Long-period Gratings Research Articles

Multiparameter Measurements for Bending, Torsion, and Temperature by Long-Period Gratings in Three-Core Fiber

Multiparameter Measurements for Bending, Torsion, and Temperature by Long-Period Gratings in Three-Core Fiber

Liquid-crystal clad tunable wavelength de-multiplexer and coupler/filter in silicon nitride rib waveguide structure

We report long-period waveguide grating (LPWG) based tunable wavelength de-multiplexer and tunable coupler/filter in liquid crystal (LC) clad rib waveguides on silicon nitride on insulator platform. High birefringence and electrically tunable refractive index of the MLC-9200-000 LC have been used to achieve a tuning range covering most of the S, C and L bands of the optical communication system. The structure consists of two parallel rib waveguides with an elevated slab. We have considered a new concept of injecting power into a higher-order slab mode. The power from the slab mode is coupled to the rib modes using LPWGs inscribed on the sidewalls of the ribs. The structure works as a tunable wavelength de-multiplexer when the grating periods in both ribs are different. The wavelength tunability of the de-multiplexer is achieved by an external voltage applied across the liquid crystal cell. Our numerical results show wavelength tuning of the de-multiplexer in the range 1.437 - 1.627 μm. The structure works as an electrically tunable coupler/filter when the grating period of both the gratings are the same. We numerically demonstrate the tuning of the coupler in the wavelength range 1.487 - 1.572 μm and that of the filter in the range 1.484 - 1.577 μm with 20-dB rejection.

Temperature and Twist Sensor Based on the Sagnac Interferometer with Long-Period Grating in Polarization-Maintaining Fiber.

We utilized a CO2 laser to carve long-period fiber gratings (LPFGs) on polarization-maintaining fibers (PMFs) along the fast and slow axes. Based on the spectra of LPFGs written along two different directions, we found that when LPFG was written along the fast axis, the spectrum had lower insertion loss and fewer side lobes. We investigated the temperature and twist characteristics of the embedded structure of the LPFG and Sagnac loop and ultimately obtained a temperature sensitivity of -0.295 nm/°C and a twist sensitivity of 0.87 nm/(rad/m) for the LPFG. Compared to the single LPFG, the embedded structure of the LPFG and Sagnac loop demonstrates a significant improvement in temperature and twist sensitivities. Additionally, it also possesses the capability to discern the direction of the twist. The embedded structure displays numerous advantages, including easy fabrication, low cost, good robustness, a wide range, and high sensitivity. These features make it highly suitable for applications in structural health monitoring and other related fields.

Gold-sputtered LPG fiber sensor pulse electroplated with iron-carbon for monitoring passivation and depassivation of carbon steel in concrete pore solutions

Monitoring the passivation and depassivation of reinforcement steel is of significant importance, particularly for certain reinforced concrete structures that define the time of steel rebar depassivation as the end of the durability service life. In this study, a gold-sputtered long-period grating (LPG) optical fiber sensor coated with pulse-electroplated iron-carbon is characterized for continuous monitoring of the surface state of carbon steel from passivation to depassivation in concrete pore solutions. Electrochemical corrosion and optical tests are performed simultaneously using an electrochemical workstation and an optical instrument. A protective passivation film with a thickness of approximately 4.05 nm was formed on the iron-carbon coating after 5 days of immersion in the simulated concrete pore solution, and its thickness is nonlinearly correlated with the wavelength shift of the LPG optical fiber sensor during the passivation process. The resonant wavelength changes in the opposite direction after depassivation, which can be used for the detection of the depassivation time. After depassivation, the wavelength shift and mass loss of the iron-carbon coating can be divided into three phases, and a linear relationship exists in each phase. The gold-sputtered fiber sensor coated with pulse-electroplated iron-carbon investigated in this study, demonstrates better performance than silver-deposited LPG fiber sensors.

Helical Long-Period Gratings in Three-Core Fiber for Directional Curvature and Torsion Measurements

Helical Long-Period Gratings in Three-Core Fiber for Directional Curvature and Torsion Measurements

High-Sensitivity Sensors Based on Long-Period Gratings Inscribed in Tapered Few-Mode Fiber

High-Sensitivity Sensors Based on Long-Period Gratings Inscribed in Tapered Few-Mode Fiber

Nonlinear mode coupling in graphene-buried optical waveguides

The photothermal effect of graphene, which refers to the effect of converting light absorbed by graphene into heat, offers an effective physical mechanism for the realization of all-optical control devices. In this paper, we explore this physical mechanism for the study of nonlinear mode-coupling effects with three graphene-buried waveguide structures: a graphene-buried long-period waveguide grating, a symmetric directional coupler with graphene buried in two cores, and a symmetric directional coupler with graphene buried in one core. We establish physical models for these graphene-buried waveguide structures based on the coupled-mode theory and experimentally implement these structures with polymer waveguides. Our experimental results agree well with the theoretical analyses. The nonlinear mode-coupling effects generated in the graphene-buried waveguide structures show similar characteristics as those achieved with Kerr nonlinearity, but the input powers required in our experiments are much lower (only several tens of milliwatts), which can be delivered by common continuous-wave lasers. The graphene-buried waveguide platform makes feasible the generation of strong nonlinear mode-coupling effects at low powers and offers much flexibility for nonlinearity engineering, which can greatly facilitate the investigation of nonlinear mode-coupling effects in different waveguide structures for practical applications.

Sensing characteristics of structural microfiber long-period gratings.

We present a detailed investigation into the sensing characteristics of a structural microfiber long-period grating (mLPG) sensor. By spirally winding a thinner microfiber to another thicker microfiber, periodic refractive index modulation is formed while the optical signal transmitted in the thicker microfiber is resonantly coupled out to the thinner microfiber, and then a 5-period four-port mLPG can be obtained with a device length of only ∼570 µm demonstrated a strong resonant dip of 25 dB. We studied the sensitivity characteristics of the four-port mLPG with surrounding strain, force, temperature and refractive index, and the obtained sensitivities were -6.4 pm/µɛ, -8418.6 nm/N, 7.62 pm/°C and 2122 nm/RIU, respectively. With the advantages of high refractive index sensitivity and wide wavelength tunable range, the four-port mLPG has great potential in applications such as tunable filters and biochemical sensor.

Distributed curvature sensing using long period fiber grating and machine learning numerical analysis.

In this numerical study, we propose a fiber distributed curvature sensor based on the analysis of the spectral transmission of a long period fiber grating (LPG) with a neural network. A simulation of the optical transmissions of a proposed 6-cm LPG structure for different curvature profiles is first performed using EigenMode Expansion and a coupled-mode theory algorithm. Both fiber curvature profiles and their corresponding optical transmission spectra are then injected into a four dense layer neural network which, after training, leads to a 0.40% relative median estimation error in the bending profiles. This paper demonstrates the efficiency of neural network-based optical sensors to analyze non-uniform perturbations, while also revealing long-period gratings to be promising candidates for such systems.

Photothermal-assisted for ultrafast response acetone sensing with ZIF-90/GO coated microfiber long period gratings

Volatile organic compound (VOC) sensors have recently been developed with sensitive coating layers to improve their performance. However, there are inevitable trade-offs in the performance of the VOC sensors with sensitive layers. It remains challenge to achieve higher sensitivity, faster response time, working at room temperature and better selectivity at the same time for real-world applications. In this work, we demonstrated a novel photothermal-assisted gas sensor using a microfiber long period grating (LPG) modified with zeolitic imidazolate framework-90 (ZIF-90)/graphene oxide (GO) for enhanced VOC acetone sensing. The GO-isolated ZIF-90 exhibited ultralow conductivity and high porosity, which significantly promoted light-to-heat energy conversion by trapping thermal and minimizing heat loss. Besides, the natural porosity and ultrahigh surface area of the ZIF-90 showed superior molecular adsorption making it a good choice for the acetone-sensitive layer. Based on the porous ZIF-90/GO nanohybrid and photothermal effects, the proposed sensor showed improved performance compared without adding laser towards acetone, with the sensitivity of 26.1 pm/ppm and low limit of detection of 760 ppb. Moreover, the proposed photothermal technology enhanced the response time down to 410 ms without sacrificing sensitivity. Thus, our proposed sensor constitutes a promising platform for effective acetone sensing systems that can be used for environmental monitoring and industry.

Wavelength Tunable Orbital Angular Momentum Mode Convertor Based on Helical Long-Period Grating Inscribed in Six-Mode Fiber Using CO2 Laser

We demonstrated the fabrication of orbital angular momentum (OAM) mode convertors (MCs) operating at 1.3 μm to 2.1 μm waveband based on the helical long-period gratings (HLPGs) inscribed in the few-mode fiber (FMF) using a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. The ±1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> , ±2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> , and ±3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> OAM mode can be directly generated by the HLPGs with efficiencies of more than 90%. The torsion sensitivities of 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> , 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order OAM modes in 2 μm waveband are 0.52 nm/(rad/m), 0.55 nm/(rad/m) and 0.36 nm/(rad/m), which has increased by 5.8 times, 2.8 times, and 1.6 times compared to that in 1.3 μm waveband, respectively. The strain sensitivities of 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> , 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order OAM modes in 2 μm waveband are - 4.2 pm/με、 - 4.0 pm/με and - 4.9 pm/με, which has increased by 2.3 times, 2 times, and 2.2 times compared to that in 1.3 μm waveband, respectively. The proposed OAM mode MCs based on the HLPGs can function as not only promotion of optical communication system but also high torsion sensitivity sensors in the sensing field.

Long Period Grating Imprinted on a Flat-Shaped Plastic Optical Fiber for Refractive Index Sensing

A corrugated surface long period grating (LPG) was fabricated on a flat-shaped plastic optical fiber (POF) as a refractive index (RI) sensor by a simple pressing with the heat pressure and mechanical die press print method. The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics. Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance, and the sensor with a thinner flat thickness, a deeper groove depth of the corrugated surface LPG, and a longer LPG exhibited better RI sensing performance. When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600 µm, an LPG with a period of 300 µm, a groove depth of 200 µm, and a length of 6 cm was fabricated on it, and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10−6 RIU. In addition, the influences of the POF cladding, tilting of LPG, and bending of the sensing structure were investigated. The results demonstrated that after removing the cladding and tilting or bending the LPG, the RI sensing performance was improved. When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/π cm, the highest sensitivity of 6 563%/RIU was achieved with a resolution of 2.487×10−9 RIU in the RI range of 1.3330–1.4230. The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication, high sensitivity, and intensity modulation at the visible wavelengths.

All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating

As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical long-period grating (PMF-HLPG) was designed and fabricated as a polarizer using the CO2-laser direct-write technique to realize the NPR effect. A homemade fiber Bragg grating (FBG) was also introduced into the laser system to enable a narrow-bandwidth lasing output and wavelength tunability. Based on the PMF-HLPG and FBG mentioned above, an all-fiber mode-locked laser with a spectra bandwidth of 0.15 nm was constructed to generate stable short pulses with a fundamental repetition rate of 12.7122 MHz and a pulse duration of 30.08 ps. In particular, its signal-to-noise ratio is up to 84.5 dB, showing the high stability of the laser. Further, the operating wavelength of the laser can be tuned from 1559.65 nm to 1560.29 nm via heating the FBG while maintaining its mode-locked state with stability. The results indicate that the PMF-HLPG could be used as a polarizer to meet the NPR mechanism for ultrashort pulse laser applications in optical communication, optical sensing, and biomedical imaging.

The Impact of Gamma Irradiation on Optical Fibers Identified Using Long Period Gratings

This paper compares the impact of gamma irradiation on various optical fibers, where the evolution in their optical properties could be identified using long period gratings (LPG) there induced. The LPGs were fabricated in broad spectrum of commercially available single-mode optical fibers - from standard to unconventional ones - using the electric arc discharge method. The fused silica fiber material contained such dopant as Ge, F, B&#x002F;Ge and P. The gamma exposure has been performed with a dose rate of 2.6 kGy&#x002F;h for 20 h (a total dose of 52 kGy). Subsequently, the recovery effect was monitored during the following 48 h. The wavelength shift and power change of the LPG attenuation bands during irradiation were investigated in real time. Moreover, to identify the impact of gamma treatment on the fiber properties, the thermal response of the gratings was studied before and after the irradiation. Depending on the fiber, significant differences have been observed in the grating responses. The highest radiation-dependent resonance wavelength shift has been found for B&#x002F;Ge fiber (&#x223C;10 nm at maximum dose). Moreover, for these LPGs no response saturation within the applied dose levels has been observed together with very low recovery effect, what differentiate these LPGs from the other tested in this experiment. Finally, concerning the changes in optical power at the resonance wavelength, the LPG induced in P-doped fiber has shown a strong irradiation-induced attenuation reaching 18 dB with a negligible recovery effect. The findings reported in this work are crucial for optical fiber and optical fiber sensors application in environments exposed to radiation.

Miniature Temperature-Independent Curvature Sensor Based on a Phase-Shifted Long-Period Fiber Grating Using Deep Tapering

A miniature temperature-independent curvature sensor was proposed and prepared by using a phase-shifted long-period fiber grating (LPG) fabricated from deeply tapered conventional single-mode fibers (SMFs). As a typical example, a sensor made from a phase-shifted deeply tapered long-period grating (PS-DTLPG) with a length of 3.4 mm and a maximum curvature sensitivity of 12.05 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was successfully demonstrated. The proposed curvature sensor is of the temperature-independence and has features of high sensitivity, high reliability, miniature size and low cost, which could have potential applications in structural health monitoring, especially for the medical industry and shape reconstruction.

Helical Long-Period Gratings in Four-Core Fiber for Multiparametric Monitoring of Directional Bending, Torsion and Temperature

We demonstrate the multiparametric monitoring of directional bending, torsion, and temperature based on helical long-period gratings (HLPGs) in a triangular four-core fiber (FCF) using CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser irradiation. The simulations investigated the stress-relaxing effect of the stress-applying parts due to CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser heating. In the experiments, the HLPGs with a period of 435 μm and a period number of 60 induced in FCF have been investigated for multiparametric monitoring. The directional bending sensing characteristics of FCF-HLPGs exhibit that at the curvature ranges A and B, the maximum bending sensitivities are -47.24 and 48.47 dB/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> obtained at -60°, respectively. And their sensitivities show the sine function behavior with curvature directions from -180° to 180°. The torsion sensitivity based on coupling efficiency variation is -0.16%/(rad/m) in the range from -10 to 10 rad/m. The temperature sensitivity based on wavelength variation is 0.123 nm/°C, and the intensity response on temperature can be negligible. The proposed FCF-HLPGs is potential in practical application for multiparametric monitoring.

Long-period gratings for monitoring the resin transfer molding of fiber-reinforced polymer composites.

The spectral characteristics of long-period gratings (LPGs) have been researched over the last two decades, and many sensing applications of LPGs have been proposed due to their spectral sensitivity to many surrounding environmental parameters such as the temperature, pressure, and refractive index. However, this sensitivity to many parameters can also be a curse due to cross-sensitivity and the inability to distinguish which environmental parameter is responsible for the LPG's spectral behavior. For the application proposed here-monitoring the progress of a resin flow front, its velocity, and the permeability of the reinforcement mats during the infusion stage of resin transfer molding-the multi-sensitivity of LPGs is a distinct advantage, as it provides the ability to monitor the mold environment at various stages of manufacturing.

Real-time monitoring and decoupling of multi parameters in vascular intervention using a cascaded LPFG-FBG sensor

During vascular interventional surgery, temperature, blood viscosity and vascular strain are very important for real-time monitoring in surgical navigation and intraoperative strategy. A highly integrated sensor for multiple index detection in real time is in great demand for robotic-assisted intraoperative surgery. In this study, we developed a cascaded long-period fiber grating-fiber Bragg grating (LPFG-FBG) sensor in which the temperature, blood concentration, and strain are decoupled and can be sensed independently in real-time. The experimental results show that when the temperature rises from 20 °C to 70 °C, the temperature sensitivity and linearity of the LPFG are significantly superior compared to those of the FBG; when the glycated hemoglobin concentration increases from 2 % to 21 %, the LPFG is more sensitive to glycated hemoglobin concentration variation, with a sensitivity of − 0.053 nm/%; when the stress increases from 0 to 300 με, the FBG is solely sensitive to strain variation, with a sensitivity of 0.000291 nm/με. The two-parameter optical fiber sensor proposed in the study does not require special inscription techniques for fabrication, and it has a simple structure, good stability, and high sensitivity. With a single cascaded LPFG-FBG inserted into a blood vessel, temperature, concentration, and strain can be calculated in real time from the sensing coefficient matrices. Its different filtering characteristics are beneficial to multi-parameter coupled measurements and decoupling in vascular surgery intraoperative monitoring.

Multiple core modes conversion using helical long-period fiber gratings.

We propose and demonstrate the fabrication of an all-fiber mode converter enabling simultaneous generation of multiple high-order core modes, which is realized by inscribing a helical long-period grating (HLPG) in a few-mode fiber (FMF) using a femtosecond laser. Helical refractive index modulation is introduced by continuously irradiating the core region with a highly focused femtosecond laser, while the fiber moves in a spiral path through a three-dimensional translation stage. Mode conversion from the LP01 mode to high-order core modes, including LP11, LP21, LP31, LP02, LP12, and LP41 modes, is achieved by controlling the inscription pitch of the grating. Moreover, first-, second-, third-, and fourth-order orbital angular momentum (OAM) modes can be directly generated using the HLPGs, and multiple OAM modes of different topological charges can be simultaneously excited using a single high diffraction order HLPG. This approach offers a new option for implementing with high-integration high-order mode converters or OAM mode generators.

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.