Biomedical Engineering Reference
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(a)
Linear rotating
polarization
Circular
polarization
Linear
polarization
Rotating
polarizer
Unpolarized
light
Quarter
wave-plate
Linear rotating
polarization
Polarizer
Elliptic
polarization
(b)
Linear
polarization
Dichroic
mirror
Rotating
polarizer
β
Unpolarized
light
α
Quarter
wave-plate
Half
wave-plate
Polarizer
FIgurE 2.10 (a) Schematic sketch of a polarization scanning equipment to be used in an SHG microscope. (b)
Schematic sketch of a polarization scanning equipment with compensation of the polarization ellipticity intro-
duced by the dichroic mirror. The half-wave plate and the quarter-wave plate are rotated in order to compensate the
phase retardation introduced by the dichroic mirror.
In fact, ellipticity produces a fluctuation in the intensity after the second polarizer, affecting the polar-
ization scanning measurement. Further, an intensity fluctuation in the excitation beam will affect the
detected SHG polarization profile introducing an intensity noise which is double with respect to the
excitation intensity fluctuation, because of the nonlinearity of the optical process generating second-
harmonic light.
All this equipment has to be placed immediately before the objective lens without any other optical
component having the potential to affect the polarization of the beam. This geometry could represent
a problem when a dichroic mirror is placed in front of the objective lens. In fact, in backward detec-
tion geometry (see Section 2.3.2.2), a dichroic mirror and the detector have to be placed as close as
possible to the objective in order to maximize the detected signal. The coating of the dichroic mir-
ror affects the polarization of the laser beam, generally producing elliptic polarization. A possible
solution has been proposed by Chou et  al. (2008). The procedure consists of placing an additional
half-wave plate in the optical path. The half-wave plate and the quarter-wave plate should be rotated
in order to introduce a compensation for the phase retardation introduced by the dichroic mirror, so
that the output beam is polarized with linear rotating polarization (Figure 2.10b). This solution allows
a compensation for a well-defined direction of the laser polarization, whereas it has to be modified
while scanning the laser polarization. So, a synchronized rotation of both polarizer and quarter wave
plate should be used. An alternative and much simpler approach has been used in the cases in which
the sample itself can be rotated about the optical axis, while keeping a constant linear polarization of
the incident laser beam (Hsieh et al., 2010).
 
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