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.
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 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.
Fresnel telescopy full-aperture synthesized imaging ladar is a new high resolution active laser imaging technique. In the
operational mode with moving target by beam scanning, spatial distributions of the complex data of the returned signals
are not regular or uniform. In order to use fast Fourier transform (FFT) algorithm in signal processing, algorithm for
Fresnel telescopy imaging system should include resampling interpolation step to change sampling data into uniform and
quadratic spatial distribution. We choose suitable resampling interpolation algorithm for Fresnel telescopy, and get
computer simulation of area target. The work is found to have substantial practical value and offers significant practical
benefit for high resolution imaging in Fresnel telescopy imaging ladar.
An efficient technique of utilizing Dammann grating and phase plate to get high power and high brightness laser beam
from phase-locked laser array is presented. The conjugate Dammann grating and the phase plate are placed in the back
and front focal plane of a Fourier lens respectively. In order to improve the beam combining efficiency, Continuous
grating of high diffraction efficiency is used to replace the Dammann grating. Analysis shows that the Continuous
grating which has a higher diffraction of efficiency is also suitable for beam combining of the presented system.
Fresnel telescopy (short for Fresnel telescopy full-aperture synthesized imaging ladar) is a new high resolution active
laser imaging technique. This technique is a variant of Fourier telescopy and optical scanning holography, which uses
Fresnel zone plates to scan target. Compare with synthetic aperture imaging ladar(SAIL), Fresnel telescopy avoids
problem of time synchronization and space synchronization, which decreasing technical difficulty. In one-dimensional
(1D) scanning operational mode for moving target, after time-to-space transformation, spatial distribution of sampling
data is non-uniform because of the relative motion between target and scanning beam. However, as we use fast Fourier
transform (FFT) in the following imaging algorithm of matched filtering, distribution of data should be regular and
uniform. We use resampling interpolation to transform the data into two-dimensional (2D) uniform distribution, and
accuracy of resampling interpolation process mainly affects the reconstruction results. Imaging algorithms with different
resampling interpolation algorithms have been analysis and computer simulation are also given. We get good
reconstruction results of the target, which proves that the designed imaging algorithm for Fresnel telescopy imaging
system is effective. This work is found to have substantial practical value and offers significant benefit for high
resolution imaging system of Fresnel telescopy laser imaging ladar.
A new method of high resolution laser imaging is developed in this paper, which is denoted as Fresnel telescopy. It
creates Fresnel zone plate (FZP) fringe at the detected target, then makes relative motion between Fresnel zone plate and
the target to accomplish the encoding process, namely, to achieve the convolution between target reflection function and
the Fresnel zone plate intensity distribution function. The design scheme of the proposed system is presented. The
proposed technique offers significant practical benefit for ground-based imaging of objects in many important military