Topographic optical profilometry of steep slope micro-optical transparent surfaces.

Optical profilometers based on light reflection may fail at surfaces presenting steep slopes and highly curved features. Missed light, interference and diffraction at steps, peaks and valleys are some of the reasons. Consequently, blind areas or profile artifacts may be observed when using common reflection micro-optical profilometers (confocal, scanning interferometers, etc…). The Topographic Optical Profilometry by Absorption in Fluids (TOPAF) essentially avoids these limitations. In this technique an absorbing fluid fills the gap between a reference surface and the surface to profile. By comparing transmission images at two different spectral bands we obtain a reliable topographic map of the surface. In this contribution we develop a model to obtain the profile under micro-optical observation, where high numerical aperture (NA) objectives are mandatory. We present several analytical and experimental results, validating the technique's capabilities for profiling steep slopes and highly curved micro-optical surfaces with nanometric height resolution.

Topographic optical profilometry by absorption in liquids.

Optical absorbance within a liquid is used as a photometric probe to measure the topography of optical surfaces relative to a reference. The liquid fills the gap between the reference surface and the measuring surface. By comparing two transmission images at different wavelengths we can profile the height distribution in a simple and reliable way. The presented method handles steep surface slopes (<90°) without difficulty. It adapts well to any field of view and height range (peak to valley). A height resolution in the order of the nanometer may be achieved and the height range can be tailored by adapting the concentration of water soluble dyes. It is especially appropriate for 3D profiling of transparent complex optical surfaces, like those found in micro-optic arrays and for Fresnel, aspheric or free-form lenses, which are very difficult to measure by other optical methods. We show some experimental results to validate its capabilities as a metrological tool and handling of steep surface slopes.

Optical cavity for auto-referenced gas detection.

An enhanced optical system design for NDIR gas detection is presented. Multiple paths lengths within the same cavity are used to auto reference the system. The system has good thermo-mechanical stability: it requires no special thermal stabilization, shows no sensitivity to thermal emitter drift and has no moving parts involved. Long term stability, virtually no zero-drift and sub-ppm level gas detection were achieved using commercial thermopile sensors and a thermal emitter modulated at low frequency (~0.5 Hz). Experimental tests were performed using carbon monoxide (CO) and a 30.5 cm cavity length. The design can be extended to allow multiple gas detection within the same optical cavity.