Photosensitive Fiber Research Articles

Simultaneous measurement of temperature, liquid level and axial strain based on torsional taper Mach-Zehnder interferometer with Vernier effect

A novel and simple highly-sensitive temperature, liquid level and axial strain sensor based on a torsional taper Mach-Zehnder interferometer (MZI) with Vernier effect is proposed. An interesting finding, which is the linear relationships between the wavelength shifts of resonance peaks (envelopes) and temperature, liquid level and axial strain surrounding the sensing fiber, is demonstrated experimentally. Another interesting feature is that the sensor with Vernier effect magnifies the temperature, liquid level and axial strain sensitivities by 11.3, 11.2 and 12.6 times, respectively. In the proposed method, the sensor is fabricated by cascading one polarization mode interferometer (PMI) as reference interferometer and one torsional taper MZI as sensing interferometer. The PMI is composed of polarization maintaining fiber (PMF), and cascaded MZI is composed of thin-core photosensitive fiber (TPSF). Combined with the characteristic of high sensitivity, the sensor can be widely used in the field of environmental and structural safety monitoring.

Experimental study of a temperature measurement system for an overhead power line using sensors based on TFBG

This article presents the use of an inclined fiber sensor Bragg grid to control the extension of wires in overhead lines. An aspect of the technological novelty of the project is the development of an innovative optoelectronic system for monitoring and diagnosing the condition of building structures based on a combination of conventional Bragg gratings. The lighting power sensor is installed directly on the controlled transmission of the power line in the form of fittings with a copper plate. A photosensitive multimode optical fiber is attached to the copper plate, at the end of which an inclined fiber Bragg grating is fixed, connected to a multimode optical fiber using a fiber-optic connector through an optical connector. An ultraviolet excimer laser and a light power detector are connected to a fiber-optic connector. The study showed that by selecting the appropriate mechanical parameters of the extension transformer, taking into account the optical parameters of the sensor, and using a special filter, the optical-mechanical system can be configured in the required range to control the sagging of the overhead line wire. During measurements to simulate the operation of a power line wire, the temperature was forced to vary in the range of 10-60°C. This led to the lengthening of the test wire from 38.987 m to 49.275 m; the error was within 4%. The deflection range depends on the distance between the supports, the type of wire, and its actual length in the span, which actually determines the deflection.

Gamma irradiation of Ge-doped and radiation-hard silica fibers at cryogenic temperatures: Mitigating the radiation-induced attenuation with 1550 and 970 nm photobleaching

We investigated the effects of gamma irradiation on radiation-induced attenuation (RIA) in photosensitive (Ge-doped) and radiation-hard (F-doped) fibers at cryogenic temperatures (77 K) under different photobleaching conditions. We show that increasing the probe power (1550 nm) and injecting lower wavelength light (970 nm) both resulted in a significant reduction in RIA in both fiber types, where radiation-hard fibers were intrinsically more resistant to the RIA. Deconvolution of RIA growth curves revealed that the RIA was composed of transient and long-term growth components that were correlated with distinct radiation-induced defects specific to each fiber composition. The 1550 nm light more effectively suppressed the transient RIA, while 970 nm more effectively suppressed the long-term RIA. Ultimately, we show that cryogenic RIA may be effectively managed in fiber optic sensing systems using radiation-hard fibers and dual-wavelength photobleaching strategies.

Highly Er/Yb-Co-Doped Photosensitive Core Fiber for the Development of Single-Frequency Telecom Lasers

A highly erbium- and ytterbium-co-doped photosensitive fiber with a germanophosphosilicate glass core was fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in-house”. Due to doping with germanium oxide (GeO2), this fiber revealed high-grade photosensitivity (without hydrogen loading) to UV laser radiation at a 193 nm wavelength. The short (28 mm) Fabry–Perot laser cavity was designed by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fabricated fiber sample. The stable single-frequency operation regime of the designed laser was observed. The laser emission peak was centered at 1540 nm, with a linewidth of 50 kHz. The slope efficiency of the laser was 10%, and the maximal output power reached a level of 35 mW.

Innovative hybrid optical sensing design to simultaneously discriminate pressure and temperature

In this work, hybrid optical fiber sensors based on Fabry-Perot (FP) interferometer and Fiber Bragg Grating (FBG) sensors were developed to simultaneously measure two external parameters, pressure, and temperature. The proposed sensor consists of a photosensitive Single-Mode Fiber (SMF), where the FBG is recorded, and spliced to a small section of a Hollow-Core Fiber (HCF). After that, the HCF tip is submerged in a UV-photosensitive polymer (RI = 1.46), creating three cavities, which will create two observable light interferences, allowing the observation of two FP responses in the spectral response. Two sensors with different HCF lengths were created to compare their sensitivities. After curing, the sensors were calibrated to both parameters in the ranges of 0.0 to 4.0 bar (steps of 1.0 bar) and 22.0 to 30.0 °C (steps of 2.0 °C), respectively. By tracking the peak shifts of the FP, it achieved higher sensitivities for the sensor with the shorter HCF tip (182.30 µm of HCF and 28.56 µm of UV-polymer lengths) of around 31.65 nm/bar and 1.53 nm/°C. On the other side, the sensor with the longer HCF tip (318.56 µm of HCF and with 52.17 µm of UV-polymer lengths) achieved 15.65 nm/bar and 1.02 nm/°C. Regarding the FBGs, they achieved 9.65 and 7.86 pm/°C for the longer and shorter sensor, respectively, while presenting insensitivity to pressure. Therefore, the shorter FP cavity produces the more sensitive sensor because, since its length is shorter and possesses a concave shape, it is more susceptible to external changes. Thus, variations of pressure and temperature could be discriminated by using the matrixial method with the FP and FBG sensitivities, given that the determinant of the coefficient matrix results is -0.31, a non-zero value. The developed sensor has the potential to integrate specific applications, such as LiBs to measure and decouple both parameters.

Evaluation of the effectiveness of the effect of photosensitization on the spectral characteristics of the fiber Bragg grating

Currently, fiber Bragg gratings obtained on the basis of photoinduced optical fibers doped with a high concentration of germanium oxide are used as highly sensitive sensors. However, it is worth noting a significant drawback – the manufacturing technology of optical fibers doped with germanium is expensive. When recording Bragg gratings in a standard telecommunication fiber, where the molar concentration of germanium in the fiber core is from 3 % to 5 %, interference occurs due to very low and insufficient light sensitivity. Thus, an important role is played by solving the problem of low photosensitivity of standard telecommunication fibers for recording Bragg gratings. This paper presents the results of studies of the spectral characteristics of fiber Bragg gratings based on standard telecommunication fibers pre-saturated with hydrogen to increase photosensitivity. According to the results obtained, it was found that under the action of UV radiation in the presence of hydrogen, the photosensitivity of the fiber increases and the Bragg wavelength shift is associated with the saturation of the fiber with hydrogen, the effective modulation amplitude of the induced refractive index is equal to 1.2 with a refractive index of 1.438. This work proves that the VBR recorded in the S pre-saturated in hydrogen for 12 days is characterized by increased photosensitivity. The experimental results obtained make it possible to use a Bragg fiber array based on a standard telecommunications optical fiber saturated with hydrogen in the field of telecommunications, seismology, engineering geology as fiber-optic sensors of pressure, deformation, temperature, rotation and rotation, including in extreme environmental conditions

NUMERICAL ANALYSIS OF FORMING UNIFORM FIBER BRAGG GRATINGS, USING PHASE MASK

In the work herein there was developed numerical analysis of forming uniform fiber Bragg gratings, using phase mask. There was developed method of recording Bragg grating, using uniform phase mask, which is characterized with a fact, that it consists of ultraviolet excimer laser, output laser light beam, directed to the system of mobile diaphragms with regulated gap between them and located behind the system of mobile diaphragms of uniform phase mask, after which there is a photosensitive multimode optical fiber with tilted Bragg grating. In the research there was applied diffraction theory to analyze intensity distribution in the fiber core during fabricating fiber Bragg grating (FBG), using uniform phase mask. There was calculated averaged distribution of diffraction field in the fiber core as a function of optic fiber position. Outcomes show, that in case of fabricating uniform FBG, averaged intensity field profile, as well, refraction index of optical fiber position behind phase mask is decreased for average intensity distribution in the fiber core along with increase of its diameter.

Fabrication and analysis of enhanced thermal stability and high-sensitivity turnaround point long-period fiber grating.

This paper presents a what we believe is a novel method to fabricate turnaround point long-period gratings (TAP-LPGs) possessing enhanced thermal stability and high sensitivity. It is shown by analysis and by experiment that LPG resonance in photosensitive fibers can be controlled partially by UV fluence and thermal annealing. TAP-LPGs with enhanced thermal stability were fabricated by following three steps: (I) finding grating period versus writing UV fluence for TAP operation; (II) writing gratings at a relatively higher period with higher fluence, in which case the resonance is out of phase; (III) controlled annealing so that the postannealed LPG operates at/near TAP. The thermal stability is enhanced. The average temperature sensitivity of dual peak resonance measured for a typical TAP-LPG in the temperature interval of 70°C-240°C is about 2.3 nm/°C. This study will be useful for the development of high temperature TAP-LPG sensors.

Respiratory fabric sensor based on the side luminescence and photosensitivity mechanism of polymer optical fibers.

It is significant to monitor respiration conveniently and in real time for people suffering from respiratory diseases. Polymer optical fibers (POFs) have the advantages of flexibility and light weight, which is highly desirable for wearable respiratory monitoring. However, in most current applications, the POFs are stitched on the textile substrates in the form of macro-bending. This method is complex to fix the bending with certain curvatures and uncomfortable compared with the POF sensors woven into the textile. In this paper, a respiratory fabric sensor based on the side luminescence and photosensitivity mechanism of POF is proposed and demonstrated. The 750µm-diameter POFs were woven into a fabric as warp and laser marking was performed at their designed positions to make them release or couple light. The spacing change between the POFs caused by the respiratory movement accordingly makes the light intensity change in the photosensitive fiber. We chose four fabric widths (10cm, 8cm, 6cm and 4cm) and four fabric weaves (plain weave, honeycomb weave, 1/3 right twill weave and 8/3 warp satin weave) to implement the full-factor experiment for exploring the measurement effect of the respiratory fabric sensor. The result is that the fabric with width of 4cm and weave of 8/3 warp satin is optimal. The calm and deep respiratory tests of the human chest and abdomen in sitting and standing posture were carried out and the test performance of the fabric sensor is almost comparable to that of the medical monitor. The proposed respiratory fabric sensor is comfortable, easily woven and high in precision, which is expected to realize industrialized scale production.

Preparation of a Photosensitive Composite Carbon Fiber for Spilled Oil Cleaning

This paper deals with preparing a functional composite carbon fiber with a large surface area for spilled oil cleaning. The composite fiber consisted of photosensitive oxide particles and polymer-derived carbon. It was made by co-spinning the polymer and metallic compounds. After heat treatment at high temperatures, an activated carbon fiber containing oxide particles was obtained. The particles were found distributed in the fiber and at the surface of the fiber. The composite fiber was found sensitive to sunlight. Fiber mats made of the composite fiber possessed a high surface area for oil absorption and removal. Cobalt(II) titanate particles were obtained from the reaction of titanium dioxide and cobalt oxide. The reaction happened in situ through the hydrolysis of metallic compounds in the spun fiber. The titanium dioxide and cobalt(II) titanate particle-containing fibers demonstrated the photoactivity in the visible light spectrum. It was concluded that particle-containing composite carbon fiber mats can be prepared successfully by co-electrospinning. Due to the oleophilic property and the high active surface area, the composites are suitable for spilled oil cleaning through fast absorption.

Simultaneous Measurement of Thermal Gradient and Average Temperature in Concrete Blocks Using Nonuniform Long-Period Optical Fiber Gratings

A recent increase in structural integrity requirements has lead to a higher demand for accurate heat flux and temperature readings of building material substrates. In the present work, simultaneous two parameter measurements of the average temperature and the temperature difference in concrete blocks are reported. To this end long period gratings (LPGs) were commissioned in a boron doped photosensitive optical fiber using a pulsed CO₂ laser. The average temperature and the temperature differences were shown to be a linear combination of the spectral shifts of the center wavelength and the changes in the variations in transmitted power <i>I</i>_c minimums. The sensitivity and cross-sensitivity coefficients of the sensitivity matrix are also reported.

Photosensitive Polymer-Based Micro-Nano Long-Period Fiber Grating for Refractive Index Sensing

In this paper, we propose and demonstrate a photosensitive polymer-based micro-nano long-period fiber grating (PPMN-LPFG) for refractive index (RI) sensing. We coat liquid photosensitive polymer on the surface of micro-nano fiber (MNF) and obtain a series of periodically distributed droplets on the MNF due to the Plateau-Rayleigh instability (PRI) property. Then we cure the polymer droplets by using the 532 nm laser illumination. We configure the cladding modulated LPFG based on the periodic polymer points along the MNF. The PPMN-LPFG combines with a strong evanescent field of MNF and cladding modulated LPFG, and thus, it is much more sensitive to the external environment. The experimental results indicate that the spectral dip redshifts from 1404 nm to 1654 nm when the RI changes from 1.3391 to 1.4164. The maximum RI sensitivity is 10419 nm/RIU in the RI range of 1.4102–1.4164. The temperature sensitivity is 0.89 nm/°C in the temperature range of 17.1 °C–50.5 °C. Compared with the RI sensitivity, the temperature crosstalk is negligible. Our simulated and experimental results verify the feasibility of the proposed sensor and show the advantages such as high sensitivity, compact size, and good stability.

Photosensitive Yb-Doped Germanophosphosilicate Artificial Rayleigh Fibers as a Base of Random Lasers

Asingle-mode Yb-doped germanophosphosilicate fiber with ultra-low optical losses (less than 2 dB/km) was fabricated by means of the MCVD method utilizing an all-gas-phase deposition technique developed “in house”. The absorption and luminescent spectral properties of the fiber were thoroughly studied. The photosensitivity of the pristine (non-hydrogenated) fiber to 248 nm-laser radiation was confirmed by means of fiber Bragg grating (FBG) inscription directly during the drawing process. The random single-frequency lasing at the 1060-nm-wavelength obtained in the 21-m-long fiber with an array of weak FBG was reported. The developed laser slope efficiency in the backward-pumping scheme was measured as high as 32%.

Raman suppression in 5 kW fiber amplifier using long period fiber grating fabricated by CO2 laser

Long period fiber grating (LPFG) can be used as a band rejection component, providing a potential effective way for Stimulated Raman scattering (SRS) suppression in high power fiber laser. Here, an improved CO2 laser scanning system with a galvanometer is built up for fabrication of LPFGs, which greatly improves the speed and consistency of grating inscribing. The characteristics of the manufactured LPFG is measured in detail, then a direct laser diode pumped 5 kW-level fiber amplifier system is established to test the validity of the LPFGs for Raman suppression for the first time. Experimental results show that the SRS suppression ratio is about 14 dB at 5.1 kW with the LPFG being inserted between the seed and the amplifier stage, meanwhile the effective output signal laser power is increased. In contrast to the ultraviolet laser exposed band rejection components, such as chirped and tilted fiber Bragg grating (CTFBGs), this type of LPFG does not need photosensitive fibers, annealing treatment and has a much lower thermal effect, which is very useful for high-power fiber lasers applications.

Special Issue on “Advancements in Fiber Bragg Grating Research”

Since the discovery of photosensitivity in optical fibers, there has been great interest in fiber Bragg grating (FBG) research [...]

Regeneration of Fiber Bragg Gratings and Their Responses Under X-Rays

Fiber Bragg gratings (FBGs)-based temperature sensors present numerous advantages such as small packaging, fast acquisition rate, and accuracy for structural health-monitoring applications in nuclear environments. Among the various classes of FBGs, Type I FBGs are inscribed with a UV continuous or pulsed laser on a photosensitive fiber or with femtosecond laser even in nonphotosensitive fibers. These gratings, however, cannot survive at temperatures exceeding 400 °C. Regenerated FBG (RFBG) gratings, instead, are derivative from type I grating: when a high thermal treatment (> 650 °C) is applied after the inscription on a prehydrogenated fiber, a new grating (RFBG) appears with different thermal properties. It withstands temperatures as high as 1000 °C, opening the way to new application fields such as the temperature monitoring of the nuclear reactor cores. This work investigates the radiation response of RFBGs originated from type I seed FBGs inscribed with an argon laser (244 nm) in two different fibers (SMF-28e fiber or in a B/Ge co-doped fiber), loaded with either H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> before the inscription to enhance the fiber photosensitivity. Regeneration was achieved at 650 °C or 900 °C depending on the fiber type. After this, the RFBGs were irradiated under 40-keV X-rays at two different dose rates [1 Gy(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s or 10 Gy/s] and at two temperatures of irradiation-25 °C or 250 °C. At room-temperature (RT) irradiation, a Bragg Wavelength Shift (BWS) of about 35 pm was observed for the SMF-28e and more than 130 pm for the B/Ge fiber at 400-kGy dose, and the combined temperature and radiation constraints reveal that the RFBGs are radiation-tolerant. Indeed, no BWS is observed anymore, at the highest temperature of irradiation.

Method for controlling temperature sensitivity of the fiber intermodal sensor and its application in the torsion sensor.

Temperature crosstalk has always been a critical problem for fiber intermodal sensors. In this work, we have proposed a novel method based on the special temperature response of photosensitive fiber to control the temperature sensitivity of the fiber intermodal sensor. The control of temperature sensitivity has been realized via adjusting the proportion of photosensitive fiber to single-mode fiber in the sensing part. The temperature sensitivity as high as -192 pm/°C, and as low as -2.6 pm/°C can be obtained, satisfying the demand in both research and application. The torsion sensor is taken as an example to illustrate feasibility of this method, showing no evident interference in the measurement of torsion parameters. The proposed method outstrips the conventional one by simple structure, facile manufacture, multiple use and low cost, which brings great promise for further employment in laboratory and industry.

Detection of Collagen-IV Using Highly Reflective Metal Nanoparticles-Immobilized Photosensitive Optical Fiber-Based MZI Structure.

In this work, a photosensitive (PS) optical fiber-based Mach-Zehnder interferometer (MZI) structure is developed to diagnose the presence of collagen-IV in human bodies. The MZI is fabricated by sequentially splicing the single mode-multimode-photosensitive-multimode-single mode (SMPMS) fiber segments. The sensing region in MZI structure is created by partially removing the cladding of photosensitive fiber by using 40% hydrofluoric (HF) acid and depositing the layers of highly reflective metal nanoparticles (NPs) over it. The used NPs are polyvinyl alcohol stabilized silver nanoparticles (PVA-AgNPs), gold nanoparticles (AuNPs), and zinc oxide nanoparticles (ZnO-NPs). The size of AuNPs, PVA-AgNPs, and ZnO-NPs are 10 ± 0.2 nm, ∼ 4 -5 nm, and < 50 nm, respectively. In order to avoid the interference of other biomolecules in the detection of collagen-IV, the sensing region is functionalized with a collagenase enzyme. The sensing ability of the probe is ascertained by sensing a wide concentration of collagen solution ranging from 0 ng/ml to [Formula: see text]/ml. It is observed that sensing performance of probe is much better on immobilizing it with PVA-AgNPs and ZnO-NPs.

Responses to temperature and thermal gradients of non-identical cascaded LPGs taking into account dispersion

Abstract In this paper we present a detailed theoretical and experimental study of the responses of non-uniform cascades of LPGs (C-LPG) to thermal gradients which cause changes in the depth of the interference minima. Simulations have been performed for cascades without and with dispersion of identical and non-identical effective refractive index differences. It has been shown that in case of non-identical LPGs the response to temperature differences shifts in a way to permit the detection of the magnitude and the sign of the thermal gradient. In the case of dispersion the side minima respond to gradients in counter phase and permit differential measurements. Sensitivities to temperature difference variations of 0.13 dB/°C and to temperature of 294 pm/°C were obtained for cascaded LPGs from photosensitive optical fibers.

Discretized superimposed optical fiber long-period gratings.

A novel technique, to the best of our knowledge, for the inscription of superimposed long-period gratings with arbitrary grating pitches is proposed and experimentally validated. The technique is based on the discretization of an ideal continuous sinusoidal refractive index (RI) pattern with a step function. The RI variation is induced by means of the irradiation of a photosensitive fiber with a 248 nm UV laser beam. The nonlinear relation between the induced RI change and the UV fluence was experimentally derived. Two superimposed long-period grating (LPGs) with different grating pitches have been realized with the discretization technique; the transmission spectrum was compared with that of two superimposed LPGs obtained with the traditional square wave RI modulation. The validity of the proposed technique was demonstrated by the better spectral characteristics of the discretized superimposed LPGs.