Optical Microsensors for Fluid Flow Diagnostics

This manuscript describes two MEMS based optical sensors for wall shear stress and velocity measurements in flow fields. Experimental results obtained with the wall shear stress sensor, the MicroS, are compared with boundary layer velocity measurements obtained with a traversing laser Doppler anemometer. Preliminary velocity measurements conducted with the time-offlight Micro Velocimeter, the MicroV, in a microchannel indicate that these microsensors offer a wide range of opportunities for the implementation of chip function monitoring and feedback control loops on microfluidics chips. Future sensor designs are described. Introduction Recent advances in micro fabrication [1] and micro-optics has enabled the development of micro optical sensors for quantities such as pressure and temperature, and for fluid flow measurements such as velocity and wall shear stress. These microsensors offer the advantage of being non-intrusive, embeddable in small wind tunnel models, and capable of remote in-situ monitoring. The sensors described in this paper are designed around diffractive optics technology. The diffractive optical elements (DOEs) surface relief profiles are fabricated in a thin film of E-beam resist (PMMA or PMGI) that is spin-coated on a fused silica or other substrate to a thickness of approximately two microns [2,3]. The diffractive optical elements, a few hundred microns in size, replace large conventional optical setup and enable the fabrication of micro sensors. MEMS wall shear stress sensors use either the displacement of a floating element, heat dissipation, or an interferometric pattern to determine the shear at the wall [4]. We describe results obtained with an optical shear stress sensor based on a technique developed by Naqui and Reynolds [5] that uses a diverging fringe pattern to measure the velocity gradient at the wall. Optical velocity measurements close to a surface are often hampered by background scattered light and accurate positioning of the probe volume near the surface. Because they are embeddable, microsensors solve the probe volume positioning and background scattered light problem. We describe preliminary measurement obtained with a time-of-flight micro velocimeter in a microchannel where the background-scattered light can be a significant obstacle. Finally, we describe an optical MEMS based particle sizing sensor capable of detecting particles as small as 0.1 microns. The MEMS Based Sensors Diffractive Optical Element: DOE The enabling optical elements in these microsensors are the diffractive optical element (DOE) that produces the desired optical pattern within such small space. The DOE takes the form of a shallow surface relief pattern that is etched into a transparent substrate. The surface relief pattern is designed to produce phase variation in the laser beam such that the light will form an Copyright© 2001 by Gharib, Modarress, Fourguette, and Wilson. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization, interference pattern that approximates the A DOE can be designed using a modified version of the iterative Fourier transform (IFT) algorithm (also known as the Gerchberg-Saxton algorithm [6]). This algorithm utili2es the well-known Fourier transform relationship between the near field (just past the DOE) and the far field (focal plane of DOE). For high efficiency and ease of fabrication, it is best if the DOE implements a phase-only transmission function, i.e. no spatial magnitude variation is required. This constraint makes it impossible to simply inverse the Fourier transform of the desired far-field image to find the near field (and hence the DOE function) because the inverse transform will generate magnitude variation as well as phase in the near field. The IFT design algorithm overcomes this problem by iterating between the near and far field planes many times, constraining the far-field intensity to the desked pattern and constraining the near field to have phase-only variation. After a sufficient number of iterations, the far field intensity approximates the desired image and the DOE is phase only. A photograph and atomic force microscope scan of a dual-line focus-laser lens DOE are shown in Figure 1. The DOE was fabricated on a 500 |Llm thick quartz substrate by analog direct-write electron-beam lithography on polymethyl methacrylate (PMMA) followed by acetone development. Figure l:Photograph (left) and AFM scan (right) of the center of the dual-line-focus laser lens DOEs are capable of shaping light in a wide range of geometries. A DOE was designed and fabricated to shape light into two elongated light spots (this DOE was used in the fabrication of a time-of-flight velocimeter described below). The comparison between modeling results and the optical pattern obtained with the fabricated DOE is shown in Figure 2. The DOE is designed to shape two elongated light spots 5 mm above the DOE surface. The DOE is illuminated with the light output of a pigtailed laser (single mode fiber output) emitting at 660 nm. The width of the light spots is slightly larger in the measured light intensity patterns and the light patterns are not as uniform however the similarity between the simulated and the measured intensity is excellent. These small discrepancies are most likely caused by a slight difference in the light source illumination pattern between the simulation and the experiment. (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.