<s>
Coherent	O
Raman	B-Architecture
scattering	I-Architecture
(	O
CRS	O
)	O
microscopy	O
is	O
a	O
multi-photon	O
microscopy	O
technique	O
based	O
on	O
Raman-active	O
vibrational	O
modes	O
of	O
molecules	O
.	O
</s>
<s>
The	O
two	O
major	O
techniques	O
in	O
CRS	O
microscopy	O
are	O
stimulated	B-Architecture
Raman	I-Architecture
scattering	I-Architecture
(	O
SRS	O
)	O
and	O
coherent	O
anti-Stokes	O
Raman	B-Architecture
scattering	I-Architecture
(	O
CARS	O
)	O
.	O
</s>
<s>
In	O
1999	O
,	O
CARS	B-Algorithm
microscopy	I-Algorithm
using	O
a	O
collinear	O
geometry	O
and	O
high	O
numerical	O
aperture	O
objective	O
were	O
developed	O
in	O
Xiaoliang	O
Sunney	O
Xie	O
's	O
lab	O
at	O
Harvard	O
University	O
.	O
</s>
<s>
CRS	O
can	O
also	O
be	O
used	O
to	O
image	O
samples	O
labeled	O
with	O
Raman	B-Architecture
tags	O
,	O
which	O
can	O
avoid	O
interference	O
from	O
other	O
molecules	O
and	O
normally	O
allows	O
for	O
stronger	O
CRS	O
signals	O
than	O
would	O
normally	O
be	O
obtained	O
for	O
common	O
biomolecules	O
.	O
</s>
<s>
Coherent	O
Raman	B-Architecture
scattering	I-Architecture
is	O
based	O
on	O
Raman	B-Architecture
scattering	I-Architecture
(	O
or	O
spontaneous	O
Raman	B-Architecture
scattering	I-Architecture
)	O
.	O
</s>
<s>
In	O
spontaneous	O
Raman	B-Architecture
,	O
only	O
one	O
monochromatic	O
excitation	O
laser	O
is	O
used	O
.	O
</s>
<s>
Spontaneous	O
Raman	B-Architecture
scattering	I-Architecture
's	O
signal	O
intensity	O
grows	O
linearly	O
with	O
the	O
average	O
power	O
of	O
a	O
continuous-wave	O
pump	O
laser	O
.	O
</s>
<s>
In	O
CARS	B-Algorithm
microscopy	I-Algorithm
,	O
there	O
are	O
normally	O
two	O
ways	O
to	O
detect	O
the	O
newly	O
generated	O
photons	O
.	O
</s>
<s>
The	O
CARS	O
photons	O
are	O
then	O
detected	O
by	O
a	O
photomultiplier	B-Algorithm
tube	I-Algorithm
(	O
PMT	O
)	O
or	O
a	O
CCD	B-Algorithm
camera	I-Algorithm
.	O
</s>
<s>
The	O
signal	O
intensity	O
of	O
CARS	O
has	O
the	O
following	O
relationship	O
with	O
the	O
pump	O
and	O
Stokes	O
laser	O
intensities	O
,	O
the	O
number	O
of	O
molecules	O
in	O
the	O
focus	O
of	O
the	O
lasers	O
and	O
the	O
third	O
order	O
Raman	B-Architecture
susceptibility	O
of	O
the	O
molecule	O
:	O
</s>
<s>
Those	O
signals	O
are	O
normally	O
called	O
non-resonant	O
(	O
NR	O
)	O
four-wave-mixing	O
(	O
FWM	O
)	O
background	O
in	O
CARS	B-Algorithm
microscopy	I-Algorithm
.	O
</s>
<s>
There	O
are	O
two	O
ways	O
to	O
measure	O
SRS	O
signals	O
,	O
one	O
is	O
to	O
measure	O
the	O
increase	O
of	O
power	O
in	O
Stokes	O
laser	O
,	O
which	O
is	O
called	O
stimulated	O
Raman	B-Architecture
gain	O
(	O
SRG	O
)	O
.	O
</s>
<s>
The	O
other	O
is	O
to	O
measure	O
the	O
decrease	O
of	O
power	O
in	O
the	O
pump	O
laser	O
,	O
which	O
is	O
called	O
stimulated	O
Raman	B-Architecture
loss	O
(	O
SRL	O
)	O
.	O
</s>
<s>
The	O
signal	O
of	O
SRS	O
is	O
free	O
from	O
the	O
non-resonant	O
background	O
which	O
plagues	O
CARS	B-Algorithm
microscopy	I-Algorithm
,	O
although	O
a	O
much	O
smaller	O
non-resonant	O
background	O
from	O
other	O
optical	O
process	O
(	O
e.g.	O
</s>
<s>
The	O
other	O
method	O
is	O
similar	O
to	O
the	O
epi-detected	O
CARS	B-Algorithm
microscopy	I-Algorithm
,	O
where	O
the	O
back-scattered	O
light	O
goes	O
through	O
the	O
objective	O
and	O
is	O
deflected	O
to	O
the	O
side	O
of	O
the	O
light	O
path	O
,	O
normally	O
with	O
the	O
combination	O
of	O
a	O
polarizing	O
beam	O
splitter	O
and	O
a	O
quarter	O
wave-plate	O
.	O
</s>
<s>
One	O
of	O
the	O
major	O
applications	O
for	O
CRS	O
is	O
label-free	O
histology	O
,	O
which	O
is	O
also	O
called	O
coherent	O
Raman	B-Architecture
histology	O
,	O
or	O
sometimes	O
stimulated	O
Raman	B-Architecture
histology	O
.	O
</s>
<s>
Even	O
though	O
CRS	O
allows	O
label-free	O
imaging	O
,	O
Raman	B-Architecture
tags	O
can	O
also	O
be	O
used	O
to	O
boost	O
signal	O
for	O
specific	O
targets	O
.	O
</s>
<s>
For	O
example	O
,	O
deuterated	O
molecules	O
are	O
used	O
to	O
shift	O
Raman	B-Architecture
signal	O
to	O
a	O
band	O
where	O
the	O
interference	O
from	O
other	O
molecules	O
is	O
absent	O
.	O
</s>
<s>
Specially	O
engineered	O
molecules	O
containing	O
isotopes	O
can	O
be	O
used	O
as	O
Raman	B-Architecture
tags	O
to	O
achieve	O
super-multiplexing	O
multi-color	O
imaging	O
with	O
SRS	O
.	O
</s>
<s>
Confocal	O
Raman	B-Architecture
microscopy	O
normally	O
uses	O
continuous	O
wave	O
lasers	O
to	O
provide	O
a	O
spontaneous	O
Raman	B-Algorithm
spectrum	I-Algorithm
over	O
a	O
broad	O
wavenumber	O
range	O
for	O
each	O
point	O
in	O
an	O
image	O
.	O
</s>
<s>
For	O
multi-channel	O
imaging	O
,	O
SRS	O
requires	O
multichannel	O
demodulation	O
while	O
CARS	O
only	O
needs	O
a	O
PMT	O
array	O
or	O
a	O
CCD	B-Algorithm
.	O
</s>
<s>
In	O
CARS	B-Algorithm
microscopy	I-Algorithm
,	O
PMT	O
,	O
APD	O
or	O
CCDs	O
are	O
used	O
as	O
detectors	O
to	O
detect	O
photons	O
generated	O
in	O
the	O
CARS	O
process	O
.	O
</s>
<s>
This	O
will	O
negatively	O
impact	O
the	O
SNR	O
of	O
CARS	B-Algorithm
microscopy	I-Algorithm
.	O
</s>
<s>
The	O
SNR	O
of	O
CARS	B-Algorithm
microscopy	I-Algorithm
is	O
normally	O
poor	O
for	O
fingerprint	O
imaging	O
(	O
400	O
–	O
1800cm−1	O
)	O
.	O
</s>
