Sensitivity Of Long-period Fiber Gratings Research Articles

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

Biosensing by Polymer-Coated Etched Long-Period Fiber Gratings Working near Mode Transition and Turn-around Point.

A methodology to enhance the sensitivity of long-period fiber gratings (LPFGs) based on the combination of three different enhancement approaches is presented; the methods here adopted are the working near mode transition (MT) of a cladding mode (CM), working near the turn-around point of a CM and the enhancement of the evanescent field of CMs by reducing the cladding diameter or by increasing the order number of CMs. In order to combine these enhancement methodologies, an electrostatic self-assembly (ESA) process was used to deposit a polymeric overlay, with a chosen thickness, onto the etched fiber. The add-layer sensitivity of the sensor was theoretically calculated, and the demonstration of the real applicability of the developed LPFG as a biosensor was performed by means of an IgG/anti-IgG immunoassay in human serum in a thermostated microfluidic system. The limits of detection (LODs) calculated by following different procedures (three times the standard deviation of the blank and the mean value of the residuals) were 6.9 × 10-8 µg/mL and 4.5 × 10-6 µg/mL, respectively. The calculated LODs demonstrate the effectiveness of the applied methodology for sensitivity enhancement.

Mode Dependence of Refractive Index Sensitivity of Long-Period Fiber Gratings Induced by Periodically Embedded Multimode Fiber

In this paper, we present and verify the mode dependence of refractive index (RI) sensitivity of long-period fiber grating induced by periodically embedded multimode fiber (PEM-LPFG). The PEM-LPFG is firstly fabricated by periodically embedding multimode fiber (MMF) into the single mode fiber (SMF). The resonant mode is translated by reducing the cladding. Numerical simulations based on beam propagation method (BPM) are applied to calculate the transmission spectrum, mode distribution, and RI sensitivities of the PEM-LPFG. The cladding diameter after etching is determined to be 89 μm and 68 μm, which corresponds to the mode translated from LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">012</sub> to LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">07</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">05</sub> , respectively. The experimental results indicate that the maximum refractive index sensitivity of 1322.26 nm/RIU in the range of 1.333 -1.437 RIU is achieved when the resonant mode is LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">07</sub> , which is 4 times as large as the original one (325.58 nm/RIU @ LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">012</sub> mode). PEM-LPFG working at LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">05</sub> mode exhibits a lower sensitivity of 844.72 nm/RIU. All of the experimental results are in good agreement with the simulation. The RI sensitivity optimization of PEM-LPFG is able to realize by mode converting. Furthermore, the temperature responses are measured to be 13 pm/°C and 4 pm/°C. This work provides a theoretical reference and experimental evidence for the dependence of the PEM-LPFG RI sensitivity on the resonant mode for the first time.

Long-Period Fiber Grating Probe: An Improved Design Suitable for Biosensing

Add-layer sensitivity of long-period fiber grating (LPFG) near mode transition (MT) has been studied in reflection configuration to realize a highly sensitive LPFG-based sensor probe suitable for biosensing. The dependency of the sensitivity on the separation between the resonant cladding modes considered during MT has also been investigated. The separation between two consecutive cladding modes was precisely increased or stretched to a desired value by reducing the cladding diameter. The probe was formed by cleaving the mode stretched LPFG (MSLPFG) through the grating region. The MSLPFG was designed to operate near MT by deposition of electrostatic self-assembled (ESA) polymeric overlay layers on the grating surface. We demonstrate that the polymeric overlay not only increases the sensitivity of the sensor but also eliminates undesirable multiple resonant bands that appear due to the introduction of arbitrary phase while cleaving the grating. Add-layer sensitivity of the MSLPFG sensor probe was obtained and found to be ~2.0 nmWL/nmTH with a significantly enhanced peak attenuation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim -30$ </tex-math></inline-formula> dB around the MT region.

Ambient refractive index sensitivity of long-period fiber grating (LPFG) with reduced cladding thickness using three-layer fiber geometry approach

Abstract This article presents the refractive index (RI) sensitivity study of cladding reduced uniform long-period fiber grating. The displacement of resonant wavelength with the change in ambient RI is observed to inspect the sensitivity. Here, the grating period is fixed at a value (550 μm) to analyze RI sensitivity of cladding modes (HE13-HE14). However, it varies in earlier mathematical studies to adjust the resonant wavelength of all modes at a specific wavelength. The three-layer fiber geometry-based mathematical approach is employed to estimate the core and cladding modes. The obtained results support decline in quantity of cladding modes sustained by a single-mode fiber. The decrease in cladding radius has shown enhancement in RI sensitivity. For a reduction of 22 μm in cladding radius, the highest ordered HE14 cladding mode has shown 5.8 times improvement in its sensitivity.

Quantification of chloride and iron in sugar factory effluent using long period fiber grating chemical sensor

Abstract This paper is an effort to introduce application of Long period fiber grating (LPG) sensors in perpetual monitoring of industrial waste water effluent. A change in ambient refractive index (RI) of an LPG causes a shift in its resonant wavelength. This RI sensitivity of LPG has been used to design a simple and highly sensitive chemical sensor to determine the concentration of Chloride (Cl) and Iron (Fe) ions present in the treated waste water effluent of sugar factories. The concentrations of Cl and Fe ions in the effluent sample determined by our sensor are 97.85 ppm and 0.274 ppm respectively. The results are in good agreement with that of two standard techniques used for the purpose, thus proving the reliability of the LPG sensor. Limit of detection of Cl and Fe ions are found to be as low as 0.038 ppm and 0.007 ppm per pm shift in wavelength respectively, reflecting enormous potential of the sensor in detection of trace elements in waste water with accuracy. Sensitivity of the sensor for Fe ions being about 5 times greater than that for Cl ions, suggests a higher sensitivity of sensor in lower range of concentration in ppm. Miniaturization, cost-effectiveness and ability of online measurement are the most important features of LPG chemical sensors that give them an edge over existing techniques for the purpose.

Effects of cladding radius on the optical characteristics of long-period fiber gratings

Using three-layer vector field model, the dispersion characteristics and spectral response of long-period fiber gratings (LPFGs) with different cladding radii are analyzed. Both the numerical and experimental results demonstrate that LPFG’s resonant wavelengths shift towards much longer wavelength and the transmission depth is strengthened significantly by decreasing cladding radius. When the cladding radius is less than 20 μm, only a single resonant peak over a wavelength range of 600 nm (from 1200 nm to 1800 nm) is achieved, and the sensitivity of LPFG to external refractive index is enhanced significantly.

Effects of boron in &lt;inline-formula&gt;&lt;math altimg="none" display="inline" overflow="scroll"&gt;&lt;mrow&gt;&lt;mi&gt;Ge&lt;/mi&gt;&lt;mtext&gt;-&lt;/mtext&gt;&lt;mi mathvariant="normal"&gt;B&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/inline-formula&gt; co-doped fiber on the spectral characteristics of optical fiber gratings

We investigate the effects of co-doping of boron on the index difference between the core and cladding of the optical fiber, the temperature and bending sensitivities of a long-period fiber grating (LPFG), and the resonance wavelength separation between the core mode and the first cladding mode of a fiber Bragg grating (FBG). We observe that the index difference between the core and cladding decreases with the slope of 1.69×10−4/SCCM and that the temperature sensitivity of the resonance wavelength shift of LPFG decreased with the slope of 0.01145 nm/°C/SCCM. The measurement results indicate that, as the amount of the co-doped boron is increased, the bending sensitivity of LPFGs increases, while the resonance wavelength separation between the core mode and the first cladding mode of FBGs decreases. These results may be used for design of Ge-B co-doped optical fibers with the desired characteristics suitable for optical fiber communication and sensing applications.

Bending sensitivity of long-period fiber gratings inscribed in holey fibers depending on an axial rotation angle

We discuss bending properties of a long-period fiber grating (LPFG) inscribed into a holey fiber (HF) depending on an axial rotation angle. High quality of the HF-based LPFG with a high extinction ratio of more than 20 dB is achieved. The proposed HF-based LPFG has bending insensitivity under a certain range of the bending curvature. As the bending curvature is higher than 4 m(-1), the center wavelength of the grating is shifted into the shorter wavelength. Bending sensitivity of the HF-based LPFG is changed by an axial rotation angle, which shows its dependence on the rotational orientation. We measure the transmission characteristics of the HF-based LPFG with the ambient index change. The HF-based LPFG has ambient index insensitivity because of the air holes in the inner cladding.

Deposition of coatings on long-period fiber gratings: tunnel effect analogy

The sensitivity of Long-period fiber gratings (LPFGs), coated with high-refractive-index thin film overlays, to the refractive index and the thickness of the overlay, and to the ambient refractive index, can be enhanced with a design based on a two-overlay coating of an LPFG. The first overlay of lower refractive index than the cladding affects the guidance of a cladding mode in the second overlay of higher refractive index than the cladding. This causes a more abrupt cladding modal redistribution than with the deposition of a unique high-refractive-index overlay. The phenomenon is analyzed with a method based on a vectorial analysis of modes and the application of coupled mode theory.

Widely tunable long-period fiber gratings with interpolymers as sensitivity-enhanced materials

By introducing an equivalent grating period, the influences of environmental index on the tuning ability and tunable sensitivity of long-period fiber gratings are analyzed and explained in this paper. Widely tunable performance is demonstrated by coating the long-period fiber gratings with acrylate-based interpolymers. Approximate 60nm wavelength tuning range and 1nm/°C thermal tunable sensitivity are implemented with the change of temperature from 15 to 70°C.

Compositional dependence of the temperature sensitivity in long-period fiber gratings with doping concentration of &lt;inline-formula&gt;&lt;math display="inline" overflow="scroll"&gt;&lt;msub&gt;&lt;mi&gt;GeO&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/math&gt;&lt;/inline-formula&gt; and &lt;inline-formula&gt;&lt;math display="inline" overflow="scroll"&gt;&lt;msub&gt;&lt;mstyle mathvariant="normal"&gt;&lt;mtext&gt;B&lt;/mtext&gt;&lt;/mstyle&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;msub&gt;&lt;mstyle mathvariant="normal"&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;/mstyle&gt;&lt;mrow&gt;&lt;mn&gt;3&lt;/mn&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/math&gt;&lt;/inline-formula&gt; and their applications

The systematic analysis of the relationship between temperature sensitivity of long-period fiber gratings and the doping concentrations of GeO2 and B2O3 in the core and cladding regions, depending on their structure of the refractive index profile, is investigated. Based on this analysis, the temperature sensitivities of fiber gratings are suppressed and enhanced to 0.002 and –0.44 nm/°C, respectively. We also discuss the versatile applications of long-period fiber gratings to gain-flattening filters of optical amplifiers and strain/temperature sensors, corresponding to their temperature sensitivity. The theoretical and experimental results are very useful for the design of optical communication devices and mechanical sensors based on long-period fiber gratings

Sensitivity characteristics of long-period fiber gratings

We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported.