Period Fiber Grating Research Articles

Tunable filter based on two concatenated symmetrical long period fiber gratings as Mach-Zehnder interferometer and its fiber lasing application

This work presents a highly stable switchable and tunable erbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) made with two symmetrical LPFGs separated by an SMF segment of 5 cm in length. The core and the cladding of the SMF central segment act like the MZI arms, whereas the LPFG1-SMF and SMF-LPFG2 splices act as optical couplers. The emission is switchable from one single emission at 1563.6 nm, a side mode suppression ratio (SMSR) of ∼55 dB, and linewidth of 0.15 nm, to a dual emission located at 1563.98 and 1573.89 nm, reaching a high SMSRs of 56 dB, and linewidths ∼0.10 nm. Moreover, the laser emission can be tuned from 1569 to 1572 nm applying torsion to the MZI from 0° to 330° obtaining a nearly linear sensitivity of 9.2 pm/° with low hysteresis. Additionally, the laser operation stability was tested over 60 min, obtaining the power and wavelength fluctuations lower than 0.086 dB and 0.028 nm respectively. To the best of our knowledge, this is the first time that symmetric LPFGs have been used as a torsion sensitive WSF. For its high wavelength stability and SMSR, the MZI can be a low-cost alternative WSF while the switchable and tunable laser can be useful as an optical source for spectroscopy and communications applications.

Mode-Locked Fiber Lasers With High-Purity High-Order Mode Generation Using a Grating Combination

We propose an all-fiber passively mode-locked laser for high-purity high-order mode generation by combining a two-mode long period fiber grating (TM-LPFG) and a two-mode fiber Bragg grating (TM-FBG). The grating combination fully plays respective advantages: highly efficient mode conversion of the LPFG, mode selection and spectrum filtering of the TM-FBG. A semiconductor saturable absorber (SESAM) is used to achieve passively mode-locking. As a result, the mode-locked cylindrical vector beam (CVB) pulse with a 3-dB spectral width of 0.08 nm at a repetition rate of 12.88 MHz has been obtained, and the signal to noise ratio (SNR) is as high as 68 dB. Moreover, the azimuthally and radially polarized CVBs obtained after degeneracy elimination have high purity of 95.6% and 98.3%, respectively. This laser may have the potential for generation of high-purity CVB pulse.

Long period fibre gratings inscribed by sinusoidal intensity modulated femtosecond laser

To the best of our knowledge, this is the first study to propose and verify a novel and simple direct inscription method for long-period fibre gratings (LPFGs) with an arbitrary grating period using a sinusoidal intensity modulated femtosecond laser. This method induces a sine refractive index modulation, which makes the LPFGs have no higher-order harmonic resonance that exists in the normal square-wave modulation method. This is beneficial for reducing the insertion loss of the LPFGs and optimising the transmission spectrum. Using this method, we also conveniently realise the inscription of superimposed, apodised, and chirped LPFGs. This work presents a simple but effective inscription scheme for various types of LPFGs using a femtosecond laser.

A compact fiber-based sensor for wide range strain measurement

In this paper, a compact strain sensor for wide range measurement is proposed and experimentally investigated. The proposed sensor is configured by jagged long period fiber grating (J-LPFG), which is fabricated by etching on two opposite sides of single mode fiber (SMF). Since the appearance of the structure seems like jag, we name it by J-LPFG. This unique physical shape brings itself a wide measurable range and linear strain response. Meanwhile, we compare the strain sensitivities of J-LPFGs with different periods through experiments. As the consequence of experiments, J-LPFG with 8 periods exhibits the best strain response which reaches 5.4 pm/με within the dynamic range of 0–6800 με. Furthermore, the strain sensitivity and measurable range of J-LPFG decrease as the number of periods increases. This theory is verified by experiments and strain distribution simulation. In addition, the temperature sensitivity of 8-periods J-LPFG is 57 pm/°C in the range of 30–150 °C. Due to low cost, simple preparation process, and wide measurable range, J-LPFG has great potential to be applied in practical engineering.

3d-printed force sensitive structure using embedded long-period fiber grating

In this work, we present a low-cost tactile sensor based on arc-induced long–period fiber grating (LPG) embedded in polylactic acid (PLA) by fused deposition modeling (FDM). The 3d printing process used to embed the LPG, offers easy customization, flexibility, fast prototyping, and low-cost fabrication. At the same time, optical fiber sensors such as fiber Bragg gratings (FBGs) and LPGs are compact, resistant, and immune to electromagnetic interference. Therefore, the easy fabrication and low cost of arc-induced LPGs coupled to the 3d printing advantages offer low-cost custom force sensing elements with all the advantages of optical fiber sensors. This way, the proposed tactile sensor is a lightweight, small-size, and cost-effective tactile sensing scheme that offers easy implementation and tailoring to specific applications. In this study, we fabricated, embedded, calibrated, and evaluated the proposed LPG sensor tactile sensor. We showed this approach provided high force sensitivity (11.723 nm.N−1) with high linearity (99.28%), sub-decimal measurement error, and no drift.

(INVITED)Chemical sensors based on long period fiber gratings: A review

Fiber optic devices are being increasingly employed in the fields of chemical and environmental sensing due to their important features, such as high accuracy, small size, chemical inertness, remote operation and multiplexing capabilities. In this work, a thorough review about the design, fabrication and characterization of fiber optic chemical sensors based on long period grating (LPG) technology is reported. The emphasis is placed on transducer designs and features as well as the techniques to enhance the sensitivity. Subsequently, coating materials to be deposited around the grating region, providing a selective response to the target analytes are described in detail. Finally, the different applications are reviewed, mainly related to the monitoring of environmental parameters, volatile organic compounds, hazardous gases, heavy metal ions, corrosion, marine salinity and food quality. The aim of this work is to deliver a comprehensive analysis regarding the state-of-the-art solutions about LPG-based chemical sensors and to summarize the current shortcomings and upcoming research paths.

Simultaneous Measurement of Curvature Vector and Temperature Based on Composite Gratings Inscribed on D-Shaped Fiber

A novel two-parameter fiber sensor has been demonstrated, consisting of D-shaped composite fiber gratings (DCFG) which includes long period fiber grating (LPFG) and fiber Bragg grating (FBG) inscribed on D-shaped fiber. LPFG is inscribed on the side of D-shaped fiber by a high-frequency CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. On account of the asymmetry of the structure in 90° and 270° and refractive index modulation (RIM) in 0° and 180°, the sensor can recognize the bending directions. In the theory, it is proved that by adjusting the thickness of D-shaped fiber, the bending sensitivity can be enhanced in 90° and 270° directions. In the experiments, the fiber sensor achieves the sensitivity of curvature up to 13.70 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , which is nearly two times as the similar structure without D-shaped fiber. The curvature and temperature sensitivities of LPFG and FBG are different in DCFG, so the proposed sensor can realize simultaneously measurement of curvature vector and temperature.

Excitation of high order orbital angular momentum modes in ultra-short chiral long period fiber gratings.

A class of ultra-short chiral long period fiber gratings (CLPFGs) are prepared by writing a spiral curve on the surface of a six-mode fiber. The CLPFGs are applied to excite ±2nd- and ±3rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the CLPFG in these modes can be as high as 99%, when the length is only 0.5cm. The polarization characteristic of the excited higher-order OAM modes in CLPFGs was theoretically analyzed and experimentally investigated. Results show that the obtained ±2nd- and ±3rd-order OAM modes are polarization independent, as expected.

A Multi-Azimuth Angle Sensor for Tilt Measuring Based on LPFG Array

A multi-azimuth angle sensor for tilt measuring based on long period fiber grating (LPFG) array is proposed. It is designed based on the sensitivity of LPFG to refractive index. The sensor consists of a sealed cylindrical device containing a half volume NaCl solution. The LPFG array which consists of four mutually perpendicular LPFGs is fixed in the container. When the sensor is tilted at different angles, the liquid level intersects with the LPFGs at different positions. By calculating the intersection point of liquid level and LPFGs, the liquid level equation after tilt can be obtained, and the tilt angle can be obtained. The array design and measurement theory of the sensor is presented, and the sensitivity of the sensor is discussed. The experimental result shows that the LPFG array can be used to measure multi-azimuth angle in tilt. The angle sensitivity varies unevenly with the tilt angle, and the range of variation is 0.0006nm/° ~ 0.0434 nm/°. The array expands the measurement range of the tilt angle sensor effectively.

Optical Fiber Sensors Based on Core-Offset Structure: A Review

This review focuses on the preparation methods and wide applications of optic fiber sensors based on core-offset structure. Firstly, the classification and preparation methods of core-offset structures are introduced. Then, the applications of different types of sensors based on the core-offset structure are investigated, including the measurement of physical parameters such as temperature, strain, refractive index (RI), bending, magnetic field by Mach-Zehnder interferometer (MZI), cascaded sensor, long period fiber grating (LPFG), fiber Bragg grating (FBG), Fabry-Perot interferometer (FPI) and Surface plasmon resonance (SPR). Compared with the traditional fiber sensor, core-offset structure sensor with a non-axisymmetric characteristic has excellent sensing characteristics, especially in the recognition of bending direction and high RI sensitivity. Furthermore, the design scheme, working principle and sensing characteristics of different core-offset sensors are also briefly described. Finally, the existing challenges and development direction of optic fiber sensors based on core-offset structure is discussed and prospected.

High sensitivity curvature sensor based on the long period fiber grating inscribed in the two-mode fiber.

The optical fiber curvature sensor is an important device that is widely used in the field of medical instruments. In this paper, we proposed a high sensitivity curvature sensor based on a long period fiber grating (LPFG), which is inscribed in the two-mode fiber (TMF) by CO2 laser pulses. Unlike the traditional LPFG in the single-mode fiber, TMF-LPFG couples the high-order mode LP11 in the core with the modes in the cladding. From the experiment results, the wavelength shifts to the short-wave direction as the sensor bends. This sensor offers a high curvature sensitivity of -23.67nm/m-1 in the range of 1.137-4.684m-1 and -18.95nm/m-1 in the range of 4.766-6.132m-1, respectively. Moreover, this sensor has the advantage of low transmission loss, which is less than -4dB.

Dynamic mode-switchable and wavelength-tunable Brillouin random fiber laser by a high-order mode pump

We experimentally investigate two schemes of Brillouin random fiber laser (RFL) by using high-order-mode (HOM) pump in a few-mode fiber (FMF). The core-mode conversion between LP01 and LP11 modes is obtained in the FMF by cascading long period fiber gratings (LPFG) working at the same wavelength region. Different transversal modes of stimulated Brillouin scattering (SBS) can be implemented based on broadband long period fiber gratings (LPFG) and acoustically induced fiber gratings (AIFG). The RFL base on two broadband LPFGs can obtain high purity LP11 mode operating in the range of 1543 nm to 1565 nm. Moreover, the output mode can be dynamically switched between LP01 mode, LP11a mode and LP11b mode by modulating frequency shift keying (FSK) signal of the AIFG. This work has potential application prospects in the fields of mode division multiplexing systems, speckle-free imaging, free-space optical communication, laser material processing.

A corrosion threshold-controllable sensing system of Fe-C coated long period fiber gratings for life-cycle mass loss measurement of steel bars with strain and temperature compensation

In this study, a corrosion threshold-controllable sensing system of long period fiber gratings (LPFG) is developed and validated for life-cycle monitoring of steel bars in corrosive environments. Three Fe-C coated LPFG sensors with two bare LPFG sensors in LP06 and LP07 modes for strain and temperature compensation were multiplexed and deployed inside three miniature, coaxial steel tubes to measure three (long-term in years) critical mass losses through the penetration of tube walls and their corresponding (short-term in hours) corrosion rates in the life span of steel bars. The strain/temperature and mass loss measurements are based on the changes in grating period and refractive index of surrounding medium, respectively. Thermal/mechanical loading and accelerated corrosion tests were conducted to validate the functionality, sensitivity, accuracy, and robustness of the proposed sensing system. Since both the steel tube and Fe-C layer represent the material composition of steel bars in the context of corrosion, the mass loss correlation among any two of the steel tube, Fe-C layer and steel bar is independent of the test conditions such as the current density and sample length, and thus applicable to engineering practices. The outer tube can notably delay and decelerate the corrosion process of its inner steel tube due to the reduced current effect.

Long Period Fiber Grating Near Turn Around Point: Suitable Design for Bio-Sensing

We report our experimental study on the add-layer sensitivity of LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,11</sub> cladding mode (CM) of Long Period Fiber Grating (LPFG) near turn around point (TAP). The CM response was tuned by controlled modification of the cladding diameter and deposition of overlay layers of polymer material. In this process an add-layer sensitivity of 2.308 nm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WL</sub> /nm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> has been achieved. It is interesting to observe that add-layer sensitivity obtained in this process is close to the sensitivity of the same CM operated near the mode transition (MT). The advantage is that, the sensor designed as per the proposed methodology will not be plagued by the basic limitations of operating a CM near MT, i.e. loss of contrast of the resonant band and limited dynamic range. In our experiment, the dynamic range has found to be ~100 nm where the contrast remains greater than -10 dB throughout the range of measurement which is practically impossible to get for any CM operating at MT. The experimental result was supported by theoretical calculations. Also the SRI sensitivity of the sensor was found out to be 6400 nm/ SRIU.

Sensing Characteristics of Collapsed Long Period Fiber Gratings in Tri-Hole Fiber

We demonstrated a novel long-period fiber grating (LPFG) written in a tri-hole fiber (THF). The LPFG was fabricated by periodically collapsed micro holes utilizing CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser irradiation integrated with pressure-assisted technology. The mode coupling between the fundamental mode and higher mode of the proposed grating is mainly caused by geometric deformation along the fiber axis. Compared with conventional LPFGs, such a grating has strong refractive index modulation due to periodic collapsed structure. We have established an approximate hypothesis to simulate the transmission spectrum of the grating. The measured spectrum is basically consistent with the simulated transmission spectrum at the resonant peak position. We investigated the sensing characteristics of the proposed LPFG, including axial strain, bending, torsion, and temperature. The sensitivity of bending, temperature and torsion are −9.56 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , 162.8 pm/(rad/m) in clockwise and −346.9 pm/(rad/m) in counterclockwise and 86.8 pm/°C, respectively. The experimental data shows that the fabricated LPFG has potential applications in the field of external high-sensitivity strain sensing with a sensitivity of −26 pm/μϵ.

Raman suppression in 5 kW fiber amplifier using long period fiber grating fabricated by CO2 laser

Long period fiber grating (LPFG) can be used as a band rejection component, providing a potential effective way for Stimulated Raman scattering (SRS) suppression in high power fiber laser. Here, an improved CO2 laser scanning system with a galvanometer is built up for fabrication of LPFGs, which greatly improves the speed and consistency of grating inscribing. The characteristics of the manufactured LPFG is measured in detail, then a direct laser diode pumped 5 kW-level fiber amplifier system is established to test the validity of the LPFGs for Raman suppression for the first time. Experimental results show that the SRS suppression ratio is about 14 dB at 5.1 kW with the LPFG being inserted between the seed and the amplifier stage, meanwhile the effective output signal laser power is increased. In contrast to the ultraviolet laser exposed band rejection components, such as chirped and tilted fiber Bragg grating (CTFBGs), this type of LPFG does not need photosensitive fibers, annealing treatment and has a much lower thermal effect, which is very useful for high-power fiber lasers applications.

Spectral Reconstruction and Bayesian Model Framework for Characterization of Long Period Fiber Gratings

The use of Long-Period Fiber Gratings (LPFGs) as sensors has been thoroughly researched, given the multitude of parameters these structures can monitor by themselves (such as temperature, strain, curvature) and the potential for combination with other materials that allow for monitoring of parameters such as humidity, pH and chemical concentration, at a low price and with easy fabrication processes available. This interest has increased the need for the development of interrogation systems for these sensors, particularly in the C-band spectral region. Given the cost and physical limitations (such as size and weight) of traditional solutions like Optical Spectrum Analyzers (OSA), the development of low-cost approaches for LPFG spectral analysis became an important topic that needed further development. The development of a simple curve fitting routine for LPFG spectra is reported in this article, along with a framework for automatic detection of certain physical phenomena such as corrosion and the presence of chemical species, among others.

Fiber Strain Sensor Based on Incline Plane-Shaped Long Period Fiber Grating Induced by CO2 Laser Polishing

In this paper, we propose the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser polishing method to prepare a novel type of incline plane-shaped long period fiber grating (IPS-LPFG) for strain measurement. This structure is achieved by periodic incline plane-shaped structures polished on single mode fiber. The incline plane-shaped structure introduced asymmetric thickness of the side plane, which enhances the strain sensitivity of the sensor effectively. Experimental results illustrate that the compact strain sensor with a length of 5 mm achieves a high strain sensitivity of 15 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> at the range of 0- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$900~\mu \varepsilon $ </tex-math></inline-formula> , and the temperature sensitivity of it is 54.5 pm/°C. Moreover, the proposed strain sensor has good mechanical properties and high repeatability, which demonstrates the potential in optical fiber sensing.

Highly sensitive vector bending sensor based on an embedded multimode D-shaped LPFG.

In this letter, a highly sensitive bending sensor based on an embedded multimode D-shaped long period fiber grating (EMD-LPFG) is proposed. The novel sensor is applied to carry out vector bending measurement. The proposed LPFG is fabricated by polishing on the prepared structure which is formed by periodically splicing between single mode fiber (SMF) and multimode fiber (MMF). Since the cross section of the embedded MMF is D-shaped, we named it EMD-LPFG. Due to the asymmetric modulation of the refractive index on the fiber by the CO2 laser, the sensor has the ability to distinguish the bending directions, and the MMFs provide higher bending response. The experimental transmission spectrum can match the simulation results well. The experimental results show that the average bending sensitivities in three orthogonal directions are 70.21 nm/m-1 (0°), 9.75 nm/m-1 (90°), -12.04 nm/m-1 (180°) and 9.98 nm/m-1 (270°), respectively. Meanwhile, the temperature sensitivity is 30 pm/°C in the range of 25 °C to 75 °C. According to the ultra-compact structure with the total length of 2.5 mm, high bending sensitivity and ability to distinguish the bending direction, the novel sensor has potential in bending measurement.

Strain, bending, refractive index independent temperature sensor based on a graded index multimode fiber embedded long period fiber grating.

A novel embedded ultra-long period fiber grating (EULPFG) based on a graded index multimode fiber (GI-MMF) is proposed for temperature measurement. Due to the small RI difference of the modes near the GI-MMF self-imaging point, the resonant peak of transmission spectrum is wavelength-insensitive to refractive index (RI), strain and bending. However, the sensor is sensitive to temperature. The experimental results show that the temperature sensitivity of the EULPFG is 90.77 pm/°C. The sensitivities of other physical parameters are suppressed, and the suppressed sensitivities are at least one order of magnitude less than those of similar sensors. The EULPFG with anti-interference from other parameters is expected to be used in ocean monitoring systems to measure the temperature of the seawater.