as the loop back light sources 126 to 128 the wdm optical signal output from the multi frequency light source 114 in the co 100 reaches the rn 104 via the optical circulator 116 and the optical fiber 102 the wdm optical signal is then wdm demultiplexed in the wdm mux demux 124 in the rn 104 the respective demultiplexed optical signals are applied to the input of the upstream light sources the loop back light sources 126 to 128 in the onus 110 to 130 via the optical fibers 106 to 108 the loop back light sources 126 to 128 output optical signals having the same wavelengths as those of the received optical signals and modulated according to upstream data signals the signals from the loop back light sources 126 to 128 are wdm multiplexed in the wdm mux demux 124 of the rn 104 and then transmitted to the co 100 the wdm optical signal is applied to the input of the wdm demux 118 via the optical circulator 116 the optical signal is wdm demultiplexed in the wdm demux 118 and the demultiplexed optical signals are fed to the respective upstream optical data receivers 120 to 122 fig2 illustrates the configuration of a wdm pon using loop back light sources according to a first aspect of the present invention the wdm pon further includes an optical wavelength tracking apparatus in a co.