Abstract: With the increasing demand for the application of fiber coupled laser ,the requirement for the coupling efficiency of fiber optic and laser increases.There are differences between the positions of the focusing spots of laser with different wavelength.The position of the laser focusing spot is one of the important factors affecting the coupling efficiency, and it has certain practicability for the precise positioning of the laser focusing spot.A focusing combined lens group focusing laser beam is designed. The CCD camera is used to collect the images of the front and back laser focusing points .The acquired images are processed by MATLAB software. The spot is fitted by circle and diameter is calculated.The position of the minimum diameter is the focus position. The results show that the laser focus position with different wavelength is different and the relative errors of focal length are 0.59% and 0.73% ,both the measured focus position and size can be accurate to 0.001 mm.The method has the advantages of simple design, automaticity and accuracy, and especially for judging the focus position of the invisible laser.

Key words: image processing, position of laser focusing point, circle fitting, dual wavelength, optical design