Fabrication of narrow bandpass filters for wavelength division multiplexing applications – A feasibility study

Thin film multilayer narrow bandpass interference filters are widely used in wavelength division multiplexing (WDM) applications in fibre optic communication systems. These filters must have sharp cut-on and cut-off on either side of the passband and practically zero transmittance outside the passband, over the wavelength range of the application. In an effort to demonstrate the feasibility of fabrication of WDM filters, the simpler designs for these filters (involving deposition of multilayers with 30-35 layers) have been examined and their fabrication has been demonstrated using a vacuum coating system with limited capabilities, at several wavelengths in the range of interest. This experience has shown that with the use of modern vacuum coating plants equipped with sophisticated coating and monitoring facilities, the fabrication of WDM filters with more complex designs and meeting commercial specifications [at least coarse WDM (CWDM) filters with somewhat less stringent specifications] can be a feasible proposition.