Mode matching for efficient laser diode to single mode fiber coupling

Abstract

This paper presents some analysis for the matching between the elliptical mode field of 1550nm high power laser diode with the circular mode field of the single mode fiber in order to obtain high coupling efficiency with relaxed misalignment tolerances. Three coupling schemes namely Butt, single ball lens and double ball lenses coupling schemes have been employed in pigtailing the butterfly laser diode module using laser welding technique with dual beams from Nd:YAG laser welding system for the attachment of coupling components. The process of fiber attachment to laser diode and welding of various coupling components, such as lens holders, fiber ferrule and welding clips have been performed in what is so called active alignment process, where the system continues measuring the coupled power during the process of coupling and welding of coupling components in their holder to each other and to the main substrate. It has been found that double ball lenses coupling scheme is efficient and more effective for mode matching of highly elliptical (large divergence ratio) laser diode mode field with the circular mode field of a single mode fiber, whereas for small divergence ratios the single ball lens is adequate.