Design and Experiment of Self-aligned Diffractive Laser Optical Encoder System

• Main Authors: Zheng -Sheng Pan, 潘政晟
• Other Authors: Chih-Kung Lee
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/15516393963611214716

碩士 === 國立臺灣大學 === 應用力學研究所 === 90 === With worldwide science and technology attention rushes from micrometer world into sub-micrometer or even nanotechnology world, methodologies as-sociated with metrology in the sub-micrometer and nanometer displacement scale is getting ever more important. Displacement sensors that possess the capabilities of high sensitivity and high accuracy have become one of the most important techniques in the metrology fields. A newly developed Diffractive Laser Encoder System (abbreviated as DiLENS) was developed during the course in this dissertation. DiLENS is one type of the grating interferometer that uses grating to code the displace-ment information to the grating pitch such that the measurement scale takes place of grating pitch instead of laser wavelength. With this conversion, DiLENS is less prone to the influence of the environmental disturbances when compared to traditional laser interferometers. DiLENS is composed of three sub-systems, which includes a polarization division system, a 1-x telescope design, and a circularly polarization interfer-ometer. The polarization division system is to produce two orthogonal linear polarization beam served as interference light source. The function of the 1-x telescope assured that diffracted beam that is diffracted by reflective grating could trace back the optical path parallel to the original one, this self-aligned feature was shown to improve the head-to-scale tolerance of DiLENS by 5 to 25 times than all leading encoders in the world. The displacement of grating will induce Doppler effect so as to modulate the phase of the laser beams. The circular polarization interferometer can transfer the grating displacement information into Doppler frequency shift, which can be retrieved by converting the coded phase information into the quadrature signal. A numerical simulation program developed based on the Rigorous Cou-pled Wave Analysis (RCWA) Theory was used to compute the desired grating scale profile in order to optimize the DiLENS performance through improving the grating diffraction efficiency. It was found that the optimal profile for the grating is a grating with 160 nm grating depth and with sinusoidal profile. The optimized grating was found to be able to improve the intensity of the in-terference signals by 3 times when compared to that of the Canon encoder, which is the current world-leading product. Analysis accomplished in this dissertation indicated that the reasons be-hind the inclined elliptical signals are the quality and the misalignment of the optical component used in DiLENS. Criteria that can be used to reduce the optical non-linearity in DiLENS were also presented to further improve the DiLENS performance. Detailed experiments were pursed to verify the theoretical predictions and the overall system specifications of the DiLENS. The calibration instrument was a HP5529A laser displacement interferometer, which is the most widely used displacement-measuring instrument. Even running the calibration at regular laboratory environment, i.e., without extensive environmental control, the displacement discrepancy between the calibration instrument and our newly developed DiLENS was a mere 37.3 nm with 25.4 nm standard deviation. In summary, the optical and optomechanical configurations, the signal processing algorithms involved, and the optimal grating profile predictions, etc. of a self-aligned laser encoder were all developed and detailed. The experi-mental results obtained matched well with the theoretical predictions. The head-to-scale tolerance of DiLENS was found to be 5 to 25 times better than all other leading encoders worldwide.