Possibilities of using fragments of M-sequences with a changing initial phase for correlation optical reflectometers

In modern optical reflectometers, a periodic sequence of single pulses is used as a probe signal. The resulting reflectogram is the response of the fiber path to a single pulse and is relatively easy to interpret, making it possible to obtain information on the distribution of attenuation and the position of inhomogeneities along the fiber-optic path. To separate the backscattered signal from the noise of the photodetector, it is necessary to increase the probe pulse energy by increasing the peak power and pulse duration, as well as to carry out multiple measurements and average the values of the backscattered signal at each point of the fiber-optic path. All of the above parameters, which determine the maximum dynamic range of the OTDR, have limitations. Peak power is limited by nonlinear phenomena in the optical path, pulse duration is limited by resolution requirements, and the number of backscattered signal accumulations is limited by the maximum measurement time. The paper considers the possibility of using a combination of the M-sequence fragments for probing a fiber-optic path. The recorded backscatter signal, presented by the calculation of the cross-correlation function of this signal with the probe sequence that generates it is processed. It is proposed to use such combinations of the M-sequence fragments, the total autocorrelation function of which is ideal (has the main lobe of maximum amplitude and does not have side lobes). The proposed improvement will increase the dynamic range when creating new devices.