Correlation Optical Time Domain Reflectometry Method and System

A computer program stored on a storage medium or downloadable from a storage via a data communication network comprising instructions to cause an optical time domain reflectometer to execute the steps of an optical time domain reflectometry method,

• wherein the optical time domain reflectometer comprises:
• (a) an optical transmitter configured to create unipolar optical probe signals and to be coupled, via an optical coupling device, to a near end of an optical path under test in order to feed these probe signal to the optical path under test;
• (b) an optical receiver configured to receive, via the optical coupling device, unipolar optical receive signals and to create, for at least a predetermined time interval of each optical receive signal, a digital receive data signal by sampling a respective opto-electrically converted electrical receive signal and digital-analog converting sampled values thereof,
• (c) wherein the optical coupling device is configured to couple the optical probe signals received from the optical transmitter to the optical path under test and to couple at least a power portion of the optical receive signals received from the optical path under test to the optical receiver;
• and
• (d) a control and processing device which is configured control the optical transmitter in such a way that it creates the optical probe signals and to control the optical receiver in such a way that it creates the digital receive data, and
• wherein the optical time domain reflectometry method comprises the steps of:
• (e) feeding the plurality of unipolar optical probe signals to the near end of an optical path under test, wherein each optical probe signal includes a single probe bit sequence of one or more differing types of probe bit sequences and wherein each optical probe signal has a predetermined length in time greater than twice the signal propagation delay between the near end and a remote end of the optical path under test;
• (f) receiving, from the near end of the optical path, a corresponding plurality of unipolar optical receive signals, wherein each optical receive signal comprises the power portion of the respective optical probe signal reflected at at least one reflective position within the optical path or at the near or remote end of the optical path;
• (g) creating, using the optical receive signals, a corresponding plurality of digital receive data signals;
• (h) calculating at least one correlation signal by correlating the digital receive data signals with a bit sequence corresponding to a respective probe bit sequence;
• and
• (i) determining, from the at least one correlation signal, the signal propagation delay between the near end of the optical path and the at least one reflective position,
• wherein:
• (j) each optical probe signal is created in such a way that it comprises, apart from a probe signal portion that comprises the respective probe bit sequence, a fill signal portion comprising a selected one of one or more differing types of optical fill signals,
• (k) two or more of the digital receive data signals or two or more averaged digital receive data signals are added or subtracted in order to form at least one sum signal comprising a resulting probe bit sequence portion and a resulting fill signal portion, wherein the bit patterns of the respective fill signals are chosen in such a way that the at least one sum signal has a constant value at each bit position of the fill signal portion, and
• (l) the at least one correlation signal is obtained by correlating the at least one sum signal with a correlation bit sequence that corresponds to the one or more probe bit sequences comprised by the digital receive data signals that have been used to create the respective sum signal.