Contents 1. Introduction
Tunable ultra-narrowband filters offer the opportunity to multiplex and demultiplex low bitrate signals or tones, such as subcarrier multiplexed monitoring, control, or routing information [1] or radio-on- fiber signals [2], to analyze modulated signals directly in the optical domain and to process individual sub-channel spectral components [3] in a cost-effective and versatile manner. A notch response with high isolation is critical for applications such as carrier suppression, sideband nulling for single-sideband detection, sideband extraction for direct detection, or subcarrier drop and add functionality. A simple ring-resonator filter is shown in Fig. 1a [4]. With this architecture, it is difficult to get high isolation since the coupling ratios must be precisely fabricated to match the expected ring loss. To provide multiple bandpass responses, several rings are cascaded and their individual outputs combined, most simply in a lossy manner. A novel architecture is demonstrated in this paper that is capable of processing multiple tones with high isolation and yielding power complementary outputs.
