Richard B. Barclay

Micromachined tunable Fabry-Perot filters for infrared astronomy

Micromachined Fabry-Perot tunable filters with a large clear aperture (12.5 to 40 mm) are being developed as an optical component for wide-field imaging spectroscopy. This program applies silicon micromachining fabrication techniques to miniaturize Fabry-Perot filters for astronomical science instruments. The filter assembly consists of two reflector plates that form a tunable Fabry-Perot etalon. One plate is fixed and the second plate is free to move along the optical axis on silicon springs. The moving plate is actuated electrostatically by capacitance pads on the stationary and moving plates. To reduce mass, both reflectors are fabricated by applying optical coatings to a thin freestanding silicon nitride film held flat in drumhead tension. In this paper, we discuss the etalon design, electromechanical modeling, fabrication, and initial results. In the current design, the transmission aperture is 11.0 mm in diameter, the moving plate is 26.3 mm in diameter, and the stationary plate is 32.6 mm in diameter. The plates and springs are nominally 350 μm thick, the electrical and mechanical spacing between plates is 18 μm, and the uncoated optical spacing is 15 μm.

Spectral contrast enhancement techniques for extrasolar planet imaging

We use analytical models to characterize the optical response of observing systems and explore spectroscopic techniques that exploit the planetary spectral signature to enhance the planet-to-star contrast in imaging. Radiative transfer model calculations of gas-giant extrasolar planets are employed in selecting spectral regions where the planetary signal shows enhancement with respect to the stellar spectrum. We specifically discuss the use of frequency switching, in conjunction with pupil plane techniques (shaping, apodizing), to optimize the detection of the planetary signal of potential planetary systems for both ground-based and space-borne observations. Expected properties of known extrasolar planets motivate the use of the near-IR spectral region. This work was motivated by the focused technology development of a MEMS tunable filter, and the application of such devices to frequency-switched imaging spectroscopy.

The balloon experimental twin telescope for infrared interferometry (BETTII): An experiment for high angular resolution in the far-infrared

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is a new balloon-borne far-infrared interferometer, being designed to provide spatially-resolved spectroscopy in the far infrared (30–90 μm). The combination of an 8-meter baseline with a double-Fourier Michelson interferometer allows the identification and separation of closely-spaced astronomical sources, while also providing a low-resolution spectrum for each source. In this wavelength range, BETTII will provide subarcsecond angular resolution, a capability unmatched by other far-infrared facilities. This paper provides an overview of the entire design of the BETTII experiment, with a short discussion of the predicted performance on flight.

Design and status of the Balloon Experimental Twin Telescope for infrared interferometry (BETTII): an interferometer at the edge of space

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to simultaneously obtain spatial and spectral information on science targets; the long baseline permits subarcsecond angular resolution, a capability unmatched by other far-infrared facilities. Here, we present key aspects of the overall design of the mission and provide an overview of the current status of the project. We also discuss briefly the implications of this experiment for future space-based far-infrared interferometers.

Electromechanical simulation of a large-aperture MOEMS Fabry-Perot tunable filter

We are developing as micro-machined electrostatically actuated Fabry-Perot tunable filter with a large clear aperture for application in high through-put wide-field imaging spectroscopy and lidar systems. In the first phase of this effort, we are developing key components based on coupled electro-mechanical simulations. In particular, the movable etalon plate design leverages high coating stress to yield a flat surface in drum- head tension over a large diameter. In this approach, the cylindrical silicon movable plate is back etched, resulting in an optically coated membrane that is suspended from a thick silicon support ring. Underestimating the interaction between the support ring, suspended membrane, and coating is critical to developing surfaces that are flat to within stringent etalon requirements. In this work, we present the simulations used to develop the movable plate, spring suspension system, and electrostatic actuation mechanism. We also present results form test of fabricated proof of concept components.

Building an interferometer at the edge of space: pointing and phase control system for BETTII

We propose an architecture for the control system of BETTII,1 a far-infrared, balloon-borne interferometer with a baseline of 8 meters. This system involves multiple synchronized control loops for real-time pointing control and precise attitude knowledge. This will enable accurate phase estimation and control, a necessity for successful interferometry. We present the overall control strategy and describe our flight hardware in detail. We also show our current test setup and the first results of our coarse pointing loop.

A near IR Fabry-Perot interferometer for wide field, Low resolution hyperspectral imaging on the Next Generation Space Telescope

We discuss work in progress on a near-infrared tunable bandpass filter for the Goddard baseline wide field camera concept of the Next Generation Space Telescope Integrated Science Instrument Module. This filter, the Demonstration Unit for Low Order Cryogenic Etalon (DULCE), is designed to demonstrate a high efficiency scanning Fabry-Perot etalon operating in interference orders 1 - 4 at 30 K with a high stability DSP based servo control system. DULCE is currently the only available tunable filter for lower order cryogenic operation in the near infrared. In this application, scanning etalons will illuminate the focal plane arrays with a single order of interference to enable wide field lower resolution hyperspectral imaging over a wide range of redshifts. We discuss why tunable filters are an important instrument component in future space-based observatories.

The balloon experimental twin telescope for infrared interferometry (BETTII): interferometry at the edge of the atmosphere

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to simultaneously obtain spatial and spectral information on science targets; the long baseline permits subarcsecond angular resolution, a capability unmatched by other far-infrared facilities. This program started in 2011, and is now in the process of building and testing components of the mission, aiming for first flight in fall of 2015. This paper will provide an overview of the BETTII experiment, with a discussion of current progress and of future plans.

SIMULATION OF AN ELE CTROSTATICALLY ACTUA TED MICRO -MACHINED FABRY -PEROT ETALON

The development of a micro -scale tunable Fabry - Perot interferometer with narrow band pass (0.3 ∝m) and application to wide -field spectroscopy is presented. In this design, a silicon wafer is micro -machined using deep reactive ion etching (DRIE) to fabricate two etalon plates. The first etalon is a monolithic structure that includes the etalon fram e, its spring suspension, and support frame. The second is a fixed reference etalon plate. A silicon nitride membrane is suspended in tension across a thick support ring on both the moveable and fixed etalon plates as a result of back etching. Due to the processes of thin film deposition, high stresses are transmitted throughout the structure. Because of stringent flatness and parallelism requirements on the etalon plates, simulations are used to aid in the design of a suitable optical coating, which is deposited onto the membrane. Tuning the optical gap is achieved via electrostatic actuation. Simulating force - deflection behavior of the tunable etalon plate is also performed to characterize mechanical stiffness of the spring suspension system, as it det ermines electrostatic stiffness. Theoretical coupled field analysis is compared with an electrostatic simulation to predict voltage required to precisely tune the optical gap (17.5 ∝m maximum) in accordance with well -defined Fabry - Perot figures of merit. Test data gathered from three different experimental setups - nanoindentation, wave front sensing interferometry, and electrostatic actuation - correlate well with predictions based on finite element analysis.