Surface profilometry with laser-diode optical feedback interferometer outside optical benches

A simple and robust interferometer with a laser diode subject to optical feedback from the interferometer is presented for surface testing of a spherical mirror. The fringe phase can be locked by the optical feedback within less than 0.2pi (peak-to-valley value) even when the interferometer is placed on a wooden table. The fringe locking is caused by the change of lasing wavelength that suppresses the net phase change to be much less than 2pi. The locked fringe pattern with spatial carriers can be analyzed by a fringe analyzer at a video rate, and the measurement results of the spherical mirror showed the same result as on an optical bench.     

Three-dimensional phase step profilometry with a multicore optical fiber

This paper demonstrates the feasibility of using phase stepping and a multicore optical fiber to calculate an object's depth profile. An interference pattern is projected by an optical fiber onto the object. The distorted interference pattern containing the object information is captured by a CCD camera and processed using a phase step interferometry method. The phase step method is less computationally intensive compared to two-dimensional Fourier transform profilometry and provides more accuracy when measuring objects of high frequency spatial variations.     

High precision dynamic multi-interface profilometry with optical coherence tomography

Optical coherence tomography (OCT) has mostly been used for high-speed volume imaging but its profilometry potentials have not been fully exploited. This paper demonstrates high precision (as good as ~50 nm) multi-interface profilometry using a Fourier domain OCT system without special antivibration devices. The precision is up to 2 orders of magnitude better than the depth resolution of the OCT. Detailed analysis of the precision achieved for different surfaces is presented. The multi-interface profiles are obtained as a by-product of the tomography data. OCT has the advantage in speed and sensitivity at detecting rough and internal interfaces versus conventional optical profilometry. An application of the technique to the dynamic monitoring of varnish drying on paintlike substrates is demonstrated, which provides a better understanding of the formation of surface roughness. The technique has potential benefits in the fields of art conservation, coatings technology, and soft matter physics.     

Adaptive whole-field optical profilometry: a study of thesystematic errors

An analysis of the sources of systematic errors in an optical whole-field profilometer based on the projection of fringes is presented here. In the system, the determination of the object profile is performed by triangulation. Both the period of the fringes and the geometrical system parameters on which triangulation operates (i.e. the distance between the object and the acquisition/projection units, and the distance between the acquisition unit and the projection unit) represent the input parameters of the profile evaluation algorithm. The influence on the height error introduced by an inaccuracy in the determination of the projected fringe period, as well as of the geometrical parameters of the system, is investigated here. The distortion in reconstructing the object shape due to the finite distance illumination scheme used in the system is also studied. The results obtained from the analysis are used to increase the accuracy at the optical profilometer by means of a suitably developed correction algorithm     

Unified calibration technique and its applications in optical triangular profilometry

A unified calibration technique based on ray tracing for optical triangular profilometry is presented. The proposed technique based on the inherent geometric relation between depth and a distorted signal is capable of speedy and accurate measurement without the determination of geometric parameters. The technique can also reduce calibration error caused by the lens distortion of the projector and the camera owing to the reasonable assumption that mapping in a small local area is a linear transformation and the coefficients of the linear transformation may be varied in different local areas. Three classical systems of triangular profilometry, spot inspection, a single-line system, and a projection-grating system, are discussed and demonstrated by experiment.     

Effect of a change in phase of the reflected wave on measurements of the shape of a surface in optical profilometry

We examine the effect of a change in phase of the reflected electromagnetic wave when measuring a surface profile using a scanning white-light interference microscope and a phase-shifting coherent light interference microscope. We present the results of experiments measuring the thickness of an aluminum coating deposited on a silicon wafer, without any phase shift correction and with phase shift correction, calculated from handbook data and from the results of ellipsometric measurements. We compare with the results of measurements on the same object on an atomic force microscope.     

高速光截法三维面形重建

在高速光截法三维面形测量中,由于光截面之间距离增加,物体表面的不连续性将容易导致错误的三角网格化,引起面形重建的畸变。由此,提出了一种新的快速面形重建方法。这种方法在三角网格化时考虑截面中心的移动,并通过引入权值函数来调整连接状态;用连接加权法构造三角网格,提高了网格连接的准确性;通过调整光截面中心消除了由三角网格化错位造成的影响。理论分析和实验结果表明,该方法具有与其它快速方法相同的运算复杂度O(N),而面形恢复的准确度明显优于传统方法。     

Effects of phase changes on reflection and their wavelength dependence in optical profilometry

The method as well as an appropriate instrumentation for measuring phase changes of reflected light is described. The phase changes on samples of Au, Al, Ag, and Cr evaporated films are measured for five wavelengths (lambda) from 442 to 633 nm, with respect to the phase change at the glass-air interface, where it should be zero. The measured results for the Au film are in fairly good agreement with values calculated by use of optical constants from a handbook or the complex refractive index measured by an ellipsometer. The phase changes for Al and Ag films are different from calculated values by ~5 degrees or a shift length of 4.4 nm at lambda = 633 nm, while those of the Cr film show large shifts as high as 16 degrees or a shift length of 9.8 nm at lambda = 442 nm.     

Color-coded optical profilometry with >10(6) resolved depth steps

A novel, to our knowledge, approach to light-stripe triangulation configuration that allows for parallel, fast, real-time three-dimensional surface topography with an extremely large number of optically resolved depth steps is presented, analyzed, and experimentally demonstrated. The method is based on a color-coding and decoding arrangement that exploits polychromatic illumination and axially dispersing optical elements. This leads to an increase of the depth-measuring range without any decrease in the axial or the lateral resolution. Our experiments yield three-dimensional surface measurements with lateral and depth optical resolutions of <40 nm, for a depth of focus of 48 mm, resulting in 1.2 x 10(6) resolving depth steps.     

Observation of nanoparticle internalization on cellular membranes by using noninterferometric widefield optical profilometry

We demonstrate the observation of gold-nanoparticle internalization in membranes of living cells by using noninterferometric widefield optical profilometry (NIWOP). The NIWOP technique can trace the height of an 80 nm gold particle on the membrane by calibrating the change of light intensity scattered from the particle along the optical axis. On the membrane, the depth resolution based on the scattering signal is similar to that based on the reflection signal, nearly 20 nm. Comparing the heights of the nanoparticle and the nearby cell membranes, we can identify the occurrence of particle internalization. Combining fluorescence microscopy with NIWOP, we also find actin aggregation around the site of the internalization process, which is an indication of endocytosis.     

Dual beam light profile microscopy: a new technique for optical absorption depth profilometry

Light profile microscopy (LPM) is a recently developed technique of optical inspection that is used to record micrometer-scale images of thin-film cross-sections on a direct basis. In single beam mode, LPM provides image contrast based on luminescence, elastic, and/or inelastic scatter. However, LPM may also be used to depth profile the optical absorption coefficient of a thin film based on a method of dual beam irradiation presented in this work. The method uses a pair of collimated laser beams to consecutively irradiate a film from two opposing directions along the depth axis. An average profile of the beam's light intensity variation through the material is recovered for each direction and used to compute a depth-dependent differential absorbance profile. This latter quantity is shown from theory to be related to the film's depth-dependent optical absorption coefficient through a simple linear model that may be inverted by standard methods of numerical linear algebra. The inverse problem is relatively well posed, showing good immunity to data errors. This profilometry method is experimentally applied to a set of well-characterized materials with known absorption properties over a scale of tens of micrometers, and the reconstructed absorption profiles were found to be highly consistent with the reference data.     

Linearly coded profilometry

A new optical profilometry-linearly coded profilometry (LCP)-is presented. It uses a sawtoothlike linear light structure to code the surface to be measured and a phase-shifting technique to decode the profile. Two kinds of coding-light structure, one with right-angle triangle teeth and the other with isosceles triangle teeth, are proposed. For coding with light with right-angle triangle teeth, a general decoding method is given. In addition, an optimum sampling manner and the measurement error are discussed with respect to a special case. For coding with light with isosceles triangle teeth, a decoding method with three samples is given. In our laboratory, an experimental system was established, and experiments that verified the reliability of the proposed methods were performed. Experimental results typical of those obtained are given. We find that LCP is similar to the widely used phase-measuring profilometry but has a faster measuring speed.     

Two-dimensional phase-measuring profilometry

An improved method is proposed to perform contouring of an object based on phase-measuring profilometry with a grid grating. Two phases unwrapped in perpendicular directions are obtained with the help of an adaptive bandpass filter and are used to unwrap the phases and to appoint the edge points of a shaded area and the object area. The height distribution of the object is obtained with the geometric relationship between coordinates and phases. The main axis of the projector and the main axis of the camera are not crossed and are also not in the same plane in order to arrange a measuring system easily and conveniently. The experimental results show that this technique is available for practical applications.     

Responses of simple optical standing wave sensors

Optical standing wave sensors have been manufactured by amorphous silicon deposition. The responses of these sensors, when subjected to standing waves, have been calculated and measured. It is shown that the responses are different depending on the way the standing wave is created. The responses also depend on the thickness and material properties of the layers used to create the sensors. Quantitative agreement between measurements and model calculations can be obtained by including alignment errors, incoherent light interaction and scaling factors. The simple construction of the sensors allows for a broad application range.     

Thermal noise from optical coatings in gravitational wave detectors

Gravitational waves are a prediction of Einstein's general theory of relativity. These waves are created by massive objects, like neutron stars or black holes, oscillating at speeds appreciable to the speed of light. The detectable effect on the Earth of these waves is extremely small, however, creating strains of the order of 10(-21). There are a number of basic physics experiments around the world designed to detect these waves by using interferometers with very long arms, up to 4 km in length. The next-generation interferometers are currently being designed, and the thermal noise in the mirrors will set the sensitivity over much of the usable bandwidth. Thermal noise arising from mechanical loss in the optical coatings put on the mirrors will be a significant source of noise. Achieving higher sensitivity through lower mechanical loss coatings, while preserving the crucial optical and thermal properties, is an area of active research right now.     

Data-dependent systems profilometry of two-dimensional surfaces

Fourier-transform profilometry (FTP) and data-dependent systems profilometry (DDSP) are two methods that are available for recovering one-dimensional fine surface profiles from the phase of a single interferogram. FTP has already been extended to two-dimensional surfaces; a similar extension of DDSP is introduced here. Inasmuch as this extension involves autoregressive modeling of the rows or columns of an interferogram, the feasibility of using a common model order is explored. The common order reduces not only the amount of computation but also the errors caused by the heterodyned phase-removal procedure. As autoregression requires masking the first few data values, the length of the mask is determined by means of a Green's function. A comparison shows that DDSP outperforms FTP in roughness measurements in terms of rms and center-line average. The comparison also shows that DDSP is able to recover a detailed surface, whereas FTP outlines only the global features. An interferogram regeneration procedure provides a reference surface for the verification of results.