Lab Introduction
One area with much recent growth in the optical industry is the development of systems based on freeform optics, which can be defined as a component with a non-rotationally symmetric optical surface about any axis. Freeform optics have shown rapid growth because of the creation of new paradigms in fields ranging from optical design and fabrication to measurement. Freeform optical surfaces lead to excellent system performance in terms of wavefront aberration, system size, and design degrees of freedom when compared to conventional optical surfaces. Freeform optics are becoming widely used in many applications such as space optics, astronomy, medical instrumentation, and instrumentation for the semiconductor industry.
This growth has been made possible because of rapid advances in manufacturing technology. However, there are still many challenges that must be overcome to realize widespread application. The measurement of freeform surfaces is particularly very challenging and considered an urgent area for research and development. Up to now, many approaches have been under development but no satisfactory general solution exists for freeform optics metrology.
For this reason, our group has started a research project of “3D surface measurements of freeform optics” since 2012. Our research group has successfully developed a new variant of lateral shearing interferometry with a tunable laser source and deflectometry based on structured light patterns. These two techniques directly reveal the slope profile of the surface under test, and the height profile is then recovered through integration without any reference. Based on accumulated knowledge and know-how related to optical techniques over recent decades, our group will provide a promising 3D metrological tool for freeform optics measurements.
Latest Research
Single-shot spectrally resolved interferometry for the simultaneous measurement of the thickness and surface profile of multilayer films
We present a single-shot spectrally-resolved interferometry for simultaneously measuring the film thickness and surface profile of each layer of a patterned multilayer film structure. For this purpose, we implemented an achromatic phase shifting method based on the geometric phase using the polarization characteristics of the light and obtained four phase-shifted interferograms in the spectrally-resolved fringe pattern at the same time by combining a pixelated polarizing camera with an imaging spectrometer. As a result, we could simultaneously measure the reflectance and phase of the sample over a wide wavelength range with a single measurement. To evaluate the validity of the proposed method, we measured a patterned five-layer film specimen and compared our measurement results with those from commercial instruments, an ellipsometer and a stylus profiler, respectively. We confirmed the results matched each other well.
One-shot deflectometry for high-speed inline inspection of specular quasi-plane surfaces
Deflectometry is a non-contact optical technique which uses a simple system setup to measure and inspect specular surfaces. Conventional deflectometry methods usually need very complicated system calibration and time-consuming phase-shifting methods, which are not practical for high-speed measurement. But if the measurement target is a quasi-plane surface where the local slope is directly modulated by phase, the complicated system calibration can be neglected. In this study, to reduce error, we investigated and analyzed error contribution when measuring quasi-plane surfaces. A simple and high speed one-shot phase retrieval method is proposed for quickly measuring surface phase. We used a single high carrier-frequency composite pattern for pattern projection, and derivation of the captured fringe pattern for phase retrieval. A phase-related slope is obtained for slope calculation and the 3D shape is reconstructed by integrating the surface slopes. We verified our proposed method by measuring representative quasi-plane surfaces and comparing results with conventional methods.
Real-time method for fabricating 3D diffractive optical elements on curved surfaces using direct laser lithography
To simplify complex optical systems, a next-generation optical component which combines a refractive element and a diffractive element is desirable. To fabricate these next-generation optical components, it is desirable to keep the focal point of the lithographic lens focused on the curved surface. Keeping the focus constant is challenging for a laser process, as well as for measuring and metrology systems. In this study, a coaxial confocal microscopic type commercialized displacement sensor was used to make it easier to fabricate the pattern required for diffractive optical elements (DOEs) on a curved surface, using direct laser lithography. The test results confirmed that a constant line width of 5 μm could be fabricated on a curved surface such as a cylinder and convex lens using the proposed auto-surface tracking system, with a position error of 1 μm. The diffraction pattern fabricated on the curved surface was analyzed for optical performance and compared with mathematical modeling.
