In this paper we report the development and test of a high sensitivity fiber Bragg grating (FBG) seismic sensor system for intrusion detection application. A high sensitivity FBG seismic sensor is designed and its sensitivity is above 1000pm/g. Unbalanced Michelson interferometer and Phase generated carrier(PGC) algorithm are adopted by the demodulation system. The system noise is below 10-3 pm/ √Hz and the minimum detectable seismic signal is below 1μg√Hz . The FBG seismic sensor has a maximum detection range of about 70 meters for human and about 200 meters for small wheeled vehicle.
An ultra-narrow linewidth fiber laser in a Brillouin ring cavity structure pumped with a distributed feedback fiber laser is presented. A total cavity length about 10m is constructed to satisfy the single longitudinal mode working condition. As a part of the ring cavity, a 3m-long Erbium doped fiber pumped with a 980nm laser diode provides an additional linear gain for intra-cavity Brillouin laser. After an intra-cavity polarization control, a single longitudinal mode Brillouin laser with an unique polarization is achieved and its linewidth is measured and deduced to be narrower than 100Hz. The threshold for single longitudinal mode operation is about 20mW distributed feedback fiber laser and 50mW 980nm pump. A maximum laser output power of over 15mW can be obtained for a maximum 980nm pump power of 250mW.
Phase-shifted grating based distributed feedback fiber lasers with different polarization characteristics were made with Ppolarized and S-polarized 244nm Argon ion laser separately. In this paper, the laser polarization characteristics from the phase-shifted gratings were investigated experimentally. The narrow linewidth laser emitted from phase-shifted grating in Erbium doped fiber showed different polarization and wavelength characteristics for incident laser in different polarization states and in different doses. By high resolution spectrum analysis, the laser always had two close wavelengths with a gap of several pm which were in two orthogonal polarization states and had similar powers, if the phase-shifted grating was made with a P-polarized (which is paralleled to the the axial fiber) incident laser. The laser showed evidently different characteristics when the incident laser is in S-polarization (which is perpendicular to the axial fiber). Here, the laser operated in single wavelength mode when irradiated dose was in a range. However, the laser operated in dual-wavelength mode and dual polarization states when irradiated dose exceeded a threshold. The wavelength gap increased from several pm to more than 10 pm with the increasing irradiated dose. The power ratio of the two wavelengths changed simultaneously.
In this paper we report the development and field test of a high sensitivity 3C fiber laser geophone for seismic acquisition application. When the fiber laser geophone senses the seismic signal, its output wavelength is proportionate to the seismic signal. By detecting the wavelength shifts using interferometric demodulation method, the micro-seismic signal can be detected. We are presenting field test results for the 3C fiber laser geophone array system and comparing its performance with regular exploration geophones. The detailed information of the acquired seismic signals, e.g. waveform, frequency spectrum, and wavelet are analyzed for assessing their performance. The 3C fiber laser geophone has advantages of wide bandwidth and good high-frequency response.
A two level 3-component distributed feed-back (DFB) fiber laser micro-seismic geophone array based on wavelength/space division multiplexing is developed. High resolution dynamic wavelength demodulation was realized with a coherent detection technology. The geophone array was tested in laboratory and showed that the detection capability of the weak vibration signals between 5-500 Hz was better than conventional moving-coil geophone. A cross-well test of the array was performed in a 100 m depth well in Changqing Oil Field in northwest China. The two level 3-component fiber laser micro-seismic geophone array was compared with the traditional in-well seismic geophone and showed better signal noise ratio (SNR) in the cross-well seismic signal acquisition. This 3C fiber laser micro-seismic geophone array system is promising in the cross-well seismic monitoring.
A fiber Bragg grating (FBG) based geophone is designed for low-frequency signal detection has high acceleration response of about 60 dB re pm/g in a low frequency range of 5 Hz ~60 Hz. To Guarantee normal operation in field test and practical application, an acceleration amplitude restriction is added in the mechanical design of the FBG geophone. Then a series of environmental and reliability test have been proceeded with online or offline monitoring of its working performance, including high and low temperature test, vibration test, shock test and free drop test. All the tests are planned according to National standard or Oil & Gas Industry Standard. And the experimental results indicate that our FBG geophone meet the criterion of oil and gas industry product and is capable of field application.
Compared with ordinary optical fiber, polyimide fiber has the characteristics of high temperature resistance and high strength, which has important application in the field of optical fiber sensing. The common methods of polyimide coating stripping were introduced in this paper, including high temperature stripping, chemical stripping and arc ablation. In order to meet the requirements of FBG writing technology, a method using argon ion laser ablation coating was proposed. The method can precisely control the stripping length of the coating and completely does not affect the tensile strength of the optical fiber. According to the experiment, the fabrication process of polyimide FBG is stripping-hydrogen loadingwriting. Under the same conditions, 10 FBG samples were fabricated with good uniformity of wavelength bandwidth and reflectivity. UV laser ablation of polyimide coating has been proved to be a safe, reliable and efficient method.
Ground surveillance system is a kind of intelligent monitoring equipment for detecting and tracking the ground target. This paper presents a fiber Bragg grating (FBG) acceleration sensor for ground surveillance, which has the characteristics of no power supply, anti-electromagnetic interference, easy large-scale networking, and small size. Which make it able to achieve the advantage of the ground surveillance system while avoiding the shortcoming of the electric sensing. The sensor has a double cantilever beam structure with a sensitivity of 1000 pm/g. Field experiment has been carried out on a flood beach to examine the sensor performance. The result shows that the detection distance on the walking of personnel reaches 70m, and the detection distance on the ordinary motor vehicle reaches 200m. The performance of the FBG sensor can satisfy the actual needs of the ground surveillance system.
For single longitudinal-mode lasers, the phase and frequency noises are very important parameters for their applications. The passive self-homodyne technique with an unbalanced Michelson interferometer is demonstrated to measure the phase and frequency noise characteristics. The Michelson interferometer for measurements is composed of a 3×3 optical fiber coupler and two Faraday rotator mirrors. The measurement performance is derived and discussed strictly from the transmission matrix of the coupler. The influence to the demodulation resulting from the asymmetry of the 3×3 optical fiber coupler is eliminated through the derived relationship between the differential phase fluctuation and the interferometer fringes. The technique is utilized to measure the phase and frequency noise features of a distributed feedback fiber laser. Based on the measured frequency noise spectrum, the lineshape, linewidth, and phase-error variance related to the laser coherence are able to be calculated and discussed well.
This paper describes a dense wavelength-division multiplexing scheme with low crosstalk for fiber Bragg grating (FBG) sensors based on interferometric detection. The scheme uses a 100-GHz spacing dense wavelength-division multiplexer with two-level optical filters to separate the returned wavelengths from FBG sensor array, enabling a high-resolution interferometric dynamic wavelength demodulation for each sensor with narrow spacing. An array with three gratings was demonstrated, and a dynamic wavelength resolution of ∼7×10−4 pm/√Hz was tested, with a bandwidth of 1 Hz to 1 kHz for each sensor. The FBGs have an operating range approaching 0.6 nm with a crosstalk level below −80 dB. The multiplexing scheme demonstrates the potential to address up to 50 FBG sensors through a single fiber with a 40-nm bandwidth optical source.
A narrow linewidth laser composed of an Erbium doped distributed feedback fiber laser (DFB-FL) and an all-single-mode-fiber amplification structure is proposed. The cavity of the DFB-FL is a phase-shifted grating in the Erbium doped fiber and fabricated by ultraviolet laser inscribing and phase mask dithering. A master oscillator power amplifier in all-single-mode-fiber structure is pumped by the residual pump laser from the DFB-FL. The amplified distributed feedback fiber laser with a 57dB signal-to-noise ratio has a linewidth of 2 kHz with low phase noise. The systematic tests about laser linewidth and phase noise are carried out. The results show that these characteristics are comparable to other commercial narrow linewidth laser products. As a stable narrow linewidth laser source, it is attractive and useful in applications in the distributed sensing and the interference sensing fields.
An intensity demodulated strain sensing scheme based on multiple phase-shifted fiber Bragg grating (FBG) is investigated. Wavelength shifts related to the external strain signal will result in various power change received by the photodetector. The reflected spectrum of the sensing multiple phase-shifted FBG are designed using the transfer matrix method to and fabricated by dithering phase mask method. The strain sensing scheme is analyzed numerically by relate the power change to reflectivity spectrum, thus help us optimize the design and fabrication of multiple phase-shifted FBG to achieve maximum sensitivity. The experimental results show that the strain sensitivity can be significantly enhanced by over 10dB by using phase-shifted FBG with many phase shifts compared to a normal FBG.
An optical fibre laser characteristic measuring system is presented in this paper. Measurement parameters include laser linewidth, relative intensity noise (RIN) and output power. Homodyne analysis method is used to achieve linewidth which resolution of linewidth can reach 1kHz. Quantization noise of the RIN is decreased greatly. And RIN measuring lower limit reaches -120dB/Hz. Digital Potentiometer is adopted in the detection circuit to adapt to high range of laser power.
The wrist-pulse is a kind of signals, from which a lot of physiological and pathological status of patients are deduced according to traditional Chinese medicine theories. This paper designs a new optic fiber wrist-pulse sensor that based on a group of FBGs. Sensitivity of the optic fiber wrist-pulse measurement system reaches 0.05% FS and the range reaches 50kPa. Frequency response is from 0 Hz to 5 kHz. A group of typical pulse signal is given out in the paper to compare different status of patient. It will improve quantification of pulse diagnosis greatly.
A high performance four-element DFB fiber laser hydrophone array system has been developed and tested. The system has the advantages of low noise floor (<10-6pm/√Hz @ 1 kHz) and wide bandwidth. Acoustic pressure sensitivity of DFB fiber laser hydrophone is 115±3dB re. Hz/Pa. Unbalanced Michelson interferometer and digital phase-generated carrier (PGC) demodulation are adopted to realize high resolution fiber laser wavelength shift demodulation. Four-element DFB fiber laser hydrophone array system performance is tested and its noise equivalent pressure (NEP) is below sea state zero (SS0). The theory analysis, design, development and test of the system are demonstrated in detail.
A compact high-sensitivity distributed feedback (DFB) fiber laser strain sensor with length of only 56 mm is investigated. The intrinsic performances including optical efficiency and acoustic sensing characteristics of bare DFB fiber laser are tested before packaging. Then polyurethane cylinder and spindle structures are applied for fiber laser packaging. By use of a Mach-Zehnder interferometer and a standard optical phase demodulator, the frequency response of DFB fiber laser before and after packaging is tested in a vibration liquid sound field and compared to standard PZT hydrophone. The experimental results show that the prestress on bare fiber laser affects the frequency response rather than strain sensitivity; the frequency sensitivity of spindle structure packaged DFB fiber laser hydrophone is about 113dB•re•Hz•Pa-1 at 1 kHz, which is 55 dB higher than bare fiber laser under the same prestress. It is remarkable that in a quite wide frequency range from 10 Hz to 10 kHz, it has a more flat frequency response with about ±8 dB fluctuation than that of cylinder structure packaged DFB FL.
An all-fiber wind anemometer and direction sensor is described in this paper. The wind anemometer probe is designed as
an encoder structure while the wind direction probe is designed as a fabry-perot cavity structure, which cavity length is
changed with the wind directions. This sensor can monitor wind velocity and directions more exactly and remotely. The
method has a good reproducibility, dispense with power supply, remote measuring and anti-electromagnetic interference.
A dual-π-phase shift distributed fiber laser (DFB FL) with symmetric structure is investigated as a strain sensor. The
sensing performances under both monolithic and local axial strain are discussed by use of transfer matrix method. While
the two lasing wavelengths of dual-π-phase shift DFB FL experience the same change as that of single-η-phase shift
DFB FL under monolithic strain, their wavelength interval changes with local strain. And the strain sensitivity of beat
frequency is calculated to be 70.6 MHz/με while one phase shift area is insensitive.
An all-fiber wind direction sensor that used in wind power generation is introduced in this paper. The system based on
Interferometer technology. The probe is designed as a fabry-perot cavity, which cavity length is changed with the wind directions.
The system can calculate the wind direction by interference fringe period. The results of experiment showed that this sensor can
monitor wind direction more exactly. The method is simple, sensitive with a good reproducibility.
A novel heterodyne continuous wave lidar system based on single-mode fiber (SMF) components and instruments is
reported. In order to improve the signal-to-noise ratio (SNR) of heterodyne lidar system, the four causes producing carrier
feed-through are presented, including: (1) the return loss of optical antenna; (2) the direction of fiber circulator; (3) the
extinction ratio of acousto-optic frequency shifter (AOFS); (4) close object's reflection. Then theoretical analysis and
experimental study for the methods of eliminating carrier feed-through are conducted. The results demonstrate that
carrier feed-through mainly arises from the non-ideality of optical components. By improving the traditional heterodyne
optical structure and enhancing the performance of optical components, the carrier feed-through power can be decreased
by more than 20dB.
Fiber optic sensors have become increasingly attractive for application in advanced intelligent coal mines, which consist
of extensive sensor network to monitor the structural integrity, environmental safety and production parameters. Fiber
optic based strain (mining pressure), temperature, water pressure, methane gas, seismic and ultrasound sensors can be
used to monitor the condition of the coal mine and provide information for accident prediction and early warning. We
report for the first time an all fiber optic comprehensive coal mine safety monitoring system. The system is capable of
methane gas monitoring, temperature monitoring, seismic event and mine pressure detection and water pressure
monitoring. The advantages of this fiber optic sensor system include intrinsic safety in explosive environment and multiparameter
monitoring. The technology potentially can be used to replace many discrete and incompatible monitoring
systems currently deployed in the coal mines and consequently greatly enhance coal mine safety.
The theory and experiment results of fiber optic vibration sensor based on over-coupled fused coupler (OCFC) is
described, 2 novel scheme to form single-end sensor is presented, i.e. OCFC sensor with a reflection mirror and OCFC
fiber loop sensor, the transmission spectrum of these OCFC sensor when applied with different strains is investigated and
interrogation system is developed, the output of these sensors when applied with vibration and shock is shown. We
demonstrate that the OCFC fiber loop sensor is feasible to have practical use to measure vibration signal because the
sensor is easy to be fabricated, it has good low and high frequency response and the interrogation system is simple, low
A remote multiplexed methane sensor system taking advantage of low cost 1.33 1&mgr;m SLD is demonstrated. it has long time stabilization, immunize from the influence of the temperature changing.
Reference light is adopted in the system to reduce the affection of attenuation caused by long distance transmission of light in fiber and the drift of the SLD output power. Resolution of 0.01% (10-s time
constant) is obtained by introducing intensity modulation and Lock-In-Amp.