Arc-induced Long-period Fiber Gratings Research Articles

Fabrication and Sensing Characteristics of Long-Period Fiber Grating in Capillary Fiber

We propose a novel fiber device based on arc-induced long-period fiber gratings (LPFGs) in a hollow core silica capillary. This device is made up of a lead-in single-mode fiber (SMF), LPFGs in capillary (CLPFGs), and a lead-out SMF, with a tapered air cavity (TAC) connecting SMF and capillary. The transmissive characteristics of capillary with and without LPFGs are analyzed theoretically and experimentally. Mode coupling of CLPFGs at longer wavelength could be enhanced, as well as the sensing performance increases times over the capillary without gratings. The refractive index (RI) sensitivity can reach to be 15 713.5 nm/RIU in the RI range from 1.451 to 1.456. Moreover, with the asymmetric index distribution induced in the capillary, the CLPFGs present vector bending properties that the bending sensitivities are 79.4 and −8.8 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{1}}$ </tex-math></inline-formula> , for bending directions at 0° and 180°, respectively. Furthermore, the proposed device shows negligible change to the external temperature variation, which could solve the cross-sensitivity problem. With the advantages of compactness, low cost, and ease of fabrication, the proposed device is potential in high-sensitivity RI sensing fields and vector bending measurement.

High Sensitivity Cryogenic Temperature Sensors Based on Arc-Induced Long-Period Fiber Gratings.

In this paper, we investigated the evolution of the dispersion curves of long-period fiber gratings (LPFGs) from room temperature down to 0 K. We considered gratings arc-induced in the SMF28 fiber and in two B/Ge co-doped fibers. Computer simulations were performed based on previously published experimental data. We found that the dispersion curves belonging to the lowest-order cladding modes are the most affected by the temperature changes, but those changes are minute when considering cladding modes with dispersion turning points (DTP) in the telecommunication windows. The temperature sensitivity is higher for gratings inscribed in the B/Ge co-doped fibers near DTP and the optimum grating period can be chosen at room temperature. A temperature sensitivity as high as −850 pm/K can be obtained in the 100–200 K temperature range, while a value of −170 pm/K is reachable at 20 K.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part II: Properties and Applications in Optical Communications and Sensing.

In this work, we review the most important achievements of INESC TEC related to the properties and applications of arc-induced long-period fiber gratings. The polarization dependence loss, the spectral behavior at temperatures ranging from cryogenic up to 1200 °C and under exposure to ultraviolet and gamma radiation is described. The dependence of gratings sensitivity on the fabrication parameters is discussed. Several applications in optical communications and sensing domains are referred.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part I: Fabrication, Characterization and Mechanisms of Formation.

In this work, we reviewed the most important achievements of INESC TEC related to the fabrication of long-period fiber gratings using the electric arc technique. We focused on the fabrication setup, the type of fiber used, and the effect of the fabrication parameters on the gratings’ transmission spectra. The theory was presented, as well as a discussion on the mechanisms responsible for the formation of the gratings, supported by the measurement of the temperature reached by the fiber during an electric arc discharge.

Fabrication and sensing characteristics of arc-induced long-period fiber gratings based on thin-cladding fiber

The fabrication of long period fiber gratings (LPFGs) based on thin-cladding fiber (TCF) has been demonstrated by adopting electric-arc discharge (EAD) technique. In order to analyze the sensing characteristics of arc-induced TCF LPFGs, the refractive index (RI), temperature and strain sensing characteristics were also experimentally investigated, respectively. Compared with conventional LPFGs, the excellent performance of the LPFGs based on TCF is that it has a higher RI sensitivity for the same order cladding mode due to the stronger evanescent field of thinner cladding. The proposed LPFGs based on this simple fabrication method have a narrow full width at half maximum (FWHM), which provides the possibility for using as filters or sensors in the applications of optical communications or fiber-optic sensing.

Comparative study of coupling to symmetric and antisymmetric cladding modes in long-period fiber gratings

ABSTRACTWe analyse the influence of coupling to symmetric and antisymmetric cladding modes in arc-induced Long-Period Fiber Gratings for temperature and strain sensing. The origin of this difference in energy coupling is related to the fabrication process of these gratings and depends on the electric arc discharge conditions, which modulates the refractive index and geometry of the optical fiber. Finally, results demonstrate the performance of different cladding modes excited in arc-induced LPFGs to temperature and strain applications and, in addition, indicate which coupling might be appropriate to certain sensing applications.

Reduction of intrinsic polarization dependence in arc-induced long-period fiber gratings

We report a production technique of reducing the polarization-dependent loss (PDL) always present in arc-induced long-period fiber gratings (LPFGs). The proposed fabrication method is based upon the rotation of the optical fiber by an axial angle of 180 deg at each electric arc discharge, therefore, inducing opposite point-by-point asymmetric modulations over the entire length of the LPFG. PDL measurements of different arc-induced LPFGs fabricated by both the conventional and proposed method are presented. Finally, the obtained results show a significant decrease of the intrinsic PDL of arc-induced LPFGs with the proposed fabrication technique.

Sensitivity-optimized long-period fiber gratings for refractive index and temperature sensing

ABSTRACTA long-period fiber grating sensor was fabricated by periodically changing the structure of single-mode fiber with an electric arc discharge technique. After the fabrication, the refractive index and temperature sensitivities were optimized by etching the cladding with hydrofluoric acid solution. The experimental results illustrate that the thinner cladding shows relatively higher refractive index and temperature sensitivities for the same order cladding mode, which are accordant with that of numerical simulation. After the long-period fiber grating was etched for 15 min, average refractive index sensitivities of 214 nm/refractive index unit (RIU) (1.3333 – 1.3931) and 1987 nm/RIU (1.4115–1.4555) were achieved. An extremely higher refractive index sensitivity of 2731 nm/RIU appears near 1.4555. By systematically studying the temperature sensing characteristic of cladding-etched long-period fiber grating in this work for the first time, the temperature sensitivity can reach as high as 144.23 pm/°C when the ambient temperature changes from 30 to 80°C. This work provides a theoretical reference for the fabrication of a high-sensitivity refractive index and temperature sensor based on arc-induced long-period fiber grating.

Electrically tunable long-period fiber gratings with low-birefringence liquid crystal near the turn-around point

In this work, an electrically tunable long-period fiber grating (LPFG) coated with liquid crystal layer (LC) is presented. As a LC layer, a prototype low-birefringence 1550A LC mixture was chosen. As a LPFG host, two types of gratings were studied: the LPFGs based on a standard telecommunication fiber, produced by an electric arc technique with a period of 222 μm, and the LPFGs based on a boron co-doped fiber written by a UV technique with a period of 226.8 μm. The relatively short period of these gratings allowed exploiting unique sensing properties of the attenuation bands associated with modes close to the turn-around point. Experiments carried out showed that for the UV-induced LPFG with a LC layer, on the powered state the attenuation band could be offset from the attenuation band measured in the unpowered state by almost 130 nm. When the arc-induced LPFG was coated with the LC, the depth of the attenuation band could be efficiently controlled by applying an external E-field. Additionally, all experimental results obtained in this work were supported by the theoretical analysis based on a model developed with Optigrating v.4.2 software.

Temperature-dependent spectral response of arc-induced long-period fiber gratings accompanied with acrylic coating

Temperature-dependent spectral response of arc-induced long-period fiber gratings accompanied with acrylic coating

Arc-discharge effects on residual stress and refractive index in single-mode optical fibers.

Arc-discharge effects on the residual stress and refractive index in single-mode optical fibers are investigated using a previously developed three-dimensional concurrent stress-index measurement method. Using commercial optical fibers and a commercial fusion splicer, the residual stress and refractive index perturbations caused by weak electrical arc discharges in single-mode fibers were measured. Refractive index changes greater than 10-4 and longitudinal perturbation lengths of less than 500μm were shown to be possible. The subsequent prospects for arc-induced long-period fiber gratings are analyzed, and a typical transmission resonance is predicted to have a depth of 56dB and a bandwidth of 0.08nm at a wavelength of 1585nm. The results of this investigation will be useful in modeling device performance and optimization of arc-induced long-period fiber grating fabrication.

Arc-Induced Long-Period Fiber Gratings in the Dispersion Turning Points

We demonstrated the possibility to inscribe long-period fiber gratings (LPFGs) in a B/Ge codoped fiber by using grating periods shorter than 150 μm. We also have arc-induced in the SMF 28 fiber an LPFG in the dispersion turning points by using a grating period of 197 μm. In previous works, the shortest periods were, respectively, of the order of 190 and 320 μm for the same fibers. To achieve such a considerable reduction in the grating periods which enables access to the higher order cladding modes (higher sensitivity), we have developed a high-voltage power supply that allows for a constant and stable electric current ranging from 10.5 up to 21 mA. Computer simulations were used to identify the cladding mode resonances for each grating inscribed in the different fibers. The fabricated LPFGs were characterized as a function of the external refractive index from 1.33 up to 1.42, and an average refractive index sensitivity of –720 nm/RIU in the 1.33–1.41 range was obtained for a 192-μm LPFG without further optimization, such as the use of etching or thin films deposition.

Asymmetric mode coupling in arc-induced long-period fiber gratings

An extensive experimental study of the transverse modal field characteristics of mircrobend arc-induced long-period fiber gratings is presented. A wavelength scanning of the near-field intensity pattern inside each loss band in the transmission spectrum, shows a clear asymmetry in the transverse intensity distribution resulting from the fabrication method. This asymmetry reflects as a 10.7dB difference in the notch depths for two orthogonal polarizations. Though a one year study, it was found that that environmental conditions during fabrication strongly affects the gratings characteristics. The best performance was obtained during the autumn season, where microbend arc-induced long-period fiber gratings produce wavelength filters with short lengths (between 10 and 30 periods for depths in excess of 20dB) and the insertion loss may be as low as 0.12dB.

Double-pass configuration to enhance the sensitivity of a polyelectrolyte-coated arc-induced long-period fiber grating

In this paper, we introduce a simple and effective method to improve the transmission power respond of a polyelectrolyte (PE)-coated arc-induced long-period fiber grating (LPFG) sensor toward external perturbation. Experiments using single-pass and double-pass configurations were conducted to measure sucrose concentrations of between 0 and 80 wt%. The results showed that the transmission power sensitivity of the PE-coated LPFG sensor in the double-pass configuration was almost double compared to the single-pass configuration.

Numerical analysis of GeO2-concentration effects in arc-induced long-period fiber gratings under external refractive-index changes

We present a numerical analysis of the response of arc-induced long-period fiber gratings (LPFGs) under ambient refractive-index changes using standard fibers with SiO2 as cladding material and different GeO2 concentration in the SiO2-GeO2 doped core. We obtain that the LPFG-sensitivity is increased using lower GeO2 concentrations and it follows a parabolic behavior. Also, we identify two linear regimes using an external index of 1.37: for GeO2 concentrations between 2 and 3.5 mol %, the LPFG-sensitivity changes with a rate of 3.614 nm per mole fraction of GeO2, while for higher concentrations the sensitivity changes with a rate of 1.39 nm/mol %. Additionally, for fibers with 2 mol% of GeO2, the sensitivity calculated is comparable to this obtained in previous works using corrugated and tapered LPFGs in SMF28 fibers. These results are of interest for the design of improved LPFG-sensors.

Arc-induced long-period fiber gratings inscribed in asymmetric transition tapers

Long-period fiber gratings (LPFGs) and tapered fibers are spectrally selective devices with important applications in sensing, especially for monitoring chemical and physical parameters. When combined, they can give rise to devices with enhanced characteristics that can be tailored by varying their fabrication parameters. We analyze the characteristics of arc-induced LPFGs written on the longer transition of asymmetric tapers, which provide a variable diameter section whose slope can be adjusted in order to tailor the device performance. In comparison with LPFGs fabricated in nontapered fibers, the grating inscription in the linear taper transition produces deeper notches (>20 dB ), whose bandwidth and separation increase with the transition slope. The simplicity of the electric arc technique to fabricate LPFGs, and the degrees of freedom added by the tapered fiber open up the possibility to realize thorough studies on the many possible combinations, to develop devices with characteristics designed on purpose.

Tuning and wavelength switching erbium-doped fiber ring lasers by controlled bending in arc-induced long-period fiber gratings

In this paper, we report a simple method for the production of an all-fiber, tunable and wavelength switchable erbium-doped fiber ring laser (EDFRL). We use the shift in resonance wavelength and notch-depth of arc-induced long-period fiber gratings (LPFGs) when it is subject to controlled bend to suppress some wavelength gain and promote lasing in a selective way in a fiber laser system. By changing the bending radius, the operating wavelength of the EDFRL can be tuned from 1526 to 1538nm and then switched and tuned from 1568 to 1557nm. The maximum separation measured between laser lines was ∼42nm. This is one of the highest laser separations reported in a wavelength switching operation of an EDFL using a single LPFG as wavelength selective filter.

Random period arc-induced long-period fiber gratings

We report the fabrication of arc-induced long-period fiber gratings with strong random variations in the period. Long-period fiber gratings with standard deviations in the period from 8.50 to 36.98 μm were fabricated. The spectral position of the resonant bands is determined by the average period value, being similar to that observed in a long-period fiber grating with a fixed period equal to the average period of the random grating. Moreover the notch bands keep the shape characteristics like wideband and depth compared with a long-period grating with a constant period. In addition, their sensitivity to external parameters such as ambient refractive index is not too different with that of fixed period long-period gratings.

Background loss minimization in arc-induced long-period fiber gratings

We demonstrate the minimization of background loss for arc-induced long-period fiber gratings in standard fiber by Taguchi's optimization method. We use Taguchi's method to determine the optimum values for parameters like electric-arc power, arc duration, and tensile strain applied over the fiber during the inscription process. With these optimal parameters, we minimize the background loss resulting from the geometrical deformations of the fiber. The experimental results show that background loss can be reduced from more than 1 dB to less than 0.3 dB at rejection bands with isolation >15 dB.

Highly sensitive cladding-etched arc-induced long-period fiber gratings for refractive index sensing

We have investigated the effects of cladding etching in arc-induced long-period fiber gratings. Comparing the position of the resonant wavelengths at each diameter with the corresponding theoretical dispersion curves of effective refractive index, it is found that some of the resonances show a better match with anti-symmetrical cladding modes, while others with symmetrical cladding modes. Etching the cladding diameter to ∼39 μm, results in coupling to a high-order cladding mode that is very sensitive to changes in the ambient refractive index. The resonant peak shifts remarkably (∼230 nm) toward shorter wavelengths as the external refractive index increases. This sensitivity is higher than any previously reported in arc-induced long-period fiber gratings.