into which signals that carry the bragg wavelength are launched these added signals are reflected by bragg gratings 12 carried through the cross path of the second coupler 14 b and join the pass signals at the pass wavelength port 22 without interfering with each other an optical add drop multiplexer may use the mach zehnder interferometer which may be tuned by heating both of the gratings 12 using heaters 24 associated with each grating 12 in one embodiment such heating may be used to initially tune the mach zehnder interferometer as a result of heating the thermo optical device 10 may be controllably operated thus the heater 24 in one embodiment of the present invention encloses the upper surfaces of each grating 12 in connection with a variable optical attenuator 10 a one arm 23 may have a heater 24 in one embodiment of the present invention as shown in fig2 referring to fig3 the grating 12 a may be defined within a region 38 in the substrate 26 in the embodiment of fig1 a cross section through the grating 12 b would be the same as fig3 the substrate 26 may include a silicon substrate 30 a sio 2 layer 26 over the substrate 30 and a boron phosphate silicon glass layer 34 over the top a trench 36 is formed through the layers 32 and 34 down to the substrate 30 formed within the boron phosphate glass layer 34 in the region within the trench 36 is a core 12 a which corresponds to the grating 12 a and which carries the signal a metal resistance heater 24 may be formed over the portions of the layers 34 and.