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

Ultrafast Laser Processing for High-Aspect-Ratio Structures

Over the past few decades, remarkable breakthroughs and progress have been achieved in ultrafast laser processing technology. Notably, the remarkable high-aspect-ratio processing capabilities of ultrafast lasers have garnered significant attention to meet the stringent performance and structural requirements of materials in specific applications. Consequently, high-aspect-ratio microstructure processing relying on nonlinear effects constitutes an indispensable aspect of this field. In the paper, we review the new features and physical mechanisms underlying ultrafast laser processing technology. It delves into the principles and research achievements of ultrafast laser-based high-aspect-ratio microstructure processing, with a particular emphasis on two pivotal technologies: filamentation processing and Bessel-like beam processing. Furthermore, the current challenges and future prospects for achieving both high precision and high aspect ratios simultaneously are discussed, aiming to provide insights and directions for the further advancement of high-aspect-ratio processing.

High-temperature strain sensor based on sapphire fiber Bragg grating

Sapphire fiber Bragg grating (SFBG) is a promising high-temperature strain sensor due to its melting point of 2045°C. However, the study on the long-term stability of SFBG under high temperature with an applied strain is still missing. In this paper, we reported for the first time to our knowledge on the critical temperature point of plastic deformation of the SFBG and demonstrated that the SFBG strain sensor can operate stably below 1200°C. At first, we experimentally investigated the topography and the spectral characteristics of the SFBG at different temperatures (i.e., 25°C, 1180°C, and 1600°C) with applied 650 µε. The reflection peak of the SFBG exhibits a redshift of about 15 nm and broadens gradually within 8 h at 1600°C, and the tensile force value decreases by 0.60 N in this process. After the test, the diameter of the SFBG region decreases from 100 to 88.6 µm, and the grating period is extended from 1.76 to 1.79 µm. This indicates that the plastic deformation of the SFBG happened indeed, and it was elongated irreversibly. Moreover, the stability of the Bragg wavelength of the SFBG under high temperature with the applied strain was evaluated. The result demonstrates the SFBG can be used to measure strain reliably below 1200°C. Furthermore, the strain experiments of SFBG at 25°C, 800°C, and 1100°C have been carried out. A linear fitting curve with high fitness (R2 > 0.99) and a lower strain measurement error (<15 µε) can be obtained. The aforementioned results make SFBG promising for high-temperature strain sensing in many fields, such as, power plants, gas turbines, and aerospace vehicles.

High-speed and high-resolution YAG fiber based distributed high temperature sensing system empowered by a 2D image restoration algorithm

High temperature monitoring is critical to the health and performance of vital pieces of infrastructure such as jet engine, fuel cells, coal gasifiers, and nuclear reactor core. However, it remains a big challenge to realize reliable distributed high temperature sensing system with high speed, high spatial and temperature resolution simultaneously. In this work, a Raman distributed high temperature sensing system with high temperature resolution and high spatial resolution was realized in a single-crystal YAG fiber. The sensing system demonstrated operation from room temperature up to 1400°C with a spatial resolution of 7 cm and response time of 1 millisecond in a 1m long YAG fiber. The average temperature sensitivity of the system is about 7.95 × 10^-4/°C. To the best of our knowledge, this is the best spatial resolution and response time reported in literature. In this system, a 2D image restoration was used to boost the signal to noise ratio of sensor. Empowered by the algorithm, the average temperature standard deviation along the sensing fiber of 7.89 °C was obtained based on a single frame data in 1 millisecond. A new record of temperature resolution of 0.62 °C was demonstrated in only 1 second frame data traces, which enables a fast response capacity.

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 with homogeneous material by side-array cladding

Optical fibers are the core elements for various fiber-optic applications in communication, lasers, sensors, tweezers, quantum optics, and bio-photonics. Current optical fibers are based on a core-cladding structure with different refractive indices and are mainly fabricated using the stack-draw method. However, such a traditional fabrication method limits the realization of fibers with various advanced optical materials, thereby restricting the utilization of excellent optical properties offered by these materials. In this study, a novel structure for side-array cladding by laser drilling on the side of the fiber with homogeneous material is proposed. Accordingly, the confinement loss, mode characteristics, birefringence, and dispersion of the side-array cladding fiber are investigated based on the numerical simulation performed via the finite element method. Subsequently, an optimal fiber structure is obtained by taking the crystal material as an example. Essentially, our proposed side-array cladding fiber can eliminate the mismatch problem of core-cladding materials in the current stack-draw fabrication method. Potentially, the proposed approach can serve as a standard design and fabrication method of optical fibers with homogeneous material, by utilizing the rapid development of laser processing. In other words, a large number of advanced optical materials can be fabricated into optical fibers with the proposed technique, thus maximizing their technical advantages for different applications.

Point-by-point inscribed sapphire parallel fiber Bragg gratings in a fully multimode system for multiplexed high-temperature sensing

We study the point-by-point inscription of sapphire parallel fiber Bragg gratings (sapphire pFBGs) in a fully multimode system. A parallel FBG is shown to be critical in enabling detectable and reliable high-order grating signals. The impacts of modal volume, spatial coherence, and grating location on reflectivity are examined. Three cascaded seventh-order pFBGs are fabricated in one sapphire fiber for wavelength multiplexed temperature sensing. Using a low-cost, fully multimode 850-nm interrogator, reliable measurement up to 1500°C is demonstrated.

Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., S_a parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average S_a value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

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.

Stabilized Ultra-High-Temperature Sensors Based on Inert Gas-Sealed Sapphire Fiber Bragg Gratings

In situ measurement of high temperature is critical in aerospace, petrochemical, metallurgical, and power industries. The single-crystal sapphire fiber is a promising material for high-temperature measurement owing to its high melting point of ∼2045 °C. Sapphire fiber Bragg gratings (SFBGs), which could be inscribed in sapphire fibers with a femtosecond laser, are widely used as high-temperature sensors. However, conventional SFBGs typically exhibit a significant deterioration in their spectra after long-term operation at ultra-high temperatures, resulting from the formation of some unwanted microstructural features, that is, lossy spots and micro-etched lines, on the surface of the sapphire fiber. Here, we report for the first time, to the best of our knowledge, a thermally stabilized ultra-high-temperature sensor based on an SFBG created by femtosecond laser inscription, inert gas-sealed packaging, and gradient temperature-elevated annealing. The results indicate that the lossy spots are essentially aluminum hydroxide induced by high-temperature oxidation, and the inert gas-sealed packaging process can effectively insulate the sapphire fiber from the ambient air. Moreover, the formation of micro-etched lines was suppressed successfully by using the gradient temperature-elevated annealing process. As a result, the surface topography of the SFBG after operating at high temperatures was improved obviously. The long-term thermal stability of such an SFBG was greatly enhanced, showing a stable operation at 1600 °C for up to 20 h. In addition, it could withstand an even higher temperature of 1800 °C with a sensitivity of 41.2 pm/°C. The aforementioned results make it promising for high-temperature sensing in chemical, aviation, smelting, and power industries.

Temporal-spatial characteristics of filament induced by a femtosecond laser pulse in transparent dielectrics

The evolution mechanism of femtosecond laser-induced filaments has been widely investigated owing to its application prospects in microprocessing. However, the material dependence of the excitation, stability, and decay of filaments is not well understood despite the importance of their precise utilization. In this study, the spatiotemporal evolution of filaments induced by a single femtosecond laser pulse in sapphire and silica glass was investigated using time-resolved pump-probe shadowgraphy on femtosecond and picosecond timescales. The results revealed that the evolution was significantly different in the two typically transparent dielectrics in terms of the electronic plasma dynamics and filament lifetimes. This difference can be attributed to the self-trapped excitons (STEs) in silica glass. Furthermore, the filament dependence on pump energy and focal position was experimentally analyzed. Divergent filaments were observed when the focal position was near the surface because of the effect of the excited plasma on beam propagation. Moreover, the evolution of filament length in the two materials was discussed. This study contributes to the applications of filaments in precise processing.

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.