
52-11Laser Output Measurement
When R is close to 1, the right-hand side of Equation 52.18 is close to zero. We have
∆
φ
FWHM
=
−2 1
2
1 2
( )
/
R
R
(5
2
.19
)
Equation
52.19 sh
ows
th
at
th
e
fr
inge
wi
dth
of I re
duces
as R is in
creased,
be
cause
la
rger
R re
sults
in
mo
re
re
ections
be
tween
th
e
tw
o
in
ner
su
rfaces
of th
e
SF
PI
. In an SF
PI,
th
e
be
am
is us
ually
ar
ranged
to
incident at normal on the inner surface. θ becomes zero. Combining Equations 52.10 and 52.16 results in
λ
=
2D
m
(5
2
.20
)
where
λ is th
e
wa
velength
at wh
ich
th
e
pe
ak
of I ap
pears
. Fi
gure
52.8 sh
ows
th
at
an SF
PI
fu
nctions
li
ke
a mu
ltibandpass
l
ter
. e ce
ntral
wa
velengths
of th
e
ba
nds
(f
ringes)
ar
e
gi