An optical time domain reflectometry method comprising the steps of (a) feeding a plurality of unipolar optical probe signals to a 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, (b) receiving, from the near end of the optical path, a corresponding plurality of unipolar optical receive signals, wherein each optical receive signal comprises a 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, (c) creating, using the optical receive signals, a corresponding plurality of digital receive data signals, (d) 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 (e) 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, characterized in (f) that 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, wherein each type of the optical fill signals has a bit pattern having no or a sufficiently small low-frequency component below a predetermined cut-off frequency in order to avoid inacceptable signal distortions, (g) that two or more of the digital receive data signals or two or more averaged digital receive data signals, each of which is created by bitwise averaging a number of 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 over the duration of the fill signal portion, and (h) that 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. (a) feeding a plurality of unipolar optical probe signals to a 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, (b) receiving, from the near end of the optical path, a corresponding plurality of unipolar optical receive signals, wherein each optical receive signal comprises a 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, (c) creating, using the optical receive signals, a corresponding plurality of digital receive data signals, (d) 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 (e) 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, characterized in (f) that 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, wherein each type of the optical fill signals has a bit pattern having no or a sufficiently small low-frequency component below a predetermined cut-off frequency in order to avoid inacceptable signal distortions, (g) that two or more of the digital receive data signals or two or more averaged digital receive data signals, each of which is created by bitwise averaging a number of 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 over the duration of the fill signal portion, and (h) that 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.