BOOK_CONTENT
From Optical Shop Testing

2.8.3.   Phase Conjugating Interferometer

Phase conjugating mirrors are very useful tools in interferometry. They eliminate the
need for a perfect reference wavefront. A Twyman–Green interferometer as shown in

FIGURE 2.39. Arrangement to simultaneously produce for interferograms with different phase differences using a pixelated CCD detector with polarizing element in front of it.


FIGURE 2.40. Oblique incidence interferometer using reflecting diffraction gratings as beam splitters.

Figure 2.41, using a phase conjugating mirror has been described by Feinberg (1983)
and Howes (1986a, 1986b). The phase conjugating mirror is formed by a BaTiO3
crystal, with the C axis parallel to one of its edges and inclined 20o with respect to a
plane perpendicular to the optical axis. The phase conjugation is obtained by four
wave mixing. These pumping beams are automatically self-generated from a 30-mW
argon laser (λ = 514.5 nm) incident beam by internal reflection at the crystal faces.
Thus, it is a self-pumped phase conjugating mirror.

The property of this self-conjugating mirror is that the wavefront incident to the
mirror is reflected back along the same ray directions that the incident wavefront has.
Thus, the wavefront deformations change sign. Since the returning rays have the
same directions as the incident rays, the quality of the focusing lens is not important.

FIGURE 2.41. Twyman-Green Interferometer with a phase conjugating mirror.

However, the quality of the light source collimator is important. Any wavefront
distortions produced by this collimator will appear in the final interferogram, but
duplicated. In other words, the wavefront is not tested against a flat reference but
against another wavefront with deformations opposite in sign.

Then the lens under test is the collimator and the sensitivity is the same as that in
the common Twyman–Green interferometer, but with only a single pass through the
lens. The advantage is that no perfect lenses are necessary. The disadvantage is that
an argon laser is required.

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Optical Prisms
Optical prisms are blocks of optical material with flat, polished sides that are arranged at precisely controlled angles to one another. They are used in optical systems to deflect or redirect beams of light. They can invert or rotate images, disperse light into component wavelengths, and separate states of polarization.
Optical Lenses
Optical lenses are transparent components made from optical-quality materials and curved to converge or diverge transmitted rays from an object. These rays then form a real or virtual image of the object.  This area includes micro lenses.
Cylindrical Lenses
Cylindrical lens have at least one surface that is formed in the shape of a cylinder. Cylindrical lenses are used to correct astigmatism in the eye, and, in rangefinders, to produce astigmatism, stretching a point of light into a line.  This area includes micro cylindrical lenses as well.
Gradient Index Lenses
Gradient index (GRIN) lenses focus light through a precisely controlled radial variation of the lens material's index of refraction from the optical axis to the edge of the lens.
Fresnel Lenses
Fresnel lenses resemble a planoconvex or planoconcave lens that is cut into narrow rings and flattened. If the steps are narrow, the surface of each step is generally made conical and not spherical.

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