7/31/2019 Lecture3 Laser

1/30

pr ng

Introduction to Biomedical Optics

Lecture 3

Laser Princi le and Technolo

- n eng

7/31/2019 Lecture3 Laser

2/30

LASER: Light Amplification by Stimulated

Spontaneous emission

is incoherent.

and has the same frequency as

the incoming light.

Stimulated emission is negligible

compared to stimulated absorption.

7/31/2019 Lecture3 Laser

3/30

Light Amplification by Population Inversion

Energy levelsF F+dF

)NF(Ndz

dF

mnnm

=

m

dz

)NF(Ndz

dFnmnm =

n m,

Nn< Nm, stimulated emissionAn active medium of length

dzamplifies the incoming

photon flux from F to F + dF

7/31/2019 Lecture3 Laser

4/30

Lasing Medium: Three-level System

Three-level energyNon-radiative

4F14F2

ecay

2E

Pump

Laser at

694.3 nm

A three-level system in a Ruby laser

ro ems o t ree- eve systems:

Requiring a significant external energy because the lower level is the ground levelMust be operated in pulsed mode because the population inversion is difficult to sustain

7/31/2019 Lecture3 Laser

5/30

Lasing Medium: Four-level System

Four-level energy

diagram

Non-radiative

decay

4

Pump Laser at

1064 nm

4I

A four-level system in a

neod mium Nd:YAG laser

Lower excitation power Can be operated in a continuous mode

7/31/2019 Lecture3 Laser

6/30

Basic Properties of Lasers

Coherent

Can be Monochromic

Directional

7/31/2019 Lecture3 Laser

7/30

Lasin medium

Gas, Dye, Semiconductor

Pumping source

Optical pumping, Electric pumping

Cavity

7/31/2019 Lecture3 Laser

8/30

Schematic of a Laser Cavit

High reflection mirror Output coupler

Output

Lasing medium

beam

Pumping source

7/31/2019 Lecture3 Laser

9/30

Pumping Source: Optical Pumping

TsunamiTi:sapphire Laser

MillenniaNd: YVO4 laser

7/31/2019 Lecture3 Laser

10/30

Pumping Source: Optical Pumping

Excitation at 532 nm from a

4

Emission 700 nm to 1000 nm

7/31/2019 Lecture3 Laser

11/30

Pumping Source: Electrical Pumping

LBO

doubling

crystalOutput

beamTi:sapphire

Nd: YVO4Diode Diode

mediumpump pump

-

or Verdi (Coherent Inc)

7/31/2019 Lecture3 Laser

12/30

Pumping Source: Electrical Pumping in a

Output

Note: Electrical pumping is also used in gas lasers.

7/31/2019 Lecture3 Laser

13/30

Laser Cavity: Longitudinal Mode

=

N: an integer

: cav y eng

: wavelength

f= c / 2L

~ z or = cm

f ~ 500 MHz for L = 30 cm

7/31/2019 Lecture3 Laser

14/30

Selecting a Single Longitudinal Mode with

ong u na mo e

selection using a

transmission Fabr -

Perot etalon.

Configuration ofetalon

longitudinal modeselection

7/31/2019 Lecture3 Laser

15/30

Laser Cavity: Transverse Mode

TEM (Transfer, Electric,

and Magnetic modes)

TEM (mn): m and nindicate the number of

minima in two

perpendicular direction

A beam with a TEM(00)

mode is called a Gaussian

eam

A TEM(00) beam can be

focused to the diffraction

limits

7/31/2019 Lecture3 Laser

16/30

Specifications of Some Lasers

Wavelength Laser medium Excitation

CO2 laser 10.6 m

400 nm to 1.9 m

CO2 gas

Semiconductors

Electrical

Nd:YAG laser 1064 nm Nd ions in a crystal

of YAG

Diode laser

Dye laser

Ar on-Ion laser

400 to 800 nm

488 nm / 514.5 nm

Dyes

Argon gas at 1 torr

Ar-ion laser

Electrical

Ti:sapphire laser

/ 453 nm

690 to 1000 nm

pressure

Ti3+ ions in a Frequency

2 3

Nd:YAG laser

7/31/2019 Lecture3 Laser

17/30

Paraxial Description of a Gaussian Beam

zw0

( ) ( ) ( )

+ zRkzkzizww /2

2/20

( )zw0,

( ) ( ) waistBeam,/1/

rangeRayleigh,2/z

ocusa ewa seam,. ..

2

00

00

0

zzwzw

kw

w

+=

=

=

( )

( ) ( ) shiftphaseGouy,/tan

radiusWavefront,/

0

1

2

0

zzz

zzzzR

=

+=

7/31/2019 Lecture3 Laser

18/30

Continuous Wave (CW) and Pulse

CW operation

Single or multi longitudinal mode

Pulse operation

Q-switch: intense nanosecond pulsescous o-op c -sw c

Electro-optic Q-switch

Mode-locking: femtosecond (10

15 s) orpicosecond (1012s ) pulses

7/31/2019 Lecture3 Laser

19/30

Q (quality)-Switch Operation

Electro-o tic Q-switch 5-20 ns

Pockels Cell

Acoustic-optic Q-switch 100 ns

7/31/2019 Lecture3 Laser

20/30

Mode-locking of Longitudinal Modes

If we lock thelongitudinal modes

0 + n oge er in phase, we have

0

1/ 2( )

0

1 / 2

( )N

i n t

N

E t I e

+

=

0

0

sin( / 2)

sin( / 2)

i t N tI e

t

=

7/31/2019 Lecture3 Laser

21/30

Mode-locking Generate Ultrashort Pulse

2

0 2

sin ( / 2)( )

N tI t I

Pulse width

~ 2 / (N )

N2I0

Average Intensity

N I0

7/31/2019 Lecture3 Laser

22/30

Repetition Rate: Number of

The time separation ( t) between adjacent

.

t = 2 n

=

t = 2 / = 2L /c

Repetition rate = 1/ t

7/31/2019 Lecture3 Laser

23/30

Compensation of Group Velocity

Dis ersion Gires-Tournois Interferometer (GTI) for picosecond

Prisms for femtosecond lasers

Prism 2 GTI

Mira 900

em osecon

Coherent Inc Prism 1

7/31/2019 Lecture3 Laser

24/30

Mode-locking Using a Acoustic-Optic

Acoustic optic

mo u a or

operating at afrequency of c / 2L

Lok-to-clock locks

80 MHzGTI

PZT feedbackadjusts the cavity

length

Product of Spectra-Physics

7/31/2019 Lecture3 Laser

25/30

Measurement of fs or ps Pulse Widths by an

-

measured by

Nonlinear crystal

x or

1 m 3.33 fs

x

Intensity autocorrelation: =>