Corner retroreflector

Figure 3.25 Cube-corner retroreflector interferometers with rotational scanning.

Figure 5.7 (a) A pair of parallel mirrors and (b) cube-corner retroreflector (only two mirrors are shown). These optical elements are slightly tilted In the lower figures, but reflected rays 2 are always parallel to incident rays I. 

In Figure 5.8, an example of an interferometer which realizes the OPD by rotating a pair of cube-corner retroreflectors is illustrated. As such an interferometer can lessen the effect of shocks and tilts to the spectrometer, restrictions on the place for siting the spectrometer can be relaxed. In this design of spectrometer, however, there is a limit to its spectral resolution, as it is difficult to attain a large OPD. Some spectrometers having such interferometers are, however, portable, and can be used for fieldwork also. 

Figure 5.8 Interferometer using rotational motion (schematically illustrated). A pair of cube-corner retroreflectors are placed on the both arms of an L-type rotator. The OPD is generated by rotating this rotator around the axis.

If a spectral measurement is disturbed, it is important to understand the origin of the disturbance first and take proper action to eliminate it. For example, if the reflectors on the two arms of the interferometer are plane mirrors (not cube-corner retroreflectors), the above case of the infrared radiation emitted by a high-temperature sample may be dealt with in the following way. It is effective to block half of the incident beam from the light source at a point close to the sample position where the incident beam is focused. Then, the infrared radiation from the sample will advance to the interferometer through the remaining half and will be reflected by the interferometer. When it comes back toward the sample. 

The principle of long-path absorption techniques is illustrated in Fig. 10.17. A laser beam is transmitted continuously into the atmosphere against a corner-cube retro-reflector (Fig. 6.21) that is placed at a distance of up to 10 km. The reflected beam is received by an optical telescope that is placed at the site of the laser and is directed towards the retroreflector. The received light mtensity is measured photoelectricaUy as a function of the laser wavelength. 

Figure 22.6. Typical cube-corner array retroreflector. An image of the camera can be seen at the center...

An alternative interferometer to the cube-corner is the cat s-eye interferometer, where the cube-corners are replaced by cat s-eye retroreflectors. These are components that consist of one parabolic and one spherical mirror, and reflect an incident ray back in the opposite direction. Cat s-eye interferometers are also used in high-resolution spectroscopy. 

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