Systems and methods provide for the addition of DWDM modules to CWDM systems in order to increase bandwidth over existing lines. This allows for the expansion of communications capacity without the need to replace existing CWDM systems. The DWDM modules can be transparently inserted into the CWDM systems by, in the CWDM system, reserving a channels for use by the DWDM. The DWDM system then occupies that channel with a scalable number of DWDM wavelength channels, for example four, eight, or sixteen. The DWDM systems can be used in both single and dual fiber systems as well as ring systems. Use of single fiber systems is obtained by implementing coupling devices, such as interleavers, passband filters, and circulators, that can used in pairs to couple bidirectional signals over a single fiber.