230 research outputs found
Real-time determination of laser beam quality by modal decomposition
We present a real-time method to determine the beam propagation ratio M2 of
laser beams. The all-optical measurement of modal amplitudes yields M2
parameters conform to the ISO standard method. The experimental technique is
simple and fast, which allows to investigate laser beams under conditions
inaccessible to other methods.Comment: 8 pages, 4 figures, published in Optics Expres
Wear-Resistant Nanostructured Sol-Gel Coatings for Functional Applications
Improvement of the wear resistance of functional surfaces is crucial in order to facilitate a variety of practical applications, such as self-cleaning or anti-fogging. This especially holds for functional surface nanostructures, whose tops can easily get worn off when exposed to even low abrasion forces. Thus, our work addresses the enhancement of the wear resistance of such fine-scale structures. We present an efficient manufacturing procedure for generating long-term durable surfaces with simultaneously tailored wetting behavior and high optical quality. Our approach is based on a sol-gel coating that consists of an alumina layer with specific nanoroughness yielding the function-relevant surface structure, and a protective thin smooth silica film providing the mechanical robustness without influencing that functional structure. The roughness of the alumina layer can be systematically adjusted, thus enabling us to achieve desired wetting effects all the way up to superhydrophilicity and, after application of an additional thin hydrophobic top coat, to superhydrophobicity. To demonstrate the enhanced robustness of these coatings we perform abrasive wear tests and investigate the impact of abrasion cycles on the wetting effects and optical properties of the coatings. Furthermore, the durability of the structures is directly revealed by advanced roughness characterization procedures based on Atomic Force Microscopy followed by power spectral density function (PSD) analysis
Total integrated scatter from surfaces with arbitrary roughness, correlation widths, and incident angles
Surface scatter effects from residual optical fabrication errors can severely degrade optical performance. The total integrated scatter (TIS) from a given mirror surface is determined by the ratio of the spatial frequency band-limited relevant root-mean-square surface roughness to the wavelength of light. For short-wavelength (extreme-ultraviolet/x-ray) applications, even state-of-the-art optical surfaces can scatter a significant fraction of the total reflected light. In this paper we first discuss how to calculate the band-limited relevant roughness from surface metrology data, then present parametric plots of the TIS for optical surfaces with arbitrary roughness, surface correlation widths, and incident angles. Surfaces with both Gaussian and ABC or K-correlation power spectral density functions have been modeled. These parametric TIS predictions provide insight that is useful in determining realistic optical fabrication tolerances necessary to satisfy specific optical performance requirements
Expected Performance of a Self-Coherent Camera
Residual wavefront errors in optical elements limit the performance of
coronagraphs. To improve their efficiency, different types of devices have been
proposed to correct or calibrate these errors. In this paper, we study one of
these techniques proposed by Baudoz et al. 2006 and called Self-Coherent Camera
(SCC). The principle of this instrument is based on the lack of coherence
between the stellar light and the planet that is searched for. After recalling
the principle of the SCC, we simulate its performance under realistic
conditions and compare it with the performance of differential imaging.Comment: 6 pages, 4 figure
Mode-resolved gain analysis and lasing in multi-supermode multi-core fiber laser
Multi-core fibers (MCFs) with coupled-cores are attractive large-mode area (LMA) specialty fiber designs that support the propagation of a few transverse modes often called supermodes (SMs). Compared to other LMA fibers, the uniqueness of MCF arises from the higher degrees of design space offered by a multitude of core-array geometries, resulting in extended flexibility to tailor SM properties. To date, the use of MCF as gain media has focused on lasers that operate in only one selected SM, typically the lowest order in-phase SM, which considerably limited the potential of these multi-core structures. Here, we expand the potential of MCF lasers by investigating multi-SM amplification and lasing schemes. Amplifier and laser systems using a 7 coupled-cores Yb-doped MCF as gain medium were successfully designed and assembled. Individual SM could be decomposed using the correlation filter technique mode analysis and the modal amplification factors (gamma(i)) were recorded. With access to amplification characteristics of individual transverse modes, a monolithic MCF laser was demonstrated that operates simultaneously on the two SMs carrying the highest optical gain
Modeling of light scattering in different regimes of surface roughness
The light scattering of rough metallic surfaces with roughness levels ranging from a few to several hundred nanometers is modeled and compared to experimental data. Different modeling approaches such as the classical Rayleigh-Rice vector perturbation theory and the new Generalized Harvey-Shack theory are used and critically assessed with respect to ranges of validity, accuracy, and practicability. Based on theoretical calculations and comparisons with Rigorous Coupled Wave Analysis for sinusoidal phase gratings, it is demonstrated that the approximate scatter models yield surprisingly accurate results and at the same time provide insight into light scattering phenomena. For stochastically rough metal surfaces, the predicted angles resolved scattering is compared to experimental results at 325 nm, 532 nm, and 1064 nm. In addition, the possibilities of retrieving roughness information from measured scattering data for different roughness regimes are discussed
Modeling of light scattering in different regimes of surface roughness
The light scattering of rough metallic surfaces with roughness levels ranging from a few to several hundred nanometers is modeled and compared to experimental data. Different modeling approaches such as the classical Rayleigh-Rice vector perturbation theory and the new Generalized Harvey-Shack theory are used and critically assessed with respect to ranges of validity, accuracy, and practicability. Based on theoretical calculations and comparisons with Rigorous Coupled Wave Analysis for sinusoidal phase gratings, it is demonstrated that the approximate scatter models yield surprisingly accurate results and at the same time provide insight into light scattering phenomena. For stochastically rough metal surfaces, the predicted angles resolved scattering is compared to experimental results at 325 nm, 532 nm, and 1064 nm. In addition, the possibilities of retrieving roughness information from measured scattering data for different roughness regimes are discussed
Speckle Temporal Stability in eXtreme Adaptive Optics Coronagraphic Images
The major noise source limiting high-contrast imaging is due to the presence
of quasi-static speckles. Speckle noise originates from wavefront errors caused
by various independent sources, and it evolves on different timescales pending
to their nature. An understanding of quasi-static speckles originating from
instrumental errors is paramount for the search of faint stellar companions.
Instrumental speckles average to a fixed pattern, which can be calibrated to a
certain extent, but their temporal evolution ultimately limit this possibility.
This study focuses on the laboratory evidence and characterization of the
quasi-static pinned speckle phenomenon. Specifically, we examine the coherent
amplification of the static speckle contribution to the noise variance in the
scientific image, through its interaction with quasi-static speckles. The
analysis of a time series of adaptively corrected, coronagraphic images
recorded in the laboratory enables the characterization of the temporal
stability of the residual speckle pattern in both direct and differential
coronagraphic images. We estimate that spoiled and fast-evolving quasi-static
speckles present in the system at the angstrom/nanometer level are affecting
the stability of the static speckle noise in the final image after the
coronagraph. The temporal evolution of the quasi-static wavefront error
exhibits linear power law, which can be used in first order to model
quasi-static speckle evolution in high-contrast imaging instruments.Comment: A&A accepte
Fundamental limitations on Earth-like planet detection with Extremely Large Telescopes
We analyse the fundamental limitations for the detection of extraterrestrial
planets with Extremely Large Telescopes. For this task, a coronagraphic device
combined to a very high order wavefront correction system is required but not
sufficient to achieve the contrast level needed for detecting an
Earth-like planet. The stellar residuals left uncorrected by the wavefront
correction system need to be calibrated and subtracted. In this paper, we
consider a general model including the dynamic phase aberrations downstream the
wavefront correction system, the static phase aberrations of the instrument and
some differential aberrations provided by the calibration unit. A rather
optimistic case of a filled circular pupil and of a perfect coronagraph is
elsewhere assumed. As a result of the analytical study, the limitation mostly
comes from the static aberrations. Using numerical simulations we confirm this
result and evaluate the requirements in terms of phase aberrations to detect
Earth-like planets on Extremely Large Telescopes.Comment: 8 pages, 8 figures, accepted in A&
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