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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 12 — Jun. 12, 2006
  • pp: 5558–5570
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Demonstrating sub-nm closed loop MEMS flattening

Julia W. Evans, Bruce Macintosh, Lisa Poyneer, Katie Morzinski, Scott Severson, Daren Dillon, Donald Gavel, and Layra Reza  »View Author Affiliations


Optics Express, Vol. 14, Issue 12, pp. 5558-5570 (2006)
http://dx.doi.org/10.1364/OE.14.005558


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Abstract

Ground based high-contrast imaging (e.g. extrasolar giant planet detection) has demanding wavefront control requirements two orders of magnitude more precise than standard adaptive optics systems. We demonstrate that these requirements can be achieved with a 1024-Micro-Electrical-Mechanical-Systems (MEMS) deformable mirror having an actuator spacing of 340 µm and a stroke of approximately 1 µm, over an active aperture 27 actuators across. We have flattened the mirror to a residual wavefront error of 0.54 nm rms within the range of controllable spatial frequencies. Individual contributors to final wavefront quality, such as voltage response and uniformity, have been identified and characterized.

© 2006 Optical Society of America

1. Introduction

The detection of over 150 extrasolar planets [1

1. G. W. Marcy, “California and Carnegie Planet Search,” U.C. Berkeley (2005). http://exoplanets.org.

] has placed planetary science at the forefront of astronomy. Most of these detections were made with radial velocity techniques, which measure the doppler shift of the parent star produced by the gravitational pull of a planet. These techniques only probe about 15% of the orbital parameter space of our solar system [2

2. C. H. Lineweaver and D. Grether, “What fraction of sun-like stars have planets?” Ap. J. 598, 1350–1360.

] meaning that planets of the size and position of our own solar system are largely unobserved by current research. Imaging extrasolar planets would open a large complimentary region to radial velocity techniques and a survey of this kind would provide valuable information about the distribution of planets in a ‘typical’ system. Imaging can also provide additional information about planets. For example, spectroscopic data could be used to investigate the material composition of exoplanets. Several observatories including the Gemini Observatory and the Very Large Telescope have recognized extrasolar planet imaging as a key science goal and funded ground-based planet imagers to meet that need. These ground based systems will require a specialized form of adaptive optics (AO) known as Extreme Adaptive Optics (ExAO) to achieve the high-contrast images needed to image extrasolar planets. The work presented here was undertaken in support of the Gemini Planet Imager (GPI).

Ground based extrasolar planet imagers will look for young Jupiter-like planets that are still glowing with the heat of formation. This type of imaging will require contrasts of between 10-6 and 10-7 [3

3. A. Burrows, M. Marley, W. B. Hubbard, J. I. Lunine, T. Guillot, D. Saumon, R. Freedman, D. Sudarsky, and C. Sharp, “A nongray theory of extrasolar giant planets and brown dwarfs,” Astrophysical J. 491, 856–875 (1997). [CrossRef]

], making these instruments technically challenging. Stringent requirements for the number of actuators, precision of flattening and frame rate make deformable mirror (DM) technology a particular risk area. GPI will require approximately 2000 actuators [4

4. B. Macintosh, J. Graham, B. Oppenheimer, L. Poyneer, A. Sivaramakrishnan, and J.-P. Veran, “MEMS-based extreme adaptive optics for planet detection,” in MEMS/MOEMS Components and thier Applications III, S. S. Olivier, S. A. Tadigadapa, and A. K. Henning, eds., Proc. SPIE 6113, pp. 48–57 (2006).

], for a clear aperture of 44 actuators across. Even one non-working actuator cannot be tolerated in the aperture because it will scatter light into the discovery region. Contrast is ultimately limited by residual static wavefront errors [5

5. A. Sivaramakrishnan, J. P. Lloyd, P. E. Hodge, and B. A. Macintosh, “Speckle decorrelation and dynamic range in speckle noise-limited imaging,” Astrophysical J. 581, L59–62 (2002). [CrossRef]

], so GPI will require wavefront control with an accuracy of better than 1 nm rms within the low- to mid-spatial frequency range [4

4. B. Macintosh, J. Graham, B. Oppenheimer, L. Poyneer, A. Sivaramakrishnan, and J.-P. Veran, “MEMS-based extreme adaptive optics for planet detection,” in MEMS/MOEMS Components and thier Applications III, S. S. Olivier, S. A. Tadigadapa, and A. K. Henning, eds., Proc. SPIE 6113, pp. 48–57 (2006).

]. Space-based planet detection architectures have similar requirements. The first step to demonstrating this is to flatten the mirror itself — in the absence of aberrations — to the <1 nm level. An extrasolar planet imager will need to correct additional aberrations, but tests without abberation identify performance limits for future more realistic tests and also demonstrate the internal calibration requirements for the DM in an ExAO system. To adequately correct the changing atmosphere the system must run at 2500 Hz [4

4. B. Macintosh, J. Graham, B. Oppenheimer, L. Poyneer, A. Sivaramakrishnan, and J.-P. Veran, “MEMS-based extreme adaptive optics for planet detection,” in MEMS/MOEMS Components and thier Applications III, S. S. Olivier, S. A. Tadigadapa, and A. K. Henning, eds., Proc. SPIE 6113, pp. 48–57 (2006).

]. Stroke requirements are reduced by the combination of two DMs. A large-stroke deformable mirror will be used for large low-order wavefront correction (woofer) and a high-order device will be used for smaller mid- to high-order correction (tweeter). A traditional deformable mirror to meet the high actuator count requirement would be prohibitively large and expensive ($1000/channel).

Micro-Electrical-Mechanical-Systems (MEMS) deformable mirrors, such as those manufactured by Boston Micromachine Corporation (BMC) [6

6. T. Bifano, P. Bierden, and J. Perreault, “Micromachined Deformable Mirrors for Dynamic Wavefront Control,” in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1–16 (2004).

], are a promising solution to the DM problem. MEMS are fabricated of polysilicon and utilize an array of independently addressable electrostatic actuators. The device discussed here has an actuator spacing of 340 µm making it about 10 mm across. The device is specified by BMC to have 2µm of stroke, although with our current operating parameters we have about 1µm. The top surface of the DM is a continuous gold-coated mirror which can be deformed by the actuators underneath. Performance testing and device characterization are ongoing at the Laboratory for Adaptive Optics (LAO) at University of California, Santa Cruz. We have focused on the precision flattening requirement using a 1024 actuator device (the largest device currently available) and the results are presented in this paper.

MEMS deformable mirrors have been used successfully in other AO systems. In a vision science system, Doble et al. [7

7. N. Doble, G. Yoon, L. Chen, P. Bierden, S. Oliver, and D. R. Williams, “Use of a microelectricalmechanical mirror for adaptive optics in the human eye,” Opt. Lett. 27, 1537–1539 (2002). [CrossRef]

] compared the performance of a 144 segmented MEMS device to a more traditional Xinetics DM. They found the MEMSDMto have comparable performance except when the MEMS DM was limited by stroke. A 1024 segmented device was tested in a horizontal path application at frame rates in excess of 800 Hz with strehl ratios >0.5 by Baker et al. [8

8. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number mircoelectricalmechanical system spatial light modulator in an interferometric phase-conjugation engine,” Opt. Lett. 29, 1781–1783 (2004). [CrossRef] [PubMed]

]. Testbed work was also done with the segmented device using Kolmogorov phase screens to introduce abberrations [9

9. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectricalmechanical systems-based spatial light modulator,” Appl. Opt. 43, 5585–5593 (2004). [CrossRef] [PubMed]

]. A major limitation of this earlier segmented MEMS device was the number of inactive actuators.

A modular electroceramic deformable mirror developed by Xinetics and tested by the Jet Propulsion Laboratory (JPL) is another possibility for a large actuator count deformable mirror. There are versions with 1024 and 4096 actuators. The 1024 version has been tested extensively in the High Contrast Imaging Testbed at JPL [12

12. J. Trauger, D. Moody, B. Gordon, Y. Gürsel, M. Ealey, and R. Bagwell, “Performance of a precision high-density deformable mirror for extremely high contrast imaging astronomy from Sapce,” in Future EUV/UV and Visible Space Astrophysics Missions and Instrumentation, J. C. Blades and O. H. W. Siegmund, eds., Proc. SPIE 4854, pp. 1–8 (2003).

], where it is kept in a temperature controlled vacuum chamber. JPL has achieved angstrom level flatness within controllable spatial frequencies and stability has been demonstrated to 0.01 angstrom [12

12. J. Trauger, D. Moody, B. Gordon, Y. Gürsel, M. Ealey, and R. Bagwell, “Performance of a precision high-density deformable mirror for extremely high contrast imaging astronomy from Sapce,” in Future EUV/UV and Visible Space Astrophysics Missions and Instrumentation, J. C. Blades and O. H. W. Siegmund, eds., Proc. SPIE 4854, pp. 1–8 (2003).

]. The 500-nm stroke and mm-pitch make it a challenging device for use in ground based astronomical systems. Smaller optics are advantageous in high-contrast systems because of improved optical quality compared to larger optics.

The ExAO testbed is uniquely suited to testing MEMS deformable mirrors in the high-contrast regime. We have already demonstrated an ability to operate at contrast levels of 10-7 to 10-8 [13

13. J. W. Evans, G. Sommargren, B. A. Macintosh, S. Severson, and D. Dillon, “Effect of Wavefront Error on 10-7 Contrast Measurements,” Opt. Lett. 31, 565–567 (2006). [CrossRef] [PubMed]

]. The extremely accurate optical metrology of the phase shifting diffraction interferometer (PSDI) [14

14. G. E. Sommargren, D. W. Phillion, M. A. Johnson, N. Q. Nguyen, A. Barty, F. J. Snell, D. R. Dillon, and L. S. Bradsher, “100-picometer interferometry for EUVL,” in Emerging Lithographic Technologies VI, R. L. Engelstad, ed., Proc. SPIE 4688, pp. 316–328 (2002).

] allows absolute measurements of theMEMS DM. Using the PSDI as the wavefront sensor we have flattened to <1 nm rms over controllable spatial frequencies. The technical challenges in achieving this result can be broken into three categories: measuring the phase, controlling the MEMS, and the stability of the system. Errors in each of these tasks leads to errors in flattening performance and are summarized in an error budget. Preliminary work with the MEMS device, including some of the engineering challenges leading up to this result, have been presented in prior works [15

15. J. W. Evans, G. Sommargren, L. Poyneer, B. Macintosh, S. Severson, D. Dillon, A. Shenis, D. Palmer, J. Kasdin, and S. Olivier, “Extreme Adaptive Optics Testbed: Results and Future Work,” in Advancements in Adaptive Optics, D. B. Calia, B. L. Ellerbroek, and R. Ragazzoni, eds., Proc. SPIE 5490, pp. 954–959 (2004).

, 16

16. J. W. Evans, K. Morzinski, L. Reza, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, G. Sommargren, D. Palmer, D. Gavel, and S. Olivier, “Extreme Adaptive Optics Testbed: High Contrast Measurements with a MEMS Deformable Mirror,” in Techniques and Instrumentation for Detection of Exoplanets II, D. R. Coulter, ed., Proc. SPIE 5905, pp. 303–310 (2005).

, 17

17. J. W. Evans, K. Morzinski, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, L. Reza, D. Gavel, D. Palmer, S. Olivier, and P. Birden, “Extreme Adaptive Optics Testbed: Performance and Charachterization of a 1024-MEMS deformable mirror,” in MEMS/MOEMS Components and their applications III, S. Olivier, ed., Proc. SPIE 6113, pp. 131–136 (2006).

]. A complete inventory of these difficulties and our mitigation techniques are included here.

2. Description of ExAO testbed

Fig. 1. Simplified schematic of interferometry mode on the ExAO testbed. A physical aperture can be placed in front of the MEMS but during closed loop operation a software aperture is used.

In interferometry mode, the testbed becomes an extremely accurate optical metrology system. The PSDI was developed at Lawrence Livermore National Laboratory for metrology of aspheric optics for use at UV wavelengths [14

14. G. E. Sommargren, D. W. Phillion, M. A. Johnson, N. Q. Nguyen, A. Barty, F. J. Snell, D. R. Dillon, and L. S. Bradsher, “100-picometer interferometry for EUVL,” in Emerging Lithographic Technologies VI, R. L. Engelstad, ed., Proc. SPIE 4688, pp. 316–328 (2002).

]. (In its original layout the PSDI has an absolute wavefront accuracy of 100 pm. In the experiment described here we estimate accuracy to be better than 250 pm.) A probe (or measurement) wavefront is injected from the upper single-mode fiber in Fig. 1. This passes through the system and is focused onto a pinhole embedded in a super-polished flat mirror (the reference pinhole). Meanwhile, a coherent reference beam passes through the pinhole and interferes with the outgoing probe wavefront. The interference pattern is recorded at a CCD located in an arbitrary location along the optical axis. Using standard phase-shifting interferometer techniques this produces a measurement of the fringe pattern at this location, which can then be converted to a wavefront. This wavefront is numerically propagated in two steps to the plane of interest using the ABCD matrix and a Huygens Integral transformation implemented with FFTs [14

14. G. E. Sommargren, D. W. Phillion, M. A. Johnson, N. Q. Nguyen, A. Barty, F. J. Snell, D. R. Dillon, and L. S. Bradsher, “100-picometer interferometry for EUVL,” in Emerging Lithographic Technologies VI, R. L. Engelstad, ed., Proc. SPIE 4688, pp. 316–328 (2002).

].

Wavefront measurements are used to control the MEMS during closed loop operations. The spatial resolution at the MEMS plane is limited by truncation effects due to an aperture at the reference pinhole. The effective resolution in the MEMS plane is ~141 µm or 41% of an actuator. For closed loop operation, programs in the interactive data language (IDL) are used to direct data acquisition (wavefront sensing with the PSDI), back propagation calculations and commanding the MEMS device through the MEMS driver. Before closed loop operation, the alignment and voltage response of the system must be calibrated. Alignment is done by activating four known actuators on the MEMS and noting their position in a wavefront measurement. For voltage calibration the response of each actuator is measured and fit with a quadratic. These calibration measurements are used to convert wavefront measurements into actuator by actuator phase and then to the corresponding voltages. Closed loop operations can also be run with a spatially filtered Shack-Hartman wavefront sensor [18

18. L. A. Poyneer and B. Macintosh, “Experimental demonstration of phase correction with a 32 x 32 microelectricalmechanical systems mirror and a spatially filtered wavefront sensor,” J. Opt. Soc. Am. A 21, 810–819 (2004). [CrossRef]

].

3. MEMS deformable mirrors

We have tested a total of ten 1024-actuator deformable MEMS mirrors fabricated by Boston Micromachines Corporation [6

6. T. Bifano, P. Bierden, and J. Perreault, “Micromachined Deformable Mirrors for Dynamic Wavefront Control,” in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1–16 (2004).

]. While a future exoplanet imager will require more actuators, the 1024 device is the largest commercially available MEMS device. Characterization and performance testing of these devices have provided feedback to the design and specification of the larger device. We have characterized voltage response, actuator uniformity and device stability as these characteristics will affect closed loop performance.

MEMS DMs are fabricated using bulk processing techniques, meaning that many are produced at once. We have tested mirrors from several such fabricating runs. Surface micromachining processes are used to fabricate the mirrors and the structures on them are made of polysilicon [6

6. T. Bifano, P. Bierden, and J. Perreault, “Micromachined Deformable Mirrors for Dynamic Wavefront Control,” in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1–16 (2004).

]. Each individually addressable actuator is composed of two electrodes which when activated are attracted due to the voltage potential. The top electrode of each actuator is held in place by a combination of springs which provide the restoring force. More complete information about the BMC mirror can be found in Bifano et al. [6

6. T. Bifano, P. Bierden, and J. Perreault, “Micromachined Deformable Mirrors for Dynamic Wavefront Control,” in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1–16 (2004).

]. MEMS DM technology for ExAO applications are still under development but over these ten devices we have seen a dramatic improvement in unpowered flatness and yield which will be crucial for the 4000 actuator device. The 1024 mirrors have 4 inactive actuators by design (they are wired to ground). The actuators are spaced 340 µm apart with a continuous face sheet as the top surface. Due to residual manufacturing stress the top surfaces of these devices have curvature. Early devices had >200 nm rms unpowered WFE, but more recent devices have had as little as 50 nm rms unpowered WFE.

In general defective actuators occur during the manufacturing process rather than failing during operation. However, a combination of high humidity and high voltage can produce oxidation in individual actuators which will eventually limit the performance of those actuators [19

19. H. R. Shea, A. Gasparyan, C. D. White, R. B. Comizzoli, D. Abushch-Magder, and S. Arney, “Anodic Oxidation and Reliability of MEMS Poly-Silicon Electrodes at High Relative Humidity and High Voltages,” in MEMS Reliability for Critical Applications, R. A. Lawton, ed., Proc. SPIE 4180, pp. 117–122 (2000).

]. To avoid humidity damage the device can be sealed under a glass window, or only operated in a controlled laboratory environment. An unpowered device is not damaged by high humidity (but if condensation occurs the mirror must be dry before it is activated). Two of the devices tested, including the device with the best closed loop performance, have windows. ‘Snap-down’ can also damage actuators. This occurs when an actuator has too much displacement and the electrical attraction compressing the actuator overcomes the mechanical force that allows the actuator to rebound. These actuators will be stuck in the maximum displacement position. Two early devices were damaged by humidity at the LAO but no damage due to snap-down has occurred.

Fig. 2. The 1024 actuator MEMS device made by Boston Micromachines Corporation, shown on the testbed with a penny for scale.

The MEMS is controlled with 13-bit D/A conversion and amplification using a system developed by Red Nun Electronics Company. The smallest voltage step allowed with these electronics is 0.025 volts for the current configuration. This corresponds to a phase step of 0.18 nm. The driver boards for many-channel systems like this are also under development. It is critical and tedious to ensure that the mapping through the driver electronics is accurate. Minor damage to the boards can be difficult to detect and will negatively affect closed loop performance.

3.1. Voltage response

One limitation of MEMS DM technology is the device’s limited stroke, especially compared with macro-DM technology. In practice, we find that MEMS stroke depends on the position of neighboring actuators as expected for their relatively broad influence functions, which have approximately 26% crosstalk. Two actuators away the crosstalk reduces to 4%. A 3 by 3 array of actuators will have more displacement at a given voltage than a single actuator at the same voltage. In Fig. 3 this difference is indicated by the 3 dotted versus solid lines. In a typical AO system the DM is operated at a bias to correct both positive and negative wavefront errors. The entire device is set at an intermediate voltage and actuators are moved by varying their voltage about the bias. We typically operate at a bias of 110 volts, that voltage being midway in our operational displacement. We set a maximum voltage limit to 160 volts to prevent snap-down both in software and on the voltage power supply. An imbalance between the electrostatic force of activating an actuator and the mechanical restoring force causes stroke at a bias to be reduced. Figure 3 summarizes the results of testing the stroke of a particular MEMS device for these situations. For this test 4 actuators were activated at several incremental voltages and their displacement relative to the flat surface of the MEMS were measured with a Zygo interferometer. The test was also done with a set of adjacent actuators moved in a 3 by 3 box. Both tests were done with a bias voltage of 0, 110 and 160 volts. In Fig. 3 the resultant curves have been re-centered so that 0 displacement is at 0 volts rather than at the bias voltage. In typical operations these devices achieve about 1 µm of stroke with our operational parameters, similar to the measured response of the 3 by 3 array because actuators are not significantly displaced from their neighbors during closed loop. The ‘snap-down’ effect is caused by too much displacement, not too much voltage. There is potential to increase the stroke of the device by increasing operational voltage above the current 160 volt maximum, while within the range of acceptable displacement. Because additional stroke was not required for our tests, increasing maximum voltage was not investigated.

Fig. 3. Stroke of a device measured with 0, 110, and 160 volt bias for an individual or group of actuators. More stroke is achieved when actuators move together without a bias voltage.

3.2. Actuator uniformity

Operating in closed loop mitigates the effect of small variations in voltage response between actuators. On the most recent device the variation in maximum displacement at 160 volts is less than 5% (excluding the outer two rows and columns of actuators), which is well within our ability to flatten. Irregular actuators, however, are unable to achieve the desired position, regardless of number of iterations, producing an in-band fitting error that limits closed loop performance. We have identified three categories of such actuators: no-response (or dead), low-response and coupled. Typically we refer to the yield of a MEMS device as percentage of working actuators. This number is particularly important in high-contrast applications as no-response actuators scatter light into the region of interest. Actuator uniformity refers to the variability of all ‘working’ actuators including low-response and coupled actuators.

Figure 4 is a representation of actuator yield and uniformity in three tested devices. No-response actuators are marked in red, other irregular actuators in yellow and normal actuators are white. The three figures exclude the outer 2 rows and columns which are outside the aperture and difficult to characterize. The left device was received in Nov 2004. It had limited performance due to the number of irregular actuators and was operated over a smaller aperture because of the number and placement of no-response actuators. The middle and right devices were received in Feb and Oct of 2005. The two no-response actuators in the top middle of all three devices are wired to ground and are excluded from the following statistics. The oldest device has 96.9 % normal actuators (33 irregular) while the most recent device has 99.5% normal actuators (5 irregular). Only 94.1 % of actuators were normal (60 irregular) in tests of the segmented device published by Baker et. al. in 2004 [9

9. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectricalmechanical systems-based spatial light modulator,” Appl. Opt. 43, 5585–5593 (2004). [CrossRef] [PubMed]

, 8

8. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number mircoelectricalmechanical system spatial light modulator in an interferometric phase-conjugation engine,” Opt. Lett. 29, 1781–1783 (2004). [CrossRef] [PubMed]

]. This dramatic improvement in actuator yield and uniformity has allowed improved performance and made MEMS deformable mirrors a feasible technology for high-contrast applications.

Fig. 4. Irregular actuators are identified for the working region of three MEMS devices. Red indicates a no-response actuator, yellow a ‘working’ irregular actuator, and white is a normal actuator. The Nov 2004 device had limited performance due to the number of irregular actuators and was operated over a smaller aperture because of the number and placement of no-response actuators. There has been a dramatic improvement in both yield and uniformity in the Feb and Oct 2005 devices. The two no-response actuators in the top middle of all three devices are wired to ground.

3.3. Stability

Fig. 5. Voltage response of two coupled actuators tested individually and together, with a bias voltage of 110 volts.
Fig. 6. Curve of growth for stability data. Of the 500 actuators tested 97% stability of better than 0.16 nm (standard deviation of surface over 60 measurements taken in 38 minutes).

4. Closed loop performance

In these closed loop tests no additional aberrations were introduced into the system. The primary source of error is the MEMS DM itself. Although a ground based extrasolar planet imager will have to correct the atmosphere the more basic test here identifies the performance limitations of future more realistic tests. A planet imager will also have a stringent internal calibration requirement of <1 nm (in mid-spatial frequencies). These experiments demonstrate we can meet that requirement. The metric we use for closed loop performance is wavefront error within the range of spatial frequencies which the DM can correct. Higher spatial frequencies will scatter to larger angles [5

5. A. Sivaramakrishnan, J. P. Lloyd, P. E. Hodge, and B. A. Macintosh, “Speckle decorrelation and dynamic range in speckle noise-limited imaging,” Astrophysical J. 581, L59–62 (2002). [CrossRef]

]. Using a numerical spatial filter to avoid aliasing a dark hole region over controllable spatial frequencies will be created in the far field image (or the power spectrum of the wavefront) [20

20. F. Malbet, J. Yu, and M. Shao, “High Dynamic Range Imaging Using a Deformable Mirror for Space Coronography,” Publications Of The Astronomical Society of the Pacific 107, 386–398 (1995). [CrossRef]

, 21

21. L. A. Poyneer, B. Bauman, B. A. Macintosh, D. Dillon, and S. Severson, “Spatially filtered wave-front sensor for high-order adaptive optics,” Opt. Lett. 31, 293–295 (2006). [CrossRef] [PubMed]

](See Fig. 9). There are some higher order effects which cause higher order aberrations to fold into the dark hole, but these are small especially in a case with no additional aberration, making in-band wavefront error a good metric for a high-contrast system.

Fig. 7. Wavefronts taken before and after a closed loop test with a 9.2 mm aperture. The initial wavefront has an RMS WFE of 148 nm, while the flattened wavefront has 12.8 nm total RMS wavefront error, which is mostly errors on the scale on an individual actuator. Inside the controlled range of spatial frequencies the rms wavefront error is 0.54 nm. This is seen more clearly in the lowpass filtered image (far right).
Fig. 8. Power spectrum generated from wavefronts taken before and after flattening. The 27 actuators across the aperture yield a highest controllable spatial frequency of 13.5 cycles per aperture. The bump at 27 cycles per aperture corresponds to physical structures on the MEMS at the scale of the individual actuator spacing.

5. Limitations to improved performance

Errors in correcting the wavefront can stem from imperfect wavefront measurements, instabilities in the system and the inability of the DM to fit the desired shape. We can summarize these errors in an error budget (See Table 1). We have disregarded errors outside of the controllable spatial frequencies of the MEMS device. Fortunately, those errors will primarily scatter light outside of the region of interest in corresponding far field measurements.

Fig. 9. Far field image simulated from the wavefront measurement shown in Fig.7. Diffraction has been suppressed with a symmetric blackman apodization for illustrating the effect of high spatial frequency errors like print-through on the image.

The effect of irregular actuators on closed loop performance is clear from the lowpass filtered image (far right of Fig. 7). Devices with more irregular actuators did not flatten as well as this device. The effect of the coupled actuators on flattening was estimated by comparing the rms WFE over 75% of the aperture to the error over the same aperture with the area around the irregular actuators removed. This does not account for any errors caused by the irregular actuators outside of their immediate vicinity. This technique for estimating error does not work well for no-response actuators or many irregular actuators within the aperture.

The remaining errors: voltage, stability and measurement are all system dependent. The voltage step size is determined by the number of bits in the electronics split over the voltage range, currently 0 to 200 volts. This voltage step size is converted to phase using a typical voltage response in the vicinity of the bias voltage. The response over the small region required to correct most of the device is quite linear, but if more stroke on the device were required this error could become larger. Voltage step size could be reduced with higher resolution drivers, or a change in the voltage range. The stability of the device was discussed previously. Measurement error is inherent to the PSDI system and is calculated by comparing two measurements taken consecutively. Alignment errors of the input fiber of the PSDI measurement leg increase measurement error.

The calculated wavefront error agrees well with the measured WFE indicating that these errors are the limiting errors for improved performance.

6. Conclusion

Our testing has demonstrated that MEMS deformable mirrors can be controlled at the level of precision needed for high-contrast AO systems. We have flattened a MEMS deformable mirror

Table 1. Error budget for best flattening result over a 9.2 mm aperture within controllable spatial frequencies. The experimental residual WFE is 0.54 nm rms within controllable spatial frequencies and corresponds well to the error budget.

table-icon
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to 0.54 nm rms over controllable spatial frequencies and identified the individual contributors to wavefront error in an error budget. The largest contributor is edge effects caused by scattered light interpreted as wavefront error by the PSDI. We have characterized the yield and actuator uniformity contributing to better calibration and providing feedback for device improvements. The most recent device has 99.5% normal actuators, which is a dramatic improvement over the early segmented device with 94.1% normal actuators[9

9. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectricalmechanical systems-based spatial light modulator,” Appl. Opt. 43, 5585–5593 (2004). [CrossRef] [PubMed]

]. In particular the most recent device has no dead actuators within an aperture 27 actuators across, greatly improving the performance. Overall the level of closed loop performance, without additional improvements, meets the precision and accuracy requirements for a high-contrast giant-planet imager and demonstrates the feasibility of MEMS technology for such an instrument.

Acknowledgments

This work has been supported by the Gordon and Betty Moore Foundation through its grant to the UCO/Lick Observatory Laboratory for Adaptive Optics and the NSF Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST-9876783. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

*P.h.D.candidate at University of California, Davis

References and links

1.

G. W. Marcy, “California and Carnegie Planet Search,” U.C. Berkeley (2005). http://exoplanets.org.

2.

C. H. Lineweaver and D. Grether, “What fraction of sun-like stars have planets?” Ap. J. 598, 1350–1360.

3.

A. Burrows, M. Marley, W. B. Hubbard, J. I. Lunine, T. Guillot, D. Saumon, R. Freedman, D. Sudarsky, and C. Sharp, “A nongray theory of extrasolar giant planets and brown dwarfs,” Astrophysical J. 491, 856–875 (1997). [CrossRef]

4.

B. Macintosh, J. Graham, B. Oppenheimer, L. Poyneer, A. Sivaramakrishnan, and J.-P. Veran, “MEMS-based extreme adaptive optics for planet detection,” in MEMS/MOEMS Components and thier Applications III, S. S. Olivier, S. A. Tadigadapa, and A. K. Henning, eds., Proc. SPIE 6113, pp. 48–57 (2006).

5.

A. Sivaramakrishnan, J. P. Lloyd, P. E. Hodge, and B. A. Macintosh, “Speckle decorrelation and dynamic range in speckle noise-limited imaging,” Astrophysical J. 581, L59–62 (2002). [CrossRef]

6.

T. Bifano, P. Bierden, and J. Perreault, “Micromachined Deformable Mirrors for Dynamic Wavefront Control,” in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1–16 (2004).

7.

N. Doble, G. Yoon, L. Chen, P. Bierden, S. Oliver, and D. R. Williams, “Use of a microelectricalmechanical mirror for adaptive optics in the human eye,” Opt. Lett. 27, 1537–1539 (2002). [CrossRef]

8.

K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number mircoelectricalmechanical system spatial light modulator in an interferometric phase-conjugation engine,” Opt. Lett. 29, 1781–1783 (2004). [CrossRef] [PubMed]

9.

K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, “Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectricalmechanical systems-based spatial light modulator,” Appl. Opt. 43, 5585–5593 (2004). [CrossRef] [PubMed]

10.

G. Vdovin, P. M. Sarro, and S. Middelhoek, “Technology and applications of micromachined adaptive mirrors,” J. Micromech. Microeng. 9, R8–R20.

11.

D. Dayton, J. Gonglewski, S. Restaino, J. Martin, J. Phillips, M. Hartman, S. Browne, P. Kervin, J. Snodgrass, N. Heimann, M. Shilko, R. Pohle, B. Carrion, C. Smith, and D. Thiel, “Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites.” Opt. Express 10, 1508–1519 (2002). [PubMed]

12.

J. Trauger, D. Moody, B. Gordon, Y. Gürsel, M. Ealey, and R. Bagwell, “Performance of a precision high-density deformable mirror for extremely high contrast imaging astronomy from Sapce,” in Future EUV/UV and Visible Space Astrophysics Missions and Instrumentation, J. C. Blades and O. H. W. Siegmund, eds., Proc. SPIE 4854, pp. 1–8 (2003).

13.

J. W. Evans, G. Sommargren, B. A. Macintosh, S. Severson, and D. Dillon, “Effect of Wavefront Error on 10-7 Contrast Measurements,” Opt. Lett. 31, 565–567 (2006). [CrossRef] [PubMed]

14.

G. E. Sommargren, D. W. Phillion, M. A. Johnson, N. Q. Nguyen, A. Barty, F. J. Snell, D. R. Dillon, and L. S. Bradsher, “100-picometer interferometry for EUVL,” in Emerging Lithographic Technologies VI, R. L. Engelstad, ed., Proc. SPIE 4688, pp. 316–328 (2002).

15.

J. W. Evans, G. Sommargren, L. Poyneer, B. Macintosh, S. Severson, D. Dillon, A. Shenis, D. Palmer, J. Kasdin, and S. Olivier, “Extreme Adaptive Optics Testbed: Results and Future Work,” in Advancements in Adaptive Optics, D. B. Calia, B. L. Ellerbroek, and R. Ragazzoni, eds., Proc. SPIE 5490, pp. 954–959 (2004).

16.

J. W. Evans, K. Morzinski, L. Reza, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, G. Sommargren, D. Palmer, D. Gavel, and S. Olivier, “Extreme Adaptive Optics Testbed: High Contrast Measurements with a MEMS Deformable Mirror,” in Techniques and Instrumentation for Detection of Exoplanets II, D. R. Coulter, ed., Proc. SPIE 5905, pp. 303–310 (2005).

17.

J. W. Evans, K. Morzinski, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, L. Reza, D. Gavel, D. Palmer, S. Olivier, and P. Birden, “Extreme Adaptive Optics Testbed: Performance and Charachterization of a 1024-MEMS deformable mirror,” in MEMS/MOEMS Components and their applications III, S. Olivier, ed., Proc. SPIE 6113, pp. 131–136 (2006).

18.

L. A. Poyneer and B. Macintosh, “Experimental demonstration of phase correction with a 32 x 32 microelectricalmechanical systems mirror and a spatially filtered wavefront sensor,” J. Opt. Soc. Am. A 21, 810–819 (2004). [CrossRef]

19.

H. R. Shea, A. Gasparyan, C. D. White, R. B. Comizzoli, D. Abushch-Magder, and S. Arney, “Anodic Oxidation and Reliability of MEMS Poly-Silicon Electrodes at High Relative Humidity and High Voltages,” in MEMS Reliability for Critical Applications, R. A. Lawton, ed., Proc. SPIE 4180, pp. 117–122 (2000).

20.

F. Malbet, J. Yu, and M. Shao, “High Dynamic Range Imaging Using a Deformable Mirror for Space Coronography,” Publications Of The Astronomical Society of the Pacific 107, 386–398 (1995). [CrossRef]

21.

L. A. Poyneer, B. Bauman, B. A. Macintosh, D. Dillon, and S. Severson, “Spatially filtered wave-front sensor for high-order adaptive optics,” Opt. Lett. 31, 293–295 (2006). [CrossRef] [PubMed]

OCIS Codes
(010.1080) Atmospheric and oceanic optics : Active or adaptive optics
(230.3990) Optical devices : Micro-optical devices
(350.1260) Other areas of optics : Astronomical optics

ToC Category:
Optical Devices

History
Original Manuscript: March 30, 2006
Revised Manuscript: May 26, 2006
Manuscript Accepted: May 29, 2006
Published: June 12, 2006

Citation
Julia W. Evans, Bruce Macintosh, Lisa Poyneer, Katie Morzinski, Scott Severson, Daren Dillon, Donald Gavel, and Layra Reza, "Demonstrating sub-nm closed loop MEMS flattening," Opt. Express 14, 5558-5570 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-12-5558


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References

  1. G. W. Marcy, "California and Carnegie Planet Search," U.C. Berkeley (2005). http://exoplanets.org.
  2. C. H. Lineweaver and D. Grether, "What fraction of sun-like stars have planets?" Ap. J. 598, 1350-1360.
  3. A. Burrows, M. Marley, W. B. Hubbard, J. I. Lunine, T. Guillot, D. Saumon, R. Freedman, D. Sudarsky, and C. Sharp, "A nongray theory of extrasolar giant planets and brown dwarfs," Astrophysical J. 491, 856-875 (1997). [CrossRef]
  4. B. Macintosh, J. Graham, B. Oppenheimer, L. Poyneer, A. Sivaramakrishnan, and J.-P. Veran, "MEMS-based extreme adaptive optics for planet detection," in MEMS/MOEMS Components and thier Applications III, S. S. Olivier, S. A. Tadigadapa, and A. K. Henning, eds., Proc. SPIE 6113, pp. 48-57 (2006).
  5. A. Sivaramakrishnan, J. P. Lloyd, P. E. Hodge, and B. A. Macintosh, "Speckle decorrelation and dynamic range in speckle noise-limited imaging," Astrophysical J. 581, L59-62 (2002). [CrossRef]
  6. T. Bifano, P. Bierden, and J. Perreault, "Micromachined Deformable Mirrors for Dynamic Wavefront Control," in Advanced Wavefront Control:Methods, Devices and Applications II, J. D. Gonglewski, M. T. Gruineisen, and M. K. Giles, eds., Proc. SPIE 5553, pp. 1-16 (2004).
  7. N. Doble, G. Yoon, L. Chen, P. Bierden, S. Oliver, and D. R. Williams, "Use of a microelectricalmechanical mirror for adaptive optics in the human eye," Opt. Lett. 27, 1537-1539 (2002). [CrossRef]
  8. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, "High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number mircoelectricalmechanical system spatial light modulator in an interferometric phase-conjugation engine," Opt. Lett. 29, 1781-1783 (2004). [CrossRef] [PubMed]
  9. K. L. Baker, E. A. Stappaerts, D. Gavel, S. C. Wilks, J. Tucker, D. A. Silva, J. Olsen, S. S. Olivier, P. E. Young, M. W. Kartz, L. M. Flath, P. Krulevitch, J. Crawford, and O. Azucena, "Breadboard testing of a phase-conjugate engine with an interferometric wave-front sensor and a microelectricalmechanical systems-based spatial light modulator," Appl. Opt. 43, 5585-5593 (2004). [CrossRef] [PubMed]
  10. G. Vdovin, P. M. Sarro, and S. Middelhoek, "Technology and applications of micromachined adaptive mirrors," J. Micromech. Microeng. 9, R8-R20.
  11. D. Dayton, J. Gonglewski, S. Restaino, J. Martin, J. Phillips, M. Hartman, S. Browne, P. Kervin, J. Snodgrass, N. Heimann,M. Shilko, R. Pohle, B. Carrion, C. Smith, and D. Thiel, "Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites." Opt. Express 10, 1508-1519 (2002). [PubMed]
  12. J. Trauger, D. Moody, B. Gordon, Y. G¨ursel, M. Ealey, and R. Bagwell, "Performance of a precision high-density deformable mirror for extremely high contrast imaging astronomy from Sapce," in Future EUV/UV and Visible Space Astrophysics Missions and Instrumentation, J. C. Blades and O. H. W. Siegmund, eds., Proc. SPIE 4854, pp. 1-8 (2003).
  13. J. W. Evans, G. Sommargren, B. A. Macintosh, S. Severson, and D. Dillon, "Effect of Wavefront Error on 10−7 Contrast Measurements," Opt. Lett. 31, 565-567 (2006). [CrossRef] [PubMed]
  14. G. E. Sommargren, D. W. Phillion, M. A. Johnson, N. Q. Nguyen, A. Barty, F. J. Snell, D. R. Dillon, and L. S. Bradsher, "100-picometer interferometry for EUVL," in Emerging Lithographic Technologies VI, R. L. Engelstad, ed., Proc. SPIE 4688, pp. 316-328 (2002).
  15. J. W. Evans, G. Sommargren, L. Poyneer, B. Macintosh, S. Severson, D. Dillon, A. Shenis, D. Palmer, J. Kasdin, and S. Olivier, "Extreme Adaptive Optics Testbed: Results and Future Work," in Advancements in Adaptive Optics, D. B. Calia, B. L. Ellerbroek, and R. Ragazzoni, eds., Proc. SPIE 5490, pp. 954-959 (2004).
  16. J. W. Evans, K. Morzinski, L. Reza, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, G. Sommargren, D. Palmer, D. Gavel, and S. Olivier, "Extreme Adaptive Optics Testbed: High Contrast Measurements with a MEMS Deformable Mirror," in Techniques and Instrumentation for Detection of Exoplanets II, D. R. Coulter, ed., Proc. SPIE 5905, pp. 303-310 (2005).
  17. J. W. Evans, K. Morzinski, S. Severson, L. Poyneer, B. Macintosh, D. Dillon, L. Reza, D. Gavel, D. Palmer, S. Olivier, and P. Birden, "Extreme Adaptive Optics Testbed: Performance and Charachterization of a 1024-MEMS deformable mirror," in MEMS/MOEMS Components and their applications III, S. Olivier, ed., Proc. SPIE 6113, pp. 131-136 (2006).
  18. L. A. Poyneer and B. Macintosh, "Experimental demonstration of phase correction with a 32 x 32 microelectricalmechanical systems mirror and a spatially filtered wavefront sensor," J. Opt. Soc. Am. A 21, 810-819 (2004). [CrossRef]
  19. H. R. Shea, A. Gasparyan, C. D. White, R. B. Comizzoli, D. Abushch-Magder, and S. Arney, "Anodic Oxidation and Reliability of MEMS Poly-Silicon Electrodes at High Relative Humidity and High Voltages," in MEMS Reliability for Critical Applications, R. A. Lawton, ed., Proc. SPIE 4180, pp. 117-122 (2000).
  20. F. Malbet, J. Yu, and M. Shao, "High Dynamic Range Imaging Using a Deformable Mirror for Space Coronography," Publications Of The Astronomical Society of the Pacific 107, 386-398 (1995). [CrossRef]
  21. L. A. Poyneer, B. Bauman, B. A. Macintosh, D. Dillon, and S. Severson, "Spatially filtered wave-front sensor for high-order adaptive optics," Opt. Lett. 31, 293-295 (2006). [CrossRef] [PubMed]

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