Zhengqing Pan

Phase-sensitive OTDR system based on digital coherent detection

The digital coherent detection method is employed into the ϕ-OTDR. The heterodyne detection offers very high optical gain while the digital signal processing serves as an effective tool to rebuild the instantaneous electric field of Rayleigh scattering light by analyzing the beating signal. Both amplitude and phase signal are obtained in our experiment. PZT vibration measurement verifies that the phase difference signal well represents the external perturbation signal and also with higher SNR. The proposed newϕ-OTDR system shows a good application foreground in the area of distributed vibration measurement.

Characteristics and Explanations of Interference Fading of a

The characteristics of interference fading in a phase sensitive optical time domain reflectometer (φ-OTDR) system with a three optical frequency source are described in this paper. It is observed in the experiments that the fading appearance is quite stable over 400 different pulse scans for each of frequencies, but different for the three frequencies. A model is proposed to explain the experimental phenomena, in which the fading waveform is expressed by an integral of propagation phase factor, taking non-uniform index distributions along the sensing fiber into account, multiplied by the random Rayleigh scattering coefficient summed within the detector response period. Simulations show that the basic characteristics of experimental observations can be explained by the model.

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A phase-sensitive optical time-domain reflectometry (Φ-OTDR) with a temporally sequenced multi-frequency (TSMF) source is proposed. This technique can improve the system detection bandwidth without the sensing range decreasing. Up to 0.5 MHz detection bandwidth over 9.6 km is experimentally demonstrated as an example. To the best of our knowledge, this is the first time that such a high detection bandwidth over such a long sensing range is reported in Φ-OTDR-based distributed vibration sensing. The technical issues of TSMF Φ-OTDR are discussed in this Letter. This technique will help Φ-OTDR find new important foreground in long-haul distributed broadband-detection applications, such as structural-health monitoring and partial-discharge online monitoring of high voltage power cables.

-OTDR With a Multi-Frequency Source

Spectral characteristics of orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings (PM-FBG) are proposed and analyzed experimentally and theoretically. Different from the polarization mode coupling in PM-FBG due to side pressure, a resonant peak at the middle of two orthogonal polarization modes is found when the PM-FBG is twisted purely which is attributed to the cross coupling of polarization modes. Its intensity increases with the twisting rate. A new coupled mode equation is built to describe the pure twist polarization mode coupling, in which both the normal strain induced by strain-applied parts and the tangential strain induced by twisting are taken into consideration and expressed in a unified coordinate. The novel phenomenon and its explanation are believed to be helpful for PM-FBG applications in fiber sensor and laser technologies.

Ultra-broadband phase-sensitive optical time-domain reflectometry with a temporally sequenced multi-frequency source.

A phase-shifted double pulse method is proposed to reduce the influence of inner-pulse interference induced fading on phase demodulation of the Φ-OTDR. The feasibility of the technique is experimentally verified and the measurement resolution of the Φ-OTDR is minimized to as low as 0.1rad by using this technique. The experimental system demonstrates a distributed phase monitoring capability over 4km range with SNR of >20 and detection bandwidth of >360Hz.

Orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings

A simple sensor system for high spatial resolution distributed strain field measurement is proposed and demonstrated experimentally. The fiber loop ringdown technique combined with a linear chirped fiber Bragg grating is used to realize the high spatial resolution. A proof-of-concept distributed strain sensor with 2 mm spatial resolution is realized. The sensor network is also explored and researched experimentally. The proposed technique suggests a broad range of applications for real-time distributed physical parameter sensing, such as strain or temperature.

Interference-fading-free phase-demodulated OTDR system

A precision and broadband laser frequency swept technique is experimentally demonstrated. Using synchronous current compensation, a slave diode laser is dynamically injection-locked to a specific high-order modulation-sideband of a narrow-linewidth master laser modulated by an electro-optic modulator (EOM), whose driven radio frequency (RF) signal can be agilely, precisely controlled by a frequency synthesizer, and the high-order modulation-sideband enables multiplied sweep range and tuning rate. By using 5th order sideband injection-locking, the original tuning range of 3 GHz and tuning rate of 0.5 THz/s is multiplied by 5 times to 15 GHz and 2.5 THz/s respectively. The slave laser has a 3 dB-linewidth of 2.5 kHz which is the same to the master laser. The settling time response of a 10 MHz frequency switching is 2.5 µs. By using higher-order modulation-sideband and optimized experiment parameters, an extended sweep range and rate could be expected.

High spatial resolution distributed strain sensor based on linear chirped fiber Bragg grating and fiber loop ringdown spectroscopy

We propose and analyze a novel orientation-free pressure sensor based on a π-shifted all-single-mode-fiber (SMF) Sagnac interferometer. Compared with a conventional pressure sensor based on high-birefringence polarization-maintaining fiber, this all-SMF structure is more simple, stable, and economical. Because an initial π shift is introduced into the Sagnac loop by adjusting actively the polarization controller, the intensity response of the sensor is independent of the birefringence orientation associated with the device under test (DUT). This configuration not only eliminates the effect of an uncertain birefringence orientation in the fiber loop, it also exhibits excellent repeatability and high sensitivity, which will ease the following demodulation of the measurand. The Jones matrix method is used to analyze the orientation-free characteristic of this sensor, and some theoretical and experimental results are also given.

Precision and broadband frequency swept laser source based on high-order modulation-sideband injection-locking

We have designed a power stabilizer based on a round-trip erbium-doped fiber amplifier (EDFA) structure to suppress the low-frequency relative intensity noise (RIN) for a narrow linewidth fiber laser. The noise suppressor is analyzed theoretically and its feasibility is verified experimentally. For a short-cavity single-frequency fiber laser with this device, about 20 dB low-frequency RIN improvement is achieved (down to −120 dB Hz−1 at 10 Hz). The corresponding frequency noise is also reduced by a factor of 1.6. The proposed method is an effective solution to achieve a low-frequency low RIN laser source for highly coherent detection applications.

Orientation-free pressure sensor based on π-shifted single-mode-fiber Sagnac interferometer

A fiber ring with an extremely low reflectivity fiber Bragg grating inserted as a slow-light component is proposed and demonstrated to be used as a cavity mirror with long delay for narrowing fiber laser linewidth. Due to the recirculating propagation and resonance in the ring composed of a low split ratio coupler, the composite ring provides not only a high enough reflectivity and extremely narrow bandwidth for laser oscillation, but also an enlarged group delay. The effective reflectance and group delay of the composite ring are discussed theoretically. Its effect of laser linewidth narrowing is verified experimentally in a laser with the composite fiber ring and a high reflectivity fiber Bragg grating as cavity mirrors. Its line shape, relative intensity noise (RIN), and frequency noise (FN) are measured, showing the full-width at half-maximum linewidth of 150 Hz, RIN and FN of less than -125 dB/Hz at 500 Hz and 30 Hz/√Hz at 500 Hz, respectively.