Bruce Martin Levine

Thin-Film Piezoelectric Unimorph Actuator-Based Deformable Mirror With a Transferred Silicon Membrane

This paper describes a proof-of-concept deformable mirror (DM) technology, with a continuous single-crystal silicon membrane reflecting surface, based on PbZr0.52Ti0.48O3 (PZT) unimorph membrane microactuators. A potential application for a terrestrial planet finder adaptive nuller is also discussed. The DM comprises a continuous, large-aperture, silicon membrane transferred onto a 20times20 piezoelectric unimorph actuator array. The actuator array was prepared on an electroded silicon substrate using chemical-solution-deposited 2-mum-thick PZT films working in a d31 mode. The substrate was subsequently bulk-micromachined to create membrane structures with residual silicon acting as the passive layer in the actuator structure. A mathematical model simulated the membrane microactuator performance and aided in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. The resulting piezoelectric unimorph actuators with patterned PZT films produced large strokes at low voltages. A PZT unimorph actuator, 2.5 mm in diameter with optimized PZT/silicon thickness and design showed a deflection of 5.7 mum at 20 V. A DM structure with a 20-mum-thick silicon membrane mirror (50 mm times50 mm area) supported by 400 PZT unimorph actuators was successfully fabricated and optically characterized. The measured maximum mirror deflection at 30 V was approximately 1 mum. An assembled DM showed an operating frequency bandwidth of 30 kHz and an influence function of approximately 30%

Planet detection in visible light with a single aperture telescope and nulling coronagraph

This paper describes the latest progress for visible direct detection of Earth like extrasolar planets using a nulling coronagraph instrument behind a 4m class telescope. Such a system is capable of satisfying the scientific objectives of the Terrestrial Planet Finder mission In our design, a 4 beam nulling interferometer is synthesized from the telescope pupil, producing a very deep null proportional to θ4 which is then filtered by a coherent array of single mode fibers to suppress the residual scattered light. With diffraction limited telescope optics and similar quality components in the optical train (λ/20), suppression of the starlight to 10-10 is achievable. Such a telescope with this nulling interferometer as back-end instrument can image and detect planets, or provide the input to a low resolution spectrometer. Shown are key features of this system in a space mission, latest results of laboratory measurements demonstrating achievable null depth, and progress toward fabrication of coherent single mode fiber arrays.

Optical Planet Discoverer: how to turn a 1.5-m class space telescope into a powerful exo-planetary systems imager

Optical Planet Discoverer (OPD) is a 1.5m class space telescope concept working as a visible nulling-interferometer imager. It is designed to detect Jupiter-like planets orbiting main sequence stars 10pc away in a few minutes of integration and carry out a low resolution (~20) spectroscopy of their atmosphere. OPD would fit in the budget envelope of a discovery class mission. It would serve as an efficient precursor to a Visible Terrestrial Planet Finder (VTPF), a scaled-up 4m class version based on the same optical scheme and allowing direct detection of 10pc Earthlike planets in a few hours. We detail here OPDs optical principle layout, which is primarily driven by an integrated stellar light attenuation of 1e-6 in the final focal plane. The optical concept is based on a double-shearing nulling interferometer followed by an array of single-mode waveguides. The waveguides array ensures high residual starlight suppression - as already demonstrated at the 1e-6 level by preliminary JPL visible LASER nulling experiments - together with diffraction limited imaging of the circumstellar environment over a 2 arcsec field. During the observations, the telescope is spun around the line of sight to allow for proper detection of fixed planetary signatures against residual off-axis speckle patterns at the 1e-9 level. Use of the single-mode waveguide array to filter out scattered starlight eliminates the requirements for pristine λ/4000 rms wavefronts anywhere in the optical train. With OPD, stringent phase requirements apply only to scales larger than 5 cm - the equivalent size of the pupil regions to be recombined and nulled in a given fiber, so that phase specifications can be met using low order active optics.

Hybrid ray trace and diffraction propagation code for analysis of optical systems

The Control Optics Modelling Package (COMP), is an optical modelling computer program capable of performing ray trace, differential ray trace and diffraction analyses for any optical design. COMP is particularly useful for optical systems that move, whether through interaction with dynamically or thermally varying structures, or optics that are actively controlled to perform particular tasks, such as steering mirrors or segmented mirrors.

Visible light Terrestrial Planet Finder: planet detection and spectroscopy by nulling interferometry with a single aperture telescope

Planet detection around a bright star is dependent on the resolution of the imaging system and the degree of light suppression of the star relative to the planet. We present a concept and a scaled precursor for a visible light Terrestrial Planet Finding (VTPF) mission. Its major feature is an imaging system for planet detection using a nulling interferometer behind a single aperture telescope. This configuration is capable of detecting earth-like planets with a 4m aperture using both imaging and spectroscopic imaging modes. We will describe the principles of the system, and show results of studies demonstrating its feasibility.

Limits on adaptive optics systems for lightweight space telescopes

Future space telescopes seek to maximize collecting aperture for increased sensitivity and high spatial resolution yet are limited in mass due to launch weight restrictions. JPL has developed the Controlled Optics Modelling Package (COMP) to easily facilitate analyses of optical systems whose elements are perturbed. Development of the computer tool, IMOS (Integrated Modeling of Advanced Optical Systems) allows modeling of structurally and thermally induced deformations to interact with optical systems. Presented here are analyses on the Segmented Reflector Telescope, (SRT), to estimate and then minimize the effect of anticipated disturbances on the resultant optical performance. Such studies are a needed prerequisite for estimating the requirements for adaptive optics due to structural movements. A simulation study estimates the space-time power spectral density of the residual telescope phases from reaction wheel disturbances. Results show that significant disturbances are concentrated in the first few Zernike polynomials with 87% of all disturbances described by the first 11 terms leaving a 0.35 micrometers rms residual. The time bandwidth of the disturbances is between 20 - 25 Hz which placed the required corrections in the adaptive optics regime.

Piezoelectric unimorph MEMS deformable mirror for ultra-large telescopes

This paper describes the results of our demonstration on a proof-of-concept piezoelectric unimorph-based deformable mirror (DM) with continuous single-crystal silicon membrane. A PZT unimorph actuator of 2.5 mm in diameter with optimized PZT/Si thickness and design showed a deflection of 5 μm at 50 V. DMs consisting of 10 μm thick single-crystal silicon membranes supported by 4×4 actuator arrays were fabricated and optically characterized. An assembled DM showed a stroke of 2.5 μm at 50 V with a resonant frequency of 42 kHz and influence function of approximately 25%.

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.

Measurement error of a Shack-Hartmann wavefront sensor in strong scintillation conditions

Adaptive optics can be used to improve the performance of optical wireless communications links degraded by atmospheric turbulence. Accurate wavefront sensing is necessary for some adaptive optics systems to compensate for the effects of atmospheric turbulence. Although the Shack- Hartmann sensor can provide accurate wavefront sensing under controlled conditions, scintillation can restrict the performance of Shack-Hartmann wavefront sensing by creating large intensity fluctuations. These intensity fluctuations can create errors in the wavefront measurement if the intensity dynamic range of the Shack-Hartmann sensor is exceeded. The result of computer simulations which model the performance of the Shack-Hartmann wavefront sensor are presented. Specifically it is shown that the intensity dynamic range of the Shack-Hartmann wavefront sensor can be increased by operating with saturated pixels without an increase in error in the measured wavefront. Operating conditions that maximize the intensity dynamic range of the Shack-Hartmann senor are presented. Experimental results are presented which support the results of the computer simulation.

Segmented mirror figure control for a space-based far-IR astronomical telescope

For segmented large mirrors to be used effectively for astronomy they must be actively aligned and controlled to extreme levels of precision. We consider the figure control problem for a spaceborne far-IR telescope, the Precision Segmented Reflector Project Focus Moderate Mission Telescope. We propose a two-stage approach. A figure initialization controller is used to achieve initial phasing and alignment of the telescope using an imaging science detector. A figure maintenance controller keeps the telescope aligned during normal operation using a laser metrology optical truss sensor system. We show that performance of any figure control system is subject to limits on the controllability of the wavefront. Maintenance controllers are additionally limited by considerations of the observability of the wavefront from the maintenance sensors. We show preliminary results for the figure initialization controller. We present a Wavefront Compensation method for figure maintenance control that minimizes wavefront errors due to misalignment errors.