Haijun He

Self-Mixing Demodulation for Coherent Phase-Sensitive OTDR System

Phase-sensitive optical time domain reflectometry (Ф-OTDR) attracts much attention due to its capability of telling the type and position of an intrusion simultaneously. In recent decades, coherent Ф-OTDR has been demonstrated to realize long-distance detection. For coherent Ф-OTDR, there are three typical demodulation schemes in the reported studies. However, they still cannot realize real-time monitoring to satisfy practical demands. A simple and effective demodulation method based on self-mixing has been put forward to demodulate the beat signal in coherent Ф-OTDR. It not only saves a local electrical oscillator and frequency locked loop, but also demodulates the beat signal without residual frequency. Several vibrations with different frequency were separately applied at the same location of a 42.5 km fiber. The spatial resolution of 10 m and frequency response range from 8 Hz to 980 Hz have been achieved. The precise location with signal-to-noise ratio of 21.4 dB and broadband measurement demonstrate the self-mixing scheme can demodulate the coherent Ф-OTDR signal effectively.

Precise Brillouin gain and phase spectra measurements in coherent BOTDA sensor with phase fluctuation cancellation

In order to cancel phase fluctuation induced Brillouin gain spectrum (BGS) and Brillouin phase spectrum (BPS) distortions, a new scheme with phase fluctuation cancellation (PFC) is proposed to realize precise BGS and BPS measurements in coherent BOTDA sensors. We present comprehensive and theoretical analysis about the effect of phase fluctuation on the shape of BGS and BPS, and further design a new experimental setup to fully cancel phase fluctuation induced measurement errors. In our new scheme, the two signals sent into the in-phase/quadrature (I/Q) demodulator are almost the same except for the additional amplitude amplification and phase shift induced by Brillouin gain, thus a strict phase synchronization between them has been realized (i.e., PFC). Experimental results show that the biggest BFS decoding error induced by phase fluctuation is reduced from 4.9MHz to 0.4MHz over a 40-km sensing fiber.

Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer

An optical fiber temperature and torsion sensor has been proposed by employing the Lyot-Sagnac interferometer, which was composed by inserting two sections of high-birefringence (HiBi) fiber into the Sagnac loop. The two inserted sections of HiBi fiber have different functions; while one section acts as the temperature sensitive region, the other can be used as reference fiber. The temperature and twist sensor based on the proposed interferometer structure have been experimentally demonstrated. The experimental results show that the envelope of the output spectrum will shift with the temperature evolution. The temperature sensitivity is calculated to be −17.99 nm/°C, which is enlarged over 12 times compared to that of the single Sagnac interferometer. Additionally, the fringe visibility of the spectrum will change due to the fiber twist, and the test results reveal that the fringe visibility and twist angle perfectly conform to a Sine relationship over a 360° twist angle. Consequently, simultaneous torsion and temperature measurement could be realized by detecting the envelope shift and fringe visibility of the spectrum.

Novel birefringence interrogation for Sagnac loop interferometer sensor with unlimited linear measurement range

A novel demodulation method for Sagnac loop interferometer based sensor has been proposed and demonstrated, by unwrapping the phase changes with birefringence interrogation. A temperature sensor based on Sagnac loop interferometer has been used to verify the feasibility of the proposed method. Several tests with 40 °C temperature range have been accomplished with a great linearity of 0.9996 in full range. The proposed scheme is universal for all Sagnac loop interferometer based sensors and it has unlimited linear measurable range which overwhelming the conventional demodulation method with peak/dip tracing. Furthermore, the influence of the wavelength sampling interval and wavelength span on the demodulation error has been discussed in this work. The proposed interrogation method has a great significance for Sagnac loop interferometer sensor and it might greatly enhance the availability of this type of sensors in practical application.

Coherent BOTDA Sensor With Single-Sideband Modulated Probe Light

In order to optimize the coherent Brillouin optical time domain analysis (BOTDA) sensor for more effective chromatic dispersion (CD) suppression, single-sideband intensity-modulated probe (IMP) light is utilized. Obvious Brillouin gain spectrum distortion due to CD observed in the double-sideband IMP case is successfully cancelled. Meanwhile, frequency preshift for local light generation in other coherent BOTDA sensors that have been reported recently can be avoided in our scheme, resulting in a reduction in system complexity. On the other hand, a logarithmic detector, instead of an inphase/quadrature demodulator, is applied to reduce half of the measurement time. A ~2-m spatial resolution and 0.97 °C temperature accuracy at the end of a 40-km sensing fiber are realized by using a 100-ns/80-ns differential pulsewidth pair.

Annealing properties of fiber Bragg grating UV-inscribed in boron-germanium codoped fiber

In this work, we mainly focus on the investigation of the feasibility of production of high-temperature stable fiber Bragg grating (FBG) based on reduplicative alternate annealing and hydrogen loading. The experimental results also can demonstrate the significance of the presence of hydrogen to the thermal regeneration of FBGs. The gratings are characterized and variations are compared after each stage, including UV fabrication, annealing, and reduplicative hydrogen-preloaded annealing. In different stages, the spectral and annealing responses of FBG are, respectively, investigated, as temperature increases, the Bragg wavelength consistently shifts to longer wavelengths; nevertheless, the reflection variations are distinctly discrepant. After reduplicative alternate annealing and hydrogen loading, the thermal stability is tremendously improved, and a reborn, stable grating is formed.

Precise Brillouin phase spectrum measurement in coherent BOTDA sensor with phase fluctuation cancellation

Its the first time to comprehensively and theoretically analyze the effect of the phase fluctuation on the shape of Brillouin phase spectrum, and the first time to design a new experimental setup to fully cancel phase fluctuation induced measurement errors.

Distributed vibration sensing with high frequency response based on frequency division multiplexing

A scheme combining phase-sensitive optical time domain reflectometry and Mach-Zehnder interferometer based on frequency division multiplexing has been demonstrated. The spatial resolution of 10m over 3Km sensing fiber and vibration frequency of 40 KHz are obtained.

Differential detection for coherent BOTDA sensor based on single sideband probe light

A differential detection method has been used in a coherent BOTDA system for DC component cancellation. 10.8-dB SNR enhancement of the Brillouin gain signal and 2.7-MHz Brillouin frequency shift accuracy improvement have been achieved.

SNR enhancement with bilateral filtering algorithm for phase-sensitive optical time domain reflectometry

A bilateral filtering algorithm has been proposed to process the phase-sensitive optical time domain reflectometry signal to enhance SNR. The maximum SNR enhancement of 8.3 dB has been achieved without spatial resolution loss.