Homodyne full-field interferometer for measuring dynamic surface phenomena in microstructures

Electromechanical devices containing vibrating microstructures like the miniaturized radio-frequency filters used in mobile communications are key components in the systems high performance capabilities. Such systems whose performance is founded on mechanical deformations and movements has triggered a necessity for advanced empirical methods to illustrate their mechanical behavior. This has created a high research and development need for the ever more sophisticated Nano and microstructures. The temporal resolution concept has been developed and has various applications including creation of high speed cameras for fast moving imaging.

In a recent paper published in Optics and Lasers in Engineering Lauri Lipiäinen and colleagues at Aalto University in Finland proposed the use of a homodyne full-field interferometer for measuring dynamic surface phenomena in microstructures. Their aim was to stabilize the interferometer to a chosen operation point by obtaining a signal feedback from a non-moving freely selectable reference region on a sample surface.

First, the research team adopted optical interferometry since it enables non-contact and direct characterization of surface vibrations down to the sub-picometer amplitude levels for frequencies up to gigahertz range. A non-moving, software-selectable stabilization technique was also opted for since it had a means for controlling the optical path length in one of the arms of the interferometer. For illumination, a light source capable of producing light of coherence length: sufficient to result in an interferogram that spreads over the height array of interest for measurements. A green light emitting diode lamp type was opted for since the height range was only up to a few micrometers and the detection was sensitive to variations and drifts of the strength of the light source, therefore requiring a highly stable direct current illumination.

The illuminated light was gathered with an aspheric condenser lens to form an approximately collimated beam which was divided into the reference and sample arms of the interferometer by a non-polarizing beam splitter cube. The double light beams incident on the sample and the mirror were reflected back and recombined in the beam splitter cube. The resultant interference pattern was recorded with a 12-bit monochrome camera equipped with a long working distance video microscope optics. Software based feedback loop was used to stabilize and control the optical path length difference between the two arms of the interferometer such that the interference signal and the non-moving reference regions on the sample stayed at a constant intensity.

From the above empirical procedure, the research team observed that under the stabilized operation the interference signal for the moving sample surface was recorded as a function for time for each camera pixel. The researchers also noted that the mapping of the position of the sample surface was unambiguous due to periodic response of the interferometer to changes in the path length difference.

From the listed empirical work, performance of the interferometer has been confirmed by taking the measurements of the chronological course of a surface deformation of the aluminum nitride membrane as a function of a time-based pressure variation applied across the membrane. As observed, it is vivid that high speed cameras can be used for high speed imaging optics since it enables micro-order temporal resolution.