Active coronagraphy is deemed to play a key role for the next generation of high-contrast instruments, notably in order to deal with large segmented mirrors that might exhibit time-dependent pupil merit function, caused by missing or defective segments. To this purpose, we recently introduced a new technological framework called digital adaptive coronagraphy (DAC), making use of liquid-crystal spatial light modulators (SLMs) display panels operating as active focal-plane phase mask coronagraphs. Here, we first review the latest contrast performance, measured in laboratory conditions with monochromatic visible light, and describe a few potential pathways to improve SLM coronagraphic nulling in the future. We then unveil a few unique capabilities of SLM-based DAC that were recently, or are currently in the process of being, demonstrated in our laboratory, including NCPA wavefront sensing, aperture-matched adaptive phase masks, coronagraphic nulling of multiple star systems, and coherent differential imaging (CDI).
The thermomechanical behavior of a standard optical fiber with double coating is theoretically analyzed at room temperature and ∼ 220 K. As the primary coating becomes stiffer at low temperature, the impact of the thermal expansion of the coatings is no longer negligible and contributes to the thermal response of the fiber sensor. The temperature sensitivity enhancement is validated by distributed fiber sensors based on Brillouin scattering and coherent Rayleigh scattering at 220 K; the experimental results agree well with the theoretical analysis.
Temperature and strain discrimination is experimentally demonstrated in an elliptical-core polarization-maintaining fiber by making use of Rayleigh-based distributed birefringence measurements and the frequency shift of the correlation peak obtained by standard coherent optical time-domain reflectometry. The high sensitivity of coherent Rayleigh sensing and the very distinct behavior of birefringence makes the two quantities clearly discriminated, resulting in temperature and strain accuracies of ~ 40 mK and ~ 0.5 με, respectively, for distributed measurements with a 2 m spatial resolution.
A novel distributed fibre sensing technique is described and experimentally validated, based on birefringence measurements using coherent Rayleigh scattering. It natively provides distributed measurements of temperature and strain with more than an order of magnitude higher sensitivity than Brillouin sensing, and requiring access to a single fibre-end. Unlike the traditional Rayleigh-based coherent optical time-domain reflectometry, this new method provides absolute measurements of the measurand and may lead to a robust discrimination between temperature and strain in combination with another technique. Since birefringence is purposely induced in the fibre by design, large degrees of freedom are offered to optimize and scale the sensitivity to a given quantity. The technique has been validated in 2 radically different types of birefringent fibres – elliptical-core and Panda polarization-maintaining fibres – with a good repeatability.
In several applications a temperature contrast between the sensing fibre and the environment is required to detect changes in the environmental heat capacity. For this purpose the process of electrical heating in metallic-coated fibres is theoretically analysed and modelled in steady-state conditions based on the thermal energy generated by resistive heating and the losses induced by convection and radiation. The impact of ambient temperature and pressure is investigated. The proposed model for the thermal exchange is experimentally validated using a high-resolution Brillouin distributed fibre sensor, which is used to measure the longitudinal profile of the temperature reached by electrical heating along an Alcoated optical fibre.
The spectrum of the temporal traces obtained from a phase-sensitive optical time-domain reflectometer is theoretically and experimentally analysed, demonstrating its dependence on the incident optical pulse shape. Numerical simulations and theoretical results are validated experimentally, showing a good matching for rectangular optical pulses. The influence of the photodetector bandwidth on the temporal trace quality is also investigated by simulation and experiment. Results show that the photodetector bandwidth needs to be ~ 40 % wider than the pulse spectrum to acquire time-domain traces of the Rayleigh backscattered light with direct detection.
The response of a distributed temperature sensor based on coherent Rayleigh scattering is experimentally studied in the temperature range from 77 K up to 300 K, using fibres with standard and ORMOCER coating. A precise and absolute frequency scan is performed to obtain the best temperature resolution that turns out to be in the mK range. Experimental results point out that heating and cooling processes, at cryogenic temperatures, exhibit different temperature sensitivities when standard single-mode fibres are used; however, specially coated fibres exhibit better repeatability.
A novel configuration for a Brillouin distributed sensor using a phase modulated probe is presented. It offers the
combined advantages of a direct implementation using simple devices and a large immunity to noise, shifting the
information from the baseband to a higher frequency, which substantially strengthens the robustness to perturbations, interferences and other optical coherent noises. It naturally facilitates the possibility to perform frequency-coding on the probe to realise dynamic and fast measurements, since all frequency tunings are realised at sub-GHz frequencies.
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