Design and modeling of InP waveguide based narrowband optical filters and add-drop multiplexers for WDM applications

Narrowband optical filters and all-optical Add-Drop Multiplexers (ADMs) are key components in dense-WDM systems. Implementation of these devices in semiconductor waveguide technology allows for integration with passive and active photonic devices for compact, high-functionality modules. This paper describes an approach based on quarter-wave shifted DBR filters. Add-Drop Multiplexers are implemented by integrating of filters with directional couplers, and the devices are analyzed using a combination of transfer matrix methods and commercial beam propagation software. Through analytical and numerical methods we demonstrate the possibility of implementing filters and ADMs with an optical bandwidth less than 25GHz. Channel cross-talk isolation better than -20dB can be achieved, with waveguide propagation loss being the critical issue. Using filter design from the theory of electron filters translated to the optical domain, higher-order optical filters with Chebyshev and Butterworth responses are designed. Reconfigurability is important for network versatility and robustness. Semiconductor waveguide devices can be tuned by injection of carriers. Based on the mode, the tuning range is evaluated and found to span several channel spacings, limited by the added waveguide propagation loss form free-carrier absorption. The design of a 25 GHz filter with 300 GHz tuning range is shown.