A method of multiplexing an optical wavelength-division multiplex signal (601, 602) in an add path of a reconfigurable optical add/drop multiplexer, comprising the steps of - providing a plurality of first optical data signals, whose wavelengths each correspond to one of first wavelength channels (A01-A80) having channel spacings according to a predetermined first wavelength grid, - providing a plurality of second optical data signals, whose wavelengths each correspond to one of second wavelength channels (B01-B20) having channel spacings according to a predetermined second wavelength grid which is different from the first, - combining the first optical data signals into a first partial multiplex signal (610, 630) using a first passive optical element, - combining the second optical data signals into a second partial multiplex signal (620, 640) using a second passive optical element, characterized by the following step: - combining the first and second partial multiplex signals to form an optical wavelength-division multiplex signal (601, 602) using a tunable edge filter element, wherein the tunable edge filter element is designed to combine signal components of a first optical input signal above an electronically adjustable edge frequency with signal components of a second optical input signal below the edge frequency to form an optical output signal, and wherein the wavelength-division multiplex signal (601, 602) in at least one first wavelength range comprises first wavelength channels (A01- A80) with channel spacings according to the predetermined first wavelength grid and in at least one second wavelength range comprises second wavelength channels (B01-B40) with channel spacings according to the predetermined second wavelength grid which is different from the first. - providing a plurality of first optical data signals, whose wavelengths each correspond to one of first wavelength channels (A01-A80) having channel spacings according to a predetermined first wavelength grid, - providing a plurality of second optical data signals, whose wavelengths each correspond to one of second wavelength channels (B01-B20) having channel spacings according to a predetermined second wavelength grid which is different from the first, - combining the first optical data signals into a first partial multiplex signal (610, 630) using a first passive optical element, - combining the second optical data signals into a second partial multiplex signal (620, 640) using a second passive optical element, characterized by the following step: - combining the first and second partial multiplex signals to form an optical wavelength-division multiplex signal (601, 602) using a tunable edge filter element, wherein the tunable edge filter element is designed to combine signal components of a first optical input signal above an electronically adjustable edge frequency with signal components of a second optical input signal below the edge frequency to form an optical output signal, and wherein the wavelength-division multiplex signal (601, 602) in at least one first wavelength range comprises first wavelength channels (A01- A80) with channel spacings according to the predetermined first wavelength grid and in at least one second wavelength range comprises second wavelength channels (B01-B40) with channel spacings according to the predetermined second wavelength grid which is different from the first.