Bragg grating (de)multiplexers with morphology engineering at 2-μm wavelengths

As an essential supplement to the telecommunication band in optical communication, 2-μm optical communication has garnered substantial research interest since its inception. One practical approach to enhancing communication capacity is the use of wavelength division multiplexing (WDM), in which Bragg gratings have been widely employed in (de)multiplexing devices. However, limited by the fabrication capability, the deformation in grating teeth typically introduces excess losses and center-wavelength shifts. Here, we significantly improve the fabrication tolerance of the Bragg gratings by morphologically engineering the envelope of the ultra-thin grating with a sinusoidal function for use in 2-μm WDM communications. The four-channel coarse wavelength division multiplexing (CWDM) (de)multiplexer centered at a wavelength of 2.25 μm has a flat-top spectral response and channel spacing of 23 nm. The insertion loss is less than 0.17 dB. The variations in the filling factors and etching widths of the Bragg gratings only introduce neglectable center-wavelength shift and bandwidth change. Moreover, by designing an apodized modulation in the grating amplitude, we further suppress the sidelobes of the Bragg grating and thus reduce the inter-channel crosstalk. The proposed Bragg grating CWDM (de)multiplexer is robust to the fabrication imperfections and holds great potential for use in 2-μm optical communication systems.