Intensity noise limit in a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

Abstract

In the present paper we perform, for the first time, the analysis of the average intensity noise power level at the output of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source. The origin of the considered intensity noise lies in random phase fluctuations of a semiconductor laser source field. These phase fluctuations are converted to intensity noise in the process of interference of backscattered light. This intensity noise inevitably emerges in every phase-OTDR spatial channel and limits its sensitivity to external phase actions. The analysis of intensity noise in a phase-OTDR was based on the study of a fiber scattered-light interferometer (FSLI) which is treated as the constituent part of OTDR. When considered independently, FSLI has a broad intensity noise spectrum at its output; when FSLI is treated as a part of a phase-OTDR, due to aliasing effect, the wide FSLI noise spectrum is folded within the spectral band, determined by the probe pulse repetition frequency. In the analysis one of the conventional phase-OTDR schemes with rectangular dual-pulse probe signal was considered, the FSLI, which corresponds to this OTDR scheme, has two scattering fiber segments with additional time delay introduced between backscattered fields. The average intensity noise power and resulting noise spectrum at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments and by the additional time delay between the scattering segments. The average intensity noise characteristics at the output of the corresponding phase-OTDR are determined by the analogous parameters: the source coherence, the lengths of the parts constituting the dual-pulse and the time interval which separates the parts of the dual-pulse. In the paper the expression for the average noise power spectral density (NPSD) at the output of FSLI was theoretically derived and experimentally verified. Based on the found average NPSD of FSLI, a simple relation connecting the phase-OTDR parameters and the limiting level of full average intensity noise power at its output was derived. This relation was verified by experimental measurement of the average noise power at the output of phase-OTDR. The limiting noise level, considered in the paper, determines the fundamental noise floor for the phase-OTDR with given parameters of the source coherence, probe pulse length and time delay between two pulses constituting the dual-pulse.

Advances in phase-sensitive optical time-domain reflectometry

“…Regarding the phase noise in Φ-OTDR, it is generally the random change in the phase caused by the laser under the action of various noises. In 2017, A. Alekseev et al conducted an in-depth study on the influence of the phase noise of the light source on Φ-OTDR, and offered an estimate of the smallest possible average noise when the coherence and pulse width of the light source were determined, which turns out to be really helpful for choosing optoelectronic components 139 . In 2019, J. Li et al analyzed and proved the influence of laser's linewidth on phase-OTDR sensing system, suggesting a way to select the appropriate laser source for phase-OTDR system 140 .…”

A Cost-Effective Distributed Acoustic Sensor for Engineering Geology

“…It is essential that the dual-pulse in these schemes is formed by the AOM from continuous wave (CW) light; as a result, the sub-pulses are separated in time and have different phase characteristics owing to inevitable phase fluctuations of the laser source. The resulting interference signal between the backscattered fields produced by the dual-pulse parts contains an excessive noise due to phase mismatch between the sub-pulses of the dual-pulse [ 27 ].…”

“…However, in the interferometer-based DAS schemes, the sub-pulses of the dual-pulse are coherent with each other, since they are generated by the MI from a single input pulse. The time delay between sub-pulses introduced by MI is compensated in the FUT; the absence of this delay makes the DAS response less sensitive to the frequency drift of the probing light and leads to the reduction of noise caused by the laser light phase fluctuations [ 8 , 27 ]. On the contrary, in the dual-pulse DAS schemes without an interferometer, where the pulses are formed by an intensity modulator from CW laser light, the sub-pulses of the dual-pulse are less coherent as the temporal delay between the sub-pulses always exists, which leads to a dependence of the DAS response on frequency stability and a stronger dependence on the coherence properties of the source.…”

Phase Noise Characterization in Phase-Sensitive OTDR Systems

“…As the CDPO systems exhibit the additional phase information and therefore the phase noise in such systems are much alleviated as compared to that of DDPO systems [11]. Recently, [15], [16] have characterized different types of noise in coherent and direct detected phase-OTDR systems except characterizing the trace-to-trace fluctuation (TTF ) or phase M. Adeel is with the Centres of Excellence in Science and Applied Technologies (CESAT), Pakistan and The Hong Kong Polytechnic University, Hong Kong. e-mail: m.adeel@connect.polyu.hk Saeed Iqbal is with depatment of computer science at Barani Institute of Information Technology, Rawalpindi, Pakistan.…”