This paper reports on the design, implementation, and validation of an SDN control plane for multi-band optical networks with externalized path computation. The SDN control plane relies on extending current open and standard interfaces to support dynamic service management and decoupled path computation services on multi-band optical networks while accounting for physical layer impairments, which is critical for successful service provisioning. We detail the multi-band optical network resource management and optimization engine for transparent and translucent networks. The system is experimentally validated in a 22-ROADM BT network with emulated hardware showing the performance of the control plane and the considered workflows.