Realisation of Butterworth-type optical filters using 3 × 3 coupler ring resonators

Butterworth-type optical filters are realised using a combination of optical resonators which exhibit a single pole and single zero transfer function, quasi-all zero and quasi-all pole optical circuits. Design procedures as well as filter structures are proposed for optical lowpass, highpass and bandpass optical filters of even and odd orders. In particular, the [3 × 3] planar optical directional coupler has been employed with a direct feedback path and a single optical delay feedback to form a single-pole single-zero optical resonator. This is the basic block of cascaded sections of optical filters. [2 × 2] optical directional couplers have also been employed to construct interferometers and resonators having quasi-all pole or all zeros characteristics for cascading with the double feedback resonators to implement odd- and even-order modular optical filters.