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Abstract
This research focuses on the performance analysis and characterization of a fiber Bragg gratings (FBGs) array, consisting of 10 first-order FBGs inscribed by a femtosecond (FS) laser in a highly multimode coreless fiber. The study evaluates the FBG array's ability to function as a distributed thermal sensing (DTS) platform, with the coreless fiber chosen as the sensing element due to its immunity to dopant migration at high temperatures. The design of a large cascaded first-order FBG array effectively eliminates unwanted harmonic peaks across a wide spectrum range. In contrast, higher-order FBGs introduce limitations due to the overlapping of Bragg peaks with harmonics. The FBG array's performance is evaluated by measuring the reflection spectrum of each grating at different temperatures, showing a high temperature sensitivity of 15.05 pm/°C at a Bragg wavelength of 1606.3 nm, with a linear response in the temperature range of 24 - 1100 °C. The FBG array was designed for a spatial resolution of 5 mm. A mode scrambler in the sensing network is employed, which suppresses multimodal interference, characterizes FBG peak visibility, and stabilizes the interference spectrum. The stability of the FBG array is also assessed over 24 hrs at 1100 °C, and it is observed to be stable during thermal treatment. Heat treatment at 1100°C improves the signal to noise ratio of the FBG array, demonstrating the robustness and suitability of the proposed FBG array on highly multimode coreless fiber as a potential sensing platform for DTS applications in harsh environmental conditions, overcoming the issues of dopant migration presented by dopes silica optical fibers at high temperatures.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
FBGs are widely used in various applications [1,2], including harsh environment distributing sensing and communication [3], due to their unique properties. Point-by-point laser writing is a technique used to create FBGs with a high degree of precision and flexibility [4,5]. Compared to other types of FBGs, point-by-point laser-written FBGs offer several advantages that make them ideal for a wide range of applications. The technique allows for a high degree of flexibility in designing FBGs, resulting in complex and arbitrary structures that offer specific spectral properties [6,7]. The grating structure and spacing are precisely controlled, leading to FBGs with high accuracy and reproducibility. These FBGs also typically have higher reflectivity and lower insertion loss than other types of FBGs [8]. Additionally, point-by-point laser-written FBGs have high temperature stability, making them suitable for use in high-temperature sensing applications. Point-by-point technique enables the fabrication of complex, non-uniform FBGs with high precision and repeatability. This has significant implications for various applications and offers a quick and easy method for producing such structures. [7]. Overall, point-by-point laser-written FBGs offer a range of benefits that make them a popular choice for many applications in sensing, communication, and other fields [9].
Extreme temperature sensing is a fundamental aspect of many industries [10–13], including steel making, aerospace, nuclear power plants, and oil and gas production. In harsh environments, such as those found in deep-sea drilling or outer space, it is essential to accurately monitor temperature changes to ensure safety and optimize performance. DTS is an emerging field that offers a promising solution for monitoring temperature changes over large distances [14]. One of the key technologies used in DTS is the FBG sensor [15], which offers several advantages over traditional sensors, including high sensitivity, immunity to electromagnetic interference, and the ability to measure temperature changes in real-time. FBG sensors operate based on the principle of wavelength-encoded sensing, which allows them to measure temperature changes by analyzing the reflection spectrum of light in response to temperature changes [16]. The FBG sensor consists of a periodic modulation of the refractive index of the fiber core, which acts as a wavelength filter for a specific wavelength, known as the Bragg wavelength and depends on the period of the grating. When the FBG is subjected to external physical parameters such as temperature, pressure, or strain, the reflected wavelength changes due to the changes in the periodicity of the FBG. To detect the Bragg wavelength, a broadband light source is typically launched into one end of the fiber, and the reflected signal is measured using a spectrometer or an optical spectrum analyzer. The Bragg wavelength can then be determined from the spectral position of the reflected signal peak.
Some studies discuss FBGs inscribed in multimode optical fibers. Markus et al. [17] demonstrated FBGs in multimode optical fibers to simplify and reduce costs in optical sensor systems, but parasitic mode effects can affect the measured reflection spectrum. They addressed this by using a mode transfer matrix formalism and demonstrated its accordance through two experiments, providing a tool to optimize mode effects and improve the performance of multimode fiber optic systems. Marius et al. [18] demonstrated that chirping combined with a new scanning technique could produce high-reflectivity FBGs in a 105/125 µm multimode fiber with NA of 0.1. This approach resulted in reflectivity of 61% and transmittance of 77% compared to 10% using linear scanning. However, the technique requires more process time due to the readjustment of fiber after every scanning step, making it suitable only for high-reflectivity FBGs.
To date, a variety of methodologies have been proposed and implemented for the purpose of inscribing FBGs into optical fibers, with a particular emphasis on facilitating sensing applications in high-temperature environments. Xu et al. [19] reported research focuses of ultra-weak FBG arrays inscribed with a FS laser for the purpose of distributed high-temperature sensing, involves fabricating FBG arrays through a point-by-point inscription technique in single mode fiber with ultra-low reflectivity. The resulting FBG array exhibit the ability to endure temperatures of up to 1000°C. Grobnic et al. [20] fabricated FBGs using an ultrafast Ti:sapphire 800 nm laser and a phase mask in Ge-doped SMF-28 fiber to create high thermal stability for sensor applications. Their experiments showed that the FBGs maintained over 99.95% reflectivity even after long annealing tests at 1000°C, and performed well in cycling experiments up to 1000°C without hysteresis, indicating their potential suitability for high temperature sensing. He et. al. [21] fabricated negative-index FBGs using FS laser overexposure and thermal regeneration, with a reflectivity of 99.22%, low insertion loss of 0.08 dB, and operating temperature up to 1000 °C for over 10 hours. The grating behavior was comparable with thermally regenerated type IIA photosensitivity. Hoffman et al. [22] demonstrated a method for creating long-lasting and resilient FBGs with the use of highly photosensitive photo-thermo-refractive (PTR) glass optical fiber. The fiber contains anions and cations, which change the refractive index upon exposure to near-UV radiation and thermal treatment, thereby exhibiting remarkable photosensitivity, thermal stability, and resistance to optical and ionizing radiation. Lee et. al. [23] reported 3D fiber shape sensor using FS laser direct-written optical and Bragg grating waveguides within a single coreless optical fiber. Nine Bragg gratings, distributed along three laterally offset waveguides, were simultaneously interrogated in real-time through a single waveguide port to deduce the shape and temperature profile along the fiber length for biomedical application. Bernier et al. [24] investigated the one-dimensional filamentation process for inscribing high-quality first-order FBGs using femtosecond pulses and a phase mask. They observed average photo-induced refractive index changes of the order of 5 × 10−3, and successfully inscribed -50 dB FBGs in both silica and fluoride glass fibers, thereby providing a physical insight into the inscription process. Su et al. [25] reported the direct writing of type II fiber gratings in the cladding of single-mode fibers using FS laser point-by-point technology as an attractive alternative to traditional fabrication methods. However, distributed FBGs fabricated using this technique may face drift at high temperatures due to dopant migration issues.
Recent advances in FS laser writing technology have facilitated the development of highly precise FBG sensors on various fiber types, including coreless optical fiber that have been gaining interest due to their resistance to dopant migration and ability to withstand high temperature in harsh environments. Prior research has identified limitations of FBGs in air clad multimode fibers, as they typically exhibit reflection degradation and can only support 3 to 5 cascaded FBGs [26–28]. These constraints in a highly multimode optical fiber arise from the following factors: 1) FBGs on multimode optical fibers may be higher order harmonic modes, particularly for line-by-line inscribed FBGs, leading to a broader bandwidth in the FBG spectrum and closer frequency separation between harmonic FBGs; 2) Fabricated FBGs on multimode optical fibers exhibit significant insertion loss; and 3) Multimodal interference in a highly multimode optical fiber results in poor fringe visibility and signal instability. This study introduces a solution by fabricating a first-order FBG array consisting of 10 FBGs on a highly multimode coreless fiber, which, to the best of our knowledge, is the first time such an array has been reported. A mode scrambler is also proposed within the interrogation system to stabilize the FBG reflection spectrum. The performance of the array for distributed thermal sensing applications was evaluated by examining its temperature sensitivity, spatial resolution, and stability. The methodology, results, and implications of employing FBG sensors in DTS applications are discussed in the subsequent sections.
2. Theoretical background
FBGs can be fabricated on single-mode or multimode fibers. In single-mode fibers, only the fundamental mode is guided, which simplifies the grating design and analysis. However, in multimode fibers, several modes are guided, and the modal interference causes additional complexity in the grating design and analysis. When light propagates through a multimode fiber, it can excite several modes that propagate with different velocities and accumulate different phases over a certain distance. As a result, the modal power distribution varies along the fiber length that can be exploited to create a multimode FBG. The grating reflectivity in a multimode FBG can be expressed by [29],
The phase-matching condition of a uniform Bragg grating with a period Λ can be described by the equation βp – βq = 2π/Λ, where βp and βq represent the propagation constants of the forward and backward propagating modes, respectively. In order to achieve reflection within the same mode, the condition βp = - βq must be satisfied. For a step-index multimode fiber, the propagation constant of the K-th principal mode can be determined by [30],
where bK represents the normalized propagation constant for the Kth principal mode, and its value is determined by [31],FBG exhibit reflection dips that can be determined by the propagation constants and grating period. The separations between resonance peaks in FBGs can be adjusted by modifying the fiber's waveguide profiles, enabling the design of multi-wavelength filters. During the grating fabrication process, changes in refractive index and imperfect phase matching cause shifts in the reflection dips towards longer wavelengths (red shift). Additionally, new reflection dips may appear when the phase matching condition of a new mode is nearly satisfied, but they may disappear with further irradiation. The combination of multiple resonance peaks in FBGs results in a significantly wider bandwidth compared to individual peaks, making them suitable for wide-band filter applications.
The properties of the grating in a multimode fiber can be analyzed using the coupled-mode theory. By expanding the total electric field in terms of the electric fields of the fiber modes, the behavior of the m-th mode propagating in the z direction can be described. In the absence of Bragg gratings, the mode amplitude of the m-th mode follows the coupled-mode equations as [32],
The order of an FBG refers to the number of half-wavelengths of the grating period that are contained within the length of the grating. The order of an FBG is given by [2],
where m = 1 for fundamental mode FBG that utilized in this study and neff represent effective refractive index of the fiber. Multimode fiber-based FBGs have several advantages over single-mode FBGs, such as higher sensitivity to temperature and strain due to the larger mode field diameter and the higher overlap between the modes and the grating. They can also be used as distributed sensors for temperature or strain measurements along the length of the fiber. However, the modal interference and dispersion can complicate the analysis and limit the performance of the FBG.3. FBG fabrications
The FS laser system employed in this study was the Spirit One from Spectra-Physics, which was integrated with an efficient second harmonic generation (SHG) module for switching the output central wavelength between 1040 nm and 520 nm. The laser was configured to work at 520 nm at a 5 kHz repetition rate and 500 nJ/pulse. A custom-built fs-laser micromachining system, comprising a FS laser and a commercial workstation (femtoFBG, Newport Corporation), was used for FBG fabrication using a point-by-point method. The initial step was to prepare the coreless fiber by stripping away the polymer coating layer, leaving only the bare fiber. The coreless fiber was then mounted onto a precision positioning system, which allowed for precise control of the laser beam during the fabrication process. Silica gel (n = 1.45) was used to focus the laser beam onto the coreless fiber via 40X non-immersion objective lens, and a computer-controlled translation stage was used to align the fiber relative to the beam. The laser pulse duration was typically on the order of femtoseconds, which enabled precise control of the pulse energy and the resulting refractive index modulation. The period and depth of the refractive index modulation were controlled by adjusting the laser power and the translation speed of the fiber relative to the laser beam. The depth of the refractive index modulation was typically on the order of tens of nanometers, while the period could range from several hundred nanometers to several microns.
Figure 1(a) illustrates the schematic of the FBG array that was fabricated with two FBGs inscribed side-by-side layers to enhance the reflection intensity by creating a ± 20 µm offset from the central axis. The fabrication process of each layer of FBG involved precise control of the velocity of a continuous moving translation stage. Upon completion of the writing process for the first layer, the positioning of the FS-laser was adjusted to introduce a 20 µm offset from the center-axis. Subsequently, the second layer of FBG, referred to as FBG-1, was inscribed using identical parameters as those employed for the initial layer. This sequential approach was then iterated to fabricate the subsequent FBG layers, denoted as FBG-2 to FBG-10, following a similar procedure. Figure 1(b) illustrates the FBG inscription platform when the inscription of the proposed FBG was in-process. Figure 1(c) presents a microscopic image of the coreless fiber with a diameter of 125 µm, showcasing the two side by side layers of fabricated FBG. To provide a more detailed view, Figs. 1(d) & (e) show The enlarged inset inset of Fig. 1(c), offering a top view and a lateral view, respectively, of the point-by-point pattern resulting from the irradiation of the FS-laser on the FBG. It is important to note that due to the diffraction limit of the microscope, further focus beyond the captured image was not possible. The image was captured using a LEICA DM500 optical microscope, equipped with a 100X/1.25 Oil immersion lens. The FBG array comprised FBG-1 to FBG-10, with the length of FBG-1 starting from 6500 µm and ending at the last FBG, which was 8750 µm long. The separation distance (S) between each FBG was 5 mm. A higher laser power (AOM intensity) with Gaussian profile of FBG was utilized to create deep reflectors and attain a higher reflection from Bragg reflectors written in coreless fiber. The FBG arrays exhibit distinct Bragg wavelengths (λBragg) that are determined by specific fabrication parameters, as presented in Table 1. The reflection spectra of the FBG arrays were also characterized by their period and depth, which are crucial parameters for their performance. The effective refractive index of a mode depends on various factors, including the mode's spatial distribution, propagation constants, and the refractive index profile of the fiber. Different modes have different effective refractive indices, resulting in distinct interactions with the FBG. The specific effective refractive index of a mode interacting with the FBG can be determined experimentally or through numerical simulations by analyzing the mode's behavior and response to the grating structure. In this study, the effective refractive index (neff) for each Bragg wavelength is evaluated both experimentally and analytically using eq (6). The step size, expressed in micrometers, represents the spacing between two neighboring points on the Gaussian profile of the FBG during the laser inscription process. This step size determines the variation in AOM intensity along the length of the FBG. These findings have been summarized in Table 1 for ease of reference. The fabrication of FBGs on multimode coreless fiber using FS laser is a precise and controlled process that offers the creation of gratings with customized reflection spectrum, as shown in Fig. 2. Such a technology has immense potential in various fields of photonics, including sensing, telecommunications, and beyond.
Fig. 1. showcases various aspects of the proposed sensor. Panel (a) presents a schematic of the sensor, which incorporates an FBG array. Panel (b) displays FS-laser writing platform while inscription of FBG. Panel (c) displays a microscopic cross-sectional image of a first-order dual-layer FBG, specifically FBG-10, written using an FS laser. The enlarged inset in panel (d) exhibits a top view, and in panel (e) exhibits a lateral view of the point-by-point pattern produced by the FS laser writing process.
Fig. 2. The spectrum captures the characteristic interference pattern resulting from the interaction of light with the FBGs within the array.
Table 1. FS-laser fabrication parameters for FBGs array
4. Result and discussions
4.1 Experimental setup
In Fig. 3(a), the camera image of the FBG array sensors is shown, where each FBG is illuminated through a laser beam, and the coreless fiber is measured as 13 inches. A schematic of the experimental setup used for high-temperature measurements is presented in Fig. 3(b). The setup includes a supercontinuum laser source (SC-5, Wuhan Yangtze Soton Laser) with a wavelength range of 400-2200 nm, a mechanical multimode scrambler (Newport FM-1), a Bay-spec multimode interrogator integrated with a laptop, and a 3 dB multimode coupler (Thorlabs, model #TM105R5F1B) to acquire the reflection spectra of the FBG array. The lead-in multimode fiber used had a core and cladding diameter of 105 µm and 125 µm, respectively. A mode scrambler was utilized to minimize multimode interferences within the highly multimode coreless fiber. For temperature testing, the FBG array sensors were heated from room temperature to 1100°C in a high-temperature tube furnace (Thermo Scientific TF55030A-1 Lindberg/Blue M), while monitoring the Bragg wavelength shift of the FBGs. In order to maintain precision and accuracy of temperature measurements, real-time monitoring of thermal profiles was conducted using a thermocouple data logger (Model# KIB-006). For calibration purposes, a K-type thermocouple was positioned near FBG-10 to serve as a calibration reference. This experimental setup allowed for accurate measurement of the FBG array's temperature sensitivity and characterization of its performance under high-temperature conditions.
Fig. 3. (a) shows an image captured by a camera displaying the FBG array with each FBG location illuminated by a laser beam, (b) depicts the schematic experimental setup utilized for monitoring the distributed thermal response via the FBG array, and (c) presents an enlarged inset of the tube furnace, demonstrating the positioning of the FBG array sensor inside.
4.2 Impact of relative phase between FBG layers
The relative phase between two similar FBGs corresponds to the phase difference or phase relationship observed in the reflected or transmitted optical signals from these FBGs. FBGs are periodic structures embedded in an optical fiber that selectively reflect specific wavelengths of light. When two FBGs are fabricated with comparable characteristics, such as resembling grating periods and refractive index modulation profiles, they exhibit similar reflection spectra. The relative phase between these FBGs can be defined based on their individual reflection phases. The phase of an optical signal indicates the position of the wave oscillation at a specific moment in time, usually measured in radians or degrees. Perfect alignment of the reflection spectra implies that the reflected waves from the two FBGs are in phase, resulting in constructive interference. This alignment enhances the signals, reinforcing each other and generating a more robust combined signal. Conversely, a phase difference between the two FBGs leads to out-of-phase reflected waves, causing a misalignment that can result in destructive interference. This misalignment diminishes the combined signal's strength as the signals cancel each other out.
To determine the relative phase between two symmetrical layers of an FBG, the reflection spectrum is measured initially with a single-layer FBG and then with a dual-layer FBG. As illustrated in Fig. 4, the dual-layer FBG exhibits constructive interference, thereby enhancing the fringe visibility from 5 dB to 7 dB. This enhancement in reflectivity demonstrates that the dual-layer FBG configuration is suitable for the proposed FBG array, as it augments the reflectivity of the structure. Therefore, utilizing FBGs with dual layers not only enhances the reflectivity but also proves to be advantageous for the proposed FBG array based on the observed constructive interference in the reflection spectra.
Fig. 4. showcases the FBG spectra comparison between single and dual layers, demonstrating the superior peak intensity achieved with dual-layer FBG.
The depth of refractive index modulation plays a crucial role in the performance of FBGs. In this study, we utilized a high FS-laser power with a dual-layer FBG design to increase the depth of refractive index modulation, resulting in enhanced reflectivity, as illustrated in Fig. 4. A deeper modulation facilitates a stronger interaction between the guided modes and the grating structure, leading to higher reflection efficiency. This characteristic is particularly beneficial for applications requiring narrow bandwidth and high reflectivity, such as wavelength-division multiplexing systems and fiber lasers. Additionally, the depth of modulation influences the bandwidth of the FBG. By increasing the depth, the bandwidth narrows, resulting in sharper reflection peaks and improved wavelength selectivity. Conversely, when utilizing low FS-laser power, weak reflectors with shallow modulation depth are promoted, leading to broader reflection bands suitable for wideband filtering or dispersion compensation applications. Therefore, careful design and optimization of the depth of refractive index modulation are necessary to achieve the desired performance characteristics while minimizing undesirable effects.
4.3 Advantage of mode scrambler in the interrogation system
The reflection spectrum was obtained using an Optical Spectrum Analyzer to illustrate the impact of using a mode scrambler in the interrogation system. Figure 5 clearly depicts the comparison between using and not using a mode scrambler during the experimental investigations. When the mode scrambler was employed, the reflection spectrum exhibited reduced noise and greater stability compared to the scenario without the mode scrambler. The mode scrambler plays a crucial role in randomizing the light from the laser source over time, introducing a specific frequency that ensures a uniform and stationary light distribution at the output. This randomization helps minimize multimodal interferences, leading to a more reliable and consistent reflection spectrum. By utilizing a mode scrambler in the interrogation system, the noise level in the reflection spectrum is effectively reduced, enabling improved signal quality and enhanced measurement accuracy.
Fig. 5. illustrates the interference signal of an FBG in the interrogation system, comparing the results with and without the use of a mode scrambler.
4.4 Difference between first-order and higher-order FBG
Various factors demonstrate the superiority of first-order coreless FBGs over their higher-order counterparts. Firstly, first-order coreless FBGs exhibit superior reflectivity in point-by-point configurations compared to higher-order FBGs. Additionally, the lower and higher-order harmonics of first-order coreless FBGs are well-separated from the FBG peaks in terms of wavelength span. On the contrary, higher-order coreless FBGs have higher and lower-order harmonics that reside close to the FBG peak, limiting the number of FBGs that can be effectively cascaded. As shown in Figs. 6(a)&(b), a first-order coreless FBG fabricated using a FS laser displays a singular FBG peak centered at 1294 nm within the 1000 to 1700nm wavelength range. In contrast, a fifth-order coreless FBG exhibits not only a main FBG peak centered at 1336 nm but also includes both higher and lower-order harmonics peaks. During the interrogation process, the limitation of the number of FBGs is significantly influenced by the overlapping of harmonics and center Bragg wavelength peaks. This poses challenges for a multiplexed large-scale cascaded coreless FBG-based system.
4.5 Thermal performance of FBG array
FBG array composed of tens of FBGs with varying center wavelengths was fabricated using specific parameters, as described in the fabrication section. The FBG array was housed within a ceramic tube (alumina) with a 0.5 mm inner diameter and 2.5 mm outer diameter, to avoid unnecessary bending and external strain. The Bay-spec interrogator was used to measure the Bragg wavelength of each FBG at different temperature intervals, starting from room temperature up to 1100°C, with a 50°C step size and a 10 minutes stabilization period at each step. The temperature was ramped up at a rate of 10°C/min to prevent thermal shock. The temperature was ramped up at a rate of 10°C/min to prevent thermal shock. This experimental setup allowed for accurate measurement of the FBG array's temperature sensitivity and characterization of its performance under high-temperature conditions. Spectral evolution was recorded for increasing and decreasing spectral regression, as shown in Fig. 7(a-b). The temperature-induced changes in the FBG array's reflection spectrum were characterized in the temperature range from room temperature to 1100°C. The experimental results revealed that the reflection spectrum red-shifted when the temperature was increased, and blue-shifted when it was decreased. The stability of the FBG array sensors was observed by maintaining it at harsh environment temperature (1100°C) for 24 hours. The study found that the signal-to-noise ratio of the fundamental mode FBG array sensors significantly improved after heat treatment. A comparison of the spectra before and after heat treatment of the FBG array confirmed that the annealing process at higher temperatures released the stresses incorporated during the laser fabrication process, as shown in Fig. 8. The results confirm the efficacy of the annealing process in enhancing the performance of the array.
Fig. 7. illustrates the spectral evolution of an FBG array during heat treatment, presented separately for (a) heating and (b) cooling.
Fig. 8. Improvement in signal-to-noise ratio of the FBG array after annealing, as shown by the state of the array (a) before and (b) after heat treatment at 1100°C.
4.6 Annealing of coreless FBGs at high temperatures
Annealing is a critical process in the fabrication of coreless fiber-based FBGs, especially when intended for operation in extreme temperature environments, such as 1100°C. The annealing process involves controlled temperature cycling of the coreless fiber. The fiber is heated to the target annealing temperature (e.g., 1100°C) and held at this temperature for a specific duration (e.g., 24 hrs). Subsequently, it is slowly cooled back to room temperature. During the FBG fabrication process, residual stresses can be introduced in the coreless fiber due to temperature fluctuations and mechanical handling. Annealing at high temperatures helps in stress relief by allowing the fiber to relax and release these internal stresses. Additionally, the thermal cycling process promotes structural rearrangements, leading to a more uniform refractive index profile. This is clearly depicted in Fig. 8, which illustrates the interference spectra before and after the heat treatment.
At high temperatures, the coreless fiber undergoes structural reorganization [34], resulting in a more homogenized refractive index profile. This improved uniformity of the refractive index contributes to a smoother grating structure and enhances the spectral characteristics and efficiency of the FBGs. The annealing process significantly improves the FBG's response to temperature changes. Annealed coreless FBGs exhibit reduced sensitivity to temperature fluctuations and a more predictable response, making them suitable for high-temperature sensing applications. By undergoing annealing at high temperatures, the coreless fiber-based FBGs gain enhanced mechanical and thermal stability. This increased reliability ensures their resilience to environmental factors and mechanical stresses, making them well-suited for applications in harsh and demanding environments.
4.7 Sensitivity and fitting function
The Bragg wavelength shift for an FBG for coreless multimode fiber can be calculated by [35],
At certain Bragg wavelength the temperature change can be estimated as [37],
Fig. 9. Temperature sensitivity slope and fitting correlation with heating and cooling for (a) FBG-1 to FBG-5 and (b) FBG-6 to FBG-10.
4.8 Stability response of FBG array
In a coreless multimode fiber structure, the absence of a bounded layer of cladding, unlike typical core/clad multimode fibers, eliminates the potential issues associated with low-index contrast cladding. In traditional multimode fibers, the cladding material with low index contrast induces stress at high temperatures, such as 1100°C, leading to dopant migrations [38] and wavelength drift [39]. However, in a coreless structure with an air cladding, there is no risk of stress-induced effects or wavelength drifting at elevated temperatures. By eliminating the stress induced by thermal expansion mismatch between the core and cladding materials, the coreless fiber reduces stress and improves the stability of its optical characteristics. This results in enhanced signal propagation and reduced signal degradation. The absence of stress-induced effects, such as dopant migrations and wavelength drift, ensures that the coreless fiber maintains its optical performance and integrity even under high-temperature conditions.
The stability of the FBG array used in this study was evaluated by subjecting it to harsh environmental conditions, including high temperatures of up to 1100°C for 24 hours. The study found that the FBG array maintained its stability and functionality under these conditions, indicating its robustness and suitability for high-temperature sensing applications, as shown in Fig. 10. It can be observed from the transient profile of FBG array that all ten FBGs in array hold wavelength shift till stabilized temperature at harsh environment (1100°C).
Fig. 10. illustrates transient plot of the stability test conducted on the FBG array, wherein the array was exposed to a temperature of 1100°C for a duration of 24 hours.
It is important to note that the FBG array used in this study was a multimode FBG array with a coreless structure, designed to enhance sensitivity and performance at high temperatures. The coreless structure reduces the mechanical strain on the FBGs and enhances their temperature sensitivity. Additionally, the multimode feature of the array allows for the detection of multiple wavelengths and modes, further enhancing the accuracy and reliability of the sensing measurements. The stability of the multimode coreless FBG array under high-temperature conditions demonstrated in this study suggests that it can reliably operate in harsh environments and withstand prolonged exposure to high temperatures. The findings highlight the potential of FBG arrays with multimode coreless structures as reliable and stable sensing platforms for a range of high-temperature applications.
5. Conclusion
In conclusion, this study showcased the successful fabrication and characterization of an FBG array consisting of 10 fundamental mode FBGs in a highly multimode coreless fiber using a high-power FS-laser. The FBG array demonstrated high stability and sensitivity under harsh environmental conditions, with no observable wavelength drift at stable temperatures. The use of coreless fiber design provided an effective solution to mitigate dopant migration issues, making it a promising choice for high-temperature sensing applications. Importantly, this is the first demonstration of side-by-side writing of FBGs in a multimode coreless fiber, which offers a unique approach to achieving a large number of sensing points in a single fiber. These findings have significant implications for the development of robust and reliable sensing platforms for distributed high-temperature applications, with potential applications in industries such as aerospace, energy, and manufacturing.
Funding
U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office (DE-EE0009119, DE-EE0009392).
Acknowledgment
This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office (AMO) Award # DE-EE0009119 and DE-EE0009392. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Access to the underlying data supporting the results reported in this article can be requested from the corresponding author in a reasonable manner.
References
1. Q. H. Bui, A. Wada, K. Fukushima, K. Nakaya, M. G. Soares, and S. Tanaka, “Simultaneous multi-point measurement of strain and temperature utilizing Fabry-Perot interferometric sensors composed of low reflective fiber Bragg gratings in a polarization-maintaining fiber,” Opt. Express 28(9), 13104 (2020). [CrossRef]
2. F. Mumtaz, D. R. Alla, M. Roman, B. Zhang, J. D. Smith, R. E. Gerald, R. J. O’Malley, and J. Huang, “Thermally robust and highly stable method for splicing silica glass fiber to crystalline sapphire fiber,” Appl. Opt. 62(5), 1392 (2023). [CrossRef]
3. F. Mumtaz, T. Hanok, B. Zhang, D. S. Jeffrey, R. J. O’Malley, and J. Huang, “Highly-cascaded first-order sapphire optical fiber Bragg gratings fabricated by femtosecond Laser,” Opt Lett 48(16) (2023).
4. J. Burgmeier, C. Waltermann, G. Flachenecker, and W. Schade, “Point-by-point inscription of phase-shifted fiber Bragg gratings with electro-optic amplitude modulated femtosecond laser pulses,” Opt. Lett. 39(3), 540 (2014). [CrossRef]
5. E. Ertorer, M. Haque, J. Li, and P. R. Herman, “Femtosecond laser filaments for rapid and flexible writing of fiber Bragg grating,” Opt. Express 26(7), 9323 (2018). [CrossRef]
6. I. Ulyanov, D. V. Przhiialkovskii, and O. V. Butov, “Point-by-point inscription of chirped apodized fiber Bragg gratings for application as ultrashort pulse stretchers,” Results Phys. 32, 105101 (2022). [CrossRef]
7. G. D. Marshall, R. J. Williams, N. Jovanovic, M. J. Steel, and M. J. Withford, “Point-by-point written fiber-Bragg gratings and their application in complex grating designs,” Opt. Express 18(19), 19844 (2010). [CrossRef]
8. T. Baghdasaryan, T. Geernaert, K. Chah, C. Caucheteur, K. Schuster, J. Kobelke, H. Thienpont, and F. Berghmans, “Anomalous transparency in photonic crystals and its application to point-by-point grating inscription in photonic crystal fibers,” Sci. Rep. 8(1), 5470 (2018). [CrossRef]
9. A. Wolf, A. Dostovalov, K. Bronnikov, M. Skvortsov, S. Wabnitz, and S. Babin, “Advances in femtosecond laser direct writing of fiber Bragg gratings in multicore fibers: technology, sensor and laser applications,” Opto-Electron. Adv. 5(4), 210055 (2022). [CrossRef]
10. M. Javaid, A. Haleem, R. P. Singh, S. Rab, and R. Suman, “Significance of sensors for industry 4.0: Roles, capabilities, and applications,” Sensors International 2(May), 100110 (2021). [CrossRef]
11. S. J. Mihailov, D. Grobnic, C. Hnatovsky, R. B. Walker, P. Lu, D. Coulas, and H. Ding, “Extreme environment sensing using femtosecond laser-inscribed fiber bragg gratings,” Sensors 17(12), 2909 (2017). [CrossRef]
12. M. Roman, H. Tekle, D. R. Alla, F. Mumtaz, J. D. Smith, L. Bartlett, R. J. O’Malley, R. E. Gerald, and J. Huang, “Temperature Monitoring in the Refractory Lining of a Continuous Casting Tundish Using Distributed Optical Fiber Sensors,” IEEE Trans. Instrum. Meas. 72, 1–11 (2023). [CrossRef]
13. F. Mumtaz, M. Aqueel Ashraf, Y. Dai, and W. Hu, “Numerical solution of strongly guided modes propagating in sapphire crystal fibers (α-Al2O3) for UV, VIS/IR wave-guiding,” Results Phys. 18(June), 103311 (2020). [CrossRef]
14. P. Rajeev, J. Kodikara, W. K. Chiu, and T. Kuen, “Distributed optical fibre sensors and their applications in pipeline monitoring,” Key Eng. Mater. 558, 424–434 (2013). [CrossRef]
15. J. Wang, Z. Li, X. Fu, H. Wang, and D. Jiang, “Distributed temperature sensing system based on a densely spaced FBG array for small fire recognition,” Measurement 179(February), 109406 (2021). [CrossRef]
16. Y. Zhang, D. Qiao, Y. Zhu, and P. Jiang, “High-order fiber Bragg grating fabricated by femtosecond laser pulses for high-sensitivity temperature and strain sensing,” Optik (Munich, Ger.) 222(August), 165423 (2020). [CrossRef]
17. M. J. Schmid and M. S. Müller, “Measuring Bragg gratings in multimode optical fibers,” Opt. Express 23(6), 8087 (2015). [CrossRef]
18. M. Raguse, S. Klein, P. Baer, M. Giesberts, M. Traub, and H.-D. Hoffmann, “Investigations on high-reflective Fiber-Bragg-Gratings in multimode fibers,” Opt. Continuum 1(5), 965 (2022). [CrossRef]
19. B. Xu, J. He, B. Du, X. Xiao, X. Xu, C. Fu, J. He, C. Liao, and Y. Wang, “Femtosecond laser point-by-point inscription of an ultra-weak fiber Bragg grating array for distributed high-temperature sensing,” Opt. Express 29(20), 32615 (2021). [CrossRef]
20. D. Grobnic, C. W. Smelser, S. J. Mihailov, and R. B. Walker, “Long-term thermal stability tests at 1000 °C of silica fibre Bragg gratings made with ultrafast laser radiation,” Meas. Sci. Technol. 17(5), 1009–1013 (2006). [CrossRef]
21. J. He, Y. Wang, C. Liao, C. Wang, S. Liu, K. Yang, and Y. Wang, “Negative-index gratings formed by femtosecond laser overexposure and thermal regeneration,” Sci Rep 6(1), 23379–9 (2016). [CrossRef]
22. P. Hofmann, R. Amezcua-Correa, E. Antonio-Lopez, D. Ott, M. Segall, I. Divliansky, J. Lumeau, L. Glebova, L. Glebov, N. Peyghambarian, and A. Schülzgen, “Strong bragg gratings in highly photosensitive photo-thermo-refractive- glass optical fiber,” IEEE Photon. Technol. Lett. 25(1), 25–28 (2013). [CrossRef]
23. K. K. C. Lee, A. Mariampillai, M. Haque, B. A. Standish, V. X. D. Yang, and P. R. Herman, “Temperature-compensated fiber-optic 3D shape sensor based on femtosecond laser direct-written Bragg grating waveguides,” Opt. Express 21(20), 24076 (2013). [CrossRef]
24. M. Bernier, S. Gagnon, and R. Vallée, “Role of the 1D optical filamentation process in the writing of first order fiber Bragg gratings with femtosecond pulses at 800 nm [Invited],” Opt. Mater. Express 1(5), 832 (2011). [CrossRef]
25. D. Su and X. Qiao, “High thermal stability of ultra-low insertion loss type II cladding fiber Bragg grating based on femtosecond laser point-by-point technology,” Opt. Express 30(6), 9156 (2022). [CrossRef]
26. S. Yang, D. Hu, and A. Wang, “Point-by-point fabrication and characterization of sapphire fiber Bragg gratings,” Opt. Lett. 42(20), 4219 (2017). [CrossRef]
27. C. Zhu, R. E. Gerald, and J. Huang, “Progress Toward Sapphire Optical Fiber Sensors for High-Temperature Applications,” IEEE Trans. Instrum. Meas. 69(11), 8639–8655 (2020). [CrossRef]
28. X. Xu, J. He, C. Liao, K. Yang, K. Guo, C. Li, Y. Zhang, Z. Ouyang, and Y. Wang, “Sapphire fiber Bragg gratings inscribed with a femtosecond laser line-by-line scanning technique,” Opt. Lett. 43(19), 4562 (2018). [CrossRef]
29. K. H. Wanser, K. F. Voss, and A. D. Kersey, “Novel fiber devices and sensors based on multimode fiber Bragg gratings,” in Tenth International Conference on Optical Fibre Sensors (1994), 2360, pp. 265–268.
30. A. Ghatak and K. Thyagarajan, An Introduction to Fiber Optics. Ch-8 (Cambridge university press, 1998).
31. X. Sang, C. Yu, and B. Yan, “Bragg gratings in multimode optical fibers and their applications,” J. Lightwave Technol. 18(2), 230–235 (2000). [CrossRef]
32. O. Katsunari, Fundamentals of Optical Waveguides (Second Edition) Chapter 4 - Coupled Mode Theory, 2nd ed. (Academic Press, 2006).
33. J. Zhang, “Multimode optical fiber Bragg gratings: modeling, simulation, and experiments,” (2004).
34. D. R. Viveiros, J. M. Maia, V. A. Amorim, P. A. Jorge, and P. V. Marques, “Fabrication of periodic structures in optical fibers by femtosecond laser micromachining for sensing applications,” in In Fourth International Conference on Applications of Optics and Photonics (2019), p. 69.
35. D. Samiappan, A. V. S. Kesarikiran, V. Chakravartula, C. R. U. Kumari, K. Shubham, B. Aakash, and R. Kumar, “Enhancing Sensitivity of Fiber Bragg Grating-Based Temperature Sensors through Teflon Coating,” Wirel Pers Commun 110(2), 593–604 (2020). [CrossRef]
36. C. V. N. Bhaskar, S. Pal, and P. K. Pattnaik, “Recent advancements in fiber Bragg gratings based temperature and strain measurement,” Results in Optics 5, 100130 (2021). [CrossRef]
37. W. Zhao and R. O. Claus, “Optical fiber grating sensors in multimode fibers,” Smart Mater. Struct. 9(2), 212–214 (2000). [CrossRef]
38. B. Liu, M. P. Buric, B. T. Chorpening, Z. Yu, D. S. Homa, G. R. Pickrell, and A. Wang, “Design and Implementation of Distributed Ultra-High Temperature Sensing System with a Single Crystal Fiber,” J. Lightwave Technol. 36(23), 5511–5520 (2018). [CrossRef]
39. L. Hua, Y. Song, J. Huang, X. Lan, Y. Li, and H. Xiao, “Microwave interrogated large core fused silica fiber Michelson interferometer for strain sensing,” Appl. Opt. 54(24), 7181 (2015). [CrossRef]
