SAW events were synchronized to the middle linear
fringe region with a technique similar to that reported by
the authors previously (Chow et al., 2014). In brief, a piezoelectric
stage was mounted on the reference mirror and to
set to oscillate. This produced an interference pattern with
consistent and repeatable temporal wavelengths (Fig. 2)
that could be tuned to the timescale of the generation laser
firing operation (several hundred microseconds) by adjusting
the piezoelectric vibration frequency. An oscilloscope
monitoring signal rise and fall times could then detect
selected wavelengths and use them to trigger and time
the laser output to the desired linear fringe region. This
synchronization technique relies on the ability to oscillate
the mirror fast enough to overcome background noise, yet
slow enough so as to not impede on the SAW timescale.
Typically, a frequency of 100–200 Hz with a mirror movement
of several microns was sufficient.