3D micro profiles are often needed for measurement in many fields, e.g., binary optics, electronic industry, mechanical manufacturing, aeronautic and space industry, etc. In the case where height difference between two neighboring points of a test profile is equal to or greater than λ / 4, microscopic interferometry based on laser source will no longer be applicable because of the uncertainty in phase unwrapping. As white light possesses the characteristic of interference length approximate to zero, applying it for micro profilometry can avoid the trouble and can yield accurate results. Using self-developed Mirau-type scanning interference microscope, a step-like sample was tested twice, with 128 scanning interferograms recorded for each test. To process each set of the interferograms, the method of spatial frequency domain analysis was adopted. That is, for each point, by use of Furrier transform, white-light interference intensities were decomposed in spatial frequency domain, thus obtaining phase values corresponding to different wavenumbers; by using least square fitting on phases and wave numbers, a group-velocity OPD was gained for the very point; and finally in terms of the relation between relative height and the group-velocity OPD, the profile of the test sample was obtained. Two tests yielded same profile result for the sample, and step heights obtained were 50.88 nm and 50.94 nm, respectively. Meantime, the sample was also measured with a Zygo Newview 7200 topography instrument, with same profile result obtained and step height differing by 0.9 nm. In addition, data processing results indicate that chromatic dispersion equal to and higher than 2nd order is negligible when applying spatial frequency domain analysis method.
Spherically optical components of small diameters have found wide applications in many fields. Radius of curvature
(ROC) is an important parameter as it determines their optical characteristics. For measuring the ROC of a mini spherical
surface, a static microscopic carrier interferogram is taken, and virtual grating phase shift technique (VGPT) is adopted
in this paper. By using the computer, four virtual gratings are generated, their reference frequency being the same as the
carrier frequency of the test interferogram, and their modulation phases being 0, π/2, π and 3π/2, respectively. By
multiplying the test interferogram separately with the four virtual gratings in intensity, four moiré fringe images are
obtained. By applying Furrier transform and low pass filtering, a set of four carrier-free phase shift fringe images are
obtained. And finally by applying four step phase shifting algorithm and least square fitting (LSF), the ROC of the test
spherical surface is obtained. Furthermore, the result agrees well with its nominal value and measurement result by Zygo
interferometer. This indicates that adopting VGPT for measuring ROCs of mini spherical surfaces is practicable and has
relatively a high accuracy.
In temporal phase-shifting interferometer, environmental vibrations can induce the wobbling of fringes pattern that usually results in erroneous surface measurement. In order to actively compensating for the external vibration, wave-front phase variation, in proportion to the variation of optical path difference between reference and test arm of the interferometer, should be measured firstly. So, an optical heterodyne interferometric vibration measuring system is embedded in the interferometer, the real-time wave-front phase variation due to vibration is measured by comparing phase difference of two heterodyne signals at 40MHz, one is from reference arm, another is from test arm. An acousto-optical modulator (AOM) is used as optical frequency shifter. After wave-front phase measuring, secondly, a vibration servo compensating system is built. In this system, if real-time wave-front phase is differ from preset phase, an adaptive feedback signal is formed to drive a piezoelectric transducer (PZT), and the PZT drives the standard reference mirror to shift optical path difference in order to pulling wave-front phase to the preset value, thus, vibration is compensated and fringes pattern is stabilized. Thirdly, if preset phase value is periodically changed, a series of phase-shifting interferograms can be sampled. In the end, vibration-compensated effect of this adaptive interferometer is evaluated by an actual optical surface testing, results show this system works very well in the presence of vibrations which amplitude-frequency product is greater than 100 wave-Hertz. Frequency response of this system is restricted by PZT driven reference mirror.
With the rapid development of fiber communications, the fiber optic connector has become an indispensable no-source device, among which PC-type connector is most commonly used. As radius of curvature (ROC) of the connector end face is one of the three main geometric parameters affecting insertion and return losses, how to measure accurately the ROC has become an important issue in the field. In this paper, an algorithm, overlapping averaging 4-bucket method (OAF), is introduced to measure the parameter. Correspondingly, calculation of the phase principal values of the object wave-front and reconstruction of the spherical profile of the connector end face are analyzed, with simulation results given. Also analyzed is the influence of the shifting error of the phase shifter on the ROC measurement, with a curved surface figure for their relationship obtained. Meanwhile, a comparison is made between the errors developed by the 4-bucket algorithm (FBA) and OAF. It is indicated by the results that, OAF is superior to FBA in the suppression of the shifting error, and that, over a general range of calibration error and nonlinear displacement error both equal to or less than 8%, the ROC measurement error with OAF is less than 2.3×10-4.