Efficient laser diode to single-mode fiber coupling using a combination of two lenses in confocal condition

A new coupling method of a laser diode to a single-mode fiber is described. It consists of a spherical ruby lens and a graded index (GRIN) rod lens positioned under a confocal condition. A high coupling efficiency of more than 40 percent has been easily obtained for InGaAsP laser diodes. Alignment sensitivities of the proposed coupling circuit have been studied both experimentally and theoretically. The dependence of the coupling efficiency on misalignments has been found to be in agreement with the theoretical consideration, and it has been apparent that the stringent alignment tolerances required for conventional coupling methods have been overcome. A laser diode module using the proposed circuit was also fabricated with high coupling efficiency into the single-mode fiber, and good reproducibility and performance have been obtained.