Mozza manual
Chapter 2 : Mozza Theory
Figure 2.1:
Mozza principle
2.3
Lock-in detection
Two beams come out from the acousto-optic crystal: the diffracted beam, which is the narrow-
band part of the incident spectrum, and the transmitted beam which is the beam not affected
by the acousto-optic interaction. Consequently, for broadband sources, the transmitted beam
can be orders of magnitude higher in energy than the diffracted.
To record the spectrum properly, it is required to record on the photodiode after the acousto-
optic crystal only the diffracted beam and no signal corresponding to the transmitted. To do
so, two techniques are used in the Mozza:
first, the transmitted and the diffracted beam are spatially separated (the diffracted
beam is deflected) and then, by using a slit before the diffracted photodiode, most of the
transmitted beam is removed from the detector.
a broadband polarizer is used to select P-polarization, which corresponds to the polariza-
tion of the diffracted beam, the transmitted beam being S-polarized.
However, as mentionned before, the transmitted beam can be orders of magnitude higher
in energy and, for broadband sources, despite these precautions, some part of the transmitted
beam can reach the detector. Furthermore, scattering of the transmitted beam can also be
responsible of misleading results.
To remove all possible contamination in the spectrum caused by scatterring or transmission
of the transmitted beam, a lock-in detection technique is implemented in the Mozza spectrome-
ter. To perform the measurement of the spectrum, the Mozza record the signal on the diffracted
photodiode with and without acoustic wave in the acousto-optic crystal. Substracting the sig-
nal without acoustic wave to the signal with acoustic wave, only the part corresponding to the
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