Optical fiber-based sensing is uniquely qualified for a wide range of harsh environment applications due to the ability of optical fiber to withstand high temperature or chemically corrosive conditions. While off-the-shelf silica fiber is stable up to 800-900°C under a wide range of conditions, single crystal fiber offers a pathway to operation above 1000°C. Current single crystal fiber growth techniques are limited to producing multimode fiber, which limits interrogation approaches to primarily time-domain techniques. Raman-based distributed temperature sensing is one time-domain technique which has demonstrated significant utility for distributed temperature sensing in conjunction with multimode, single crystal fiber.
A distributed temperature sensor based on Raman scattering and consisting of a single crystal probe spliced to an arbitrarily long silica lead is considered for operational environments up to 1400°C. The impact of temperature and wavelength-dependent optical loss on the measured temperature is investigated, particularly at temperatures above 1000°C. Strategies for improved performance at extreme temperatures are 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.
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.
Rare-earth doped single crystal (SC) yttrium aluminum garnet (YAG) fibers have great potential as high-power laser gain media. SC fibers combine the superior material properties of crystals with the advantages of a fiber geometry. Improving processing techniques, growth of low-loss YAG SC fibers have been reported. A low-cost technique that allows for the growth of optical quality Ho:YAG single crystal (SC) fibers with different dopant concentrations have been developed and discussed. This technique is a low-cost sol-gel based method which offers greater flexibility in terms of dopant concentration. Self-segregation of Nd ions in YAG SC fibers have been observed. Such a phenomenon can be utilized to fabricate monolithic SC fibers with graded index.
Rare-earth doped single-crystal (SC) Yttrium Aluminum Garnet (YAG) fibers are excellent candidates for high power lasers. These SC fiber optics combine the favorable low Stimulated Brillouin Scattering (SBS) gain coefficient and excellent thermal properties to make them an attractive alternative to glass fiber lasers and amplifiers. Various rare-earth doped SC fibers have been grown using the laser heated pedestal growth (LHPG) technique. Several cladding methods, including in-situ and post-growth cladding techniques, are discussed in this paper. A rod-in-tube approach has been used by to grow a fiber with an Erbium doped SC YAG fiber core inserted in a SC YAG tube. The result is a radial gradient in the distribution of rare-earth ions. Post cladding methods include sol-gel deposited polycrystalline.
Single crystal (SC) yttrium aluminum garnet (YAG, Y3Al5O12) as a host material has the ability to be doped with high
concentrations of Er3+ ions. We utilize this ability to grow a 50% Er3+ doped YAG SC fiber, which was inserted into
a SC YAG tube. This rod-in-tube was used as a preform in our laser-heated pedestal growth (LHPG) apparatus to
grow a fiber with a radial distribution of Er3+ ions. The work shows that there is a distribution of Er3+ ions from their
fluorescence and two different techniques were used to measure the index of refraction.
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