Passive alignment of optical elements in a printed circuit board

Abstract

A successful implementation of optics into PCBs (printed circuit boards) requires a precise passive alignment of optical elements relative to the optical waveguides in the board. We tackled this challenge with a novel concept that allows the passive alignment onto a PCB of any optical or optoelectronic building block with a precision of a few micrometers. Markers, structured into a copper layer during manufacturing, are used as a position reference for the polymer waveguide fabrication and for the formation of mechanical alignment features. To form the latter, laser drilling, a standard process for via formation in PCBs, is used. Thereby, we exploit the fact that the laser light is reflected at the copper surface, such that the copper marker can act as a mask for the laser beam. An opening in the copper marker can then be used to define an accurately positioned alignment slot, independently of the low positioning accuracy of the drilling laser. We were able to demonstrate repeated insertions of adapter elements into these alignment slots with a standard deviation of 3 pm for in-plane displacements. Afterwards, optical modules were mounted onto the adapters, using a standard MT interface provided by the adapter. We measured a standard deviation of the order of 5 pm for the in-plane and out-of-plane misalignments of the module with respect to the optical waveguides. The passive alignment concept demonstrated enables accurate and simple plug-in of any kind of element, in particular of optical and opto-electronic elements, into a PCB. The concept is based on established PCB manufacturing processes, which is crucial for the development towards a low-cost optical interconnect technology. © 2006 IEEE.