Improved Packaging of Silica Optical Fibers in Sensor Heads

Lewis Research Center, Cleveland, Ohio

A process for the fabrication of improved high-temperature fiber-optic sensor heads has been proposed. Fiber-optic sensor heads that can withstand hot, dirty, noisy environments are needed for advanced jet engines and stationary power-generating turbine engines.

A typical optical fiber for high-temperature service is made of fused silica. Also typically, the surface layer of the fiber is doped with germanium to alter its index of refraction in such a way as to achieve a specific numerical aperture needed for directionally selective sensing. Engine operating temperatures to which silica optical fibers are likely to be exposed range up to somewhat above 250 °C, and the fibers can withstand higher temperatures up to 700 °C. Prolonged exposure to temperatures above 700 °C causes diffusion of germanium from the surface layers into the pure fused silica cores of the fibers, with consequent loss of the desired numerical apertures. Accordingly, any heating that must be performed in fabricating a sensor head should be limited to temperatures below 700 °C.

A fiber-optic sensor is made with multiple optical fibers to provide (1) a cross-sectional area large enough for reliable sensing and (2) redundancy for protection against breakage of one or more fiber(s). To form a sensor head, the fibers are bundled at one end, where they are sealed rigidly and hermetically in a ring sized to fit an instrumentation port in an engine. The ring is made of Kovar (or equivalent) iron/nickel/cobalt alloy.

Silica Optical Fibers Would Be Sealed into a metal ring by applying a paste of low-melting-temperature glass, then fusing the glass.

The proposed fabrication process would include a step for sealing the fibers into the ring by use of a glass that melts at a temperature below 700 °C. The process (see figure) would comprise the following steps:

Prepare the requisite number of silica fibers of equal length. Such fibers are supplied with polyimide or nylon coats. Strip off about 1 in. (about 2.5 cm) of the coat at one end of each fiber. The polymeric coating material will not survive the high temperature to which the stripped ends will subsequently be exposed, but elsewhere than at the stripped ends it provides mechanical support and thus should be left intact.
Pack the fibers in the ring. Fill the interstices between fibers and the space between the fibers and the ring with a boric-glass paste. If necessary, install fixtures to hold the fibers steady in the ring.
Using a controllable source of heat, raise the temperature of the glass paste to at least the melting temperature of the boric glass (580 °C) but no more than 700 °C. Continue heating until the paste fuses and bonds to the fiber surfaces.
Anneal the package by bringing down the temperature very slowly. Prolonged heating at intermediate temperatures (e.g., 500 °C) might be necessary to prevent cracking.
Clean off excess glass and polish the ends of the fibers in the ring. The other ends of the fibers can be bundled as a conventional fiber-optic cable because they will not be exposed to high operating temperatures.

This work was done by E. Shu, W. Daum, and L. Petrucco of General Electric Co. for Lewis Research Center. Inquiries concerning rights for the commercial use of this invention should be addressed to