Long-period Grating Structure Research Articles

In-Fiber Breathing Monitoring Sensor by Micro-Tapered Long-Period Grating Structure

A simple in-fiber breathing monitoring sensor based on a micro-tapered long-period fiber grating (MT-LPFG) structure was proposed and demonstrated. The MT-LPFG structure was manufactured through the periodical tapering of an optical fiber utilizing a CO2 laser fusion splicer. Without the use of a humidity sensitive film, the MT-LPFG structure can directly and dynamically monitoring relative humidity with a fast response, based on which, the breathing process can be monitored in real time through the monitoring of its transmission affected by the humidity. Experiments show that breathing rate and breathing status during respiration were successfully monitored by placing the sensor structure nearby the nose in a breathing mask. A rise time of 0.237 s and a fall time of 0.299 s were realized in the breathing process monitoring. For its advantages of simple fabrication, miniature size, and fast response, the proposed sensor may found applications in visual observation of the patient’s breathing condition.

Potential of Mechanically Induced Cascaded Long-Period Grating Structure for Reflectometric Pressure, Strain, and Temperature Sensing

We present the first report on reflectometric optical fiber sensing based on mechanically induced cascaded long-period grating (LPG) structure. This method utilizes in-house designed mechanical casings and a bare fiber. When a fiber is sandwiched between the casings and pressed by tightening screws, an LPG is induced due to a photo-elastic effect. By exploiting Fresnel refection at a fiber end, a cascaded LPG structure can be implemented using a single LPG, enabling reflectometric configuration. When this sensor is subjected to external physical processes, the attenuation bands change clearly, and their shift can be used for non-destructive evaluations. We show experimentally that this sensor can measure loads of up to 10 N without producing any permanent change in the fiber properties, indicating its pressure sensing capability. We also show that this sensor can potentially measure strain and temperature. In our experiments, the strain and temperature sensitivities are 9.4 nm/% and 0.045 nm/°C, respectively.

Towards the design of a wideband reflective long period grating distributed sensor

In this paper, we computationally investigate the effects of metal coating length and coating coverage on the reflected spectrum of a long period grating (LPG) over a broad bandwidth. Simulation results indicate that coating the tail end of the fiber between the LPG and the end facet of the fiber provides a reflected spectrum that mimics the LPG transmission spectrum shape over a 400 nm bandwidth. Based on single LPG simulation results, we present the design of a distributed LPG structure containing a multiple number (n) of LPGs in reflection mode for the first time. Simulation results for n = 1, 2, and 3 are presented here to demonstrate the concept of a distributed reflective LPG design. It is expected that such a sensor will open a new window for distributed sensing using reflective LPGs.

Functionalized fiber-optic long-period grating with reduced cladding size for humidity sensing

Relative humidity (RH) is an important factor in the field of structural health monitoring, especially during the early stage of corrosion. Many methods have been proposed for humidity sensing, and one of the attractive sensors is fiber-optic long-period grating (LPG) sensors. Unfortunately, the current sensing sensitivity of this kind of sensors is limited. A thin fiber-optic LPG sensor with a self-assembled thin film of PAH + / PAA − is proposed and demonstrated for humidity measurements. The LPG sensor is inscribed in a single-mode fiber using a CO2 laser, and the cladding size is reduced to about 27 μm. It shows that an LPG sensor with a reduced cladding size has an enhanced refractive index sensing characteristic, compared to those with normal cladding size due to the enhancement of the evanescent field. In a next step, selectivity is added to the LPG sensor coated with a film of PAH + / PAA − for functionalization to be sensitive to humidity. The resonance spectral responses of LPGs are experimentally investigated with respect to its sensitivity to a change in humidity that modifies the index of the nanolayer and the cladding, leading to a resonant wavelength shift. The experimental results show that the coated thin LPG has a highly sensitive resonance wavelength shift of −220.75 pm / % RH for an RH variation from 25% to 80%, of which the sensitivity is enhanced thrice compared to those with a normal cladding size. The proposed sensing setup opens LPG structures for a variety of sensing and detection applications.

Periodic micro-structures in optical microfibers induced by Plateau-Rayleigh instability and its applications

A periodic micro-structure on optical microfibers induced by Plateau-Rayleigh instability (PRI) was investigated and a potential application for long period gratings (LPGs) fabrication was given. The linear relation between the average periods of micro-structures and the diameters of optical microfibers was demonstrated first. By brushing a glass rod with a Teflon droplet suspended at the end tip along microfibers, a continuous film of Teflon was formed at once, then the film broke up into a series of periodic droplets due to PRI. Periodic Teflon nodes were left after the evaporation of the solvent. A LPG structure based on polymer was finally formed by this method on a microfiber with a diameter of 5.5 μm. An attenuation transmission dip of 15 dB around 1447 nm was achieved. Investigation of the strain and temperature response characteristics of the grating presented a strain sensitivity of -2.5 pm/με and a temperature sensitivity of -157 pm/°C. The technique proposed here provides a versatile technique for polymer-based LPGs fabrication. Benefiting from the high sensitivities, LPGs based on numerous polymers fabricated in this way could have potential applications in optical and biological sensing.

Improved Refractive Index Sensitivity Utilizing Long-Period Gratings with Periodic Corrugations on Cladding

A new structure of Long-Period Gratings (LPGs) sensor is introduced as a sensitive ambient RI sensor. This structure consists of creating periodic corrugations on the cladding of the LPG. The experimental results show that this LPG structure has good performances in terms of linearity and sensitivity and serves as a highly sensitive and cost-effective sensor. It also has the advantage of portability as the corrugation can also serve as the reservoir for the specimen collection to be tested.

Long-Period Grating Fiber Sensor With In Situ Optical Source for Remote Sensing

A concept of long-period-based optical fiber sensors, with broadband light illumination generated just after the sensing structure, is presented in this work. This new approach allows the interrogation in transmission of the sensing head while integrated in a reflective configuration, which means the long-period grating (LPG) sensor is seen in transmission by the optical source but in reflection by the measurement system. Also, it is shown that with this illumination layout the optical power balance is more favorable when compared with the standard configurations, allowing better sensor performances, particularly when the sensing head is located far away from the photodetection and processing unit. This is demonstrated for the case of the LPG structure applied to measure strain and using ratiometric interrogation based on the readout of the optical power reflected by two fiber Bragg gratings spectrally located in each side of the LPG resonance.

Azimuthally symmetric long-period fibre grating fabrication with a TEM01∗-mode CO2 laser

We present a fabrication method that produces azimuthally symmetric long-period fibre gratings using a carbon dioxide laser operating in the TEM01∗-mode. The outlay and optimization of the optical system are introduced, and the long-period fibre grating fabrication method is outlined. A uniform long-period grating written in a boron-doped fibre exhibited a maximum transmission loss of 10.83 dB at 1538.5 nm. The peak transmission loss of a non-uniform long-period grating also written in the boron-doped fibre yielded a value of 10.25 dB at 1552.6 nm. The polarization properties of the fabricated long-period fibre gratings were investigated to determine the quality of the grating structures. The uniform long-period grating structure exhibited a maximum polarization-dependent loss of 0.97 dB at 1542.1 nm, and the non-uniform long-period grating yielded a maximum polarization-dependent loss of 2.48 dB at 1551.4 nm.

Refractive index sensing of fiber optic long-period grating structures coated with a plasma deposited diamond-like carbon thin film

Long-period grating (LPG) structures including cascaded LPGs on step index fibers and photonic crystal fibers were coated with thin films of diamond-like carbon (DLC) using plasma deposition techniques. Improvements in the coating procedures increased sensitivity to external refractive index variations indicating significant improvements in sensing capability of the hybrid structures. DLC films in the range of tens of nanometers significantly increased sensitivity of all the structures tested.

Simultaneous Measurement of Refractive Index and Temperature by Using Dual Long-Period Gratings With an Etching Process

A new structure of long-period grating (LPG) sensor is introduced for simultaneous measurement of the refractive index (RI) and temperature. This type of grating device consists of two LPG sections, one of which is post etched by hydrofluoric acid (HF) solution and, therefore, has an improved RI sensitivity (the demonstrated improvement of sensitivity is 3.6 times). The experimental results show that this LPG sensor has a good performance in terms of linearity and sensitivity.

Optical effect of diamond‐like carbon film coating of long‐period optical fiber gratings

AbstractThe paper presents diamond‐like carbon (DLC) film as a coating for long‐period grating (LPG) optical fiber structure. The spectral response of a DLC‐coated LPG structure is strictly dependent on Radio Frequency Plasma Chemical Vapour Deposition (RF PCVD) process parameters. The paper discusses the influence of DLC deposition time and self‐bias voltage to shift of the LPG resonance peak and explores the possibility of using the structure as a refractometric sensor. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrashort pulse propagation in uniform and nonuniform waveguide long-period gratings

We conduct a detailed theoretical analysis of ultrashort pulse propagation through waveguide long-period grating (LPG) structures operating in the linear regime. We first consider the case of uniform LPGs, and we also investigate the effect of the typical grating nonuniformities, e.g., grating profile apodization, grating period chirping, and discrete phase shifts, on the spectral and temporal behavior of LPG structures. The two interacting modes are analyzed separately, and advanced representation tools, namely, space-wavelength and space-time diagrams (where space refers to the longitudinal grating dimension), are used to provide a deeper insight into the physics that determines the pulse evolution dynamics through the grating structures under analysis. In addition to its intrinsic physical interest, our study reveals the strong potential of LPG-based devices for optical pulse reshaping operations in the subpicosecond regime.