Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf ·...

35
Laser Basics What happens when light (or photon) interact with a matter? Assume photon energy is compatible with energy transition levels. Electron energy levels in an hydrogen atom n=5 + h υ ] eV [ 6 . 13 E = 2 n=3 n=4 - ] eV [ n E 2 n = n=1 n=2 - Energy levels inside every matter are quantized; details depend on the matter Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Transcript of Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf ·...

Page 1: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

What happens when light (or photon) interact with a matter? Assume photon energy is compatible with energy transition levels.

Electron energy levels in an hydrogen atom

n=5

+h υ

]eV[6.13E −=2

n=3

n=4

-]eV[

nE 2n =

n=1

n=2

-

Energy levels inside every matter are quantized; details depend on the matter

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 2: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Consider for simplicity only two energy levels: ground and excited statesAssume hν = E2 – E1

Three interaction processes are possible

E2h υ

E1

E2h υ

E2

h υ

E2

Absorption Spontaneous Emission Stimulated Emission

h υh υ

h υ In h υ

Out

h υ

E1E1 E1

out photons are “identical”t i h t lifi ti

out photons are“random”

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

to in photons: amplification random

Page 3: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Determine the rate for each process

E2h υ

E2h υ

E2

Absorption Spontaneous Emission Stimulated Emission

h υh υ

h υ h υ

h υ

E1E1 E

1

221sp NAR ⋅=ρ⋅⋅= 11212 NBR ρ⋅⋅= 22121 NBR

: photon densityl d i EN

ρ

221spρ11212 ρ22121

B 38A h

It can be shown

1,2 1,2

12 sp 21

: electron density at E, , : constants

NB B B

21

12

1BB

=3

213

12

8A hB c

π υ=

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 4: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

E E E

Absorption Spontaneous Emission Stimulated Emission

E2

h υh υ

E2

h υ h υ

E2

E1E1

h υ

E1

221sp NAR ⋅=ρ⋅⋅= 11212 NBR ρ⋅⋅= 22121 NBR

21 1B Interpretations:21

12

1B

=

38A h

Interpretations

-Absorption and simulated emission have the same coefficeintThe only difference is N1 and N2

S t i i d ti l t d i i321

312

8A hB c

π υ=

-Spontaneous emission and stimulated emission are intrinsically related

Spontaneous emission is simulated emission d t fl t ti

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

due to vacuum fluctuation

Page 5: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Which process is dominant at equalibrium?

21 21 2 2 2 1 1R B N N E Eρ −⎛ ⎞

Stimulated emission vs. absorption

21 21 2 2 2 1

12 12 1 1

exp 1R B N N E ER B N N kT

ρρ

⎛ ⎞= = = − <<⎜ ⎟⎝ ⎠

E2λ=1.55μm921 2 0.8 2 10R N eV⎛ ⎞

⎜ ⎟E1

E2-E1=1.55μm921 2

12 1

0.8exp ~ 2 100.04

N eVR N eV

−⎛ ⎞= = − ×⎜ ⎟⎝ ⎠

Virtually no possibility for stimulated emission at equalibrium

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 6: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Which process is dominant at equalibrium?

3 321 21 2 21 8 1R B N B c hρ π υρ

Stimulated emission vs. spontaneous emission

21 21 2 213

3 2 121 2 21 8 exp 1exp 1sp E ER A N A h hckTkT

ρπ υ υ

= = = =−⎡ ⎤ ⎛ ⎞⎛ ⎞ −− ⎜ ⎟⎜ ⎟⎢ ⎥ ⎝ ⎠⎝ ⎠⎣ ⎦

9218

1 1 ~ 2 100 8

RV

−= = ×⎛ ⎞

E2 λ=1.55μm

80.8 4.84 10 1exp 10.04

sp eVReV

× −⎛ ⎞ −⎜ ⎟⎝ ⎠E1

Virtually all photon emission is due to spontaneous emission at equalibrium

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 7: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

How can we induce stimulated emission?

E2h υ

E2h υ

E2

Absorption Spontaneous Emission Stimulated Emission

h υh υ

h υ h υ

h υ

E1E1 E

1

221sp NAR ⋅=ρ⋅⋅= 11212 NBR ρ⋅⋅= 22121 NBR221spρ11212 ρ22121

Make N2 larger than N1: Break equalibrium and “pump” carriers into E2

N2 = N1 : transparent

N2 > N1 : population inversion

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

N2 N1 population inversion

Page 8: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Optical Amplifier

EPin Pout = G Pin

E2

E1

Which process is useful for optical amplifier?

How can we make stimulated emission dominant over absorption?How can we make stimulated emission dominant over absorption?

Pump carriers into N2 so that N2 > N1

Optical Pumping and Electrical Pumping

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 9: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Optical Pumping: Consider Er

Energy levels in Er -Pump light is absorbed at E3

generating carriers

1.27 eV E3

g g

- Carriers at E3 rapidly transfer to E2

N2 builds up

0.80 eV E21550 nm 1550 nm

980 nm

Non-radiative decay

Pump- When N2>N1 (population inversion),

ti l t d i i > b ti

N2 builds up

E10In

Out stimulated emission > absorptionfor 1550nm light

Er can be easily added to core of Silica fiber

EDF (Er-Doped Fiber)

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 10: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser BasicsLect. 18: Optical Amplifiers

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. ChoiHigh-Speed Circuits and Systems LaboratoryOptoelectronics and Photonics

Page 11: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Electron energy levels in semiconductorsSi lattice structure

C o v a l e n t b o n dS i i o n c o r e ( + 4 e )

Electrons in each Si atom have discrete energy levels.

But in Si crystal, energy bands are formed.

+ ++ +

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 12: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

C du ti B d

Band diagram

Conduction Band E

kBand Gap cE

vEkp

(=Ec-Ev)

Valence Band

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 13: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Direct semiconductor Indirect Semiconductor

E E

k k

M t ti t iblMomentum conservation not possible by photon emission=> No emission (Examaple: Si)

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 14: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Absorption Spontaneous Emission Stimulated Emission

kh υ

k

12 12 1 1 2 2( ) ( ) ( ) ( ) R h B N E P E hν ρ ν= ⋅ ⋅ ⋅ 21 2 2 1 1( ) ( ) ( )spR h A N E P Eν = ⋅ ⋅ 21 21 2 2 1 1( ) ( ) ( ) ( )R h B N E P E hν ρ ν= ⋅ ⋅ ⋅

For population inversion, 2 1

1 2

1N PN P

⋅>

⋅Electron and hole injection needed.

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 15: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

How to pump electrons and holes into a semiconductor? Forward-bias PN junction

Electron in CBHole in VB

Light emitting diode (LED)

Light output

Eg

p n+

Insulator (oxide)p

n+

hυ - Eg

g

n+Substrate

VMetal electrode

Light emission by spontaneous emission

Does any semiconductor emit light?

What determines the color of LED?

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 16: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Bandgap energies for major LED materials: III-V compound semiconductor

Indirectbandgap

GaAs1-yPy

In1-xGaxAs1-yPyAlxGa1 xAs

x = 0.43InGaN

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6λ

1.7I f d

x 1-x

In0.49AlxGa0.51-xP

Infrared

Free space wavelength coverage by different LED materials from the visible spectrum to theinfrared including wavelengths used in optical communications. Hatched region and dashedli i di t E t i l

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

lines are indirect Eg materials.

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

Page 17: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Current injection into PN Junction can be used for SOA (Semiconductor Optical Amplifier)

Current Injection

P P = G PPin Pout = G Pin

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 18: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Gain spectrum for SOA

O i l i

Energy

Optical gain EFn − EFp

E

CBElectronsi CB

EFn

hυEg

0

Ec in CB

H l i VB

eV

g

Optical absorption

Ev

VB

Holes in VB= Empty statesEFp

At T > 0

At T = 0Optical absorption

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 19: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Optical Amplifier

EPin Pout = G Pin

E2

E1

Light source based on stimulated emission?

- Use photons produced by spontaneous emission as initial seeds

- Recycle output photons as seeds for further stimulated emission

U i f li t t h t- Use mirror for recycling output photons

LASER: Light Amplification by Stimulated Emission Radiation

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 20: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

LASER: Optical Amplifier + Mirror

Pump

Gain Medium

RRL

RR

Optical property of gain medium: n, g

0 2gk nk j= + g depends on λ and the amount of pumping

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 21: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Pump

gGain Medium

Z LRR

0 2gk nk j= +

Z=LAssume there is an initial photon moving in z-direction inside gain medium.

What is the condition that this photon is sustained?

0 0jkL jkLE e r e r E− −⋅ ⋅ ⋅ ⋅ =

p

No loss after one round trip.

0 0E e r e r E2 2 1j kLr e−⋅ = 2

2

1 1j kLer R

− = =

02 1j nk L gLe eR

− = 021 and 1j nk LgLe eR

−∴ = =

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 22: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

021 and 1j nk LgLe eR

−= = Pump

th1 1 1From , lngLe gR L R

= =Gain Medium

RR

==> Sufficient gain to compensate mirror lossL

02From 1,j nk Le− = 02 2nk L mπ= 2Ln mλ

⇒ = or 2

L mnλ

=

Id ti l h t ti l d d t t t t

cavity length should be multiples of half wavelength

Identical photons are continuously produced at two outputs

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 23: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

1 1 2Two conditions for lasing: (1) ln and (2) Lg λ= =thTwo conditions for lasing: (1) ln and (2) g

L R n m= =

<= (1)gth

λ

gth

λ

<= (2)

λ

Lasing spectrum

Lasing peaks (modes) has non-zero linewidth

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 24: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Various LASERsAny optical gain material with mirrors can form a laser

First Laser: Ruby doped with Cr (Al2O3:Cr3+)

Maiman with first laser in 1960.

Optical Gain: Cr in Al2O3

Pump: Flash Lamp

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 25: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Gas Laser (HeNe)

(1s12s1)

He(2p55s1)

NeCollisions

20 66 VFlat mirror (Reflectivity = 0.999) Concave mirror (Reflectivity = 0 98 ( )20.61 eV

(2p53p1)

Lasing emission632.8 nm

~600 nm

Fast spontaneous decay

20.66 eVFlat mirror (Reflectivity 0.999) Concave mirror (Reflectivity = 0.98

Laser beam

Very thin tube

(2p53s1)

Collisions with the walls

Electron impactHe-Ne gas mixture

Laser beam

(1s2)0 (2p6)

Current regulated HV power supply

A schematic illustration of the He-Ne laser

Ground states

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 26: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Semiconductor Laser Structure: PN Junction + Mirror (Cleaved Facets)

WCleaved reflecting surface

Stripe electrode L

Oxide insulator

GaAs (Active layer)p-AlxGa1-xAs (Confining layer)

SubstrateElectrode

n-GaAs (Substrate)

( y )

Currentpaths

n-AlxGa1-xAs (Confining layer) 12 3 Substrate

Active region where J > Jth.(Emission region)

Cleaved reflecting surfaceEllipticallaserbeam

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

(Emission region)

Page 27: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Efficient carrier confinement: PIN structure with large Eg for P, N regions

P N P NI

Injected carriers are spread-out => smaller density

Double heterojunction: Confinement ofInjected carriers

=> larger density

For population inversion, 2 1

1 2

1N PN P

⋅>

=> larger density

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

1 2

Page 28: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Efficient photon confinement: PIN structure with smaller n for P, N regions

Smaller Eg material has larger n (n1>n2)

Dielectric waveguide!

=> More photons interacting with injected

Active(n1)

cladding(n2)

cladding(n2) p g j

electrons and holes in the active region

larger ΓWith Γ<1,

th m1 1ln (mirror loss)g α= = th m

1 1=> lng αΓ = =th m ( )gL R th mg

L R2L

n mλ=

ff

2 ;Ln mλ

==> = effnkβ

=

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

n m effn m 0k

Page 29: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

LaserOptical Power

O i l P ( W)

LEDOptical Power

Optical Power (mW)

λ

SpontaneousSlope Efficiency:dP/dI

λ

I (mA)0

Spontaneousemission

th0 I

Threshold current: Ith

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 30: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

th m intff

2Two conditions for lasing: (1) and (2) Lgn mλα αΓ = + =effn m

There can be several lasing modes: several λ’s satisfying above conditions.

- Multiple values for neff if there are multiple waveguide modes

Different modes have different neff

p eff p g

Active(n )

cladding( )

cladding( )

Design for single guided mode.

(n1)(n2) (n2)

TE, TM modes?

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 31: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

- Multiple cavity modes Mode separation

02eff 0

eff 0 0

From 1 2 2

( ) => ( )

j nk Le n k L m

n k L kn L

πππ

− = ⇒ =

Δ = Δ =

λ

gtheff

2

0 0 0220 0 0

2 2 2 22 eff

n L

k k kk k k n Lπ δλ π π λλ λ

δ π= ∴Δ = Δ = − Δ = − Δ = −

⎛ ⎞⎜ ⎟

0 0 0 eff

λ⎛ ⎞⎜ ⎟⎝ ⎠

With typical semiconductor lasers with cleaved facets, Δλ is less than gain bandwidth => multi lasing modes

2 f ( )Lλ λ

λLaserOptical PowerFabry-Perot laser

theff

for ( )g gn m

λ= >

λ

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

λ

Page 32: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Problems with multi-mode laser?

How to make single-mode laser?

Modal dispersion even with single-mode fiber

Use another type of mirror: Grating

R b

( )sin sin id mθ θ λ− = ⋅

Incidentiθ θ

Remember

Incidentlight wave For mirror, 90 and 90 ,iθ θ= = −o o

d λ2

d m=

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 33: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

How to implement diffraction grating within semiconductor laser?

Distributed Feedback (DFB) Laser

dOptical power

d

( )

d

Active layer

grating

Guiding layer

λ (nm)

d

λ (nm)

(typically m=1)d m λ=

eff

(typically m 1)2

d mn

=

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 34: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Another approach: Make L very small so that Δλ larger than gain bandwidth

g

Δλ larger than gain bandwidth

gain bandwidth: in the order of 10nm

λ

gthgain bandwidth: in the order of 10nm

: 1.5 m: 3.5 effn

λ μ

Not easy to fabricate by cleavi~ 30 m;

ng

ff

L μ

λ

2λ m too mu1 1From ch mir ln ror lo ss,L R

α =

2 effn LλλΔ = L R

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi

Page 35: Laser Basics - tera.yonsei.ac.krtera.yonsei.ac.kr/class/2011_1_STOC/lecture/Laser Basics.pdf · Laser Basics How can we induce stimulated emission? E 2 h ...1999 S.O. Kasap, ... Light

Laser Basics

Solution: Very short cavity vertical lasers with very high reflectivity mirrors(VCSEL: Vertical Cavity Surface Emitting Laser)

Contact In semiconductor fabrication,vertical thickness can bevery precisely controlled

λ/4n1

Active layer

λ/4n2 Dielectric mirror

PN

very precisely controlled.

Dielectric mirror can have hi h fl tivity hi R 1

Dielectric mirror

Active layerN high reflectivity approaching R=1.

m1 1From ln ,α =

Contact

Substrate

m

m

o ,

can be made small if approaches 1

L R

R

α

α

Surface emission

if approaches 1.R

VCSELs are cheaper because it is more mass-producible.

Special Topics in Optical Comm.: Si Photonics (11/1) W.-Y. Choi