3-lecture

20-Aug-11

1

Functional Devices Functional Devices EE5433R: Optical Devices: Optical Devices

A/Prof. Hong Minghui

Room No: E2-04-09 Tel: 6516-1636

E-mail: [email protected]

Lecture 3Chapter 4 Laser

4 1 h d l

1

4.1 Laser, its characteristics and applications 4.2 Laser physics4.3 Laser diode4.4 Other laser sources and laser safety

Quiz (30 minutes)

Chapter 4 Laser4.1 Laser, its characteristics and applications

• Light Amplification by Stimulated Emission of Radiation;

• A device that creates & amplifies a narrow & intense beam of

coherent light. h ν1

E2

E

hE2

Absorption E1

E2h

E1Spontaneous Emission

E1

E2

hh h

Stimulated Emission 2

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4.1 Laser, its characteristics and applicationsLaser Laser -- a Special Light Sourcea Special Light Source

Pure - light rays are nearly the same colour;Well-collimated - rays are headed in the same direction;Coherent - monochromatic, photons in same phases;Versatile – its applications extensively improving our lifestyles.

Data Storage

Telecommunications

Manufacturing

Sensor Device Holography

Decorative Uses

Lasers

Military

Bio-MedicalEntertainment

Environmental

PrintingResearch Nanotechnology

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Discover the Wonder of Laser

4ManufacturingManufacturing

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出淤泥

Self-cleaningIn Eastern cultures, lotus is

a symbol of purity. Although lotuses prefer to grow in

muddy rivers and lakes, the leaves and flowers remain

clean.

而不染

5Microfabrication Microfabrication

Lotus lilyLotus lily

Nano-Venus on hair

6Diffraction pattern

Nanoengineering Nanoengineering

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7

Entertainment Entertainment –– Laser ShowLaser Show


Google Automatic Self-Driving Car

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Laser Range finder (LRF)


∆t

Distance = c ∆t /2Distance = c ∆t /2

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4.2 Laser physics

Lasers

. Light Amplification by Stimulated Emission of Radiation

. Light emitted at narrow wavelength bands (monochromatic)

. Light emitted in a directed beamg

. Light is coherent (in phase)

. Light often polarized

History of Lasers

. 1917: Einstein's paper showing "Stimulated Emission"

11

. 1957: MASER discovered: Townes & Schawlow

. 1960: First laser using Ruby rods: Maiman first solid state laser

. 1961: Gas laser, 1962: GaAs semiconductor laser, 1964: CO2 laser;

. 1972: Fiber optics really take off

. 1983: Laser CD introduced

. 1997: DVD laser video disks

4.2 Laser physics • Assume gas in thermal equilibrium at temperature T;

• Some atoms in a gas are in an excited state;

• Quantization means discrete energy levels;Q gy

• Atoms density Ni (atoms/m-3) at a given energy level Ei;• E0 is the ground state (unexcited);

• Fraction at a given energy follows

Boltzmann distribution

12Equilibrium Energy Populations

T: temperature (k),

K: Boltzmann constant 1.38 x 10-23 J/K

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4.2 Laser physics

Before After

Stimulated Absorption

h

Spontaneous Emissionh

13Spontaneous and Stimulated Emission

Stimulated Emissionh

2h

4.2 Laser physics Spontaneous and Stimulated Emission

14

Animation 1

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4.2 Laser physics Two level system

.In thermal equilibrium lower level always has a greater population: N1 >> N2

P i dd l i i . Pumping: suddenly inject energy into system, now not a equilibrium condition;

. If pumped hard enough get "Population Inversion“:

N2 >> N1. Population Inversion: f d i f l i

15Population Inversion

foundation of laser operation, creates the condition for high stimulated emission;

. In practice difficult to get 2 level population inversion.

4.2 Laser physics Three level system

. Pump from E1 level to E3, E3 short lifetime, rapid decay to E2;

. E2 much longer lifetime, Meta-stable;

. Population inversion obtained with enough pumping;

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Animation 2

Population Inversion

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4.2 Laser physics Four level system

. Pump from E0to level E3, but require E3 to have a short lifetime;

. Rapid decay to E2;. Rap d decay to E2;

. E2 has long lifetime: meta-stable;

. E1 has short lifetime for decay to E0;

. If population inversion, stimulated emission dominates: Lasing;

. In principal easier to get population i i

1717

inversion;. Problem: energy losses from E3 to E2

and E1 to E0.

Population Inversion

Key Components for Laser• Optical resonator cavity;• Laser gain medium of 2, 3 or 4 level types in the Cavity;• Sufficient excitation, pumping by light, current, chemical reaction;

4.2 Laser physics

uff c nt c tat n, pump ng y ght, curr nt, ch m ca r act n;• Population inversion in the gain medium due to pumping.

Laser Types• Two types depending on time operation: continuous wave (CW) or pulsed;• Different types depending on gain medium: Solid State Laser (solid rods): ruby, Nd:YAG;

1818

tat La r ( r ) ru y, N Y G;Gas laser: He-Ne, excimer; femtosecond laser: pulse stretching, amplifying and compressing;Semiconductor Laser: GaAs laser diodeDye, Chemical Laser: chemical medium Fiber laser: doped fiber

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• Light confined, bounce back/forth within two mirrors:

• Back end mirror highly reflective, f d f ll

Resonator Cavity4.2 Laser physics

Cooling System

Laser Medium Front Mirror

front end not fully transparent;• Pumped medium between two mirrors, curved mirror to focus beam at radius.

Animation 3

1919Optical Resonator Cavity

Laser Medium Front MirrorR=95%Rear Mirror

R=100%Output Laser BeamPumping

System

• Energy level distribution: width of emission, usually in Gaussian shape;• Width broadened by many mechanisms:

Doppler Broadening (movement of molecules)

4.2 Laser physics

Collision Broadening (atomic/molecular collisions)Radiative Lifetime Broadening (finite lifetime of transitions)

A few nm

2020Laser Output Line Shape

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Transverse ElectroMagnetic mode TEMqr, cylindrical geometry cavity (q, r: number of horizontal & vertical phase reversal orientations running null)

4.2 Laser physics

Laser beam profile of TEM00

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21

Gaussian Beam

Laser Transverse Modes

4.3 Laser diodeSemiconductor Lasers

• Small, efficient, cheap;

• Only red and green wavelengths available at reasonable prices;

• Mostly made of Gallium aluminum arsenide (GaAlAs);

• Emission can be controlled by varying

ration of gallium to aluminum in semiconductor;

• Main use in CD players/laser printers/pointers;

• Problem is poor beam profiles;

A. Laser diode

• Noise levels are generally ≤ 0.05%,

~ 1% for air cooled and ~ 0.02%

with water cooled argon lasers

• Operation Current ~ 60mA

• Typical Threshold current ~30mA

• Typical Operating Voltage ~ 2.7V

• Power Output ~5mW

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Semiconductor Lasers

• Laser diode is similar in principle to an LED.• Optical cavity is an added geometry required for laser diode:

4.3 Laser diode

to facilitate feedback for stimulated emission.• Fundamental of laser diode:

1). Edge emitting LED: suitable for adaptation to feedback waveguide.2). Polish the sides of the structure that is radiating.3). reflecting mechanism to return radiation to active region.

A. Laser diode

• Drawback: low Q, excessive absorption of radiation in p and n layers.• Remedy: to add confinement layers on both sides at different

refractive indice for radiation to reflect back to active region.

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4.3 Laser diode

• Polishing the light emitting

sides of cavity. A large

Current

Cleaved surface mirror

Semiconductor Lasers

percentage of radiation is

reflected back due to

difference in reflective

indexes between air and

GaAs. So mirror coating is

not necessary.

LElectrode

GaAs

GaAsn+

p+

Electrode

Active region

L

A. Laser diode

• Note: radiation propagates

from both sides of the

device.

(stimulated emission region)

A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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4.3 Laser diodeSemiconductor Lasers

• Lasing occurs when the supply of free electrons exceeds the

l i id th itloss inside the cavity.

• Current going through the junction and the electrons supply are

directly proportional. ITH must be exceeded before the laser

action occurs.

• Drawback (temperature coefficient): Threshold current ITH

A. Laser diode

increases with temperature and leads to possible shutdown.

• Remedy: 1). cooling mechanism. (to install cooling mount), 2).

constant current power supply coupled with a photodetector.

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4.3 Laser diodeWorking mechanism

p+ n+

Eg

Ec

Junction

p+ n+

EF nEc

eVo

EF n

(a)

Ev

Ev

Holes in V BElectrons in C BElectrons

Ec

Eg

(b)

F n

eV

EF p

Invers ionreg ion

EF p

c

Ec

A. Laser diode

V

The energy band diagram of a degenerately doped p-n with no bias. (b) Banddiagram with a sufficiently large forward bias to cause population inversion andhence stimulated emission.


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4.3 Laser diode

• Degenerately doped direct bandgap semiconductor p-n junction.

• Degenerate doping: EFP on P-side is in valence band (VB) and EFN

Working mechanism

D g n rat op ng EFP on s s n a nc an (V ) an EFN

on N-side is in conduction band (CB).

• Energy levels up to Fermi level are occupied by electrons.

• When there is no an applied voltage, Fermi level is continuous

across the diode. EFP = EFN

S h l (SCL) i

A. Laser diode

• Space charge layer (SCL) is very narrow.

• V0 (built in voltage, barrier) prevents electrons/holes in n+

CB/VB P+ side) from diffusing into P+ CB/VB n+ side.

. 27

4.3 Laser diode

• When an external voltage (eV) is applied and greater than the

bandgap energy Eg EgeVEE FPFN

Working mechanism

g p gy g.

• EFN and EFP are now separated by eV.

• This applied voltage diminishes the barrier potential to 0,

allowing electrons to flow from n+ side into SCL then to p+ side

to form diode current.

A i il d ti i th b i t ti l f h l t fl

A. Laser diode

• A similar reduction in the barrier potential for holes to flow

from p+ side into SCL then to n+ side occurs.

• Result SCL is no longer depleted.

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4.3 Laser diodePopulation Inversion

Optical gain EF n EF p

Energy

Ec

CBElectronsin CB

EF n

hEg

Optical absorption

0

Ev

VB

(a) The density of states and energy distribution of electrons and holes in

Density of states

Holes in VB= Empty statesEF p

eV

At T > 0

At T = 0

(a) (b)

( ) y gythe conduction and valence bands respectively at T 0 in the SCLunder forward bias such that EFn EFp > Eg. Holes in the VB are emptystates. (b) Gain vs. photon energy.


More electrons in CB at energies near Ec than electrons in VB near Ev. This

results in Population Inversion . The region where population inversion occurs

develops a layer along the junction called Inversion layer or Active region.29

4.3 Laser diodeStimulated Emission

• An incoming photon with an energy of EC – Ev will not see electrons

to excite from Ev to EC due to the absence of electrons at Ev.v C v

• A photon can cause an electron to fall down from Ec to Ev .

• The incoming photon is stimulating direct recombination.

• The region where there is more Stimulated emission than

Absorption results in Optical gain.

O i l i d d h h d h l h

A. Laser diode

• Optical gain depends upon the photon energy and thus wavelength.

*Photons with energy > Eg but < EFN - EFP (eV) cause stimulated emission;

*Photons with energy > EFN - EFP (eV) are absorbed.

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4.3 Laser diodePumping

• Fermi-Dirac function spreads energy distributions of electrons in CB

to above EFN and holes below EFP in VB, resulting in a reduction in

i l ioptical gain.

• Optical gain depends on EFN - EFP, which depends on the applied

voltage eV. In turn, it depends on diode current.

• An adequate forward bias is required to develop enough injection

carriers across a junction to initiate a population inversion between

A. Laser diode

energies at EC and energies at Ev.

• Forward diode current is used to achieve the pumping mechanism.

The process is called injection pumping.

31

4.3 Laser diodePumping

LaserOptical Power

LEDO ti l P

Optical Power

Laser

Optical Power

I0

LEDOptical Power

Ith

Spontaneousemission

Stimulatedemission

A. Laser diode

Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

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4.3 Laser diodeOptical cavity

• In addition to population inversion, laser oscillation must be sustained.

A ti l it ( ti l t ) i i l t d t l t th • An optical cavity (optical resonator) is implemented to elevate the

intensity of stimulated emission.

• It provides an output of continuous coherent radiation.

• A homojunction laser diode: the pn junction uses the same direct

bandgap semiconductor material (e.g. GaAs) .

A. Laser diode

• The ends of the crystal are cleaved to a flatness and the ends polished

to provide reflection.

33

4.3 Laser diodeOptical cavity

• Photons reflected from the cleaved surface stimulate more photons

at the same frequency at the same frequency.

• Wavelength of radiation that escalates in the cavity is dependant

on cavity length L. (Resonant length, only multiples of /2 exist).

frequency)resonant or (modeinteger :2

anismwhereL

nm

A. Laser diode

h wavelengtspace free theis

torsemiconduc theofindex refractive theis 2

n

n

nLm 2

2

34

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4.3 Laser diodeModes

• Separation between potential modes that can develop, or allowed

wavelengths, is determined by the equation in previous slide as m.g y q p m

• The output spectrum of a laser diode depends upon the nature of the

optical cavity and optical gain versus wavelength.

• Note: lasing radiation occurs when optical gain in the medium can

overcome photon losses from the cavity which requires diode current

to exceed a threshold current I

A. Laser diode

to exceed a threshold current ITH.

• Light that exists below ITH is due to spontaneous emission. Incoherent

photons are emitted randomly and device behaves like an LED.

35

4.3 Laser diodeOutput

• Lasing oscillations occur when optical gain exceeds photon losses

and this is where optical gain reaches threshold gain at ITH.p g g TH

• This is the point where modes or resonant frequencies resonate

within the cavity.

• The polished cavity ends are not perfectly reflecting with

approximately 32% transmitting out of the cleaved ends.

Th b f d th t i t i th t t t d

A. Laser diode

• The number of modes that exist in the output spectrum and

their magnitudes depend on the diode current.

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4.3 Laser diodeHetero-structure

• The drawback of a homo-junction structure is the threshold current

density JTH is too high and thus restricts it to operate at low

temperatures.

• Remedy: Hetero-structure semiconductor laser diodes.

• To reduce threshold current to a usable level needs an improvement

of stimulated emission rate and efficiency of the optical cavity.

A. Laser diode

of stimulated emission rate and efficiency of the optical cavity.

37

4.3 Laser diodeHetero-structure

• Methods for improvement:

1) C i fi t t fi i j t d l t d h l t 1). Carrier confinement: to confine injected electrons and holes to a

narrow region near pn junction. It requires less current to achieve

the required concentration of electrons for population inversion.

2). Photon confinement : to construct a dielectric waveguide around

the optical gain region to increase photon concentration and elevate

th b bilit f ti l t d i i Thi d th b f

A. Laser diode

the probability of stimulated emission. This reduces the number of

electrons lost traveling off the cavity axis.

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4.3 Laser diodeDouble Hetero-structure

AlGaAsAlGaAs(a)

pn p

(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(G A d AlG A )( 0 1 m)

GaAs

Refractiveindex

Active n ~ 5%

2 eV

Holes in VB

Electrons in CB

1.4 eV

Ec

Ev

Ec

Ev

(b)

Ec

(GaAs and AlGaAs).

2 eV

(b) Simplified energyband diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer

(~0.1 m)

(c) Higher bandgapmaterials have alower refractiveinde

(c)

A. Laser diode

Photondensity

region index

(d) AlGaAs layersprovide lateral opticalconfinement.

( )

(d)


39

4.3 Laser diode• AlGaAs has Eg of 2 eV, GaAs has Eg of 1.4 eV. P-GaAs is a thin layer (0.1

~ 0.2 µm), active layer where lasing recombination occurs.• Both p regions are heavily doped and are degenereated with EF in VB.

At d t f d bi E f AlG A b E f G A • At an adequate forward bias, Ec of n-AlGaAs moves above Ec of p-GaAs, a large injection of electrons from CB of n-AlGaAs into CB of p-GaAs.

• These electrons are confined inside CB of p-GaAs due to difference in barrier potential of two materials.

• Due to thin p-GaAs layer, a minimal amount of current needs to increase concentration of injected carriers at a fast rate. Threshold current is

A. Laser diode

reduced for the purpose of population inversion and optical gain.• Semiconductor with wider bandgap (AlGaAs) has lower refractive index.

The difference in refractive index forms an optical dielectric waveguide that confines photons to active region.

40

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4.3 Laser diodeDouble Hetero-structure

Stripe electrode L

WCleaved reflecting surface

• Substrate: n-GaAs: Confining layers: n-AlGaAs & p-AlGaAs.

Oxide insulator

SubstrateElectrode

Active region where J > Jth.(Emission region)

p-GaAs (Contacting layer)

n-GaAs (Substrate)

p-GaAs (Active layer)

Currentpaths

Cleaved reflecting surfaceEllipticallaserbeam

p-AlxGa

1-xAs (Confining layer)

n-AlxGa

1-xAs (Confining layer) 12 3 Substrate

Active layer is p-GaAs (870~900nm).

• Additional contacting layer: p-GaAs for better electrode contact and avoids Schottky junctions, which limits current.

A. Laser diode

Schematic illustration of the the structure of a double heterojunction strcontact laser diode


• P and n-AlGaAs layers provide carriers and optical confinement by forming hetero-junctions with p-GaAs.

41

4.3 Laser diode• Advantage of AlGaAs/GaAs hetero-junction: to offer a small lattice

mismatch between their crystal structures. It introduces negligiblestrain induced interfacial defects (dislocations). Defects of thisnature act as non radiative recombination centersnature act as non-radiative recombination centers.

• Stripe Geometry:1). current density J is not uniform laterally from stripe contact.2). current is maximum along central path and diminishes on either side with confinement between paths 2 and 3. (gain guided).3). Population inversion and then optical gain occur where current

A. Laser diode

) p p gdensity exceeds ITH. 4). Advantages: to reduce contact and ITH; to reduce emission area so light coupling to fibre easier. (stripe width at a few µm develop ITH

of tens of mA)

42

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4.3 Laser diode

Homogeneous Semiconductor“Bandgap Engineering”

Choose alloy of semiconductors to

“design in” the desired bandgap energy.

F b i t d ti ll b d siti t

Electrons accumulate in minimum energy regionHetero-junction

Excess electronsSi only, GaAs only etc.

Fabricated vertically by deposition to

confine Excess Carrier Density.

Semiconductor A

Semiconductor B

A. Laser diode

Eg(A) Eg(B) Eg(A)

Semiconductor BSemiconductor A Semiconductor A

Semiconductor A

Vertical stack of different bandgap semiconductors

43

4.3 Laser diode“Refractive Index Engineering”

Choose the center layer at a higher

refraction index than the surrounding

l s It s lts i b ilt i id layers. It results in a built-in waveguide

to confine light in active region.

n

nAnB

Semiconductor A

Semiconductor B

Semiconductor C

Index nA

Index nB

Index nC

A. Laser diode

Inde

x of

refr

actio

n

Light guided in high index region

nB > nA

nB > nC

Vertical stack of different index semiconductors

44

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4.3 Laser diode• Traditional laser diodes are “edge-emitters”. Light perpendicular to direction of layer

structures. Cavity is formed by cleaving the end faces of diodes. VCSEL: designed with

mirrors at top and bottom, surrounding a thin active region for small optical cross-section.

Mirrors highly reflective (~90%; ~ 30% of cleaved faces in edge emitters) Early designs Mirrors highly reflective ( 90%; 30% of cleaved faces in edge emitters). Early designs

with metallic mirrors to reduce ITH. Now mirrors consist of alternating layers of

semiconductor materials at different refraction indices, creating a distributed Bragg

reflector (DBR) with reflectivity ~ 99%.

• Typical 850-nm VCSEL: a bottom DBR constructed

from tens of layers of AlGaAs, then a few GaAs

quantum wells a layer of AlGaAs with 97~ 98% Al

Vertical cavity surface emitting lasers (VCSELs)

quantum wells, a layer of AlGaAs with 97 98% Al,

and a final dozen layers of AlGaAs for top Bragg

reflector. Ability to easily create arrays of

transmitters is another advantage for VCSEL for

unprecedented integration of telecommunications. From: Research & Device Magazine, 2007: Vertical Cavity Lasers Push into the Future

45

4.3 Laser diodeFundamental characteristics

What factors determine LD output spectrum?

a) Feature of optical resonator that develops laser oscillations.

b) Optical gain curve (line-shape of active medium).

Optical resonator is a Fabry-Perot cavity:

a) Length determines longitudinal modes where width and height of the

cavity determines transverse or lateral modes.

b) ff l ll W & H l l d TEM

A. Laser diode

b) At sufficiently small W & H, only lowest transverse mode exits TEM00.

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4.3 Laser diode

Fabry-Perot cavity

Dielectric mirror • Laser diode has longitudinal modes. Its separation depends on cavity length.

Height, H Width W

Length, L

The laser ca ity definitions and the out ut laser bea

Diffractionlimited laserbeam

• Laser beam displays a diverging field due to diffraction at cavity ends. The smaller the aperture, the greater the diffraction.

• Light spectrum is either

A. Laser diode

The laser cavity definitions and the output laser beamcharacteristics.


g pmultimode or single mode determined by optical resonator geometry and pumping current level.

47

4.3 Laser diode

Po = 5 mW

Relative optical power Fundamental characteristics

778 780 782

Po = 1 mW

(nm)

Po = 3 mW

A. Laser diode

778

Output spectra of lasing emission from an index guided LD.At sufficiently high diode currents corresponding to highoptical power, the operation becomes single mode. (Note:Relative power scale applies to each spectrum individually andnot between spectra)

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall) 48

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4.3 Laser diodeTemperature characteristics

• The output characteristics of an LD are sensitive to temperature.

As temperature increases I increases exponentially• As temperature increases, ITH increases exponentially.

• Output spectrum also changes.

• A single mode LD will mode hop (jump to a different mode) at

certain temperatures.

• This results in a change of laser oscillation wavelength.

A. Laser diode

• 0 increases slowly due to small change in refractive index and

cavity length.

49

4.3 Laser diodeQuantum Cascade Laser

interband transition:Eappl

intersubband transition:

A. Laser diode

Tunneling rate >> 3 = 1 psand 2 = 0.3 ps << 32 > 1 ps population inversion

50

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• Discovered in 1960s. First laser: Ted Maiman;

4.4 Other laser sources and laser safety

B. Ruby laser

• Ruby (Al2O3) monocrystal, Cr doped.

Optical pumping: 510-600nm, 360-450nm.Fast transition on 2E. Lasing: 2E on 4A2, 694nm. 51

• Gas sealed in a tube with brewster window;• Electric arc in tube causes glowing of gas;• Glow indication of pumping.

He-Ne: three level system


52C. Gas Lasers

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Nobel Gas Ion Lasers


•Visible light with gas ionization (plasma);•Magnetic field keeps ions away from walls;

53

•Magnetic field keeps ions away from walls;•Water cooled with efficiency about 5%;•Prism for wavelength selection, in laser show: Argons for Green, Blues, UV; Krypton for Red, Yellow.

C. Gas Lasers

Molecular Lasers• Operation by molecular vibration transitions;• Generally Infra-red ~ 10.6 m;• Power from 10 ~ 10,000 Watts

Hi h ffi i 30%

CO2: 10.6 m (CW, pulsed)


• High efficiency: up to 30%

54C. Gas Lasers

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Excimer Lasers

• Two atom types repel each other:

b l s & h lid id ;

F2, ArF, KrF, XeCl excimer lasersDeep UV : 157 - 350 nm


nobel gas & halide or oxide;

• When excited/ionized, atoms attract;

• Bound together/separated short distance.

• Excited state Dimer: Excimer.

55C. Gas LasersOptical Lithography Today (32 nm)

4.3 Laser sources and structures

• Gain medium: solid matrix, crystal doped with transition metal, rare earth ions;

• Pumped by flash lamp or diode laser;

Four level system

• Light absorbed by doped ion: emitted laser light.

56D. Nd:YAG Lasers

Nd+3 doped yttrium aluminum garnet crystal

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4.3 Laser sources and structures

57E. Fiber Lasers

Ytterbium-doped large-core fiber laser (1 kW CW output power)

4.4 Laser Safety

Class 1: Not hazardous for continuous viewing; Class 2: < 1 mw visible lasers, potential hazard

if viewed directly for a period of timef w y f p f mClass 3a: Would not injure eyes if viewed for

only momentary periods, a hazard if viewed

by collecting optics. Power: < 5mW

58

58

Classification of Laser

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4.4 Laser Safety

Class 3b: hazard if viewed directly, not hazardous for diffuse reflection from a matte target. Power: < 500mW laser pointers & He-Ne laser;

Class 4: hazard from direct/diffuse reflection skin and fire hazards Class 4: hazard from direct/diffuse reflection, skin and fire hazards.

CO2, Nd:YAG, Excimer, fiber & fs lasers for micromachining.

Hazard Classification:Exposure of eyes or skin; fire/fume hazard; lethal high voltage shock hazard.

59

59

Classification of Laser

4.4 Laser Safety Laser Safety

• To get laser operation license/pass certification

for laser operation;• To wear proper protective eyewear at all times;• To post “Laser in operation” sign on all doors;• To turn on vent hose, remove fumes/hazardous gas;• Not to touch exposed electric connectors (high

voltage) of power supply.

60

60

Protection measurement

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Quiz (30 minutes)

3-lecture

Documents

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laser sources

laser physics4

laser diode4

applicationslaser laser

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laser cd

laser physics lasers