Previously, we demonstrated useful and novel features of the General Dynamics QuickStar adaptive-optics testbed
utilizing Phase Diversity (PD) as the wavefront sensor operating on a point object. Point objects are relatively easy to
produce in the laboratory and simplify the calibration procedure. However, for some applications, natural or artificial
beacons may not be readily available and a wavefront sensor that operates on extended scenes is required. Accordingly,
the QuickStar testbed has been augmented to allow PD to operate on natural three-dimensional solar-illuminated scenes
external to the QuickStar laboratory. In addition, a computationally efficient chip-selection strategy has been developed
that allows PD to operate on chips with favorable scene content. Finally, a covariance matrix has been developed that
provides an accuracy estimate for PD wavefront-parameter estimates. The covariance can be used by the controls
algorithm to properly weight the correction applied according to the accuracy of the estimates. These advances suggest
that PD is a sufficiently mature technology for use in adaptive optics systems that require operation with extended
scenes.
This paper will report on efforts to automatically calibrate in situ a phase-diversity (PD) wavefront sensing and control
(WFS&C) system, the results of which are demonstrated on the General Dynamics Advanced Information System's
(GDAIS') QuickStar testbed1, a dual deformable mirror (DM) system which operates at 100Hz sampling rate. The
iterative automatic calibration (AutoCal) process includes both coarse and fine calibration modes, initial closed-loop
flattening of the commercial-off-the-shelf (COTS) DMs, estimation of the system's static wavefront - including DM
print-through, determination of PD-derived actuator influence functions, formulating the resulting system matrix and the
resulting forward-model parameters. Analyses of the system after the calibration routines shows low-order WFS
accuracy of ~0.005λ RMS and closed-loop residual wavefront measurement of ~0.002λ. All of these results were
accomplished with a software package that takes on the order of one hour to operate.
KEYWORDS: Wavefronts, Process control, Calibration, Imaging systems, Image processing, Fermium, Frequency modulation, Beam splitters, Cameras, Control systems
A proof-of-concept phase diversity (PD) wavefront sensing and control (WFS&C) testbed has been developed that
displays 5/1,000 wave RMS accuracy, operates at a sample rate of 100Hz, uses the extended scene of interest in lieu of a
guide star, and is comprised of all low-cost commercial-off-the-shelf (COTS) parts - including the PD processor. This testbed allows closed-loop
operation via a dual deformable-mirror (DM) concept where two DMs are optically conjugate to the exit pupil: one
acting as an independent disturbance and the other reacting to PD WFS&C commands in order to correct the system
wavefront. The use of low-cost, COTS components demonstrated the flexibility of a PD-only
WFS&C approach, and additionally allowed for this system to be conceived, designed, assembled and brought to
operation in approximately nine months. Automatic calibration efforts begun on this testbed have allowed for the quick
discrimination of prominent PD forward-model parameters and a more rapid verification and validation (V&V) process.
Also aiding the V&V process is a novel spatial-heterodyning optical interferometer that collects all information in a
single snapshot and may be made synchronous with the fast PD sample rate. This demonstration proves a PD-only
WFS&C subsystem capability suitable for use on a wide variety of adaptive-optics imaging systems.
Terahertz imaging has been shown to be a powerful tool for analyzing a variety of materials. From the amount of water in a leaf over time to looking at the spectroscopic species in a flame, this technique shows great potential for commercial applications. However, in order to work in a commercial environment, the present free-space optical systems must be abandoned in favor of fiber-optic delivery. To this end, we have developed a compact, fiber-pigtailed terahertz imaging system that utilizes a hermetically sealed, photoconductive, transmitter and receiver. The receiver uses an integrated amplifier to obtain a 1000:1 S/N with only 1 mW of power on both the transmitter and receiver and with a one second integration time. This system has usable energy extending from 0.04 to 2 THz and has both a rapid (20 Hz) scanner for short, 40-ps, scans as well as a long rail for scans up to 1 ns. The system hardware is contained in a 1.5 cu. ft. box with fibers feeding both the transceiver units. These units can be configured into either a transmission or reflection mode depending on the user's application. An advanced software system controls the hardware, collects the data, and does image processing.
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