Better centering increases fiber stability

John Wallace

Developments in integrated passive and active optical devices have not eliminated the need for fiber alignment-for example, lining up an optical fiber to a semiconductor laser to achieve maximum light coupling. Even small improvements in this basic procedure can have a major impact on the performance of a fiberoptic system. Researchers at National Sun Yat-sen University (Kaohsiung, Taiwan) and Chunghwa Telecom Laboratories (Taoyuan, Taiwan) have studied one commonly used fiber alignment technique based on soldering fibers into metal ferrules and figured out how to carry it out with higher precision.

In the technique, a fiber is soldered into a cylindrical ferrule and the ferrule aligned and welded to a metal structure containing a laser diode. The alignment must be done to submicron accuracy or the coupling of light into the fiber will suffer. If done well, the resulting assembly has a high coupling efficiency. However, thermal cycling of the assembly can cause coupling efficiency to decrease over time. To see why, the researchers studied fiber-solder-ferrule (FSF) joints having inner and outer ferrule diameters of 0.4 and 0.9 mm and a fiber diameter of 0.125 mm, with solder filling the gap between the fiber and the ferrule. Both finite-element-method (FEM) modeling and experimentation on real FSF joints were carried out.

When repeatedly cycled through a temperature range of -40°C to +85°C, the FSF joints showed fiber shifts-as seen through an optical microscope-that were proportional to the eccentric offset of the fiber from the center. For example, a fiber offset of 100 µm produced a shift of 0.8 µm. (Such eccentric offsets are introduced in the soldering process when the solder shrinks as it solidifies within the FSF joint.) As the FEM model made clear, the shift in fiber position was a result of redistribution of thermal stresses in the solder. In addition, high-temperature creep in the solder during cycling may have contributed to the shifts. The results led the researchers to propose a simple solution: find a way to solder the fiber so that it is positioned at the center of the ferrule. For more information, contact Wood-Hi Cheng at gaex@mail.nsysu.edu.tw.

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