Showing papers by "Liang Chen published in 2011"

Recent progress in Brillouin scattering based fiber sensors.

Abstract: Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures.

High Sensitivity Distributed Vibration Sensor Based on Polarization-Maintaining Configurations of Phase-OTDR

Abstract: A system based on all-polarization-maintaining configurations of the phase-sensitive optical time-domain reflectometry (OTDR) is presented for vibration sensing. Both polarization-induced signal fading and noise are mitigated via polarization-maintaining components. The detectable frequency response is increased to ~2.25 kHz. Moreover, only a straight sensing fiber segment of ~0.13 m instead of several closed fiber loops is needed to sense the vibration event with a spatial resolution of 1 m, which makes it convenient for vibration monitoring in practical applications. The maximum distance between the sensing fiber and vibration event can be 18 cm.

Time-division multiplexing-based BOTDA over 100km sensing length

Abstract: We propose and demonstrate a high-performance and long-range Brillouin optical time-domain analysis (BOTDA) based on time-division multiplexing measurement, where a probe pulse and a pump pulse are used to perform the measurement on a selected sensing section, and the measurement of the entire sensing fiber is realized by combining the series measurements over different sections through changing the delay time between the two pulses. In experiment, a 100km sensing fiber is divided into 11 sections based on the gain-controlled principle, and spatial resolutions of 0.6m and 2m are obtained at the end of 75km and 100km, respectively.

A Single Longitudinal-Mode Tunable Fiber Ring Laser Based on Stimulated Rayleigh Scattering in a Nonuniform Optical Fiber

Abstract: We have realized a novel tunable narrow linewidth fiber laser with single-longitudinal-mode operation based on the homogenous gain broadening effect of Er-doped fiber and the stimulated Rayleigh scattering (STRS) in a nonuniform optical fiber in a ring laser configuration. A 3 dB linewidth of less than 2.0 kHz is achieved with 70 dB extinction. The minimum tunable laser bandwidth is determined by the balance of the bandwidth of tunable bandpass filter (in GHz), relaxation time and gain of the Er-doped fiber, and cavity loss. The tuning range is determined by the range of tuning filter within Er-doped fiber gain spectrum. This technique is applicable to any fiber with much higher stimulated Brillouin scattering threshold than that of STRS.

Recent Progress in Optical Fiber Sensors Based on Brillouin Scattering at University of Ottawa

Abstract: The distributed sensor is proven to be a powerful tool for civil structural and material process monitoring. Brillouin scattering in fiber can be used as point sensors or distributed sensors for measurement of temperature, strain, birefringence and vibration over centimeters (Brillouin grating length) for point sensor or the pulse length for the distributed sensor. Simultaneous strain and temperature measurement with a spatial resolution of 20 cm is demonstrated in a Panda fiber using Brillouin grating technique with the temperature accuracy and strain accuracy of 0.4 °C and 9 μɛ. This technique can also be used for distributed birefringence measurement. For Brillouin optical time domain analysis (BOTDA), we have developed a new technique to measure differential Brillouin gain instead of Brillouin gain itself. This technique allows high precision temperature and strain measurement over long sensing length with sub-meter spatial resolution: 50-cm spatial resolution for 50-km length, using return-to-zero coded optical pulses of BOTDA with the temperature resolution of 0.7 °C, which is equivalent to strain accuracy of 12 μɛ. For over 50-km sensing length, we proposed and demonstrated frequency-division-multiplexing (FDM) and time-division-multiplexing (TDM) based BOTDA technique for 75-km and 100-km sensing length without inline amplification within the sensing length. The spatial resolution of 2 m (100 km) and Brillouin frequency shift accuracy of 1.5 MHz have been obtained for TDM based BOTDA and 1-m resolution (75 km) with Brillouin frequency shift accuracy of 1 MHz using FDM based BOTDA. The civil structural health monitoring with BOTDA technique has been demonstrated.

High sensitivity optical fiber current sensor based on polarization diversity and a Faraday rotation mirror cavity.

Abstract: A novel high sensitivity optical fiber current sensor (OFCS) based on polarization diversity and a Faraday rotation mirror cavity is proposed and demonstrated. Comparing with single-channel detection in a conventional OFCS, a signal power gain of 6 dB and a signal-to-noise ratio improvement of over 30 dB have been achieved in the new scheme. The cavity amplifies magnetic field-induced nonreciprocal phase modulation, while the Faraday rotation mirrors suppress the reciprocal birefringence. A linear response is obtained for current amplitude as low as several mA at an AC frequency of 1 kHz.

Four-wave mixing analysis of Brillouin dynamic grating in a polarization-maintaining fiber: theory and experiment

Abstract: We investigate the Brillouin dynamic grating generation and detection process in polarization-maintaining fibers for the case of continuous wave operation both theoretically and experimentally. The four interacting light waves couple together through the material density variation due to stimulated Brillouin scattering. The four coupled equations describing this process are derived and solved analytically for two cases: moving fiber Bragg grating approximation and undepleted pump and probe waves approximation. We show that the conventional grating model oversimplifies the Brillouin dynamic grating generation and detection process, since it neglects the coupling between all the four waves, while the four-wave mixing model clearly demonstrates this coupling process and it is verified experimentally by measuring the reflection of the Brillouin dynamic grating. The trends of the theoretical calculation and experimental results agree well with each other confirming that the Brillouin dynamic grating generation and detection process is indeed a four-wave mixing process.

Tunable Fabry-Perot filter using hollow-core photonic bandgap fiber and micro-fiber for a narrow-linewidth laser.

Abstract: A novel tunable fiber Fabry-Perot (FP) filter is proposed and demonstrated by using a hollow-core photonic bandgap fiber (HC-PBF) and a micro-fiber. The interference cavity is a hollow core of HC-PBF. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other reflection mirror is a gold-coated end of micro-fiber that uses chemical etching process to obtain the similar diameter as the core of HC-PBF. Hence the movable mirror can be adjusted with long distance inside the hollow core of HC-PBF. Tunable FP filter is used as a mode selecting component in the reflection mode to implement stable single longitudinal mode (SLM) operation in a ring laser. With FP cavity length of 0.25 ± 0.14 mm, the wavelength of SLM laser can be tuned over 1554-1562 nm with a tuning step of 0.2-0.3 nm, a side-mode suppression ratio (SMSR) of 32-36 dB and a linewidth of 3.0-5.1 kHz. With FP cavity length of 2.37 ± 0.37 mm, the SLM laser can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm, a SMSR of 44-51 dB and a linewidth of 1.8-3.0 kHz.

Vibration monitoring with high frequency response based on coherent phase-sensitive OTDR method

Abstract: A coherent phase-sensitive OTDR system to detect distributed weak vibration is reported via heterodyne detection combined with moving averaging and moving differential signal processing. The location and high frequency response of weak vibration signal, such as pencil-break vibration which would match the characteristics of crack as a source of an acoustic emission signals, are easily monitored. Our experimental results show that spatial resolution of vibration is about 5m with 50 ns pump pulses, and the signal to noise ratio could be up to 6.5 dB. Also the frequency response could be up to 1 KHz. Both consecutive multiple events at the same location and multiple simultaneous events at different location with frequency components are identified which means our vibration testing system could be used for distributed multiple vibration events detection.

2-km-range and 2-cm-spatial-resolution Brillouin optical fiber sensor using a transient differential pulse pair

Abstract: We report a long-range and high-spatial-resolution Brillouin optical time-domain analysis based on differential pulse-width pair technique. A 2-cm spatial resolution with a 2°C temperature accuracy is realized over 2-km sensing length by using an optimized 8/8.2 ns transient pulse pair.

High performance Brillouin strain and temperature sensor based on frequency division multiplexing using nonuniform fibers over 75km fiber

Abstract: We report a high-performance 75-km Brillouin optical time-domain analysis (BOTDA) based on frequency-division multiplexing using multi-segment nonuniform fibers, realizing a spatial resolution of 1 m and an accuracy of 1°C/20Νe at the end of 75 km, and a spatial resolution of 0.5 m and an accuracy of 0.7°C/14μe; at the end of 50 km. Nonuniform fibers with different Brillouin frequency shifts reduce the effective Brillouin amplification length to one resonant Brillouin frequency segment rather than the entire sensing fiber, and thus the Brillouin interaction can be enhanced in individual segments and signal-to-noise ratio can be improved without the pump pulse depletion or excess amplification on probe pulse.

Distributed fiber beat length, birefringence and differential group delay measurement using BOTDA technique

Abstract: A novel method for distributed fiber beat length, birefringence and differential group delay (DGD) measurement based on Brillouin optical time domain analysis (BOTDA) technique is proposed. The difference of local maximum and minimum Brillouin gain is determined by scanning input state of polarization (SOP) of pulsed probe wave to recover local beat length. The average beat length, birefringence and DGD measured on 100m SMF28 at 1550nm wavelength with 1m spatial resolution is 11.8m, 7 1.4 10 − × and 0.21ps respectively. Key word: beat length, birefringence, differential group delay, polarization mode dispersion, BOTDA

Characteristics of stimulated Rayleigh scattering in optical fibers

Abstract: The linewidth and the threshold of the stimulated Rayleigh scattering (STRS) in single mode fiber (SMF-28e), large effective area fiber (LEAF) and polarization maintaining fiber (PMF) have been studied using heterodyne detection to separate the Brillouin scattering with a fiber laser for the first time to the best of our knowledge. Experimental results show that the linewidth of STRS and spontaneous Rayleigh scattering are ~9 kHz, ~10 kHz, and ~11 kHz, and ~25 kHz, ~30 kHz, and ~27 kHz for SMF-28e, LEAF and PMF, respectively. The threshold power for STRS for 2km SMF-28e, 7km LEAF, and 100m PMF are 11dBm, 4.5dBm and 16.5dBm, respectively. The measured Rayleigh gain coefficient is a 2×10-13m/W for SMF-28e. Because of the properties of narrower bandwidth and lower threshold power of STRS in fibers, some of applications, such as narrower filter, could be realized.

100-km sensing range Brillouin optical time domain analysis based on time-division multiplexing

Abstract: We propose and demonstrate a novel high-performance 100-km range Brillouin optical time-domain analysis (BOTDA) based on time-division multiplexing. To avoid the limitation of pump depletion or excess amplification in a conventional long-range BOTDA, in our scheme instead of a pulse and a CW wave but two pulses, i. e., a probe pulse and a pump pulse are used to perform the measurement. The delay between the probe and pump pulses can be changed to select the sensing section where the probe pulse interacts with the pump pulse. The measurement of the entire sensing fiber is realized by implementing the measurement for each sensing section through changing the delay between the two pulses. In experiment, a 100-km sensing fiber is divided into 11 sections based on gain-controlled principle, and a spatial resolution of 0.6 m and 2 m are obtained at the end of 75 km and 100 km, respectively.

Online monitoring of the distributed lateral displacement in large AC power generators using a high spatial resolution Brillouin optical fiber sensor

Abstract: We report for the first time, to the best of our knowledge, online monitoring of the distributed lateral displacement in large AC power generators using high spatial resolution differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). To perform the measurement of distributed lateral displacements with periods of only a few cm in large AC power generators, a 2?cm spatial resolution strain measurement is realized using the differential pulse pair of 8/8.2 ns in DPP-BOTDA, and then the lateral displacements are reconstructed according to the strain?displacement relation with the assumption of a sine shape function. Using different fiberglass ripple springs, two types of lateral displacement with periods of 3 and 3.25?cm are demonstrated, obtaining a maximum displacement of 0.43?mm with a measurement accuracy of ~ 40??m. This provides the information on the stator coil tightness through online monitoring of the distributed lateral displacement caused by the fiberglass ripple springs, and ensures safe operating conditions for large AC power generators. In addition, the large number of sensing points associated with distributed optical fiber sensors make it economically and technically practical to monitor large numbers of key components in a generator without any interference from the large magnetic and electrical fields.

A novel optical fiber current sensor using polarization diversity and a Faraday rotation mirror cavity

Abstract: A novel high sensitivity optical fiber current sensor (OFCS) based on polarization diversity (PD) and a Faraday rotation mirror cavity (FRMC) is proposed and demonstrated. The PD detection doubles the sensitivity and effectively removes the DC background compared with the single-channel detection, while the FRMC suppresses the reciprocal birefringence and amplifies non-reciprocal phase modulation caused by the current-induced magnetic field. Compared with the results obtained by single-channel detection, a signal gain of 6dB and a signal-to-noise ratio improvement of over 30dB have been achieved in the new scheme. A linear response is obtained for current amplitude as low as mA.

Tunable Fabry-Perot filter based on hollow-core photonic bandgap fiber and micro-fiber and its application

Abstract: A novel tunable Fabry-Perot (F-P) filter is reported in this paper. The interference cavity is the hollow core of hollowcore photonic bandgap fiber (HC-PBF), which supports fewer modes than the conventional cavities. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other one is a cleaved end of a section of micro-fiber, which is inserted into the hollow core of HC-PBF. The cavity length of the F-P device can be tuned by adjusting the position of the micro-fiber in the hollow core. Because of the low loss of HC-PBF, the F-P cavity length can reach several millimeters, even up to the order of centimeters, which results in its very narrow linewidth and high multiplexing capability. The experimental results show that the F-P filter has a fringe contrast of over 4 dB over a wide wavelength range, which can be used as the tunable filter in a fiber laser to obtain a very narrow linewidth.

Tunable narrow linewidth and stable frequency laser based on stimulated Rayleigh scattering in non-uniform optical fiber

Abstract: A novel tunable and stable frequency fiber laser with ~4 kilohertz linewidth based on stimulated Rayleigh scattering (STRS) in non-uniform optical fiber by using a broadband light source and a tunable filter is proposed in this paper for the first time to our knowledge without ring laser configuration. Because of the variable core size and dispersion along the fiber for non-uniform optical fiber, the threshold value of stimulated Brillouin scattering (SBS) was increased by ~7 dB; hence the phenomena of STRS can be clearly demonstrated. The frequency stability of the tunable laser only depends on the tunable filter because the frequency of Rayleigh scattering in optical fiber is not sensitive to the change of environmental conditions. Such kind of narrow tunable laser sources could find widely applications in optical communication, optical sensing, optical precision metrology, and high-resolution spectroscopy.

High performance BOTDA for long range sensing

Abstract: The recent progress in long range sensor based on Brillouin scattering has been summarized, the limitation on sensing length, spatial resolution, different approaches to improve the limitation have been discussed. Two examples on frequency division multiplexing (FDM) and time-division-multiplexing (TDM) to extend the sensing range of the distributed Brillouin sensor via BOTDA without inline amplifiers have been proposed and demonstrated. Using FDM concept we demonstrate a 75 km BOTDA with three types of 25 km fiber achieving a spatial resolution of 1.1 m and an accuracy of 1°C/20μe at the end of 75 km, and a spatial resolution of 0.5 m and an accuracy of 0.7°C/14μe at the end of 50 km. Using TDM technique, we demonstrate a 100 km sensing fiber of 0.6 m and 2 m spatial resolution at the end of 75 km, and at 100 km achieving a Brillouin frequency shift accuracy of 1.5 MHz, this is equivalent to 1.5°C temperature resolution and strain resolution of 30μe, respectively.

Refractive index and temperature sensor based on double-pass in-line Mach-Zehnder interferometer

Abstract: Different from the conventional optical fiber in-line Mach-Zehnder interferometer (MZI), we proposed and demonstrated a novel double-pass in-line fiber taper MZI as a refractive index and temperature sensor in this study. The double-pass MZI is composed of a reflecting mirror, a circulator and an in-line fiber MZI. The attenuation peaks of the double-pass MZI spectra have higher contrasts than the peaks of single-pass spectra. Using our recently proposed spectrum differential integration (SDI) analyzing method, the double-pass MZI sensors' sensitivities can be made nearly twice as large as those of the single-pass ones.

transient differential pulse pair

Abstract: We report a long-range and high-spatial-res olution Brillouin optical time-domain analysis based on differential pulse-width pair technique. A 2-cm spatial resolution with a 2°C temperature accuracy is realized over 2-km sensing length by using an optimized 8/8.2 ns transient pulse pair. Keywords-distrib uted optical fiber sensing; stmulated Brillouin scattering; differential technique; long-range; high­ spatial-resoluti on

High-axial-resolution distributed lateral displacement measurement based on differential pulse-width pair BOTDA

Abstract: We report for the first time to the best of our knowledge a distributed lateral displacement sensor through measuring high-spatial-resolution axial strain based on differential pulse-width pair Brillouin optical time-domain analysis. With the assumption of a sine-shape displacement caused by a small surface deformation, the relationship between the axial strain and the lateral displacement of a flat plate is established, based on which a distributed lateral displacement measurement is realized. In experiment, a periodic lateral displacement of a flat plate is constructed through a ripple spring by applying side pressure on them, and the axial strain measurement with a 2-cm spatial resolution is performed by using a differential pulse pair of 8/8.2 ns. Using two types of ripple springs, measurements of lateral displacements with periods of 30 and 32.5 mm are demonstrated with a maximum displacement of 0.43 mm and a minimum measurable displacement of ~40 μm.