
51-6 Optical
To reduce any possible heating of the sample due to absorption, the beam is then passed through a
ne
utral
de
nsity
l
ter,
wh
ich
at
tenuates
th
e
be
am
fu
rther
to a le
vel
of 50
–100
μW. e ne
xt
it
em
in th
e
be
am
li
ne
de
pends
on th
e
de
tection
sc
heme
. Fo
r
ac op
eration,
a li
ght
ch
opper
is in
serted
in
to
th
e
be
am
li
ne,
wh
ile
fo
r
dc de
tection,
no
thing
el
se
is re
quired
be
fore
th
e
sa
mple
. An ou
tput
fr
om
th
e
ch
opper
is
us
ed
fo
r
sy
nchronous
de
tection
. e sa
mple
ch
amber
th
en
fo
llows
. i
s
ch
amber
re
quires
an in
put
an
d
an ou
tput
po
rt
an
d
a de
sign
th
at
pr
ecludes
in
terference
of an
y
be
am
wi
th
ot
her
re
ected
be
ams
. Ot
her
de
sign
de
tails
ar
e
de
termined