In this book wavelength division multiplexing (WDM) and dense WDM (DWDM) systems are designed. The practical components of WDM system models have been simulated through software to understand the behavior of these components and overall system considerations because of the role played by these components in the transmission processes (e.g., optical fiber, continuous wave laser diode (CW) and multiplexer (MUX)). Two types of amplifiers (Raman Amplifiers and Erbium Doped Fiber Amplifiers) are studied that are used to amplify the optical signals. Ultra wideband WDM system is proposed in this book. The total 3-dB spectral range obtained is equal to 83-nm as compared with 75-nm that is obtained today. An improved gain-flatness (7 dB as compared with the same value that is obtained today but with short distance) of a Raman amplifier with wide gain-bandwidth and a high gain-output power is obtained by using multipump configuration. 96 channels are simulated with channel spacing equal to 100 GHz (0.8 nm); distance more than 700 km and with more than distance between amplifiers 71.75 km (as compared with 50 km that is used today).