Mechanically Induced Long-period Fiber Grating Research Articles

Using Mechanically-Induced Long-Period Fiber Gratings for OAM Modes Generation Based on Anti-Resonant Mechanisms in Ring-Core Fibers

A flexible first-order OAM modes generation in ring-core fibers (RCFs) based on a mechanically-induced long-period fiber grating (MLPFG) is proposed. By directly fusion splicing a standard single-mode fiber (SMF) and RCFs, the anti-resonant reflection guidance mechanism (ARRGM) is introduced in the RCF, leading to the guided-core modes, so-called anti-resonant modes (AR modes), which have a Gaussian field distribution and are confined in low-refractive index central core of RCFs. A mechanical long-period fiber grating is used to couple the AR modes into the OAM modes propagated in the high-index ring core of the RCF. Results show that the generated OAM modes are the combination of two orthogonal linearly polarized (LP) modes with ±π/2 phase shift in the ring core. The mode conversion efficiency of OAM_+1 and OAM_−1 can reach 95% and 94%, respectively, indicating promising applications in sensing and communication of the RCFs.

Transverse Load and Temperature Sensing Using Multiplexed Long-Period Fiber Gratings

The simultaneous measurement of transverse load and temperature using two long-period fiber gratings multiplexed in the wavelength domain is presented experimentally. For this, a mechanically induced long-period fiber grating (MI-LPFG) and a long-period fiber grating inscribed by a continuous-wave CO2 laser (CO2 LPFG) are connected in cascade. First, the transverse load and the temperature measurements were individually performed by the multiplexed long-period fiber gratings configuration. The MI-LPFG is subject to a transverse load variation from 0–2000 g with steps of 500 g, whereas the CO2 LPFG is unloaded and they are kept at room temperature. Similarly, the CO2 LPFG is subject to a temperature variation from 30 to 110 °C by increments of 20 °C, while the MI-LPFG with a constant transverse load of 2000 g is kept at room temperature. Subsequently, the simultaneous measurement of the transverse load and the temperature is performed by the multiplexed long-period fiber grating following the steps outlined above. According to the experimental results, the transverse load and temperature measurement present high repeatability for the individual and simultaneous process. Moreover, the multiplexed LPFGs exhibit low cladding-mode crosstalk of transverse load and temperature. The coarse wavelength-division multiplexing (CWDM) of long-period fiber gratings is an attractive alternative technique in optical fiber distributed sensing applications.

Cladding mode loss effect on transmission characteristics of mechanically induced long‐period fiber gratings

AbstractIn the mechanically induced long‐period fiber grating (MLPFG) fabricated by applying the periodical pressure on an optical fiber, the small bending occurs due to the periodical pressure, which influences the losses of the core and cladding modes. We propose the theoretical model of the MLPFG for estimating its transmittance based on the transfer matrix method and investigate the effect of the losses of the core and cladding modes in the MLPFG on the transmittance, theoretically and experimentally. We numerically clarify that the transmitted light spectrum of MLPFG shows only the main attenuation lobe, no side lobe, and the attenuation bandwidth is broadened as the cladding mode loss increases. We also clarify that the coupling coefficient, and the losses of the core and cladding modes in the fabricated MLPFG can be estimated from the measured transmitted spectrum based on our model. Moreover, we measure the transmittance of two types of MLPFGs that fabricated with a screw, weights, and metallic plates, and that fabricated with a screw and a heat‐shrinkable tube. The measured spectra show good agreements with the calculated ones using our model. Our model will be useful for designing MLPFGs applied to the sensors.

Mechanically-Induced Long-Period Fiber Gratings Using Laminated Plates.

This work presents a formation method of mechanically-induced long-period fiber gratings using laminated plates. The mechanically-induced long-period fiber grating is temporarily inscribed by compressing the optical fiber between a flat plate and the proposed laminated plate. In turn, the new laminated plate consists of a parallel assembling of single-edged utility blades. We present the experimental characterization of mechanically-induced long-period fiber gratings while employing three laminated plates with a period of 480 ± 20 µm and low duty cycles. These mechanically-induced long-period fiber gratings display a leading rejection band (>15 dB) with a couple of shallow rejection bands (<2 dB) in the range of 1100–1700 nm. This spectral behavior is due to the new mechanical fabrication process that is based on laminated plates that we have proposed, which consists of piling multiple blades with trapezoidal edges that are polished with different levels to obtain different duty-cycles. With the proposed method, we can obtain values of duty-cycles around 10%, much lower than those obtained using traditional methods. Additionally, with this new method, the required mechanical pressure to form the grating is remarkably reduced, which minimizes the probability of the optical fiber failure in the mechanically-induced long-period fiber gratings (MI-LPFGs). Moreover, the proposed mechanically-induced long-period fiber gratings with a single rejection band open the feasibility to implement coarse wavelength division multiplexing systems that are based on long-period fiber gratings.

Liquid-Level Sensor Based on Reflective Mechanically Induced Long-Period Grating Using Double-Cladding Fiber

A reflection-type liquid-level sensor based on mechanically induced long-period fiber grating (MLPFG) is proposed and experimentally demonstrated. The sensor, acting as a Michelson interferometer, includes MLPFG with double-cladding fiber (DCF) whose end-face is coated with a silver film. DCF is used here in order to be more conducive for interference-spectrum generation. The liquid level can be measured by monitoring the shift of the interference fringes. By theoretical and experimental analysis of the interference characteristics, it is concluded that there are certain relationships between the interference-fringe spacing and the distance from the grating to the reflector, and that the fringe contrast ratio is influenced either by the stress or the position of the MLPFG. After the appropriate parameters are selected for the liquid-level sensor, the sensor sensitivity of 0.03136 nm/mm is obtained, which is considerably superior to that of conventional interferometric liquid-level sensors. In addition, the device has advantages, such as low cost and structural simplicity.

Modal interferometer based on a single mechanically induced long-period fiber grating and a nanoparticles-coated film section

A modal interferometer by a single mechanically induced long-period fiber grating (MI-LPFG) using a half-length coating fiber is presented. The coating material used for this Letter is a film of silica nanoparticles doped with an organic chromophore. The silica nanoparticles, with diameters within the range of 40-50 nm, were deposited over 3.5 cm length of fiber by the dip-coating method, forming a film with a thickness between 500 and 1250 nm. Then the modal interferometer was implemented by inscribing the MI-LPFG over the coated fiber section and a similar fiber length of the uncoated fiber. The experimental results show high-contrast transmission bands, where the position and depth of the absorption envelope band are finely selected by the grating period, the pressure applied, and the film thickness. The novel modal interferometer architecture based on a single MI-LPFG, combined with a functionalized nanoparticles coating film, offers an attractive platform for the development of fiber sensors and other fiber-based devices.

Polarimetric Mueller-Stokes analysis of photonic crystal fibers with mechanically induced long-period gratings

A Mueller-Stokes analysis of the polarization characteristics of a photonic crystal fiber (PCF) with a mechanically induced long-period fiber grating (MLPFG) is presented. Results show that the diattenuation and the polarizance parameters, and the anisotropic degree of depolarization, increase greatly with the LPFG in the PCF. The depolarization index, the Q (M) depolarization scalar metric, the theorem of Gil-Bernabeu, and the degree of polarization, provide consistent results that indicate the existence and the increasing of depolarization effects due to the presence of the LPFG in the PCF. Finally, the polarization dependent loss also increases when the LPFG is present in the fiber for a wavelength illumination centered at 1064 nm. As a conclusion, the PCF with LPFG cannot be described by the Jones matricial formulism. One important result we have found here is that the PCF we employed has an intrinsic anisotropic degree of depolarization.

Second-order optical differentiator using mechanically-induced LPFG with a single π-shift

An all-optical second-order temporal differentiator using a mechanically-induced long-period fiber grating (MI-LPFG) with a single π-shift was demonstrated. The MI-LPFG was created by pressing a fiber between two periodically grooved plates with a π-shift located at the 3/4 length from the input end of LPFG. The coupling coefficient (κ) can be adjusted by changing the pressure applied on the fiber. The experimental results show that the transfer function of the proposed MI-LPFG can be adjusted to have a transfer function as an ideal second-order differentiator. The differential performance of the designed differentiator to a Gaussian pulse is also analyzed.

Mechanically induced long-period fiber grating array sensor

This article presents the experimental characterization of a mechanically induced long-period fiber grating (MLPFG) array sensor for measuring the amplitude and width of an applied weight as well as detecting its position. The MLPFG array sensor consists of an optical fiber placed between a rubber cover and a 45-cm-long metal bar with round grooves on its upper surface divided into 10 different grating periods. The grating periods are sequentially increased by 10 μm from 690 to 780 μm, and the number of grooves per grating period is 60. Experimental results show that the MLPFG array sensor can be used for measuring the amplitude of an applied weight from the optical attenuation at the resonance wavelength, measuring the loading width from the full width at half maximum of the dip, and detecting the position of the applied weight from the resonance wavelength. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2295–2298, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26289

High-sensitivity pressure sensors based on mechanically induced long-period fiber gratings and fiber loop ring-down

A transverse pressure sensor with high-sensitivity based on a mechanically induced long-period fiber grating (MLPFG) and fiber loop ring-down technique is presented. When a MLPFG is spliced into a fiber loop, an extra loss is introduced, which leads to a decrease of the ring-down time. The results demonstrate that the difference between the reciprocals of the ring-down time with and without pressure increases exponentially with increasing the pressure in the range of 0–23.4 MPa. This sensor shows good repeatability, and the least detectable pressure is only 0.0068 MPa which is about 18 times less than detecting the output light intensity directly.

高精度机械感生长周期光纤光栅横向压力传感系统

高精度机械感生长周期光纤光栅横向压力传感系统

Mechanically induced long-period fiber gratings on tapered fibers

The characteristics of a mechanically induced long-period fiber grating (MLPFG) made by pressing a pair of grooved plates over single-mode fiber tapers are analyzed. Fiber tapers with a waist length of 80 mm and diameter ranging from 90 to 125 μm, fabricated using the heating and puling method, were used. We observed that the resonance wavelengths shift toward shorter wavelengths as the fiber taper waist diameter is reduced. A maximum shift of 254 nm in the position of the resonance peaks was observed when the fiber diameter was reduced to 90 μm. This technique is particularly suitable for tuning the resonance wavelengths to shorter wavelengths below the limit imposed by the grooved plate period.

Amplified spontaneous emission–induced thermal nonlinearities in a mechanically induced long-period fiber grating

Using the amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) at zero signal condition, the shifting of resonant wavelengths in a mechanically induced long-period fiber grating (MLPFG), resulting from the intensity induced effects is demonstrated. The theoretical and experimental investigations on the ASE-induced effects in an MLPFG are presented. The developed theory is concise and is useful for differentiating the index changes due to Kerr and other intensity-dependent effects in a long-period fiber grating (LPFG). It is demonstrated that the induced index will be high for the LP11 resonance near the third telecommunication window and is negligible for the LP12 resonance. Because of the increase in microbend loss at each groove of the MLPFG, the thermally induced nonlinear effects lessen at higher values of applied pressures

Interrogation of a Fiber Bragg Grating Using a Mechanically Induced Long-Period Fiber Grating

A fiber-sensing scheme with controlled sensitivity comprising a fiber Bragg grating (FBG) and a mechanically induced long-period fiber grating (MLPFG) is presented. The FBG was written by exposing the fiber to 248-nm UV laser radiation such that the Bragg wavelength is localized on the slope of a resonant band of a mechanical grating, which was produced by winding a nylon string around a fiber/grooved tube set. The strength of that resonant band was altered by applying loads to the MLPFG. For different loads, the FBG was submitted to strain values of up to 2200 muepsiv, in steps of 200 muepsiv, during which the Bragg wavelength and the respective transmitted peak power through the MLPFG were recorded. It was demonstrated that by applying a weight with a value of 0.78 kg to the MLPFG, the sensitivity of the FBG interrogation technique to strain variations increased from 2.23 (without load) to 3.20 pW/muepsiv

Spectral Filtering in OCT System Implemented by Mechanically Induced Long-Period Fiber Grating

A spectral filtering method that can enhance the axial resolution and suppress sidelobes of the point spread function in Optical Coherence Tomography (OCT) system is demonstrated. A mechanically induced long-period fiber grating (MLPG) is used to tailor the spectral shape of a homemade Er-doped fiber amplified spontaneous emission (ASE) source. By using a MLPG, the high and narrow characteristic peak, centered at 1533 nm, of the ASE source was fairly well removed, which enabled us to use a broader spectral bandwidth of the source. The experimental results show that the proposed spectral filtering method can suppress the sidelobes in the axial point spread function up to 5.4 dB and thus enhance the axial resolution of an OCT system by four times. The OCT images of an extracted human premolar taken with and without the spectral filtering are presented and compared with each other.

PDL-Compensated Mechanically Induced Long-Period Grating for EDFA Gain Flattening

We present and demonstrate a polarization-insensitive mechanically induced long-period fiber grating (MLPG) for EDFA gain flattening. By forming a 90° fiber rotator in the middle of the MLPG the PDL of the MLPG was substantially reduced. The PDL-compensated MLPG was able to flatten the ASE spectrum of an EDFA successfully without deterioration of the polarization characteristics.