and 411 the output of tx 2 402 is the input to mux 411 and 412 the output of tx 3 403 is the input to mux 412 and 413 finally the output of tx 4 404 is the input to mux 413 and 414 the outputs of the muxes 410 414 comprises 5 channels connected to five laser diodes ld 1 ld 5 labeled 420 421 422 423 and 424 respectively each 2 u00d7 1 multiplexer 410 414 simply forwards one of its input signals to its respective laser diode ld 1 ld 5 and ignores the other input signal controlled by appropriate switching signals the four signals from the tx 1 tx 4 may be directed to four of the optical channels for example 420 423 leaving the remaining channel ld 5 424 redundant in the event of a laser failure ld 1 ld 5 the signal from one or more of the transmitters may simply be switch or shifted to be directed to a different laser diode for example if ld 4 423 fails then mux 414 can simply switch the signal output by tx 4 404 to working ld 5 424 if for example ld 2 421 fails then mux 412 can switch the output signal from tx 2 402 to ld 3 422 mux 413 can output the signal from tx 3 403 to ld 4 423 and mux 414 can switch the signal output by tx 4 404 to working redundant.