The theoretical analysis and experimental research of the optical parametric oscillator (OPO) based on periodically poled LiNbO<sub>3</sub> (PPLN) crystal are presented. The wavelength tuning curves of PPLN-OPO are calculated through the coupling equations of the three-wave mixing. An optical parametric oscillator with the output signal at 1.49μm and idler at 3.8μm, which is pumped by a Nd:YAG laser at 1.064μm, is obtained based on PPLN of length 20mm and thickness 0.5mm. When the pump power of the Nd:YAG laser is 5.2W with a repetition rate of 5kHz, the output of the idler at 3.8μm reaches 307mW.
A synthesis of the calorimetric and photoelectric method on the high energy laser beams measurement is presented. Data fusion of the two kinds of detector units is achieved with real-time scaling onsite. A set of compound diagnostic system is developed for the large area laser beam intensity distribution measurement, which is mainly composed of 256 calorimetric detectors, 120 photoelectric detectors, multi-channel data sampling module and one central processing computer. The total energy of the laser beam is accurately measured with calorimetric detectors, and the spatial intensity distribution with high temporal resolution is given by the photoelectric detectors. With the merits of energy accuracy and the temporal resolution based on the two kinds of detector units, the compound diagnostic system can be used to measure accurately the far-field temporal and spatial distribution of high energy laser beams.
Along with the incessant advancement in optics, electronics and computer technologies during the last three decades,
commercial digital video cameras have experienced a remarkable evolution, and can now be employed to measure
complex motions of objects with sufficient accuracy, which render great assistance to structural displacement
measurement in civil engineering. This paper proposes a computer vision-based approach for dynamic measurement of
structures. One digital camera is used to capture image sequences of planar targets mounted on vibrating structures. The
mathematical relationship between image plane and real space is established based on computer vision theory. Then, the
structural dynamic displacement at the target locations can be quantified using point reconstruction rules. Compared with
other tradition displacement measurement methods using sensors, such as accelerometers,
linear-variable-differential-transducers (LVDTs) and global position system (GPS), the proposed approach gives the
main advantages of great flexibility, a non-contact working mode and ease of increasing measurement points. To
validate, four tests of sinusoidal motion of a point, free vibration of a cantilever beam, wind tunnel test of a cross-section
bridge model, and field test of bridge displacement measurement, are performed. Results show that the proposed
approach can attain excellent accuracy compared with the analytical ones or the measurements using conventional
transducers, and proves to deliver an innovative and low cost solution to structural displacement measurement.
Image sequences recorded by high-resolution digital video cameras contain vast amount of spatial-temporal information
of targeted objects. With the rapid increase of image resolution, these digital cameras can now be used to measure
three-dimensional complex motion of structures with sufficient accuracy for the purpose of modal analysis or health
monitoring applications. In this paper, a measurement technique based on image sequence analysis is proposed for
extracting spatial-temporal responses of continuous structures. Two digital cameras are used to record two-dimensional
(2D) image sequences of a three-dimensional (3D) structural vibration response. This structural vibration response is
reconstructed using the two 2D image sequences through the epipolar geometry theory without the use of any
pre-installed targets. The obtained displacement response provides a more direct way to quantify the modal properties of
the structure. To demonstrate, a laboratory test was conducted to measure the free vibration of a cantilever steel rod.
Results show that the proposed technique can obtain excellent results as compared to the analytical solution. The
proposed technique provides a low-cost alternative to measure 3D vibration of low-frequency flexible structures such as
bridge cables in a non-contact and non-target fashion.
The availability of inexpensive and high-resolution commercial digital video cameras has brought forth a new area of
application that based on the processing of high quality of digital images. Videogrammetry is a three-dimensional
measurement technique that combines the traditional photogrammetry and the computer vision technique. This technique
has been previously demonstrated to provide reliable accuracy comparable to that of the traditional sensors for dynamic
measurement. Potentially, the technique can measure three-dimensional deformation time history of either a few selected
targets or a continuous spatial profile on a structure. In this paper, a novel technique based on videogrammetry is
proposed for investigation of structural dynamic behavior, which can measure deformation with sub-pixel accuracy is
first established to extract the temporal-spatial deformation of a structure. A simple modal identification method in
frequency domain is then applied to extract structural vibration parameters using dynamic responses reconstructed from
the image sequence. For demonstration of proof-of-concept, a lab test of identifying the free vibration of a steel
cantilever beam is performed. Results have indicated that the proposed technique can achieve a good agreement with the
analytical analysis, and show its significant potential for vibration-based real applications.
The past few years have seen unprecedented technological advancement in commercial digital cameras. The image resolution of these cameras has increased from below 1 million pixels a few years ago to over 10 million pixels today, with little increase in cost. These low cost high-resolution digital cameras have opened up new areas of application for various engineering disciplines, including civil engineering. The objective of this study is to investigate the application of videogrammetric principle for measuring structural response. A general videogrammetric framework for high precision measurement of three-dimensional structural response is proposed using two commercial digital cameras. Some important issues such as camera calibration, feature point detection and 3D point reconstruction are discussed. In order to evaluate the performance of the technique, three experiments involving capturing the trajectories of different types of motion are performed. The test results indicate that the videogrammetric technique can provide sub-pixel measurement accuracy and can be used to measure both static and dynamic responses of structures in laboratory.