Low-crosstalk and fabrication-tolerant four-channel CWDM filter based on dispersion-engineered Mach-Zehnder interferometers.

The O-band coarse wavelength-division (de)multiplexing (CWDM) has been extensively used in data-center optical communications, whereas it's still challenging to reduce crosstalk and enhance fabrication tolerances for a CWDM filter. In this paper, we propose and experimentally demonstrate a low-crosstalk and fabrication-tolerant four-channel CWDM filter by utilizing dispersion-engineered Mach-Zehnder interferometers. The multi-sectional phase shifters are exploited to eliminate the phase errors induced by width deviations, leading to ultra-precise phase shifts and ultra-large width-error tolerances. The random-phase errors are also inhibited by using multi-mode waveguides as phase-shifting sections. The two-stage-coupler scheme is utilized to flatten the strong coupling-ratio dispersions for directional couplers, so that low crosstalk can be achieved over the whole O-band. The experimental results show both low insertion losses (< 1.2 dB) and low crosstalk (< -22.2 dB) over the whole working wavelength range. The measured width-error tolerance is also as large as ≈ 70 nm.