WDM itself is a technology, not a single device, and it can use passive optical components such as multiplexers and demultiplexers.

Wavelength-Division Multiplexing (WDM) is a method for combining multiple optical signals of different wavelengths onto a single fiber and separating them at the receiver end. The core WDM devices, such as multiplexers (Mux), demultiplexers (Demux), and optical add-drop multiplexers (OADMs), can be passive, meaning they do not require electrical power to operate and rely on optical filtering techniques like bandpass filters, prisms, or fused couplers to separate or combine wavelengths . Passive WDM devices are widely used in both Coarse WDM (CWDM) and Dense WDM (DWDM) systems. They are typically installed at network endpoints or intermediate nodes and provide high reliability because they do not depend on active electronics or software integration . Examples include:

• CWDM Mux/Demux units: Combine or separate a small number of widely spaced wavelengths for short-range applications.
• DWDM Mux/Demux units: Handle many closely spaced wavelengths for high-capacity, long-distance networks.
• OADMs: Allow selective adding or dropping of specific wavelengths while passing others through, often implemented with passive optical filters . However, WDM systems can also include active components, such as optical amplifiers (EDFAs), which boost signal strength over long distances. These active elements are not passive devices, but they complement passive WDM components in a complete optical network . In summary, WDM is a technology that can employ both passive and active devices. The multiplexing and demultiplexing functions themselves are often passive, while amplification or signal processing may require active components. This distinction is important when designing optical networks for reliability, cost, and power consumption.