A wavelength division multiplexed, WDM, optical system comprising: a trunk terminal configured to provide a WDM trunk input signal on a trunk path, said WDM trunk input signal comprising one or more trunk signals occupying one or more of a plurality of trunk channels; a branch terminal configured to provide a WDM branch input signal on a branch path, said WDM branch input signal comprising one or more branch signals occupying one or more of a plurality of branch channels; and a branching unit coupled to said trunk path and said branch path for receiving said WDM trunk input signal and said WDM branch input signal and providing a WDM trunk output signal on said trunk path, said WDM trunk output signal comprising said WDM trunk input signal if a fault is detected on said branch path, said WDM branch input signal if a fault is detected on said trunk path, or at least one trunk signal and at least one branch signal if no fault is detected, said branching unit comprising a trunk input switch having an input coupled to said WDM trunk input signal, a branch input switch having an input coupled to said WDM branch input signal, a trunk output switch having an output for providing said WDM trunk output signal, a filter switch having an output coupled to said trunk output switch, and a filter stage coupled between said trunk input switch and said filter switch, said filter stage comprising at least one filter for passing only selected ones of said trunk signals or said branch signals to said WDM trunk output signal, said trunk input switch having a first output coupled to said filter stage and a second output directly coupled to said filter switch, said branch input switch having a first output coupled to said filter stage and a second output coupled directly to said trunk output switch, whereby if said fault is detected on said branch path, said trunk input switch is configured to bypass said filter stage and provide said WDM trunk input signal as said WDM trunk output signal through said filter switch, and if said fault is detected on said trunk path, said branch input switch is configured to bypass said filter stage and provide said WDM branch input signal as said WDM trunk output signal through said trunk output switch. a trunk terminal configured to provide a WDM trunk input signal on a trunk path, said WDM trunk input signal comprising one or more trunk signals occupying one or more of a plurality of trunk channels; a branch terminal configured to provide a WDM branch input signal on a branch path, said WDM branch input signal comprising one or more branch signals occupying one or more of a plurality of branch channels; and a branching unit coupled to said trunk path and said branch path for receiving said WDM trunk input signal and said WDM branch input signal and providing a WDM trunk output signal on said trunk path, said WDM trunk output signal comprising said WDM trunk input signal if a fault is detected on said branch path, said WDM branch input signal if a fault is detected on said trunk path, or at least one trunk signal and at least one branch signal if no fault is detected, said branching unit comprising a trunk input switch having an input coupled to said WDM trunk input signal, a branch input switch having an input coupled to said WDM branch input signal, a trunk output switch having an output for providing said WDM trunk output signal, a filter switch having an output coupled to said trunk output switch, and a filter stage coupled between said trunk input switch and said filter switch, said filter stage comprising at least one filter for passing only selected ones of said trunk signals or said branch signals to said WDM trunk output signal, said trunk input switch having a first output coupled to said filter stage and a second output directly coupled to said filter switch, said branch input switch having a first output coupled to said filter stage and a second output coupled directly to said trunk output switch, whereby if said fault is detected on said branch path, said trunk input switch is configured to bypass said filter stage and provide said WDM trunk input signal as said WDM trunk output signal through said filter switch, and if said fault is detected on said trunk path, said branch input switch is configured to bypass said filter stage and provide said WDM branch input signal as said WDM trunk output signal through said trunk output switch.