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. 2024 May 23;24(11):3338.
doi: 10.3390/s24113338.

Instability Compensation of Recording Interferometer in Phase-Sensitive OTDR

Konstantin V Stepanov  1 , Andrey A Zhirnov  1 , Tatyana V Gritsenko  1 , Roman I Khan  1 , Kirill I Koshelev  1 , Cesare Svelto  2 , Alexey B Pnev  1
Affiliations

Instability Compensation of Recording Interferometer in Phase-Sensitive OTDR

Konstantin V Stepanov et al. Sensors (Basel). .

Abstract

In the paper, a new method of phase measurement error suppression in a phase-sensitive optical time domain reflectometer is proposed and experimentally proved. The main causes of phase measurement errors are identified and considered, such as the influence of the recording interferometer instabilities and laser wavelength instability, which can cause inaccuracies in phase unwrapping. The use of a Mach-Zender interferometer made by 3 × 3 fiber couplers is proposed and tested to provide insensitivity to the recording interferometer and laser source instabilities. It is shown that using all three available photodetectors of the interferometer, instead of just one pair, achieves significantly better accuracy in the phase unwrapping. A novel compensation scheme for accurate phase measurements in a phase-sensitive optical time domain reflectometer is proposed, and a comparison of the measurement signals with or without such compensation is shown and discussed. The proposed method, using three photodetectors, allows for very good compensation of the phase measurement errors arising from common-mode noise from the interferometer and laser source, providing a significant improvement in signal detection. In addition, the method allows the tracking of slow temperature changes in the monitored fiber/object, which is not obtainable when using a simple low-pass filter for phase unwrapping error reduction, as is customary in several systems of this kind.

Keywords: acoustic monitoring; distributed fiber optic sensor; fiber optic sensor; phase-sensitive OTDR; weak fiber Bragg gratings (WFBG).

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Conflict of interest statement

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Figures

Figure 1
Figure 1
Scheme of a φ-OTDR with phase recovery (coherent): (a) Heterodyne, with frequency shift; (b) Homodyne, with baseband detection.
Figure 2
Figure 2
Scheme of a φ-OTDR with direct detection phase recovery: (a) With phase difference and time delay between pair of pulses; (b) With interference of two reflectograms shifted in time at the receiver.
Figure 3
Figure 3
The temperature gradients and the overall vibration influences on the recording interferometer: (a) Registration scheme; (b) MZI is not placed in an isolation box; (c) MZI is placed in a special cumbersome isolation box.
Figure 4
Figure 4
Scheme of a sensor based on a φ-OTDR.
Figure 5
Figure 5
Scheme (a) and algorithm (b) of the experiment.
Figure 6
Figure 6
Scheme of the interference signal formation at the PD, after passing through the unbalanced MZI: (a) ΔLMZI = 2ΔLWFBGs; (b) ΔLMZI < 2ΔLWFBGs.
Figure 7
Figure 7
Time signal at the point of perturbation (PZT-wounded fiber region) registered by the two photodiodes in the MZI forward direction and converted by ADC1.
Figure 8
Figure 8
Time signal registered by ADC2.
Figure 9
Figure 9
Unwrapped signals from ADC1 and ADC2, differences between the signal from the φ-OTDR and the signals on the photodetectors connected to ADC2.
Figure 10
Figure 10
Comparison of the signal obtained by low-pass filtering (blue trace) and the signal obtained by the proposed method (red trace).
See this image and copyright information in PMC

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