silica fiber-based high-power Raman fiber laser

In this paper, we experimentally investigated the extreme frequency shift in high-power Raman fiber laser (RFL). The RFL was developed by using a pair of fiber Bragg gratings with fixed and matched central wavelength (1120 nm) combined with a piece of 31-m-long polarization maintaining (PM) passive fiber adopted as Raman gain medium. The pump source was a homemade high-power, linearly polarized (LP) wavelength-tunable master oscillator power amplifier (MOPA) source with ${\sim}25~\text{nm}$ tunable working range (1055–1080 nm). High-power and high-efficiency RFL with extreme frequency shift between the pump and Stokes light was explored. It is found that frequency shift located within 10.6 THz and 15.2 THz can ensure efficient Raman lasing, where the conversion efficiency is more than 95% of the maximal value, 71.3%. In addition, a maximum output power of 147.1 W was obtained with an optical efficiency of 71.3%, which is the highest power ever reported in LP RFLs to the best of our knowledge.

Temperature-Insensitive Contact Force Sensing in Bi-Directional Catheter Using Fiber Bragg Grating Pair

Bi-directional catheters are commonly used in electrophysiology mapping and ablation procedures. We have designed and demonstrated a temperature-insensitive contact force sensor for bi-directional catheters with variable curve diameter during deflection. The compact force sensor is based on a fiber Bragg grating (FBG) pair, where both FBGs have the same center wavelength and are fixed at two opposite deflection arcs of the bi-directional catheter. When the catheter is deflected or when contact force is applied to the catheter, bifurcation of the FBG pair spectrum is observed. The amount of spectral bifurcation and the applied contact force has a linear relationship. Contact force measurement, therefore, is accomplished by measuring the change in the spectral bifurcation of the FBG pair, i.e., the change in the Bragg wavelength difference between the two FBGs. Since the approach relies solely on the amount of spectral bifurcation of the FBG pair instead of the Bragg wavelength shift of one single FBG, the proposed contact force sensor is temperature insensitive.

Broadband Vibration Detection in Tissue Phantoms Using a Fiber Fabry-Perot Cavity.

A fiber optic vibration sensor is developed and characterized with an ultrawide dynamic sensing range, from less than 1 Hz to clinical ultrasound frequencies near 6 MHz. The vibration sensor consists of a matched pair of fiber Bragg gratings coupled to a custom-built signal processing circuit. The wavelength of a laser diode is locked to one of the many cavity resonances using the Pound-Drever-Hall scheme. A calibrated piezoelectric vibration element was used to characterize the sensor's strain, temperature, and noise responses. To demonstrate its sensing capability, an ultrasound phantom with built-in low frequency vibration actuation was constructed. The fiber optic senor was shown to simultaneously capture the low frequency vibration and the clinical ultrasound transmission waveforms with nanostrain sensitivity. This miniaturized and sensitive vibration sensor can provide comprehensive information regarding strain response and the resultant ultrasound waveforms.