Steffen Koehler was appointed vice president of marketing for Sparkolor Corporation, a Silicon Valley-based developer of tunable optical networking components and modules.   ...   AIXTRON will open an R&D subsidiary in the Science-Based Industrial Park in Hsin-chu, Taiwan, on Jan. 1, 2002.   ...   AT&T Wireless Services Inc. has started its higher-speed next generation wireless service in Detroit, Flint and Ann Arbor in Michigan and Toledo in Ohio. The Redmond, Washington-based firm said it plans to offer the upgraded service in Florida in the next several weeks.