Curvature sensor

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Curvature sensor
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AFRL/Kestrel Fast Wavefront Sensor

• 90,000 frames per second
• Approximately 1 second of data per run
• Raw data analyzed within minutes
• Thanks to AFRL/DES for loan of equipment!

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STTR Phase I Results

• With the help of Kestrel, raw curvature data was
  converted to OPD data and delivered to
  AgilOptics.
• AgilOptics was able to take raw OPD data, remove
  the tare, and translate it to mirror voltages
  which were played back on an AgilOptics Aeri.

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Overview
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Compressible Shear Layer Facility

• Effects of Acoustic Forcing
• Regularization of most-unstable Kelvin-Helmholtz
  instability
• Forces first vortex roll-up to occur at 800 Hz
• Causes OPD pattern to become predictable and
  repeatable

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Compressible Shear Layer Facility
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Test Results from NDU

• Two tunnels measured Boundary Layer and Forced
  Flow Shear Layer
• Best boundary layer data at m 0.8
• Best shear layer data at 750 Hz
• Disturbances on the order of 0.1-0.2 microns, P-P

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Movie of m 0.8 Boundary Flow
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Analysis of Data

• Typically 1000 or more frames of data
• Data taken at 90,000 FPS
• Features move at 7000-8000 cycles per second
  across view port
• For some data, tip/tilt is subtracted on site
• Start with 40 by 40 OPD data, comma separated
  values are best seen in Excel
• OPD is Optical Path Difference
• Remove average tare due to large scale errors
  in optical paths (typically astigmatism at 1-5
  microns)
• tare is background optical aberrations
• Residual aero-optical OPDs are 0.1 - 0.2 microns
• Create voltage maps for driving DMs, frame by
  frame, using DMT

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MASS Project TasksWFS

• WFS camera will be a Photron FastCam Ultima SE
  with a capture rate of 27,000 fps with the
  ability to collect 4.9 sec of data and a 60x60
  data array.
• Camera processor has 1 GB of on board memory
  allowing for 131,072 frames captured per run.
• Efforts will be made to extract the data from the
  processor in a different way in order to reduce
  data download time.

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MASS WFS Camera Options
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MASS Project TasksWFS
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MASS Project TasksWFS

• WFS will be a curvature sensor consisting of a
  distorted grating.

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MASS Project TasksWFS

• Using an off-axis portion of a zone plate two
  defocused spots can be acquired on the same
  detector.
• Calculate the wavefront curvature from the two
  defocused spots a simple subtraction of
  intensities.

Zone Plate
Grating Centerline
Offset


Beam Input
Lens
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MASS Project TasksWFS
Sensor Plane
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MASS Project TasksWFS

• AgilOptics has demonstrated through other
  projects the ability to design distorted
  gratings.
• Fabrication of original grating mask done by
  Infinite Graphics Inc.
• Duplicates made in house at AgilOptics.

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Distorted Grating

• The grating was distorted according to a
  quadratic function such that a different focal
  length could be associated with each diffraction
  order.

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• If a known transformation T exists between the
  complex amplitudes u1 and u2 on two surfaces,
  such that u2 T(u1), it may be possible to
  determine the phase, and thus the complex
  amplitude, of u from measurements of the
  intensity u12 and u22. When the
  transformation arises from the inclusion of an
  additional phase function in the imaging system,
  this principle is known as phase-diversity
  wave-front sensing.

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Magnification of the Mth order diffraction
nu is the ratio of image distance to object
distance R is the radius of the grating
aperture Mo is the magnification associated with
the 0th order W is the standard defocus power of
the grating
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Magnification of 1 (-1) orders
Location of 1 (-1) images relative to grating
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• Defocus B) Astigmatism
• C) Coma D) Trefoil E) Spherical
• Aberration

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