We summarize our development of a low-cost instrument for the detection of fabrication errors in diffraction gratings. Our instrument applies a low-cost digital camera and high dynamic range (HDR) imaging in the focal plane of a lens to analyze light reflected by a diffraction grating. A dynamic range close to 1010 is achieved for a spatial frequency range that is relevant for types of errors that occur in the lithographic fabrication of gratings. We also describe a modification of the instrument in which a spatial filter is used to block the large fraction of light that does not carry information on fabrication errors. This “coronagraph” modification reduces measurement times by about an order of magnitude. A unique pseudologarithmic transformation is described for the visualization of wide range data that include zero or negative values, such as HDR images.
We describe a coronagraph-like instrument for the characterization of fabrication errors in diffraction gratings and diffractive optics. A diffraction grating under test is illuminated with collimated laser light, and a reflected diffraction order is focused by a lens. A beam stop placed at the focal plane of the lens blocks the primary diffracted beam, passing only the small fraction of light scattered into nearby angles due to fabrication errors. The transmitted light fraction is focused by a second lens onto a digital camera and the light distribution is measured with a sensitive high dynamic range imaging method. Preliminary measurements with the coronagraph setup are compared to focal-plane images made without beam stop.
We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics on bulk InAs, InGaAs and GaAs that leads to terahertz emission after ultrafast photoexcitation. This detailed model has allowed us to understand various aspects of the THz emission process, including the near-field distribution which has been experimentally observed, the role of the excess excitation photon energy, and the relative importance of the surface field driven, diffusive (photo-Dember) and ballistic currents.
In order to understand the near-field emission we coupled a finite-difference time-domain routine to the carrier dynamics simulation, by doing this, we were able to analyse the near terahertz field emission caused by the motion of such carriers even when the excitation is performed at normal incidence. We found that both the current parallel, which has traditionally been assumed not to take part in the emission, and normal to the interface take a relevant role in the terahertz generation. We performed another set of simulations for different bandgaps and excitation-photon energies in order to compare the emission power of all three semiconductors as function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their motilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.
We report on recent progress in the development of our focal plane imaging system for the detection and characterization of small fabrication errors in diffraction gratings. The instrument uses a purpose-designed high dynamic range imaging method in conjunction with a low-cost digital camera to acquire images with a dynamic range that can now exceed eight orders of magnitude. The sensitivity and utility of the instrument is demonstrated with measurements of three different diffraction gratings. Avenues for further possible improvements of the instrument are discussed.
We present a straight forward and practical method for joining pneumatically floated optical tables with no
previous preparation. In order to demonstrate this method we joined two optical tables in an uncentered "T-shape"
using twenty four stainless steel plates (SSP), and used a Michelson interferometer to compare the stability
of the entire "T"-structure versus one of its parts alone, finding that they both show similar rigidity. We also
evaluated the performance of two different master-salve leg configurations by calculating the stress on the joint
and confirmed the calculations by Michelson interferometry. In terms of floor vibration damping, it was observed
that the performance of the system for the joined "T"-table seemed to be comparable to that of a single segment.
This method can significantly reduce costs of large optical tables and will be useful to extend existing optical
tables without manufacturer modification.
We present a simple and practical method for joining pneumatically floated optical tables. In order to demonstrate
this method we joined two optical tables in an uncentered "T-shape", and used a Michelson interferometer to
compare the stability of the entire "T-structure" versus one of its parts alone finding that they both show similar
rigidity. We also found the optimal master-slave leg configuration by calculating the stress on the joint and
confirmed the calculations by Michelson interferometry. The vibration damping for the "T-structure" against
the unjoined tables was measured finding comparable results. This method can significantly reduce costs of large
optical tables and will be useful to extend existing optical tables without manufacturer modification.
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