Focusing is one of the important techniques in the optical microscope systems. With the recent development in science and technology, the requirement of high precision and automation of focusing are proposed. Besides the image quality of a microscope system itself, the focusing precision often influences the measurement precision to a great extent. So, confocal scanning optical microscope with high depth resolution have been used for focus detection technique.
This dissertation is concerned with autofocus of optical microscopes by confocal principle. The confocal design is embodied from replacement of pinhole by single mode optical fiber. For simplicity of system design, reciprocal scheme and infinity-corrected design is applied. To acquire the focus error signal, lock-in technique based on source modulation, which has many advantages, such as mechanical stability and insensitivity to noise, is utilized.
The system modeling is executed to define the function of each component forming the whole autofocus system and estimate the design of controller to improve the system response. Experimental results show that the autofocus method proposed in this thesis is capable of providing a control range of 15~20㎛ and 0.5㎛ focusing repeatability for mirror surface. The robustness of autofocus system is confirmed by the variation of photo-detector signal and lock-in amp. signal with change of surface reflectance and inclination. The lateral scanning result for Cr surface shows the possibility of application about 3-D profiler.
