Design of a Low-Loss Narrowband Frequency Demultiplexer Using Multimode Waveguide Gratings

We propose a narrowband, low-insertion-loss frequency demultiplexer based on a multimode waveguide grating (MWG) implemented on an x-cut lithium niobate on insulator platform with a 600-nm-thick film for quantum photonic integrated circuits. The device is designed to address the trade-off between photon-pair generation rate and spectral purity in integrated PPLN-based sources by enabling efficient and selective frequency demultiplexing. By optimizing the MWG geometry, the proposed structure simultaneously achieves low insertion loss, narrow 1-dB bandwidth, and high sidelobe suppression ratio. Numerical results show that an MWG with a waveguide width of 2 μm, etching depth of 300 nm, corrugation depth $\delta_{0}=150 \text{nm}$, apodization strength b = 10, and grating number N = 1500 provides a 1-dB bandwidth of 4 nm, an insertion loss of 0.003 dB.