In coherent optical time-domain reflectometry, external modulation is used to maintain the coherence of laser probe pulses launched into optical fibers. However, the residual continuous wave (CW) component produced by modulation may considerably degrade the system sensitivity. The backscattered signal from the pulse must be dominant compared to the CW signal. We discuss the effects of the finite extinction ratio (ER) on the instrument’s sensing range. A model analyzing the impact of the CW component on the backscattered signal as a function of the ER, fiber length, and pulse widths is proposed. It is also shown that acousto-optic modulation is more suitable than electro-optic modulation in optical fiber for longer distances. The results are confirmed experimentally in a 31–km-long fiber. A 1-Hz vibration was applied at 25.5 km, and the resulting signal-to-noise ratio of ∼13 dB was measured using 75-ns pulses. This result is in agreement with the performance predicted by the model.
The utilization of an external modulation allows the coherence of laser probe pulses launched to the optical fiber to be maintained in coherent optical time domain reflectometry. However, continuous-wave (CW) component product of external modulation could considerably degrade the sensitivity of the system. In this paper, we discuss the effects of finite extinction ratio in different external modulation methods and the attempts that have been made in order to overcome these effects. A model is proposed to analyze the impact of external modulation CW component on backscattered optical signal of probe pulses from optical fiber as a function of extinction ratio, fiber length and pulse widths. External modulation can be achieved by acousto-optic modulation or electro-optic modulation. From the results obtained with the model, the advantages and drawbacks of each technology are discussed, subsequently leading to required prospective developments and refinements.
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