Haiwen Cai

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.

\phi

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 novel fiber Bragg grating temperature sensor is proposed and experimentally demonstrated with a long-period grating as a linear response edge filter to convert wavelength into intensity-encoded information for interrogation. The sensor is embedded into an aluminum substrate with a larger coefficient of thermal expansion to enhance its temperature sensitivity. A large dynamic range of 110 degreesC and a high resolution of 0.02 degreesC are obtained in the experiments. The technique can be used for multiplexed measurements with one broadband source and one long-period grating, and therefore is low Cost

Interference-fading-free phase-demodulated OTDR system

A dissolved oxygen sensor made of plastic optical fiber as the substrate and dichlorotris (1, 10-phenanthroline) ruthenium as a fluorescence indicator is studied. Oxygen quenching characteristics of both intensity and phase were measured; the obtained characteristics showed deviation from the linear relation described by the Stern-Volmer equation. A two-layer model is proposed to explain the deviation, and main parameters can be deduced with the model.

Fiber Bragg grating temperature sensor for multiplexed measurement with high resolution

A specklegram in a multimode fiber (MMF) has successfully been used as a sensor for detecting external disturbance. Our experiments showed that the sensitivity in the sensor with a multiple longitudinal-mode laser as its source was much higher than that with a single longitudinal-mode laser. In addition, the near-field pattern observations indicated that the coupling between different transverse modes in the MMF is quite weak. Based on the experimental results, a theoretical model for the speckle formation is proposed, taking a bend-caused phase factor into consideration. It is shown in the theoretical analysis that the interferences between different longitudinal modes make a larger contribution to the specklegram signals.

Characterization of a dissolved oxygen sensor made of plastic optical fiber coated with ruthenium-incorporated solgel

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.

Specklegram in a multiple-mode fiber and its dependence on longitudinal modes of the laser source

A laser phase and frequency noise measurement method by an unbalanced Michelson interferometer composed of a 3 × 3 optical fiber coupler is proposed. The relations and differences of the power spectral density (PSD) of differential phase and frequency fluctuation, PSD of instantaneous phase and frequency fluctuation, phase noise and linewidth are derived strictly and discussed carefully. The method obtains the noise features of a narrow linewidth laser conveniently without any specific assumptions or noise models. The technique is also used to characterize the noise features of a narrow linewidth external-cavity semiconductor laser, which confirms the correction and robustness of the method.