Elastic Stability of the Glass Fibers in a Micromachined Fiber Optic Switch Packaged into a Dual-In-Line Ceramic Package

We address the elastic stability of the optical fibers in a micromachined fiber-optic switch, packaged into a dual-in-line ceramic package. The ends of the fibers are adhesively bonded with a UV-curable epoxy adhesive to a ceramic substrate and to a silicon cyip. When the package is subjected to low temperature conditions during temperature cycling tests, the fibers experience compression caused by the thermal contraction mismatch of the glass and the ceramic materials (we show that the favorable role of the silicon chip is small and need not be accounted for). Such a compression can possibly result in the buckling of the fibers, thereby having an adverse effect on the reliability of both the fibers and the adhesively bonded joints. We suggest that the elastic stability of the fibers be judged based on the calculated ratio of the actual thermally induced compressive force, arising in the glass fiber at low temperature conditions, to the critical (Euler) force. The critical force is computed using an assumption that the fiber ends on the ceramic substrate are rigidly clamped, while the ends of the fibers on the silicon chip might experience minor angular rotations (i.e., should be considered elastically clamped), because of the possible elevated compliance of the joints on the chip. The numerical data are obtained for a fiber with an unbonded span (length) of 6.5mm. The fiber is adhesively bonded to a ceramic substrate with a CTE of 7.5ppm, and the package is subjected to a temperature drop of only 65C. The computed data indicate that although no buckling is expected to occur for the fiber in question, the "margin of safety" is small: a minor increase in the length of the unbonded portion of the fiber to, say, 7.0mm, will lead to its buckling. The results of the performed analysis can be used for the prediction and prevention of fiber buckling in micromachined fiber-optic switch designs.