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

Abstract

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.

Single Crystal Fibers: Diversified Functional Crystal Material

“…We can achieve total reflection by adjusting the refractive index of the claddings so as to reduce the propagation loss caused by particulates and contaminants from the surrounding media [40]. Excellent cladding materials must possess a refractive index lower than the core and a thermal expansion coefficient close to that of core materials, while simultaneously exhibit thermo-chemical and mechanical stability [41]. The best approaches thus far are summarized below.…”

“…The researchers also calculated through numerical simulations the optimal size of the voids needed to improve light propagation. The ion implantation cladding method changes the refractive index of the surface of the crystal fiber and avoids the delamination of claddings, which is beneficial for applications in harsh environments [41].…”

“…The etching solution was a mixture of sulfuric acids and phosphoric acid and the etching rate was about 2.3 μm/h [54]. However, the higher cost and lower etching efficiency of the wet etching method have limited its commercial implementation [41].…”

Tailoring the nanostructure of anodic aluminum oxide cladding on optical fiber

“…Porous fiber cladding, especially with axially aligned pore channels such as in microstructured optical fiber (also termed photonic crystal fiber [PCF]), affords endless degrees of freedom in structural and hence fiber optic property manipulation . This breakthrough development has led to the advent and continued expansion of new classes of special optical fiber with novel functionalities for applications not possible with conventional optical fiber . We have recently reported that anodic aluminum oxide (AAO) cladding with radially aligned nanoscopic pore channels can be produced on silica and sapphire optical fiber by anodization of aluminum deposited on the fiber surface .…”

“…4,5 This breakthrough development has led to the advent and continued expansion of new classes of special optical fiber with novel functionalities for applications not possible with conventional optical fiber. [6][7][8][9] We have recently reported that anodic aluminum oxide (AAO) cladding with radially aligned nanoscopic pore channels can be produced on silica and sapphire optical fiber by anodization of aluminum deposited on the fiber surface. [10][11][12] Compared with conventional optical fiber and PCF, AAO-clad fiber, given the unique cladding structure and tunable index of refraction, has the following distinct advantages.…”