Optical EngineeringInsertion loss comparison of microcollimators used to propagate light in and out of single-mode fibers
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Most telecom components use lenses to collimate the beam emerging from a single-mode fiber and again, after some manipulation of the beam, to focus the beam back into an output fiber. In response to the drive for smaller components, collimator lenses have shrunk and a number of new microcollimator types been introduced. Several of the newer collimator types involve lenses that are fused to the fiber. Optical software was used to model six different types of microcollimators divided into three different size categories. Because not all of the six types were applicable in all three of the size categories, a total of eleven different collimator systems were modeled. A collimator system consists of an input collimator and an identical output collimator. Each collimator includes a single-mode fiber and a collimating lens. In order to achieve equivalence, all of the collimators in the same size category produce an intermediate waist with the same mode field diameter and distance from the lens surface to the intermediate waist. An algorithm is presented to determine the necessary physical parameters (lengths and radii) for a collimator to produce the appropriate beam criteria for a given size category. Once the physical parameters for the collimator systems were determined, all of the collimator systems were examined for insertion loss in their ideal configuration as well as in the presence of fabrication and alignment errors. All of the insertion loss analyses were performed using coherent field propagation starting from the fundamental fiber mode of the input fiber, through the collimating lenses, and finishing with the calculation of the overlap integral of the field inside the output fiber with the fundamental fiber mode. No thin lens or paraxial propagation assumptions are made...The results indicate that the collimator types involving lenses that are fused to the fiber are less sensitive to longitudinal fiber and collimator shifts than the components with the fiber and lens not in direct contact.