The Design Of Crystal Nonlinear Measuring Instrument Based On Digital Image Processing

Z-Scan technology can measure the nonlinear refractive coefficient, can distinguish the symbols of the nonlinear refractive coefficient through the transmittance curves. This technology has many advantages, such as, simple experiment device, high sensitivity and easy operation, which make it occupies an important place in measuring nonlinear coefficient of the material. Sheik-Bahae proposed single-beam Z-Scan techniques in 1989, from then on, Z-Scan technique has been improved, appeared a variety of styles of Z-scan.Based on the traditional Z-Scan optical path and combined with digital image processing techniques, we proposed an optical path based on digital image processing. In this paper, improved the new optical path, analyzed the new optical path in theory, deduced the equation of nonlinear coefficient based on new optical path.In the optical path based on digital image processing, replaced the power meter for CCD camera, removed the small aperture. In the traditional Z-Scan, the beam is divided into excitation beam and reference beam by a splitting mirror. The power with a small aperture and the power without aperture must be gain with two-time Z-Scan. The reference beam is the beam before the sample in theoretical analysis. In the new light path, the position far from the focus, the gauss light spot is flat enough, the change of far field power due to nonlinear is small, which can be ignored, so, the power of this position can consider as the reference power, so we can gain the reference power without divide the beam. In this paper, we replaced the laser for frequency stabilized laser, to gain a better reference light, joined the beam expanded device in the optical path, so we can gain the reference light not far away from the focus. We choose a range of pixels near the center of the gauss light spot, the sum of those pixels as the power with a small aperture; the sum of whole image pixels as the power without aperture, the hole power and opening power can once gained.We collected data with CCD camera and processed data with the method of digital image processing. We used C++to realize digital image processing:Used gray histogram to determine whether the light is too high; Used image addition to eliminate the system random error; Used image subtraction to eliminate the influence of the background light; Used nonlinear filter to eliminate the frequency component in the picture; Used two-dimensional Gaussian fitting method to get the center of the light spot. Then obtained obturator transmittance curve, opening transmittance curve, obturator/opening transmittance curve, finally obtained nonlinear refractive coefficient.We constructed the measurement system and used C++builder write the software part. Used MSComm to control stepping motor to realize the movement of the sample, using CCD interface function control the CDD camera to take pictures. Integrated data collecting and data processing into one software. Every picture (1280x1024) have to eliminate background light, image filtering, gain light spot center and so on. In the meantime, the software have to process many pictures. The computational cost of the image processing part is huge, so we join the multithreaded way in the part of image processing. When measure nonlinear coefficient, the operators of gain background light, gain light spot power, whether the spot light is overflow, eliminate background light, image filtering, gain center, obturator power, opening power, transmittance curve requires certain order. In order to prevent unexpected error, these operators are in turn enabled in the software.