Angular scan optical coherence tomography imaging and metrology of spherical gradient refractive index preforms

Abstract

The fabrication of high-performance spherical gradient refractive index (S-GRIN) optics requires nondestructive metrology techniques to inspect the samples. We have developed an angular-scan, swept-source-based, Fourier-domain optical coherence tomography (OCT) system centered at 1318 nm with 5 mm imaging depth capable of 180° polar scan and 360° azimuthal scan to investigate polymeric S-GRIN preforms. We demonstrate a method that enables simultaneous mapping of the group optical thickness, physical thickness, the radially-averaged group refractive index, and the transmitted wavefront of the S-GRIN preforms. The angular scan OCT imaging and metrology enables direct visualization, molding uniformity characterization, and optical property evaluations of the preforms. The results on two generations of S-GRIN preforms are discussed that showcase the evolution of the manufacturing process in response to the OCT metrology feedback.

An approach to viscoelastic characterization of dispersive media by inversion of a general wave propagation model

“…The maximum sensitivity of the system was measured to be 112 dB. 32 The imaging depth of the system was measured to be 5 mm in air (-10 dB sensitive fall-o®). The optical lateral resolution was approximately 30 m, and the FWHM of the axial point spread function after dispersion compensation was 10 m. The synchronized control of the galvanometer and the OCT data acquisition was conducted through a LabVIEW platform (National Instruments, Austin, TX, USA) connected to a workstation.…”

Three-dimensional measurement of small inner surface profiles using feature-based 3-D panoramic registration

“…The advantages of using our approach to currently used metrology: (1) ability of noncontact inspection; (2) accessibility of variable data; (3) independence from operators’ skills; (4) high automatability with robotics; (5) reduced labor and maintenance cost; (6) potential high efficiency with high-power computer; (7) feasibility of in-line measurement; and (8) vision of the threaded surface for point, line (scratch), and spectroscopic (color) inspection. Direct vision is a big advantage of our method compared with other technologies that are currently feasible for the 3-D measurement of small internal surfaces: x-ray CT, 8 OCT, 27,28 and laser ring beam scanning. 30 Duplicating human vision has a long-term major advantage because it can rapidly harness the incredible capabilities being generated in computer vision and machine learning using deep neural networks for artificial intelligence analyses.…”

“…26 Although OCT was developed to image below the surface of translucent biological tissue, the technique can be applied to measuring opaque surface structures with lateral resolutions in the 10- to 30-µm range, but with limited depth of focus. 27 With higher-speed 3-D imaging using Fourier domain OCT, new inline inspection applications are being developed for industry, 28 although no 3-D surface reconstruction of threaded holes in shiny metal has been reported for these OCT techniques. Strong specular reflection of the shiny metal surface does lead to large artifacts in 3-D reconstructions.…”

Point-cloud noncontact metrology of freeform optical surfaces

“…In this paper, we investigate a point-cloud method that leverages Fourier-domain sweptsource optical coherence tomography (SS-OCT), a technique based on low-coherence interferometry, for precision freeform metrology. As a high-resolution, high-sensitivity and high-speed imaging technique [31], SS-OCT has been utilized for nondestructive material characterization such as qualifying gradient refractive index materials [32][33][34][35][36][37]. Different optical coherence tomography (OCT) systems with various noise sources have been characterized [38].…”