Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation

Optical Fiber Sensing-Aided 3D-Printed Replacement Parts for Enhancing the Sensing Ability of Architectural Heritage

This study discussed the application of optical fibers in addressing the problem of insufficient light harvesting and sensing health monitoring in ancient buildings. Based on three-dimensional (3D) printing technology to fix the light-harvesting lens and conducting optical fiber, develop the replacement parts that can be buried into the optical fiber of ancient buildings. By introducing the experimental application to improve the experimental quality of research and teaching. Firstly, it highlights the advantage that the optical fiber plus lens structure design can make the natural light introduced for a long time; secondly, it points out that the buried optical fiber structure design does not affect the warmth and sound insulation of the building; finally, the health monitoring of the building is realized through the proposed method of buried optical fiber sensing. The design scheme adopts a fiber optic light transmission and sensing system, which can realize the whole system's corrosion resistance, after laying buried and low-cost operation.

Multifunctional Integration of Optical Fibers and Nanomaterials for Aircraft Systems

Smart sensing for aeronautical applications is a multidisciplinary process that involves the development of various sensor elements and advancements in the nanomaterials field. The expansion of research has fueled the development of commercial and military aircrafts in the aeronautical field. Optical technology is one of the supporting pillars for this, as well as the fact that the unique high-tech qualities of aircrafts align with sustainability criteria. In this study, a multidisciplinary investigation of airplane monitoring systems employing optical technologies based on optical fiber and nanomaterials that are incorporated into essential systems is presented. This manuscript reports the multifunctional integration of optical fibers and nanomaterials for aircraft sector discussing topics, such as airframe monitoring, flight environment sensing (from temperature and humidity to pressure sensing), sensors for navigation (such as gyroscopes and displacement or position sensors), pilot vital health monitoring, and novel nanomaterials for aerospace applications. The primary objective of this review is to provide researchers with direction and motivation to design and fabricate the future of the aeronautical industry, based on the actual state of the art of such vital technology, thereby aiding their future research.

High Temperature Measurement with Low Cost, VCSEL-Based, Interrogation System Using Femtosecond Bragg Gratings

In this article, a cost-effective and fast interrogating system for wide temperature measurement with Fiber Bragg Gratings is presented. The system consists of a Vertical Cavity Surface Emitting Laser (VCSEL) with a High Contrast Grating (HCG)-based cavity that allows for the fast tuning of the output wavelength. The work focuses on methods of bypassing the limitations of the used VCSEL laser, especially its relatively narrow tuning range. Moreover, an error analysis is provided by means of the VCSEL temperature instability and its influence on the system performance. A simple proof of concept of the measurement system is shown, where two femtosecond Bragg gratings were used to measure temperature in the range of 25 to 800 °C. In addition, an exemplary simulation of a system with sapphire Bragg gratings is provided, where we propose multiplexation in the wavelength and reflectance domains. The presented concept can be further used to measure a wide range of temperatures with scanning frequencies up to hundreds of kHz.

Optical Fiber Sensors for High-Temperature Monitoring: A Review

High-temperature measurements above 1000 °C are critical in harsh environments such as aerospace, metallurgy, fossil fuel, and power production. Fiber-optic high-temperature sensors are gradually replacing traditional electronic sensors due to their small size, resistance to electromagnetic interference, remote detection, multiplexing, and distributed measurement advantages. This paper reviews the sensing principle, structural design, and temperature measurement performance of fiber-optic high-temperature sensors, as well as recent significant progress in the transition of sensing solutions from glass to crystal fiber. Finally, future prospects and challenges in developing fiber-optic high-temperature sensors are also discussed.

Single-mode sapphire fiber Bragg grating

Sapphire optical fiber has the ability to withstand ultrahigh temperatures and high radiation, but it is multimoded which prevents its use in many sensing applications. Problematically, Bragg gratings in such fiber exhibit multiple reflection peaks with a fluctuating power distribution. In this work, we write single-mode waveguides with Bragg gratings in sapphire using a novel multi-layer depressed cladding design in the 1550 nm telecommunications waveband. The Bragg gratings have a narrow bandwidth (<0.5 nm) and have survived annealing at 1000°C. The structures are inscribed with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. A single-mode sapphire fiber Bragg grating is created by writing a waveguide with a Bragg grating within a 425 µm diameter sapphire optical fiber, providing significant potential for accurate remote sensing in ultra-extreme environments.

Metrological Characterization of a High-Temperature Hybrid Sensor Using Thermal Radiation and Calibrated Sapphire Fiber Bragg Grating for Process Monitoring in Harsh Environments

Fiber Bragg gratings inscribed in single crystalline multimode sapphire fibers (S-FBG) are suitable for monitoring applications in harsh environments up to 1900 °C. Despite many approaches to optimize the S-FBG sensor, a metrological investigation of the achievable temperature uncertainties is still missing. In this paper, we developed a hybrid optical temperature sensor using S-FBG and thermal radiation signals. In addition, the sensor also includes a thermocouple for reference and process control during a field test. We analyzed the influence of the thermal gradient and hotspot position along the sensor for all three detection methods using an industrial draw tower and fixed point cells. Moreover, the signal processing of the reflected S-FBG spectrum was investigated and enhanced to determine the reachable measurement repeatability and uncertainty. For that purpose, we developed an analytical expression for the long-wavelength edge of the peak. Our findings show a higher stability against mechanical-caused mode variations for this method to measure the wavelength shift compared to established methods. Additionally, our approach offers a high robustness against aging effects caused by high-temperature processes (above 1700 °C) or harsh environments. Using temperature-fixed points, directly traceable to the International System of Units, we calibrated the S-FBG and thermocouple of the hybrid sensor, including the corresponding uncertainty budgets. Within the scope of an over 3-weeks-long field trial, 25 production cycles of an industrial silicon manufacturing process with temperatures up to 1600 °C were monitored with over 100,000 single measurements. The absolute calibrated thermocouple (Uk=2≈1K…4K) and S-FBG (Uk=2≈10K…14K) measurements agreed within their combined uncertainty. We also discuss possible strategies to significantly reduce the uncertainty of the S-FBG calibration. A follow-up measurement of the sensor after the long-term operation at high temperatures and the transport of the measuring system together with the sensor resulted in a change of less than 0.5 K. Thus, both the presented hybrid sensor and the measuring principle are very robust for applications in harsh environments.

Femtosecond laser inscribed helical sapphire fiber Bragg gratings

This Letter reports a novel helical sapphire fiber Bragg grating (HSFBG) in a single crystal sapphire fiber with diameter of 60 µm fabricated by a 515 nm femtosecond laser. Due to the large refractive index modulation region and high structural symmetry of the HSFBGs, high-reflectivity and high-quality spectra can be prepared and additionally have good bending resistance. The spectral properties of HSFBGs with different helical diameters are studied. When the helical diameter is 30 µm, the reflectivity of HSFBG is 40%, the full width at half-maximum is 1.56 nm, and the signal-to-noise ratio is 16 dB. For the HSFBG bending test, the minimum bending radius is 5 mm, which can still maintain relatively good spectral quality. In addition, the HSFBG array with different periods has been successfully cascaded in a sapphire fiber. The experimental results of the HSFBG high-temperature test show that this HSFBG can work reliably at 1600°C, and the temperature sensitivity in the high-temperature range can reach 35.55 pm/°C. This HSFBG can be used in high-temperature and harsh environments, such as metal smelting and aeroengine structural health monitoring.

Efficient point-by-point Bragg grating inscription in sapphire fiber using femtosecond laser filaments

Sapphire fiber Bragg gratings (SFBGs) inscribed by using femtosecond laser point-by-point (PbP) technology typically have an extremely low reflectivity due to the limited cross-sectional area of refractive index modulations (RIMs) created in sapphire fiber. Hence, we propose and experimentally demonstrate a filamentation process for fabricating PbP SFBGs. This approach provides an efficient method for producing SFBGs at various Bragg wavelengths with a higher reflectivity, since the filament tracks could enlarge the cross-sectional area of RIMs. The influences of the pulse energy and the focal depth on the generation and morphology of the filament tracks were studied, and after optimizing these parameters, high-quality filament tracks with a length of 90 µm and a width of 1.4 µm were produced into sapphire fiber with a diameter of 100 µm. These filament tracks were precisely assembled in sapphire fiber, generating an SFBG with a reflectivity of 2.3%. The total fabrication time for this SFBG only requires ${\sim}{1.1}\;{\rm s}$. Subsequently, a wavelength-division-multiplexed (WDM) SFBG array consisting of five SFBGs was efficiently constructed. Moreover, the high-temperature response of the SFBG array was investigated and the experimental results showed that the SFBG array can withstand a high temperature of 1600°C. Such a WDM SFBG array could serve as quasi-distributed high-temperature sensor which will be promising in many areas, i.e., metallurgical, chemical, and aviation industries.

Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing

High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200 °C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900–1000 °C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.

Multi-layer, offset-coupled sapphire fiber Bragg gratings for high-temperature measurements

We demonstrate the fabrication of multi-layer sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser line-by-line scanning technique. This multi-layer grating structure enlarges the index modulation area and can effectively increase the reflectivity of SFBGs. The spectral characteristics of multi-layer SFBGs with different layer quantities and spacings were studied. A double-layer SFBG with a layer spacing of 5 μm exhibits a reflectivity of 34.1%, which is much higher than that of a single-layer SFBG (i.e., ∼6.3%). Moreover, the higher-order modes of multi-layer SFBG could be suppressed by offset coupling, leading to a reduced SFBG bandwidth (full width at half-maximum [FWHM]) of 1.32 nm. In addition, the high-temperature response of the multi-layer, offset-coupled SFBGs was also studied. The SFBG can withstand a high temperature of 1612°C and exhibits a sensitivity of 45.2 pm/°C at high temperatures. Such multi-layer, offset-coupled SFBGs could be developed for high-temperature sensing in metallurgical, chemical, and aviation industries.

Application of Sapphire-Fiber-Bragg-Grating-Based Multi-Point Temperature Sensor in Boilers at a Commercial Power Plant

Readily available temperature sensing in boilers is necessary to improve efficiencies, minimize downtime, and reduce toxic emissions for a power plant. The current techniques are typically deployed as a single-point measurement and are primarily used for detection and prevention of catastrophic events due to the harsh environment. In this work, a multi-point temperature sensor based on wavelength-multiplexed sapphire fiber Bragg gratings (SFBGs) were fabricated via the point-by-point method with a femtosecond laser. The sensor was packaged and calibrated in the lab, including thermally equilibrating at 1200 °C, followed by a 110-h, 1000 °C stability test. After laboratory testing, the sensor system was deployed in both a commercial coal-fired and a gas-fired boiler for 42 days and 48 days, respectively. The performance of the sensor was consistent during the entire test duration, over the course of which it measured temperatures up to 950 °C (with some excursions over 1000 °C), showing the survivability of the sensor in a field environment. The sensor has a demonstrated measurement range from room temperature to 1200 °C, but the maximum temperature limit is expected to be up to 1900 °C, based on previous work with other sapphire based temperature sensors.

Multifunctional Smart Optical Fibers: Materials, Fabrication, and Sensing Applications

This paper presents a review of the development of optical fibers made of multiple materials, particularly including silica glass, soft glass, polymers, hydrogels, biomaterials, Polydimethylsiloxane (PDMS), and Polyperfluoro-Butenylvinyleth (CYTOP). The properties of the materials are discussed according to their various applications. Typical fabrication techniques for specialty optical fibers based on these materials are introduced, which are mainly focused on extrusion, drilling, and stacking methods depending on the materials’ thermal properties. Microstructures render multiple functions of optical fibers and bring more flexibility in fiber design and device fabrication. In particular, micro-structured optical fibers made from different types of materials are reviewed. The sensing capability of optical fibers enables smart monitoring. Widely used techniques to develop fiber sensors, i.e., fiber Bragg grating and interferometry, are discussed in terms of sensing principles and fabrication methods. Lastly, sensing applications in oil/gas, optofluidics, and particularly healthcare monitoring using specialty optical fibers are demonstrated. In comparison with conventional silica-glass single-mode fiber, state-of-the-art specialty optical fibers provide promising prospects in sensing applications due to flexible choices in materials and microstructures.

Crystallization-induced refractive index modulation on sapphire-derived fiber for ultrahigh temperature sensing

We have demonstrated crystallization-induced refractive index (RI) modulation on sapphire-derived fiber (SDF) showing superheat resistance and developed the SDF based Fabry-Perot interferometers (FPIs) for ultrahigh temperature sensing. The SDF is a special fiber with high concentration of alumina to silica in the fiber core region. Reheating and cooling the SDF by arc discharge generates mullite particles in the core region, which achieves RI modulation up to ~0.015. Such crystallized region in the SDF is explored as mirrors for FPI, showing a good linear response to temperature with sensitivity of ~13.2 pm/°C. Benefiting from superheat resistance of the crystallized SDF being mirrors, the developed SDF-FPI sensor is capable to withstand high temperature up to 1600°C, which is the highest working temperature for amorphous fiber. Moreover, the SDF-FPI sensor exhibits 6-hour stability at 1200°C. The crystallized SDF-FPIs with compactness, wide temperature working range, high sensitivity, and robustness show great potential application in harsh environment such as turbine engines, power plants, petrochemical, gas industry, etc.

Review of Fiber Optic Sensors for Structural Fire Engineering

Reliable and accurate measurements of temperature and strain in structures subjected to fire can be difficult to obtain using traditional sensing technologies based on electrical signals. Fiber optic sensors, which are based on light signals, solve many of the problems of monitoring structures in high temperature environments; however, they present their own challenges. This paper, which is intended for structural engineers new to fiber optic sensors, reviews various fiber optic sensors that have been used to make measurements in structure fires, including the sensing principles, fabrication, key characteristics, and recently-reported applications. Three categories of fiber optic sensors are reviewed: Grating-based sensors, interferometer sensors, and distributed sensors.

Femtosecond laser inscription of depressed cladding single-mode mid-infrared waveguides in sapphire

Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515 nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850 nm and propagation loss of <0.37  dB/cm are reported for a 33 mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn=∼2.5×10^-3) up to 1400°C.

Ordinary Optical Fiber Sensor for Ultra-High Temperature Measurement Based on Infrared Radiation

An ordinary optical fiber ultra-high temperature sensor based on infrared radiation with the advantages of simple structure and compact is presented. The sensing system consists of a detection fiber and a common transmission fiber. The detector fiber is formed by annealing a piece of ordinary fiber at high temperature twice, which changes the properties of the fiber and breaks the temperature limit of ordinary fiber. The transmission fiber is a bending insensitive optical fiber. A static calibration system was set up to determine the performance of the sensor and three heating experiments were carried out. The temperature response sensitivities were 0.010 dBm/K, 0.009 dBm/K and 0.010 dBm/K, respectively, which indicate that the sensor has good repeatability. The sensor can withstand a high temperature of 1823 K for 58 h with an error of less than 1%. The main reason why the developed ordinary optical fiber sensor can work steadily for a long time at high temperature is the formation of β-cristobalite, which is stable at high-temperature.

Sapphire fiber Bragg gratings inscribed with a femtosecond laser line-by-line scanning technique

We propose and demonstrate the fabrication of single-crystal sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser line-by-line scanning technique. This approach provides a robust method for producing SFBGs at various Bragg wavelengths with an acceptable reflectivity. The spectrum characteristics of the SFBGs with various fiber diameters, track lengths, and grating pitch quantities were investigated. An SFBG with a reflectivity of 6.3% was obtained via optimization of fabrication parameters. Additionally, a serial array consisting of five SFBGs at different wavelengths was successfully constructed. The high-temperature response of these SFBGs was tested and the experimental results showed the SFBGs could withstand a high temperature of 1612°C. Moreover, a temperature sensitivity of 36.5 pm/°C was achieved in the high-temperature region. Such SFBGs could be developed for promising high-temperature sensors in aero engines.

A IR-Femtosecond Laser Hybrid Sensor to Measure the Thermal Expansion and Thermo-Optical Coefficient of Silica-Based FBG at High Temperatures

In this paper, a hybrid sensor was fabricated using a IR-femtosecond laser to measure the thermal expansion and thermo-optical coefficient of silica-based fiber Bragg gratings (FBGs). The hybrid sensor was composed of an inline fiber Fabry-Perot interferometer (FFPI) cavity and a type-II FBG. Experiment results showed that the type-II FBG had three high reflectivity resonances in the wavelength ranging from 1100 to 1600 nm, showing the peaks in 1.1, 1.3 and 1.5 μm, respectively. The thermal expansion and thermo-optical coefficient (1.3 μm, 1.5 μm) of silica-based FBG, under temperatures ranging from 30 to 1100 °C, had been simultaneously calculated by measuring the wavelength of the type-II FBG and FFPI cavity length.

Fiber Bragg grating fabricated in micro-single-crystal sapphire fiber

This Letter introduces a fiber Bragg grating (FBG) in a micro-single-crystal sapphire fiber (micro-SFBG) for sensing applications in high-temperature and harsh environments. The FBG was fabricated by a point-by-point method via an IR-femtosecond laser in a large-diameter sapphire fiber that was then wet-hot acid etched to achieve microfiber size, which culminated in fabricating and characterizing a 9.6 μm diameter micro-SFBG. The refractive index measurement ranging from 1 to 1.75 and temperature measurement from room temperature to 1400°C are also reported.

Extreme Environment Sensing Using Femtosecond Laser-Inscribed Fiber Bragg Gratings

The femtosecond laser-induced fiber Bragg grating is an effective sensor technology that can be deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices suitable for high temperature, pressure, ionizing radiation and strain sensor applications are possible. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

Point-by-point fabrication and characterization of sapphire fiber Bragg gratings

The Letter reports the inscription of fiber Bragg gratings (FBGs) in a single-crystal sapphire optical fiber via a point-by-point method by 780 nm infrared-femtosecond laser pulses. Compared to phase mask exposure, the use of the point-by-point method for the inscription provides a flexible way to fabricate sapphire FBGs and to make wavelength division multiplexing in sapphire fiber more practicable. The multiplexing of three cascade gratings is demonstrated, and their performance up to 1400°C is tested. The permanent enhancement of reflectivity by a factor of about 5 after heat treatment and the nearly linear temperature response with a slope of 25.8 pm/°C are demonstrated.

Temperature sensing up to 1300°C using suspended-core microstructured optical fibers

We demonstrate a new approach to high temperature sensing using femtosecond laser ablation gratings within silica suspended-core microstructured optical fibers. The simple geometry of the suspended-core fiber allows for femtosecond laser processing directly through the fiber cladding. Pure silica glass is used, allowing the sensor to be used up to temperatures as high as 1300°C while still allowing the fibre to be spliced to conventional fiber. The sensor can also be wavelength division multiplexed, with three sensors in a single fiber demonstrated.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

A thermally annealed Mach-Zehnder interferometer for high temperature measurement

An in-fiber Mach-Zehnder interferometer (MZI) for high temperature measurement is proposed and experimentally demonstrated. The device is constructed of a piece of thin-core fiber (TCF) sandwiched between two short sections of multimode fiber (MMF), i.e., a MMF-TCF-MMF structure. A well-defined interference spectrum is obtained owing to the core-mismatch, and the interference dips are sensitive to the ambient temperature. The experimental results show that the proposed interferometer is capable of high temperature measurement up to 875 °C with a sensitivity of 92 pm/°C over repeated measurements. The explored wavelength drop point may limit the measurement range, which can be improved by repeated thermal annealing.