Xinyu Fan

Phase-Noise-Compensated Optical Frequency-Domain Reflectometry

The theory of phase-noise-compensated optical frequency-domain reflectometry (PNC-OFDR), a novel type of optical frequency-domain reflectometry (OFDR) with a measurement range much longer than the laser coherence length, is described, and the signal and noise spectral densities are deduced for a discussion of signal-to-noise ratio (SNR). The analysis of PNC-OFDR shows the possibility of obtaining a high SNR by using many reference signals for phase-noise compensation. By using a ldquoconcatenately generated phaserdquo (CGP), only a single auxiliary interferometer is needed for phase-noise compensation, and other reference signals can be easily generated by performing a calculation based on signal use obtained from the single auxiliary interferometer. An experimental investigation shows the feasibility of using CGPs for PNC-OFDR by dividing the fiber under test into several sections for phase-noise compensation. Moreover, the influence of strong reflection events on Rayleigh backscattering is discussed by considering the dead zone caused by a fiber/air Fresnel reflection. It is shown theoretically that a dead zone that has no influence on the neighboring section can be achieved by using suitable parameters in an actual system.

Long Range and cm-Level Spatial Resolution Measurement Using Coherent Optical Frequency Domain Reflectometry With SSB-SC Modulator and Narrow Linewidth Fiber Laser

This paper describes coherent optical frequency domain reflectometry (C-OFDR) with cm-level spatial resolution over 5 km. We propose the use of a single sideband with a suppressed carrier (SSB-SC) modulator and a narrow linewidth fiber laser as a tunable light source for C-OFDR. The advantage of using SSB-SC modulation is clarified theoretically by comparison with a double sideband with suppressed carrier modulation. We achieved cm-level spatial resolution over a 5-km measurement range and high sensitivity with a noise level 30 dB lower than the Rayleigh backscattering level.

Long-Range Coherent OFDR With Light Source Phase Noise Compensation

Recent progress on novel long-range coherent optical frequency domain reflectometry is reviewed along with its applications to diagnosing problems with optical fiber cables. The measurement range of a conventional C-OFDR is limited to the coherence length of the laser used as the light source, since the phase noise of the laser degrades the sharpness of the beat spectrum. We have developed phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) to overcome this limitation by introducing a novel phase-noise compensation technique, and we achieved a very high-resolution measurement over the fiber link length. We describe the principle of PNC-OFDR and recent related developments, and discuss its use in diagnosing issues with fiber networks.

Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method

A novel type of optical frequency domain reflectometry with a measurement range much longer than the laser coherence length is proposed and experimentally demonstrated. To reduce the influence of laser phase noise, the measurement signal is compensated by using reference signals generated from a single auxiliary interferometer supported by a newly proposed compensation process. The compensation is accomplished numerically with a computer for each section of the delay fiber length in an auxiliary interferometer after only one data acquisition. By using the proposed technique, it is confirmed experimentally that the laser phase noise is well compensated even beyond the coherence length.

Distributed fiber-optic vibration sensing based on phase extraction from time-gated digital OFDR.

A novel distributed fiber vibration sensing technique based on phase extraction from time-gated digital optical frequency domain reflectometry (TGD-OFDR) which can achieve quantitative vibration measurement with high spatial resolution and long measurement range is proposed. A 90 degree optical hybrid is used to extract phase information. By increasing frequency sweeping speed, the influence of environmental phase disturbance on TGD-OFDR is mitigated significantly, which makes phase extraction in our new scheme more reliable than that in conventional OFDR-based method, leading to the realization of long distance quantitative vibration measurement. By using the proposed technique, a distributed vibration sensor that has a measurement range of 40 km, a spatial resolution of 3.5 m, a measurable vibration frequency up to 600 Hz, and a minimal measurable vibration acceleration of 0.08g is demonstrated.

Bandwidth-adjustable dynamic grating in erbium-doped fiber by synthesis of optical coherence function

We present an approach for bandwidth-adjustable optical filter with the dynamic grating in erbium-doped fiber (EDF). The dynamic grating is introduced by the interference of two coherent light beams counter-propagating in the pumped EDF per the phenomenon of gain saturation. The bandwidth of the grating is determined by the length of the grating, i.e., the length of the interference region. With the technique of synthesis of optical coherence function (SOCF), we localize the interference into a range at an arbitrary position along the fiber by modulating the frequency of the two interfering light beams. The length of the range is controlled by adjusting the frequency modulation parameter. In this way, the length of the dynamic grating is controlled and its reflection bandwidth then adjusted. The experimental demonstration is given.

Centimeter-level spatial resolution over 40 km realized by bandwidth-division phase-noise-compensated OFDR.

We present a bandwidth-division phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) technique, which permits a fast sweep of the optical source frequency. This method makes it possible to reduce the influence of environmental perturbation, which is the dominant factor degrading the spatial resolution of frequency-domain reflectometry at a long measurement range after compensation of the optical source phase noise. By using this approach, we realize a sub-cm spatial resolution over 40 km in a normal laboratory environment, and a 5 cm spatial resolution at 39.2 km in a field trial.

Ultrahigh resolution optical fiber strain sensor using dual Pound–Drever–Hall feedback loops

We present an ultrahigh resolution optical fiber strain sensor with a broad frequency range from quasi-static to several hundred hertz. The sensor consists of a π-phase shifted fiber Bragg grating for strain sensing and a fiber Fabry-Perot interferometer as reference. The laser carrier and sideband are locked to the reference and sensing elements, respectively, via two individual feedback loops, in which the Pound-Drever-Hall technique is employed to generate the error signals. The sampling rate is up to 500 samples/s in the demonstrational experiments, only limited by the updating rate of the frequency counter. The strain resolution exhibits a 1/f characteristic in the bandwidth of 0.01-250 Hz, and is better than 0.01 nϵ at 10 Hz with a dynamic range up to 149 dB. Compared with the traditional static strain sensors, the proposed sensor shows a great improvement in both resolution and sensing bandwidth, and can be a powerful tool for geophysical applications.

Time-gated digital optical frequency domain reflectometry with 1.6-m spatial resolution over entire 110-km range

A novel time-gated digital optical frequency domain reflectometry (TGD-OFDR) technique with high spatial resolution over long measurement range is proposed and experimentally demonstrated. To solve the contradictory between the tuning rate of lightwave frequency, which determines the spatial resolution, and the measurable distance range in traditional OFDR, our proposed scheme sweeps the frequency of probe beam only within a time window, while the local reference remains a frequency-stable continuous lightwave. The frequency-to-distance mapping is digitally realized with equivalent references in data domain. In demonstrational experiments, a 1.6-m spatial resolution is obtained over an entire 110-km long fiber link, proving that the phase noises of the laser source as well as environmental perturbations are well suppressed. Meanwhile, the dynamic range was 26 dB with an average of only 373 measurements. The proposed reflectometry provides a simple-structure and high-performance solution for the applications where both high spatial resolution and long distance range are required.

Novel strain- and temperature-sensing mechanism based on dynamic grating in polarization-maintaining erbium-doped fiber

The first experimental observation of a dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) is reported and a novel fiber-optic strain- and temperature-sensing mechanism based on the dynamic grating in PM-EDF is demonstrated experimentally. The dynamic grating is written with light beams in one primary polarization axis of the PM-EDF, and read with a light beam in the other primary polarization axis. The readout Bragg reflection wavelength of the grating differs from the writing wavelength and the wavelength difference is proportional to the birefringence between the two polarization axes. Making use of the dependence of the birefringence on strain or temperature, strain- and temperature-sensing is realized by measuring the Bragg reflection wavelength (frequency) shift. In order to detect the weak reflection from the dynamic grating, a dual-stage synchronous detection scheme is adopted in the experiment. The results show a strain-sensitivity of 1.4 MHz/μe and a temperature-sensitivity of 60 MHz/°C, respectively.