The research objective of this Grant Opportunity for Academic Liaison with Industry (GOALI) project is to apply frequency modulated continuous wave laser radar, also known as ladar, to high resolution, non-contact, large-volume, three-dimensional metrology. Non-contact methods of optical three-dimensional metrology are limited at longer ranges due to optical diffraction from finite sized receiver optics. Tunable laser sources, however, can now provide sufficient optical bandwidth to achieve ranging resolution of sub 50 microns with measurement precisions to diffuse targets of less than an optical wavelength in short integration times at tens of meter distances. The approach of this project is use the ultra-high range resolution provided by tunable laser sources, and apply trilateration and synthetic aperture imaging techniques to leverage this resolution for measurements transverse to the optical propagation. The project will investigate the formation and accurate registration of ultra-high resolution three-dimensional synthetic aperture images from multiple moving and stationary apertures. For metrology applications the tuning rate of the laser source must be stable and accurately calibrated. Therefore, a sub-objective will research the fundamental and practical limits to calibrating the measurement system.
If successful, the benefits of the research project will include demonstration of a novel new method for accurate, large-scale, non-contact, industrial metrology. This method could benefit industries such as aerospace and energy that manufacture very large precision machinery, improving the speed and cost of manufacturing and improve machine life through tighter tolerances. This will help strengthen the United States' manufacturing base and benefit the national economy. This project will also help educate and train students in optics and engineering at Montana State University-Spectrum Lab a unique university research environment that includes close collaboration with the local optics industry in largely rural Montana, which is an EPSCoR member state.
This NSF Grant Opportunities for Academic Liaison with Industry (GOALI) project investigated the application of ultra-broadband Frequency Modulated Continuous Wave (FMCW) ladar (also known as laser radar) to 3D industrial metrology (study of measurement). The FMCW ladar technology measures absolute distances with a time-of-flight method that utilizes optical linear chirped waveforms that can exceed 4 THz of bandwidth. The technology is capable of measuring distances with precision of less than 10 micrometers at distances exceeding 10 meters with an accuracy of 1 part per million. This project investigated and developed techniques to calibrate the FMCW ladar sources to ensure high accuracy measurements and experimentally compared the technology to more traditional optical interferometry techniques that measure displacement rather than absolute distances. The project also investigated an advanced imaging technique enabled by the FMCW ladar technology named Synthetic Aperture Ladar (SAL) that provides for the formation of images by processing ladar range profiles from many perspectives provided by relative motion between the object of interest and the ladar measurement system. The SAL imaging technique is the direct optical analogue of Synthetic Aperture Radar (SAR), which is utilized in remote sensing and imaging. During the project the SAL technique was extended to include measurement of 3D surface profiles using the coherent properties of the image using a method called interferometric SAL. The SAL technique is of interest in applications because it can increase the resolution of a ladar imaging system beyond the standard diffraction limit of the physical optics used by the system. For this project SAL was of interest to improve the precision of transverse coordinate measurements in a 3D metrology system. Finally, the project provided proof-of-concept demonstrations of a non-contact multi-dimensional metrology system design based on the trilateration technique, which uses only absolute distance measurements to provide 3D coordinates. Trilateration forms the basis of the Global Positioning System (GPS), so the demonstrated metrology system can be described as a kind of "optical GPS". This proof-of-concept system demonstration showed that coordinates on the surface of a diffuse scattering target could be measured with a precision of less than 200 micrometers. As manufacturing and industrial construction advances, the need for highly accurate and automated measurement (metrology) systems increases to ensure that parts manufactured in separate locations will fit properly when assembled. Improvements in industrial metrology will strengthen these industries by enabling new manufacturing and construction techniques improving productivity and thus benefit the economy. In all, this project advanced the application and understanding of the FMCW ladar technology to 3D coordinate metrology. Additionally, the advances in SAL imaging made during this project could find application in national defense and the space sciences.
