the correct direction to allow it to be coupled out of the ring via coupler 46 yet another embodiment of the invention is shown in fig5 in this embodiment a thin film type coupler 50 is again used however the coupler 50 provides both the input and the output coupling for the ring the coupler has an input path connected to it in one direction and has an output path connected to it from an opposite direction a wavelength selective component in the coupler 50 has two wavelength selective coatings one on either side of it on one side is a coating that is reflective at wavelengths at and below a wavelength in the range of u03bb 0 such that the input wavelength from input port 12 is reflected by the coupler 50 into the ring on the other side is a coating that is reflective at wavelengths at and above a wavelength in the range of the output wavelength u03bb n this characteristic is indicated by the two arrows showing u03bb 0 being coupled from input port 12 into the ring and u03bb n being coupled from the ring to output port 34 and by the double headed arrow showing wavelengths in between i e u03bb 0 lt u03bb lt u03bb n passing through the coupler and remaining in the ring a coupler structure that may be used for the coupler 50 of the fig5 embodiment is shown in fig5 a two grin.