We have built a laboratory testbed called the Exoplanet Imaging System Testbed (EXIST) to develop future highcontrast imaging technologies. The main objective of the EXIST is the development of broadband coronagraph and wavefront control techniques. The EXIST is equipped with several fiber-coupled laser and broadband light sources to model star and planets. A spatial light modulator (SLM) is used to carry out the wavefront control in front of the coronagraph. We incorporated a variety of coronagraphic masks, including four-quadrant, eightoctant, and 12-sector phase masks. These masks exhibit second-, fourth-, and sixth-order starlight suppression properties, respectively. When combined with the wavefront control, higher-order coronagraphic masks provided better dark hole contrast. This paper reports on recent experimental results and prospects for future technological development at the EXIST.
Direct detection of Earth-like exoplanets requires a high-contrast imaging system to suppress bright stellar light that prevents the detection. The wavefront sensing and control technique which is one component of the high-contrast imaging system can suppress stellar scattered light (speckles) caused by wavefront aberrations. However, deformation of the system due to temperature changes in space telescopes or atmospheric turbulence in ground-based telescopes cause speckles that fluctuate faster than the wavefront sensing and control. As the post-processing technique, the Coherent Differential Imaging on Speckle Area Nulling (CDI-SAN) method was proposed to suppress the fast-fluctuating speckles. We are conducting the laboratory demonstration of the CDISAN method using two types of experimental facilities. One of them is equipped with a deformable mirror and a field programmable gate array. In our initial laboratory demonstration, we achieved 10−8 level contrast. To achieve higher contrast, we are updating our facility. The other facility is equipped with a spatial light modulator (SLM). In this facility, the contrast was improved by 10−1 using the CDI-SAN method.
High-contrast coronagraphs have been developed to detect and characterize exo-planets with contrasts of 10-8~10-10. One method used for coronagraphs is the vector vortex phase mask, and a 6th-order one is required to sufficiently suppress the light from stars with finite apparent diameters using future large telescopes at the ground and in space. We manufactured 12- and 24-segmented sixth-order vector vortex phase masks with photonic crystal waveplates, and their fast axis orientations are 90 degrees and 45 degrees in adjacent regions, respectively. A three-layer structure was designed to cover a broad band of 600-1000nm. The design contrast for the phase mask alone is 8e-6, and we expect to achieve the final contrast by polarization filtering. We found that the retardation of the phase mask was almost coincident to the design curve. As for the 24-segment phase mask, a shaped pupil was designed as a combined element to exhibit the performance of the phase mask in the pupil shape of the TMT, which has the obstructions of the secondary mirror, the spiders, and the segment boundaries. The shaped pupil design has a transmittance of about 70% and a contrast of 10-7 within the outer working angle of 10 lambda/D. A shaped pupil was manufactured by chromium etching on a quartz substrate, and we observed the reduction of the diffracted light within 8 lambda/D.
Various types of high-contrast imaging instruments have been proposed and developed for direct detection of exoplanets by suppressing nearby stellar light. Stellar speckles due to wavefront aberration can be suppressed by the appropriate wavefront control, called the dark hole control. However, the speckles, which fluctuate faster than the dark hole control due to atmospheric turbulence in ground-based telescopes or instrument deformation caused by temperature changes in space telescopes, cannot be suppressed by the control and remain in focal plane images. The Coherent Differential Imaging on Speckle Area Nulling (CDI-SAN) method was proposed to overcome such fast fluctuating speckles and detect exoplanetary light. We constructed an optical setup in a laboratory to demonstrate the CDI-SAN method. With the dark hole control and the CDI-SAN method, we achieved 10−8 level of contrasts.
High-contrast imaging techniques are essential for future space missions aimed at directly detecting and characterizing exoplanets. We constructed a new testbed called the facility for coronagraphic elemental technologies (FACET) for the development of high-contrast imaging techniques. FACET has three optical paths (A, B, and C). Path A is used for developing focal-plane phase mask (FPM) coronagraphs. Currently, we have been developing photonic-crystal multi-layer phase masks for suppressing stellar light over a broad wavelength range. In path B, we demonstrate high-contrast observation combining FPM coronagraphs with a dark hole (DH) control technique. A spatial light modulator (SLM) is used as a wavefront control device. We are developing DH control techniques that take advantage of the large pixel format of the SLM. In path C, we demonstrate DH control for detecting exoplanets not only around single stars but also around binary-star systems. We install the SLM into the common-path visible nulling coronagraph to test the multiple-star DH control. We report details of FACET and recent progress of our activities at FACET. We have recently made significant progress with the demonstration of observation for a single star for which we achieved a monochromatic contrast of 2.9 × 10−9 at path C.
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