Comprehensive and Detailed Explanation From Nokia Optical Networking Fundamentals:

In the context of the Nokia 1830 PSS (Photonic Service Switch) and general WDM principles, the statement that OADMs always require O-E-O (Optical-Electrical-Optical) conversion for pass-through channels is fundamentally incorrect. The primary purpose of an OADM is to provide the ability to 'add' or 'drop' specific wavelengths while allowing other wavelengths (known as express or pass-through channels) to continue through the node entirely in the photonic domain.

By remaining in the optical layer, these express channels avoid the latency and cost associated with O-E-O conversion. FOADMs (Fixed OADMs) use static filters to achieve this, while ROADMs (Reconfigurable OADMs) use Wavelength Selective Switches (WSS) to dynamically route traffic. O-E-O conversion only occurs at the transponder or muxponder level when a service is terminated (dropped) or initiated (added) to convert the client signal into a compliant DWDM wavelength. Therefore, the efficiency of an optical network relies on the fact that pass-through traffic stays as light, bypassing the need for electrical processing at every node.