In many cases, line discontinuities must be recognized in optical transmission systems and lasers and/or optical amplifications switched off, so as to avoid any danger to persons. Under the term, “automatic laser shutdown (ALS)”, such measures form part of international standards.
DE 10046104.2 discloses a method and a device for recognizing line discontinuities in an optical WDM system with at least one optical amplifier on an optical transmission path, whereby a pilot signal is modulated to at least one of the optical signals to be transmitted.
However, the use of methods with a pilot signal still remains costly. Moreover, the signals can be disadvantageously influenced by experiencing a spectral broadening through modulation. In such methods, evaluation is only made after existing amplification.
From WO 99/48229 a method is known for the equivalent network in optical transmission devices, whereby apart from a working signal and a protection signal, respective check-back signals are transmitted with information about the state of seizure and evaluated at the receiving end. The check-back signals are transmitted via a monitoring channel even when the useable signal is switched off. In FIG. 6 of this publication a description is given of an arrangement for switching an amplifying station on or off, in which station a check-back signal as monitoring channel and a useable signal are separated into two paths using a demultiplexer. In the path of the check-back signal, the level is regenerated by means of an opto-electrical modulator, a regenerator and an electro-optical modulator. An amplifier with a subsequent level circuit-breaker is arranged in the path of the useable signal, said level circuit-breaker switching off the output signal on the amplifier if there is no useable signal. Here the regenerated check-back signal always continues to be transmitted at a low level. Provision is also made for decision logic modules, which check the presence or absence of an useable signal. Combined with a check-back signal, the transmission is rerouted to a transmission path that is not switched off and the laser in the interrupted line is switched off.
From US 2001/0033406 A1 a method is known for avoiding damage to the eye when a fiber-optic light guide is switched off visually, whereby, in addition to a data signal, a check-back signal is detected in a narrow-band spectral range. If the data signal has small signal-to-noise ratios or if the pump source used is switched off, there is a danger that the check-back signal is suppressed in its signal noise by amplitude limiting. Rather, comparators are used to compare the outputs between data signal and check-back signal, whereby the outputs of the check-back signal and of the data signal are chosen to be as similar as possible.