Research On Precision Measuring Method Of Lens Thickness Based On Laser Interference Technology

With the rapid development of modern science and technology,optical system are becoming more and more widely used in defense,aerospace,civil and other industries.At present,the center thickness of the lens as an important optical system parameters,which detection is still in the traditional contact measurement methods in our country.Because of its low efficiency,such as poor accuracy and easy to scratch objects to be measured,the contact measurement can not meet the requirements of the measurement of the center thickness of the lens,which seriously hampers the development of China's optical industry.In recent years,the rapid development of optical fiber low coherence interferometry technology provides a good technical basis for the detection of lens center thickness.In this paper,a non-contact lens center thickness detection system is proposed based on the principle of fiber coherence interferometry.In addition,the main factors that affect the detection precision of the detection system are analyzed,and the methods of improving the detection precision of the detection system are put forward from the aspects of optical circuit,circuit design and signal processing.The specific work of this paper is as follows:In order to improve the intensity of the interference signal,a Michelson interferometer with a different proportion of incident light between the sample arm and the reference arm is designed for the characteristic that the interference signal is not obvious due to the difference of the intensity of the reflected light.This is because the lens under test generally has a very low reflectivity,while the plane mirror has a strong reflectivity.In addition,in order to reduce the influence of the system noise on the interference signal,we designed a band-pass filter to effectively suppress thesystem noise.Furthemore,In order to ensure that the thickness gauges can be detected quickly and improve the efficiency of the system,an image sensor(CCD)is added near the stage of the lens to be measured to observe whether the reflected light from the front and back surfaces of the lens to be measured is completely into the sample arm,which is convenient to adjust the lens position to meet the interference condition of the detection state.In order to improve the measurement efficiency and accuracy,a method of measuring the positive and negative of the moving mirror is proposed in this paper.Furthermore,in order to improve the repeatability and accuracy of the measurement,the measurement error is reduced by the method of multiple measurements.In the signal processing method,the K-means clustering algorithm is applied to the multi-group interference signal peak identification and extraction,effectively improve the system measurement repeat accuracy.At the same time,taking into account the actual measurement of the existence of random noise,the zero-path difference of the peak position may be noise point interference.The zero-path difference of the interference signal is found by the method of obtaining the integral in the coherence length of the symmetry of the peak position.Experimental results show that the method can effectively reduce the system error and measurement error,and the measurement repeatability is obviously improved.Moreover,the system has the advantages of simple structure,fast measurement and so on.Besides,the measuring accuracy is better than 1um and the measuring range is up to 60 mm.