Sapphire Fiber Research Articles

Dispersion properties of a single-mode windmill single crystal Sapphire optical fiber and its broadband infrared supercontinuum generation

The effective refractive indices of each mode of a windmill single crystal sapphire fiber as function of wavelength can be obtained by finite element method. Recently, it is shown that this fiber operates in single-mode condition for a wide wavelength range. The dispersion and the slope of dispersion for the fundamental mode of a windmill single crystal sapphire fiber are properly obtained making use of the measured values of the refractive index as a function of the wavelength. Subsequently, the pulse evolution along this windmill fiber is investigated for low peak power pulses. Due to high applicability of broadband light sources, here, supercontinuum generation process is also studied when high peak power pulses are launched into the single-mode windmill single crystal sapphire fiber. Due to its unique optical properties, the single-mode windmill single crystal sapphire fiber can be used to generate broadband IR supercontinuum light compared with the conventional single crystal sapphire fiber. Our simulation results indicate that generated output spectrum contains dense frequency combs in a wide range. This broadband IR light source facilitates IR spectroscopy while its frequency combs are important in the development and improvement of the optical metrology.

Miniature Uncooled and Unchopped Fiber Optic Infrared Thermometer for Application to Cutting Tool Temperature Measurement.

A new infrared thermometer, sensitive to wavelengths between 3 μm and 3.5 μm, has been developed. It is based on an Indium Arsenide Antimony (InAsSb) photodiode, a transimpedance amplifier, and a sapphire fiber optic cable. The thermometer used an uncooled photodiode sensor and received infrared radiation that did not undergo any form of optical chopping, thereby, minimizing the physical size of the device and affording its attachment to a milling machine tool holder. The thermometer is intended for applications requiring that the electronics are located remotely from high-temperature conditions incurred during machining but also affording the potential for use in other harsh conditions. Other example applications include: processes involving chemical reactions and abrasion or fluids that would otherwise present problems for invasive contact sensors to achieve reliable and accurate measurements. The prototype thermometer was capable of measuring temperatures between 200 °C and 1000 °C with sapphire fiber optic cable coupling to high temperature conditions. Future versions of the device will afford temperature measurements on a milling machine cutting tool and could substitute for the standard method of embedding thermocouple wires into the cutting tool inserts. Similarly, other objects within harsh conditions could be measured using these techniques and accelerate developments of the thermometer to suit particular applications.

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 36.5pm/°C was achieved in the high-temperature region. Such SFBGs could be developed for promising high-temperature sensors in aero engines.

Femtosecond Laser-Inscribed High-Order Bragg Gratings in Large-Diameter Sapphire Fibers for High-Temperature and Strain Sensing

Aiming for structural health monitoring applications in harsh environments, a high-order sapphire fiber Bragg grating (HO-SFBG) written with a femtosecond (fs) laser in a large-diameter sapphire fiber is demonstrated. The radial refractive index modulation area induced by scanning exposure is greater than 60% of the fiber cross-section area. The large mode overlapping area leads to a transmission resonance strength of approximately 3.0 dB. As demonstrated by numerical simulations of mode excitation and multimode resonance, each mode resonance is significantly superposed when the coupling coefficient is above 0.0005. A study of the high-temperature and strain sensing characteristics of the HO-SFBG shows that each high-order resonance has different response characteristics. This type of large-diameter SFBG has important application potential for high-temperature smart materials with embedded fiber sensors.

High‐temperature effects on the light transmission through sapphire optical fiber

AbstractSingle crystal sapphire optical fiber was tested at high temperatures (1500°C) to determine its suitability for optical instrumentation in high‐temperature environments. Broadband light transmission (450‐2300 nm) through sapphire fiber was measured as a function of temperature as a test of the fiber's ability to survive and operate in high‐temperature environments. Upon heating sapphire fiber to 1400°C, large amounts of light attenuation were measured across the entire range of light wavelengths that were tested. SEM and TEM images of the heated sapphire fiber indicated that a layer had formed at the surface of the fiber, most likely due to a chemical change at high temperatures. The microscopy results suggest that the surface layer may be in the form of aluminum hydroxide. Subsequent tests of sapphire fiber in an inert atmosphere showed minimal light attenuation at high temperatures along with the elimination of any surface layers on the fiber, indicating that the air atmosphere is indeed responsible for the increased attenuation and surface layer formation at high temperatures.

Review and perspective: Sapphire optical fiber cladding development for harsh environment sensing

The potential to use single-crystal sapphire optical fiber as an alternative to silica optical fibers for sensing in high-temperature, high-pressure, and chemically aggressive harsh environments has been recognized for several decades. A key technological barrier to the widespread deployment of harsh environment sensors constructed with sapphire optical fibers has been the lack of an optical cladding that is durable under these conditions. However, researchers have not yet succeeded in incorporating a high-temperature cladding process into the typical fabrication process for single-crystal sapphire fibers, which generally involves seed-initiated fiber growth from the molten oxide state. While a number of advances in fabrication of a cladding after fiber-growth have been made over the last four decades, none have successfully transitioned to a commercial manufacturing process. This paper reviews the various strategies and techniques for fabricating an optically clad sapphire fiber which have been proposed and explored in published research. The limitations of current approaches and future prospects for sapphire fiber cladding are discussed, including fabrication methods and materials. The aim is to provide an understanding of the past research into optical cladding of sapphire fibers and to assess possible material systems for future research on this challenging problem for harsh environment sensors.

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 9.6μm 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.

Composite Oxide Fibres Grown by Internal Crystallisation Method

The internal crystallisation method (ICM) allows producing bundles containing hundreds and thousands of the fibres of a limited length. It is shown in the present paper that ICM can be used to produce fibres composed of a sapphire matrix and inclusions of calcium hexaaluminate. The fabrication of such composite fibres is described in the present paper. An effect of the calcium hexaaluminate inclusions in sapphire fibres is evaluated by testing model composites with brittle molybdenum matrices. The critical stress intensity factors of composite-fibre/molybdenum-matrix specimens are higher than those of specimens reinforced with sapphire fibres. The model specimens reveal non-linear stress/strain behaviour. The results of this work show the effectiveness of composite fibres as a reinforcement for brittle matrices.

Creation of an Internal Cladding in Sapphire Optical Fiber Using the $^{6}$ Li(n, $\alpha)^{3}$ H Reaction

The focus of this work is to extend optical frequency domain reflectometry sensing to sapphire optical fiber, by creating a cladding in the fiber. A cladding was created by irradiating a sapphire optical fiber, which was surrounded by an annulus of Li-6 enriched lithium carbonate (Li2CO3) powder, in The Ohio State University Research Reactor. The 6Li(n, $\alpha)^{3}\text{H}$ reaction created high energy alpha particles and tritons that irradiated the fiber simultaneously to a depth of 24 microns along the entire periphery of the sapphire fiber, thereby slightly reducing the index of refraction in the fiber’s periphery and creating a cladding within the fiber. Transmitted light intensity profiles show that the ion implanted cladding made the fiber’s intensity profile nearly single mode. The cladding survived to the highest temperature that was tested (1500 °C). The cladding made the fiber sufficiently single mode that the cladded sapphire fiber, when read out with an optical backscatter reflectometer, produced distributed temperature measurements along the entire length of the fiber.

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 780nm 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 25.8pm/°C are demonstrated.

A penetrating Raman probe with self‐referencing capability

A penetrating Raman probe having a hypodermic needle probe head is desirable for measurements inside soft materials. The Raman probe reported in this paper not only is designed for penetration but also provides for self‐referencing. This is a significant advantage, because the measurement of Raman intensity relative to some standard is not always feasible with conventional probes. In the Raman probe described here, this capability is realized through the use of a c‐axis single‐crystal sapphire fiber as an interface between the probe body and the sample. As an illustration of how this probe can be put to use, the Raman spectrum from the interior of porcine fat has been obtained. Situations where this probe may be the preferred choice include subcutaneous and subterranean measurements as well as forensic investigations. Copyright © 2017 John Wiley & Sons, Ltd.

Single Mode Air-Clad Single Crystal Sapphire Optical Fiber

The observation of single mode propagation in an air-clad single crystal sapphire optical fiber at wavelengths at and above 783 nm is presented for the first time. A high-temperature wet acid etching method was used to reduce the diameter of a 10 cm length of commercially-sourced sapphire fiber from 125 micrometers to 6.5 micrometers, and far-field imaging provided modal information at intervals as the fiber diameter decreased. Modal volume was shown to decrease with decreasing diameter, and single mode behavior was observed at the minimum diameter achieved. While weakly-guiding approximations are generally inaccurate for low modal volume optical fiber with high core-cladding refractive index disparity, consistency between these approximations and experimental results was observed when the effective numerical aperture was measured and substituted for the theoretical numerical aperture in weakly-guiding approximation calculations. With the demonstration of very low modal volume in sapphire at fiber diameters much larger than anticipated by legacy calculations, the resolution of sapphire fiber distributed sensors may be increased and other sensing schemes requiring very low modal volume, such as fiber Bragg gratings, may be realized in extreme environment applications.

High-temperature sensor instrumentation with a thin-film-based sapphire fiber.

A novel sapphire fiber-optic high-temperature sensor has been designed and fabricated based on blackbody radiation theory. Metallic molybdenum has been used as the film material to develop the blackbody cavity, owing to its relatively high melting point compared to that of sapphire. More importantly, the fabrication process for the blackbody cavity is simple, efficient, and economical. Thermal radiation emitted from such a blackbody cavity is transmitted via optical fiber to a remote place for detection. The operating principle, the sensor structure, and the fabrication process are described here in detail. The developed high-temperature sensor was calibrated through a calibration blackbody furnace at temperatures from 900°C to 1200°C and tested by a sapphire crystal growth furnace up to 1880°C. The experimental results of our system agree well with those from a commercial Rayteck MR1SCCF infrared pyrometer, and the maximum residual is approximately 5°C, paving the way for high-accuracy temperature measurement especially for extremely harsh environments.

Advanced ceramic components with embedded sapphire optical fiber sensors for high temperature applications

This paper describes an extrusion-based additive manufacturing process that has been developed to enable embedment of sapphire optical fiber sensors in ceramic components during the part fabrication. In this process, an aqueous paste of ceramic particles is extruded through a moving nozzle to build the part layer-by-layer. In the case of sensor embedment, the fabrication process is halted after a certain number of layers have been deposited; the sensors are placed in their predetermined locations, and the remaining layers are deposited until the part fabrication is completed. Because the sensors are embedded during the fabrication process, they are fully integrated with the part and the problems of traditional sensor embedment can be eliminated. Scanning electron microscopy was used to observe the embedded sensors and to detect any possible flaws in the part or embedded sensor. Attenuation of the sensors was measured in near-infrared region (1500–1600nm wavelength). Standard test methods were employed to examine the effect of embedded fibers on the strength and hardness of the parts. The results indicated that the sapphire fiber sensors with diameters smaller than 250 micrometers were able to endure the freeform extrusion fabrication process and the post-processing without compromising the part properties.

Sapphire-fiber-based distributed high-temperature sensing system.

We present, for the first time to our knowledge, a sapphire-fiber-based distributed high-temperature sensing system based on a Raman distributed sensing technique. High peak power laser pulses at 532 nm were coupled into the sapphire fiber to generate the Raman signal. The returned Raman Stokes and anti-Stokes signals were measured in the time domain to determine the temperature distribution along the fiber. The sensor was demonstrated from room temperature up to 1200°C in which the average standard deviation is about 3.7°C and a spatial resolution of about 14 cm was achieved.

A heat-sinking self-referencing fiber optic emission probe.

A novel heat-sinking, self-referencing fiber optic emission probe having a sapphire fiber probe head is described. The laser heating effect in a GaAs wafer (on a polytetrafluoroethylene (PTFE) platform) has been measured with the probe in both the noncontact proximity mode and the contact mode. The GaAs/PTFE composite was selected to simulate the thermal conductivity of animal tissues. It was found that for the same laser power delivered to the wafer, the temperature rise in the contact mode was only 42% of that in the proximity mode. Additionally, a demonstration of the self-referencing capability of the probe is also presented.

Biological oxygen sensing via two-photon absorption by an Ir(III) complex using a femtosecond fiber laser

Near-infrared two-photon absorption of the phosphorescent Ir(III) complex (2,4-pentanedionato-κO2,κO4)bis[2-(6-phenanthridinyl-κN)benzo[b]thien-3-yl-κC]iridium (BTPHSA) was characterized. It exhibited a 800–1200 nm two-photon absorption band, and thus could be electronically excited by 1030-nm femtosecond Ti:sapphire and Yb-doped fiber lasers. By using BTPHSA, oxygen concentrations in human embryonic kidney 293 (HEK293) cells were imaged. These results demonstrate two-photon oxygen sensing of live tissues via easily operable excitation sources.

Facet Appearance on the Lateral Face of Sapphire Single-Crystal Fibers during LHPG Growth

Results of the study of the lateral surface of single-crystal (SC) sapphire fibers grown along crystallographic directions [ 0001 ] and [ 11 2 ¯ 0 ] by the LHPG method are presented. The appearance or absence of faceting of the lateral surface of the fibers depending on the growth direction is analyzed. The crystallographic orientation of the facets is investigated. The microstructure of the samples is investigated with the help of an optical microscope and a JSM-5910LV scanning electronic microscope (JEOL). The crystallographic orientations of the facets on the SC sapphire fiber surface are determined by electron backscatter diffraction (EBSD). The seed orientation is studied by means of XRD techniques.

High-Temperature Strain Sensing Using Sapphire Fibers With Inscribed First-Order Bragg Gratings

Strain sensor designs and strain measurements based on single-crystal sapphire fibers with inscribed first-order fiber Bragg gratings for applications up to 600 °C are presented. We report on all the details of two different sensor designs; for instance, we show that the resolution of multimode sapphire fiber Bragg grating (SFBG) strain sensors is about $\Delta{l/l}=10^{-5}$ (10 $\mu\text{strain}$ ), which is comparable with state-of-the-art high-temperature sensors. We apply our sensors for the determination of the thermal expansion coefficients of high-temperature steel alloys, showing a good match to known values. Hence, we believe that SFBG sensors may represent a promising alternative to currently used non-optic-based strain-detecting devices.

Design and analysis of large-core single-mode windmill single crystal sapphire optical fiber

We present a large-core single-mode “windmill” single crystal sapphire optical fiber (SCSF) design, which exhibits single-mode operation by stripping off the higher-order modes (HOMs) while maintaining the fundamental mode. The “windmill” SCSF design was analyzed using the finite element analysis method, in which all the HOMs are leaky. The numerical simulation results show single-mode operation in the spectral range from 0.4 to 2 μm in the windmill SCSF, with an effective core diameter as large as 14 μm. Such fiber is expected to improve the performance of many of the current sapphire fiber optic sensor structures.