Long-period Fiber Gratings Research Articles

FBG-LPFG-Based Sensor to Monitor 3-D Strain in Ice During Freezing–Melting Processes

Three-dimensional strain (3D strain) and the principal strain in ice are very important for glacier movement research and glacier-related disaster prediction. The internal 3D strain of ice is sensitive to temperature, and it is very complex during the whole freezing and melting process including phase transformation. In this study, we present a 3D strain sensor (3D-S Sensor) based on fiber Bragg grating (FBG) and long period fiber grating (LPFG) that can measure the 3D strain in ice. The LPFG (the temperature sensitivity coefficient is 56.3 pm/°C) was fabricated by the laser etching method, which was used for the temperature compensation of FBGs to improve the accuracy of strain measurement. The 3D principal strain can be calculated by the value of 6 direction strains from FBGs. A freeze-melt experiment is designed to verify the performance of 3D-S Sensor. The experimental results show that the sensor can measure the complex 3D deformation of ice from a more precise angle. Among the 6 directions of strain measured by the sensor, the strain value in the Z direction is the largest (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>z</i></sub> = 6232.9 με). The first principal strain in ice is close to the negative direction of the Z axis (θ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>Z</i></sub> = 160.7°), and the maximum value is 6638.2 με. The measured results are consistent with the actual deformation of ice, indicating that the sensor can effectively measure the 3D strain and principal strain inside the ice.

A Seawater Salinity Sensor Based on Optimized Long Period Fiber Grating in the Dispersion Turning Point.

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‱ can be achieved when the salinity increases from 5.001‱ to 39.996‱, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing.

Ultrasensitive magnetic field sensor based on cladding-etched long-period grating.

We demonstrate a high-sensitivity fiber-optic magnetic field sensor, which consists of a cladding-etched long-period fiber grating (LPFG) near the dispersion turning point (DTP) integrated with a magnetic fluid (MF). By reducing the cladding diameter of the LPFG, the fundamental mode is coupled to the lowest order cladding mode (LP0,2) near the DTP, which has a much higher surrounding refractive index sensitivity. Thanks to the excellent magneto-optical characteristics of the MF, the proposed sensor can achieve a magnetic field intensity sensitivity of 44.69 nm/mT in the range of 3-7.4 mT. The minimum magnetic field intensity that can be detected is 0.45 µT due to the 0.02-nm wavelength resolution of the optical spectrum analyzer. The proposed etched DTP-LPFG-based sensor with ultrahigh magnetic field sensitivity could have potential applications in magnetic fields and electrical systems.

Sub-kHz high-order mode Brillouin random fiber laser based on long-period fiber grating and distributed Rayleigh scattering in a half-open linear cavity.

We demonstrate a narrow-linewidth high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on a long-period fiber grating (LPFG) and distributed Rayleigh random feedback in a half-open linear cavity. The single-mode operation of the laser radiation with sub-kilohertz linewidth is achieved thanks to distributed Brillouin amplification and Rayleigh scattering along kilometer-long single mode fibers whilst a few mode fiber-based LPFGs enable the transverse mode conversion among a broadband wavelength range. Meanwhile, a dynamic fiber grating (DFG) is embedded and incorporated to manipulate and purify the random modes, which hence suppresses the frequency drift resulting from random mode hopping. Consequently, the random laser emission with either high-order scalar or vector modes can be generated with a high laser efficiency of 25.5% and an ultra-narrow 3-dB linewidth of 230 Hz. Furthermore, the dependence of the laser efficiency and frequency stability on the gain fiber length are also experimentally investigated. It is believed that our approach could provide a promising platform for a wide range of applications such as coherent optical communication, high-resolution imaging, highly sensitive sensing, etc.

Bacterial Detection and Differentiation of Staphylococcus aureus and Escherichia coli Utilizing Long-Period Fiber Gratings Functionalized with Nanoporous Coated Structures

A biosensor utilizing long-period fiber gratings (LPFG) functionalized with nanoporous coated structures was developed for the rapid detection of Staphylococcus aureus (S. aureus) bacteria. The nanoporous structure coatings on the LPFG surface facilitated specific adhesion and interaction with S. aureus, resulting in an instantaneous shift in the resonance wavelength (RW) in the transmission spectrum of the LPFG. The LPFG with nanoporous polyelectrolyte coatings exhibited an approximately seven-fold RW shift compared to the bare LPFG under the optimal experiment conditions. By tracking the RW shifts, we were able to monitor the real-time S. aureus adhesion to study the interaction process in detail. The bacterial differentiation and S. aureus specificity of the method was confirmed through a series of studies using Escherichia coli (E. coli). This nanoporous structure-enabled LPFG-based biosensor scheme holds significant promise for rapid, reliable, and low-cost detection of S. aureus for biomedical applications.

High-Order OAM Mode Generator Using Multi-Cascaded Long-Period Fiber Gratings

We propose and demonstrate the inscription of multi-cascaded long-period fiber gratings (LPFGs) in a few-mode fiber using a CO2-laser. The multi-cascaded LPFGs containing three gratings were fabricated successfully. The first LPFG was used to realize the mode conversion from LP01 mode to LP11 mode, the second one was used to convert LP11 mode to LP21 mode, and the third one achieved the higher-order mode conversion from LP21 mode to LP31 mode. The second-order and third-order orbital angular momentum (OAM) modes can be generated using the cascaded LPFGs with the second and third gratings, respectively. The multi-cascaded LPFGs have the promising potential to be adopted as mode converters and mode scramblers in the mode division multiplexing system thanks to the flexible mode conversion among different higher-order modes.

A Highly Sensitive Curvature Sensor Based on Omega-Shaped Long-Period Fiber Grating

A compact Omega shaped long-period fiber grating (OS-LPFG) for vector bending measurement is proposed and investigated in this paper. This sensor is fabricated by periodically splicing single-mode fiber (SMF) and multi-mode fiber (MMF) into Omega shape to get the purpose of sensitivity enhancing. The proposed OS-LPFG can distinguish the bending direction and achieves the curvature sensitivities in X, Y, -X directions are 12.03 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , –69.95 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and –19.84 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the range of 0 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 1.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Meanwhile, the sensor has a small temperature crosstalk of 0.0011 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> /°C at the temperature range of 20 °C to 110 °C. In addition, the sensor features a compact structure with the length of 1.5 mm due to the strong refractive index modulation induced by fabricating method, which proposes a feasible idea for compact fiber curvature sensor design.

Helical Long-Period Fiber Grating-Based OAM Interferometer and Its Application to Fiber Sensing

Orbital-angular-momentum (OAM) beams-based interferometers have recently been attracted significant interest and found various applications in the field of optical precision measurement. Whereas all-fiber OAM interferometers featured with the superior characteristics of the compact size, low cost, extremely-low insertion-loss, and the inherent compatibility with other fiber devices, have less been studied and especially demonstrated in experiment. In this study, we proposed and experimentally demonstrated an all-fiber OAM beams-based interferometer, where the two conjugated OAM beams generated by helical long-period fiber gratings (HLPGs) but with opposite helicities, were particularly utilized as the tested and referenced beams, respectively. The resulted interferograms are of the polarization independence, robust, and particularly suitable for phase-demodulation in azimuthal angle domain. As application examples of the proposed all-fiber OAM beams-based interferometer, changes in temperature ranging from 30 °C to 60 °C and changes in the strain ranging from 0 to 100 μϵ were measured, respectively. The obtained maximum sensitivity in temperature and strain are 26.585 rad/°C and 2.130 rad/μϵ, respectively. The accuracies for temperature and strain measurements are estimated to be 0.0016 °C and 0.0195 μϵ, respectively, which are about 2–3 order higher than those obtained from the conventional optical fiber-based sensors. It is believed that the proposed all-fiber OAM beams-based interferometer would find potential applications to ultra-high precision and sensing measurements.

108.96 nm Broadband Generation of Second-Order OAM Beam Using an Angular Modulated Cascading LPFG

Broadband generation of orbital angular momentum (OAM) beam in fiber has a lot of applications in optical communications and fiber sensing. However, the efficient broadband generation of second-order OAM beam using long-period fiber grating (LPFG) is still a great challenge due to the low intermodal overlap. In this paper, we achieved a 108.96 nm efficient broadband generation of second-order OAM beams with a conversion efficiency over 95% in experiment using an angular modulated cascading LPFG with low insertion loss. By changing the refractive index modulation area in certain angle, not only the coupling between fundamental mode and second-order mode in fiber can be greatly enhanced, but also second-order even and odd modes are generated with approximately equal magnitudes, which is very beneficial to high-purity OAM beams generation. Finally, by choosing proper cascading parameters and an adaptive refractive index modulation fabrication strategy, we achieved a LPFG mode converter with a conversion efficiency over 95% in a 108.96 nm from 1520.96 nm to 1629.92 nm. The insertion loss of the LPFG is measured only ∼0.48 dB.

Strain-Insensitive Simultaneous Measurements of Torsion and Temperature Using Long-Period Fiber Grating Inscribed on Twisted Double-Clad Fiber

Here, we report an optical fiber sensor capable of performing strain-insensitive simultaneous measurements of torsion and temperature using a long-period fiber grating (LPFG) inscribed on twisted double-clad fiber (DCF) with a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. The LPFG written on twisted DCF, referred to as TDC-LPFG, exhibited multiple resonance dips with different cladding-mode orders, and two of them were selected as sensor indicators for the simultaneous measurement of torsion and temperature. The measured torsion sensitivities of the sensor indicators were approximately –2.167×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and –4.667×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pm/(rad/m) in a twist angle range of –180° to 180°, and their temperature sensitivities were measured as ~6.580×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> and ~7.189×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> pm/°C in a temperature range of 25–100 °C. Their linear and independent responses to torsion and temperature made possible the discrimination of the two measurands. Moreover, the strain sensitivity of the TDC-LPFG was measured and found to be less than 2.750×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> pm/με in magnitude, over four times less than that of a standard LPFG. This strain insensitiveness contributes strongly to diminishing strain-induced measurement uncertainties associated with torsion and temperature. Finally, we validated the simultaneous measurement capability of twenty-five applied points of torsion and temperature variations. The average torsion and temperature standard deviations were ~3.44×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> rad/m and ~3.01×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> °C, respectively.

Long-Period Fiber Grating Probe: An Improved Design Suitable for Biosensing

Add-layer sensitivity of long-period fiber grating (LPFG) near mode transition (MT) has been studied in reflection configuration to realize a highly sensitive LPFG-based sensor probe suitable for biosensing. The dependency of the sensitivity on the separation between the resonant cladding modes considered during MT has also been investigated. The separation between two consecutive cladding modes was precisely increased or stretched to a desired value by reducing the cladding diameter. The probe was formed by cleaving the mode stretched LPFG (MSLPFG) through the grating region. The MSLPFG was designed to operate near MT by deposition of electrostatic self-assembled (ESA) polymeric overlay layers on the grating surface. We demonstrate that the polymeric overlay not only increases the sensitivity of the sensor but also eliminates undesirable multiple resonant bands that appear due to the introduction of arbitrary phase while cleaving the grating. Add-layer sensitivity of the MSLPFG sensor probe was obtained and found to be ~2.0 nmWL/nmTH with a significantly enhanced peak attenuation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim -30$ </tex-math></inline-formula> dB around the MT region.

OAM mode generation in helically twisted hollow-core antiresonant fiber.

We investigate the orbital angular momentum (OAM) mode-generating process of a long-period onefold chiral fiber grating (L-1-CFG) based on a helically twisted hollow-core antiresonant fiber (HC-ARF). Taking the right-handed L-1-CFG for an example, we theoretically and experimentally prove that the first-order OAM+1 mode can be generated by only inputting a Gaussian beam. We fabricated three right-handed L-1-CFG samples based on the helically twisted HC-ARF with twist rates (α) of -0.42, -0.50, and -0.60 rad/mm, where the twisted HC-ARF with α of -0.42 rad/mm can achieve high OAM+1 mode purity of 94%. Subsequently, we present simulated and experimental transmission spectra at the C-band, and sufficient modulation depths were obtained at wavelengths of 1550 nm and 1561.5 nm in the experiment.

Fabrication of High-Sensitivity Optical Fiber Sensor by an Improved Arc-Discharge Heating System.

We proposed a high-sensitivity optical fiber sensor based on a dual-resonance helical long-period fiber grating (HLPG). The grating is fabricated in a single-mode fiber (SMF) by using an improved arc-discharge heating system. The transmission spectra and the dual-resonance characteristics of the SMF-HLPG near the dispersion turning point (DTP) were studied through simulation. In the experiment, a four-electrode arc-discharge heating system was developed. The system can keep the surface temperature of optical fiber relatively constant during the grating preparation process, which shows an advantage in preparing high-quality triple- and single-helix HLPGs. In particular, benefiting from this manufacturing system, the SMF-HLPG operating near the DTP was successfully prepared directly by arc-discharge technology, without secondary processing of the grating. As a typical application example of the proposed SMF-HLPG, physical parameters such as temperature, torsion, curvature and strain can be measured with high sensitivity by monitoring the variation of the wavelength separation in the transmission spectrum. Therefore, the proposed sensor and its fabrication technology have potential application prospects in practical sensing measurement.

Radiation Sensitive Long-Period Fiber Grating Based on Tb-Doped Silica Fiber

We demonstrate a radiation sensitive long-period fiber grating (LPG) based on terbium (Tb)-doped silica fiber, fabricated by dual-sided CO2 laser exposure. At a total dose of 6.0 ± 0.2 kGy of gamma radiation, the maximum radiation-induced resonance wavelength shift (RIRWS) of LPG in Tb-doped silica fiber is up to 3.98 nm, which is much higher than that of 1.14 nm in standard single-mode fiber. To characterize the degree of modification of fibers by radiation, we calculate the radiation-induced refractive index change (RIRIC), which is as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.05\times 10^{-5}$ </tex-math></inline-formula> for Tb-doped silica fiber, approximately 276.3% of that of standard single-mode fiber. In addition, the growth of RIRWS with increasing cladding mode order is analyzed, and the theoretical and experimental RIRWS of LPG are compared. This new type of LPG has great promise in the field of high-dose radiation sensing, such as dose monitoring in nuclear reactors, particle collision experiments, and so on.

Exoskeleton type long-period fiber grating for cross-talk-resistance single-parameter measurement.

This paper proposes a new, to the best of our knowledge, design framework of long-period fiber grating (LPFG) sensors resistant to multi-parameter cross talk. A section of hollow quartz capillary (HQC), which acts as an exoskeleton, is periodically merged with a single-mode fiber (SMF) by the arc-discharge method. The mechanical stress in the SMF is released while the thermal stress is enhanced after a high-temperature fusion process. Under the influence of the elastic-optical effect, the refractive index of the core is periodically modulated along the axial direction to form an exoskeleton long-period fiber grating (Es-LPFG). The unique exoskeleton structure not only induces mode coupling but also enables the proposed device to resist cross talk among the strain, ambient refractive index, and vector bending. The temperature is able to be measured independently with a sensitivity of 74 pm/ ∘C. The novel Es-LPFG is promising in single-parameter sensing, mode-locked lasers, and frequency-locked gain flattening.

High power vortex random lasing in an all-fiber structure

A vortex beam has broad development prospects in free space optical communication, optical tweezers, super-resolution imaging due to its unique helical phase distribution and orbital angular momentum. Traditional free-space based methods of generating vortex beam involve complex bulky structures and it is extremely difficult to acquire high power output of vortex beam. In this paper, a mode converter based on a long period fiber grating (LPFG) is used to excite the vortex beam. By further combining the LPFG and the structure of random fiber laser (RFL), we demonstrate an all-fiber structure vortex RFL. Our vortex RFL presents the typical intensity distribution of doughnut shape of vortex beam. The maximum output power of the vortex RFL reaches 3.77 W benefiting from the all-fiber structure, which also improves the integration of the laser system markedly. In addition, we measure the output properties of the vortex RFL, which show excellent temporal stability, modeless output and low relative intensity fluctuation. This work may not only offer a potential scheme for producing high power vortex beams, but also promote the application prospect in free space optical communication, lidar, biological imaging and remote sensing.

Fabrication and Sensing Characteristics of Long-Period Fiber Grating in Capillary Fiber

We propose a novel fiber device based on arc-induced long-period fiber gratings (LPFGs) in a hollow core silica capillary. This device is made up of a lead-in single-mode fiber (SMF), LPFGs in capillary (CLPFGs), and a lead-out SMF, with a tapered air cavity (TAC) connecting SMF and capillary. The transmissive characteristics of capillary with and without LPFGs are analyzed theoretically and experimentally. Mode coupling of CLPFGs at longer wavelength could be enhanced, as well as the sensing performance increases times over the capillary without gratings. The refractive index (RI) sensitivity can reach to be 15 713.5 nm/RIU in the RI range from 1.451 to 1.456. Moreover, with the asymmetric index distribution induced in the capillary, the CLPFGs present vector bending properties that the bending sensitivities are 79.4 and −8.8 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , for bending directions at 0° and 180°, respectively. Furthermore, the proposed device shows negligible change to the external temperature variation, which could solve the cross-sensitivity problem. With the advantages of compactness, low cost, and ease of fabrication, the proposed device is potential in high-sensitivity RI sensing fields and vector bending measurement.

Ultrabroadband Long-Period Fiber Gratings for Wavelength-Tunable Doughnut Beam Generation

An ultrabroadband long-period fiber grating with 10-dB bandwidth of 155 nm and 3-dB bandwidth of 415 nm is demonstrated around an optical wavelength of 1060 nm in a few-mode fiber. Combined with a wavelength-tunable ytterbium-doped fiber laser, the application of the force-gradient method achieves the generation of optical doughnuts in the spectral range of 1030 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~1099$ </tex-math></inline-formula> nm. This long-period fiber grating has also been used with an amplified-spontaneous-emission source to obtain a broadband optical doughnut. Instead of varying the grating period, the ultrabroad bandwidth is obtained by adjusting the effective index difference between the core mode and the higher-order modes, which is realized by the variation of an applied force in the few-mode fiber, and the force gradient along the grating is quantitatively determined to be 2.11 N/cm.

Spatiotemporal mode-locked fiber laser based on dual-resonance coupling long-period fiber grating.

Spatiotemporal mode-locked (STML) fiber lasers have become an excellent platform in nonlinear optics research due to the rich nonlinear evolution process. In order to overcome modal walk-off and realize phase locking of different transverse modes, it is usually crucial to reduce the modal group delay difference in the cavity. In this paper, we use long-period fiber grating (LPFG) to compensate the large modal dispersion and differential modal gain in the cavity, realizing the spatiotemporal mode-locking in step-index fibers cavity. The LPFG inscribed in few-mode fiber could induce strong mode coupling, which has wide operation bandwidth based on dual-resonance coupling mechanism. By using dispersive Fourier transform involved intermodal interference, we show that there is a stable phase difference between the transverse modes constituting the spatiotemporal soliton. These results would be beneficial for the study of spatiotemporal mode-locked fiber lasers.

A long period fiber grating seawater salinity sensor based on bend insensitive single mode fiber

In this work, we proposed a long period fiber grating (LPFG) seawater salinity sensor based on a bend insensitive single mode fiber (SMF). The average curvature sensitivity of LPFG in the most sensitive direction is 0.889 nm/m−1 in curvature range of 0 m−1–2.25 m−1. A comprehensive design method was proposed to optimize the sensitivity of seawater salinity by fully combining mode transition (MT) and reducing cladding diameter. The enhancing effects of MT and reducing cladding on surrounding refractive index (SRI) sensitivity were discussed, respectively. After reducing the cladding diameter by hydrofluoric acid (HF) etching, a layer of titanium dioxide (TiO2) nanofilm was deposited on the surface of LPFG by electrostatic self-assembly technology. The average seawater salinity sensitivity of 163.299 pm/‰ can be achieved in salinity range of 5.001 ‰–39.996 ‰. The experimental results are well agreement with theoretical analysis.