Zhonghua Ou

A carrier removal method in phase measuring deflectometry based on the analytical carrier phase description.

In phase measuring deflectometry (PMD), a camera observes a sinusoidal fringe pattern via the surface of a specular object under test. Any slope variations of the surface lead to distortions of the observed pattern. 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. However, in the usual measurement system, the carrier phase will be nonlinear due to the restrictions of system geometries. In this paper, based on the analytical carrier phase description in PMD, a carrier removal method is proposed to remove the nonlinear carrier phase. Both the theoretical analysis and the experiment results are presented. By comparison with reference-subtraction method and series-expansion method, this proposed method can achieve carrier removal process with only the measurement of one single object, as well as high accuracy and time-saving.

Landslide monitoring based on high-resolution distributed fiber optic stress sensor

We present a landslide monitoring application using a high-resolution distributed fiber optic stress sensor. The sensor is used to monitor the intra-stress distribution and variations in landslide bodies, and can be used for the early warning of the occurrence of the landslides. The principle of distributed fiber optic stress sensing and the intra-stress monitoring method for landslides were described in detail in this paper. By measuring the distributed polarization mode coupling in the polarization-maintaining fiber, the distributed fiber stress sensor with stress measuring range 0-15 Mpa, spatial resolution 10 cm and measuring range 0.5 km, was designed. The warning system was also investigated experimentally in the field trial.

Optical fiber sensors for landslide monitoring

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.

Study of spectrum flattening of ASE fiber source based on long period fiber grating

Flattened ASE fiber sources with C+L bandwidth(1520-1620nm) are attractive. Long period fiber gratings (LPFG) have the characteristics of band-stopped, which can be used to flatten the spectrum of amplified spontaneous emission (ASE) light source. In this paper, spectrum flattening of ASE light source covering C-band and L-band based on a long period fiber grating is studied. As a flattening filter, LPFG is often placed in the end of the output port. The results in this paper show that the placed position of LPFG has great influence on the spectrum flattening of ASE light source. Output spectrum measured of filtered ASE source with the LPFG at different position is given, and the detailed theoretical analysis to explain the experimental results is followed. In the dual-stage double-pass structure, based on a long period fiber grating and two 980 nm high power semiconductor lasers, high power and ultra bandwidth ASE fiber source has been gained. The source has a bandwidth of 77 nm (1525.5 ~1602.5 nm) with ±1.6 dB ripple and 42 mw of output power.

A high precision metrology method and system for thin film's parameters based on reflectance spectrum

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.

High-accuracy inspection of defects and profile of wafers by phase measuring deflectometry

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.

Accuracy enhancement of three-dimensional surface shape measurement using curvelet transform

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.

Study of the polarization maintaining optical fiber beat length variation with external force

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.

Carrier frequency removal in phase measuring deflectometry with non-telecentric imaging

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.

Error-reduction methods for shape measurement of low reflectivity specular surfaces by temporal phase unwrapping

In phase measuring deflectometry, the measurement accuracy is relatively low with low reflectivity surface. Based on temporal phase unwrapping, two main noises are reduced to improve accuracy by least-square method and 3-frame method.