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Chapter 2
Development of the Fabry-Perot
Doppler Imaging System
2.1 Basic principle of the Fabry-Perot interferometer
The Fabry-Perot interferometer (FPI) is one of the main tools for observing wind veloc-
ities in the upper atmosphere. The basic principle of the FPI is as follows. A standard FPI
system consists of a collimation lens, a Fabry-Perot etalon and a focusing lens as shown in
Figure 2.1. The Fabry-Perot etalon is a combination of two optical flats which are placed in
parallel with a separation d. Their inner surfaces are coated with semi-transparent material
of high reflectance, such as dielectric multi-layered films. When a monochromatic light with
a wavelength A falls upon the etalon at an angle 475 to the normal, the light undergoes a series
of transmission and reflections, as shown in Figure 2.1, and pairs of adjoining emergent rays
differ in their path lengths by A1, where
At = 2d cos (2.1)
Constructive interference between the emerging rays will then occur at those wavelength for
which
i.e.,
nAl = mA (2.2)
2nd cos 4 = mA (2.3)
where n is the reflective index of the medium between the plates A and B and 771, is an
integer. Since the image of a point source P1 is still a point at P2, only the image of a point
source of which the optical path holds equation 2.3 becomes bright.