A multiplexer for multiplexing signals with different wavelengths from a plurality of ports to a single port, comprising N optical multiplexing modules, wherein the N optical multiplexing modules correspond to the signals with different wavelengths; and in the N optical multiplexing modules, an accumulated negative dispersion amount and an accumulated insertion loss at an M th optical multiplexing module are less than those at an (M+1) th optical multiplexing module, wherein N is a positive integer, and M is a positive integer less than or equal to N, wherein each one of the N optical multiplexing modules comprises a narrow-band filter and a reflector, wherein the reflector is a negative dispersion optical element configured to provide a negative dispersion amount; and an M th narrow-band filter of the M th optical multiplexing module is configured to: filter and receive a first signal with a corresponding wavelength; receive a second signal reflected by an (M+1) th reflector in the (M+1) th optical multiplexing module; and reflect the first signal and the second signal as a compound signal to an M th reflector of the M th optical multiplexing module, wherein the M th reflector of the M th optical multiplexing module is configured to reflect the compound signal into an (M-1) th narrow-band filter of an (M-1) th optical multiplexing module, and a 1 st reflector of a 1 st optical multiplexing module is adapted to output a corresponding compound signal reflected by a 1 st narrow-band filter of a 1 st optical multiplexing module as an output of the multiplexer. the N optical multiplexing modules correspond to the signals with different wavelengths; and in the N optical multiplexing modules, an accumulated negative dispersion amount and an accumulated insertion loss at an M th optical multiplexing module are less than those at an (M+1) th optical multiplexing module, wherein N is a positive integer, and M is a positive integer less than or equal to N, wherein each one of the N optical multiplexing modules comprises a narrow-band filter and a reflector, wherein the reflector is a negative dispersion optical element configured to provide a negative dispersion amount; and an M th narrow-band filter of the M th optical multiplexing module is configured to: filter and receive a first signal with a corresponding wavelength; receive a second signal reflected by an (M+1) th reflector in the (M+1) th optical multiplexing module; and reflect the first signal and the second signal as a compound signal to an M th reflector of the M th optical multiplexing module, wherein the M th reflector of the M th optical multiplexing module is configured to reflect the compound signal into an (M-1) th narrow-band filter of an (M-1) th optical multiplexing module, and a 1 st reflector of a 1 st optical multiplexing module is adapted to output a corresponding compound signal reflected by a 1 st narrow-band filter of a 1 st optical multiplexing module as an output of the multiplexer. filter and receive a first signal with a corresponding wavelength; receive a second signal reflected by an (M+1) th reflector in the (M+1) th optical multiplexing module; and reflect the first signal and the second signal as a compound signal to an M th reflector of the M th optical multiplexing module, wherein the M th reflector of the M th optical multiplexing module is configured to reflect the compound signal into an (M-1) th narrow-band filter of an (M-1) th optical multiplexing module, and a 1 st reflector of a 1 st optical multiplexing module is adapted to output a corresponding compound signal reflected by a 1 st narrow-band filter of a 1 st optical multiplexing module as an output of the multiplexer.