Frequency shift differential pulse modulation phase sensitive optical time domain reflectometer

Abstract

The invention belongs to the technical field of optical fiber sensing, and discloses an active frequency shift differential pulse modulation phase sensitive optical time domain reflectometer, which comprises a narrow linewidth laser, laser emitted by a narrow linewidth laser is divided into first detection light, second detection light and reference light, and the first detection light and the second detection light are respectively and sequentially subjected to pulse modulation, delay amplification modulation and re-amplification and then are incident to an active frequency shift annular cavity to generate two multi-order harmonic pulse trains with aligned pulse trains and unequal pulse widths; the two multi-order harmonic pulse strings are combined and then are incident to the sensing optical fiber; after the output sensing light in the sensing optical fiber and the reference light are subjected to beat frequency, a beat frequency signal is detected by the balanced photoelectric detector. According to the invention, the extinction ratio of the modulation pulse can be improved, the intensity of the backward Rayleigh scattering light signal can be effectively enhanced through the multi-frequency detection light pulse signal of frequency diversity, the interference noise of the backward Rayleigh scattering light is weakened, the signal-to-noise ratio of the system is optimized, and the ultrahigh spatial resolution is realized.

Description

technical field [0001] The invention relates to the technical field of optical fiber sensing, in particular to a phase sensitive optical time domain reflectometer with active frequency shift differential pulse modulation. Background technique [0002] As a typical representative of distributed optical fiber vibration sensing technology, phase sensitive optical time domain reflectometer (Ф-OTDR) has been widely used in perimeter security early warning, Oil and gas pipeline third-party intrusion monitoring and other fields. Spatial resolution is a key parameter of the Ф-OTDR system, usually determined by the width of the detection laser pulse. Conventional laser pulse modulation devices have physical limits. Taking the acousto-optic modulator widely used in the Ф-OTDR system as an example, only the rising edge of the laser pulse can reach 10 nanoseconds, which makes it difficult for the laser pulse width to reach the nanosecond level, which in turn makes it difficult to achi...

AI Technical Summary

Problems solved by technology

However, high spatial resolution needs to be achieved by using extremely narrow laser pulses with a small pulse width, and extremely narrow laser pulses usually lead to a decrease in the signal-to-noise ratio of the system. The main reasons are: First, due to insufficient extinction ratio, pulse modulation is required The extinction ratio is improved by optimizing the mode; the second is that due to the influence of interference fading noise, it is necessary to optimize the frequency diversity mode to suppress interference fading

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