Input/output fiber configuration in a laser package design: optimization for lower stresses

Abstract

Mechanically and thermally induced stresses in the input/output (I/O) fiber of a laser package design are evaluated for different configurations of this fiber. It is found that if the fiber exhibits bending deformations, mechanical stresses can be minimized if a proper end offset is applied and thermal stresses can be reduced if the fiber is mechanically prestressed. It is found also that if the optical device can be rotated by a small angle around the transverse axis, this rotation can be used effectively to minimize the stresses in both categories. It is shown that the smallest fiber span can be obtained if one makes the end planes of the device perpendicular to the package axis, i.e., by making the fiber straight. Clearly in this case the fiber should be made short enough to avoid buckling under the compressive action of thermally induced stresses. Such a configuration is the most feasible because it results in the shortest fiber span (length) and in minimal optical losses. Such a configuration should be employed in all cases when rotation of the optical device is possible, when the fiber ends can be easily aligned, and if the support structures are strong enough to withstand the higher thermally induced forces from the compressed fiber. Thermal stresses can be brought down by the use of low-expansion materials such as Kovar or Invar for the package enclosure. It should be pointed out that although the results of this analysis provide designers with a useful theoretical guide for optimizing the I/O fiber configuration, the final configuration can be selected only after the allowable stress and the achievable alignment (in the case of straight fiber) are evaluated experimentally.