Method for detecting discontinuities in an optical channel, in particular in a fibre optic cable, - having two laser units (10, 20), each comprising a laser (11, 21) and an electroabsorption modulator (12, 22), - wherein the reference frequencies (f 1, f 2) of the two lasers (11, 21) has a predetermined reference frequency difference (δf), in particular in a range from 1 GHz to 10 GHz, - wherein the optical ports (120, 12p; 220, 22p) of the electroabsorption modulator (12, 22) are guided to the two inputs (41, 42) of a polarisation beam splitter and the output of the polarisation beam splitter is connected to the optical channel, - wherein a test pulse, in particular with a length of 1 to 10 ns, is applied simultaneously to the two electroabsorption modulators (12, 22) and, in such a way, during the time of the test pulse laser light from the lasers (11, 21) is transmitted through the respective electroabsorption modulators (12, 22), - wherein the test pulse is divided into two, in particular equally long, time periods (T 1, T 2), and during the first time period (T 1) the lasers (11, 21) are operated at their respective reference frequency (f 1, f 2), and during the second time period (T 2) the emission frequencies of the lasers are detuned relative to the reference frequencies, in particular by modification of the respective laser current, preferably by a frequency of 1 to 10 GHz, by a predetermined detuning frequency difference (Δf), - wherein the two electroabsorption modulators (12, 22) are switched to an opaque state after the end of the test pulse, wherein the electroabsorption modulators (12, 22) are illuminated from one side by the lasers (11, 22) at their respective reference frequency (f 1, f 2) and simultaneously absorb the light of the test pulse reflected via the optical channel at a discontinuity, and - wherein the two electroabsorption modulators (12, 22) make available at their optical port (120, 12p; 220, 22p) a photocurrent corresponding to the total light incident on them, and the photocurrents thus determined, optionally after filtering and/or amplification, are considered characteristic of the respective channel. - having two laser units (10, 20), each comprising a laser (11, 21) and an electroabsorption modulator (12, 22), - wherein the reference frequencies (f 1, f 2) of the two lasers (11, 21) has a predetermined reference frequency difference (δf), in particular in a range from 1 GHz to 10 GHz, - wherein the optical ports (120, 12p; 220, 22p) of the electroabsorption modulator (12, 22) are guided to the two inputs (41, 42) of a polarisation beam splitter and the output of the polarisation beam splitter is connected to the optical channel, - wherein a test pulse, in particular with a length of 1 to 10 ns, is applied simultaneously to the two electroabsorption modulators (12, 22) and, in such a way, during the time of the test pulse laser light from the lasers (11, 21) is transmitted through the respective electroabsorption modulators (12, 22), - wherein the test pulse is divided into two, in particular equally long, time periods (T 1, T 2), and during the first time period (T 1) the lasers (11, 21) are operated at their respective reference frequency (f 1, f 2), and during the second time period (T 2) the emission frequencies of the lasers are detuned relative to the reference frequencies, in particular by modification of the respective laser current, preferably by a frequency of 1 to 10 GHz, by a predetermined detuning frequency difference (Δf), - wherein the two electroabsorption modulators (12, 22) are switched to an opaque state after the end of the test pulse, wherein the electroabsorption modulators (12, 22) are illuminated from one side by the lasers (11, 22) at their respective reference frequency (f 1, f 2) and simultaneously absorb the light of the test pulse reflected via the optical channel at a discontinuity, and - wherein the two electroabsorption modulators (12, 22) make available at their optical port (120, 12p; 220, 22p) a photocurrent corresponding to the total light incident on them, and the photocurrents thus determined, optionally after filtering and/or amplification, are considered characteristic of the respective channel.