Echelle grating demux/mux in SiN

An Echelle grating wavelength division multiplexing (WDM) device, comprising:

• a first waveguide comprising an input waveguide or an output waveguide;
• a slab waveguide comprising silicon nitride (SiN);
• multiple second waveguides optically coupled to the first waveguide through the slab waveguide, the multiple second waveguides comprising multiple output waveguides if the first waveguide comprises the input waveguide or multiple input waveguides if the first waveguide comprises the output waveguide;
• an Echelle grating;
• and
• a trench that forms a mirror to reflect incident light from the first waveguide toward the multiple second waveguides or from the multiple second waveguides to the first waveguide, the trench defined by a toothed wall formed by a cladding material and spaced apart from a toothed wall formed by the slab waveguide.

An Echelle grating multiplexer (mux), comprising:

• an output waveguide;
• a slab waveguide;
• multiple input waveguides optically coupled to the output waveguide through the slab waveguide;
• an Echelle grating formed in a silicon nitride (SiN) layer of a silicon (Si) photonic integrated circuit (PIC), the Echelle grating including multiple grating teeth formed in the slab waveguide, the multiple grating teeth having a pitch of less than 300 nanometers (nm);
• and
• a broadband mirror located at the multiple grating teeth and encapsulated below a top surface of the Si PIC.