Polymer Fiber Bragg Gratings Research Articles

Highly sensitive liquid level monitoring system utilizing polymer fiber Bragg gratings.

A novel and highly sensitive liquid level sensor based on a polymer optical fiber Bragg grating (POFBG) is experimentally demonstrated. Two different configurations are studied and both configurations show the potential to interrogate liquid level by measuring the strain induced in a POFBG embedded in a silicone rubber diaphragm, which deforms due to hydrostatic pressure variations. The sensor exhibits a highly linear response over the sensing range and a good repeatability. For comparison, a similar sensor using a FBG inscribed in silica fiber is fabricated, which displays a sensitivity that is a factor of 5 smaller than the POFBG. The temperature sensitivity is studied and a novel multi-sensor arrangement proposed which has the potential to provide level readings independent of temperature and the liquid density.

A self-referenced optical intensity sensor network using POFBGs for biomedical applications.

This work bridges the gap between the remote interrogation of multiple optical sensors and the advantages of using inherently biocompatible low-cost polymer optical fiber (POF)-based photonic sensing. A novel hybrid sensor network combining both silica fiber Bragg gratings (FBG) and polymer FBGs (POFBG) is analyzed. The topology is compatible with WDM networks so multiple remote sensors can be addressed providing high scalability. A central monitoring unit with virtual data processing is implemented, which could be remotely located up to units of km away. The feasibility of the proposed solution for potential medical environments and biomedical applications is shown.

Experimental Study and Analysis of a Polymer Fiber Bragg Grating Embedded in a Composite Material

The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding. With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously.

Soft Fiber Optic Sensors for Precision Measurement of Shear Stress and Pressure

This paper reports a soft fiber optic sensor based on polymer fiber Bragg gratings (PFBGs) for simultaneous measurement of shear and normal stresses. The sensor comprises two PFBGs embedded in soft polydimethylsiloxanes (PDMS) matrix, and one of them is horizontally placed while the other is tilted. The transduction principle is that applied normal and/or shear stresses lead to deformation of the matrix and induce strains on gratings. Finite element simulation reveals non-uniform strain distribution along two fibers for direct imbedding that is verified by theoretical analysis, and therefore four gaskets are used to stretch polymer optical fibers from two ends and to produce strains on gratings. The fabricated sensor is tested by concurrently applying normal and shear forces, and Bragg wavelength shifts of two gratings were found to be linearly dependent on stresses in two directions. The measured pressure sensitivity is 0.8 pm/Pa in the range of 2.4 kPa and the shear stress sensitivity is 1.3 pm/Pa for a full range of 0.6 kPa. Such soft sensors are especially suitable for high precision measurement of contact stresses involving human skins and tissues because they are similar in Young's modulus to the used PDMS.

Acousto-Optic Effect in Microstructured Polymer Fiber Bragg Gratings: Simulation and Experimental Overview

A fine control of the microstructured polymer fiber Bragg grating spectrum properties, such as maximum reflected power and 3-dB bandwidth, through acousto-optic modulation is presented. For simulation purposes, the device is modelled as a single structure, comprising a silica horn and a fiber Bragg grating. For similar sized structures a good correlation between the numerical results and the experimental data is obtained, allowing the strain field to be completely characterized along the whole structure. It is also shown that the microstructured polymer fiber Bragg grating requires less effort from the piezoelectric actuator to produce modification in the grating spectrum when compared with a silica fiber Bragg grating. This technique has potential to be applied on tunable optical filters and tunable cavities for photonic applications.

High Sensitivity Force and Pressure Measurements Using Etched Singlemode Polymer Fiber Bragg Gratings

Singlemode polymer fiber Bragg gratings (FBGs) are etched down to a diameter of 30 $\mu{\rm m}$ to be used for high sensitivity tensile force measurements. The singlemode polymer fiber and the Bragg gratings are fabricated using the in-house fabrication facility. The fabricated cladding etched polymer FBGs are characterized for low value tensile force measurements and the sensitivity of the sensors is measured and compared with the theoretically estimated values. A sensitivity of 643 nm/N is obtained for the polymer Bragg grating with a diameter of 30 $\mu{\rm m}$ . A pressure sensor based on a singlemode polymer fiber grating is also demonstrated. Low pressure in the range of 290–790 Pa is measured and the observed sensitivity of the pressure sensor is 1.32 pm/Pa. Given the bio-compatible nature of polymer fibers, such as high sensitivity force and pressure sensors can find applications in bio-medical field where high sensitivity low value force/pressure measurements are required.

Intrinsic Temperature Sensitivity of Fiber Bragg Gratings in PMMA-Based Optical Fibers

Thermal response of fiber Bragg gratings (FBGs) in polymethyl methacrylate-based optical fibers doped with trans-4-stilbenemethanol is studied at 0% relative humidity (RH) from -20°C to 60°C. The Bragg wavelength is found to negatively shift upon heating in a nonlinear manner. The higher the temperature, the larger the magnitude of wavelength shift is. The absolute value of calculated temperature sensitivity also increased with temperature. The sensitivity at room temperature is about -1 pm/°C, which is one order of magnitude lower than that measured at constant RH for polymer FBGs and that of silica FBGs. Such low temperature sensitivity challenges the common notion of high temperature sensitivity of polymer FBGs. In addition, it is found that the dependence of Bragg wavelength of the polymer FBGs on RH is nonlinear. Below 30% RH, the Bragg wavelength shift upon RH increase is minimal compared to that above 30% RH.

Three-dimensional sub-wavelength fabrication by integration of additive and subtractive femtosecond-laser direct writing

ABSTRACTAdditive nanofabrication by two-photon polymerization (TPP) has recently drawn increased attention due to its sub-100 nm resolution and truly three-dimensional (3D) structuring capability. However, besides additive processes, subtractive process is also demanded for many 3D fabrications. Method possessing both additive and subtractive fabrication capabilities was rarely reported. In this study, we developed a complementary 3D micro/nano-fabrication process by integrating both additive two-photon polymerization (TPP) and subtractive multi-photon ablation (MPA) into a single platform of femtosecond-laser direct writing process. Functional device structures were successfully fabricated including: polymer fiber Bragg gratings containing periodic holes of 500-nm diameter and 3D micro-fluidic systems containing arrays of channels of 1-µm diameter. The integration of TPP and MPA processes enhances the nanofabrication efficiency and enables the fabrication of complex 3D micro/nano-structures that are impractical to produce by either TPP or MPA alone, which is promising for a wide range of applications including integrated optics, metamaterials, MEMS, and micro-fluidics.

Phase-Shifted Fiber Bragg Gratings for Terahertz Range

We demonstrate notch filters in polymer fiber Bragg gratings for use at THz frequencies. The filters were created by introducing a π/2-phase shift into narrowband Bragg gratings manufactured in Topas polymer subwavelength fiber. Experimental results are presented and compared with a model of the filter. Excess resonator loss of the notch filter is discussed.

Analysis of multimode BDK doped POF gratings for temperature sensing

We report a temperature sensor based on a Bragg grating written in a benzil dimethyl ketal (BDK) doped multimode (MM) polymer optical fiber (POF) for the first time to our knowledge. The thermal response was further analyzed in view of theory and experiment. In theory, with the order of the reflected mode increasing from 1st to 60th order, for MM silica fiber Bragg grating (FBG) the temperature sensitivity will increase linearly from 16.2pm/°C to 17.5pm/°C, while for MM polymer FBG the temperature sensitivity (absolute value) will increase linearly from −79.5pm/°C to −104.4pm/°C. In addition, temperature sensitivity of MM polymer FBG exhibits almost 1 order larger mode order dependence than that of MM silica FBG. In experiment, the Bragg wavelength shift will decline linearly as the temperature rises, contrary to that of MM silica FBG. The temperature sensitivity of MM polymer FBG is ranged from −0.097nm/°C to −0.111nm/°C, more than 8 times that of MM silica FBG, showing great potential used as a temperature sensor.

High Sensitivity Polymer Optical Fiber-Bragg-Grating-Based Accelerometer

We report on the fabrication and characterization of the first accelerometer based on a polymer optical fiber Bragg grating (FBG) for operation at both 850 and 1550 nm. The devices have a flat frequency response over a 1-kHz bandwidth and a resonance frequency of about 3 kHz. The response is linear up to at least 15 g and sensitivities as high as 19 pm/g (shift in resonance wavelength per unit acceleration) have been demonstrated. Given that 15 g corresponds to a strain of less than 0.02% and that polymer fibers have an elastic limit of more than 1%, the polymer FBG accelerometer can measure very strong accelerations. We compare with corresponding silica FBG accelerometers and demonstrate that using polymer FBGs improves the sensitivity by more than a factor of four and increases the figure of merit, defined as the sensitivity times the resonance frequency squared.

Investigation Into Time Response of Polymer Fiber Bragg Grating Based Humidity Sensors

In this work we experimentally investigate the response time of humidity sensors based on polymer optical fiber Bragg gratings. By the use of etching with acetone we can control the poly (methyl methacrylate) based fiber in order to reduce the diffusion time of water into the polymer and hence speed up the relative wavelength change caused by humidity variations. A much improved response time of 12 minutes for humidity decrease and 7 minutes for humidity increase, has been achieved by using a polymer optical fiber Bragg grating with a reduced diameter of 135 microns.

Tunable Polymer Fiber Bragg Grating (FBG) Inscription: Fabrication of Dual-FBG Temperature Compensated Polymer Optical Fiber Strain Sensors

We demonstrate stable wavelength tunable inscription of polymer optical fiber Bragg gratings (FBGs). By straining the fiber during FBG inscription, we linearly tune the center wavelength over 7 nm with less than 1% strain. Above 1% strain, the tuning curve saturates and we show a maximum tuning of 12 nm with 2.25% strain. We use this inscription method to fabricate a dual-FBG strain sensor in a poly (methyl methacrylate) single-mode microstructured polymer optical fiber and demonstrate temperature compensated strain sensing around 850 nm.

Analysis of multimode POF gratings in stress and strain sensing applications

Polymer fiber Bragg gratings (FBGs) are made using the modified sagnac system with a 355 nm pulsed laser from a photosensitive polymer optical fiber (POF) with external and core diameters of 290.6 and 21.0 μm, respectively. Multimodes are reflected based on the reflection spectra of the gratings. The reflectivity spectra are also studied when such multimode polymer FBGs are subjected to axial static stress and strain. The respective effects of stress and strain on the sensor are decoupled and analyzed independently. Experiments and regression show that the effect of stress and the relaxation of stress in multimode FBGs (MM FBGs) in POF during loading and unloading have a more evident non-linear effect than that of strain. These non-linear properties make FBGs attractive for mechanical sensing applications.

Optical fibre Bragg grating recorded in TOPAS cyclic olefin copolymer

A report is presented on the inscription of a fibre Bragg grating into a microstructured polymer optical fibre fabricated from TOPAS cyclic olefin copolymer. This material offers two important advantages over poly (methyl methacrylate), which up to now has formed the basis for polymer fibre Bragg gratings: TOPAS has a much lower water affinity and has useful properties for biosensing. The grating had a Bragg wavelength of 1569 nm and a temperature sensitivity of -36.5±0.3 pm/°C.

Thermo-optical modeling of polymer fiber Bragg grating illuminated by light emitting diode

The development of a thermo-optical model of an illuminated polymer fiber Bragg grating (PFBG), combining use of the modified coupled-mode theory with thermal conduction theory and the modified transfer matrix method (TMM), is presented. This model is applied to the prediction of the thermo-optical behavior of an intrinsically heated and passively cooled PMMA fiber Bragg grating illuminated with a LED light source and operating over a range of ambient temperatures. Parametric influences on the thermo-optic characteristics and the predictive accuracy of several simplifications in the Bragg grating relations are also explored.

Thermo-optical modeling of an intrinsically heated polymer fiber Bragg grating

A fully concatenated thermo-optical model is presented to predict the thermo-optical behavior of an intrinsically heated polymer fiber Bragg grating (PFBG). Coupled-mode theory and heat-conduction theory are first used to determine the axial heat generation and temperature distribution of a PFBG and the transfer matrix method (TMM) is subsequently employed to predict its thermo-optical behavior. The validity of the TMM is corroborated experimentally using an externally heated glass fiber Bragg grating (FBG) with an axially decaying temperature field. The verified model is utilized to investigate the thermo-optical behavior of a poly(methyl methacrylate) (PMMA) FBG. The counteracting thermally driven changes in the refractive index and the grating pitch, respectively, are found to be of comparable magnitude and to result in very modest net shifts in the Bragg wavelengths despite the considerable temperature changes induced by the absorption of the incident light.

Tensile strain characterization of polymer optical fibre Bragg gratings

The systematic investigation on the tensile strain characterization of polymer optical fibre Bragg gratings is reported in this paper. Experimental results indicate that a very large Bragg wavelength tuning range (32 nm) with good reproducibility, reversibility and repeatability can be achieved by applying the simple tension on polymer fibre Bragg gratings. It is also demonstrated that the strain sensitivity of polymer fibre Bragg gratings is larger than that of silica fibre Bragg gratings. As a result, the strain characterization study shows the great potential of polymer fibre Bragg gratings in both WDM optical communications and fibre strain sensing applications.

Novel Growth Behaviors of Fiber Bragg Gratings in Polymer Optical Fiber Under UV Irradiation With Low Power

Some novel behaviors in fiber Bragg gratings (FBGs) growth have been observed when the polymer optical fiber are exposed to low-power level of ultraviolet (UV) irradiation. For the first time, we observed the growing and erasing of polymer FBGs under UV exposure, and regrowing after the UV exposure is off. The growth behaviors are quite different in contrast with the previously reported Type I and Type II polymer optical FBGs behaviors. This discloses that polymer FBG growth is a writing-power-dependent process.

Strain and temperature sensor using a combination of polymer and silica fibre Bragg gratings

We show that a sensor scheme consisting of a combination of a polymer fibre Bragg grating and a silica fibre Bragg grating gives large discrimination against temperature and strain. It provides large sensitivity and dynamic range for sensing temperature and strain changes simultaneously and independently.