
POLARIZED LIGHT 173
From source
To detector
To and from
target
QWP
Figure 6.24 The optical T/R switch. We want to transmit as much light as possible through
the beam splitter and detect as much as possible on reflection.
where F
target
is the fraction of the light incident on the target that is returned to the system.
The goal is to maximize
(
1 −R
)
R. Solving d
[
(
1 −R
)
R
]
/dR = 0, we obtain a maximum at
R = 1/2 where
(
1 −R
)
R = 1/4, which is a loss of 6 dB. However, the conservation law
only holds for a given polarization. We have already seen polarization-splitting cubes, where
T
p
and R
s
both approach unity. All that is needed is a quarter-wave plate in the target ...