Preface
This
manual
contains
information
you
will
need
for
day-to-day
operation
and
maintenance
of your
QuantaRay®
OCR
Series
Nd:YAG
Laser
System. You
will
find
instructions
for installation,
operation,
preventive
maintenance,
a
brief
description
of
its
circuitiy,
and
a
troubleshooting
and
repair
guide.
The
system
comprises
three
elements—the
laser
head
power
supply,
and
remote
control. In
addi
tion
to instructions
for these
components,
the
manual
describes
the
installation
and
operation
of
the
HG-2
harmonic
generator.
While
this
manual
contains
a
brief
installation
procedure,
it
is
not
intended
as
a
guide
to
the
initial
installation and
set-up
of
your
laser.
Please
wait
for
the
Spectra-Physics
service
engineer
who
has
been
assigned
this
task
as
part
of
your
purchase agreement.
Allow
only
those
qualified
and
authorized
by
Spectra-Physics
to
install
and
set
up
your
laser
system.
The
Service
and
Repair
section
is
intended
both
as
an
aid
to
help
you
guide your
field
service
engineer
to the
source
of
problems,
and
as
a
guide
to
repairs
you
may
choose
to
do
yourself.
Do not
attempt
to
repair
the
unit
while
it
is
under
warranty;
instead,
report
all
system
failures
to
Spectra-Physics
customer
service
for
warranty repair.
The
OCR
Series
lasers
emit
laser
radiation
that
can
permanently
damage
eyes
and
skin,
ignite
fires,
and vaporize substances.
More
over,
focused
back
reflections
of
even
a
small
percentage
of
its
out
put
energy
can
destroy
expensive
internal
optical
components.
The
Laser
Safety
section
contains
information
and
guidance
about
these
hazards.
To
minimize
the
risk
of
injury,
death,
or
expensive
repairs,
carefully
follow
these
instructions.
If
you
encounter
any
difficulty
with
the
content
or
style
of
this
manual,
please let
us
know.
The
last
page
is
a
form
to aid
in
bringing
such
problems
to
our
attention.
Thank
you
for
your
purchase of
Spectra
Physics
instruments.
III
Table
of
Contents
1—1
1—1
1-3
1-4
1-6
1-7
1-8
1-10
1—11
1-12
1-14
2-1
2-1
2-3
2-3
2-4
3—1
3-1
3-1
3-1
3-2
3-3
3-5
3-5
3-5
3-6
3-6
3-6
3-7
3-7
Chapter
1
Introduction
Emission
and
Absorption
of
Light
Population
Inversion
Nd:YAG
as
an
Excitation
Medium
Q-switching
Resonant
Optical
Cavity
Longitudinal
Modes
and
Linewidth
Producing
Other
Wavelengths
Resonator
Structural
Considerations
Pulse
Triggering
Sequence
and
Timing
Specifications
Chapter
2
Laser
Safety
Precautions
for
the
Safe
Operation
of
Class
IV-High
Power
Lasers
Focused
Back
Reflection
Safety
Maintenance
Required to
keep
this
Laser
Product...
in
Compliance
with
Center
for
Devices
and
Radiological
Health
(CDRH)
Regulations
Sources
of
Laser
Safety
Standards
Chapter
3
Installation
and
Operation
Unpacking
your
Laser
Installing
the
Laser
Connecting
the
Electrical
Service
Filling
the
Cooling
System
Controls
and
Connections—Remote
Control Module
Controls
and
Connections—Power Supply
OUTPUT
Connectors
lNPUTConnectors
REMOTE
Connector
COMPUTER
Connector
POWER
Controls
PURGE
Controls
Power
Supply
Rear
Panel
V
Table of
Contents
(cont.)
Chapter
3
3-8
3-8
3-8
3-8
3-9
Installation
and
Operation
(cont..)
3-1
Controls
and
Connections—Laser
Head
Q-switch
Driver
(Marx
Bank)
Box
Emission Indicator
Convenience
Receptacle
Starting
the
Laser
Chapter
4
Computer
Interface
Module
4-1
Functional
Overview
4-1
Computer
Control
and
Diagnostic
Functions
4-3
Power-On
Default
State
and
System
Initialization
4-3
Computer
Safety (Watchdog)
Interlock
4-4
IEEE
488
Interface
4-4
Operation
4-4
Remote
Reset
4-4
Serial
Poll
Status
Byte
4-5
SW2
DIP
Switch
Setting 4-6
RS-232-C Serial
Interface
4-6
Operation
4-6
Data
Transfer
and
Handshaking
4-7
SW1
DIP
Switch
Setting
4-8
SW3
DIP
Switch
Setting
4-8
Message
Formats
4-8
Command
Format
4-8
Response
Format
4-9
CIM-1
Commands
4-10
CONFIGURE
p,
OUTPUT,
NONCLOCKED
4-10
4-11
SELECTc
4-11
WRITEp,n
4-11
SELECT
1,
WRITE 4,
n
4-13
SELECT
1,
WRITE 5, n
4-13
SELECT
1,
WRITE 6, n
4-13
SELECT
1,
WRITE
7,
n
4-13
SETd,n
4-15
Examples
4-15
External
lamp
fire
4-15
Variable
rep
rate
4-15
Q-switch
Advance
Sync
4-16
Sample
Analog-to-Digital
Conversion
4-17
SAMPLE
a
Example
Commands
Using
GW
BASIC
on
a
Personal
Computer
4-18
VI
Table
of
Contents
(cont.)
Chapter
5
Installing
and
Operating
the
5-1
HG-2
Harmonic
Generator
HG-2
Controls
5-3
Installing
the
HG-2
5-3
Operation
5-4
Type
I
and
II
Crystals
5-5
HG-2
Temperature
Controller
5-6
Controls
5-6
Operating
Voltage
5-6
Second
Harmonic
(Types
I
and
II)
5-7
Third
and
Fourth
Harmonic
Generation
Chapter6
Maintenance
6—1
Maintaining
the
Cooling
System
6-1
Maintaining
the
Air
Purge
System
6-i
Maintaining
the
HG-2
6-i
Replacing
the
Deionizing
Water
Filter
6-2
Procedure
6-2
Replacing
the
Air
Filters
6-3
Procedure
6-3
Replacing
the
Flash
Lamps
6-4
Procedure
6-4
Chapter7ServiceandRepair
7-1
System
Description
7-i
Computer/Internal
Switch
7-1
Enabling
Signals
7-i
Analog
Signals
7-1
Q-switch
Delay
7-2
Q-switch
Advanced
Sync
Generator
7-2
Mode
Switch
(Ui
1)
7-2
Q-switch
Drivers
7-3
Single-Shot
Operation
7-3
Inhibit
Switch
7-3
OFF
[STOP]/ON
[ENABLE]
buttons
7-3
Interlock
Logic
7-4
Pulse
Forming
Network 7-4
Flash
Lamp
Simmer
Supply
7-5
System
Start-up
Tests
7-5
VII
Table
of
Contents
(cont.)
ChapterlOCustomerService
8-1
Warranty
8-1
Return
of
the
Instrument
for
Repair
8-2
Service
Centers
8-2
List
of
Figures
Figure
1-1:
Electrons
occupy
distinct
orbitats
defined
by
the
1-2
probability
of
finding
an
electron
at a
given
position,
the
shape
of
the
orbital
being
determined
by
the
radial
and
angular
dependence
of
the
probability.
Figure
1-2:
A
Typical
Four-level
Transition
Scheme
(a)
1-4
Compared
to
that
of
Nd:YAG
(b)
Figure
1-3:
Energy
Level
Scheme
for
the
Nd:YAG
Laser
Source
1-5
Figure
1-4:
The
Q-switch
comprises
a
polarizer,
a
quarter-wave
1-6
polarization
rotator,
and
a
Pockets
cell.
Figure
1-5:
Stable
and
Unstable
Resonator
Configurations
1-7
Figure
1-6:
Frequency
Distribution
of
Longitudinal
Modes
for
a
Single
Line
1-9
Figure
1-7:
Etaton
Loss
Minimum
Tuned
to
Laser
Gain
Maximum
1-9
Figure
1-8:
Simplified
Block
Diagram
of
GCR
Series
Electronics
1-12
Figure
2-1:
Radiation
Control
Drawing 2-5
Figure
2-2:
Warning
Labels
2-6
Figure
3-1:
Main
autotransformer
is
tapped
for
several
operating
voltages
3-2
Figure
3-2:
Cooling
System
Component
Identification
3-3
Figure
3-3:
Remote
Control
Panel
3-4
Figure
3-4:
Power
Supply
Control
Panel 3-6
Figure
3-5:
Q-switch
Driver
(Marx
Bank)
Box
3-8
Figure
3-6:
Head
Emission
Indicator
3-9
Figure
4-1:
Location
of
CIM-1
PC
Boards
4-2
Figure
4-2:
Diagram
of
the
serial
poll
status
byte
4-5
Figure
4-3:
Standard
RS-232-C
Interconnections
4-7
Figure
4-4:
Command
Word
Abbreviations 4-10
Figure
4-5:
CIM-1
Proprietary
PC
Board
4-14
Figure
5-1:
HG-2
Component
Identification
5-1
Figure
5-2:
Temperature
Control
Panel
5-6
Figure
6-1:
Short
together
posts
A
and
B
to
prevent
shock
6-5
when
servicing
the
flash
lamps.
VIII
Table
of
Contents
(cont.)
List
of
Tables
Table
4—1:
SW2
DIP
Switch
Settings
for
Selecting
Device
Address
4-6
Table
4-2:
SW1
Baud
Rate
Settings
4-8
Table
4—3:
SW1
Mode
Select
Settings
4-8
Table
4—4:
Sample
a
Command
Functions
4-11
Table
4—5:
SELECT
1,
WRITE
Command
Functions
4-12
Table
4-6:
SELECT
2,
WRITE
Command
Functions
4-13
Table
5—1:
Summary
of
Translation
Arm
Positions
5-2
Table
5—2:
Summary
of
HG-2
Positions
5-2
Table
7—1:
System
Start-up
Tests 7-7
Table
7—2:
Replacement
Parts
7-13
SI
Units
The
following
System
International
(SI)
units,
abbreviations,
and
prefixes
are
used
in Spectra-Physics
Lasers
manuals:
Quantity
Unit
Abbreviation
mass
gram
g
length
meter
m
time
second
s
frequency
hertz
Hz
force
newton
N
energy
joule
J
power watt
W
electric
current ampere
A
electric
charge
coulomb
C
electric
potential
volt
V
resistance
ohm
W
inductance henry
H
magnetic
flux
weber
Wb
magnetic
flux
density tesla
T
luminous intensity
candela
cd
temperature
kelvin
K
pressure
pascal
Pa
capacitance
farad
F
angle
radian
rad
Prefixes
tera
(1012)
T
deci
(10-1)
d
nano
(10)
n
giga
(1O)
G
centi (102)
c
pico
(1012)
p
mega
(106)
M
milli
(10)
m
femto
(10-15)
f
kilo
(10)
k
micro
(10-6)
atto
(10-18)
a
xi
Chapter
1
Introduction
Emission and
Absorption
of
Light*
Laser
is
an
acronym
derived
from
“Light
Amplification
by
Stimu
lated
Emission
of
Radiation.”
Thermal
radiators,
such
as
the
sun,
emit
light
in
all
directions,
the
individual
photons
having
no
definite
relationship
with
one
another.
But
because the
laser
is
an
oscillating
amplifier
of
light,
and
because
its
output
comprises
photons
that
are
identical
in
phase,
direction,
and
amplitude,
it
is
unique
among
light
sources.
Its
output
beam
is
singularly
directional,
intense,
monochro
matic,
and coherent.
Radiant
emission
and
absorption
take
place
within
the
atomic
or
molecular
structure
of
materials.
The
contemporary model of
atomic
structure
describes an
electrically
neutral
system
composed
of
a
nucleus
with
one
or
more
electrons
bound
to
it.
Each electron
occupies
a
distinct
orbital
that
represents the
probability
of
finding
the
electron
at
agiven
position
relative
to
the
nucleus.
Each
orbital
has
a
charac
teristic shape
that
is
defined
by
the radial
and
angular
dependence of
that
probability,
e.g.,
all
“s”
orbitals
are
spherically
symmetrical, and
all
“p”
orbitals surround
the
x,
y,
and
z
axes
of
the
nucleus
in
a
double-
lobed
configuration
(Figure
1—1).
The
energy
of
an
electron
is
deter
mined
by
the
orbital
that
it
occupies,
and the
over-all
energy
of
an
atom—its energy
level—depends
on
the
distribution
of
its
electrons
throughout
the
available
orbitals.
Each
atom
has
an
array
of
energy
levels:
the
level with
the
lowest
possible energy
is
called
the
ground
state,
and
higher
energy
levels
are
called
excited
states.
If
an
atom
is
in
its
ground state,
it
will
stay
there
until
it
is
excited
by
external
forces.
*
“Light”
will
be
used
to
describe
the
portion
of
the electromagnetic
spectrum from
far
infrared
to ultraviolet.
1—1
Quanta-Ray
GCR
Series
Figure
1—1:
Electrons
occupy
distinct
orbitals
that
are
defined
by
the
probability
of
finding
an
electron
at
a
given
position,
the
shape
of
the
orbital
being
determined
by
the
radial
and
angular
depen
dence
of
the
probabiit
Movement
from
one
energy
level
to
another—a
transition—happens
when
the
atom
either
absorbs
or
emits
energy.
Upward
transitions
can
be
caused
by
collision
with
a
free
electron or
an
excited
atom,
and
transitions
in
both
directions
occur
as
a
result
of
interaction
with a
photon
of
light.
Consider
a
transition
from
a
lower
level
whose energy
content
is
E
1
to
a
higher
one
with
energy
E
2
.
It
will
only
occur
if
the
energy
of
the incident
photon
matches
the
energy
difference
between
levels,
i.e.,
hv=E
2
—E
1
[1]
where
h
is
Planck’s
constant,
and
v
is
the
frequency
of
the
photon.
Likewise,
when
an
atom
excited
to
E
2
decays
to
E
1
,
it
loses
energy
equal
to
E
2
—E
1
.
Because
its
tendency
is
toward
the lower energy
state,
the
atom
may
decay
spontaneously,
emitting
a
photon
with
energy
hv
and
frequency
[2]
Spontaneous
decay
can
also
occur
without
emission
of
a
photon,
the
lost
energy
taking
another
form,
e.g.,
transfer
of
kinetic
energy
by
collision
with
another
atom.
An
atom
excited
to
E
2
can
also
be
stim
ulated
to decay to
E
1
by
interacting
with
a
photon
of frequency
v,
shedding
energy
in
the
form
of
a
pair
of
photons
that
are
identical
to
the incident
one
in
phase,
frequency,
and
direction.
By
contrast,
spontaneous
emission
produces
photons that
have
no
directional
or
phase
relationship
with
one
another.
x.
x
1-2
Introduction
Alaser
is
designed
to
take
advantage of
absorption,
and
both
spon
taneous
and
stimulated emission
phenomena,
using
them
to
create
conditions favorable
to
light amplification.
The
following
paragraphs
describe
these
conditions.
Population
Inversion
The
absorption
coefficient
at
a
given
frequency
is
the difference
be
tween the
rates
of
emission
and absorption
at that
frequency.
It
can
be
shown
that
the
rate
of excitation from
E
1
to
E
2
is
proportional
to
both
the number
of
atoms
in
the
lower
level
(N
1
)
and the
transition
probability.
Similarly,
the
rate
of
stimulated
emission
is
proportional
to
the population of the
upper
level
(N
2
)
and
the
transition
probability.
Moreover,
the
transition
probability
depends
on
the
flux
of the
inci
dent
wave
and
a
characteristic
of
the
transition
called
its
“cross
sec
tion.”
It
can
also
be
shown
that
the
transition
cross
section
is
the
same
regardless
of
direction.
Therefore,
the
absorption
coefficient
depends
only on
the difference
between
the
populations
involved,
N
1
and
N
2
,
and
the
flux
of
the
incident
wave.
When
a
material
is
at
thermal
equilibrium,
a
Boltzmann
distribution
of
its
atoms
over
the array
of
available
energy
levels
exists
with
nearly
all
atoms
in
the
ground
state.
Since
the
rate
of
absorption of
all
frequencies
exceeds
that
of
emission,
the absorption
coefficient at
any
frequency
is
positive.
If
enough
light
of
frequency
v
is
supplied,
the
populations
can
be
shifted
until
N
2
=
N
1
.
Under
these
conditions
the
rates of
absorption
and
stimulated
emission
are
equal,
and
the
absorption
coefficient
at
frequency
v
is
zero.
If
the transition
scheme
is
limited
to
two
energy
levels,
it
is
impossible
to
drive
the populations
involved
beyond
equality;
that
is,
N
2
can
never
exceed
N
1
because
every
upward
tran
sition
is
matched
by
one
in
the opposite
direction.
However,
if
three
or
more
energy
levels
are
employed,
and if
their
relationship
satisfies
certain
requirements
described
below,
additional
excitation
can
create
a
population
inversion,
in
which
N
2
>
N
1
.
A
model
four-level
laser
transition
scheme
is
depicted
in
Figure
1—2
(a).
A
photon
of
frequency
v1
excites—or
“pumps”—an
atom
from
E
1
to
E
4
.
If
the
E
4
to
E
3
transition probability
is
greater
than
that
of
E
4
to
E
1
,
and
if
E
4
is
unstable,
the atom
will
decay
almost
immediately
to
E
3
.
If
E
3
is
metastable,
i.e.,
atoms
that
occupy
it
have
a
relatively long
lifetime,
the
population
will
grow
rapidly
as
excited
atoms
cascade
from
above.
The
E
3
atom
will
eventually
decay
to
E
2
,
emitting
a
pho
ton of
frequency. Finally,
if
E
2
is
unstable,
its
atoms
will
rapidly
return
to
the
ground
state,
E
1
,
keeping the
population
of
E
2
small
and
reduc
ing
the
rate
of
absorption
of
v2
.
In
this
way
the population
of
E
3
is
kept
large and
that
of
E
2
remains
low,
thus
establishing
a
population
inversion
between
E
3
and
E
2
.
Under
these
conditions,
the absorption
coefficient
at
v2
becomes
negative.
Light
is
amplified
as
it
passes
through
the
material,
which
is
now
called
an “active
medium.”
The
greater
the
population
inversion,
the
greater
the
gain.
1-3
Quanta-Ray
GCR
Series
Figure
1—2:
A
Typical
Four-level
Transition
Scheme (a)
Compared
to
that
of
Nd:YAG
(b).
A
four-level
scheme
has
adistinct
advantage
over
three-level
systems,
where
E
1
is
both
the
origin
of
the
pumping
transition
and
the
terminus
of
the
lasing
transition.
In
the four-level
arrangement,
the
first
atom
that
is
pumped
contributes
to
the
population
inversion,
while
over
half of
the
atoms must
be
pumped
from
E
1
before
an inversion
is
established
in
the
three-level
system.
In
commercial
laser
designs
the source
of
excitation
energy
is
usually
optical
or
electrical:
arc
lamps
are often
employed to
pump
solid-
state
lasers;
the
output
of
one
laser
can
be
used
to
pump
another,
e.g.,
a
liquid
dye
laser
can
be
pumped
by
a
pulsed
Nd:YAG
laser;
and
an
electric discharge
is
generally
used
to
excite
gaseous
media
like
argon
or
krypton.
Nd:YAG
as
an
Excitation
Medium
The
properties
of
neodymium-doped
yttrium
aluminum
garnet
(Nd:YAG)
are
the
most
widely
studied and
best understood
of
all
solid-state laser
media.
Its
transition
scheme
is
compared to
the model
in
Figure
1—2(b)
and
its
energy
IeveJ
diagram
is
depicted
in
Figure
1-3.
The
active
medium
is
triply
ionized
neodymium,
which
is
optically
pumped
by
a
flash
lamp
whose
output
matches principle
absorption
bands
in
the
red
and
near
infrared.
Excited
electrons
quickly
drop
to
the
F
372
level,
the
upper
level
of
the
lasing
transition,
where
they
remain
for
a
relatively
long
time—about
230
lisec.
E
3
E
2
11502
cm
1
2111
ctrf
1
(a)
I9/2
Nd
+
3
(b)
1-4
Introduction
Figure
1—3:
Energy
Level
Scheme
for
the
Nd:YAG
Laser
Source
The most
probable
lasing
transition
is
to the
‘1i12
state,
emitting
a
pho
ton
at
1064
nm.
Because
electrons
in
that
state
quickly
relax
to
the
ground
state,
its
population
remains
low.
Hence, it
is
easy
to
build
a
population
inversion.
At
room
temperature
the
emission
cross
section
of
this
transition
is
high,
so
its
lasing
threshold
is
low.
While
there
are
competing
transitions
from
the
same
upper
state—most notably
at
1319, 1338,
and
946
nm—all
have lower
gain
and
a
higher
threshold
than
the
1064
nm
transition.
In
normal
operation,
these
factors
and
wavelength-selective
optics
limit oscillation
to
1064
nm.
A laser
made
up
of
just
the
active
medium and
resonator
will
emit
a
pulse
of laser
light each
time
the
flash
lamp
fires.
However,
the
pulse
duration
will
be
long,
about
the
same
as
the
flash
lamp,
and
its
peak
power
will
be
low.
A
Q-switch
is
used
to shorten
the
pulse
and
raise
its
peak
power.
20
—
Pump
FZ
Bands
18
16
-
14
-
12-
-
-
-_____
E
C.)
10
-
_—
11502
cm
R
2
—------.
11414
R
1
Laser
Transition
8-
Laser
I
15
/
2
,
Transition
______
6-
115/2
-
-
---
113/2,:
_______
4.
6000
cm
1
“4000
cm
2-
0-
___
--
__4
___
—2526
13/2
- -
-.
111/2
,:-
—---—--
2473
______
-.
-‘:_______
______
-
-
-
- -
- -
_______
_________
%/2
_________
.-.-.-.-
Ground
Level
311
\
134
0
1—5
Quanta-Ray
GCR
Series
0-switching
Because
the
upper
level
of
the
transition
has
a
long
lifetime,
a
large
population
of
excited
neodymium
ions
can
build
up
in
the
YAG
rod,
much
as
a
capacitor
stores
electrical
energy.
If
oscillation
be
prevented
while
the
population
inversion
builds,
and
if
the
stored
energy
can
be
quickly
released,
the
laser
will
emit
a
short
pulse
of
high
intensity
light.
An
electro-optic
0-switch
introduces
high
cavity
loss
to
prevent
Os
cillation.
As shown
in
Figure
1—4
the
Q-switch
comprises
a
polarizer,
a
quarter-wave
plate,
and
a
Pockels
cell.
Applying
high
voltage to
the
Pockels
cell crystal
changes
its
polarization
retardation
character
istics,
which
determine whether
the
0-switch
is
open
(low loss)
or
closed
(high loss).
5
sec
—i’
—
ctor
Quarter-Wave
Pockels
Cell
Polarizer
Plate
Figure
1—4:
The
Q-switch
comprises
a
polarizer,
aquarter-wave
polarization
rotator,
and
a
Pockels
cell.
With
no voltage applied,
the
Pockels
cell
does
not
affect
the
polariza
tion of
light
passing
through
it,
and
the
Q-switch
functions
as
follows.
The
polarizer
vertically
polarizes
light
entering
the
Q-switch,
and
the
quarter-wave
plate
converts
it
to
circular
polarization.
As
the
circularly
polarized
light
returns
from
the
high
reflector, the
quarter-wave
plate
converts
it
to
horizontal polarization. Because
the
polarizer
only
trans
mits
vertically
polarized
light,
it
reflects
the
light
out
of
the
resonator,
so
the
cavity
loss
is
high.
With
voltage
applied,
the
Pockels
cell
cancels
the
polarization
retardation of
the
quarter-wave
plate,
so
the
light
re
mains
vertically
polarized
and
suffers
minimal
loss.
During
0-switched
operation
the
flash
lamp
excites
the
Nd
ions
for
approximately
200
jisec
to
build
up
a
large
population
inversion.
At
the
point
of
maximum
population
inversion,
a
fast
high-voltage
pulse
applied
to
the
Pockels
cell
changes
the
0-switch
from
high
to
low
loss.
The
resultant
pulse
width
is
<10
nsec,
and the
peak
optical
power
is
tens of
megawatts.
1—6
Introduction
This
short
pulse
of
high
peak
power
is
the
key
to
the usefulness
of
the
pulsed
Nd:YAG
laser.
Its
high
peak
power permits
wavelength
conversion
through
several
nonlinear
processes,
e.g.,
frequency
doubling,
frequency
mixing,
dye
laser pumping, or
Raman
frequency
conversion.
A short
pulse
provides
excellent
temporal
resolution
of
fast
phenomena
like
rapid
chemical
reactions or
high-speed
motion.
An
alternative
“long
pulse”
mode
of
operation
is
built
in
to
the
GCR.
Voltage
is
applied
to
the
Pockels
cell
as
soon
as
the
flash
lamp
fires,
and
the
Q-switch
is
held
open
for
the
entire
lamp
firing.
The
result
is
a
train
of
pulses
about
200
i.zsec
long,
with
a
separation
between
individual
pulses
of
2
to
4
isec.
The
total
energy
of
the
pulse
train
is
similar
to
that
of
a
single
Q-switched
pulse.
This
long
pulse
mode
allows
safer
alignment
and
set-up,
and
is
useful
in
experi
ments
where
total
pulse
energy,
not
its
distribution
in
time,
is
the
critical
factor.
Resonant
Optical
Cavity
A
resonant
cavity,
which
is
defined
by
two
mirrors,
provides
feedback
to
the
active
medium.
Photons
emitted
parallel
to the
optical
axis
of
the
cavity
are
reflected,
returning
to
interact
with
other
excited
ions.
Stimulated
emission
produces
two
photons
of
equal
energy,
phase
and
direction
from
each
interaction.
The
two
become
four,
four
become
eight,
and
the
numbers
continue
to
increase geometrically
until
an
equilibrium
between
excitation
and
emission
is
reached.
Both mirrors
are
coated to
reflect the
wavelength,
or
wavelengths,
of
interest
while
transmitting
all
others.
One
of the
mirrors—the
output
coupler—transmits
a
fraction
of
the
energy
stored
in
the
cavity,
and
the
escaping
radiation
becomes
the
output
beam
of
the
laser.
There
are
two
major
types
of
optical
resonators:
stable
and
unstable
(Figure
1—5).
The
difference
between
them
lies
in
what
happens
to
a
ray
of
light
traveling
close
to,
and
parallel
with
the
optical
axis.
In
the stable
resonator
the
ray
is
reflected toward
the
optical
axis
by
its
cavity
mirrors,
so
it
is
always
contained along
the
primary
axis
of
the
laser.
By
contrast,
a
ray
travelling
in
an
unstable
resonator
can
be
reflected
away
from
the
axis
by
one
of
the
cavity
mirrors.
Stabl
Unstable
Figure
1—5:
Stable
and
Unstable
Resonator Configurations
1—7
Quanta—Ray GCR
Series
Stable
resonators
can only
extract energy
from
a
small
volume
near
the
optical
axis
of
the
resonator,
which
limits
the
energy
of the
out
put. Conversely,
unstable
resonators
can
have
large
beam
diameters.
Thus,
they
can
efficiently
extract
energy from
active
media
whose
cross-sectional
area
is
large,
like
that
of
typical
Nd:YAG
laser
rods.
The
output
coupler
in
an
unstable
resonator
can
take
one
of
three
forms.
In
the
first
case,
a
small
high
reflector
is
mounted
on
a
clear
substrate
and
placed
on
the optical
axis
of
the
resonator.
Energy
escapes
the
resonator
by
diffracting
around
this
dot,
which
gives
the
“diffraction
coupled
resonator”
(DCR)
its
name.
A
second
form
employs
a
partially
reflective
coating
that
uniformly
covers
the
whole
substrate.
The
third
is
a
variation
on
the
first,
where
the
small high
reflector
is
replaced
by
a
partial
reflector
with
radially
variable
reflec
tivity
(an
RVR
optic).
This
reflector
is
capable
of
producing
gaussian
or
near-gaussian
spatial
profile
at
the laser
output,
and
gives
the
“gaus
sian
coupled
resonator”
(OCR)
its
name.
If
the
energy
of
the
output
beam
is
to
be
uniformly
distributed,
the
Nd:YAG
rod
must
be
uniformly
illuminated.
Placing
the
flash
lamp
at
one
focus
of
an elliptical
chamber
causes
all
the
light
it
produces
to
be
reflected
through
the
rod,
which
is
placed
at
the
other
focus.
Uniform
cooling
is
also
essential
to
optimal
performance
of
pulsed
Nd:YAG lasers.
When
heated,
the
Nd:YAG
rod
becomes
a
lens
whose
focal
length depends on
the
average
power
absorbed.
For
optimal
per
formance,
the
high
reflector
must
be
matched
to
the
focal
length
of
the
rod,
which
must
remain
stable
during
operation.
The
thermal
gradient
of
the
rod
also
causes
a
radially
variable
polarization
rotation
that
must
be
carefully
controlled
for
the
best
beam
quality.
Longitudinal
Modes
and
Linewidth
The
laser
oscillates
within
a
narrow
range
of frequencies
around
the
transition
frequency.
The
width
of
the
frequency distribution—the
Iinewidth—and
its
amplitude
depend
on
the
active
medium,
its
tem
perature,
and
the
magnitude
of the
population
inversion. Linewidth
is
determined
by
plotting the
net
gain
of
each
frequency
and
measuring
the
width
of
the
curve
where
the
gain
has
fallen
to
one-half
maximum
(full
width
at
half
maximum,
Figure
1—6).
The
output
of
the
laser
is
discontinuous
within
the
homogeneously
broadened
line.
A
standing
wave
propagates
within
the
optical
cav
ity,
and
any
frequency
that
satisfies
the resonance condition
Vm
=
mc/2L
[4]
will
oscillate.
Vm
is
the
frequency,
c
is
the
speed
of
light,
L
is
the
opti
cal
cavity
length,
and
m
is
an
integer.
Thus,
the
output
of
a
given
line
is
a
set
of
discrete
frequencies—called
“longitudinal modes”—spaced
such
that
zv=c/2L.
[51
1—8
Introduction
I
Longitudinal
/
Modes
/
220
MHz
-/
Spacing
I
/
_____
_____
30GHz
Linewidth
Figure
1—6:
Frequency
Distribution
of
Longitudinal
Modes
for
a
Single
Line
An
etalon,
which
is
a
frequency-selecting
element,
must be
inserted
in
the
cavity
in
order
to reduce
the
linewidth. Spectra-Physics
utilizes
an
optional Fabry-Perot
interferometer that
acts
as
a
bandpass
filter,
introducing
enough
loss
in
other
modes
to
prevent
them
from
reach
ing
the
lasing
threshold
(Figure
1—7).
The
coherence
length—the
distance
over
which
the
output beam
maintains
a
fixed
phase
rela
tionship—is
inversely
proportional
to
the
linewidth:
1=
c/v.
[4]
Reducing
the
linewidth
increases the
coherence
length.
If
the
line-
width
is
reduced
from
30
0Hz
to
about
6
0Hz,
the
coherence
length
increases
from
10
mm
to
50
mm.
Longitudinal
z
Etalon
Loss
Modes
\
Curve
Figure
1—7:
Etalon
Loss
Minimum
Tuned
to
Laser
Gain
Maximum
1—9
Quanta-Ray
GCR
Series
Producing
Other
Wavelengths
The
high
peak
power
of
the
Q-switched
pulses
permit
frequency
con
version
in
nonlinear
crystals
like
potassium
dideuterium
phosphate
(KD*p).
In
the
simplest
case
the
1064
nm
Nd:YAG
fundamental
interacts
with
the
crystal
to
produce
a
secondary
wave
with
half
the
fundamental
wavelength.
For
maximum
efficiency
the
waves
must
maintain
the
same
speed
and
phase relationship
throughout
the
crystal.
The
index
of
refrac
tion
of
most
materials
depends
on
the
wavelength,
decreasing
as
the
wavelength
gets
longer.
However,
some
materials are
birefringent:
their
index of
refraction
depends
on
the
polarization
of
the
propa
gating
waves.
In
these
materials,
if
the ordinary
index
of
one
wave
length
matches
the
extraordinary
index of
the
other, the
waves
propagate
in
phase
and
at
the same
speed.
Frequency
conversion
is
most
efficient
under
these
“phase
matching”
conditions.
Phase
matching
is
critically
dependent
on
the
temperature
of
the
crystal
and
the
angle
between
the
direction
of polarization
and the
axes
of
the
crystal.
With
KD*P
two
phase
matching
alternatives
exist.
In
Type
I
phase
matching, the
input
is
along
the
ordinary
axis,
and
the
output
is
po
larized
along
the extraordinary
axis.
This
leaves
the
residual
input
wavelength linearly
polarized.
In
Type
LI
the
input
polarization
is
at
an
angle
between the extraordinary
and
ordinary
axes,
while
the
out
put
remains
polarized
along
the extraordinary
axis.
The
residual
input
wavelength
is
elliptically
polarized. Although
either
type
of
phase
matching
can
be
used
to
generate
the
second
harmonic
of
Nd:YAG
in
K1)*P
Type
II
is
more
widely
used
because of
its
higher
conversion
efficiency.
However,
some
experiments
require
linear
polarization
of
the
residual
1064
nm light
for
highest
efficiency,
and
Type
I
doubling
may
yield
better
overall
system
performance.
The
resultant
532
nm
wave
can
be
doubled
again
by
passing
it
through
a
second
crystal, which
yields
a
266
nm
wave.
It
can
also
be
mixed
in
KD*P
with
the residual
1064
nm
fundamental
to
produce
a
355
nm
wave.
These
four
wavelengths—
1064,
532,
355,
and
266
nm
—cover
the
electromagnetic
spectrum
from
the
near infrared
to
the
ultraviolet,
which
enhances
the
usefulness
of
the
Nd:YAG
laser.
532
and
355
nm
will
pump
dye
lasers
with
high
conversion
efficiency.
355
and
266
nm
are
useful
for
dissociation
and
photodestructive
studies
of
many
molecules.
1064
nm
and
266
nm
are
widely
used
for
optical
modification
of
materials
and
probing
of semiconductors.
These
fixed
frequencies
can
be
extended
further
through
Raman
shifting
or
by
using
them
to
pump
a
dye
laser. The
result
of
the
latter
is
continuously
tunable
output
over
a
wide
range
of
wavelengths.
1—10
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