Figure 1 shows our infrared (IR) Fresnel sapphire
prism spectrometer optical layout. The
implementation of this prism is basically like any other prism spectrometer in
a Czerny Turner configuration. Light
enters the system through a slit and is collimated by the first lens. The IR light is directed through the prism
which “bends” each wavelength at a slightly different angle. The dispersed beams are then focused onto the
detector array by a second set of lenses.
The detector thus records the spectra of the IR radiation illuminating
the entrance slit.
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| Figure 1 - Fresnel Sapphire Prism Spectrometer Layout |
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The focal plan dispersion of the prism spectrometer
is given by:
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where f is the focal length of the focusing lenses, B is the prism base dimension, W is the beam width, and dn/dλ
is the spectral dispersion of the prism material. Equation 1 can
be used to determine the requisite focal length, f, needed to disperse
a designated spectral range, dλ, over the physical range, dx, spanned by the detector array.
We chose sapphire for this system because of its extremely strong
dispersion that is about four times that of calcium fluoride or zinc
selenide over the designed wavelength region (Figure 2). However,
given this choice, we had to carefully take into account both the
birefringence of the sapphire. During our analysis we discovered
that as long as the crystal axis of the sapphire is oriented parallel
to the optical axis of the system, the resulting blur from the
birefringence is less than that associated with the aberrations of the
imaging system. In fact, the effects of the birefringence are
really only noticeable as the axis mis-orientation becomes fairly
large.
The size of the individual prisms that make up the Fresnel prism was
set such that the associated diffraction limit was less than the width
of a single detector element.
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| Figure 2 - Dispersion of Sapphire, Calcium Fluoride, and Zinc Selenide |
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