M. S. Thesis Abstract
Development of A Grating Interferometer for Non-Contact Relative Displacement Measurement
Harry Douglas Garner, Jr.
The measurement of nanometer-scale displacements is becoming more common in various high-tech manufacturing processes. One example is in hard disk drive servo-track writing, where a conventional interferometer is used to measure the displacement of a retro-reflective target on the actuator arm in order to position it with a pushpin. To eliminate contact between the pushpin and actuator arm, and the need to reopen the hard drive in a clean room to apply a retro-reflective target, a technique has been developed in this thesis to make non-contact sub-wavelength relative displacement measurements of a moving target using a grating interferometer. Unlike conventional interferometers that require the application of an external target to the object to be measured, this technique utilizes an integrated feature on the moving object that conditions an incident light beam in a known fashion.
The grating interferometer system design that has been developed in this thesis consists of a coherent collimated light source, diffraction grating configuration including a small reflection grating mounted to the actuator arm of a hard disk drive, and a photosensitive detector to measure the interference fringe pattern. The light source used is a collimated linear light source that is designed to extend over the entire range of motion of the small arm-mounted grating. The configuration of diffraction gratings allows the beams reflected from the arm-mounted grating to form an interference fringe pattern on the photosensitive detector whose phase shift is proportional to the relative displacement between the light source and the grating configuration. This grating interferometer system has the advantages of adding only a small reflection grating to the actuator arm and a stationary light source and detector arrangement.
The theory for designing and modeling a practical grating interferometer for measuring the relative displacement of a hard disk drive actuator arm has been developed, which utilizes elements of electromagnetic theory and grating diffraction to determine characteristics of the interference fringe pattern formed by the grating interferometer system. A recursive diffracted beam ray tracing routine has been implemented as a tool to trace the paths of diffracted beams through various grating interferometer systems. This routine has been very useful in the grating interferometer system design because of its ability to visualize the diffracted beams that are present in a dimensionally accurate fashion, which is very important for the placement of various system components. The interference fringe pattern signal formed on the photosensitive detector has also been modeled in order to provide a means of recovering the phase of the fringe pattern and to examine the expected resolution characteristics of various grating interferometer configurations.
The components necessary to build a breadboard experimental setup of the grating interferometer design have been fabricated and assembled. These include various optical components and mounting hardware, a collimated linear laser source, CCD camera detector, and phase-type diffraction gratings, which have been fabricated using photolithography techniques. This breadboard setup has been used to perform a number of experiments to characterize the grating interferometer system and to demonstrate its feasibility for use in the non-contact relative displacement measurement of a hard disk drive actuator arm.
This thesis will contribute to the understanding of the grating interferometer in several ways. First, the necessary theory has been developed for the analysis of various grating interferometer systems. This theory has also been implemented as a set of design tools for the development of a specific grating interferometer design for the measurement of a hard disk drive actuator arm for the servo-track writing application. These design tools would also be useful for the development of other grating interferometer systems for various measurement applications. A breadboard system based on the grating interferometer design developed in this thesis has been constructed to verify the grating interferometer concept through fundamental experiments. Experimental results from motion tests with the breadboard setup using a high-resolution rotary stage and an actual hard disk drive have demonstrated the capability of the grating interferometer system to closely track the position of the actuator arm with an angular resolution of up to 70 nanoradians.