Patent ID: 7778544

Claim:
A method for monitoring an optical transmission line in which digital optical signals are transmitted bidirectionally between a first end point of the transmission line and a second end point of the transmission line, the method including: (a) optically modulating the amplitude of a first digital optical signal to produce an amplitude-modulated digital optical signal at the first end point, and transmitting the amplitude-modulated digital optical signal over the transmission line in the direction from the first end point to the second end point, the amplitude-modulated digital optical signal carrying both the amplitude modulation provided by optically modulating the amplitude of the first digital optical signal and digital information of the first digital optical signal; (b) at the second end point, passively directing a fraction of the power of the amplitude-modulated digital optical signal onto the optical transmission line as an amplitude-modulated signal component propagating in the direction from the second end point towards the first end point, and transmitting the amplitude-modulated signal component to the first end point together with a second digital optical signal being transmitted from the second end point to the first end point; (c) detecting the amplitude-modulated signal component at the first end point in a phase-sensitive way; and (d) comparing a characteristic parameter associated with the amplitude-modulated signal component with a limit value for that parameter, the characteristic parameter associated with the amplitude-modulated signal component comprising the power level of the amplitude-modulated signal component or an optical or electrical characteristic derived from the amplitude-modulated signal component, wherein the comparison indicates an interruption of the optical transmission line when the characteristic parameter associated with the amplitude-modulated signal component is less than the limit value for that parameter.