Development and characterization of a differential interferometer setup using ultra-wideband SAW devices
Résumé
Optical interferometry is a well-known technique for studying wave propagation and is often preferred over other methods for its high
precision in studying wave profiles, amplitude and phase variations, standing wave patterns, group delay measurements, etc., of waves like surface acoustic waves, Scholte- Stoneley waves, dilatational waves, etc... In the past, different interferometer setups have been employed to detect and measure surface acoustic waves. Differential interferometers have proved to be useful in various fields, like piezoelectric thin film characterization, angular measurements of the order of nano-radians and displacement measurements in the picometre range, of acoustic signals, etc. High sensitivity to the phase and amplitude of the acoustic signals and easy calibration of the setup are the key advantages of a differential interferometer. They are a solution to a common disadvantage of interferometry, i.e., path length sensitivity. Our objective is to construct an interferometer that allows to study transient surface acoustic waves in the time domain.
The differential interferometer used in this work is an extended version of a Michelson’s interferometer, where the laser beam
reflected from the SAW device, carrying phase and amplitude information of the surface acoustic waves, is divided into two orthogonally polarized beams that traverse different arm lengths, and are recombined using a polarizing beam splitter. This causes
shearing in the time domain, with the change in phase along the two arms of the interferometer being a function of the path lengths of
the two orthogonal beams causing interference. The signal received on the photodiode as a function of time is proportional to the
normal velocity at the surface of the sample. The experimental setup is designed to measure displacements of the order of a few
nanometers, of a SAW device producing transient short pulses with a chirped input IDT. An output IDT with only one finger-pair is
used for comparison purposes.
The setup being extremely sensitive, requires noise calibration. In order to improve the alignment and test the accuracy of the
response, different parameters of the experimental setup are varied. An input chirp is used to excite SAW pulses within a frequency
range of 200 – 400 MHz. The Fourier transform limited pulse duration is about 5 ns. The response measured at the output IDT,
directly and by using the interferometer setup, are compared, and the change in amplitude and phase for the two measurements are
studied for two cases: varying position of focus between the two IDTs on the SAW device, and varying path length.