Sapphire Fiber Research Articles

Research progress and development tendency of sapphire fiber Bragg grating-based high-temperature sensors (invited)

Research progress and development tendency of sapphire fiber Bragg grating-based high-temperature sensors (invited)

Исследование зависимости прочности при изгибе от длины и диаметра сапфировых волокон, полученных методом Степанова

Bending tests of sapphire fibers with different diameters from 220 to 410 μm at room temperature were investigated. Experimental dependences of strain at failure and fiber strength on length for different cross-section diameters have been obtained. Using the statistical Weibull distribution, the parameters of these dependences have been determined. On the basis of these parameters, a comparison of the dependences of the strength and strain of fibers at fracture on their diameter at different lengths has been investigated. It is established that the increase of strength occurs with the increase of fiber diameter. It is shown that, at a fiber length of 1 mm, the dependence of fiber strength on its diameter is the steepest, and the dependences are “smoothed out” with increasing fiber length up to 100 mm.

Локализация энергии лазерного излучения в оптико-терагерцевом преобразователе ультракоротких ИК-импульсов при помощи профилированных сапфировых волокон

We report the seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via the focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at the certain ratio between the fiber diameter and the gap size, i.e. d/g = 22.5, one can attain a 35-fold enhancement of the laser irradiation in vicinity of the PCA electrodes. This provides the formation of subwavelength EM wave caustics located at the edges of the PCA electrodes which potentially gives rise to an increase of optical-to-terahertz conversion efficiency.

Fabrication and evaluation of sol-gel derived magnesium aluminate spinel-clad sapphire fiber

Even though the high melting point of sapphire fibers offers the potential for applications in temperatures as high as 1800 °C, the absence of an effective and stable cladding has been a major hurdle in utilizing single-crystal fibers for harsh environment sensing applications. This work discusses the development of a high temperature stable magnesium aluminate spinel (MgAl2O4) cladding for sapphire fibers using wet chemical methods. Stability of spinel coatings on sapphire substrates has been demonstrated up to 1400 °C. Transmission electron microscopy and X-ray diffraction analysis indicate clear film-substrate boundary, suggesting very limited solid-state reaction. 125 µm sapphire optical fiber with 340 nm thick spinel cladding exhibited improved protection from contamination under different application relevant gas environments at elevated temperatures. Spinel cladding of sapphire fibers also reduced numerical aperture by 55% compared to the uncoated fiber.

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.

Single-mode helical Bragg grating waveguide created in a multimode coreless fiber by femtosecond laser direct writing

We demonstrate the fabrication of single-mode helical Bragg grating waveguides (HBGWs) in a multimode coreless fiber by using a femtosecond laser direct writing technique. This approach provides a single-step method for creating Bragg grating waveguides. Specifically, the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity. Effects of pulse energy, helical diameter, and helical pitch used for fabricating HBGWs were studied, and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters, including a diameter of 10 μm and a helical pitch of 1.07 μm. The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW. Moreover, five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch, and this technique could be used for achieving a wavelength-division-multiplexed HBGW array. Then, the temperature and strain responses of the fabricated single-mode HBGW were tested, exhibiting a temperature sensitivity of 11.65 pm/°C and a strain sensitivity of 1.29 pm/με, respectively. In addition, the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700°C for 15 h. The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process, and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of ∼ 7 dB and a “blue” shift of 0.36 nm. Hence, such a femtosecond laser directly written single-mode HBGW could be used in many applications, such as sapphire fiber sensors, photonic integrated circuits, and monolithic waveguide lasers.

Sapphire-Derived Fiber Bragg Gratings for High Temperature Sensing

In this paper, a sapphire-derived fiber (SDF) with a core diameter of 10 μm and a cladding diameter of 125 μm is fabricated by the melt-in-tube method, and fiber Bragg gratings (FBGs) with reflectivity over 80% are prepared by the femtosecond laser point-by-point direct writing method. By analyzing the refractive index distribution and reflection spectral characteristics of the SDF, it can be seen that the SDF is a graded refractive index few-mode fiber. In order to study the element composition of the SDF core, the end-face element distribution of the SDF is analyzed, which indicates that element diffusion occurred between the core and the cladding materials. The temperature and stress of the SDF gratings are measured and the highest temperature is tested to 1000 °C. The temperature and strain sensitivities are 15.64 pm/°C and 1.33 pm/με, respectively, which are higher than the temperature sensitivity of the quartz single-mode fiber. As a kind of special fiber, the SDF expands the application range of sapphire fiber, and has important applications in the fields of high-temperature sensing and high-power lasers.

Review of high temperature measurement technology based on sapphire optical fiber

A review is presented on high temperature measurement technology based on sapphire optical fiber. This review paper focuses on the sensing theory, sensor structures and sensing performances of different sapphire fiber sensors. Firstly, there is discussion on pyrometer made from sapphire fiber and description on its corresponding sensing properties. Secondly, there are summary on sapphire fiber Bragg grating (SFBG) inscribed by different methods for high temperature measurement. Thirdly, there are analysis on sapphire fiber sensors based on Fabry Perot (FP) interference with different structures. All above, subsequently, lead to the discussion on microwave interferometry and Raman scattering-based sapphire fiber sensors. Then a sensing performance comparison of sapphire pyrometers based on different detection principles are discussed. On the premise of all above, a future prospect and development trend of high-temperature sapphire fiber sensor are addressed as well.

Hybrid Sapphire Dual-Fabry—Perot-Cavities Sensor for High Temperature and Refractive Index Measurement

We presented a hybrid sapphire dual-Fabry—Perot- cavities sensor for high temperature and refractive index measurement. A double-sided polished sapphire solid wafer is connected in series with a double-sided parallel sapphire C-ring and a single-sided polished sapphire mirror to form the hybrid Fabry—Perot interferometer. The double-sided polished sapphire solid wafer acts as the first Fabry—Perot cavity for temperature sensing, and the sapphire hollow C-ring cavity constitutes the second Fabry—Perot cavity for measuring the refractive index of external environment medium. The single-sided polished sapphire mirror serves as the most distal reflector. Heterogeneous fiber splicing between sapphire fiber and multimode silica fiber is realized to achieve long-distance transmission of interference signals. A series of high temperature measurement experiments range of room temperature to 1080 °C in an air environment and refractive index measurement experiments in a liquid environment were carried out. Based on the changing rates of corresponding Fabry—Perot cavity optical path difference against refractive index and temperature, we calculated the sensitivities of the sensor. The refractive index and temperature sensitivities were 2.0659 × 105 nm/RIU in the RI range of 1.3330∼1.3741 and 7.837 nm/ °C at 1080 °C.

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.

High sensitivity sapphire FBG temperature sensors for the signal processing of data communications technology

<p>This study has outlined the fiber bragg grating (FBG) temperature sensors signal processing for data communications by using OptiGrating simulation software. The reflectivity of the silica and sapphire fiber grating spectrum is reported against the grating wavelength for internal and external temperature variations. As well as apodized Gaussian reflectivity of the silica and sapphire fiber grating spectrum is simulated and clarified against the grating wavelength for high temperature variations. The temperature sensitivity of sapphire FBG nearly 0.11 pm/<sup>0</sup>C, where its value is three times higher than silica FBG. It is observed that silica and Sapphire FBG sensors were tested up to 1000 <sup>0</sup>C by using Gaussian apodization type, side lobes in reflectivity spectrum are totally suppressed.</p>

Machine learning-assited optical thermometer for continuous temperature analysis inside molten metal

This paper demonstrates a robust optical fiber thermometer (OFT) for temperature measurement under extreme environments. To date, the development of sensors for continuous temperature measurement in environments with temperatures over 1000 °C, severe electromagnetic interferences, and strong oxidizing agents has been very challenging. The proposed nano-OFT system consists of a ceramic tube, a nanorod coated sapphire fiber, and a near-infrared (NIR) spectrum analyzer for continuous measurement of molten steel temperature in furnaces. The nanorod layer functions as an effective cladding material for the sapphire fiber to sustain a reliable transmission of NIR thermal emissions. The thermal radiation from the ceramic tube's tip was coupled out of the nano-OFT probe via the sapphire fiber and measured using the NIR spectrometer. The NIR emissions were analyzed using a convolution neural network to determine the probe temperature. Our results show that the nano-OFT probe can measure furnace temperature in the temperature range from 1,000–1,650 °C, with the error percentage as low as 0.5 %. The nano-OFT system can be employed by the steel industry to monitor steel temperature continuously, and thus enhance steel production efficiency and reduce energy consumption.

Terahertz transmission-mode scanning-probe near-field optical microscopy based on a flexible step-index sapphire fiber

Terahertz scanning-probe near-field optical microscope (THz-SNOM), operating in transmission mode and relying on a flexible sapphire fiber, was developed. For this aim, a 300-μm-diameter step-index sapphire optical fiber with flat ends, high surface, and volume quality was fabricated using the edge-defined film-fed growth technique. For the lowest-order guided mode of such fiber, numerical analysis revealed small-to-moderate THz-wave propagation loss, as well as strongly subwavelength guided mode confinement in a fiber core thanks to a high refractive index of sapphire in the THz range. Experimental setup realizing the THz-SNOM principles was assembled based on thus fabricated fiber, as a probe, a backward-wave oscillator, as a continuous-wave emitter with the output wavelength of λ = 1.2 mm, and a Golay cell, as a THz beam power detector. By combining numerical analysis and experimental study, a subwavelength resolution of our THz-SNOM setup was demonstrated. The resolution depends on orientation of the guided mode polarization toward the image plane, but always remains strongly subwavelengths and overcomes the Abbe diffraction limit. For certain orientations, it can be as low as 0.23λ. In our opinion, thanks to the subwavelength resolution, high-energy efficiency, and flexibility of design, the developed sapphire-fiber-based THz-SNOM holds strong potential in different branches of THz science and technology.

Review of femtosecond laser fabricated optical fiber high temperature sensors [Invited

The femtosecond laser has been an efficient tool for optical fiber high temperature sensor construction. Here, we review the progress of optical fiber high temperature sensors based on femtosecond laser fabricated fiber gratings and various types of fiber in-line interferometers in silica fibers and sapphire fibers.

Matrixless fibrous oxide composites

Modern composite materials are normally composed of at least two components. The fibres in ceramic matrix composites are often coated with a “weak” material that is the interphase arresting a macrocrack and providing the composite with fracture toughness. In many biological materials of a ceramic type an organic weak material of a tiny quantity plays actually two roles those being the matrix and interphase. Oxide fibres produced by the internal crystallization method have two plane and two concave surfaces. It is shown in the present work that the composite microstructure composed only of sapphire fibres contacting one another by plane surfaces is characterized by the non-brittle behaviour.

Layered-Fibrous Composite with a Niobium-Based Matrix Reinforced with Single-Crystal Sapphire Fibers

In this paper, we obtain samples of composite materials with a layered niobium-based matrix unidirectionally reinforced with single-crystal sapphire fibers using the solid-phase method of diffusion welding. Sapphire fibers are obtained by the Stepanov method (EFG). As a result of tests, the dependences of the strength of such fibers on the length are obtained for the first time. The strength values ​​correspond to the global level. X-ray spectral microanalysis of the obtained materials is performed. The formation of solid solutions and intermetallic compounds of the Nb-Al system during composite-structure formation is established. Strength tests (500–700 MPa) are carried out and the effective surface energy of fracture of the composites is determined ((9–49) × 103 J/m2), and their correlation with the fabrication modes is established. The features of the fracture structure of such materials and the processes inhibiting its fracturing under load are studied.

Progress Toward Sapphire Optical Fiber Sensors for High-Temperature Applications

High-temperature measurements are of significant importance in various harsh-environment engineering fields, such as fossil fuel production, and the metallurgical and aviation industries. In recent years, there is a steady trend to shift from conventional electronic sensors to optical fiber sensors for high-temperature applications. In particular, optical fiber sensors are small in size, immune to electromagnetic interference, readily applicable for remote sensing, have high elasticity, and incorporate capabilities for multiplexing and distributed sensing. However, commonly used fused silica optical fiber sensors exhibit severe limitations at ultrahigh temperatures due to significantly degraded optical and mechanical properties at temperatures 1000 °C. The excellent optical transparency, thermal and chemical stability, mechanical robustness, and high melting temperature (~2040 °C) of single-crystal sapphire fibers (SFs) make them a strong candidate for sensing applications in high-temperature environments. Translation of the sensing schemes from mature silica fiber sensors to SF sensors has undergone tremendous growth and advancements in the past two decades. However, hurdles to the development of a near-term deployable SF sensing system have proven persistent due to the highly multimodal nature of SFs. This article reviews sensing techniques that have been implemented with SFs recently. The aim is to provide a comprehensive summary of past research on SF sensing systems. Perspectives on further research into the challenging yet promising arena are also discussed.

Experimental Study on the Gas Bubble Temperature Around an Arc Under Insulation Oil

The internal temperature of a gas bubble caused by a fault arc is a key basic parameter for investigating the bubble behavior and the pressure rise inside oil-filled power equipment. Due to the constraints imposed by the measurement methods and experimental conditions, this temperature has always been estimated as a constant value of 2000 K in past decades. In this letter, we present direct experimental evidence of the gas bubble temperature around an arc. A large oil-filled transformer was reformed and used to conduct arcing fault tests. A sapphire fiber optic infrared thermometer was established and used to detect the internal gas bubble temperature. The experimental results confirm that this internal temperature is not constant but rather varies over a wide range from 2100 K to 1500 K, plummeting with rapid bubble growth and then exhibiting small variations until arc extinction. Furthermore, the bubble growth process throughout the arc duration can be approximately regarded as first polytropic and then isothermal.

Monitoring Corrosion Processes via Visible Fiber-Optic Evanescent Wave Sensor

Ferrous objects, especially those that are additionally exposed to harsh environments, e.g., high humidity, have the common problem of suffering aggressive corrosion processes. Without a precaution, this leads in many cases to a limited functionality followed by treatment steps, and expensive repairing costs, as well as to defects/uselessness and even to safety problems, e.g., bridge-collapsing, escaping gas and liquids from pipelines, or leaking oil tankers destroying the ecological system. Conventionally applied methods are confined to laboratory use due to bulky instruments, and are, therefore, time-intensive and may cause erroneous results. Therefore, a sensor based on fiber-optic evanescent wave spectroscopy (FEWS) working in the visible spectral range was developed. The sensor system is comprised of an uncoated single crystal sapphire fiber as a waveguide operating in the visible spectral range in combination with a laser diode as a light source at a specific wavelength and a photodiode for signal detection. Within this study, the potential of the developed sensor was investigated. The corrosion process was simulated by implementing a corroded iron bar inside the measuring cell. When corrosion starts, iron ions are released leading to a complexation reaction with the dye. The results showed the potential use of the developed sensor system enabling implementation for online and on-site detection and monitoring of components susceptible to corrosion.

Sapphire optical fiber with SiBCN coating prepared by chemical vapor deposition for high-temperature sensing applications

To prevent the oxidation of BN-coated sapphire optical fiber (SOF) at 800 °C, a SiBCN ceramic coating was used as an outer coating owing to its excellent high-temperature oxidation resistance. Compared with BN/SOF, the tensile strength of SiBCN/BN/SOF increases from 500 to 723 MPa at 1200 °C. The optical properties of double coating indicate that the addition of SiBCN does not diminish the total reflection of the BN coating. Furthermore, SiBCN-coated fibers indicate lower residual stress and performed better compared with other coated fibers in terms of mechanical and optical properties. Hence, SiBCN is highly suitable as an outer coating of SOF.