An apparatus comprising: a first wavelength demultiplexer having an optical input and N optical outputs, each to output a respective optical signal having a first WDM component (312n,1) and a second WDM component (312n,2); a first wavelength multiplexer having an optical output and N optical inputs, wherein the first wavelength multiplexer is configured to receive N data modulated optical signals on the N optical inputs thereof and generate a WDM signal therefrom; characterized by N optical drop filters, each having a respective optical input, a respective first optical output, and a respective second optical output; N first optical modulators optically connected between the N optical drop filters and the first wavelength multiplexer; a second wavelength multiplexer having an optical output and 2N optical inputs; and N second optical modulators optically connected to apply modulated light to N of the optical inputs of the second wavelength multiplexer; wherein the optical output of the second wavelength multiplexer is fiber-connected to the optical input of the first wavelength demultiplexer; wherein the respective optical input of each of the N optical drop filters is optically connected to a respective one of the N optical outputs of the first wavelength demultiplexer; wherein the respective first optical output of each of the N optical drop filters is to output the first WDM component (312n,1) of the respective optical signal; and wherein the respective second optical output of each of the N optical drop filters is to output the second WDM component (312n,2) of the respective optical signal and is optically connected to a respective one of the N optical inputs of the first wavelength multiplexer by way of a respective one of the N first optical modulators. a first wavelength demultiplexer having an optical input and N optical outputs, each to output a respective optical signal having a first WDM component (312n,1) and a second WDM component (312n,2); a first wavelength multiplexer having an optical output and N optical inputs, wherein the first wavelength multiplexer is configured to receive N data modulated optical signals on the N optical inputs thereof and generate a WDM signal therefrom; characterized by N optical drop filters, each having a respective optical input, a respective first optical output, and a respective second optical output; N first optical modulators optically connected between the N optical drop filters and the first wavelength multiplexer; a second wavelength multiplexer having an optical output and 2N optical inputs; and N second optical modulators optically connected to apply modulated light to N of the optical inputs of the second wavelength multiplexer; wherein the optical output of the second wavelength multiplexer is fiber-connected to the optical input of the first wavelength demultiplexer; wherein the respective optical input of each of the N optical drop filters is optically connected to a respective one of the N optical outputs of the first wavelength demultiplexer; wherein the respective first optical output of each of the N optical drop filters is to output the first WDM component (312n,1) of the respective optical signal; and wherein the respective second optical output of each of the N optical drop filters is to output the second WDM component (312n,2) of the respective optical signal and is optically connected to a respective one of the N optical inputs of the first wavelength multiplexer by way of a respective one of the N first optical modulators.