A communication module comprising : an optical transmitter configured to emit an outbound optical signal representative of an outbound electrical signal received from a host, the outbound optical signal transmitted into an optical communication system through a module-side port of a multiplexer/demultiplexer of the optical communication system; an optical receiver configured to receive an inbound optical signal through the same module-side port of the multiplexer/demultiplexer through which the outbound optical signal is transmitted and to convert the inbound optical signal into an inbound electrical signal to provide to the host; wherein a first frequency of the outbound optical signal transmitted through the module-side port of the multiplexer/demultiplexer and a second frequency of the inbound optical signal received through the module-side port of the multiplexer/demultiplexer are offset from each other and both assigned to a common transmission peak of a transmission spectrum of the multiplexer/demultiplexer ; and a periodical filter positioned in an optical path of the outbound optical signal and in an optical path of the inbound optical signal, the periodical filter having a transmission spectrum with periodic transmission peaks, at which the optical signal passes through the periodical filter, and troughs, at which the optical signal is reflected by the periodical filter, wherein: the first frequency of the outbound optical signal is aligned to one of the transmission peaks and the second frequency of the inbound optical signal is aligned to one of the transmission troughs; or the first frequency of the outbound optical signal is aligned to one of the transmission troughs and the second frequency of the inbound optical signal is aligned to one of the transmission peaks. an optical transmitter configured to emit an outbound optical signal representative of an outbound electrical signal received from a host, the outbound optical signal transmitted into an optical communication system through a module-side port of a multiplexer/demultiplexer of the optical communication system; an optical receiver configured to receive an inbound optical signal through the same module-side port of the multiplexer/demultiplexer through which the outbound optical signal is transmitted and to convert the inbound optical signal into an inbound electrical signal to provide to the host; wherein a first frequency of the outbound optical signal transmitted through the module-side port of the multiplexer/demultiplexer and a second frequency of the inbound optical signal received through the module-side port of the multiplexer/demultiplexer are offset from each other and both assigned to a common transmission peak of a transmission spectrum of the multiplexer/demultiplexer ; and a periodical filter positioned in an optical path of the outbound optical signal and in an optical path of the inbound optical signal, the periodical filter having a transmission spectrum with periodic transmission peaks, at which the optical signal passes through the periodical filter, and troughs, at which the optical signal is reflected by the periodical filter, wherein: the first frequency of the outbound optical signal is aligned to one of the transmission peaks and the second frequency of the inbound optical signal is aligned to one of the transmission troughs; or the first frequency of the outbound optical signal is aligned to one of the transmission troughs and the second frequency of the inbound optical signal is aligned to one of the transmission peaks. the first frequency of the outbound optical signal is aligned to one of the transmission peaks and the second frequency of the inbound optical signal is aligned to one of the transmission troughs; or the first frequency of the outbound optical signal is aligned to one of the transmission troughs and the second frequency of the inbound optical signal is aligned to one of the transmission peaks. <EOS>