Polymer Optical Fiber Bragg Gratings Research Articles

Artificial Skin Based on Polymer Optical Fiber Bragg Grating Arrays for Robotic Tactile Perception

Artificial Skin Based on Polymer Optical Fiber Bragg Grating Arrays for Robotic Tactile Perception

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review.

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

In vivo brain temperature mapping using polymer optical fiber Bragg grating sensors.

Variation of the brain temperature is strongly affected by blood flow, oxygen supply, and neural cell metabolism. Localized monitoring of the brain temperature is one of the most effective ways to correlate brain functions and diseases such as stroke, epilepsy, and mood disorders. While polymer optical fibers (POFs) are considered ideal candidates for temperature sensing in the brain, they have never been used so far in vivo. Here, we developed for the first, to the best of our knowledge, time an implantable probe based on a microstructured polymer optical fiber Bragg grating (FBG) sensor for intracranial brain temperature mapping. The temperature at different depths of the brain (starting from the cerebral cortex) and the correlation between the brain and body core temperature of a rat were recorded with a sensitivity of 33 pm/°C and accuracy <0.2°C. Our in vivo experimental results suggest that the proposed device can achieve real-time and high-resolution local temperature measurement in the brain, as well as being integrated with existing neural interfaces.

Point-by-Point Induced High Birefringence Polymer Optical Fiber Bragg Grating for Strain Measurement

In this paper, the first- and fourth-order fiber Bragg grating (FBG)-based axial strain sensors are proposed. The FBGs are inscribed in step-index polymer optical fibers (POFs) (TOPAS core and ZEONEX cladding) via the point-by-point (PbP) direct-writing technique. A first-order FBG with a single peak is obtained with a pulse fluence of 7.16 J/cm2, showing a strain sensitivity of 1.17 pm/με. After that, a fourth-order FBG with seven peaks is obtained with a pulse fluence of 1.81 J/cm2 with a strain sensitivity between 1.249 pm/με and 1.296 pm/με. With a higher fluence of 2.41 J/cm2, a second fourth-order FBG with five peaks is obtained, each of which is split into two peaks due to high birefringence (Hi-Bi) of ~5.4 × 10−4. The two split peaks present a strain sensitivity of ~1.44 pm/με and ~1.55 pm/με, respectively. The peak difference corresponding to Hi-Bi presents a strain sensitivity of ~0.11 pm/με and could potentially be used for simultaneous dual-parameter measurement, such as temperature and strain.

Polymeric fiber sensors for insertion forces and trajectory determination of cochlear implants in hearing preservation

The level of hearing restoration in patients with severe to profound sensorineural hearing loss by means of cochlear implants (CIs) has drastically risen since the introduction of these neuroprosthetics. The proposed CI integrated with polymer optical fiber Bragg gratings (POFBGs) enables real-time evaluation of insertion forces and trajectory determination during implantation irrespective of the speed of insertion, as well as provides high signal quality, low stiffness levels, minimum induced stress even under forces of high magnitudes and exhibits significant reduction of the risk of fiber breakage inside the constricted cochlear geometry. As such, the proposed device opens new avenues towards atraumatic cochlear implantations and provides a direct route for the next generation of CIs with intraoperative insertion force assessment and path planning capacity crucial for surgical navigation. Hence, adaptation of this technology to clinical reality holds promising prospects for the hearing impaired.

Temperature and Humidity Sensitivity of Polymer Optical Fibre Sensors Tuned by Pre-Strain.

Polymer optical fibre Bragg grating (POFBG) sensors are of high interest due to their enhanced fracture toughness, flexibility in bending, and sensitivity in stress and pressure monitoring applications compared to silica-based sensors. The POFBG sensors can also detect humidity due to the hydrophilic nature of some polymers. However, multi-parameter sensing can cause cross-sensitivity issues in certain applications if the temperature and humidity measurements are not adequately compensated. In this work, we demonstrate the possibility of selectively tuning sensors’ temperature and humidity sensitivities to the desired level by applying a certain amount of fibre pre-strain. The temperature sensitivity of POFBG sensors fabricated in perfluoropolymers (CYTOP) can be selectively tuned from positive to negative values, having the option for insensitivity in specific temperature ranges depending on the amount of the applied pre-strain. The humidity sensitivity of sensors can also be changed from positive values to insensitivity. The importance of thermal annealing treatment of POFBG sensors for improved repeatability in temperature measurements is also reported. An array of 4 multiplexed POFBGs was fabricated, and each sensor was pre-strained accordingly to demonstrate the possibility of having targeted temperature and humidity sensitivities along the same fibre.

Nearshore Contamination Monitoring in Sandy Soils Using Polymer Optical Fibre Bragg Grating Sensing Systems.

Civil engineering assets and geo-structures continually deteriorate during their lifetime, particularly in harsh environments that may be contaminated with corrosive substances. However, efficient and constant structural health monitoring and accurate prediction of the service-life of these assets can help to ensure their safety, performance, and health conditions and enable proper maintenance and rehabilitation. Nowadays, many of the largest cities throughout the world are situated in coastal zones, leading to a dramatic increase in the construction of nearshore geo-structures/infrastructures which are vulnerable to corrosion attacks resulting from salinity contamination. Additionally, seawater intrusion can threaten the quality and the sustainability of fresh groundwater resources, which are a crucial resource in coastal areas. To address these issues, detection of salinity in soil utilizing a novel polymer optical fibre Bragg grating (POFBG) sensor was investigated in this research. Experiments were carried out at different soil water contents with different salinities to assess the sensor’s response in a representative soil environment. The sensitivity of the POFBG sensor to salinity concentrations in water and soil environment is estimated as 58 ± 2 pm/%. The average standard error value in salinity is calculated as 0.43% for the samples with different soil water contents. The results demonstrate that the sensor is a promising and practical tool for the measurement and monitoring with high precision of salinity contamination in soil.

Smart Carbon-Fiber Reinforced Polymer Optical Fiber Bragg Grating for Monitoring Fault Detection in Bearing

This article describes the development of a fiber Bragg grating (FBG) sensor encapsulated in carbon-fiber reinforced polymer to detect faults in rotating electrical machine bearings. The proposed sensor was based on the temperature and strain sensitivity of the FBG sensor and the robustness of carbon fiber as an encapsulation. The sensor was composed of three FBGs, one of which was packaged in a glass tube. The behavior characteristics of the developed sensor were evaluated in the calibration process. The experimental setup and measurement system were discussed and analyzed under dynamic conditions. The sensor was implemented in an induction motor operating at a controlled full load. The tests were performed with new and faulty bearings. The results, for the faulty bearing frequencies of 147.5, 296.6, and 439.0 Hz, were identified. These frequency components were generated by defects in the inner raceway. Comparing the amplitude spectrum between the signal of the new and faulty bearing, a variation of approximately 96% was found. The FBG reinforced sensor can identify bearing integrity in real-time and be a preventive maintenance tool for bearing monitoring.

Regenerated polymer optical fiber Bragg gratings with thermal treatment for high temperature measurements

We report for the first time, to the best of our knowledge, regenerated polymer optical fiber Bragg gratings (RPOFBGs) in ZEONEX-based polymer optical fibers (POFs). The regeneration temperature can be adjusted using a heat treatment process on the POF before FBG inscription, enabling a scalable improvement of the thermal stability of the RPOFBGs. Thermal sustainability of the RPOFBGs at high temperature conditions was investigated for their prolonged use in diverse environments. Furthermore, these RPOFBGs can withstand strain levels up to 2.8% while maintaining a good linearity, even at temperature of 110°C. The RPOFBGs are capable of short-term operation at elevated temperatures of up to 132°C, which is the standard temperature for steam sterilization with at least a 4 min exposure period. The distinction in the morphologies of the two grades of ZEONEX (E48R and 480R, ZEON Corp.) used to fabricate the optical fiber together with the characteristics of UV irradiated and regenerated gratings is explained using micro-Raman spectroscopy. Collectively, these findings provide new heights for long-term operation of POF Bragg gratings (POFBGs) at elevated temperature environments and would be applicable to a wide range of disciplines.

Humidity-Sensitive PMMA Fiber Bragg Grating Sensor Probe for Soil Temperature and Moisture Measurement Based on Its Intrinsic Water Affinity.

Soil moisture measurement is very important for soil system monitoring. Compared to the traditional thermo-gravimetric technique, which is time-consuming and can be only performed in labs, the optic-fiber technique has unique advantages, such as small size, remote application in fields, fast response time and immunity to electromagnetic fields. In this paper, the soil moisture is measured by using a polymer optical fiber Bragg grating (POFBG) probe with a packaged dimension of 40 mm × 15 mm × 8 mm. Due to the intrinsic water-absorbing property of poly (methyl methacrylate) (PMMA), optical fiber Bragg gratings based on PMMA have been widely investigated for humidity measurement. Taking advantage of this, a sensor based on the POFBG is investigated to verify the soil condition. The POFBG is protectively integrated inside a stainless-steel package. A window is opened with a thin polypropylene mat as a filter, which allows the air to go through but prevents the soil from going inside to pollute the POFBG. The sensor probe is embedded in soils with different gravimetric soil moisture contents (SMCs) ranging from 0% to 40% and, then, insulated by polyethylene films to minimize the impact from the external environment, showing an average temperature cross sensitivity of −0.080 nm/°C. For a constant temperature, an exponential relationship between the Bragg wavelength and the SMC is obtained. For the SMCs between 8% and 24%, linear relationships are presented showing a temperature-corresponded sensitivity between 0.011 nm/% and 0.018 nm/%. The maximal sensitivity is calculated to be 0.018 nm/% at 20 °C, which is 28 times as high as that in the previous work. For the SMC over 24%, the sensor becomes insensitive because of humidity saturation in the cavity of the sensor probe. Though temperature cross sensitivity is problematic for SMC measurement, the influence could be eliminated by integrating another humidity-insensitive temperature sensor, such as a silica FBG temperature sensor.

Large refractive index modulation based on a BDK-doped step-index PMMA optical fiber for highly reflective Bragg grating inscription.

We experimentally report high reflectivity on the poly(methyl methacrylate) (PMMA)-based polymer optical fiber Bragg gratings by means of a 266 nm pulsed laser and phase mask technique. In the first recipe, fiber Bragg gratings (FBGs) were manufactured with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2×10-4 was obtained. In the second recipe, FBGs were inscribed by 22 pulses with a lower pulse energy of 1.4 mJ, showing a stable refractive index change of 6.2×10-4. Both behaviors may mainly be attributed to the movement of initiating radicals arising from benzyl dimethyl ketal (BDK) under UV irradiation. The high refractive index change in step-index fibers paves the way to tilted FBG manufacturing with large tilt angles potentially for biomedical applications.

Chirped POF Bragg grating production utilizing UV cure adhesive coating for multiparameter sensing

In this work, we obtained chirped Bragg gratings by post-coating uniform poly (methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs) with an UV cure adhesive. The main advantage of the proposed method consists on being a cost-effective and simple solution, since no special phase mask, additional etching or gradient thermal annealing are required. A comparison between the fabricated chirped Bragg grating’s performance and previous reported methods in the literature is presented. As a compact sensor with two different active areas (adhesive coated and uncoated sections of the POFBG), the developed sensing device can be used for multiparameter sensing, namely for strain, humidity and temperature monitoring. Allying the proposed flexible and low-cost fabrication technique with the intrinsic characteristics of POFs, one may conclude that the developed sensor has high potential for multiparameter sensing in possible future engineering and biomedical applications.

High-sensitivity, high-resolution polymer fiber Bragg grating humidity sensor harnessing microwave photonic filtering response analysis.

Polymer optical fiber Bragg gratings (POFBGs) provide an efficient, robust, and cost-effective approach for humidity sensing. In this Letter, we propose and experimentally demonstrate a high-sensitivity and high-resolution POFBG-based humidity sensor that utilizes microwave photonic filtering response analysis. A POFBG and a silica FBG are used as the sensing probes, and a two-tap microwave photonic filter is developed to enable the high precision interrogation. Since only the poly (methyl methacrylate) (PMMA) based fiber is sensitive to humidity, the wavelength shift due to the humidity change is converted into the change of frequency response of the microwave photonic filter. A relationship between the humidity and the free spectral range (FSR) of the microwave photonic filter is established as the interrogation technique. Compared with the traditional approach by directly monitoring the wavelength shift using an optical spectrum analyzer, our proposed scheme can increase the humidity sensor resolution by two orders of magnitude, which greatly increases the accuracy of the humidity sensor, making it possible and feasible in many potential high-precision applications.

Single nanosecond-pulse production of polymeric fiber Bragg gratings for biomedical applications.

In this study, we present first-time fabrication of FBGs in all ZEONEX-based SMPOFs with a single 25 ns pulse of 248 nm UV irradiation over a 12-month period, which opens up new frontiers in optics and photonics for the effective fabrication of polymer optical fiber Bragg gratings (POFBGs), permitting mass producibility of them. POFBGs were characterized by subjecting them to various physical parameters including temperature and tensile strain. Strain responses of FBGs with similar grating strengths fabricated with 248 nm and 325 nm He-Cd laser irradiations were explored over a year to demonstrate their long-term stability and applicability. Owing to the unique features of the proposed sensing device fabricated by embedding POFBGs in silicone rubber, a good performance in the detection of human heart rate with an amplitude of 4 pm, which is 4 times higher compared to that of silica single mode fiber (SMF) was demonstrated. The response of the sensing device during a human respiration process was also explored where exhalation and inhalation were monitored and distinguished while the breath was held. These revelations signify the importance of ZEONEX-based POFBGs, which allow consistent and effective grating fabrication and are highly promising in the foreseeable future for biomedical applications.

Gas Pressure Sensor Based on BDK-Doped Polymer Optical Fiber

This paper presents a high sensitivity gas pressure sensor with benzyl-dimethylketal (BDK)-doped polymer optical fiber Bragg grating (POFBG), whose sensitivity is up to 8.12 pm/kPa and 12.12 pm/kPa in positive and negative pressure atmosphere, respectively. The high sensitivity can be explained by its porous chemical structure. The stability and response behavior under air pressure atmosphere has also been investigated. The new understanding of the air pressure response principle and sensitivity difference for the presented sensor can be a worthy reference.

Twist dependencies of strain and temperature sensitivities of perfluorinated graded-index polymer optical fiber Bragg gratings

We measure the twist dependence of the reflected spectrum of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI) polymer optical fiber (POF). With increasing twist, the strain sensitivity decreases with a coefficient of –34.2 pm/%/turns/m, while the temperature sensitivity increases with a coefficient of 0.39 pm/°C/turns/m. At a twist of 150 turns m−1, the temperature sensitivity is 3 times higher than that with no twist. The controllability of the strain and temperature coefficients without considerable cost is beneficial not only to highly sensitive temperature sensing but also discriminative sensing of strain and temperature using two PFGI-POF-FBGs.

Small and Robust All-Polymer Fiber Bragg Grating Based pH Sensor

The smallest all-polymer optical fiber Bragg grating based transducer element for pH sensing is presented. We show that, considering it's size and robustness, it out-performs similar state-of-the-art fiber Bragg grating based pH sensors regarding both sensitivity and response time. A 5-10 μm thick pH sensitive hydrogel coating is placed on a PMMA-based microstructured polymer optical fiber Bragg grating. The hydrogel expands or contracts depending on the pH and thus changes in pH are monitored by following the fiber strain induced changes in the reflected Bragg wavelength λB. Prior to applying the hydrogel coating the mPOF is etched from 150 to 80 μm to enhance sensitivity and surface crazing is introduced with a 50/50 vol% solution of acetone and methanol to enhance spreading of the hydrogel during the application and adhesion after cure. With this design, we achieved a sensitivity of Δλ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> = 73 pm/pH ± 2 pm/pH and response times below 4.5 mins. for pH 5-7 and 4-7, respectively, and a thermal cross sensitivity of 31.4 pm/°C ± 0.4 pm/°C.

Effects of Solvent Etching on PMMA Microstructured Optical Fiber Bragg Grating

Polymer optical fiber Bragg gratings (POFBGs) can be solvent etched to achieve higher sensitivity sensors. To analyze the influence of etch on the mechanical and thermal performance of PMMA-based microstructured POFBGs (mPOFBGs), we etched annealed and unannealed samples in pure acetone, while at the same time monitoring the induced Bragg wavelength shifts. Optical-thermodynamical interpretation of these shifts together with thermal and mechanical testing of etched fibers, reveal that solvent etching can lead to trapping of residual plasticizing solvent in the polymer matrix and consequently to change of Young's modulus and thermal instability of the etched mPOFBG. Removal of these solvent residuals by thermal treatment results in a stable linear response with a thermal sensitivity indistinguishable of that of the unetched mPOFBG. The maximum found change of Young's modulus due to acetone plasticization during etch is ~5% for both unannealed and annealed mPOFBGs. The mPOFBG force sensitivity increases greatly with reduced diameter and the relatively small changes in strain sensitivity and Young's modulus introduced by the etching process in this case do not have a big effect.

Hot water-assisted fabrication of chirped polymer optical fiber Bragg gratings.

We obtained chirped gratings by performing hot water gradient thermal annealing of uniform poly (methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs). The proposed method's simplicity is one of its main advantages because no special phase mask or additional etching are needed. It not only enables easy control tuning of the central wavelength and chirp characteristics, but it also leads to obtain flexible grating response, compared with tapered chirped POFBGs. Therefore, a flexible and low-cost chirped POFBG devices fabrication technique has been presented by using a single uniform phase mask.

Toward Commercial Polymer Fiber Bragg Grating Sensors: Review and Applications

Interest in polymer optical fiber Bragg gratings (POFBGs) arises from the different material properties and sensing modalities brought by polymers relative to silica. Polymer fibers typically offer twice the sensitivity to temperature of conventional silica fiber and increased sensitivity to strain overall. In addition, polymer fibers have higher elastic limits and as a result a larger range of operation for physical constraints. While some polymers are effectively humidity insensitive, others present inherent humidity sensitivity. Their organic properties also allow a variety of chemical processes to create (bio)chemical sensors, with the consequences of fiber breakage in situ being less hazardous than silica. These attributes have led to the use of POFBGs for applications that remain complex using silica fibers. This review paper covers the progress toward commercialization and the increasing number of specific applications.