An optical communications system has a selective block/add capability of an optical channel. An optical beam separator receives and separates an input mixed optical beam into a separated optical channel and a remainder optical beam. A block/add unit receives as an input the separated optical channel and produces as an output a controllably modified separated optical channel. The block/add unit includes a source of an add input optical signal, and a controllable mirror. The controllable mirror is controllable between a first state in which the controllable mirror does not block the path of the separated optical channel, so that the separated optical channel is the output of the block/add unit, and a second state in which the controllable mirror blocks the path of the separated optical channel, and also reflects the add input optical signal as the output of the block/add unit. An optical beam combiner receives the output of the block/add unit and combines the output of the block/add unit with the remainder optical beam.