Period Fiber Grating Sensor Research Articles

Theoretical analysis of dual-mode long period fiber grating sensor based on dual-peak resonance for dual-parameter measurement

A dual-mode long period fiber grating (DM-LPFG) sensor for simultaneous measurement of the surrounding refractive index (SRI) and temperature is proposed and demonstrated theoretically. It is made by inscribing a LPFG based on the dual-peak resonance in a dual-mode fiber. Known from coupled mode theory, the sensing principle of the sensor is originated from the sensibility of the dual peaks to the SRI. There are three peaks corresponding to the dual-peak resonance and single-peak resonance in different core modes in the transmission spectrum, respectively. And the three peaks have a different response to the SRI and temperature. Through the monitoring of the distance change between dual peaks and the wavelength shift of single peak, the dual-wavelength matrix method can be used to simultaneously measure the SRI and temperature. As the results show, the SRI sensitivities corresponding to the distance between the dual peaks and the resonant wavelength of the single peak are 1940.0nm/RIU and 590.0nm/RIU in the SRI range of 1.33 to 1.37, respectively. Besides, the temperature sensitivities corresponding to the distance between the dual peaks and the wavelength of the single peak are -0.42nm/°C and 0.23nm/°C with the temperature changing from 20°C to 120°C, respectively. It has great potential application in chemical and biological sensing fields, owing to its advantages of high sensitivity and multi-parameter measurement.

Monitoring Passivation, Pitting Corrosion Initiation, and Propagation of Steel Bar with Iron–Carbon Electroplated Long Period Fiber-Grating Sensor

AbstractAn iron–carbon electroplated long period grating (LPG) fiber sensor to monitor the passivation, initiation, and propagation of the pitting corrosion of steel bars is characterized in simula...

Characteristics of novel tail ring-shaped cascaded long period fiber grating sensor

Abstract In this paper, a novel tail ring-shaped cascaded long period fiber grating (TR-CLPFG) sensor is fabricated by connecting two LPFGs with ring-shaped optical fiber. The results show that the sensor can effectively reduce full width at half maxima (FWHM) and improve refractive index sensitivity. The parallel fibers in the ring-shaped structure make the two LPFGs almost identical and improve the fabrication efficiency. The smaller the sensing area leads to fewer samples to be measured. The TR-CLPFG sensor has compact structure, which is conducive to its application in narrow space. In addition, this U-shaped structure allows the sensor to be inserted into the solution measurement, which changes the previous measurement methods of LPFG sensor and makes the sensor more flexible in application. In general, this novel TR-CLPFG sensor not only maintains the advantages of traditional linear cascaded long period fiber grating (CLPFG) sensor, but also gets many superiorities in the fabrication process and application. This TR-CLPFG sensor has great potential in future detection applications.

Layer-by-layer assembly of polyelectrolytes-wrapped multi-walled carbon nanotubes on long period fiber grating sensors

A new type of long period fiber grating (LPG) sensor coated with polyelectrolytes-wrapped multi-walled carbon nanotube (MWCNT) (PE-CNT) thin film is reported in this study. The novel nanostructured thin film is deposited on the LPG grating region using a layer-by-layer assembly technique. The aim of using polyelectrolytes (PE) including poly(ethylenimine) (PEI) and poly(acrylic acid) (PAA) in the coating layer is to improve the dispersion of MWCNTs and to enhance the adhesion of functionalized MWCNTs on the LPG surface. The structures and morphologies of the MWCNT-based nanostructured thin film are observed by scanning electron microscopy characterization. The refractive index and pH sensing behaviors of LPGs with pure (PE)10 thin film and with (PE-CNT)10 thin film, respectively are compared. Results show that for LPG with (PE-CNT)10 thin film the transmission intensity at the resonance wavelength decreases noticeably with the increase of the surrounding refractive index. This is distinct from the sensing mechanism of LPG with (PE)10 thin film, which is based on the resonance wavelength shift. The LPG with a (PE-CNT)10 thin film provides enhanced pH sensitivity compared with that of a (PE)10 thin film. The LPG with (PE-CNT)10 thin film has a fast response rate and a good repeatability, thus demonstrating its strong potentiality for practical monitoring applications.

Theoretical analysis for the influence of the core radius on long period fiber grating sensors

The influence of core radius on refractive index sensitivity of long period fiber grating (LPFG) sensor has been studied theoretically for the first time, and a new method to fabricate LPFG sensor with high sensitivity is proposed. The structure and parameters of the optical fiber, the period of the gratings, and the coupling order of the cladding modes are all held consistent in the process of studying the change of the core radius. The resonant wavelength of LPFG sensor shifts with changes in the external environment, which is simulated by the transfer matrix method. The sensitivity of LPFG sensor is enhanced by more than 200 times when the core radius increases from 3.1 to 5.1 μm. The results show that LPFG sensors with larger core radii have higher sensitivity. Selecting a fiber with a large core radius to fabricate LPFG sensors is proposed as a method to improve the sensitivity of these devices. This new simple and effective method is expected to be widely used in the manufacture of high sensitivity LPFG sensors in the future.

Molybdenum sulfide/citric acid composite membrane-coated long period fiber grating sensor for measuring trace hydrogen sulfide gas

A molybdenum sulfide/citric acid composite membrane-coated long period fiber grating (LPFG) sensor was demonstrated and applied to detect the trace hydrogen sulfide gas. The sensor was fabricated by growing molybdenum sulfide/citric acid composite film on the surface of optical fiber grating by using the sol-gel and dip-coating techniques. The sensor measures the hydrogen sulfide concentration by monitoring the molecular adsorption-induced resonant wavelength shift of LPFG in near infrared (∼1555 nm) region. Experimental results show that with the increasing concentration of hydrogen sulfide, the transmission spectrum appears blue shift with time response of 89 s. In addition, a high sensitivity of 10.52 pm/ppm and a good linear relationship are achieved within a measurement range from 0 to 70 ppm. Furthermore, there is an excellent selectivity for H2S, which has also been confirmed by the surface adsorption energy results of molybdenum sulfide with five common gases in the air (H2S, N2, CO2, O2 and Ar) based on the density functional theory calculations. The proposed sensor has an excellent detection limit of hydrogen sulfide which is as low as 0.5 ppm. This sensor has the advantages of simple structure, easy manufacture, high sensitivity and low cost, and can be used in hydrogen sulfide gas sensing places such as factories, sewers, and so on.

Detection of mercury (II) ions in water by polyelectrolyte–gold nanoparticles coated long period fiber grating sensor

This paper presents mercury (II) ions detection based on long period fiber grating (LPFG) sensor written on a single mode optical fiber by electrical arc induced technique that is suitable to be used for long term monitoring purpose. In the work, the LPFG was coated with both polyelectrolyte (PE) layers to enhance its sensitivity as well as a layer of gold nanoparticles (AuNP) for reaction to the mercury (II) ions. Experiments were conducted using double-pass configurations with mercury (II) ions concentrations varied between 0.5 ppm to 10 ppm. The results showed that the resonance wavelength of the PE–AuNP coated LPFG notch shifted to the longer wavelength, with a total shift of 1.34 nm and transmission power increment of 1.74 dBm over a period of 5 h. The results were then compared with uncoated as well as PE-only coated LPFGs, where no significant changes in resonance wavelength and transmission power were observed for these LPFGs. A novel PE–AuNP coated LPFG sensor that is suitable to be used for in-situ, long term and remote monitoring has been successfully demonstrated and tested for the detection of mercury (II) ions in water.

Highly reflective long period fiber grating sensor and its application in refractive index sensing

In this work a highly reflective long period fiber grating (RLPG) sensor is proposed. The sensor was fabricated by depositing the silver atom on both surfaces of the flat end-face and the tail profile of long period fiber grating (LPG), which makes the LPG work in high reflection mode without interferometric fringe and enhances the coupling at the resonant wavelength of original LPG. The RI-response characteristic of the RLPG was investigated. The reflection spectrum remains the resonant dip without interference infringe and the resonant wavelength has obvious blue shift with the increase of external RI. By utilizing the above characteristic, we propose an application for long distance RI measurement combining a standard optical time domain reflectometer (OTDR). Preliminary results of using the sensor to measure the refractive index of glycerol/water solutions in a distance of 100km were successfully demonstrated.

Long period fiber grating sensors coated with nano iron/silica particles for corrosion monitoring

In this study, a novel long period fiber grating (LPFG) sensor coated with a thin film of nano iron and silica particles is proposed, designed, and tested for corrosion and environmental monitoring. Nano iron particles were introduced to facilitate the monitoring of the corrosion rate while nano silica particles were added to improve coating transparency and robustness. Proof-of-concept tests have demonstrated that the coated thin film did not affect the sensitivities of either the intensity or wavelength of the LPFG sensor to temperature and pH values. However, the resonant wavelength of the coated LPFG sensor increased as iron particles were gradually corroded away. Therefore, with a parallel LPFG sensor for nearby temperature and pH measurements, the proposed novel sensor can measure the corrosion rate of nano iron particles and, once calibrated and correlated, provide critical information about a nearby reinforcing steel bar and structural member.

Zeolite thin film-coated long period fiber grating sensor for measuring trace organic vapors

A zeolite thin film-coated long period fiber grating (LPFG) sensor was developed and tested for direct measurement of trace organic vapors with straightforward operation. The sensor was fabricated by growing pure silica MFI-type zeolite (i.e. silicalite) thin film on the optical fiber grating by in situ hydrothermal crystallization. The sensor measures chemical vapor concentration by monitoring the molecular adsorption-induced shift of LPFG resonant wavelength (λR) in near infrared (IR) region. Upon loading analyte molecules, the zeolite's refractive index changes in the close vicinity of the fiber index where the λR responds with extremely high sensitivity. Because the silicalite is hydrophobic and preferentially adsorbs organic molecules, the zeolite-LPFG sensor is capable of quantifying organic vapor concentrations in lower ppm or ppb ranges without sample pre-concentration and rigorous control of operating conditions. The sensor had a much faster response speed for isopropanol than for toluene as the former has a smaller molecular kinetic size and greater diffusivity in the zeolite pores than the latter.

Zeolite thin film-coated long period fiber grating sensor for measuring trace chemical

This paper reports the development of a new zeolite thin film-coated long period fiber grating (LPFG) sensor for direct measurement of trace organic vapors. The sensor was fabricated by growing pure silica MFI-type zeolite thin film on the optical fiber grating by in situ hydrothermal crystallization. The sensor measures chemical vapor concentration by monitoring the molecular adsorption-induced shift of LPFG resonant wavelength (lambda(R)) in near infrared (IR) region. Upon loading analyte molecules, the zeolite's refractive index changes in the close vicinity of the fiber index where the LPFG has a large response to achieve high sensitivity.

A novel long period fiber grating sensor measuring curvature and determining bend-direction simultaneously

A novel long period fiber-grating (LPFG) sensor that can not only measure curvature directly, but also determine every bend-direction within the circular range of 0/spl deg/-360/spl deg/, is proposed and demonstrated in this paper. Such a bend-sensor consists of one LPFG induced by a UV laser and two LPFGs induced by high-frequency CO/sub 2/ laser pulses. The curvature is measured by the UV laser-induced LPFG whose bend-sensitivity is independent of the bend-directions, and the bend-direction is determined by the CO/sub 2/ laser-induced LPFGs whose bend-sensitivities depend strongly on the curved directions. In addition, the unique bend-characteristics of LPFGs induced by high-frequency CO/sub 2/ laser pulses are demonstrated.

Comparison of single and double cladding long period fiber grating sensor using an intensity modulation interrogation system

The performance of long period fiber grating (LPFG) sensors written in single cladding and double cladding fibers have been compared by using a fast responding interrogation system based on intensity modulation. Temperature and dynamic strain monitoring using this system have been demonstrated. This system is capable of resolving strain to 0.2 and 0.4 μ ε at a loading frequency of 20 Hz, and temperature resolution to 0.02 and 0.19 °C by using LPFG in the single cladding (SC-LPFG) and double cladding (DC-LPFG), respectively.