Mechanically Induced Long-period Fiber Gratings 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.

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.

Optical Fiber Torsion Sensor with Mechanically Induced Long Period Fiber Gratings in Rare-Earth Doped Fibers

In this work wavelength sensitivity in mechanically induced long period fiber gratings (MLPFG) is analyzed. This analysis is first carried out both in standard single-mode fiber SMF-28 and in Er-doped fibers. The mechanical analysis for both types of fibers under different torsion conditions is presented. In order to apply the torsion one of the fiber ends is fixed while torsion is applied on the other end. A MLPFG whose period is 503 μm is used to press the fiber after torsion is applied. This allows for micro curvatures to be formed on the fiber, which in turn generates a periodical index perturbation on it. Here, it was noted that the sensitive wavelength shift of the rejection bands is bigger for Er-doped fibers. For a torsion of 6 turns applied to 10 cm of doped fiber the wavelength peaks can be moved up to 25 nm, which is longer to what was detected on standard fibers. Therefore, by using Er-doped fibers to monitor torsion on structures will give more sensitive and accurate results than using standard fibers. These results can be employed for sensing applications, especially for small to medium size structures, which can be mechanical, civil or aeronautics.

Temperature impact on mechanically induced long-period fiber gratings

The spectral response of mechanically induced long-period fiber gratings (MLPFG) to the ambient temperature variation was experimentally study. In the MLPFG setting, a pressure rig with aluminum grooved plates on standard telecommunication fiber was used. We found that a relatively low change in temperature can produce a severe impact on the characteristic parameters of the attenuation bands, such as a critical decrease in the contrast and fast shift saturation. When the temperature increases from −10 to 70 °C, all the bands are shifted toward longer wavelengths with a mean sensitivity of approximately 180 pm/°C from 0 to 40 °C in the near-linear region, then they present a flattened zone beyond this temperature. Meanwhile, the contrast of the attenuation bands rapidly decreases from the maximum value to 0 dB with a quasi-cosinoidal behavior. These results are important and have to be considered when MLPFG are applied in a medium with ambient temperature variation. Furthermore, we show that MLPFG can be used as low cost ambient temperature sensors through intensity based measurements.

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.

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.

机械感生长周期光纤光栅实验研究

A full experimental characterization of mechanically induced long-period fiber gratings (MLPFGs) fabricated by pressing a plate with periodic grooves against a short length of fiber is presented. This technique enables a good control over the gratings' isolation loss peaks and has high repeatability. The spectra characteristics of MLPFGs are studied with the change of the parameters including pressure, period, and temperature. The produced MLPFGs have low insertion loss (l0.2 dB) and the loss peaks can be higher than 20 dB. A large tunability of the resonant wavelength (g14 nm) is achieved through adjusting pressure grooves' period. The center wavelength temperature sensitivities of 0.057, 0.086 nm/oC, and resonant peak temperature sensitivities of 0.230, 0.312 dB/oC, are achieved for jacketed and unjacketed fibers, respectively. These MLPFGs, which are simple and inexpensive, also offer the unique advantages of being tunable, erasable, and reconfigurable.

A birefringence compensation method for mechanically induced long-period fiber gratings

We present a new method of compensating linear birefringence always present in mechanically induced long-period fiber gratings (MLPFGs). With the proposed compensation method the MLPFGs became polarization insensitive and the polarization mode dispersion of the MLPFGs was substantially reduced.