The RB filter, or Residual Band filter, is used in Dense Wavelength Division Multiplexing (DWDM) transmission systems to effectively remove residual optical noise that may be present in the optical signal. This filter helps to improve the signal quality and reduce the level of interference in the transmission, ensuring a clearer and more reliable communication channel. The RB filter plays a crucial role in optimizing the performance of DWDM systems by enhancing signal integrity and minimizing signal degradation, ultimately leading to improved overall network efficiency and reliability