Huimin Yue

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.

A study on carrier phase distortion 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. However, the difficulty lies in the fact that the nonlinearity is generally contained in the carrier frequency due to the restrictions of system geometries. This paper investigates nonlinear carrier components introduced by the generalized imaging process in PMD. Furthermore, the analytical expression of carrier components in PMD is presented for the first time. The presented analytical form of carrier components can be extended to analyze and describe various effects of system parameters on carrier distortion. Assuming a pinhole perspective model, carrier phase distribution of arbitrary geometric arrangement is modeled as a function of spatial variables by exploring ray tracing method. As shown by simulation and experimental results, the carrier distortion is greatly affected by non-telecentric camera operation. Experimental results on the basis of reference subtraction technique further demonstrate that restrictions on reflection system geometry can be eliminated when the carrier phase is removed elaborately.

The VCNR properties of nano-structured ceria thin films

Abstract Nano-structured ceria thin films were prepared by using the sol–gel dip-coating method. The films were heated at various temperatures. A constant voltage source is used to measure current–voltage (I–V) properties. We found voltage-controlled differential negative resistance (VCNR) properties in these ceria thin films. The VCNR properties exhibit two distinct characteristics. One is that the VCNR behavior is very obvious for the negative I–V characteristics and is not symmetric about the voltage. The other is that in the negative I–V characteristics region, the current increases sharply at low voltage until it reaches a maximum, and subsequently the current decreases to a minimum but not zero, and then the current continues to rise linearly with increasing of voltage. The conduction mechanism of the above phenomena is discussed and the influence of operating temperature, dopant concentration and heating temperature on the VCNR properties of ceria thin films is analyzed.

Dynamic specular surface measurement based on color-encoded fringe reflection technique

Abstract. A color-encoded fringe reflection technique is presented for dynamic specular surface measurement. Only one color-encoded fringe pattern is required in this method. In comparison with the reported dynamic specular surface measuring method (the composite fringe pattern method), the proposed color-encoded fringe technique has higher phase accuracy. The color intensity crosstalk problem between the three channels is discussed. As a result, this problem will seldom affect the phase accuracy of the proposed method. This turns out to be the main reason why the presented method can achieve a higher measuring accuracy than the existing dynamic measurement method. In addition, the proposed color-encoded fringe technique is proven to be more suitable than the existing method for the complex tested surface. The vibrating measuring experiment of a wafer proves the ability of the proposed method to achieve dynamic measurement.

Phase error analysis and reduction in phase measuring deflectometry

Abstract. In phase measuring deflectometry (PMD), the inspection accuracy of the defects and height of the specular surface are related to the level of phase errors. The usage of numeric integration in reconstructing the shape and the defocusing capture of the fringe pattern, which will amplify the phase errors, make error discussion more significant in PMD than other shape measurement techniques. Phase error analysis and reduction in PMD are presented. The random noises, nonlinear response function, the nontelecentric imaging of the charge-coupled device camera, and the nonlinear response function of the liquid crystal display screen are the main phase error sources in PMD. The analytical relation between the random phase error and its influence factors in PMD is deduced. From the relation formulation, the influence factors of random phase error are analyzed, and the results are proven by the simulation and experiment. A possible phase error-reduction method, which integrates several methods for congeneric errors in fringe projection profilometry, is investigated to reduce phase errors in PMD. This composite method is proven to have a good performance by a plane mirror experiment.

Phase unwrapping method based on reliability and digital point array

Phase unwrapping, which usually involves complicated and time-consuming procedures, is crucial for phase-measuring profilometry. Unwrapping isolated phase regions with global phase discontinuity is challenging for traditional reliability ordering based phase unwrapping methods. This paper presents a novel method that combines the advantages of digital point array based active triangulation profilometry with a reliability of modulation ordering algorithm. Table entries that store spatial coordinates of sampling points are established, and the true phases measured by geometrical relationships are used to direct phase unwrapping. Different from the proposed 2D spatial unwrapping algorithms until now, the fringe order can be easily identified by referencing the patterns of multiple points. Additionally, unwrapping from the interior part of isolated regions following an optimized path is guaranteed. The prevention of penetration through invalid pixels into isolated regions ensures this new method will alleviate errors due to phase ambiguity. Simulation and experimental results demonstrate that the proposed technique can feasibly measure objects with a wide range of height variation.

Characterization of the phase modulation property of a free-space electro-optic modulator by interframe intensity correlation matrix

Characterization of a phase modulator or phase shifter has always been an integral part of phase-modulating or phase-adjusting applications. We propose a simplified approach to characterize a phase modulator by investigating the performance of phase shifts from grabbed interferograms using the phase extraction method. After reviewing some phase analysis techniques, the interframe intensity correlation (IIC) matrix method is introduced to the investigation. The proposed strategy is illustrated by the measurement of a free-space electro-optic modulator (EOM). Placing the modulator in one arm of a Michelson interferometer, the global phase shifts are estimated by the IIC method from the phase-stepped interferograms. Experimental results demonstrate the tested EOM has a phase modulation response of at least 2π  rad with a π/20  rad modulation precision for λ=1064  nm. In addition, our method is applicable to various types of phase modulator or phase shifter calibration, e.g., electro-optic phase modulator, spatial light modulator, or piezoelectric transducer (PZT).

Flexible global calibration technique for an arbitrarily arranged fringe projection profilometry system

Abstract. Calibration is a crucial step in fringe projection profilometry, which establishes the relationship between unwrapped phase and (FPP) three-dimensional (3-D) shape data (X,Y,h). For an arbitrarily arranged FPP system, a simple geometrical model and mathematical descriptions of the relationships among phase, height distribution, and transverse coordinate are presented. Based on this, a flexible global calibration method is presented to reconstruct 3-D shape by just using a checkerboard with known separation and alternating white and blue. The calibration board is placed at several random positions to determine the relationship between phase and height, and the relationship between pixel position and X, Y coordinates. To get high accuracy, distortion for each pixel is considered. The validity, flexibility, and practicality of this system and calibration technique are verified by experiments.

Single-shot three-dimensional shape measurement of specular surfaces by orthogonal color fringe pattern reflection technique

A convenient method based on fringe reflection technique with a single color fringe pattern is presented in this paper for dynamic measurements. A color screen and a color CCD camera are required in the system. The orthogonal color fringe pattern, which is composed with a horizontal fringe pattern in the red channel and a vertical fringe pattern in the blue channel, is displayed by the screen. The CCD camera captures the distorted color fringe pattern via the tested specular surface. The horizontal and vertical fringe patterns will be distinguished directly once the composite color fringe pattern is read by the software like MATLAB. After we get the phase of the horizontal and vertical fringe patterns by Fourier transform profilometry, the two directions’ slope distributions of the tested specular surface can be acquired by the slope-phase relation of fringe reflection technique, and the shape can be reconstructed by intergral of the slope. The whole shape measurement can be completed by a single fringe pattern. The experiment of measuring a plane mirror shows the phase error of the presented method is several times smaller than the existing method, and a vibrating wafer measuring experiment proves the ability of the proposed method to reach dynamic measurement.

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.