Patent ID: 7941048

Claim:
An apparatus for detecting loss of optical signal (LOS) at a repeater site of a Wavelength Division Multiplexed (WDM) optical transmission system, said apparatus comprising: a periodic filter for splitting an input WDM optical signal having a plurality of wavelength channels into two output signals, said filter having an input port for receiving the input WDM optical signal, and two output ports each directing a respective one of the output signals, said output ports exhibiting complementary periodic passbands transmission characteristics in the frequency domain; a first and a second photodiode, each one for receiving a respective one of the output signals, and for generating a respective photocurrent, said photocurrents to be used for measuring optical powers P 1 and P 2 , respectively, of the received output signals; and a LOS computing unit for receiving measured optical powers from said photodiodes, computing the ratio R=(P 1 /r)/P 2 , where r is a normalization factor, and comparing said ratio R with a predetermined threshold value V, such that If R≦V, LOS=‘YES’; else, LOS=‘NO’; wherein the predetermined threshold value V is related to common-mode rejection ratio (CMRR) of the detection apparatus as V=1+n/CMRR where n is a constant between 2 and 10.