Abstract

For potential applications in dense wavelength division multiplexing networks, great efforts have been made to improve the specifications of narrow bandpass filter coatings. Besides environmental stability requirements, which have been met by utilizing ion- or plasma-assisted deposition techniques and appropriate substrate selection [1,2], the spectrum shape requirements of the narrow bandpass filters are especially critical for WDM applications. Multi-cavity design and layer optimization have been adopted to achieve better theoretical results. On the practical side, layer thickness and index monitoring is key to achieving high quality narrow bandpass filters. For multi-cavity Fabry-Perot filters, previous studies [3,4] showed that transmission turning point monitoring is the best monitoring approach due to its error compensation mechanism. However, total compensation is only achievable when the monitoring light is monochromatic. In a practical coating situation, monitoring light always has a finite bandwidth. The bandwidth of the optical monitoring light comes from three sources as depicted in Figure 1. These are the bandwidth of the monochromator, the effective bandwidth caused by non-uniformity thickness inside the monitoring light spot on the filter, and the effective bandwidth caused by the incident angle span of the monitoring light on the filter. For error compensation, smaller bandwidth is preferable. However, monitoring light intensity decreases proportionally with bandwidth (except for a laser light source, whose long coherent length is problematic). Thus, the turning point error increases. A trade-off must be taken to optimize the coating process. Due to analytical complexity, a numerical method approach was taken here.

© 1998 Optical Society of America