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Nonlinear optical phase conjugation, obtained by four-wave degenerate mixing is utilized to create a new form of optical
pointing error sensor-theActiveOptical Self-Referencing Component(AO-SRC). Thisdevicewas conceived and constructed
during a 1988 nonlinearoptic initiative thatwas designed to demonstrate the expanded degrees of freedom available for designs
that incorporate nonlinear optic elements.
The AO-SRC concept was successfully demonstrated and verified that an absolute, seff-referenced form of pointing sensor
couldbeachievedwith the use of nonlinear photorefractive design elements. The self-referencing capabiity is obtained from the
ideal and generalized phase-conjugate reflectance from nonlinear mirrors. The absolute capability is related to the optical
relationship between the phase-conjugate and the Fresnel (specular) reflected energy from a common nonlinear medium. It is
seen that the same physical mechanism responsible for the well known self-referencing capability of phase-conjugate
interferometers (-i.e. elimination ofthe reference mirror) can be used to constructa high sensitivity, self-referencing, pointing
error sensor.
Current nonlinear materials such as barium titanate are limited to maximum input apertures ofabout 12 millimeters diameter.
Since sensitivity (minimum resolvable angle) is inversely related to aperture alze, the maximum size of nonlinear media may
represent a severe limiting factor for some AO-SRC device forms. Two mechanisms associated with nonlinear optic materials
were used to ease this constraint, specifically:
1. Electric Field Enhancement: Application of an electric field seross the mdium was used to construct a phase-shifting,
phase-conjugate interferometer. This construction will yield an increase in sensitivity of 1-2 orders of magnitude.
2. Nonlinear Synthetic Aperture: Utilization of the self-coupling capability of discrete nonlinear materials was shown to yield
effective aperture sizes of arbitrary dimension.
It is shown that the AO-SRC concept can be applied to such critical tasks as the assessment of the optical coherence achieved in
phasing (pointing error and piston) segmented or synthetic aperture optical systems.
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