Recent advances in the field of ultra-short pulse lasers have led to the development of reliable sources of carrier envelope phase stabilized femtosecond pulses. The pulse train generated by such a source has a frequency spectrum that consists of discrete, regularly spaced lines known as a frequency comb. In this case both the frequency repetition and the carrier-envelope-offset frequency are referenced to a frequency standard, like an atomic clock. As a result the accuracy of the frequency standard is transferred to the optical domain, with the frequency comb as transfer oscillator. These unique properties allow the frequency comb to be applied as a versatile tool, not only for time and frequency metrology, but also in fundamental physics, high-precision spectroscopy, and laser noise characterization. The pulse-to-pulse phase relationship of the light emitted by the frequency comb has opened up new directions for long range highly accurate distance measurement.
We demonstrate the measurement of an arbitrary absolute distance with a mode-resolved frequency comb laser. By resolving the frequency comb modes individually with a virtually imaged phase array (VIPA), thousands of lasers modes are available that can be exploited for massively parallel homodyne interferometry. With this method a non-ambiguity range of 15 cm is obtained, allowing for non-incremental distance measurement in an interferometric scheme.
We are developing an experimental setup to characterize dynamic scattering in presence of static scattering. We attempt
to retrieve the flow parameters like fluid concentration and velocity in presence of phantoms providing static scattering
mimicking the characteristics of skin. Our measurement relies on an optimally-designed optical setup coupled to a high
speed detector and the use of appropriate light sources. The flow of particles causes a time varying effect on the speckle
pattern which can be measured quantitatively by the speckle contrast term. The speckle contrast is defined as the ratio of
standard deviation and mean intensity of speckle variation. Depending on the concentration and velocity of moving
particles, the speckle pattern will decorrelate and this results in a drop in the contrast which can also be seen in the
recorded images as blurring of the speckle pattern. In literature, measured contrast is related to the velocity and the
concentration of the scatterers as it plays a major role in the speckle correlation time (ι<i>c</i>). In our experimental setup we
attempt to recover the properties of the moving scatterers in presence of static scatterers. In parallel we present
experimental simulations of our experiment comparing it with theoretical studies describing dynamic speckle in presence
of static scatterers.
The invention of the femto-second frequency comb laser has revolutionized the field of high-resolution spectroscopy, by
providing very accurate reference frequencies in the optical domain, acting as a 'frequency ruler'. Similarly, a frequency
comb can be viewed as a ruler for distance measurement, which is based on the fact that the vacuum distance between
subsequent pulses is known with the accuracy of the used time standard. We have recently demonstrated absolute distance
measurements using a FC laser applying a cross-correlation technique, which was supported by a theoretical and a numerical
study on the formation of cross-correlation in dispersive media. In this contribution a measurement scheme based on
dispersive (spectral) interferometry is presented. For the measurement of distances up to 50 meter, sub-micrometer accuracy
A Cavity-Enhanced Direct Frequency Comb Spectroscopy (CE-DFCS) setup developed in our laboratory is described.
We focus on the broadband spectrometer which is based on a Virtually Imaged Phased Array (VIPA). It can detect 3500
independent channels simultaneously, covering a bandwidth of 20 nm with a resolution of 800 MHz around a central
frequency of 1.5 μm in a few microseconds. The bandwidth can be extended to 200 nm by rotating the grating of the
Frequency and distance metrology have been revolutionized with the arrival of stabilized frequency comb lasers. We
discuss several aspects of distance metrology specially the contribution of the dispersion in by air.