Period Fiber Research Articles

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.

Fabrication and application of a novel long period fiber grating with arched fiber cores

In this paper, the fabrication of arched fiber core based on single-mode fiber is first proposed and experimentally studied about the relevant sensing characteristics. A novel kind of long-period fiber grating (LPFG) with good sensing performance is formed by increasing the number of arched cores with a fixed cycle. The arched core long-period fiber grating (AC-LPFG) has excellent bending sensitivities with −25.06 nm/m−1, −4.26 nm/m−1, 16.84 nm/m−1, −4.92 nm/m−1 in four different directions, and it shows that the AC-LPFG can distinguish the bending direction very well. Meanwhile, the strain sensitivity and temperature sensitivity of the AC-LPFG are also experimentally tested to be −5.9 pm/μm and 84.5 pm/℃ respectively. The excellent bending and strain characteristics of the AC-LPFG indicate that the arched fiber core has similar effects as some special fibers, which will surely get closer attention and more research applications in the future.

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.

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.

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.

Hybrid hierarchical homogenization theory for unidirectional CNTs-coated fuzzy fiber composites undergoing inelastic deformations

A new hybrid homogenization approach is proposed for simulating the homogenized and local response of unidirectional fuzzy fiber nanocomposites undergoing inelastic deformations. Fuzzy fiber composites are hierarchical reinforcing structures where the fibers coated with radially aligned carbon nanotubes (CNTs) are embedded in the matrix. In this spirit, the fuzzy fiber composites are modeled as a three-scale medium. At the microscale, the CNTs-reinforced matrix is homogenized as nanocomposite interphase (NCP) attached to the main fiber via the asymptotic expansion homogenization (AEH). At the mesoscale, an intermediate equivalent fiber that substitutes for the NCP and the main fiber is constructed using the composite cylinder assemblage (CCA) and the transformation field analysis (TFA) techniques. At the macroscale, homogenization of the equivalent fiber and the surrounding matrix is handled by AEH which yields the effective response of the whole fuzzy fiber composites. The new technique facilitates accurate and efficient studies of the inelastic deformation mechanisms of periodic fuzzy fiber arrays with single or multiple inclusions under biaxial and triaxial loading conditions, eliminating exhausting interphase mesh discretizations encountered in the classical full-field homogenization. An added advantage is that the proposed theory captures the fiber-fiber interaction neglected in the classical CCA-TFA, an issue that leads to the exceptionally stiff post-yielding stress-strain response common in the mean-field micromechanics approaches.

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.

Label-free detection of vitamin D by optical biosensing based on long period fiber grating

In this work, we report a fiber optic biosensor for the label-free detection of vitamin D, by targeting the major circulating form of D3 in human body, i.e., 25-hydroxyvitamin D3 (25(OH)D3). The sensing platform relies on a long period grating (LPG) written into an unconventional fiber, i.e., double cladding fiber (DCF) with W-type refractive index profile. The sensitivity to surrounding medium is enhanced by chemical etching of the outer cladding as the working point of the device is tuned to the mode transition region. The DCF structure allows to combine the enhancement of the sensitivity (up to −1400 nm/RIU) with a good visibility of the grating resonance band (>10 dB) and long-term stability due to an all-silica structure. Moreover, the LPG transducer is coated with a nanosized layer of graphene oxide to provide carboxylic functional groups (-COOH) for the grafting of the biological recognition element for vitamin D3, i.e., a 25(OH)D3 specific antibody. To the best of our knowledge, this is the first evidence of a fiber optic biosensor for the detection of vitamin D3, which was performed using concentrations within the clinically relevant range of 1–1000 ng/mL and achieving a low limit of detection below 1.0 ng/mL in buffer solution that is well comparable with the state of the art. Finally, the performance of the biosensor was also evaluated in a complex medium with interfering proteins at physiological concentration obtaining promising results.

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.

Mode converter with C+L band coverage based on the femtosecond laser inscribed long period fiber grating.

The mode converter (MC) is one of the key components for mode division multiplexing (MDM). Here, we experimentally demonstrate an all-fiber long period grating (LPG) based MC inscribed in the few-mode fiber (FMF) with the line-by-line femtosecond laser irradiation technique. Experimental characterization results agree well with the theoretical calculations, and a clear mode evolution from the LP01 mode to the LP11 mode is observed with the tuning of the operation wavelength. An average mode conversion efficiency of more than 90% and an average insertion loss of less than 5 dB, together with a polarization-dependent loss of less than 3 dB, are achieved over the C+L band with a good repeatability. The proposed MC based on the LPG inscribed into the FMF has the advantages of mode scalability, compact size, and wideband operation, which is desired for the wavelength division multiplexing (WDM) and MDM hybrid transmission.

Application of the principal components analysis technique to optical fiber sensors for acetone detection

Generally, the data analysis of sensors based on spectral measurements, for instance, fiber Bragg gratings (FBG), long period fiber gratings (LPFG) is performed observing the shift or attenuation of a single wavelength. However, sometimes such spectrum variations are not evident or clearly perceptible; therefore, it becomes necessary to use other analysis techniques. One of them is principal components analysis (PCA) which allows to explore the behavior of the complete transmission spectrum pattern. In this work, we show the application of PCA to the response of fiber optical sensors for acetone detection. Two kinds of acetone sensors based upon single long period fiber gratings (LPFG) and Mach-Zehnder fiber interferometers (MZI), formed with a pair of cascaded LPFGs, coated with polydimethylsiloxane (PDMS) were developed. The sensor response was measured inside a Teflon chamber injecting a liquid acetone sample. The transmission spectrum variations at the devices output were measured with an optical spectrum analyzer, stored in a computer and analyzed by PCA. A comparison between the performance of both kinds of sensors was carried out and it was found that the sensitivity was increased up to 13 times (from 0.7 × 10−4 ppm−1 to 9.4 × 10−4 ppm−1) for the MZI sensors compared with that of single LPFG ones, which means, at best, a limit of detection (LOD) of 170 ppm.

Raman suppression in high-power fiber laser oscillator by long period fiber grating

Stimulated Raman scattering (SRS) is a major limiting factor for better performance of high-power fiber lasers. Several spectral filtering methods have been used for Raman suppression, among which long period fiber grating (LPFG) is relatively simple and effective due to its all-fiber structure and broadband rejection properties. Here, a broadband, low insertion loss and low thermal effect LPFG is easily fabricated using CO2 laser by galvanometer scanning, then is used for SRS suppression within a high-power fiber oscillator for the first time. Results show that a maximum SRS suppression ratio of ~16 dB was achieved and the extractable Raman-free output power was increased by about 13% with the insertion of LPFG. This work has demonstrated a potentially applicable method for Raman suppression in high-power fiber oscillators.

Single- and dual-wavelength fiber laser with multi-transverse modes.

A single- and dual-wavelength fiber laser with multi-transverse modes is proposed. Mode interference is realized in the core of an optical fiber by writing a long period fiber grating on a few-mode fiber to obtain the LP01 mode and the LP11 mode simultaneously. A mode interferometer based on a few-mode long period fiber grating (FM-LPFG) is used as a comb filter in the ring-cavity fiber laser. Single- and dual-wavelength outputs can be achieved in the fiber laser by adjusting a polarization controller (PC). A mode-selective photonic lantern is used to realize mode conversion, and six LP modes, LP01, LP11a, LP11b, LP21a, LP21b, and LP02, can be generated. A single- and dual-wavelength fiber laser with multi-transverse modes can be achieved by combining a mode interferometer with a mode-selective photonic lantern. This work has potential applications in mode division multiplexing (MDM) systems to enlarge the capacity of optical communications.

High-Order Mode Mode-Locked Fiber Laser Based on Few-Mode Saturable Absorber

We presented two kinds of high-order mode (HOM) mode-locked fiber lasers based on few-mode (FM) saturable absorbers for direct HOM oscillation. One is a ring fiber laser based on a carbon nanotube (CNT) saturable absorber (SA), in which the combination of a long period fiber grating (LPFG) and a mode selective coupler (MSC) is used for the cycle of mode conversion between LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes. The other is a linear-cavity laser using a FM-based SESAM with the HOM oscillation, in which a MSC is used for mode conversion inside the cavity. The presented mode-locked fiber lasers can achieve the saturable absorption of the HOM and the cylindrical vector beams (CVBs). And we also compared the nonlinear saturation absorption characteristics of two SAs operating at LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes.

A Sensitive Ammonia Sensor Using Long Period Fiber Grating Coated With Graphene Oxide/Cellulose Acetate

A novel functionalized long period fiber grating (LPFG) sensor based on graphene oxide (GO)/cellulose acetate (CA) composites was developed for ammonia sensing. The adsorption of ammonia molecules by GO/CA composite materials changes the coupling efficiency of long period grating, further leads to the resonance wavelength shift. The surface morphology and structure of GO/CA composites material were characterized using the Scanning Electron Microscope, Raman spectroscopy, X-ray diffractometer, thermal analyzer and Fourier transformation Infrared Spectrometer. The effect of humidity on the sensor probe efficiency was also investigated. The composite mechanism between GO and CA and the mechanism of ammonia adsorption by composite membrane were studied using the density functional theory (DFT). The DFT calculation shows that there is no chemical bond forming between CA and GO, but there is a tendency to form hydrogen bond. The ammonia optic fiber sensor exhibits excellent performance in response time (32 s), recovery time (68 s) and exhibits excellent sensitivity (98.32 pm/ppm).