Wavelength-division multiplexing (WDM) is a technology which originates from the telecommunication industry and defines the use of multiple optical signals onto a single optical fiber by combining different wavelengths (i.e., colors) of light. In the case of optical fiber sensing it describes the use of Fiber Bragg Gratings (FBGs) with different reflections wavelengths (Bragg wavelengths) in one optical fiber. These multiplexed FBG sensors are mostly used in the S, C, and L telecommunication window which is between 1460nm and 1625nm. Typical measurement devices (Interrogator) for WDM FBG sensors are operating between 1510nm-1590nm. These interrogators with an 80 nm wavelength span are fitting typically up to 70 temperature sensors and up to 40 strain sensors into one optical fiber. By using interrogators with multiple optical channels one can build up networks of more than 500 sensing points. WDM based FBG sensing is today the most commonly used sensing technology offering cost effective solution for high sensor count which can be distributed of several fiber kilometers.