or drops a channel corresponding to the bragg grating 39 s wavelength of reflection because fiber bragg grating 18 is tunable the network provider can select which channel i e a specific optical signal 39 s wavelength is being dropped or passed through by the roadm 10 this can be done by either remotely located network operator or via a routing algorithm in the event of drop another channel at the same wavelength may be added to the signal channels generally the added signal would be provided to the wss device 15 through its output port 15 b for example via the circulator 16 as stated above the add drop channel corresponds to the specific reflection wavelength of the bragg grating 18 the bragg grating 18 is tuned by temperature or strain or other means so as to select a different drop channel corresponding to the reflected wavelength u03bb d thus active temperature tuning or stress tuning can be used to achieve wavelength selection more specifically the local refractive index of the grating 18 is modified thereby shifting the bragg wavelength reflection wavelength u03bb d by stretching compressing or heating of the optical fiber containing the bragg grating 18 the tunable roadm 10 includes a pass through path 25 which enables non interruptive reconfiguration and an alternative pass 23 the term u201c non interrupting u201d reconfiguration means that when a particular add drop channel is switched between the two states the two states being add drop and pass through states the power of channels that are not being dropped or added is.