The spatial resolution of conventional Brillouin optical time domain reflectometry (BOTDR) is intrinsically limited by the relaxation time of phonons in the fiber. To make use of the narrow pulse width for higher spatial resolution, the double-pulse probe can induce interference between two acoustic waves, making it possible to determine the Brillouin frequency shift from the oscillating Brillouin gain spectrum. However, the signal-to-noise ratio (SNR) is restricted since the pulse width is quite narrow. In this paper, we propose to combine the double-pulse technique with coding pulse to improve the SNR while maintaining high spatial resolution. The complementary return-to-zero (RZ) Golay codes are employed in the experiment, and every code element is expanded to contain the entire form of double-pulse. Thus, the interference pattern from the double-pulse interaction is guaranteed, and the decoding process can be correctly conducted. Experimental results demonstrate a significant SNR enhancement with a submeter-level spatial resolution.
Phase sensitive optical time-domain reflectometry (Φ-OTDR) based on ultra-weak fiber Bragg grating (UWFBG) array measures the dynamic strain by extracting the phase of the reflective signals. However, the performance is challenged by the trade-off between spatial resolution and sensing distance. In this paper, we propose a wavelength division multiplexed (WDM) Φ-OTDR based on a cross-arranged dual-wavelength UWFBG array to overcome this limitation. By combining the vibration positioning results of two wavelengths, high resolution and long sensing distance can be achieved at the same time.
Due to green building railway requirements, sound barrier, as an effective noise reduction measure, has been widely constructed. However, the sound barrier may fall off if its components are damaged or aging. It is critical to accurately detect the early faults of sound barrier. But existing technical solutions have low recognition rate and lack of real-time performance. To solve these problems, a sound barrier fault identification method based on phase-sensitive optical time-domain reflectometry (ϕ-OTDR) is proposed. We propose a novel method based on optimized multi-domain features for feature extraction and feature screening to describe intrinsic information of the vibration signal. A field experiment was carried out in the Hu-Hang Railway. A total of 405 sets of data were obtained. With the help of quadratic discriminant classifier and 5-fold cross-validation, the average recognition accuracy is 82.3% even under complex field environments.
A novel hybrid BOTDR and Φ-OTDR system for simultaneous static and dynamic measurements is proposed. Employing the division of both backscattering light and local light, a double coherent detection configuration is achieved to extend the sensing range and demodulate the phase quantitively. With the independent detection of Brillouin scattering light and Rayleigh scattering light, the demodulation error in BOTDR is avoided while the signal-to-noise ratio of Φ-OTDR is enhanced. Experiment for simultaneously measuring temperature and vibration is carried out, which proves the sensing range of the hybrid system is 49.5km with 20m spatial resolution. The measurement uncertainty of BOTDR is verified to be 2.821MHz, and the maximum frequency response range of Φ-OTDR is 900Hz. Experimental results indicate the proposed system is specialized in dealing with both static and dynamic situations simultaneously.
A simple and effective interference fading suppression method for Φ-OTDR using optimal peak-seeking is proposed. This method can reconstruct the vibration signal with high fidelity under the premise of using only ordinary single-mode sensing fiber without changing the structure of the traditional Φ-OTDR system. Based on the data after interference suppression, we applied different machine learning models to recognize the invasive events category. The promising results show potential applications of Φ-OTDR equipment and future implementation with machine learning algorithms.
The phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been developed rapidly as a fully fiber-optic distributed vibration sensing technology. However, the demodulation technique based on the phase term would induce a serious false alarm problem due to the signal fading effect. An effective method to suppress fading-induced false alarms in the Φ-OTDR system is proposed, which is based on the suppression mask and numerical relationship between phase and amplitudes of Rayleigh backscattering. The performance of the proposed method has been experimentally demonstrated in both laboratory environment and in-field situation test. Without any hardware addition in a traditional Φ-OTDR equipment, false alarms rate can be reduced from 4.81% to 0.15%, whereas low missing alarms rate can be achieved at the same time. In-field results show that this work provides a low-cost solution to enhance the performance for real-life engineering application of the phase-discrimination Φ-OTDR system.
Phase-sensitive optical time-domain reflectometry (Ф-OTDR) has been widely used to interrogate multi-point vibration events in the health monitoring and damage detection of civil infrastructures, mechanical processes and aerospace transportation. Due to the different lengths of the events, the spatial resolution (SR) requirements would also be different. This paper presents and demonstrates two kinds of Φ-OTDR sensing systems with multi-spatial resolution (MSR) analysis characteristics. Frequency division multiplexing (FDM) and multi-scale matching filtering schemes were introduced to provide MSR ability for the phase-discrimination and amplitude-discrimination Φ-OTDR systems respectively. The proposed methods could restore vibration events of different scales with optimum signal-to-noise ratio (SNR) in merely a single measurement while maintaining the detectable frequency range, which offered promising solutions for the performance optimization of Φ-OTDR sensing systems.
KEYWORDS: Signal to noise ratio, Filtering (signal processing), Electronic filtering, Digital signal processing, Signal processing, Reflectometry, Optical filters, Interference (communication), Wavelets, Denoising
Phase-sensitive optical time-domain reflectometry (ϕ-OTDR) has been widely used to interrogate multipoint vibration events in the health monitoring of large-scale infrastructures. Since high signal-to-noise ratio (SNR) is the core parameter for evaluating the performance of ϕ-OTDR, many researchers have presented digital signal processing (DSP) methods for SNR enhancement. However, the DSP methods using fixed parameters cannot achieve the optimum SNR for different vibration events. In addition, although transform domain analysis methods such as wavelet transform could provide multiscale observation for target events, a large amount of computation would be required simultaneously. Matched filtering is a commonly used technique to extract noisy signals in traditional wireless radar systems. Mathematical theory has indicated that matched filtering is also suitable for signal extraction in ϕ-OTDR. The optimization of system performance could be realized when the filter scale matches the length of vibration events. For different lengths of disturbance region, a multiscale matching filtering method has been proposed, which makes it possible to choose appropriate filter scale and finally obtain the optimal SNR. Experimental results have shown that the proposed multiscale matching filtering method could improve the SNR by over 6 dB even under strong noise influence and reach the lowest locating uncertainty of 0.49 m with low time consumption, compared to conventional methods.
We propose and experimentally demonstrate a technique using a composite-double-probe-pulse (CDPP) to eliminate the effect of polarization fading for phase-sensitive optical time-domain reflectometry (Φ-OTDR) based on ultra-weak FBG (UWFBG) array. The CDPP is composed of two optical pulses whose spatial interval is equal to twice the spatial interval of adjacent UWFBGs in the UWFBG array. One optical pulse is a long optical pulse, and the other optical pulse is composed of two continuous short optical pulses, whose polarization states are orthogonal to each other. The width of the short pulse is equal to half of the width of the normal pulse and their frequencies are different from the long pulse. By using such a method to perform the sensing for the UWFBG array, distributed quantitative measurement can be realized with only direct detection scheme and the influence of polarization fading in the demodulation of signal is thoroughly eliminated.
An improved data processing and analysis method is proposed to realize simultaneous monitor of multiple vibrations using polarization optical time domain reflectometry system. In our method, a differential trace of the frequency component along the fiber is got by doing subtraction of the distance traces with different number of vibrations at a certain frequency, and the vibrations vibrating at different time can be located by analyzing the response power variation of the differential trace. For multiple vibration points vibrating with the same frequency and at the same time, multiple vibration events can also be distinguished by extracting the modulated frequency component, and the frequency component is obtained at the starting or ceasing state of the vibrations because the initial phases of vibration sources are unsteady and different. With our method, a POTDR sensing system which can simultaneously monitor multiple vibration points over 3km with 10m spatial resolution is demonstrated.
KEYWORDS: Optical fibers, Sensing systems, Reflectors, Aluminum, Ferroelectric materials, Connectors, Amplitude modulation, Avalanche photodetectors, Linear filtering, Signal to noise ratio
Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in many applications for its distributed sensing ability on weak disturbance all along the sensing fiber. However, traditional Φ-OTDR cannot make quantitative measurement on the external disturbance due to the randomly distributed position and reflectivity of scatters within the optical fiber. Recently, some methods have been proposed to realize quantitative measurement of dynamic strain. In these literatures, the fiber with or without FBGs in practice was easily damaged and with difficulty of maintenance. PZT is employed to generate strain event in the fiber. There is a large gap compared with the real detecting environment, which will not reveal the full performance of the sensing system. In this paper, a distributed optical fiber sensing (DOFS) system for dynamic strain measurement based on artificial reflector is proposed and demonstrated experimentally. The fiber under test (FUT) is composed by four 20-meter long single mode optical fiber patch cords (OFPCs), which are cascaded with ferrule contactor/physical contact (FC/PC) connectors via fiber flanges. The fiber facet of FC/PC connector forms an artificial reflector. When the interval between the two reflectors is changed, the phase of the interference signal will also be changed. A symmetric 3×3 coupler with table-look-up scheme is introduced to discriminate the phase change through interference intensity. In our experiment, the center 10m section of the second OFPC is attached to the bottom of an aluminum alloy plate. An ordinary loudspeaker box was located on the top of the aluminum alloy plate. The dynamic strain generated by the loudspeaker box is transmitted from the aluminum alloy plate to the OFPC. Experimental results show that the proposed method has a good frequency response characteristic up to 3.2 kHz and a linear intensity response of R2=0.9986 while the optical probe pulse width and repetition rate were 100ns and 10 kHz respectively. Meanwhile, triangle and cosine amplitude-modulated (AM) dynamic strain applied on the fiber are successfully discriminated. The artificial reflectors based on FC/PCs were easily assembled and maintained, and the method of vibration transmission closely resembled the real circumstance than PZT. Therefore, these advantages will extend the potential of this Φ-OTDR technology in structure health monitoring.
Optical time domain reflectometry (OTDR) is one of the most successful diagnostic tools for nondestructive attenuation measurement of a fiber link. To achieve better sensitivity, spatial resolution, and avoid dead-zone in conversional OTDR, a single-photon detector has been introduced to form the photon-counting OTDR (ν-OTDR). We have proposed a ν-OTDR system using a gigahertz sinusoidally gated InGaAs/InP single-photon avalanche detector (SPAD). Benefiting from the superior performance of a sinusoidal gated SPAD on dark count probability, gating frequency, and gate duration, our ν-OTDR system has achieved a dynamic range (DR) of 33.4 dB with 1 μs probe pulse width after an equivalent measurement time of 51 s. This obtainable DR corresponds to a sensing length over 150 km. Our system has also obtained a spatial resolution of 5 cm at the end of a 5-km standard single-mode fiber. By employing a sinusoidal gating technique, we have improved the ν-OTDR spatial resolution and significantly reduced the measurement time.
Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire sensing fiber. Commonly, the sensing system is considered to be only sensitive to the phase change and capable of detecting multiple vibration events. In application, once any of the vibration events leads to a local birefringence change, the polarization evolution of the signal will be disturbed along the following fiber, which will result in the generation of polarization-related noise and the failure of identification for multipoint vibration events. We will reveal the polarization-dependence of Φ-OTDR both theoretically and experimentally. To suppress the polarization-dependence of Φ-OTDR, an orthogonal-state of polarization pulse pair method has been proposed, making the sensing system purely phase-sensitive. The experiment result has shown that maximum noise suppression ratio of 11.2 dB and mean noise suppression ratio of 4.9 dB could be achieved, which confirmed the validity of the proposed method.
Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance. However, traditional Φ-OTDR cannot realize quantitative measurement of the strain variation introduced by the external disturbance. Recently, an ultra-weak fiber Bragg grating (UWFBG) array-based Φ-OTDR system has been proposed to realize quantitative measurement. Unfortunately, the spatial resolution in this system is not satisfactory, since spatial resolution is equal to UWFBGs interval and the interval cannot be too small. We have proposed an enhanced system to realize quantitative strain measurement and high spatial resolution monitoring by combining ordinary Φ-OTDR system with the UWFBG array-based Φ-OTDR system. Experimental results showed that quantitative strain measurement at the end of a 5-km long sensing fiber could be realized with a spatial resolution of 4 m while the interval between adjacent UWFGBs was 50 m.
The quality inspection process is usually carried out after first processing of the raw materials such as cutting and milling. This is because the parts of the materials to be used are unidentified until they have been trimmed. If the quality of the material is assessed before the laser process, then the energy and efforts wasted on defected materials can be saved. We proposed a new production scheme that can achieve quantitative quality inspection prior to primitive laser cutting by means of three-dimensional (3-D) vision measurement. First, the 3-D model of the object is reconstructed by the stereo cameras, from which the spatial cutting path is derived. Second, collaborating with another rear camera, the 3-D cutting path is reprojected to both the frontal and rear views of the object and thus generates the regions-of-interest (ROIs) for surface defect analysis. An accurate visual guided laser process and reprojection-based ROI segmentation are enabled by a global-optimization–based trinocular calibration method. The prototype system was built and tested with the processing of raw duck feathers for high-quality badminton shuttle manufacture. Incorporating with a two-dimensional wavelet-decomposition–based defect analysis algorithm, both the geometrical and appearance features of the raw feathers are quantified before they are cut into small patches, which result in fully automatic feather cutting and sorting.
Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire length of sensing fiber. In our previous research, the capacity of Φ-OTDR sensing system has been enhanced with ultra-weak fiber Bragg grating (UWFBG) array for high-precision quantitative measurement on the external disturbance. Multipoint nε level dynamic strain variation has been fully captured with 2m spatial resolution. However, only 5 identical UWFBGs were used in the demonstration. For large-scale sensing network, the detectable fiber length would be limited by the signal-to-noise ratio (SNR) and the dynamic range of the reflection signal. In this paper, the performance limit of the UWFGB array enhanced Φ-OTDR sensing system has been investigated. A numerical simulation was performed to illustrate the relationships between the SNR, sensing length, spatial resolution and the UWFBG reflectivity. The simulation results have shown that when the reflectivity is relatively low, the noise is mainly contributed from Rayleigh scattering. On the other hand, the multiple reflections will dominate the overall noise characteristic with the growing of UWFBG reflectivity. The requirement of detection dynamic range was also analyzed. Simulation has shown that lower UWFBG reflectivity will lead to smaller signal dynamic range, making the selection of detector more practicable. The analysis result is quite useful for designing large-scale sensing network based on UWFGB array enhanced Φ-OTDR sensing system in practical situation.
The leakage light of an electro-optic modulator (EOM) induced by its finite extinction ratio (ER) may degrade the
performance of Brillouin optical time domain reflectometer sensing system, especially for long distance measurement. In
this letter, the configuration of a high ER probe pulse generator assisted by synchronous optical switch has been
presented. A dual pulses interferometric method was also proposed to determine the dynamic ER value (DER) of the
generated probe pulse. Contrast experiments have been performed to verify the effect of the proposed method in a
BOTDR system and the results have shown that the performance of a long distance BOTDR sensing system can be
improved observably with the proposed high ER probe pulse generator. At the end of a 48.5km sensing fiber, the
maximum uncertainty of temperature measurement has been reduced from 5.2℃ to 0.8℃ with 25m spatial resolution
after we improved the extinction ratio of probe pulse from 35dB to 65dB.
Capacitive gate transient noise has been problematic for the high-speed single photon avalanche photodiode (SPAD), especially when the operating frequency extends to the gigahertz level. We proposed an electro-optic modulator based gate transient noise suppression method for sine-wave gated InGaAs/InP SPAD. With the modulator, gate transient is up-converted to its higher-order harmonics that can be easily removed by low pass filtering. The proposed method enables online tuning of the operating rate without modification of the hardware setup. At 250 K, detection efficiency of 14.7% was obtained with 4.8×10−6 per gate dark count and 3.6% after-pulse probabilities for 1550-nm optical signal under 1-GHz gating frequency. Experimental results have shown that the performance of the detector can be maintained within a designated frequency range from 0.97 to 1.03 GHz, which is quite suitable for practical high-speed SPAD applications operated around the gigahertz level.
Stereo vision based 3D metrology technique is an effective approach for relatively large scale object’s 3D geometric detection. In this paper, we present a specified image capture system, which implements LVDS interface embedded CMOS sensor and CAN bus to ensure synchronous trigger and exposure. We made an error analysis for structured light vision measurement in large scale condition, based on which we built and tested the system prototype both indoor and outfield. The result shows that the system is very suitable for large scale metrology applications.
Chip packaging is an important process during IC manufacturing. Wire bonders are key equipments of chip packaging.
The manually type of wire bonders can be updated to automatic with a pattern recognition system (PRS). However, the
resolution of PRS is quite sensitive to the mechanism's trembling. Additional waiting time has to be taken before the
image sensor can take a shoot, which limits the wire bonder's productivity. In this paper, method for motion blurred
image restoration is presented. The method uses point spread function (PSF) learning to estimate the degradation
function and power spectrum analyze to estimate the image's signal to noise rate (SNR). Experimental results are given
to illustrate the proposed image restoration algorithm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.