Long-period Fiber Gratings Research Articles

Environmentally robust all-polarization-maintaining random fiber laser of a cylindrical vector beam.

Random lasers with vector modes have garnered widespread attention due to their unique polarization and phase properties and the absence of cavity-defined longitudinal modes. In this work, we propose and demonstrate an all-polarization-maintaining (PM) random fiber laser (RFL) based on a half-open cavity that can simultaneously generate linearly polarized fundamental modes and higher-order modes. Then a cylindrical vector beam (CVB) of azimuthal polarization is generated by the mode superposition method with mode purity exceeding 93.4%. The mode conversion process is realized with a homemade long-period fiber grating and a fiber Bragg grating based on a PM few-mode (FM) fiber. The proposed compact and simple all-PM random laser is capable of generating a CVB without polarization controllers; thus, the environmental stability and self-starting ability will be greatly improved. This laser scheme will be beneficial for the design of vector mode random lasers and is expected to find diverse applications in modal division multiplexing systems, time-domain ghost imaging, and secure communication.

Potential Multiparameter Sensor Based on Thin-Cladding Fiber Helical Long-Period Fiber Gratings

Potential Multiparameter Sensor Based on Thin-Cladding Fiber Helical Long-Period Fiber Gratings

Projection to latent structures regression and its application to Mach–Zehnder interferometer optical fiber sensors for acetone detection

The techniques of multivariate analysis methods as prediction models open a new vision to the analysis of applied data. The method used in this work was projection to latent structures regression, which is a regression method that uses the latent variables generated by calculating the linear relationship between the dependent and independent variables giving them equal importance and thus obtaining the maximum variance and correlation of these variables. This method was applied to optical fiber sensors for acetone detection. It is important to detect acetone because it is a biomarker of diabetes mellitus. The sensor was fabricated with two cascaded long-period fiber gratings (LPFG) with a 515 µm grating period and separated 1 cm to form a Mach–Zehnder interferometer (MZI). Clinical studies for the diagnosis of diabetes are usually invasive, the development of these sensors proposes a new non-invasive option through the detection of acetone in human breath whose concentrations are in the order of 1.25–2.5 ppm. To study the response of the sensors to acetone, different sensing films, such as polydimethylsiloxane (PDMS), polymethyl methacrylate, Apiezon L and Apiezon T were used, which have a good affinity to this compound. Spectral changes in the transmission spectrum of the MZI were obtained due to the modes interference together with an increment of the sensitivity, since the interaction between the acetone concentration and the sensing film provokes a change in the effective index of the cladding, which in turn is amplified by the LPFGs separation. The analysis showed that the best results were obtained for the sensor with PDMS as sensing film, with the lowest limit of detection, 1.7 ppm using 4 latent structures.

Femtosecond Laser-Inscribed Cladding Mode Small-Period Long-Period Fiber Gratings for Temperature and Strain Sensing

Femtosecond Laser-Inscribed Cladding Mode Small-Period Long-Period Fiber Gratings for Temperature and Strain Sensing

Multi-wavelength Erbium-doped Fiber Laser with High-purity LP11 Mode Output Based on Mechanically Induced Long-period Fiber Gratings and Double Sagnac Fiber Filter

Multi-wavelength Erbium-doped Fiber Laser with High-purity LP11 Mode Output Based on Mechanically Induced Long-period Fiber Gratings and Double Sagnac Fiber Filter

Real-time optical fibre near-infrared chromatic dispersion analyser using collocated optical fibre gratings.

We present what we believe to be a novel optical fibre device for real-time measurements of near-infrared chromatic dispersion of a liquid medium from 1100 nm to 1700 nm. This inline optical fibre chromatic dispersion analyser is based upon collocated fibre long-period gratings written adjacently in the core by femtosecond laser inscription yielding 8 attenuation bands associated with different cladding modes, yielding 8 independent measurements. This fibre device is tested on a series of chemical compounds associated with wines. The results for each compound yielded distinctive chromatic responses. A machine learning pipeline was developed that successfully clustered and classified spectral responses of the different chemicals to illustrate the distinctive responses. The limits of detection for these fibre devices ranged from 1×10-3 to 6×10-5 mol/L for the compounds associated with wines, demonstrating the potential usefulness of an optical fibre chromatic dispersion analyser is capable of monitoring in real-time effective chromatic dispersion that is not offered by any current technology.

Highly sensitive threaded LPFG strain sensor

Long Period Fiber Grating (LPFG) strain sensors have been used to monitor the structural health of bridges and buildings, and there is a need to further improve their sensing sensitivity. This article proposed a threaded LPFG strain sensor, which used a CO2 laser to etch a long groove at the top of the cladding of a single-mode fiber. The fiber in the long groove area was heated and twisted to fabricate a long period fiber grating (TH-LPFG) strain sensor with a threaded groove on the cladding. When strain is applied, the strain is concentrated in the threaded area on the cladding, increasing and decreasing the effective refractive index changes of the cladding mode and core mode respectively, achieving enhanced strain sensing. The experimental outcomes demonstrate that the TH-LPFG strain sensor achieves a sensitivity of 44.9 pm/µε to strain, while its cross-sensitivity to temperature is minimal, at just 1.28 µε/°C. TH-LPFG provides a new solution for enhancing the sensitivity of LPFG strain sensing, with low production cost and low temperature crosstalk. It can be used in the field of highly sensitive strain monitoring of building structures.

Generation of High-Quality Cylindrical Vector Beams from All-Few-Mode Fiber Laser

Transverse mode control of laser intracavity oscillation is crucial for generating high-purity cylindrical vector beams (CVBs). We utilized the mode conversion and mode selection properties of two-mode long-period fiber gratings (TM-LPFGs) and two-mode fiber Bragg gratings (TM-FBGs) to achieve intracavity hybrid-mode oscillations of LP01 and LP11 from an all-few-mode fiber laser. A mode-locked pulse output with a repetition rate of 12.46 MHz and a signal-to-noise ratio of 53 dB was achieved with a semiconductor saturable absorber mirror (SESAM) for mode-locking, at a wavelength of 1550.32 nm. The 30 dB spectrum bandwidth of the mode-locked pulse was 0.13 nm. Furthermore, a high-purity CVB containing radially polarized and azimuthally polarized LP11 modes was generated. The purity of the obtained CVB was greater than 99%. The high-purity CVB pulses have great potential for applications in optical tweezers, high-speed mode-division multiplexing communication, and more.

Mode conversion in graded-index few-mode fiber via hollow cylindrical long-period fiber gratings.

We demonstrate the realization of mode conversion using hollow cylindrical long-period fiber gratings (LPFGs) inscribed in graded-index few-mode fibers (FMFs) by a femtosecond laser. By precisely shaping the refractive index modulation into a hollow cylindrical structure, we enable efficient coupling from the fundamental mode to high-order modes (LP11, LP02, LP21, LP12, LP31, LP03, and LP22 modes). The method achieves high-efficiency and low-loss mode conversion across various mode groups, marking a first for graded-index FMFs with different grating periods. The influence of the hollow cylindrical LPFG radius on mode conversion efficiency and spectral characteristics is thoroughly investigated. Additionally, incorporating a linear polarizer allowed for distinguishing between LP modes within a mode group, enhancing the tunability of the mode converter. These mode conversion devices have significant potential for applications in mode gain equalization and mode scrambling for mode division multiplexing (MDM) systems.

A small-period long-period fiber grating biosensor based on immobilization of concanavalin A on polydopamine nanospheres for simultaneous detection of D-glucose and temperature

The accurate and sensitive monitoring of glucose levels plays a crucial role in diagnosing diabetes in clinical settings. In this work, a novel small-period long-period fiber grating (SP-LPG) biosensor was first developed for D-glucose detection based on the immobilization of concanavalin A (Con A) on polydopamine (PDA) nanospheres. SP-LPG was fabricated by using femtosecond laser direct writing technology and the obtained grating region was only 2.4 mm. The recognition element of D-glucose, Con A, was attached to the surface of PDA nanospheres. Due to the high specific surface area of PDA nanospheres, a large number of binding sites were available for Con A. The as-prepared sensor showed selective and sensitive to D-glucose measurement in the range of 10 nM to 10 mM and a low detection limit of 1.95 nM (S/N = 3) was achieved. Moreover, the small grating period of SP-LPG allows for the observation of Bragg reflection, which can be utilized for temperature sensing. This property eliminates the issue of temperature cross-sensitivity present in refractive index sensing. And the sensor exhibited a thermal sensitivity of 10 pm/°C within the range of 30 ∼ 100 °C. The developed sensor offers a convenient and efficient platform of simultaneously measuring multiple parameters, including target molecules and temperature, for real-time medical monitoring applications.

Phage-modified dual-peak long-period fiber grating biosensor for ultrasensitive, rapid and specific detection of pathogen strains

Herein, we present an ultrasensitive, rapid and specific phage-modified dual-peak long-period fiber grating (Phage-DP-LPFG) biosensor for detecting foodborne pathogens. Compared to the single-peak resonance LPFG with a grating period of 360 μm, the DP-LPFG with a grating period of 163 μm offers enhanced sensitivity to refractive index changes caused by bacterial adsorption. By directionally immobilising phages on the DP-LPFG, the sensor achieves high specificity for target bacterial strains. Upon capturing the target strain, the short-wavelength near-infrared transmission loss peak undergoes a blue shift, whereas the long-wavelength transmission loss peak exhibits a red shift. The shift in spacing between the two peaks (Δλ) increases with higher bacterial capture, enabling the quantification of target strains based on Δλ changes. Using Escherichia coli O157:H7 (E. coli O157:H7) as a model, the Phage-DP-LPFG biosensor demonstrates a linear detection range of 25–108 CFU/mL with a low limit of detection of 8 CFU/mL in 10 min under optimal conditions. The biosensor was also tested on food samples such as fish and milk, showing no notable differences from standard methods but with reduced detection time. It was further extended to the detection of another foodborne pathogen strain Hafnia paralvei (MCCC 1K06097) (H. paralvei (MCCC 1K06097)) with satisfactory results. This study offers a novel approach for the rapid, sensitive and specific detection of trace pathogenic strains in foods.

Observation of the enhanced dual-split photonic spin Hall effect in wavelength domain via a helical fiber grating

To date, the photonic spin Hall effect (PSHE) has been studied and observed only in the space and momentum domains. Direct observation of the PSHE in the wavelength domain remains unexplored. In this work, we demonstrate both theoretically and experimentally the enhanced PSHE in the wavelength domain via a helical long-period fiber grating (HLPG), where the spin-dependent dual-split in the transmission spectrum of the utilized HLPG has been observed. This is unlike the PSHEs reported thus far, where a mono-splitting of the light is observed in either the space or the momentum domains. The proposed method provides an additional degree of freedom to observe the PSHE, which paves the way for exploiting all fiber-based HLPGs in chiral photonics, chiral sensors, and fine precise measurements.

Temperature self-compensated dual core fiber-optic sensor integrated with moisture sensitive MIL-96(Al) film for breath detection

In this paper, a dual-core fiber optic sensor has been proposed for dynamic monitoring of temperature and humidity. The side core is polished into a D-type optical fiber, and a layer of nano-silver film and a layer of polydimethylsiloxane (PDMS) are deposited to construct a surface plasmon resonance (SPR) temperature compensator. The middle core is constructed as the long-period fiber grating (LPFG) sensing channel. The mode conversion of LPFG is achieved by TiO2 film, and its refractive index (RI) sensitivity is increased by about 4 times. Meanwhile, a layer of moisture sensitive MIL-96(Al) film is grown outside the TiO2 film. Due to the particular pore structure for water molecules, it is possible to change the effective RI of MIL-96(Al) film by absorbing water molecules. Research has shown that the LPFG sensor can achieve a relative humidity sensitivity of −0.411 nm/%RH. Finally, the sensor successfully achieved real-time dynamic monitoring of respiratory rate.

Relative humidity sensor based on a nano-film deposited LPFG with hydrophilic polymer composite coating

A novel high-performance relative humidity (RH) sensor is proposed by using a hydrophilic polymer composite film-coated nano-film deposition deposited long-period fiber grating (LPFG). A high refractive index (RI) TiO2 nano-film is deposited on the LPFG as an inner layer. The film thickness is selected to maximize the surrounding refractive index (SRI) sensitivity of the grating when the SRI is approaching the cladding index. A composite film made of polyvinyl alcohol (PVA) and polyethylene glycol (PEG) is used as the an external humidity-to-RI transducer. The concentration and proportion of the PVA/PEG solutions are appropriately chosen to tune the refractive index (RI) RI of the polymer coating to the transition range during the RH varying from 60 % to 95 %. For the LPFG coated with two layers film, the a RH sensitivity up to 3.3 nm/% is achieved. The variation in RH is found to have a linear response. The proposed device offers a reliable alternative for higher RH environment sensing.

A Simultaneous Measurement Sensor for Temperature and Curvature Based on Congruent Quasi-Helical Long-Period Fiber Grating.

This article presents a long-period fiber-grating sensor based on a congruent quasi-helical structure (CQH-LPFG) with the two-parameter measurement of both temperature and curvature. The CQH-LPFG sensor was manufactured using a high-frequency CO2 laser, and an innovative quasi-helical structure was introduced into the two-parameter measurement of the temperature and curvature of the optical fiber sensor with excellent results. The experiment and analysis demonstrate that the curvature sensitivities of the three resonance peaks in the 1440 nm to 1540 nm transmission spectrum were 11.88 nm/m-1, 8.05 nm/m-1, and 11.11 nm/m-1, and the curvature varied ranging from 0.156 m-1 to 0.494 m-1. The three resonance peaks showed temperature responsivities of 29.87 pm/°C, 24.65 pm/°C, and 36.85 pm/°C, respectively, and the linear fit was of excellent quality. In the case of measuring both curvature and temperature changes simultaneously, the resonant peak wavelength of the CQH-LPFG sensor was demodulated through matrix analysis, with dip A and dip C providing superior simultaneous measurements. These features make it a promising candidate for applications such as engineering machinery and the health inspection of buildings.

A novel single mode fiber optic temperature sensor combined with the FLRDS technique

A single mode optical fiber loop was employed as a temperature sensor to observe changes in optical loss regarding to ringdown time (RDT) by high sensitive the fiber loop ringdown spectroscopy (FLRDS) technique which has real-time and fast response measurement capability due to allowing trapped light pulse multiple interactions with the measurands. Two different fiber loops of 45 ± 5 m and 120 ± 5 m lengths were embedded one by one into a copper, circular and closed housing. Continuous monitoring of RDTs was carried out by changing the temperature in the range of 25–200 °C with the steps of 25 °C for the first time by using bare fiber without any modification as a temperature sensor. The FLRDS system for temperature sensing has simple design without extra components such as an optical time domain reflectometer (OTDR), long-period fiber grating (LPFG) or fiber Bragg grating (FBG) as sensorhead. The FLRDS system was diligently optimized to achieve the lowest baseline as %0.49. Since the RDT of the FLRDS system was changed due to the thermal expansion of the fiber, continuous monitoring of the temperature was the first time recorded by utilizing this kind of FLRDS temperature sensor. These kind of FLRDS temperature sensors have high potential to be employed in mining, nuclear facilities, railways, underwater structures, biomedical, medicine, structural health monitoring, transportation and communication applications with simple system setup, lower cost, higher sensitivity, portability, real-time and continuous monitoring for early detection.

Spacing mode multi-wavelength erbium doped fiber laser based on a symmetrical long-period fiber grating

We present a multiwavelength erbium-doped fiber laser based on a symmetrical long-period fiber grating (LPFG) with tunable spacing mode between single lasing mode and multiwavelength spectra. The LPFG was manufactured using a thermal expansion technique using an LZM-100 glass processing system equipped with a CO2 laser, and it was used as a wavelength-selective filter (WSF) in the laser ring cavity. The laser can emit single, double, triple, quadruple, quintuple, or sextuple lines in the range from 1546 to 1563 nm, which can be tunable by controlling the radius of curvature of the LPFG in the range 0–0.2662 m −1. A minimal spacing mode of 1.92 nm was observed in the multiwavelength region; meanwhile, the fiber laser offers an average spacing mode of 6.945 nm between the multiwavelength region and the single emission. This laser has a 3 dB linewidth of 0.11 nm and a side mode suppression ratio (SMSR) of 55.56 dB. Finally, according to experimental results, the laser has high wavelength stability at room temperature for 88 min.

Novel beer bitterness measurement instrument using optical fiber sensor

This paper presents a novel optical fiber sensor for measuring the bitterness of beer samples. The sensor is based on a Mach–Zehnder interferometer (MZI) composed of two long-period fiber gratings (LPFGs). The sensor exploits the sensitivity of the LPFGs to the refractive index of the surrounding medium, which varies according to the concentration of iso-α-acids that cause bitterness in beer. The sensor uses a two-point on-line self-calibrating procedure to estimate the bitterness of unknown samples by comparison with reference samples of known bitterness. The sensor performance was evaluated in both laboratory and factory settings, showing good repeatability, reproducibility, and low bias. The proposed sensor offers a fast, simple, and low-cost alternative to the conventional method of measuring beer bitterness based on UV spectroscopy. The sensor can be integrated into the Industry 4.0 framework to improve the quality and consistency of beer production.

Femtosecond laser direct writing annular small-period long-period fiber gratings for simultaneous refractive index and temperature sensing.

A new type of small-period long-period fiber grating (SP-LPFG) consisting of a series of annuli inscribed by a femtosecond laser in a single-mode fiber is proposed and demonstrated. The effects of the annuli radii and the number of annuli in each period on the transmission spectrum are studied. The transmission spectrum of the annular SP-LPFG exhibits both strong Bragg resonances and cladding mode resonances, which have similar temperature sensitivities of 9.26 and 9.75 pm/°C, respectively. The Bragg resonances are insensitive to the change of environment refractive index (RI), while the cladding mode resonances show a RI sensitivity of 141.89 nm/RIU. The proposed sensor offers a potential solution for biomolecular sensing applications by effectively eliminating the issue of temperature cross-sensitivity. Its compact design and ease of spectral acquisition enhance its practicality and convenience.

Application of Long-Period Fiber Grating Sensors in Structural Health Monitoring: A Review

Structural health monitoring (SHM) is crucial for preventing and detecting corrosion, leaks, and other risks in reinforced concrete (RC) structures, ensuring environmental safety and structural integrity. Optical fiber sensors (OFS), particularly long-period fiber gratings (LPFG), have emerged as a promising method for SHM. Various LPFG sensors have been widely used in SHM due to their high sensitivity, durability, immunity to electromagnetic interference (EMI) and compact size. This review explores recent advancements in LPFG sensors and offers insights into their potential applications in SHM.