Argon Ion Laser 1

Argon Gas Laser

Bonny Kuipers

Argon Ion Laser 2

Basic Gas Ion LaserKrypton Laser: same story

• Plasma tube• Separate resonator

Argon Ion Laser 3

Introduction Gas Lasers

• Cylindrical tube with low-pressure gas (Argon 0.1 Torr)

internal diameter ~ 1 - 2 mmlength 10 cm to 2 mlifetime 2000 to 5000 hours

• Two electrodes on opposite ends high voltage ignition & discharge current

Argon Ion Laser 4

Introduction Argon Laser• Continuous wave

• 25 spectral lines in visible, 10 in UV

• Strong spectral lines: 488 nm (blue) & 514.5 nm (green)• Power up to 100 W multi lines, 10 W single line

Efficiency: Medium power2 W @ single line 514.5 nm17 kW power supply needed !

• Intensity feedback control: stability & noise < 0.5 %

Argon Ion Laser 5

Three Level System

Argon Ion Laser 6

Level Scheme I: 488 nm

Ground state = ionized state (Ar+) of Ar atom

VIS: Ionized

2 steps electron collision !

UV: Double Ionized

3 steps electron collision !!

Argon Ion Laser 7

Level Scheme II: 488 nm detailed

• Population inversion owning to rapid decay of lower level !

Argon Ion Laser 8

Broadening mechanism

Homogeneous versus inhomogeneous (@ 488 nm ):

• DnNatural = 4.5 x 108 Hz < Dn Doppler = 2.7 x 109 HzDoppler broadening dominant !

• Range of cavity mode frequencies

• Relative narrow frequency line width

Argon Ion Laser 9

Pumpingacceleration free electrons

electron collision

<ve> = ◊ ( 8 kb Te / me p)

Low pressure gas:

electron temperature (Te) related to :

• U electrodes• 1/ Pgas

Argon Ion Laser 10

Plasma Discharge

Low pressure Argon gas& high discharge current density up to 1500 A/cm2 :

High plasma electron temperature 20,000 to 30,000 K

Gas temperature 1500 KWater cooling 9.5 liter/min needed !

Axial magnetic field (B ~ 0.15 Tesla)to avoid electrons striking the tube walls

Argon Ion Laser 11

Typical Argon Ion Laser Parameters

• Stimulated emission cross section (sul) = 2.6 x 10-16 m2

• Inversion density ( D Nul ) = 2 x 1015 /m3

• Small signal gain (g0) = 0.5 /m• Single-pass gain

1.05-1.65

• Beam divergence 0.5 mrad @ 514.5 nm

LNulule ∆σ

Argon Ion Laser 12

Resonator I

• Temperature compensated resonator structure design:

Longitudinal (single) mode stabilityTypically mode spacing 140 MHzBeam pointing stability

typically < 5 mrad / oC• Traverse Laser Cavity Mode: turn wheel with

differenced sized apertures influence TEM modes(besides mirror radii)

Argon Ion Laser 13

Resonator II Brewster window

• Brewster windows (UV : quartz)Fixes polarization (extinction ratio > 1:100)

Argon Ion Laser 14

Resonator II

Argon Ion Laser 15

Etalon I

• Etalon (fixed Fabry Perot Resonator):single frequency mode operation

Argon Ion Laser 16

Etalon IITypically order 2.7 GHzreduced to 3 MHz

linewidth

clcoherence υ∆=

spatial coherence length increases from 10 cm to 20 m(example: generation of interference ‘fringe’ pattern on

long distance)

Argon Ion Laser 17

Application: Light Scattering

Colloidal suspension:particles ~ 10 - 500 nm in fluid

• Too small for optical microscopes• Light Scattering • Static & Dynamic

Argon Ion Laser 18

Static Light Scattering

Argon Ion Laser 19

Dynamic Light Scattering

Coherence: wave front fixed phase relation

Brownian motion