Patent ID: 7447441

Claim:
A signal transmission apparatus, comprising: a) a signal source for generating a plurality of polarized signals having amplitudes and different wavelengths; b) controllable gain-control means, including a dynamic filter for differential adjustment of the amplitudes of the polarized signals, having a signal-input port connected to receive signals from the signal source and a signal-output port for delivering amplitude-controlled signals, the controllable gain-control means having a control-input port through which a gain of the dynamic filter is adjustable for respective ones of the plurality of polarized signals; c) a length of signal transmission cable including an input port at one end and an output port at another end, the input port of the length of signal transmission cable being connected to receive signals from the signal-output port of the controllable gain-control means, the length of signal transmission cable, in operation, changing polarization of the polarized signals passing through it in accordance with the amplitudes and wavelengths of the polarized signals; d) a polarizing element having a signal-input port connected to receive signals from the output port of the length of transmission cable and a signal-output port for delivering, to a receiving device, signals conveyed from the length of transmission cable; e) a signal monitoring device for determining respective states of polarization of the plurality of polarized signals present at its signal-input port which is connected to receive signals from the signal-output port of the polarizing element, the signal monitoring device having an output port connected to deliver, to the control-input port of the controllable gain-control means, a result of a determination which, in operation, it makes for the signals it receives; and f) the signal monitoring device and the controllable gain-control means so operating together as to move the polarization states of the polarized signals, by controlling the amplitudes of the polarized signals as they pass through the controllable gain-control means, towards alignment with a low-loss axis of the polarizing element.