Absolute measurement of the fiber-to-waveguide distance during a coupling process by low-coherence interferometry

Absolute measurement ofthe fiber to waveguide distance during a coupling process by low-coherence interferometiyHenri Porte, Wilhelm Efflein, Richard FerriereLaboratoire d'Optique P.M.Duffieux, TJRA CNRS 214, UIFR Sciences et TechniquesUniversite de Franche Comte, 16, route de Gray, F-25030 Besancon cedex, FranceABSTRACTDuring a fiber-to-waveguide connection process, it can be important to determine accurately the absolute distancebetween the end ofthe fiber and the edge ofthe optical waveguide without visual control. We investigate here the use of alow-coherence optical source whose light, transmitted by the fiber, is launched into the waveguide in order to control thealigmnent. The end faces ofthe waveguide and ofthe fiber form a Fabiy-Perot cavity for the transmitted light beam. Theoptical transmission versus the distance exhibits a fringe pattern whose visibility is related to the coherence properties of thebroadband source. We deduce the absolute distance from visibility measurements, we assess the accuracy ofthe method, andthe results are compared with the direct distance measurements.1. iNTRODUCTIONThe problem ofthe coupling ofan optical fiber to an optical waveguide for packaging has been investigated since thebeginning ofthe research in the field of integrated optics. Nevertheless, few simple, automatic and low-cost solutionsappeared, slowing down the development ofthe integrated optic devices and systems, which find the widest applications inthe field ofoptical telecommunications networks (switching, routing, multi/demultiplexing, modulation).In the frame ofa project dealing with the development ofan automatic positioning and alignment system, one of theproblems met consists in achieving the final contact between the fiber and the waveguide once the optical transmission isoptimized in the transverse plane [X,Y] and electronically locked on the maximum oftransmitted light. Without any visualcontrol, the system will steer the motion ofthe fiber to the waveguide and will stop the shifting when contact occurs. Thisrequires the accurate knowledge ofthe absolute value ofthe gap between the two devices before moving any of it, for instancethe fiber.The approach can be done by using a motorized set-up or using piezoelectric transducers. The accuracy of thedisplacement should be about 0. 1tm. As in available commercial systems, one can imagine to stop the advance along the Z-axis as soon as the optical transmission becomes constant and maximum, after a progressive increasing. However, thismethod brings with it the risks to, either, damage the edge ofthe waveguide, for instance in the case offragile ciystallinesubstrates, or more often, the risk to modify the position of the waveguide in the transverse plane [X,Y] .