KEYWORDS: Signal detection, Signal processing, Sensing systems, Signal to noise ratio, Filtering (signal processing), Rayleigh scattering, Heterodyning
A phase-sensitive optical time-domain reflectometric (Φ-OTDR) system based on the direct detection is demonstrated. It uses amplitude difference and two-dimensional Fourier transform method to extract the vibration information with a frequency response range from 5Hz to 2.5kHz, with a frequency resolution of 1 Hz, and the longest sensing length has reaches 9.8km. Then, a vibration positioning scheme based on hybrid Gaussian model is introduced, which is able to overcome the system error caused by laser power drift and successfully reduce the positioning error in the vibration-free region. A Φ-OTDR system based on the heterodyne detection is also proposed. Using self-mixing as demodulation method and the bilateral filtering algorithm in image processing as signal filtering method, this system preserves the information of vibration while filtering the background noise and improves the signal to noise ratio. Compared with the positioning result under direct detection scheme, there is a great improvement of result under this heterodyne detection scheme, and a better frequency response as low as 3 Hz is obtained.
A one-dimensional Fourier transform of a Rayleigh backscattering traces matrix along the traces direction method has been proposed to simultaneously extract location and frequency information of vibration in the distributed vibration sensing system based on phase-sensitive optical time domain reflectometry. Meanwhile, the signal-to-noise ratio (SNR) of the proposed method also can be improved as the signals are processed in the frequency domain since in the frequency domain, noise is “slow change” compared with the vibration. Then, experiments on two-point vibrations have been done. An SNR of 9.5 dB was achieved, and the spatial resolution is also improved to 3.7 m with a 50 ns pulse width and 2.7 km long fiber owing to the improved SNR.
The demands of the less-defective and high-flatness wafers are urgent in many wafer based technologies ranging from micro-electronics to the current photovoltaic industry. As the wafer becomes thinner and larger to cope with the advances in those industries, there is an increasing possibility of the emerging of crack and warp on the wafer surface. High-accuracy inspection of defects and profile are thus necessary to ensure the reliability of device. Phase measuring deflectometry(PMD) is a fast, cost-effective and high accuracy measurement technology which has been developed in recent years. As a slope measurement technology, PMD possesses a high sensitivity. Very small slope variation will lead to a large variation of the phase. PMD is very possible to have a good performance in the wafer inspection. In this paper, the requirements of the wafer inspection in the industries are discussed, and compatibility of PMD and those requirements is analyzed. In the experimental work, PMD gets the slope information of the wafer surface directly. The curvature or height information can be acquired simply by the derivation or integral of the slope. PMD is proved to make a superior result in high-precision defect detecting and shape measurement of wafer by the analysis of experiment results.
Fringe projection profilometry (FPP) has been widely used for 3-D surface shape measurement with the features of high accuracy, non-contact and fast speed. In FPP, the phase distribution is extracted from the captured distorted fringe pattern, and the height information could subsequently be obtained by the phase-height relation. In actual measurement, the captured pattern usually contains noises, which will influence the precision of the reconstructed result. In order to increase the accuracy of measurement, noise reduction procedure to these fringe patterns is required. The existing noise reducing methods (such as Fourier transform, Wavelet transform) have certain effect. However, they will eliminate some high frequencies generated by a surface with sharp change and make the image blurring. In this paper, we use Curvelet transform to enhance the accuracy of measurement in FPP. The Curvelet transform has the ability of multiscale and multidirection analysis in image processing. It has better descriptions of edges and detailed information of images. Simulations and the experimental results show that the Curvelet transform has an excellent performance in image denoising and it has a wonderful effect on accuracy enhancement of complex surface shape measurement in FPP.
The polarization maintaining fiber has been playing an important role in the fields of optical fiber sensing, communication, and so on. The beat length is one of the main parameters of polarization maintaining fiber, and it usually represents its polarization maintaining performance. In this paper, the beat length variation of Panda fiber with external force is investigated. The simulation results indicate that the beat length variation was determined both by the external force value F and the angle θ between the external force direction and the slow axis of Panda fiber. When F is a constant, the beat length of polarization maintaining fiber is changed in sinusoidal form whose various cycle is π with the variation of θ. Meanwhile, the minimum and maximum values of beat length will be obtained when the angles are even multiple of π/2 or odd multiple of π/2, respectively. When θ is a constant, the beat length is changed in linear form with the increasing of external force value. Finally, the experimental system of beat length measurement based on Sagnac interferometer loop is illustrated, and the result shows an excellent agreement with the theoretical analysis.
In phase measuring deflectometry (PMD), the fringe pattern deformed according to slope deviation of a specular surface is digitized employing a phase-shift technique. Without height-angle ambiguity, carrier-removal process is adopted to evaluate the variation of surface slope from phase distribution when a quasi-plane is measured. This paper investigates nonlinear carrier components introduced by the generalized imaging process in PMD and the nonlinear carrier removal methods. To remove the nonlinear carrier components in PMD, the reference subtraction technique, series-expansion technique and Zernike polynomials which are normally used in fringe projection profilometry are analyzed on accuracy, processing time and experimental simplicity. What’s more, a new nonlinear carrier removal technique is proposed according to the analytical expression of carrier phase. The theoretical analysis and the experiment results show that the new technique is accurate, simple and time-saving.
We present a landslide monitoring approach using a high-resolution distributed fiber stress sensor based on polarizationsensitive
optical frequency reflectometry (P-OFDR) technology. The sensing system consists of a polarizationmaintaining
(PM) fiber and an OFDR with a high spatial resolution. The PM fiber is used as a distributed sensing
element. The OFDR is used to measure the polarization mode coupling loss in PM fiber causing by the external pressure
along the PM fiber. With the advantages of frequency domain technique and coherent detection, the sensing system can
achieve high spatial resolution, high sensitivity and large dynamic range. By monitoring the mechanical property
distribution and variations in the landslide body, the occurrence of the landslides can be predicted accurately. We
demonstrate an early landslide warning system based on polarization-sensitive distributed fiber stress sensor, which has a
spatial resolution of 5cm, dynamic range of about 70dB and theoretical measuring range of 10km. The warning system is
also investigated experimentally in the field trial.
A novel method of narrow line-width Er3+-doped fiber ring laser based on FBG F-P etalon and FBG Sagnac loop is
presented in this paper. The all-fiber single frequency and narrow line-width Er3+-doped fiber ring laser has been
designed in which two 976 nm laser diodes are used as the pump sources, the high concentration Er3+-doped fiber as the
gain medium, the fiber Faraday rotator is adopted to eliminate the spatial hole burning effect, the FBG F-P etalon and
FBG Sagnac loop filter can discriminate and select laser longitudinal modes efficiently. The experiment system using 3m
long Er3+-doped fibers is presented, when the maximum pump power of two 976nm laser diodes is 146mW, the fiber
laser exhibits 16mW threshold and stable single frequency 1550nm laser with the output powers of 45mW is acquired,
and the slope efficiency is about 34.6%. The 3dB line-width is less than 9.3 kHz, measured by the delayed selfheterodyne
method with 15km single-mode fiber, and no mode hopping is observed. The fiber laser has the advantages
of simple structure, high efficiency and high reliability and it has great potential applications in the fields of optical fiber
sensing system.
Narrow linewidth fiber lasers have become more and more necessary in field of coherent communication, fiber optic
sensor, and high resolutions optical spectrum analysis, especially long-range distributed fiber sensing system. In this
paper, a simple and stable narrow linewidth Er3+ -doped ring fiber laser is first proposed and experimentally
demonstrated. In the cavity, a passive fiber Bragg grating Fabry-Perot (FBG F-P) etalon acts as mode-selecting device to
greatly reduce the longitudinal-mode density, while a section un-pumped Er3+ -doped fiber serves as a saturable absorber
to ensure the single longitudinal-mode (SLM) operation. Meanwhile the stability of lasing frequency is further improved
by controlling the temperature of the FBG F-P, and then through varying the temperature the lasing wavelength tuning is
achieved also. Finally stable SLM laser at 1550nm with linewidth about 7.5 kHz, maximum output power 39 mW and
the corresponding slope efficiency 30% is acquired.
An optical fiber hydrogen sensor based on the measuring principle of surface plasma resonance is introduced. The
structure of the hydrogen-sensitive head which is coated with Pd-Ag alloy film on the surface of the etched optical fiber
is investigated theoretically. When hydrogen gas is absorbed into the Pd thin layer of the sensing head, the Pd hydride is
formed and then the refraction index of the etched optical fiber surface will be changed with different hydrogen gas
concentration. The surface plasma wave is stimulated by the light wave in optical fiber and the surface plasma resonance
occurs between the thin metal layer and the medium surface of hydrogen gas. The Pd-Ag alloy film thickness versus the
sensitivity of hydrogen sensing head is analyzed and optimized via the numerical method. The sensing head which is
based on surface plasma resonance is manufactured and used in the experiment system of hydrogen gas detecting, and
the experiment results demonstrate that the detecting system has high sensitivity with the hydrogen concentration in the
range of 0%-4%, the accuracy, resolution and response time are respectively 5%, 0.1% and 30s. This sensor structure can
be applied to detecting the low concentration of hydrogen gas.
How to determine the parameters of thin film precisely and rapidly becomes more and more important. In this paper, a
new method based on reflectance spectrum combining complex-shape method to determine multi parameters of thin film
simultaneously by dwindling the polyhedron is first presented. On the one hand, this new method fix out the problem of
multiple solutions well. On the other hand, for there is no dependence on interferential spectrum and transparent area, the
method can be applied to diverse sorts of reflectance spectrum with a fast speed, nice convergence, and high precision. In
the meantime, the metrology system based on this method is built up and measured experiment for standard thin film
sample is implemented also. Comparing the measured results to the standard values of the standard thin film sample, the
results show that the error degree is about ± 3%. The experimental results prove this method has a nice application
value.
In this paper, an early warning monitoring system for landslides, based on high spatial resolution distributed fiber optic
stress sensing and monitoring the intra-stress distribution and changes in landslide bodies, was first presented. The
principle of distributed fiber optic stress sensing and the monitoring method for landslides were described in detail.
Through measuring the distributed polarization mode coupling in the polarization-maintaining fiber, the distributed fiber
stress sensor with stress measuring range 0-15Mpa, 10cm spatial resolutions and measuring range 0.5km, was designed.
The warning system was investigated in detail through the field test also.
A novel method for direct optical detection of spontaneous Brillouin scattering in optical fiber by using a fiber Bragg
grating (FBG) Sagnac loop is introduced. The transmission character as an optical filter of FBG Sagnac loop is
investigated theoretically. The filter which is based on an asymmetric grating Sagnac loop is manufactured and used in
the measurement of spontaneous Brillouin scattering sensing system, and the separation of backscattered spontaneous
Brillouin from Rayleigh is achieved effectively. It is demonstrated that the fiber grating Sagnac loop filter can be applied
in the distributed sensing system based on spontaneous Brillouin scattering.
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