Types of Diode Lasers

DFB, DBR and External Cavity

Joachim SacherJoachim Sacher

Outline

� Principles of Semicondutor Diode Lasers

� Types of Single Mode Semicondutor Diode Lasers� DFB Laser� DBR Laser� External Cavity Laser

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� ICL and QCL Laser

� Application Examples� CO2 Absoption @ 2002nm� C2H2 Absoption @ 1530nm� CH4 and H2O Absoption @ 3µm� N2O Absorption @ 8.7µm

Semiconductor Band Structure

� Si � GaAs

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� Indirect Semiconductor � Direct Semiconductor

P-N Junction

� Semiconductor Diode

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Quantum Well Structure

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Ridge Waveguide Laser

� Fabry Perot Laser

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Semicondutor Laser Prodution

� Laser Wafer � Wafer Growth� Common Materials

� GaN (blue)� GaAs (730nm -1100nm)� InP (1200nm - 2100nm)� GaSb (1800nm - 3400nm)

� Stucturing via Lithographie

� Breaking into Laser Bars� Facet Coating� Testing on Bar Level

� Breaking into Laser Chips� Soldering to Subcarriers� Mounting into packages

Surface grating

Laser facet

substrate

Quantum wells

Cladding

Single Mode Diode Lasers

� DFB Lasers

� DRB Lasers

Position

Intensity

Emitter width

Bragg Grating AR-Coating

Intensity

se S

egm

ent

g S

egm

ent

Seg

me

nt

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� External Cavity Laser

Gain

Phase

DBR

PIN 4Position

Emitter width

Bragg Grating AR-Coating

Ph

as

Bra

gg

Gai

n

Types of DFB Lasers

� 1. Classic Type of DFB Laser

� Realization� Bragg Grating Segments etched

into waveguide� Back Facet HR Coated� Front Facet 0% AR Coated

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� Front Facet 0% AR Coated

� Features� Single Mode Emission� Sensitive to Optical Feedback� Linewidth sensitive to Laser

Current Noise

(confidential technical documentation)

Types of DFB Lasers

� 2. Weak Coupled DFB Lasers

� Realization� Bragg Grating weakly coupled

sidewise to waveguide� Back Facet HR Coated� Front Facet 10% AR Coated

Position

Intensity

Emitter width

Bragg Grating 5-10% AR-Coating

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� Front Facet 10% AR Coated

� Features� Single Mode Emission� Reduced Sensitivty on Optical

Feedback� Linewidth sensitive to Laser

Current Noise

(confidential technical documentation)

Types of DFB Lasers

� 3. Digital Type of DFB Laser

� Realization� 2 Bragg Grating Segments

etched into waveguide� Back Facet HR Coated� Front Facet 5% AR Coated

Position

Intensity

Emitter width

Bragg Grating 5-10% Coating

Bra

gg

Se

gmen

t 2

Bra

gg

Se

gmen

t 1

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� Front Facet 5% AR Coated

� Features� Single Mode Emission� Reduced Sensitivity to Optical

Feedback� Linewidth Reduced Sensitivity to

Laser Current Noise

(confidential technical documentation)

Digital DFB Lasers – Principles of Operation

� Principles of Operation

DIGITAL DFB

| 12� Etched Features select one FP Mode

while all others are suppressed

Suppressed FP Modes

Selected Mode

TMM model

(confidential technical documentation)

Digital DFB Lasers – Realization

� GaSb DFB-Laser Siode on Subcarrierwithin a TO39-Package (Match as size Reference)

� Spectrum with SMSR better then 40 dB� Wellenlength approximately 2002 nm

Digital DFB Lasers – Realization

� Measured and simulated (Hitran) Absorption Spectrum of CO2

� Tuning Curve

Graphs Graphs publishedpublished in in thethe November November IssueIssue of Applied of Applied OpticsOptics!!

DFR Lasers

� Schematic

� Tuning Behavior

(Graphics from Photodigm.com)

External Cavity Laser Systems

� Littrow Cavity

� Littman Cavity

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� VBG Cavity

External Cavity Laser Systems

� Antireflection Coating Requirementsro

babi

lity

of

ultim

ode

pera

tion

100% Class A

Class B

Class C

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� For best performance, a reflectivity <1E-5 is required (Class D)

Wavelength / a.u.

Pro

Mu

Op

0%Class D

External Cavity Laser Systems

� Synchronisation of Grating and Cavity Determined Wavelength� Mode-Hop Free Tuning

Grating(a)

(b)

� True Pivot Point Selection

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Grating

Grating(c)

(b)

� Exact Choice of x0, x1, x2

� Synchronization� Cavity Defined Wavelength� Grating Defined Wavelength

� Result: � Fully Mode-Hop Free Tuning

External Cavity Laser Systems

� Littrow Cavity Laser Application (Most Common): � Alcaline Spectroscopy (Rb, Cs, Li)� Optical Cooling & Trapping� Bose Einstein Condansate� Optical Clocks

� Most Common Optimization� High Stability

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� High Stability� Unsensitive against ambient conditons

� Rb D2 Line

Littrow Laser Products – Industrial Grade

� Temperature Cycling� Operation Performance

� On – Off Repeatability

793,4025

793,4030

793,4035

793,4040

793,4045

793,4050

Wav

elen

gth

/ nm

780,026

780,027

780,028

22,8

23,0

23,2

23,4

Tem

pera

ture

(B

asep

late

) / °

C

Wav

elen

gth

/Vac

uum

) / n

m

Full StartupTemp & Current

Partial StartupLaser Current

| 20| 20

� Excellent wavelength stability over >24 hours temperature cycling

� No laser stabilization during measurement

� Simple compensation via Locking electronics

(confidential technical documentation)

� Excellent repeatability after start-up sequences.

� Back to the absorption line within a few minutes after restarting laser current,

� no loss of the absorption line

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5793,4010

793,4015

793,4020

Wav

elen

gth

/ nm

Time / hours

0 5 10 15 20 25 30780,024

780,025

22,6

22,8

Tem

pera

ture

(B

asep

late

) / °

C

Wav

elen

gth

/Vac

uum

) / n

m

Time / hours

Littrow Laser Products – Industrial Grade

� High Power Model @ 780nm� PI Curve � Wavelength Tuning

200

250

300

350

400

Pow

er (

mW

)

780,030

780,032

780,034

780,036

780,038

780,040

Wav

elen

gth

/ nm

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� Efficiency� 1mW/mA

� Tuning Factor� 1pm / mA

0 100 200 300 400 500 6000

50

100

150

Pow

er (

mW

)

Injection Current (mA)

(confidential technical documentation)

200 220 240 260 280 300780,020

780,022

780,024

780,026

780,028

Wav

elen

gth

/ nm

Injection Current / mA

External Cavity Laser Systems

� Littman Cavity Laser Application (Most Common):� Molecular Spectroscopy (CH4, C2H2)� Plasma Spectrosopy� Quantum Dot Spectroscopy

� Most Common Optimization� Large Mode-Hop Free Tunability

0,00

| 22200000 220000 240000 260000 280000 300000 320000

-0,04

-0,03

-0,02

-0,01

0,00

Abs

orpt

ion

Sig

nal /

a.u

.

Wavelength / a.u.

Hyperfine Transition (50mTorr)

Littrow Laser Products – Industrial Grade

� Linewidth� Beat Experiment

180

200

220

VBG Pilot rauscharm

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� Linewidth << 100kHz< 15kHz (typ.)

(confidential technical documentation)

395 400 405 410

40

60

80

100

120

140

160

Inte

nsity

/ dB

Frequency / MHz

Interband Cascade Laser

� Band Structure

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Interband Cascade Laser

� Test Data at 3300nm: CH4 and H2O Absorption

1

2

3

4

5

Pow

er /

mW

25°C 30°C 35°C

0

20

40

60

80

100

120

140

160

180

Sig

nal /

a.u

.

125mA 150mA 175mA 200mA

25°C

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0 20 40 60 80 100 120 140 160 180 2000

Current / mA

3290 3292 3294 3296 3298 3300 3302 3304-20

0

Wavelength / mn

120 130 140 150 160 170 180 190 200 2103295

3296

3297

3298

3299

3300

3301

3302

3303

Wav

elen

gth

/ nm

Current / mA

25°C nm 30°C nm 35°C nm

25 30 35 400

2

4

0

1

2

3

4

5

6

H2O

tube 150cm room

Pow

er /

mW

Temperature / °C

H2O

CH4

2cm room

Quantum Cascade Laser

� Band Structure

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� Quantum Cascade External Cavity Laser

Quantum Cascade Laser

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Quantum Cascade Laser

QCL: Wavelength vs. Intensity

� Overlap of different Wavelength

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Quantum Cascade Laser

QCL: Emissionsleistung-Stromstärke Diagramm

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Quantum Cascade Laser

� EC QCL Spectroscopy

� N2O Line @ 8,70087 µm (1149.31cm-1)

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Thank You For Your Attention!

Sacher Lasertechnik GmbHRudolf-Breitscheid-Str. 1-535037 MarburgGermany

http://www.sacher-laser.com© 2009 Sacher Lasertechnik GmbH. All rights reserved.

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