Showing papers by "Liang Chen published in 2014"

Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry

Abstract: A novel approach to realize long-range distributed temperature and strain measurement with high spatial resolution, as well as high temperature and strain resolution, is proposed based on optical frequency-domain reflectometry (OFDR). To maintain the high measurement resolution and accuracy while increasing the sensing length, an optimized nonlinearity compensation algorithm is implemented to ensure a large wavelength tuning range. The compensated OFDR trace exhibits improved sensing resolution at a short distance, and the spatial resolution gradually deteriorates at the far end due to accumulated phase noise induced by fast tuning of the laser wavelength. We demonstrated the spatial resolution of 0.3 mm over a single-mode fiber sensing length of over 300 m, and temperature and strain resolution of 0.7 $^{\circ}\hbox{C}$ and 2.3 $\mu\varepsilon$ with spatial resolution of up to 7 cm, respectively.

Sub-MHz ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings

Abstract: We propose and demonstrate an ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings (BDGs). Taking advantage of creating a long grating in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a long single-mode fiber (SMF), and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser. The BDG-based optical spectrometry features ultrahigh resolution, large wavelength coverage, and a simple direction-detection scheme. In the experiment, a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m SMF, and the wavelength coverage can be readily extended to C+L bands with a commercial tunable laser.

Characterization of evolution of mode coupling in a graded-index polymer optical fiber by using Brillouin optical time-domain analysis

Abstract: A narrow bandwidth (2GHz) π-phase-shift flattop fiber Bragg grating (FBG) is proposed to achieve Brillouin optical time-domain analysis (BOTDA) for perfluorinated graded-index polymer optical fibers (GI-POFs) for the first time to best of our knowledge. Using the technique of BOTDA, we explore the evolution of mode coupling in perfluorinated GI-POFs by analyzing the Brillouin frequency shift (BFS) variation along the whole fiber, and compare them with that of silica graded index multimode fibers (GI-MMFs). The characteristics of mode coupling of GI-POFs and GI-MMFs were also investigated in terms of the speckle patterns at the output face of the two fibers. The results show that compared with silica GI-MMFs, GI-POFs exhibit more efficient mode coupling and the excellent ablility of mode scrambling regardless of alignment conditions.

Suppression of thermal frequency noise in erbium-doped fiber random lasers

Abstract: Frequency and intensity noise are characterized for erbium-doped fiber (EDF) random lasers based on Rayleigh distributed feedback mechanism. We propose a theoretical model for the frequency noise of such random lasers using the property of random phase modulations from multiple scattering points in ultralong fibers. We find that the Rayleigh feedback suppresses the noise at higher frequencies by introducing a Lorentzian envelope over the thermal frequency noise of a long fiber cavity. The theoretical model and measured frequency noise agree quantitatively with two fitting parameters. The random laser exhibits a noise level of 6 Hz²/Hz at 2 kHz, which is lower than what is found in conventional narrow-linewidth EDF fiber lasers and nonplanar ring laser oscillators (NPROs) by a factor of 166 and 2, respectively. The frequency noise has a minimum value for an optimum length of the Rayleigh scattering fiber.

Ultranarrow Linewidth Brillouin Fiber Laser

Abstract: Brillouin fiber laser with ultranarrow linewidth of 30 Hz is demonstrated for the first time without phase locking loop, the laser operates single longitudinal mode, while mode selection is achieved by a Fabry-Perot interferometer and a section of coreless fiber with Farday rotation mirror through optical phase conjugation, the Brillouin gain is provided by a 25-km fiber. The output linewidth is a factor of 100 reductions comparing with pump laser. Moreover, the laser exhibits an ultralow-frequency noise (<;15 Hz/Hz 1/2 at 10 Hz). The proposed laser configuration provides an ideal tool to achieve linewidth reduction of any laser from kilohertz level to subhundred hertz with lower frequency noise.

Characterization of high nonlinearity in Brillouin amplification in optical fibers with applications in fiber sensing and photonic logic

Abstract: A highly accurate, fully analytic solution for the continuous wave and the probe wave in Brillouin amplification, in lossless optical fibers, is given It is experimentally confirmed that the reported analytic solution can account for spectral distortion and pump depletion in the parameter space that is relevant to Brillouin fiber sensor applications, as well as applications in photonic logic The analytic solutions are valid characterizations of Brillouin amplification in both the low and high nonlinearity regime, for short fiber lengths

Narrow line-width laser characterization based on bi-directional pumped brillouin random fiber laser

Abstract: A method and apparatus for launching optical pump signals into a disordered gain medium at two points thereby producing a narrow line-width random fiber laser within the disordered gain medium. The optical pump signals are propagated in a direction through the disordered gain medium towards one another. The apparatus may comprise an optical gain fiber having a first end configured to receive a first fiber pump signal and a second end configured to receive a second fiber pump signal such that the first fiber pump signal and the second fiber pump signal propagate within the optical gain fiber in a direction towards one another. An optical loss device is coupled to the optical gain fiber, wherein the optical loss device isolates a narrow line-width random fiber laser signal from a signal generated within the optical gain fiber.

Effects of polarization on stimulated Brillouin scattering in a birefringent optical fiber

Abstract: The most general model of elliptical birefringence in an optical fiber has been developed for a steady-state and transient stimulated Brillouin scattering interaction. The impact of the elliptical birefringence is to induce a Brillouin frequency shift and distort the Brillouin spectrum—which varies with different light polarizations and pulsewidths. The model investigates the effects of birefringence and the corresponding evolution of spectral distortion effects along the fiber, providing a valuable prediction tool for distributed sensing applications.

Frequency stabilized coherent brillouin fiber laser

Abstract: A high-finesse Fabry-Perot interferometer (FPI) is introduced into a coherent Brillouin RFL configuration to thereby produce a frequency stabilized random laser

OTDR and OFDR for distributed multi-parameter sensing

Abstract: The drive for high spatial resolution (millimeters) distributed fiber sensors has renewed the interest in optical frequency domain reflectrometry (OFDR) systems. Because millimeters equivalent spatial resolution in optical time domain reflectrometry (OTDR) systems would require a data acquisition card with a bandwidth of 10 GHz and a sampling rate of tens of G Samples/s, such a digitizer or data acquisition card plus the pulse generator and detection system will make a distributed sensors very expensive, while a tunable laser with a wide tuning range can provide millimeters resolution with short sensing range (<100m). We developed a high precision temperature (0.1°C) and strain (1μ strain) resolution and 2.5mm spatial resolution over 180m range by auto and cross-correlation of OFDR in PMF. The dual modes of PMF allow the discrimination of the temperature and strain with distinct dependency. The application of this sensor for internal crack detection of concrete beam has been demonstrated. For distributed dynamic measurement, the upper frequency is limited by the repetition rate of the laser pulse in sensing fiber; in addition the weak Rayleigh scattering signal demands many averaging to improve SNR. The continuous wavelet transform approach has been introduced in phase OTDR sensor system to suppress random noise, and multiple vibration disturbances have been measured simultaneously for power generator monitoring. For the high frequency vibration detection, the coherent detection combined with polarization diversity scheme is implemented.

Frequency stabilized coherent brillouin random fiber laser

Abstract: A high-finesse Fabry-Perot interferometer (FPI) is introduced into a coherent Brillouin RFL configuration to thereby produce a frequency stabilized random laser.

Tapered polarization-maintaining fiber sensor based on analysis of polarization evolution

Abstract: We proposed and experimentally demonstrated a novel tapered polarization-maintaining fiber (PMF) sensor based on analysis of polarization evolution. The unique structure of the tapered PMF demonstrates extraordinary polarization characteristics under external disturbance such as twists and magnetic fields, exhibiting great advantages of compactness, high sensitivity, and versatility.

High-resolution high-sensitivity and truly distributed optical frequency domain reflectometry for structural crack detection

Abstract: Optical Frequency Domain Reflectometry (OFDR) with the use of polarization maintaining fiber (PMF) is capable of distinguishing strain and temperature, which is critical for successful field applications such as structural health monitoring (SHM) and smart material. Location-dependent measurement sensitivities along PMF are compensated by cross- and auto-correlations measurements of the spectra form a distributed parameter matrix. Simultaneous temperature and strain measurement accuracy of 1μstrain and 0.1°C is achieved with 2.5mm spatial resolution in over 180m range.

Investigation of combined Brillouin gain and loss in a birefringent fiber with applications in sensing

Abstract: The most general model of elliptical birefringence in an optical fiber is extended to describe a transient Brillouin interaction including both gain and loss. The effects of elliptical birefringence cause a Brillouin spectral shape distortion, which is detrimental for fiber sensing techniques. The model investigates the effects of birefringence and the corresponding evolution of spectral distortion effects along the fiber, and also investigates regimes where this distortion is minimal.

Ultrahigh resolution optical spectrometry based on Brillouin dynamic grating

Abstract: We demonstrate an ultrahigh resolution optical spectrometry based on Brillouin dynamic grating (BDG). Taking advantage of creating a long grating in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a long single-mode fiber (SMF), and the optical spectrometry is performed by sweeping the center wavelength of the filter through swept-tuned laser. In experiment, a 4-fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400-m SMF, and the wavelength coverage can be readily extended to C+L band with a commercial tunable laser.