Interferometric surface tests of stigmatic aspherics can be carried out in a null test configuration. Null tests require
reference null elements either plane or spherical surfaces or both. A parabolic reflector transforms a plane into a spherical
wave which converges to the focus of the paraboloid. Therefore, a spherical ball lens or a steel ball can be placed into the
focus enabling a double-pass geometry for the null test. Here a Fizeau interferometer geometry has been selected in order
to guarantee invariance against polarization distortions under the assumption that radially polarized laser light is used for
the interferometer. Radial polarized light is necessary to mimic a Hertzian dipole field. Due to the extreme solid angle
produced by the paraboloid the alignment of the setup is very critical and needs auxiliary systems for the control.
Aberrations caused by misalignments are removed via fitting of suitable functionals provided through ray-trace
simulations. It turned out that the usual vector approximations fail under these extreme circumstances. Test results are
given for a paraboloid with 2mm focal length transforming a plane wave into a near dipole wave comprising a solid angle
of about 3,4π.
The polarization dynamics of vertical-cavity surface-emitting
lasers operating close to threshold is investigated
experimentally. For free-running devices a characterization of the
dynamics is given for a scenario when two modes are excited at
threshold due to a small net gain anisotropy. The polarization
dynamics in this regime are found to be governed by the relaxation
oscillations and to exhibit an anticorrelation of the two modes.
The level of anticorrelation is strongly depending on the
injection current. If isotropic feedback by a distant
reflector is added, the dynamics are governed by the external
cavity round-trip time scale and low frequency fluctuations are
observed for both modes. These are shown to occur also for large
net gain anisotropy, but without excitation of polarization
degrees of freedom.
Polarization selection in small-area vertical-cavity
surface-emitting lasers (VCSELs) is studied experimentally in
dependence on injection current and substrate temperature in the
vicinity of the minimum threshold condition. Different types of
polarization switching (PS) are observed and analyzed: PS from the
high to the low frequency mode, PS from low to high frequency and
double switching (from high to low and back to high frequency).
Whereas PS from the high to the low frequency mode is due to a
change of sign of linear dichroism, the optical spectra show
dynamical transition states for the other case which hint to the
relevance of nonlinear effects. A comparison to the predictions of
the SFM-model based on phase-amplitude coupling is given. The
interplay of spatial and polarization effects can depend strongly
on the ambient temperature.
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