The optical surface profiler offers fast non-contact and high-precision 3D metrology for complex surface features, which
are widely used in the field of precision machining manufacturing. The optical surface profiler traditionally adopts the
white light interference (WLI) technique which mainly includes optical interference system and high-precision
displacement stage. The accuracy of the displacement table determines the longitudinal resolution of the instrument. In
this paper, a novel WLI technique is proposed, i.e. full-field heterodyne WLI, which combines common displacement
stage, low differential-frequency heterodyne system and optical interferometry system. The low differential-frequency
heterodyne system generates heterodyne signal in the range of laser coherence length. By using the digital phase shift in
substitution for the mechanical phase shift, the vertical resolution increases from the sub-nanometer level to the
sub-angstrom level. Due to the low difference frequency technique, the common area array detector acquisition is
available. A fixed displacement stage position obtains a set of three-dimensional data cubes. Through Fourier-Transform
process of the time series data, the initial phase of each pixel at a specific heterodyne frequency is calculated and
transformed into surface height information. By using phase unwrapping, the object surface profile can be restored
within the laser coherence length. Through digital phase-shifting, phase extraction technology replaces the intensity
extraction technology, the moving distance of the displacement can be calibrated with high precision. Thus it can achieve
a large range of high-precision contour measurement and reduce the cost of the instrument.
The full-field heterodyne interferometric measurement technology is beginning better applied by employing low
frequency heterodyne acousto-optical modulators instead of complex electro-mechanical scanning devices. The optical element surface could be directly acquired by synchronously detecting the received signal phases of each pixel, because standard matrix detector as CCD and CMOS cameras could be used in heterodyne interferometer. Instead of the traditional four-step phase shifting phase calculating, Fourier spectral analysis method is used for phase extracting which brings lower sensitivity to sources of uncertainty and higher measurement accuracy. In this paper, two types of full-field heterodyne interferometer are described whose advantages and disadvantages are also specified.
Heterodyne interferometer has to combine two different frequency beams to produce interference, which brings a variety of optical heterodyne frequency errors. Frequency mixing error and beat frequency error are two different kinds of inescapable heterodyne frequency errors. In this paper, the effects of frequency mixing error to surface measurement are derived. The relationship between the phase extraction accuracy and the errors are calculated.
The tolerance of the extinction ratio of polarization splitting prism and the signal-to-noise ratio of stray light is given. The error of phase extraction by Fourier analysis that caused by beat frequency shifting is derived and calculated. We also propose an improved phase extraction method based on spectrum correction. An amplitude ratio spectrum correction algorithm with using Hanning window is used to correct the heterodyne signal phase extraction. The simulation results show that this method can effectively suppress the degradation of phase extracting caused by beat frequency error and reduce the measurement uncertainty of full-field heterodyne interferometer.
Proc. SPIE. 9142, Selected Papers from Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics: Optical Imaging, Remote Sensing, and Laser-Matter Interaction 2013
In the field of Fourier-transform spectroscopy, tilt and shearing problems caused by the moving components in a translational type of spectrometer reduce the quality of the interferogram dramatically. While, the spectrometer based on rotational motion can avoid these problems. In this paper, a novel rotational type of interferometer, called rotating parallel-mirror-pair interferometer (RPMPI), is presented. Its principle and properties are studied. This interferometer consists of one beam splitter, two fixed flat mirrors, and one rotating wedged parallel-mirror-pair (PMP). The optical path difference (OPD) is obtained by the rotational motion of the PMP. Factors that affect the maximum OPD include the wedged angle of the rotating PMP, the distance between the two parallel mirrors, the direction of the incident ray, and the range of rotating angle. This interferometer can operate either in swinging mode or continuous rotary mode depending on the range of the rotating angle. In swinging mode, the OPD function is linear. In continuous rotary mode, the sampling efficiency is higher and it can operate as an ultra rapid scanning interferometer.
As a novel detection approach which simultaneously acquires two-dimensional visual picture and one-dimensional
spectral information, spectral imaging offers promising applications on biomedical imaging, conservation and
identification of artworks, surveillance of food safety, and so forth. A novel moderate-resolution spectral imaging system
consisting of merely two optical elements is illustrated in this paper. It can realize the function of a relay imaging system
as well as a 10nm spectral resolution spectroscopy. Compared to conventional prismatic imaging spectrometers, this
design is compact and concise with only two special curved prisms by utilizing two reflective surfaces. In contrast to
spectral imagers based on diffractive grating, the usage of compound-prism possesses characteristics of higher energy
utilization and wider free spectral range. The seidel aberration theory and dispersive principle of this special prism are
analyzed at first. According to the results, the optical system of this design is simulated, and the performance evaluation
including spot diagram, MTF and distortion, is presented. In the end, considering the difficulty and particularity of
manufacture and alignment, an available method for fabrication and measurement is proposed.
The effect of beam position error on the imaging quality of a Fourier telescope is analyzed in this paper. First, the origin of the transmitting beam position error and the error types are discussed. Second, a numerical analysis is performed. To focus on the transmitting beam position error, other noise sources exclusive of the reconstruction process are neglected. The Strehl ratio is set to be the objective function and the transfer function of the position error is constructed. Based on the numerical model, the features of Strehl ratio reduction caused by position error are deduced. Third, simulations are performed to study the position error effect on the imaging quality. A plot of the Strehl ratio versus the different levels of position errors is obtained and the simulation results validate the numerical model to a certain extent. According to the simulation results, a high value of the transmitting beam position error obviously degrades the imaging quality of the system; thus, it is essential to contain the position error within a relatively low level.
Fourier telescopy is an active unconventional imaging technique. Three or more beams from different spatially separated transmitters are pointed at a distant and faint object. The spatial Fourier spectrum of the object is carried on the reflected temporally modulated signals. The image of the target can be reconstructed from the back signals by demodulation and phase closure algorithm. The conventional demodulation processing is calculating spectrum directly by inverse Fourier transform of the signal. However spectrum estimated by inverse Fourier transform has non-negligible errors caused by frequency shift error of the Acoustic-optical modulator, the noise and the relative motion between beams and the target. An improved demodulation method based on spectrum correction of FT is proposed. The method corrects the amplitude and the phase on the demodulated frequency of the signal by which better reconstructed image can be obtained. In this paper, the effect of the frequency shift error in Fourier telescopy demodulation is investigated. The degradation of the reconstructed image is simulated. We summarize the new demodulation method based on spectrum correction and give the simulated comparison between the conventional demodulation and the developed method. The result confirms the effectiveness of the improved demodulation method.