THORLABS SA210 Series, Руководство пользователя

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Fabry-Perot Interferometer Chapter 4: Fabry-Perot Interferometry
Page 4 19502-D02
Chapter 4 Fabry-Perot Interferometry
4.1. Free Spectral Range
To scan the spectra of the laser beam entering the Scanning Fabry–Perot interferometer small displacement is
applied to one of the cavity mirror mounted on piezoelectric transducers. This operation is done by fine tuning the
ramp voltage applied to the Piezoelectric elements using the controller SA201. When the mirror spacing becomes
equal to an integral number of half the wavelength of the laser, constructive interferences occur. That spectral
response of the signal can be visualized with a scope. A series of periodical peaks appear on the screen of the
scope. The distance between consecutive peaks is called the free spectral range (FSR) of the instrument.
From a users perspective a confocal cavity has a FSR that is given by c/4d instead of c/2d (where c is the speed
of light and d is the cavity length) as would be the case for a plano-plano cavity; the factor of 2 in the denominator
can be understood by inspecting the ray trace shown below in Figure 1. Note that a ray entering the cavity at a
height ‘h’ parallel to the optical axis of the cavity makes a triangular figure eight pattern as it traverses the cavity.
From this pattern it is clear that the ray makes four reflections from the cavity mirrors instead of the two that would
result in a plano-plano cavity. Hence the total round-trip path through the cavity is given as 4d instead of 2d.
Figure 1
This figure shows a simplified ray-trace for a ray entering the cavity at height ‘h’. The curvature of the
mirrors ‘R’ and the separation being set precisely to ‘R’ ensures that the input ray is imaged back onto itself after
traveling a distance of approximately 4R.
Additionally, in this configuration if a paraxial ray is traced through the system as shown in Figure 1, it is apparent
that in the confocal configuration each mirror serves to image the other mirror back onto itself so that a ray
entering the cavity will, after four traverses of the cavity, fall back onto itself, (note that the focal length of a
spherical mirror is R/2). This imaging of the beam back onto itself greatly simplifies the alignment of the cavity;
just align your input to within a few tenths of a millimeter of the center of the mirror set and restrict your input
angles to less than a few degrees. The SA200 series interferometer has two iris diaphragms that simplify this
alignment requirement.