Spectra-Physics Quanta-Ray GCR-12 User manual

—©

Quanta-Ray

Pulsed

Nd:YAG

Lasers

Instruction

Manual

,

.

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GCR-12

GCR-L4

GCR-18

Spectra-Physics

SpectraPhysics

Lasers

4.’—

—©

Quanta-Ray

Pulsed

Nd:YAG

Lasers

Instruction

Manual

GCR-12

GCR-14

GCR-16

GCR-18

Spectra-Physics

Lasers

1330

Terra

Bella

Avenue

Post

Office

Box

7013

Mountain

View, CA

94039-7013

International

Headquarters

Siemensstrasse

20

D-61

00

Darmstadt

Germany

Part

Number

0000-225A,

Rev.

A

April

1992

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

1-3

1-4

1-6

1-7

1-8

1-10

1—11

1-12

1-14

2-1

2-3

2-4

3—1

3-1

3-2

3-3

3-5

3-6

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

3-9

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

Warning

Conventions

NOTE

Statement

to

cover

exceptional

circumstances

or

reference.

CAUTION

Statement

to

warn

against

or to

prevent

poor

perfomance

or

error.

WARNING

Statement

to

warn

of

possible

damage

to

equipment.

DANGER

Statement

to

cover

situation

involving

personal

safety

or

injury.

XII

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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Spectra-Physics Quanta-Ray GCR-12 User manual

Spectra-Physics Quanta-Ray GCR-12 User manual

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