Apparatus and method for data tag signal recovery

In an optical communications system, it is known for system diagnosis, to label a data stream with a data tag signal comprising information such as the originating laser device of the data stream. In order to recover the data tag signal from the data stream it is known to remove the data tag signal using a sense resistor in the voltage bias path of a photodetector employed to receive the data stream. Such a method complicates accurate biasing of the photodetector. The present invention therefore provides a data tag recovery apparatus (50) comprising a photodetector (54) to receive an optical signal (120) comprising a data input signal and a data tag input signal. The photodetector (54) is coupled to an amplifier (58) having outputs (60, 62) coupled to a data output module (64) and a data tag output module (90) arranged to recover the data input signal and the data tag input signal respectively from the data path of the photodetector (54).