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
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.