This study presents a reflective-type optical encoder based on fractional Talbot self-image effect using a phase grating.
An amplitude grating image is produced by the phase grating at a quarter or three quarters of Talbot distance. The encoder
has the advantages of a large gap and more light efficiency. This work use a 850nm Light-Emitting Diode (LED) as light
source, the light source will be collimated by a collimator ,and incident on a phase grating by 45° angle. The fractional
Talbot self-image of phase grating is produced and projected on an amplitude index grating. The Moiré interference is
occurred when the both gratings displaced from each other. The encoder includes a reflective-type phase-grating scale
with period 20 μm and an index-amplitude grating. The gap between the phase index grating and mail scale was increased
to three quarters of Talbot distance. The tolerance of gap between the index and main grating is 0.1 mm. The tolerance of
yaw motion is ± 0.25°.
Gap uniformity between two plates is an important parameter in many optical devices or instruments. The proposed
method combines low coherence tandem interferometry and the concept of distributed fiber sensors for in-situ multipoint
gap measurement. No gratins or special marks are required on the surfaces of the plates. A superluminescent diode
(SLD) is used as the light source because of its low temporal coherence, high spatial coherence, and high optical power.
Therefore, the proposed technique can also be easily implemented in free space. A collimated light beam from the SLD
first passes through a scanning Michelson interferometer. The output of the interferometer is split into several separate
beams, and then each beam is normally incident to the test sample at different positions. Gap distribution measurement is
performed by sequentially opening the shutter behind each incident beam. A detector is employed to receive the reflected
lights from the sample. The interferometric signal is recorded as a function of the arm-length difference of the scanning
interferometer. The gap is eventually derived from the separation of the two side fringe packets, whose peak positions
are determined by the centroid algorithm. Preliminary experimental results validate the feasibility of the presented
This study proposes the auto-focusing procedure and the scan-range determining algorithm for white-light scanning
interferometry. During white-light scanning interferometry, the interference fringe must be located and to the best-focus
interferogram identified. The vertical-scan range must also be determined prior to the scanning procedure. A series of
images, either in-focus or out-of-focus, are collected in a proposed interference-fringe searching step. The contrast and
the sharpness indices of each image are calculated and applied in the auto-focusing scheme, and the vertical-scan range
is determined accordingly. Some preliminary experiments are performed to demonstrate that the best-focus
interferogram can be located precisely and the vertical-scan range can be determined.
The results of combining the wrapped phase with the fringe order of this phase to increase the precision of white-light interferometry at high scanning speed are presented. Monochromatic phase data are calculated using the Fourier method and the fringe order is determined using a general coherence peak sensing method. A wide scanning interval of 5λ/8 and a narrow-band color filter with a bandwidth of 70 nm are adopted to acquire interferograms. Experiments with an rms repeatability of step height measurement of below 1 nm and a scanning speed of 40 μm/s are performed.
A static and dynamic 3-D surface profilometer with nano-scale measurement resolution was successfully developed using stroboscopic illumination and white-light vertical scanning techniques. Microscopic interferometry is a powerful technique for static and dynamic characterization of micro electromechanical systems (MEMS). As MEMS devices move rapidly towards commercialization, the issue of accurate dynamic characterization has emerged as a major challenge in design and fabrication. In view of this need, an interferometric microscopy based on white-light stroboscopic interferometry using vertical scanning principle was developed to achieve static and dynamic full-field profilometry and characterization of MEMS devices. A micro cantilever beam used in AFM was characterized using the developed instrument to analyze its full-field resonant vibratory behavior. The first five mode resonant vibration can be fully characterized and 3-5 nm of vertical measurement accuracy as well as tens micrometers of vertical measurement range can be achieved. The experimental results were consistent with the theoretical simulation outcomes from ANSYS. Using white-light stroboscopic illumination and white-light vertical scanning techniques, our approach has demonstrated that static and dynamic 3-D nano-scale surface profilometry of MEMS devices with tens-micrometer measurement range and a dynamic bandwidth up to 1MHz resonance frequency can be achieved.
A planar encoder using conjugate optics is proposed for sensing the 2-D displacement of a 2-D grating. A Doppler frequency shift of diffracted light is generated when the grating moves. The optical conjugate path can compensate for the error arising from the relative tilt between the optical head and the scale (head-to-scale tilt). Additionally, the optical head is easily integrated, having high tolerance in component-to-component placement. The 2-D displacement system with the 2-D grating, which has period of 1.6 µm in both the X and Y directions, provides a signal period of 0.4 µm by using a double-diffraction configuration. This system and associated electronics provide interpolation with a factor of 400, corresponding to a measurement resolution of 1 nm.
A new profiling algorithm is proposed for the scanning white-light interferometry. A series of white-light interferograms are acquired by traditional vertical scanning process. The collected intensity data of the interferograms are then Fourier-Transformed with respect to the ordinate, or the scanning axis, into the wave number domain, where two or more wave numbers are selected for further calculation. The multi-wavelength phase-unwrapping technique is then used to solve for the surface profile. Preliminary experiment has been carried out with a Mirau-type white-light interferometer on two sets of step-height standards. The proposed algorithm works as well even when the spectrum of the white-light source is not Gaussian distributed, while the conventional peak sensing algorithms do not.
Laser encoders overcome the fundamental resolution limit of geometrical optical encoders by cleverly converting the diffraction limit to phase coded information so as to facilitate nanometer displacement measurement. As positioning information is coded within the optical wavefront of laser encoders, interferometry principles must be adopted in the design of the laser encoders. This effect has posed a very strong alignment tolerance among various components of the whole laser encoder, which in turn imposes a serious user adaptation bottleneck. Out of all alignment tolerances, the head-to-scale alignment tolerance represents the most important hindrance for wider ap-plications. This paper presents a novel laser planar encoder, which serves as a two-dimensional position detection apparatus for precision machine applications and can provide a measuring resolution less than 1 nm. Improving the IBM laser optical encoder design by taking into consideration manufacturing tolerance of various optical components, an innovative two-dimensional laser encoder with ultra high head-to-scale tolerance is presented. It was identified that this newly proposed laser encoder design could avoid the effect of differences in polarization diffraction efficiencies for the 2-D grating scale used. Optimizing the system performance by cleverly designing the profile of the 2-D grating scale was also detailed. The effect of non-uniform temperature field within the head-to-scale range that can yield a nonzero initial phase so as to decrease the system measurement accuracy was analyzed. In addition, misalignment of the polarizers located in front the photodiodes were identified to be the main cause for imperfect Lissajous circles, which may lower the measuring resolution when traditional arctangent algorithm was adopted for circular polarization interferometers. The resolution of the newly developed laser planar encoder was verified by experiments and was found to agree well with the theoretical predictions.