Frank Loya

Mid-infrared nuller for Terrestrial Planet Finder: design, progress, and results

Nulling interferometry shows promise as a technique enabling investigation of faint objects such as planets and exo-zodiacal dust around nearby stars. At Jet Propulsion Laboratory, a nulling beam combiner has been built for the Terrestrial Planet Finder project and has been used to pursue deep and stable narrowband nulls. We describe the design and layout of the modified Mach Zehnder TPF nuller, and the results achieved in the laboratory to date. We report stabilized nulls at about the 10-6 level achieved using a CO2 laser operating at 10.6 μm, and discuss the alignment steps needed to produce good performance. A pair of similar nullers has been built for the Keck Observatory, for planned observations of exo-zodiacal dust clouds. We also show briefly a result from the Keck breadboard experiments: passively stabilized nulls centered around 10.6 micron of about 2 10-4 have been achieved at bandwidths of 29%.

Exoplanet Exploration Program, Planet Detection Test-bed: Latest results of planet light detection in the presence of starlight

The Planet Detection Test-bed is a lab based simulation of the optics and control systems for an interferometer based Terrestrial Exoplanet characterization mission. The test-bed supports starlight nulling at 10um infrared wavelengths, with fringe tracking at 2um wavelengths and angle and shear tracking at visible wavelengths. It further allows injection of simulated planet light in the presence of the nulled star light, to allow testing of planet detection methods. We will describe the detailed construction and operation of the test-bed from an optical and control system perspective. We will also report the latest results for narrow band nulls, and the detection of broad band planet light in the presence of nulled starlight.

The Potential of Rotating-Baseline Nulling Interferometers Operating within Large Single-Telescope Apertures

The use of a rotating-baseline nulling interferometer for exoplanet detection was proposed several decades ago, but the technique has not yet been fully demonstrated in practice. Here we consider the faint companion and exozodiacal disk detection capabilities of rotating-baseline nulling interferometers, such as are envisioned for space-based infrared nullers, but operating instead within the aperture of large single telescopes. In particular, a nulling interferometer on a large aperture corrected by a next-generation extreme adaptive optics system can provide deep interferometric contrasts, and also reach smaller angles (sub λ/D) than classical coronagraphs. Such rotating nullers also provide validation for an eventual space-based rotating-baseline nulling interferometer. As practical examples, we describe ongoing experiments with rotating nullers at Palomar and Keck, and consider briefly the case of the Thirty Meter Telescope.

Testing exo-planet signal extraction using the Terrestrial Planet Finder planet detection testbed

The Planet Detection Testbed developed at the Jet Propulsion Laboratory is being used to test direct optical detection of an Earth-like planet using nulling interferometry. Operating at infrared wavelengths, the testbed produces four near-identical beams simulating a distant star and planet. The testbed is reconfigurable to simulate different telescope array designs that are being studied. Many of the systems which will be needed for the space application of nulling stellar interferometry are incorporated. The goal of the testbed is to simulate the planet detection process which requires both a long detection period of many hours to overcome the thermal background noise and also high instrument stability to avoid introducing noise signals that could be mistaken for a planet. Numerous control systems are needed to maintain the optical path differences to about 2 nm and maintain beam alignments in shear and tilt. The testbed emulates functions of the fringe-tracking and metrology systems envisioned for the flight system including finding and tracking the fringe, controlling vibration and allowing for changing conditions. The relationship of the testbed to flight conditions is discussed and the latest results are presented showing planet detection in the presence of bright starlight.

A Phase-Shifting Zernike Wavefront Sensor for the Palomar P3K Adaptive Optics System

A phase-shifting Zernike wavefront sensor has distinct advantages over other types of wavefront sensors. Chief among them are: 1) improved sensitivity to low-order aberrations and 2) efficient use of photons (hence reduced sensitivity to photon noise). We are in the process of deploying a phase-shifting Zernike wavefront sensor to be used with the real-time adaptive optics system for Palomar. Here we present the current state of the Zernike wavefront sensor to be integrated into the high-order adaptive optics system at Mount Palomar’s Hale Telescope.

Experimental results from the optical planet detector interferometer

Researches have suggested several techniques (ie.: pupil masking, coronography, nulling interferometry) for high contrast imaging that permit the direct detection and characterization of extrasolar planets. Our team at JPL, in previous papers, has described an instrument that will combine the best of several of these techniques: a single aperture visible nulling corograph. The elegant simplicity of this design enables a powerful planet-imaging instrument at modest cost. The heart of this instrument is the visible light nulling interferometer for producing deep, achromatic nulls over a wide optical band pass, and a coherent array of single mode optical fibers 2 that is key to suppressing the level of scattered light. Both of these key components are currently being developed and have produced intial results. This paper will review, in detail, the design of the nulling interferometer experiment and review the latest experimental results. These results illustrate that we are well on our way to developing the fundamental components necessary for planned mission. Likewise, our results demonstrate that the current nulling levels are already consistent with final requirements.

Progress in testing exo-planet signal extraction on the TPF-I Planet Detection Testbed

The Terrestrial Planet Finder Interferometer (TPF-I) concept is being studied at the Jet Propulsion Laboratory and the TPF-I Planet Detection Testbed has been developed to simulate the detection process for an earthlike planet orbiting a star within about 15 pc. The testbed combines four beams of infrared light simulating the operation of a dual chopped Bracewell interferometer observing a star and a faint planet. This paper describes the results obtained this year including nulling of the starlight on four input beams at contrast ratios up to 250,000 to 1, and detection of faint planet signals at contrast ratios with the star of 2 million to 1.

Progress in broadband infrared nulling technology for TPF

The Terrestrial Planet Finder Interferometer Project (TPF-I) has set for itself a host of challenging technical milestones along its path to demonstrating the feasibility of infrared nulling for planet detection. Our activities are focused solely upon the experimental demonstration that deep nulling in the mid-IR over a wide bandpass can be accomplished. Specifically, we have the near-term goal of demonstrating a contrast of 10-6 at 10 μm with a 25% spectral bandwidth. To meet these goals, several areas of technical development are required. These include: single-mode infrared fibers, bright infrared sources, laser path-length and tip/tilt metrology, and improvements to null detection. Progress in each of these areas of technical development will be reviewed as well as their impact on the overarching technical milestones.

Exploring 5-40 AU scales around AB Aurigae with an upgraded Palomar Fiber Nuller

With a null precision of a few 10-4 at all azimuth angles inside a field-of-view extending from 35 to 275 mas, the Palomar Fiber Nuller (PFN) is able to explore angular scales intermediate between those accessed by coronagraphic imaging and by long baseline interferometry. We first briefly summarize the recent performance improvements of the PFN (sensitivity, azimuthal coverage, duty cycle efficiency on-sky) over the 2011-2014 time period. Then we report on recent K-band observations of the young pre-main sequence star AB Aurigae obtained with the PFN. It is shown that a mean astrophysical null of 1.52% was detected around AB Aur at all probed azimuthal angles, and this inside a field-of-view corresponding to projected separations between 5 and 40 AU. In addition, we also report a slight ±0.2% modulation in addition to this average null level. The isotropic astrophysical null is indicative of circumstellar emission dominated by an azimuthally extended source, possibly a halo or one or more rings of dust. The modest azimuthal variation may be explained by some skewness or anisotropy of the spatially-extended source, e.g. with an elliptical or spiral geometry, or clumping, but it could also be due to the presence of a point-source located at a separation of ~120 mas (17AU) and carrying ~6*10-3 of the stellar flux.

Current progress on TPF-I mid-infrared achromatic nulling at the Jet Propulsion Laboratory

Infrared interferometric nulling is a promising technology for exoplanet detection. Nulling research for the Terrestrial Planet Finder Interferometer has explored several interferometer architectures at the Jet Propulsion Laboratory (JPL). The most recent efforts have focused on an architecture which employs a geometric field flip to achieve the necessary π phase delay in the interferometer. The periscope design currently in use allows for a completely achromatic phase flip. Deep interferometric nulling requires optical path stability, precision optical alignment, intensity balancing, and dispersion correction. This paper will discuss recent efforts to implement a precision optical alignment, stabilize the interferometer environment, implement optical path metrology, control intensity balance, and compensate for dispersion introduced by beamsplitter mismatch.