CHAPTER
2
Mozza Theory
2.1
Acousto-optic Tunable Filter (AOTF)
An Acousto-optic Tunable Filter (AOTF) is a electronically tunable bandpass filter; this tech-
nique uses the acousto-optic interaction inside an anisotropic medium to select a line from a
broadband laser source.
The Mozza spectrometer uses a solid-state TeO
2
crystal as the acousto-optic tunable filter
to select a narrow band of the incident pulse. The center of the narrow band depends on
the acoustic frequency; therefore, by scanning the acoustic frequency, the diffracted beam is
recorded for each acoustic frequency and the optical spectrum is eventually reconstructed by
mapping the acoustic frequencies to the optical frequencies. The acousto-optic interaction is
sensitive to the polarization of the incident beam and in the Mozza spectrometer, the source
must be S-polarized.
Using Acousto-Optic Tunable Filter provides many advantages over other scanning tech-
niques. Using a solid-state crystal and selecting the wavelength electronically, no moving part
is used in this setup; as a consequence, it is reliable, fast and stable and appears as a very
valuable tool, especially when a pulsed laser is the source to analyze.
2.2
Mozza principle
The principle of the Mozza spectrometer is shown in Figure
. The broadband incident beam
is split in two parts:
the first part goes through the acousto-optic crystal and the diffracted narrowband pulse
coming out of the crystal is recorded by a single point detector (MCT), it provides the
spectral density at the given wavelength (related to the current acoustic frequency) in a
5 cm
−
1
spectral bandwidth.
the second part is directed to a second single point detector (MCT). Since the measure-
ment is not single-shot, it can disturbed by the energy fluctuation of the source. To
counterbalance this effect, the second part of the beam records the energy fluctuation of
the source so that the spectrum recorded on the first photodiode can be normalized.
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