Graded-index Polymer Optical Fiber Research Articles

Potential of high-sensitivity tactile sensing using polymer optical fiber gratings

We characterize fiber gratings inscribed line-by-line in perfluorinated graded-index polymer optical fibers (PFGI-POFs) for tactile sensing. First, we measure the dependence of the wideband transmitted light spectrum on the distance between the fiber grating and an object composed of metal or rubber, clarifying that the spectral power is significantly dependent on the object position in a wide wavelength range. Subsequently, we experimentally show that this behavior is caused by the light scattering at the fiber grating, indicating that PFGI-POF gratings are potentially applicable to simple low-cost tactile sensors or high-sensitivity short-range displacement sensors with no need for additional supportive tools.

Amplification of whispering gallery microlaser emission using dye-doped graded-index polymer optical fiber

PMMA-based Rhodamine-640 perchlorate doped microring embedded hollow polymer optical fiber (MEHPOF), as well as graded-index polymer optical fiber (GIPOF), were fabricated. At a threshold pump pulse energy (PPE) of 0.23 mJ, a whispering gallery mode lasing was observed from the MEHPOF. The quality factor and free spectral range of the fabricated microcavity were found to be 3.69×103 and 0.25 nm respectively. The lasing modes from the MEHPOF were amplified using a GI fiber doped with the same gain material. The maximum gain achieved from the experiment is 17.5 dB. The optimum length of the amplifier fiber for maximum gain was found to be 8 cm.

Evaluation of Radio over Plastic Optical Fiber Communications

In this work, we have experimentally evaluated the performance of a Radio over Plastic Optical Fiber (RoPOF) communications link by simultaneously transmitting Long-Term Evolution (LTE) and Narrow-Band Internet of Things (NB-IoT) signals over 75-meters of PMMA large-core Graded-Index POF (GI-POF).

Optical Bend Sensor Based on Eccentrically Micro-Structured Multimode Polymer Optical Fibers

We report on a novel bend sensor with high flexibility and elasticity based on Bragg grating structures in polymer optical fibers to detect bending for the measurement of movement. The concept is very simple and relies on the inscription of eccentrical Bragg gratings into multimode graded-index polymer optical fibers via contact exposure with a krypton fluoride excimer laser in the ultraviolet region and an optimized phase mask. Depending on the fiber deformation, the lattice constant of the inscribed Bragg grating is strained or compressed due to its position relative to the fiber core. This in turn results in a specific shift of the Bragg wavelength of up to $1.3\text{ nm}$ to the red or blue wavelength region, respectively, which is sufficiently large to be reliably detected. Therefore, as proof of principle, deformation along one axis can be observed with a single Bragg grating with a maximum sensitivity of up to $65\text{ pm/m}^{-1}$ . Moreover, multiple Bragg gratings inscribed into the same polymer optical fiber at different positions around the fiber axis allow to determine the shape deformation of the fiber relative to a reference frame with similar accuracy. Consequently, this technology could form the basis for new applications in the areas of medical diagnostics, robotics or augmented reality, which are lacking affordable sensor systems to date.

Refractive-Index Profile Reconstruction in Graded-Index Polymer Optical Fibers Using Raman Spectroscopy.

This work reports a novel method to create a 3D map of the refractive index of different graded-index polymer optical fibers (GI-POF), measuring the Raman spectra at different points of their transverse sections. Raman fingerprints provide accurate molecular information of the sample with high spatial resolution. The refractive index of GI-POFs is modified by adding a dopant in the preform; therefore, by recording the intensities of the Raman peaks related to the dopant material, a 3D map of the refractive index is rendered. In order to demonstrate the usefulness of the method, three different GI-POFs were characterized and the obtained results were compared with the information provided by the manufacturers. The results show accurate 3D maps of the refractive index taken in the actual GI-POF end faces, showing different imperfections that manufacturers do not take into account, such as the slight deviations of the azimuthal symmetry. The simplicity and the feasibility of the technique mean this method has high potential for fiber characterization purposes.

Scattering Into Guided Modes Due to Imperfect Graded-Index Structure in Polymer Optical Fibers

Rayleigh scattering in graded-index polymer optical fibers (GI-POF) is studied under the assumption that spherical scatterers are uniformly distributed within the layers of concentric ring structure. This approximation allows to describe typical imperfections occurring during the fabrication of GI-POFs. For the purpose of experimental characterization by side illumination, an analytical model of the light conversion into waveguide modes has been developed. This model describes the mode excitation by a multiplicative contribution of three different physical mechanisms. The corresponding experimental analysis of the light scattering has been performed and compared with numerical simulation results for the fibers with single-ring and multi-ring structures. Besides the application to GI-POFs, this article can be also applied to multi-step index fibers.

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.

Potential of Discriminative Sensing of Strain and Temperature Using Perfluorinated Polymer FBG

Though the strain characteristics of fiber Bragg gratings (FBGs) inscribed in perfluorinated graded-index (PFGI) polymer optical fibers (POFs) have been reported, their temperature characteristics have not yet been detailed. In this paper, we experimentally investigate the temperature dependence of the Bragg wavelength of a PFGI-POF-FBG. With the increasing temperature, each peak of the FBG-reflected spectrum shifted to longer wavelength with different coefficients. The temperature coefficient of one of the clearest peaks was 0.09 nm/°C, which was eight times larger than those of FBGs in silica single-mode fibers and almost the same as those of FBGs in polymethyl methacrylate POFs. A temperature-independent but strain-dependent peak was also observed, which indicates the potential of discriminative sensing of strain and temperature.

Investigation on the Influence of Humidity on Stimulated Brillouin Backscattering in Perfluorinated Polymer Optical Fibers.

In this paper perfluorinated graded-index polymer optical fibers are characterized with respect to the influence of relative humidity changes on spectral transmission absorption and Rayleigh backscattering. The hygroscopic and thermal expansion coefficient of the fiber are determined to be = (7.4 ± 0.1) %r.h.−1 and = (22.7 ± 0.3) K−1, respectively. The influence of humidity on the Brillouin backscattering power and linewidth are presented for the first time to our knowledge. The Brillouin backscattering power at a pump wavelength of 1319 nm is affected by temperature and humidity. The Brillouin linewidth is observed to be a function of temperature but not of humidity. The strain coefficient of the BFS is determined to be (−146.5 ± 0.9) MHz/% for a wavelength of 1319 nm within a strain range from 0.1% to 1.5%. The obtained results demonstrate that the humidity-induced Brillouin frequency shift is predominantly caused by the swelling of the fiber over-cladding that leads to fiber straining.

Pump-Polarization Effects in Dye-Doped Polymer Optical Fibers

This paper presents a study on the influence of the polarization of the pump beam on the emission properties of dye-doped polymer optical fibers. The fiber samples, which are pumped transversely with linearly polarized light, are graded-index polymer optical fibers doped with rhodamine 6G. The variation of the intensity and angular distribution of the output emission are theoretically and experimentally analyzed as the polarization direction of the pump is changed. The analysis is carried out both in the regime of fluorescence and in the regime of amplified spontaneous emission. It is shown that a pump polarization perpendicular to the fiber symmetry axis yields not only higher emission efficiency but also narrower axial angular width. The effect of the polarization of the pump beam on the properties of the amplified spontaneous emission is also measured, including its threshold-like energy, relative efficiency, and optical gain. The results indicate that the pump polarization determines the light-emitting performance of dye-doped polymer optical fibers.

Humidity-induced Brillouin frequency shift in perfluorinated polymer optical fibers.

We report, to our knowledge, for the first time on humidity-induced Brillouin frequency shifts in perfluorinated graded index polymer optical fibers. A linear relation between Brillouin frequency shift and humidity was observed. Furthermore, the humidity coefficient of the Brillouin frequency shift is demonstrated to be a function of temperature (-107 to -64 kHz/%r.h. or -426 to -49 kHz m3/g in the range of 20 to 60 °C). An analytical description proves temperature and humidity as two mutually independent effects on the Brillouin frequency shift.

Displacement sensing based on modal interference in polymer optical fibers with partially applied strain

Strain sensing based on modal interference in multimode fibers (MMFs) has been extensively studied, but no experimental or theoretical reports have been given as to how the system works when strain is applied not to the whole MMF but only to part of the MMF. Here, using a perfluorinated graded-index polymer optical fiber as the MMF, we investigate the strain sensing characteristics of this type of sensor when strain is partially applied to fiber sections with different lengths. The strain sensitivity dependence on the length of the strained section reveals that this strain sensor actually behaves as a displacement sensor.

Strain dependence of perfluorinated polymer optical fiber Bragg grating measured at different wavelengths

We measure the strain dependence of multiple Bragg wavelengths (corresponding to different diffraction orders) of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index polymer optical fiber (PFGI-POF) in the wavelength range up to 1550 nm. On the basis of this result, we show that the fractional sensitivity, which has been conventionally used as a wavelength-independent index for fair comparison of the FBG performance measured at different wavelengths, is dependent on wavelength in this range. The reason for this behavior seems to originate from the non-negligible wavelength dependence of refractive index and its strain-dependence coefficient. Using the wavelength dependence of the refractive index already reported for bulk, we deduce the wavelength dependence of the strain coefficient of the refractive index. This information will be a useful archive in implementing PFGI-POF-based strain sensors based on not only FBGs but also Brillouin scattering in the future.

Pressure Dependence of Fiber Bragg Grating Inscribed in Perfluorinated Polymer Fiber

We investigate the hydrostatic pressure dependence of the Bragg wavelength of a fiber Bragg grating (FBG) inscribed in a perfluorinated graded-index (PFGI-) polymer optical fiber (POF) at 1550 nm. At 0.5 MPa, the Bragg wavelength increased with time and became almost constant ~150 min later. Such a long time constant probably originates from the unique structure of the PFGI-POF, which has a thick overcladding around its core and cladding. The pressure-dependence coefficient without considering the time constant was estimated to be 1.3 nm/MPa; this is over five times the values of other types of POF-FBGs. This indicates that by removing the overcladding of the PFGI-POF, fast-response high-sensitivity pressure sensing will be feasible. Once the Bragg wavelength became constant at 0.5 MPa, the pressure-dependence coefficient of the Bragg wavelength was measured to be −0.13 nm/MPa, the absolute value of which was comparable with those of other POF-FBGs, but with an opposite sign.

Characterization of the optical gain in doped polymer optical fibres

The properties of the amplified spontaneous emission in rhodamine-6G doped graded-index polymer optical fibres of two different dopant concentrations have been analyzed by using the Variable Stripe Length method, both theoretically and experimentally. A theoretical model based on the laser rate equations describes the dependence of the emitted spectrum on the pumped length successfully. The fibre gains and their dependence on the dopant concentration have been analyzed.

The response of polymer optical fiber (POF) to bending and axial tension for the application of a POF sensor for automotive seat occupancy sensing

The automotive industry is a promising area for innovations in the field of polymer optical fiber (POF) sensors as the industry currently uses the POF mostly for data transmissions. Since an optical fiber sensor has a high bandwidth, is small in size, is lightweight, and is immune to electromagnetic interference, it offers higher performance than that of its electrical-based counterparts such as the strain gage, elastomeric bladder, and resistive sensor systems. This enhanced performance makes an optical fiber sensor a suitable material for sensing seat occupancy for improved safety features in automobiles. The overall goal of this research is to develop a textile-based optical fiber sensor for automotive seat occupancy with high accuracy and reproducibility. In this study, the bending and tensile loading responses of POF were investigated, where two perfluorinated (PF) graded index (GI) POFs with two different core/cladding diameters, 62.5/750 and 62.5/490 μm, were used. The bending loss and the light attenuation against the applied axial stress were measured by a photon counting optical time-domain reflectometer. The critical bending diameters were analyzed: Cytop-1 (62.5/750 μm) ≥ 38.10 mm, Cytop-2 (62.5/490 μm) ≥ 44.45 mm. Furthermore, the elastic sensitive strain regions (x), where the stress-induced loss was recoverable, of the POFs at a 76.2 mm gage length at a strain rate of 4 mm/min were determined: Cytop-1: 3% ≤ x ≤ 3.5%, Cytop-2: 3.1% ≤ x ≤ 3.3%. The Cytop-1 was found to be less sensitive to bending and to have greater elastic sensitive strain range relative to the Cytop-2. In this study, a theoretical approach of the PF GI POF behavior to bending and axial tension was provided. The results demonstrated the feasibility of POFs as optical fiber sensors for automotive seat occupancy sensing.

A textile- based optical fiber sensor design for automotive seat occupancy sensing

In our previous publications, the response of perfluorinated (PF) graded index (GI) POFs (62.5/750, 62.5/490 μm) to bending, tensile loading, and cyclic loading was investigated. The results showed that Cytop-1 (62.5/750 μm) was more appropriate to be used as an optical fiber sensor for automotive seat occupancy sensing relative to Cytop-2 (62.5/490 μm). In this study, a textile-based optical fiber sensor was designed and the effect of automotive seat covering including face material and foam backing on a sensor’s performance was analyzed. The pressure interval under which the proposed POF sensor design could perform well was found to be between 0.18 and 0.21 N/cm2, where PF GI POF (62.5/750 μm) was used as the POF material. The responses of the sensor in this interval were observed to be accurate and reproducible. The face fabric structure and the thickness of foam backing were not found to be significant factors to change the sensor response. Artificial neural network (ANN) was used for data analysis, and Qwiknet (version 2.23) software was used to develop ANNs. According to the results of Qwiknet, the prediction performances for training and testing data-sets were 75 and 83.33%, respectively.

The response of polymer optical fiber (POF) to cyclic loading for the application of a POF sensor for automotive seat occupancy sensing

The goal of this research aimed to develop an accurate and reproducible textile-based optical fiber sensor for automotive seat occupancy. In our previous publication, the response of perfluorinated (PF) graded index (GI) polymer optical fibers (POFs) (62.5/750 and 62.5/490 μm) to bending and tensile loading was investigated. In this study, the response of the PF GI POFs to cyclic loading was investigated. The repeated loading and unloading the POF sensor would experience due to car vibrations and multiple uses by seat occupants, might cause fatigue failure to the POF sensor. The results showed that the Cytop-1 did not show any permanent deformation up to 500 cycles at strain rates 4 and 60 mm/min at a gage length of 76.2 mm in its elastic sensitive strain region. The Cytop-2 showed permanent deformation at 3.5% strain after 500 cycles at a gage length of 76.2 mm. Thus, the Cytop-1 was found out to be more appropriate to be used as an optical fiber sensor for automotive seat occupancy sensing relative to the Cytop-2. In this study, a theoretical approach of the behavior of PF GI POF to cyclic loading was also provided.

Drastic sensitivity enhancement of temperature sensing based on multimodal interference in polymer optical fibers

It has been reported that temperature sensors based on modal interference in perfluorinated graded-index polymer optical fibers show extremely high temperature sensitivity at room temperature. In this work, we confirm that the temperature sensitivity (absolute value) is significantly enhanced when the temperature increases toward ∼70 °C, which is close to the glass-transition temperature of the core polymer. When the core diameter is 62.5 µm, the sensitivity at 72 °C at 1300 nm is 202 nm/°C/m, which is approximately 26 times the value obtained at room temperature and >7000 times the highest value previously reported using a silica multimode fiber.

Polymer optical fiber tapering without the use of external heat source and its application to refractive index sensing

We perform a pilot trial of the highly convenient taper fabrication of perfluorinated graded-index polymer optical fibers. Instead of conventional external heating, we utilize internal heating caused by high-power propagating light (500 mW in this experiment). An approximately 4-mm-long section of a polymer fiber is tapered, and the outer diameter of the ∼2-mm-long waist around its midpoint is approximately 200 µm, which is quite uniform with a standard deviation of 4.3 µm. The polymer fiber taper fabricated by this technique is shown to be capable of generating evanescent waves and thus measuring the refractive indices of liquids from 1.333 to 1.410.