Abstract
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.
Highlights
IntroductionSingle-crystal sapphire optical fiber is frequently used in high temperature and corrosive environments for sensors monitoring properties such as temperature [1,2,3] or gas concentration [4].While the resilience of sapphire in extreme environments is rivaled by very few optical materials, its large refractive index relative to air results in high modal volume at typical sensing wavelengths.This contributes to higher loss, increased modal dispersion, decreased distributed sensing resolution, and decreased detection sensitivity than can be achieved by silica-based counterparts [5,6] or preclude the application of some sensing schemes altogether [7].A cladding is employed in most optical fiber applications to regulate the index disparity between the core and its surroundings to reduce modal volume
We previously demonstrated a wet-acid etching method which was successfully applied to reduce the diameter of a sapphire optical fiber to less than 800 nm while maintaining a smooth, consistent surface [12]
15 cm of single crystal sapphire optical fiber with an original diameter of 125 μm was cut from a 100 cm segment from Micromaterials, Inc. (Tampa, FL, USA)
Summary
Single-crystal sapphire optical fiber is frequently used in high temperature and corrosive environments for sensors monitoring properties such as temperature [1,2,3] or gas concentration [4].While the resilience of sapphire in extreme environments is rivaled by very few optical materials, its large refractive index relative to air results in high modal volume at typical sensing wavelengths.This contributes to higher loss, increased modal dispersion, decreased distributed sensing resolution, and decreased detection sensitivity than can be achieved by silica-based counterparts [5,6] or preclude the application of some sensing schemes altogether [7].A cladding is employed in most optical fiber applications to regulate the index disparity between the core and its surroundings to reduce modal volume. Single-crystal sapphire optical fiber is frequently used in high temperature and corrosive environments for sensors monitoring properties such as temperature [1,2,3] or gas concentration [4]. While the resilience of sapphire in extreme environments is rivaled by very few optical materials, its large refractive index relative to air results in high modal volume at typical sensing wavelengths. This contributes to higher loss, increased modal dispersion, decreased distributed sensing resolution, and decreased detection sensitivity than can be achieved by silica-based counterparts [5,6] or preclude the application of some sensing schemes altogether [7]. At temperatures approaching or exceeding 2000 ◦ C, many candidate claddings will delaminate due to thermal expansion mismatch or diffuse into the core, altering optical properties (not to mention those materials which would form an excellent sapphire cladding from an optics perspective but cannot survive hot and corrosive environments)
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