Zhaohui Hu

Modeling and algorithm of attitude measurement of moving target by laser tracking systems

We demonstrate theoretically the feasibility of the full-attitude measurement of a moving target by laser tracking systems. Based on the combination of retroreflector target, laser tracking, and interferometry techniques, two kinds of laser tracking systems known as distance- angle-measurement (DAM) and distance-only-measurement (DOM) sys- tems are proposed for the attitude measurement of a moving target in real time. Mathematical models and algorithms of both systems are built to determine and calculate the dynamic attitude of the target under the world coordinate system and the targets initial coordinate system re- spectively. The coordinate transform between different coordinate sys- tems and the initial geometric parameters of the laser tracking stations can be self-calibrated. The coordinate transform self-calibration algo- rithm can be solved by the singular matrix decomposition method, while the self-calibration algorithm of the initial geometric parameters of the DOM system can be solved by the Levenberg-Marquardt method. The simulation results show that the self-calibration algorithm converges rap- idly and its accuracy is satisfactory under noise measurement.

Theoretical and experimental investigation of the optical trapping force in single lensed fibre trapping

A numerical calculation method for the optical trapping force which is appropriate for single lensed fibre trapping (SLFT) is proposed. The piconewton optical trapping forces on a yeast cell in SLFT as a function of the position along two horizontal orthogonal axes are measured experimentally by static and dynamic methods, respectively. The order of magnitude and the characteristics of the theoretical trapping force curve calculated by our method are the same as those of the experimental measurement curve. The theoretical and experimental results for the angle of inclination of the fibre probe also coincide with each other.

An Optical Fiber Lateral Displacement Measurement Method and Experiments Based on Reflective Grating Panel

An optical fiber sensing method based on a reflective grating panel is demonstrated for lateral displacement measurement. The reflective panel is a homemade grating with a periodic variation of its refractive index, which is used to modulate the reflected light intensity. The system structure and operation principle are illustrated in detail. The intensity calculation and simulation of the optical path are carried out to theoretically analyze the measurement performance. A distinctive fiber optic grating ruler with a special fiber optic measuring probe and reflective grating panel is set up. Experiments with different grating pitches are conducted, and long-distance measurements are executed to accomplish the functions of counting optical signals, subdivision, and discerning direction. Experimental results show that the proposed measurement method can be used to detect lateral displacement, especially for applications in working environments with high temperatures.

Experimental research on laser tracking system with galvanometer scanner for measuring spatial coordinates of moving target

The spatial position of industrial object, such as robot end- effector, is an important geometric parameter whose accuracy determines whether robot can perform accurately. Therefore, we have established a laser tracking and coordinate measuring system with galvanometer scanner for high accuracy, large range, non- contact, and spatial dynamic measurement. In this paper, the laser tracking system and its setup are illuminated at first. Then, the formulae for calculating coordinates are deduced, and the calibration method of the initial distance from tracking mirror to target is presented. After that, two preliminary experiments in different distances are described. One is on CMM; the other is with grating ruler as reference. In the former, the maximum measurement error of coordinates is 70micrometers and the maximum error of length is 35micrometers in the 85x100x100mm3 measurement volume, and in the 1m initial distance. In the later, the maximum error of length is 140micrometers in the range of 480mm, and in the 5m initial distance. At the end of the paper, the error sources are analyzed and simulated.

Novel synthetical instructional system for principles of lasers

For the general existing experimental systems of laser teaching, it is hard to generate lots of transverse modes and to show the process of mode competition. In the paper a novel synthetical instructional system based on a cats eye cavity He-Ne laser is proposed. Lots of transverse modes from fundamental to high-order modes can be generated by adjusting the cats eye. A quartz crystal or a stress plate in intra-cavity can split one-frequency laser into dual-frequency laser. Based on the frequency splitting, the dynamic process of mode competition is observed. Almost all the important concepts in lasers are demonstrated experimentally. The instructional system provides a powerful aid for laser teaching. On the other hand the real frequency splitting and the polarization direction of the laser output versus the rotation angle of crystal are measured and discussed, and their values at very large rotation angle are forecasted.

Position and attitude measurement of moving target using laser tracking system with multiple measuring stations

Dynamic geometric parameter measurement plays an important role in most industries. Research and development on this technology have attracted great attention. We proposed a laser tracking system for measuring development of laser tracking technology, a laser tracking system consisting of three tracking and measuring stations is described in detail. The three stations track respectively three retro reflectors on the moving target, and measure the position and attitude. We built the mathematical model of measurement and developed the algorithm for processing data. According to the homogeneous coordinate transformation, we deduced the formulae for computing coordinates and attitude under different coordinate systems. Some key techniques of the measuring system are discussed at the end of the paper.

Experimental measurement of the trapping force acting on a yeast cell with a lensed optical fiber probe

In conventional optical tweezers system a high numerical aperture (NA) objective is employed both to image and to generate a gradient force toward the focus on sample particles, so the system is complex and expensive especially for the multi-optical-tweezers system. We built a novel simple optical trapping system based on a lensed optical fiber probe. This new method offers several other advantages over the conventional optical tweezers. The trapping system we built includes a laser coupling unit, a multi-dimensional probe manipulating unit, a sample nano-positioning unit, and a microscopy imaging unit. Based on the system, a yeast cell is trapped and manipulated on the chamber bottom by the lensed fiber probe, and the optical trapping forces acting on the yeast cell as a function of the offset are measured and discussed in different directions by the static method and the dynamic one respectively with various powers. The results by the two measurement methods coincide with each other, and the detail experimental procedure and the data processing of the two methods are introduced in this paper.

Field distribution of optical fiber probe for trapping particle

The technique ofoptical fiber trapping has been successfully used to manipulate small particles, while near field optical tweezers based on the sub-wavelength size fiber tip has been proposed to trap nanometric particles. With some similar physical mechanism and properties they are investigated and analyzed respectively in this paper. Firstly the optical field distributions of uncoated fiber probe with different radii of tip, from several microns down to zero, are calculated and analyzed by 3D finite-difference time-domain (FDTD) method. Then the metal-coated fiber probe is also discussed in the same way. In conclusion, the light from the fiber probe with the radius larger than wavelength is converged at a cometic spot, whatever with or without metal coating. When the radius of metal-coated tip is smaller than halfofwavelength, the evanescent field occurs and decays rapidly. The local field enhancement dominates in the metal-coated probe ifthe radius is smaller.