Fabricating Sapphire Optical Fibers for High-Temperature Use
Polycrystalline cladding layers are formed on sapphire cores.
John H. Glenn Research Center, Cleveland, Ohio
A process for the fabrication of coated sapphire multimode optical fibers has been developed. Sapphire multimode optical fibers are suitable for use as embedded sensors for measuring mechanical and thermal stresses and strains in material specimens at temperatures up to about 1,500 °C.
The core of each optical fiber of this type is made of optical-grade single-crystal sapphire (which is basically an allotrope of alumina). The sapphire core is clad with a layer that has an index of refraction smaller than that of sapphire, in order to confine propagating optical signals predominantly in the core and limit attenuation at the surface of the core. The cladding material preferred for thermal durability and for chemical compatibility with the core is polycrystalline alumina; however, one or more other optical materials (e.g., SiO2, MgO, AlxSiyOz) could be used for cladding. The cladding should be at least a few tens of microns thick, the exact required thickness depending on the optical properties needed for a specific application. The cladding is coated with an outer protective layer of SiC, ZrO2, Ta2O5, HfO2, or other strong, high-temperature-resistant material.
The techniques used to apply the cladding and protective layers are critical to the success of the fabrication process. The techniques and the fabrication sequence are chosen to ensure adhesion between adjacent layers. Although the process admits of a number of variations, the following steps are typical:
1. The surface of a sapphire optical fiber is prepared by cleaning in hydrochloric acid followed by rinsing in distilled water.
2. Alumina particles no larger than about 20 nm are suspended in a suitable monomer (e.g., acrylic acid) and a suitable accelerator and initiator are added to the suspension.
3. The fiber is immersed in the suspension for about half an hour to allow a thin sublayer of polymer and alumina particles to begin to form on its surface, and the fiber is allowed to stand out of the suspension for another 24 hours, during which time the monomer polymerizes. All of the foregoing steps take place at room temperature.
4. The coated fiber is heated to a temperature of 600 °C to remove the polymeric binder, leaving the alumina particles in place.
5. The coated fiber is heated further to a temperature between 1,300 and 1,500 °C for a time between 10 and 60 minutes, in order to sinter the alumina particles around the sapphire core. The precise temperature-vs.-time sintering schedule is chosen to suit the intended application.
6. Steps 2 through 5 are repeated, as needed, in order to accumulate cladding sublayers to the required cladding-layer thickness.
7. A suspension of particles of the protective material is prepared as in step 2, then applied to the fiber and consolidated as in steps 3 through 5.
This work was done by Don J. Roth of Glenn Research Center and Mahmoud A. El-Sherif, Ihab L. Kamel, and Frank K. Ko of Drexel University. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17054.Download detailed Technical Support Package for this Brief