By a combination of quarter-wave plates made of different birefringent materials it is possible to produce achromatic quarter-wave plates whose degree of achromatism is dependant on the dispersions of birefringence and on the thicknesses of the individual quarter-wave plates. These waveplates are widely used in optical instrumentation and the residual errors associated with these devices can be very important in high resolution spectro-polarimetry measurements. The misalignment of optic axis in a double crystal waveplate is one of the main source of error and leads to elliptical eigenpolarization modes in the retarder and the oscillation of its orientation according to the wavelength. This paper will discuss, first, how the characteristics of a quartz-MgF2 quarter-wave plate is affected by such a misalignment. A correlation with the experiment is then achieved in order to highlight the interest of taking a possible tilt error into consideration when doing polarimetric measurements.
The temporal frequency response of water supplemented with scattering particles was deduced from the measurement of the optical backscattering of a short (100 ps) blue-green (532 nm) optical pulse given by a frequency-doubled Nd:YAG laser. Indeed, this backscattering measurement gave the water impulse response from which the transfer function was computed by taking the power spectrum. This backscattering frequency response had a low-pass filter-like response which enabled us to estimate the medium cut-off frequency. As the backscattered signal shape depends on the value of the water attenuation coefficient, the ocean water cut-off frequency varies with the amount of scattering particles its contains. The aim was to define the variations of the cut-off frequency as a function of the attenuation coefficient.
We have experimentally assessed the efficiency of a new
underwater-target detection scheme, called modulated lidar. The technique
used is based on a remarkable physical property of the propagating medium;
sea-water has a low-pass transfer function in backscattering
configuration. As the target return is less frequency-dependent, the use
of a radio frequency modulated laser source along with a narrow band
filtering at the detection drastically reduces the backscattering clutter,
but does not affect the target return; the modulation frequency of the
source is substantially above the cut-off frequency of the propagating
medium. This technique allowed us to greatly improve underwater-target
contrast. We associated with this frequency detection a temporal
localization of the underwater target by using a pulse-modulated laser
source.
In underwater optical imaging using pulsed laser radiation the vector nature of the electromagnetic wave can be used to produce polarization contrast. We have analyzed different polarimetric transformations of light pulses through seawater on immersed targets and defined optimum conditions for using polarization parameters.
Laser imaging set-ups permit characterization of the vectorial nature of the optical wave which is transmitted by a medium or reflected by a target, from polarimetric formalisms such as Jones or Stokes - Mueller calculi. When the Mueller matrix of targets (or media) is experimentally determined, information about the polarizing and depolarizing properties can be obtained from a Mueller - Jones matrix, which operation requires knowledge of the experimental uncertainties in the elements of . We will discuss the possibility of a polar decomposition of the extracted Mueller - Jones matrix. We then obtain the product of a Hermitian matrix and a unitary matrix, which can be associated with it, for a polarization transformer, with an elliptical partial polarizer and a pure elliptical retarder respectively. Classification of a target is deduced from its polarimetric characteristics (depolarization, polarizing and retardation properties). Experimental Mueller matrices of polarization transformers (media or targets) are provided and analysed by this method in order to characterize the physical behaviour of these optical systems with respect to polarization phenomena.
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