Laser heated pedestal growth (LHPG) method is a unique technique to grow single crystal fibers and fibrous materials of high temperature ceramics for various photonic and electronic applications. The stability of the solidification process and the floating molten zone created at the interface between the feed and seed materials is critical for ensuring high quality of the fiber including uniformity of diameter. To maintain the molten zone volume constant throughout the dynamic growth process, the typical LHPG system control scheme will modify the relative speed control of fiber-pull rate versus the source material feed rate as constrained by mass conservation. However, due to the dynamic nature of the growth process and the floating state of the molten material, it is prone to instability due to non-uniform heating, heat loss, melt convection, seed-fiber and pedestal material alignments, and other factors which impact growth processes. Sustainable growth process demands a combination of optimized optical components and real-time process controls. Here we present a detailed optical analysis of several candidate LHPG optical designs and compare details of the illumination at the molten zone region during the growth process. In addition, we explore the potential for enhancing the typical process control by utilizing (1) active laser power control and (2) machine vision methods for real-time characterization of the molten zone profile to be integrated into active control schemes. Impacts on the quality of the fiber grown in terms of uniformity in diameters upon active laser power feedback loop to mitigate the molten zone shape variation is also discussed.
KEYWORDS: Laser crystals, Fiber lasers, Crystals, Laser development, Biological and chemical sensing, Imaging spectroscopy, High power lasers, Environmental sensing, Control systems
Single crystal fiber has a wide range of applications spanning from high temperature sensing, radiation sensing in harsh environment, high power laser and power delivery, medical and chemical application, and imaging applications. Nevertheless, the potential of single crystal fiber has not been fully explored in part because of the limited facilities available for custom growth of high quality, low loss, and custom fiber chemistries and geometries. The presentation provides an overview of recent work and current state of the art on growth of single crystal oxide fibers using various techniques. A discussion of recent progress in applications of single crystal fibers was also presented spanning harsh environment sensing, radiation sensing, and fiber lasers. In this paper, we also overview establishment of a Laser Heated Pedestal Growth system at University of Pittsburgh including the online monitoring of the fiber growth process and discuss important process parameters for future process optimization. We demonstrate the growth of single crystal fiber from a polycrystalline source rod which may be a more affordable and flexible method in the future.
Single crystal fibers have shown great potential for harsh environment sensing applications. Interrogation schemes based on temperature dependence of Raman scattering modes in crystals, offer capability of sensing at high temperatures. Yttrium aluminum garnet (YAG) is a potential alternate to sapphire as it also has similar material properties like high melting point and chemical resistance. In this work, Raman scattering in YAG samples have been measured from room temperature to 1000°C. Temperature dependence of the peak position, and peak width are measured and compared for the different peaks. A Raman based Optical time domain reflectometry (OTDR) is constructed and distributed temperature measurements of a furnace are conducted using a YAG crystal fiber.
Sapphire single crystal fibers have shown great potential in fiber-based high temperature sensing applications. As single crystals are grown without cladding, there have been a lot of different techniques that have been proposed to achieve a core-clad structured fiber. A common approach is to achieve this is by using sol-gel deposition where the cladding layer is grown on a crystal fiber via dip-coating and subsequent thermally treatment. In this work we describe the synthesis of magnesium aluminate spinel via sol-gel methods and application of it as a cladding material for sapphire crystal fibers. The thermal stability of spinel coatings on sapphire substrates is investigated as the formation of non-stoichiometric spinel at high temperature is well-documented. Sapphire fibers with the spinel cladding layers are tested under different gas atmosphere at elevated temperatures, to demonstrate the efficacy of spinel as a cladding layer.
The absence of an effective and stable cladding has been a major hurdle in utilizing single crystal fibers for harsh environment sensing applications despite the promise of sapphire for temperatures as high as 1800°C. This work discusses the development of a high temperature cladding for sapphire fibers using wet chemical methods. Magnesium aluminate spinel has been chosen as the material for the cladding as it has a lower refractive index compared to sapphire and does not undergo significant interdiffusion with sapphire at temperatures below approximately 1200°C. Different sol-gel based approaches have been pursued to develop polycrystalline cladding layers with thicknesses greater than a micron, as required to ensure adequate confinement of the guided electromagnetic radiation within the fiber core. For sapphire fibers, high temperature stability of the cladded fibers as well as the effect of the cladding layer on optical characteristics under different application relevant gas environments at elevated temperatures has been investigated.
Although single crystal sapphire fiber has been fabricated extensively for decades, many details surrounding the impacts of growth conditions on fiber quality are still unreported. Traditional fiber quality measurements require stopping the fiber growing process, cutting the fiber into short pieces, and measuring the transmission which is time consuming and highly variable. We developed a very simple method to monitor the fiber quality in real-time. During the fiber growth process, the melting pool shape becomes stable and incandescence from the molten zone can be used as an active light source. By connecting the other end of the growing fiber to a spectrometer, we can monitor the light intensity as fiber length increases continuously. Not only we are able to deduce the current fiber quality being grown, but also to identify the optimum growing conditions include the growth rate, rod-to-fiber ratio, and position of the crystallization interface, as well as monitoring impacts from laser power instability. These measurements can be further compared to cutback style measurement, or loss measurements using Raman interrogation. Different single crystal fibers, including sapphire and YAG are grown while measuring throughput during growth.
High quality single crystal sapphire optical fiber is important not only for its capacity for high laser power delivery, but also for applications in harsh environment sensing. Improving the quality of Laser Heated Pedestal Growth (LHPG) fabricated single crystal fiber has been a long-term effort for decades. The equilibrium state during crystal growth and defect formation rate are the two most important factors in single crystal fiber fabrication. In this paper, we study the theory governing the molten zone profile and verify the theoretical predictions with a high-resolution CCD camera. We also study defect formation during the crystal growth process and observed dislocation defects with transmission electron microscopy (TEM). These analyses will help to guide high quality single crystal fiber fabrication and hopefully will lead to the production of better fibers for harsh-environment sensing applications.
KEYWORDS: Raman spectroscopy, Signal to noise ratio, Spatial resolution, Signal detection, Roads, Scattering, Signal attenuation, Receivers, Sensors, Modulation
This paper proposes a new vector Brillouin optical time-domain analysis optical fiber sensor with large dynamic range and high signal-to-noise ratio that combines distributed Raman amplification with optical pulse coding. The optimized Raman pumping configurations are numerically simulated by solving the coupled differential equations of the hybrid Brillouin-Raman process, and experimentally investigated with respect to the Brillouin pump pulse. A vector network analyzer is adopted to extract both the amplitude and phase spectrograms of the Brillouin interaction in a distributed fashion which effectively lessens the impact of the Raman relative intensity noise transfer problem and achieve high accuracy measurement over a long sensing distance. Advanced pulse coding is further introduced to increase the sensing range under high spatial resolution. Initial experimental results of phase and amplitude from a custom built BOTDA system is presented. Compared to typically tens of kilometers measurement distance of conventional Brillouin optical time-domain analysis techniques, the proposed optical fiber Brillouin sensor has the potential to greatly enhances sensing range up to one hundred kilometers or greater, providing distributed temperature and strain monitoring of high spatial resolution and high sensing resolution in structures such as oil and natural gas pipelines.
Sapphire optical fiber is an excellent candidate for harsh environment sensing due to its high melting point, small size, and chemical resistance. Various optical sensors in sapphire fiber have been explored for decades. However, there is still lack of accurate data on sapphire fiber optical properties at elevated temperatures, which impedes the development of sapphire fiber sensors. In this paper, we fabricate single crystal sapphire fiber via a laser heated pedestal growth system and measure the optical properties of our fiber from room temperature to 1500 ℃ in ambient air and in different gas environments.
Harsh environment sensor applications are becoming more accessible due to the implementation of single-crystal optical materials and devices. In particular, fossil energy applications like gas turbines or coal gassifiers require new, more robust sensing technologies compatible with modern control schemes. Fabricating common devices in sapphire or YAG fibers rather than standard fused silica can extend the operating temperature range significantly beyond the current state of the art. Here, we discuss configuration of our Laser Heated Pedestal growth (LHPG) system with a novel control algorithm that permits the growth of fibers with non-uniform diameters along the fiber’s length. This algorithm controls the molten zone height, laser power, and drawing rates simultaneously to reduce the mismatch between instantaneous diameter changes and current diameter. We detail the range of structural possibilities achievable using this control technique, and subsequently evaluate the spectral properties of as-grown devices like sapphire long-period gratings. Finally, we make recommendations regarding new single-crystal sensor devices which will be shown to maintain operational stability over a wide range of operating temperatures.
Performing attenuation measurements in unclad single crystal sapphire fiber has traditionally been accomplished through use the cutback method. Because single-crystal sapphire fibers do not cleave easily like silica fibers, this method requires repeated cutting and polishing of the sapphire fiber sample; which is very time consuming and introduces uncertainty in each loss measurement. In this paper, we present a new method to measure attenuation in sapphire or other single-crystal fibers based on distributed sapphire Raman optical time domain reflectometry (OTDR). This method is both nondestructive, significantly faster than the cutback method, and capable of measuring the local loss along the entire length of the fiber.
Single crystal fibers like those made from sapphire are capable of operating at higher temperatures than conventional
silica-glass-based fibers. This work aims to construct single-crystal optical fiber sensors capable of providing
environmental data in combustion, high-temperature chemical processing, or power generation applications where
temperatures exceed 1000 °C and standard silica fibers cease to provide useful information. Here, we explore the
functionalization of single crystal fibers using methodologies intrinsic to the crystal growth process or with methods
which do not severely reduce their operating temperature range. While operating a laser-heated pedestal growth system
to produce single-crystal optical fibers from rod feedstock, we continuously vary parameters such as fiber diameter to
produce novel single-crystal linear distributed-sensing devices. The spectral characteristics of those modified devices,
along with sensing performance in a high-temperature harsh-environment are reported. Finally, a technique for
increasing the intrinsic Rayleigh backscattering using femtosecond laser irradiation is discussed for temperature sensing
applications.
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.
A type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high- and low-index regions in the azimuthal direction. The structure retains a “core” region of pure single crystal (SC) sapphire in the center of the fiber and a “cladding” region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the “windmill”-shaped cladding. The simulation results showed that the number of guided modes was significantly reduced in the windmill fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the windmill SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF.
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