Low-loss flat-topped wavelength division (de)multiplexer based on cascaded Mach-Zehnder interferometers

We propose and demonstrate a 2-channel coarse wavelength-division multiplexing (de)multiplexer with low crosstalk and flat-top passbands. The device utilizes cascaded Mach–Zehnder interferometers (MZIs) based on a planar lightwave circuit (PLC) to achieve flat passbands with wide bandwidth. By adjusting the arm length differences of each asymmetric MZI, the channel spacing is set to 50nm. As the order increases, the 14dB bandgap between channels can be reduced to as narrow as 10nm, with a bandwidth of up to 40nm—an important feature for coarse wavelength-division multiplexing (CWDM). Various splitter structures were compared, and multimode interference (MMI) waveguides and directional couplers were selected to interconnect different stages, minimizing wavelength-dependent losses. By fine-tuning the coupling ratios of the power splitters based on directional couplers, the crosstalk of the MZIs can be theoretically reduced to below -14dB. The device, fabricated on a silica-based platform with a refractive index contrast of 0.75%, features waveguides with a 6× 6μm cross-section, reducing polarization-dependent loss (PDL). Experimental results demonstrate crosstalk of approximately -10dB across a 40-nm wavelength range, with a PDL of less than 0.3dB per channel, making it suitable for optical communication systems that require large flat-top bandwidths, particularly in metropolitan all-optical networks.