This study presents a focusing type grating interferometer for precision displacement measurement. The proposed measurement technique combines the designs of grating interferometers and a focusing type optical path configuration, granting the system high resolution and high stability. This measurement system uses a helium-neon laser as a light source. A beam from the light source is passed through a Wollaston prism and is divided into p-polarized and s-polarized beams separated at 20°. The beams are then focused into the grating via a focusing lens and then diffracted. By choosing a specific combination of grating pitch and lens focal length, it is possible to partially superimpose the zero-order ppolarized beam with the first-order s-polarized beam and form interference after the beams pass through a polarizer. When the grating is displaced, a phase change is introduced into the interference signal, which is then received by a photodetector. Via a self-developed phase demodulation program, the grating displacement can be derived from the phase change of the interference signal. The experiments show that the proposed system can accurately provide displacement information, with a resolution of up to 10 nm and repeatability of up to 10 nm. In addition, the system was tested with random waveforms to verify its ability to measure irregular displacements, and the results prove that the focusing type grating interferometer possesses excellent displacement measurement capabilities.
The laser interferometer is widely used in various fields because of its high resolution, high stability, high measurement speed and large-scale measurement capabilities, thus many research groups and equipment manufacturers have devoted time and resources to its development. This study presents an innovative symmetrical double diffraction laser encoder for precision displacement measurement. The system has the advantages of not defocusing during measurement, and can provide long range dual-axis linear displacement and rotation angle measurement. The system consists of two detection configurations, each composed of a double diffraction optical configuration, grating interferometer and phase demodulation system. The light source is passed through a non-polarized beam splitter, diffraction grating and mirror to form a grating interferometer system. The positive and negative first order beams formed from grating diffraction are reflected back through the grating by mirrors, forming a symmetrical double diffraction optical configuration to effectively enhance the system resolution. When the grating moves a corresponding phase shift will be introduced into the signal. Finally, a photodetector receives the signal and the data is analyzed with a self-developed phase demodulation program to obtain the displacement information. By comparing the displacement information of the two axes, rotation information can be obtained via trigonometric calculation. It can be inferred from the measurement principles that the theoretical resolution can be as high as 15 pm. Experimental results demonstrate that for displacement and rotation measurement, the repeatability of the symmetrical double diffraction laser encoder is 5 nm and 35 nrad, respectively. The system has excellent measurement performance, and its simple structure lends to easy setup and calibration.
In this study, a heterodyne grating interferometer based on the sinusoidal phase modulation method for displacement
measurements was proposed. The interference beams were modulated using a sinusoidal oscillating grating, and the
proposed frequency-domain quadrature detection method was used to detect the optical phase of the interferometer
and determine the displacement. Experimental results were consistent with the strain gauge results for several
displacement ranges. When only high-frequency noise was considered, our method achieved a measurement
resolution of approximately 2 nm.
An innovative moiré technique for full-field wafer warpage measurement is proposed in this study. The wafer warpage
measurement technique is developed based on moiré method, Talbot effect, scanning profiling method, stroboscopic,
instantaneous phase-shift method, as well as four-step phase shift method, high resolution, high stability and full-field
measurement capabilities can be easily achieved. According to the proposed full-field optical configuration, a laser beam
is expanded into a collimated beam with a 2-inch diameter and projected onto the wafer surface. The beam is reflected
by the wafer surface and forms a moiré fringe image after passing two circular gratings, which is then focused and
captured on a CCD camera for computation. The corresponding moiré fringes reflected from the wafer surface are
obtained by overlapping the images of the measuring grating and the reference grating. The moiré fringes will shift when
wafer warpage occurs. The phase of the moiré fringes will change proportionally to the degree of warpage in the wafer,
which can be measured by detecting variations in the phase shift of the moiré fringes in each detection points on the
surface of the entire wafer. The phase shift variations of each detection points can be calculated via the instantaneous
phase-shift method and the four-step phase-shift method. By adding up the phase shift variations of each detection points
along the radii of the circular gratings, the warpage value and surface topography of the wafer can be obtained.
Experiments show that the proposed method is capable of obtaining test results similar to that of a commercial sensor, as
well as performing accurate measurements under high speed rotation of 1500rpm. As compared to current warpage
measurement methods such as the beam optical method, confocal microscopy, laser interferometry, shadow moiré
method, and structured light method, this proposed technique has the advantage of full-field measurement, high
resolution, stability and adaptability.
In this research, a novel heterodyne laser encoder for 6-DOF displacement and angle measurements is proposed. The technique combines the advantages of heterodyne interferometry, grating shearing interferometry, and Michelson interferometry. When a heterodyne light beam with two orthogonally polarized directions is used to focus on a semi-transmission grating, two detection configurations for in-plane and out-of-plane will be obtained. By means of measuring the phase variations of the interfering signals from the moving grating, the in-plane displacement can be acquired. Besides, the out-of-plane displacement can be obtained by detecting the optical path difference between the reference beam and the reflection beam. Furthermore, 6-DOF displacement and angle information can be measured simultaneously by using the beam dividing method. According to the experimental results, the measurement resolution is about 2 nm. The experimental results show that our proposed method has the ability to measure 6-DOF displacement and angle information with high system stability. Comparing with other commercial measurement instructions, this laser encoder has the advantages of high resolution, high stability, and high flexibility.