of the light signal through collimator 45 and one color of the light signal through collimator 43 fig5 is an illustration of an embodiment of a multiplexer which uses a diffraction wavelength filter to enable a drop function source fiber 70 carries all the colors of the light signal in the fiber ring and emits them through collimator 61 destination fiber 71 carries all of the colors of the light signal in the fiber ring except the wavelength of interest u03bb x having received them through collimator 62 all light from collimator 61 impinges on temperature dependent diffraction filter 63 diffractor 63 reflects the entire light incident thereon except the wavelength of interest u03bb x diffractor 63 is of such size that by heating and cooling it can be tuned to transmit any of the potential wavelengths of interest transmitted over source fiber 70 a heating element 69 is positioned to maintain diffraction filter 63 at the appropriate temperature to pass the wavelength of interest u03bb x then light at the wavelength of interest u03bb x is collected by collimator 64 and transmitted to photodiode 65 which turns the light into an electrical signal that represents the extracted data a temperature sensor 66 observes the temperature of diffraction filter 63 the signal from temperature sensor 66 is mixed with a tuning voltage in mixer 67 and amplified to act as a feedback control loop to maintain the diffraction filter 63 at an appropriate temperature for transmitting the wavelength of interest u03bb x fig6.