Synthetic hydrogen spectra of prominence oscillations

Context. Prominence oscillations have been mostly detected using Doppler velocity, although there are also claimed detections by means of periodic variations in half-width or line intensity. However, scarce observational evidence exists about simultaneous detection of oscillations in several spectral indicators. Aims. Our main aim here is to explore the relationship between spectral indicators, such as Doppler shift, line intensity, and line half-width, and the linear perturbations excited in a simple prominence model. Methods. Our equilibrium background model consists of a bounded, homogeneous slab, which is permeated by a transverse magnetic field, having prominence-like physical properties. Assuming linear perturbations, the dispersion relation for fast and slow modes has been derived, as well as the perturbations for the different physical quantities. These perturbations have been used as the input variables in a one-dimensional radiative transfer code, which calculates the full spectral profile of the hydrogen H-alpha and H-beta lines. Results. We have found that different oscillatory modes produce spectral indicator variations in different magnitudes. Detectable variations in the Doppler velocity were found for the fundamental slow mode only. Substantial variations in the H-beta line intensity were found for specific modes. Other modes lead to lower and even undetectable parameter variations. Conclusions. To perform prominence seismology, analysis of the H-alpha and H-beta spectral line parameters could be a good tool to detect and identify oscillatory modes.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in Astronomy and Astrophysic

Dark Off-limb Gap:Manifestation of a Temperature Minimum and the Dynamic Nature of the Chromosphere

We study off-limb emission of the lower solar atmosphere using high-resolution imaging spectroscopy in the H$\beta$ and Ca II 8542 \r{A} lines obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. The H$\beta$ line wing images show the dark intensity gap between the photospheric limb and chromosphere which is absent in the Ca II images. We calculate synthetic spectra of the off-limb emissions with the RH code in the one-dimension spherical geometry and find good agreement with the observations. The analysis of synthetic line profiles shows that the gap in the H$\beta$ line wing images maps the temperature minimum region between the photosphere and chromosphere due to the well known opacity and emissivity gap of Balmer lines in this layer. However, observed gap is detected farther from the line core in the outer line wing positions than in the synthetic profiles. We found that an increased microturbulence in the model chromosphere is needed to reproduce the dark gap in the outer line wing, suggesting that observed H$\beta$ gap is the manifestation of the temperature minimum and the dynamic nature of the solar chromosphere. The temperature minimum produces a small enhancement in synthetic Ca II line-wing intensities. Observed off-limb Ca II line-wing emissions show similar enhancement below temperature minimum layer near the edge of the photospheric limb.Comment: 14 pages, 8 figures, accepted in Ap

Phasemeter core for intersatellite laser heterodyne interferometry: modelling, simulations and experiments

Inter satellite laser interferometry is a central component of future space-borne gravity instruments like LISA, eLISA, NGO and future geodesy missions. The inherently small laser wavelength allows to measure distance variations with extremely high precision by interfering a reference beam with a measurement beam. The readout of such interferometers is often based on tracking phasemeters, able to measure the phase of an incoming beatnote with high precision over a wide range of frequencies. The implementation of such phasemeters is based on all digital phase-locked loops, hosted in FPGAs. Here we present a precise model of an all digital phase locked loop that allows to design such a readout algorithm and we support our analysis by numerical performance measurements and experiments with analog signals.Comment: 17 pages, 6 figures, accepted for publication in CQ

Real-time phasefront detector for heterodyne interferometers

We present a real-time differential phasefront detector sensitive to better than 3 mrad rms, which corresponds to a precision of about 500 pm. This detector performs a spatially resolving measurement of the phasefront of a heterodyne interferometer, with heterodyne frequencies up to approximately 10 kHz. This instrument was developed as part of the research for the LISA Technology Package (LTP) interferometer, and will assist in the manufacture of its flight model. Due to the advantages this instrument offers, it also has general applications in optical metrology

Compact Multifringe Interferometry with Subpicometer Precision

Deep-frequency-modulation interferometry combines optical minimalism with multifringe readout. However, precision is key for applications such as optical gradiometers for satellite geodesy or as dimensional sensors for ground-based gravity experiments. We present a single-component interferometer smaller than a cubic inch. Two of these are compared to each other to demonstrate tilt and displacement measurements with a precision of less than 20 nrad/Hz and 1 pm/Hz at frequencies below 1 Hz

Laser-Frequency Stabilization via a Quasimonolithic Mach-Zehnder Interferometer with Arms of Unequal Length and Balanced dc Readout

Low-frequency high-precision laser interferometry is subject to excess laser-frequency-noise coupling via arm-length differences which is commonly mitigated by locking the frequency to a stable reference system. This approach is crucial to achieve picometer-level sensitivities in the 0.1-mHz to 1-Hz regime, where laser-frequency noise is usually high and couples into the measurement phase via arm-length mismatches in the interferometers. Here we describe the results achieved by frequency stabilizing an external cavity diode laser to a quasimonolithic unequal arm-length Mach-Zehnder interferometer readout at midfringe via balanced detection. We find this stabilization scheme to be an elegant solution combining a minimal number of optical components, no additional laser modulations, and relatively low-frequency-noise levels. The Mach-Zehnder interferometer is designed and constructed to minimize the influence of thermal couplings and to reduce undesired stray light using the optical simulation tool ifocad. We achieve frequency-noise levels below 100  Hz/Hz at 1 Hz and are able to demonstrate the LISA frequency prestabilization requirement of 300  Hz/Hz down to frequencies of 100 mHz by beating the stabilized laser with an iodine-locked reference.DFG/SFB/112

Fiber backscatter under increasing exposure to ionizing radiation

The Laser Interferometer Space Antenna (LISA) will measure gravitational waves by utilizing inter-satellite laser links between three triangularly-arranged spacecraft in heliocentric orbits. Each spacecraft will house two separate optical benches and needs to establish a phase reference between the two optical benches which requires a bidirectional optical connection, e.g. a fiber connection. The sensitivity of the reference interferometers, and thus of the gravitational wave measurement, could be hampered by backscattering of laser light within optical fibers. It is not yet clear if the backscatter within the fibers will remain constant during the mission duration, or if it will increase due to ionizing radiation in the space environment. Here we report the results of tests on two different fiber types under increasing intensities of ionizing radiation: SM98-PS-U40D by Fujikura, a polarization maintaining fiber, and HB1060Z by Fibercore, a polarizing fiber. We found that both types react differently to the ionizing radiation: The polarization maintaining fibers show a backscatter of about 7 ppm·m−1 which remains constant over increasing exposure. The polarizing fibers show about three times as much backscatter, which also remains constant over increasing exposure. However, the polarizing fibers show a significant degradation in transmission, which is reduced to about one third. © 2020 OSA - The Optical Society. All rights reserved

Optical testbed for the LISA phasemeter

The planned spaceborne gravitational wave detector LISA will allow the detection of gravitational waves at frequencies between 0.1 mHz and 1 Hz. A breadboard model for the metrology system aka the phasemeter was developed in the scope of an ESA technology development project by a collaboration between the Albert Einstein Institute, the Technical University of Denmark and the Danish industry partner Axcon Aps. It in particular provides the electronic readout of the main interferometer phases besides auxiliary functions. These include clock noise transfer, ADC pilot tone correction, inter-satellite ranging and data transfer. Besides in LISA, the phasemeter can also be applied in future satellite geodesy missions. Here we show the planning and advances in the implementation of an optical testbed for the full metrology chain. It is based on an ultra-stable hexagonal optical bench. This bench allows the generation of three unequal heterodyne beatnotes with a zero phase combination, thus providing the possibility to probe the phase readout for non-linearities in an optical three signal test. Additionally, the utilization of three independent phasemeters will allow the testing of the auxiliary functions. Once working, components can individually be replaced with flight-qualified hardware in this setup.DLR/50 OQ 1301Bundesministerium f¨ur Wirtschaft und Technologi

Breadboard model of the LISA phasemeter

An elegant breadboard model of the LISA phasemeter is currently under development by a Danish-German consortium. The breadboard is build in the frame of an ESA technology development activity to demonstrate the feasibility and readiness of the LISA metrology baseline architecture. This article gives an overview about the breadboard design and its components, including the distribution of key functionalities.Comment: 5 pages, 3 figures, published in ASP Conference Series, Vol. 467, 9th LISA Symposium (2012), pp 271-27

Picometer-Stable Hexagonal Optical Bench to Verify LISA Phase Extraction Linearity and Precision

The Laser Interferometer Space Antenna (LISA) and its metrology chain have to fulfill stringent performance requirements to enable the space-based detection of gravitational waves. This implies the necessity of performance verification methods. In particular, the extraction of the interferometric phase, implemented by a phasemeter, needs to be probed for linearity and phase noise contributions. This Letter reports on a hexagonal quasimonolithic optical bench implementing a three-signal test for this purpose. Its characterization as sufficiently stable down to picometer levels is presented as well as its usage for a benchmark phasemeter performance measurement under LISA conditions. These results make it a candidate for the core of a LISA metrology verification facility