An optical sensor (100) comprises: a holding sleeve (11); a fixed ferrule (12) fixedly mounted in said holding sleeve (11); a movable ferrule (13) movably mounted in said holding sleeve (11), a predetermined distance existing between a first movable end of said movable ferrule (13) and a first fixed end of said fixed ferrule (12) in said holding sleeve (11); a reflection part (14) arranged at a second movable end of said movable ferrule (13) opposite to said first movable end, for reflecting light entering the movable ferrule (13); and an actuation part (15), said actuation part (15) being constructed to drive said movable ferrule (13) to move so that said first movable end moves towards said first fixed end. An optical sensor assembly and a monitoring device comprising the optical sensor (100), or another sensor (1012) can remotely detect a mechanical movement in a passive mode. A first reflector (14, 1016) is configured to provide a first reflected optical signal. The sensor (100, 1012) is connected to the first reflector and has a first position and a second position, the second position configured to attenuate the first reflected optical signal more than the first position. The sensor is configured to move between the first and second positions in response to a monitored parameter (1018).