The micro interferometer is diffraction based optical displacement measuring system. Figure 1 shows the schematic of the micro-interferometer with photo-diode integration. The main feature of the grating interferometer is a phase sensitive optical diffraction grating, in which the diffraction pattern is determined by the distance between a reflective grating and the reflecting surface. The devices can also operate in parallel with individual actuation and many MEMS devices spread over a large area can be tested simultaneously as shown in the same figure.
Figure 1: Schematic of the deformable grating micro-interferometer with integrated photo-diodes and of parallel operation of micro-interferometers.
The distance between grating and surface can be tuned by electrostatic actuation of the grating. The graph of optical intensity Vs. Distance between grating and surface is plotted in Figure 2 High sensitivity point on the optical curve shows the desired position of the grating on the optical curve. Low frequency vibration and acoustic noise disturb the position of the grating while measurement. Reduction of this noise is achieved by adjusting the distance between the grating and the surface under observation real time.
Figure 2: Optical intensity curve
Control of the grating is based on detecting the harmonic distortion of the photo detector signal. This distortion is sensed by a Field Point Gate Array (FPGA from National Instruments) and a PID loop is run to control the position of the grating. Each μSGI has a tunable membrane which enables an array of μSGIs to be used simultaneously with high sensitivity and reduced noise. Preliminary results show that the tunable grating can increase the sensitivity by 12.5 dB and reduce the positioning error by 35.5 dB.
People: O Karhade
Collaborators: Dr. T R Kurfess (Clemson University)
Funding: Manufacturing Research Center