The intensity distribution of diffraction field of two non-diffracting beams which interfere in a long distance is derived. Non-diffracting beam is generated by an axicon, which is then split into two coherent beams by a beam splitting prism, one of the two beams is regarded as reference beam, which meet the other one through reflector and another beam splitting prism after a long distance. The distribution of interference field is the coherent superposition of each diffraction field. Results show that the locus of interference fringes can be generally assumed hyperbola, the shape of which is affected by interference instance. When the two beams remain parallel, the diffraction pattern is analyzed to be hyperbolic moire fringe,in a case where there is an oblique angle between two beams, the trend of hyperbolic moire fringe change with the interference fringe trend without axicon. Experimental results are compared with theoretical analysis results. Good agreements between them is obtained.
A circuit processing method is present to restrain DC drift after analyzing the traditional signal processing method of interferometry for micro vibration measurement. At first, the circuit diagram is designed and its mathematical model is built, then the theoretical equations of the output signal are derived with the practical parameters. By using SIMULINK simulation, the process for restraining DC drift is present on the conditions of the variations of background intensity. The validity of feedback circuit was verified through analyzing the real experiment data. Theoretical predictions match simulation results, showing that this method effectively restrains DC drift for interferometry of micro vibration measurement and it greatly improves the system’s stability.
A ‘0/π’ phase pupil mask was developed to extend the depth of field of a circularly symmetric optical microscope imaging system. The modulation transfer function curves, the normalized point spread function figures and the spot diagrams of the imaging system with the optimal mask were analyzed and simulated. The results show that the large depth of field imaging system with the ‘0/π’ phase pupil mask has a high resolution in a long frequency band and can obtain clear images without any post-processing. The experimental results also demonstrate that the depth of field of the imaging system is extended successfully.
Single wavelength microscopic interferometry, driven by the Piezoelectric Transducer (PZT), is a common surface topography measurement method. Its measurement accuracy is directly determined by the original phase acquisition precision of every pixels in area array CCD. Traditional phase identification methods adopt 3 points or 4 points algorithm to obtain the phase. However, they require the displacement step, actuated by the PZT, to strictly keep the same to satisfy the 90° phase condition. Therefore, these methods are only suitable for the strict anti-vibration experimental environment or conditions with high precise closed-loop PZT actuator and strict calibration of interferometric wavelength. An auto-acquisition driving step method for the single wavelength microscopic interferometry is presented in this paper. Firstly, interference sequence diagrams, containing the surface topography information, are gathered by the CCD under open-loop PZT actuating. Next, two pixels whose phase difference is approximate 90° are selected as the calculating center to obtain smoothed gray values with regional gray average algorithm, which can reduce the influence of random noise. Finally, an optimal fitting algorithm for the ellipse, formed by the average gray values, is proposed to obtain the amplitudes and offsets of the two gray values array. According these fitting parameters and gray values, the drive step can be calculated by elliptic equations. Experiments have shown that this method can reduce the requirement conditions of measurement conditions and improve the measurement accuracy.
Ultrafast lasers, emitting ultra-short pulses of light, generally of the order of femtoseconds to ten picoseconds, are widely used in micro-processing with the advantage of very little thermal damage. Parallel micro-processing is seen significant developments in laser fabrication, thanking to the spatial light modulator (SLM) which can concert single beam to multiple beams through computer generate holograms (CGHs). However, without synchronization control, on the conditions of changing different holograms or processing on large area beyond scanning galvo’s ability, the fabrication will be interrupted constantly for changing holograms and moving the stages. Therefore, synchronization control is very important to improve the convenience and application of parallel micro-processing. A synchronization control method, carried out through two application software: SAMLight (or WaveRunner) and Labview, is presented in this paper. SAMLight is used to control the laser and the scanning galvo to implement microprocessing, and the developed program with Labview is used to control the SLM and motion stages. The synchronization signals, transmitted between the two software, are utilized by a National Instruments (NI) device USB-6008. Using optimal control methods, the synchronized system can easily and automatically accomplish complicated fabrications with minimum time. A multi-drilling application is provided to verify the affectivity of the synchronized control method. It uses multiple annular beams, generated by superimposing multi-beam CGH onto a diffractive axicon CGH, to drill multiple holes at one time, and it can automatically finish different patterns based on synchronization control. This drilling way is an optical trepanning and it avoids huge laser energy waste with attenuation. The multi-beam CGHs, generated by the Grating and Lens algorithm, are different for different patterns. The processing is over 200 times faster than traditional mechanical trepanning, moving a small laser spot in a larger orbit.
A high-accuracy signal processing algorithm was designed for the absolute distance measurement system performed with frequency scanned interferometry. The system uses frequency-modulated laser as light source and consists of two interferometers: the reference interferometer is used to compensate the errors and the measurement interferometer is used to measure the displacement. The reference interferometer and the measurement interferometer are used to measure synchronously. The principle of the measuring system and the current modulation circuit were presented. The smoothing convolution was used for processing the signals. The optical path difference of the reference interferometer has been calibrated, so the absolute distance can be measured by acquiring the phase information extracted from interference signals produced while scanning the laser frequency. Finally, measurement results of absolute distances ranging from 0.1m to 0.5m were presented. The experimental results demonstrated that the proposed algorithm had major computing advantages.
A new five-degree-of-freedom measuring system was developed as a linear guide. According to the principle of
autocollimation, the system consisted of two semiconductor lasers, two right angle prisms, two lenses, two polarization
spectroscopes and four quadrant Si-photoelectric detectors(QPD). Two axial displacements and three angular rotation
degrees are measured by comparing the position of the spot center on the QPD. Repetitive simulations show that the
accuracy of the system is 3" for measurement of angle, which proves the feasibility of this system. The advantages of the
system include simple structure, easy operation, high accuracy, low cost and real-time work.
The standard of technology without an interesting name (TWAIN) was introduced and a displacement measurement
method was presented. Based on the theories of image capture and image measurement, a displacement measurement
system was developed and subpixel and centroid technique were used to improve the measurement precision. The
Experiment results show that the proposed method has low cost and high precision.