Long-period Fiber Gratings Research Articles

High-sensitive refractive index sensor based on the long-period gratings inscribed in the tapered fiber at dispersion turning point

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the tapered single-mode fiber by CO2 laser. The phase-matching curves of the LPFGs inscribed in the tapered fiber with different waist diameters have been studied theoretically and experimentally. When the waist diameter of the tapered fiber decreased from 90 μm to 22 μm, the slope of the phase-matching curve changed dramatically. The refractive index sensitivity can be increased by inscribing the LPFGs in the tapered fiber around the dispersion turning point. A maximum refractive index sensitivity of 21702.78 nm/RIU has been achieved in the range of 1.445–1.457 for the LPFG inscribed in the tapered fiber with a waist diameter of 50 μm. This kind of LPFGs inscribing in the tapered fiber may be a good candidate for the development of refractive index based chemical and biological sensors.

The thermal diffusion blocking effect of holes enclosing core fiber

We have developed a variety of optical fibers with holes enclosing the core to increase the service life of fiber optic devices under high-temperature and high-pressure environments by blocking the diffusion of dopants in the core with the help of air holes. Simulation results show that the fiber with uniformly distributed holes enclosing the core, the more the hole number, the smaller the core hole distance, and the larger the hole radius, the more significant the blocking effect on thermal diffusion. By comparing the refractive index distribution of three kinds of holes enclosing core fibers and single-mode fibers after prolonged high-temperature heating treatment, the thermal diffusion blocking effect of the holes enclosing core fiber is verified. Long-period fiber gratings fabricated by periodic refractive index modulation of holes enclosing core fibers can realize the measurement of various physical parameters. The thermal diffusion blocking effect of the holes enclosing core fiber enables the long-period fiber grating devices to work for a long time under high-temperature and high-pressure environments, showing the potential for practical applications in various industrial processes.

Refractive Index Sensors Based on a Chirped Core Long-Period Fiber Grating

Refractive Index Sensors Based on a Chirped Core Long-Period Fiber Grating

Multi-order orbital angular momentum mode generators based on integrated long-period fiber gratings.

We propose integrated long-period fiber gratings (LPFGs) fabricated by a CO2 laser to realize a multi-channel and multi-order orbital angular momentum (OAM) mode generator. The integrated LPFG is inscribed on multiple surfaces of the few-mode fiber (FMF) by rotating the fiber in the opposite direction at an angle θ. By controlling the rotation angle, the number of integrated LPFGs can be set. The selected rotation angle is 43 ∘, which can integrate up to nine LPFGs, i.e., realizing that the number of channels for first-order orbital angular momentum (OAM) mode conversion is nine. The integrated LPFGs fabricated in this method allow a flexible design of channel spacing. In addition, the flexible selection of the integrated grating period achieves the simultaneous generation of multi-channel second-order and third-order OAM mode conversion. The multi-channel and multi-order OAM mode generators have important application in optical communication multiplexing systems and OAM sensing.

Monitoring of Curing Process of Epoxy Resin by Long-Period Fiber Gratings.

The curing of epoxy resin is a complex thermo-chemical process that is difficult to monitor using existing sensing systems. We monitored the curing process of an epoxy resin by using long-period fiber gratings. The refractive index of the epoxy resin increases during the curing process and can be measured to determine the degree of curing. We employed long-period fiber gratings that are sensitive to the refractive index of an external medium for the measurement of refractive index changes in the resin. We observed that the resonances of long-period fiber gratings increased their depth with the increased refractive index of the resin, which was well described by our simulation taking the coupling to radiation modes into account. We demonstrated that the degree of cure can be estimated from the depth of the grating resonances using a phenomenological model. At the same time, long-period fiber gratings are sensitive to temperature variations and internal strains that are induced during curing. These factors may affect the measurements of curing degree and should also be addressed.

Full C- and L-band covered second-order OAM mode generator based on a thinned helical long-period fiber grating

A full C- and L-band covered second-order orbital-angular-momentum (OAM) mode generator has been proposed and experimentally demonstrated, which is realized by using a helical long-period fiber grating (HLPG) but inscribed in a thinned four-mode fiber. By optimizing the design of grating period and fiber diameter of the proposed HLPG, an ultra-broadband rejection filter with a depth of ∼23 dB, a bandwidth of ∼156 nm @-10 dB (ranging from 1522 nm to 1678 nm) and a bandwidth of ∼58 nm @-20 dB (ranging from 1574 nm to 1632 nm), has been successfully obtained as a typical sample. To the best of our knowledge, this is the first demonstration of such ultra-broadband second-order OAM mode generator by using only one fiber component, i.e., the thinned HLPG. In addition, the proposed generator is less polarization-dependent and less temperature-sensitive than those of the conventional HLPGs, which is believed to be considerably helpful to find potential applications of the device itself in wavelength division multiplexing (WDM) and OAM mode division multiplexing (MDM) optical fiber communication systems.

All-fiber 3 kW LP02 laser output based on long-period fiber grating for precise welding

An innovative all-fiber mode converter specifically designed for high-power fiber lasers is introduced. The mode converter relies on the coupling theory of long-period fiber gratings (LPFGs) and involves the fabrication of a LPFG, where the performance is meticulously studied by examining various fabrication parameters. The LPFG is integrated into a high-power fiber laser system to achieve efficient conversion from the LP01 mode to the LP02 mode with a conversion ratio of 90 % and a low transmission loss of only 0.088 dB. Under a 3.03 kW laser output, the LP02 fiber laser exhibits a remarkable laser-to-raman peak intensity ratio, which is 15.98 dB higher than that of the signal-mode fiber laser. Furthermore, during the welding process of a thick aluminum plate, the LP02 fiber laser demonstrates a significant increase of 1.2 times in the depth of the weld pool morphology compared to the signal-mode fiber laser, particularly evident when operating at a laser welding head speed of 100 mm/s. The mode converter with LPFG exhibits lower insertion loss, enhanced power-handling capacity, making it ideal for diverse high-power fiber laser applications with promising practical prospects.

Simultaneous generation of first- to fourth-order OAM modes based on a cascaded preset-twist long-period fiber grating

Abstract We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge–Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge–Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating.

Thermally stable vector torsion sensing based on cascaded long-period fiber gratings in two-mode fiber

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP01-LP11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields.

Compact refractive index sensor based on offset splicing long-period fiber grating

Long-period fiber grating (LPFG) is one of the most important passive fiber devices. They work by coupling light from the fundamental core mode to the forward-propagating cladding mode, creating a series of resonant peaks in the transmission spectrum. In this paper, a series of no-core fiber (NCF) sections were periodically misaligned and embed in the single-mode fiber (SMF), to form a LPFG through the periodic refractive index (RI) modulation along the fiber axis. This fabricating method enhances the RI modulation to shorten the length of the proposed sensor to 2.82 mm. Experimental result shows that the highest RI sensitivity for the sensor reaches −4460.83 nm/RIU in a measuring range of 1.33–1.40 RIU at room temperature. Hopefully, the proposed study can lay some implementation methods for the design of the all-fiber compact RI sensor.

Enhancing milk quality assessment: A novel approach using an optical tongue with fiber-based Mach–Zehnder interferometry

The incorporation of advanced technologies in agricultural industry procedures has boosted operational efficiency, enabling more effective data collection and analysis. In this study, we developed an ”optical tongue” using fiber-optic Mach–Zehnder interferometers (MZIs) built from micro tapered long-period fiber gratings (MT-LPFGs). This tool was employed to collect data related to milk quality, with results indicating a high potential to improve the quality control of dairy products. These advances demonstrate the potential of optical fiber sensor technologies in the agricultural industry and quality control.

Multiparameter Sensor Based on Cascaded Micro-Tapered Long-Period Fiber Gratings

Multiparameter Sensor Based on Cascaded Micro-Tapered Long-Period Fiber Gratings

Simultaneous measurement of the surrounding refractive index and temperature using a local-micro-structured long-period fiber grating sensor operating near the phase-matching turning point

We have proposed a local-micro-structured long-period fiber grating (LMSLPFG) sensor near the phase-matching turning point (PMTP) for simultaneous measurement of the surrounding refractive index (SRI) and temperature. By using the HF solution or a femtosecond laser to partially change the cladding radius of LPFG, the LMSLPFG is realized. Such change affects the effective refractive index of cladding mode and further causes a new resonant peak in the spectrum. By using the transfer matrix method, the formation mechanisms of the peak in the middle of the LMSLPFG transmission spectrum corresponding to a little off-resonant state (ALOR) near PMTP and the other two peaks on both sides corresponding to the dual-peak resonance are studied. On this basis, the sensing characteristics of such three resonant peaks to SRI and temperature are analyzed. When the SRI and temperature change, the transmissivity of the peak in ALOR changes without a response occurring at resonant wavelength, while the wavelength of the other two dual peaks will respond proportionately. The numerical simulation results demonstrate that the sensitivities of the distance between the dual peaks to SRI and temperature are 777.14 nm/RIU and −0.22nm/∘C, respectively, and the sensitivities of single peak transmissivity to SRI and temperature are −394.06dB/RIU and 0.05 dB/°C, respectively. Therefore, the sensitivity matrix can be used to measure SRI and temperature simultaneously with a single LMSLPFG.

Optically tunable mode-locked fiber laser using long-period grating coated with multiwall carbon nanotubes

We demonstrate an optically tunable mode-locked fiber laser using long-period fiber grating (LPFG) coated with multi-walled carbon nanotubes (CNTs). The multi-walled CNTs can absorb light to convert it into thermal energy, and the resonance wavelength of the grating can be easily turned by varying the external modulated light power. This multi-walled CNT coated LPFG-based all-optical fast and efficient spectrum tunable filter enables continuous tuning of the central wavelength of the laser by manipulating the loss of the mode-locked laser, ensuring the stability of the mode-locking state. In the absence of modulated light on multi-walled CNTs, the soliton laser could generate 890 fs pulses at 1546.7 nm with a spectrum bandwidth of 3.26 nm and a signal-to-noise ratio of 73.1 dB. Through adjustment of the pump power of the modulation light on multi-walled CNTs, the mode-locked fiber laser can be continuously tuned from 1546.71 to 1563.15 nm. The response time of the optically tunable system was measured to be in the order of hundreds of milliseconds. The presented optical tuning filter shows great potential in the fiber laser system, offering a repeatable, straightforward, and rapidly responsive laser tuning technique.

Improvement of Temperature Sensitivity of Long-period Fiber Gratings Using Liquids with High Thermo-optic Coefficients

Improvement of Temperature Sensitivity of Long-period Fiber Gratings Using Liquids with High Thermo-optic Coefficients

Optical pH Sensor Based on a Long-Period Fiber Grating Coated with a Polymeric Layer-by-Layer Electrostatic Self-Assembled Nanofilm.

An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92-9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry-Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

Polyvinyl Alcohol Nanofibers Wrapped Microtapered Long-Period Fiber Grating for High-Linearity Humidity and Temperature Sensing

Polyvinyl Alcohol Nanofibers Wrapped Microtapered Long-Period Fiber Grating for High-Linearity Humidity and Temperature Sensing

Mechanically Induced Long-Period Fiber Gratings and Applications

Long-period fiber gratings (LPFGs) functioning as band-reject filters have played a pivotal role in the realm of optical communication. Since their initial documentation in 1996, LPFGs have witnessed rapid advancements in areas such as optical sensing, the equalization of optical amplification, and optical band-pass filtering, etc. The unique attributes of optical fiber-based grating, including their miniaturized size, cost-effectiveness, and immunity to electromagnetic interference, have contributed significantly to various sectors over the last two decades. This paper presents a review of the evolution of LPFGs, with a specific focus on the progression and current trends of mechanically induced long-period fiber gratings. It offers a concise overview of coupled-mode theory, the fabrication processes, the merits, and the limitations associated with mechanically induced LPFGs. Moreover, this review elucidates the application methodologies of mechanically induced LPFGs and anticipates future directions in this field.

A dual-wavelength erbium-doped fiber laser for high resolution strain sensing with compound filter of a Sagnac loop and a long period fiber grating

A dual-wavelength erbium-doped fiber laser (EDFL) strain sensing system composed of compound filter of a long period fiber grating (LPFG) and a Sagnac loop is proposed. Polarization maintaining fiber (PMF) in Sagnac loop acts both an interference filter and a strain sensing probe with high sensitivity, low cost, and low loss. LPFG is used to modulate the transmission wavelength and bandwidth of EDFL through adjusting its bending curvature. Flexible laser output with single wavelength and dual-wavelength tunable wavelength band of 10 nm is achieved in EDFL. The measurement sensitivity of strain reaches 8.33 pm/με, while the 3 dB linewidth is less than 2 nm and the signal-to-noise ratio (SNR) exceed 36 dB. This sensor has a high resolution (R) of 0.11 nm and a high quality factor (Q) of 38.13. The experimental results suggest that the system is suitable for precise strain measurement and has significant potential in fields like biomedicine, environmental monitoring, health monitoring, and aerospace.

High-power polarization-maintaining LP11-mode fiber laser based on long-period fiber grating for precise welding.

An LP11-mode output all-fiber laser was presented, utilizing long-period fiber gratings (LPFGs) and polarization-maintaining optical fiber (PMF). The LPFG was designed and fabricated, achieving a 90.56% efficiency in LP01 to LP11 mode conversion. Furthermore, the transmission stability of LP11-mode in the PMF was also explored, with the spatial mode overlap ratio exceeding 0.95. Ultimately, the high-power polarization-maintaining (PM) fiber laser, capable of the LP11 mode output, was constructed, with the output power of 600 W and the beam quality M2 of 2.84. During the process of welding a thick Al-plate, the LP11 fiber laser exhibits a notable 1.88 times greater depth of fusion compared to the commercial single-mode fiber laser, when operating at the laser welding head speed of 100 mm/s. For applications demanding non-circular symmetric high-order modes, this research holds substantial potential for widespread adoption within the field of industrial processing.