Normal WDM (sometimes called BWDM) uses the two normal wavelengths 1310 and 1550 nm on one fiber. Dense WDM (DWDM) uses the C-Band (1530 nm-1565 nm) transmission window but with denser. Wavelength division multiplexers are fundamental to the functioning and performance of integrated photonic circuits, with applications ranging from optical interconnects to sensing and quantum technologies. Current solutions are limited by trade-offs between channel spacing, crosstalk, insertion. In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i. This technique enables bidirectional communications over a. This article introduces topology optimization theory into the design of topological photonic crystals, aiming to achieve the inverse design of microwave wavelength division multiplexers. This collection encompasses a variety of research papers, conference proceedings, and technical articles that explore both foundational.