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Chirped-pulse phase sensitive (CP-Φ) OTDR is a distributed sensing technology that allows for quantitative measurement of strain and temperature along an optical fiber by simply direct detection of the Rayleigh backscattering. Typically, chirped pulses have a linear frequency modulation covering few GHz. Backscattered traces must be amplified before detection, which introduces noise and limits the signal-to-noise ratio (SNR) and, therefore, the maximum measurable range. To increase the SNR, an optical filter is usually placed before photodetection aimed at reducing broadband optical noise caused by amplified spontaneous emission. However, narrow-band filters (e.g., 10 GHz bandwidth) are not easily compatible with multi-wavelength approaches, used to improve the long-term stability. Furthermore, in practice it is not straightforward to find narrowband optical filters that continuously match the central frequency of the laser, considering laser wavelength drifts. In this study, the influence of the optical filter bandwidth on the range in CP-ΦOTDR is theoretically investigated for two types of photodetection: direct and coherent. The results show that when using coherent detection, the SNR does not depend on the filter bandwidth. Therefore, it is possible to achieve an equivalent measurement range by using a wide optical filter (e.g., 100 GHz) as compared to that obtained when using direct detection with a narrowband filter. This finding suggests that coherent detection can be used to increase the range in CP-ΦOTDR and could be compatible with the use of multi-wavelength techniques to improve the long-term stability for applications such as civil engineering and seismology.
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