Contents I. Introduction
For the purpose of expanding communication capacity of photonic integrated circuits, combined use of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) attracts attention [1], [2]. In order to build the MDM network, a mode multi/demultiplexer (MUX/DEMUX) is necessary. Asymmetric directional couplers (ADCs) [3], [4] are often used for mode MUX/DEMUXs. However, the loss and the modal crosstalk of ADC increase as the operation wavelength shifts from the center wavelength since ADC has parabolic spectral shape [5]. Therefore, when WDM and MDM are used in combination, the degradation of the optical signal for the wavelength far from the center will be large. Mach-Zehnder (MZ) filter [6], [7] is frequently used for WDM multiplexer, which is composed of two 3dB coupler (usually multimode interference (MMI) waveguide) and delay line waveguides. MZ filter has periodic transmission spectrum and the wavelength separation (free-spectral range: FSR) can be freely tuned by the length of delay line.
