We present a synthesized sub-ps dual-wavelength laser source for digital holographic interferometry with a wide
reconstruction range. The developed laser source generates two spectrally separated parts within one pulse. The sub-ps
pulse duration desensitizes the holographic setup to environmental impacts. A center wavelength distance of only 12 nm
with a high contrast was demonstrated by spectral shaping of the 50 nm broad seed spectrum of a CPA Ti:sapphire laser
system centered at 800 nm.
Time-resolved two-wavelength contouring requires the simultaneous and separable recording of two holograms. In
general, a single CCD-camera is applied, and the spectral separation is realized by different reference wave tilts, which
requires ambitious interferometric setups. Contrary to this, we introduce two CCD-cameras for digital holographic
recording, thus essentially simplifying the interferometric setup by the need of only one propagation direction of the
reference wave. To separate the holograms for the simultaneous recording process, a Mach-Zehnder interferometer was
extended by a polarization encoding sequence.
To study our approach of time-resolved digital holographic two-wavelength contouring, an adaptive fluidic PDMS-lens
with integrated piezoelectric actuator served as test object. The PDMS-lens consists of an oil-filled lens chamber and a
pump actuator. If a voltage is applied to the piezoelectric bending actuator the fluid is pumped into the lens chamber
which causes a curvature change of the 60-μm thick lens membrane and thus a shift of the focal length. The dynamic
behavior of the PDMS-lens, driven at a frequency of 1 Hz, was investigated at a frame rate of 410 frames per second.
The measured temporal change of the lens focal length between 98 and 44 mm followed the modulation of the
piezoelectric voltage with a 30 V peak-to-peak amplitude. Due to the performed time-resolved two wavelength
contouring, we are able to extract the optical path length differences between center and perimeter of the lens. From the
calculated phase difference maps we estimated large optical path differences of larger than 10 μm, corresponding to
more than 15 times of the source wavelength.