High Power Diode Lasers

Bio-Photonics ’03 Summer School High Power Diode Lasers Fabrication Characterization Results in the near IR (732-980 nm) Applications Michel Krakows...
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Bio-Photonics ’03 Summer School

High Power Diode Lasers Fabrication Characterization Results in the near IR (732-980 nm) Applications

Michel Krakowski Sophie-Charlotte Auzanneau Thales Research and Technology France

Fabrication • Materials

Laser diodes

• Epitaxy • Process Characterization • power and spectrum measurement • far-field and near-field measurement • The M² factor : definition, measurement • Saturation of the power : cause and solution = the large optical cavity Main epitaxial structures : at 808 nm, 940 nm, 980 nm and 732 nm Examples of lasers diodes • Ridge and broad-area lasers at 980 nm • high brigthness lasers near at 980 nm • state of the art • external approach : four-wave mixing • internal approach : the tapered laser diode • Tapered lasers bars at 980 nm coupled into an optical fiber • Laser diodes at 732 nm : broad-area laser results and aging test • Application to photodynamic therapy of laser diodes at 732 nm • Application to surgery and dentistry of laser diodes at 980 nm

How is a laser diode? Max. size : 3 mm long and 100-200 µm large

18 Au wires On Indium

6 mm

Laser

3 mm Laser

Output facet

Alumina

Copper holder

Schematic of a laser diode = stack of semiconductor materials layers 100-200 µm

Anti-reflection coating (AR 3%)

High reflection coating (HR 95%) m m 3 2Electrodes Confinement layers Separated confinement layers Cleaved facets Active region Lateral passivation

Current injection Photon guiding Optical losses are reduced Mirrors Photon generating Electrical isolation

Which material system for which wavelength?

Diode lasers

II-VI compounds

GaAs/GaInAs/GaAlAs GaN/AlInGaN 0

InP/GaInAsP

GaAs/GaInAsP

0

0.5

1.0

1.5

2.0

Wavelength ( µ m)

Quantum Cascade lasers

GaAs/AlGaAs GaInAs/AlInAs/InP

THz and mm wave

GaSb/ AlGaInSbAs

0

0

5

10

Wavelength ( µ m)

15

20

Steps of the fabrication of a laser diode

1- SUBSTRAT (wafer)

4-

FACETS CLEAVING

2- EPITAXY : growth

of layers

3- LASER PROCESSING

5-chip cleaving 6- MOUNTING, BONDING

A 2’’ wafer ≈ 1000-2000 chips

Schematic of the growth of semiconductor layers Dissociated molecules

Dissociated molecules

substrate

Layers growth : 2 kinds of epitaxy

MBE : Molecular Beam Epitaxy

Molecules fluxes, with ≠ rates

MOCVD : Metallo-Organic Chemical Vapor Deposition

Gas fluxes, with ≠ rates

Wafer en rotation 400-500°

Dissociated molecules Unused gas

Rotating wafer ∼ 650°

Liquid or solid sources

Gas sources

Layers growth : 2 kinds of epitaxy

MBE : Molecular Beam Epitaxy

MOCVD : Metallo-Organic Chemical Vapor Deposition

 slow growth : good controle of the interfaces quality  atomic layers.

 faster growth : layers thicker  50-70 nm (no quantum cascade laser).

 ultravacuum (10-10 Torr)

 under weak pressure or ambiant pressure.

 Controle in-situ of the thicknesses of the layers (diffraction X)

 A usual laser diode = 5-6h

 A usual laser diode = 6h  Quantum cascade laser diode = 40h  Machines : Riber, Varian (en salle blanche)….  No phosphide.

 Machines : Aixtron (white room)….  phosphore allowed.  organo-métallic = gas giving elements III ( Triméthyl In, Ga, Al…)  Arsine (As), phosphine (P), Silane (Si) : toxiques

1 epitaxy machine = a small truck

Process : why ? To define a surface where the current is injected Typical threshold current density of a laser diode Jth~ 100 A/cm2 to 1 kA/cm²

To minimize it, we reduce the injection surface!

Active region

contact

Si3N4, SiO2

~ ~

Substrate

~ ~

process

EPITAXY

Laser surface = 1 cm2

~ ~

~ ~

Substrate

size = 1 - 100

mm

Ith = 100A to 1000 A !

Laser surface = 100 µm x 1 mm = 10-3 cm2

Ith = 100 mA to 1 A

Process Three steps of the planar process

 Photolithography Masquage et Insolation UV

Développement et ouverture

Ionic etching (dry etching) Chemical etching

 Etching

 Coating

(metallic and dielectric)

Then mounting on copper holder … p-down

p-up

xxxxxxxxx

xxxxxxxxx

N side xxxxxxxxx

xxxxxxxxx

P side

soldering

N side

soldering

current

current

Thermal regulation (Peltier)

Thermal regulation (Peltier)

The heat is well dissipated because the active region is near the thermal regulation

 high power laser diodes

P side

For specific technologies

And electrical wires soldering …

Scale : the micron !!

Now we can characterize the laser diode

... ha ha ha

...

Fabrication • Materials

Laser diodes

• Epitaxy • Process Characterization • power and spectrum measurement • far-field and near-field measurement • The M² factor : definition, measurement • Saturation of the power : cause and solution = the large optical cavity Main epitaxial structures : at 808 nm, 940 nm, 980 nm and 732 nm Examples of lasers diodes • Ridge and broad-area lasers at 980 nm • high brigthness lasers near at 980 nm • state of the art • external approach : four-wave mixing • internal approach : the tapered laser diode • Tapered lasers bars at 980 nm coupled into an optical fiber • Laser diodes at 732 nm : broad-area laser results and aging test • Application to photodynamic therapy of laser diodes at 732 nm • Application to surgery and dentistry of laser diodes at 980 nm

The laser diode : assymetric device  elliptic beam Perpendicular (fast) axis

Near-field (µm) ∼ 1 µm

Diode laser

θx

cavité optique HR

θy

AR Parallel (slow) axis

We characterize according to each axis (no correlation)

Far-field (rad)

In the ⊥ direction, monomode beam. In the // direction, it’s different …

Polarization of a laser diode Point for electrical contact

With a p-down laser diode :

Pol. -

Pol. +

Thermal regulation

• power measurement : high divergence of the beam ⇒ we use a sphere.

z Laser diode

Thermal regulation

Photodiode : power calculation

+ spectrometer for λ

• Near-field measurement Microscope objective z Laser diode

Beam analyser

CCD camera

Camera linked to a PC (beam analyser)

• far-field measurement power

Laser diode

Diverging beam

Rotating photodiode

0.0

0.2

0.4

-20 -10

θ

z

a n g0 l e 10

θ1/e²

20

θ

θ1/2

0.6

0.8

1.0

Luminance (brightness) Brightness 3D (W.cm- 2 .sr - 1) =

emissive

Optical Power (W) area (cm²) * solid angle (sr)

Optical Power (W) Optical Power (W) -1 -1 Brightness 1D (W.cm .rad ) = ∝ W (cm) * θ (rad) M² 1D Perpendicular Near- field (fast) axis

θy

Laser diode Wx

θx

Optical cavity

Parallel (slow) axis

Far- field

Luminance ⇒ spatial beam quality ⇒ M² ⇒ near-field size and far-field angle of the beam.

The Gaussian beam  2 M λ(z − z 0 )   W(z) = W0 1 +    πW02  

2

π M =  θ1/e² W01/e² > 1 λ 2

W(z) 1

z

1.0



1.0

0.8 0.8 0.6 0.6

0.4

W0 1/e²

0.2 0.0 -20

-10

0 Taille u.a.

1 0

Near-field at the waist

2 0

u.a.

0

Waist : 2 w0

0.4

θ1/e²

0.2 0.0 -20

-10 0 10 Divergence u.a.

Far-field Ref. : A.E.Siegman, « New developments in laser resonators », SPIE Vol. 1224, 1990

20

What is the limitation of the emitted power? • Diode heating : power saturation , normally non-destructive (thermal roll-over)

400 350 300

P (mW)

250 200 150 100 50

• Catastrophic optical mirror damage (COMD) : destructive

0 0

200

400

600

I (mA)

Recombinaison non-radiative électron-trou de surface

BC hν

Elévation de température

BV

COMD

Réabsorption optique

Filamentation Face-miroir

800

1000

• Catastrophic optical mirror damage (COMD) : destructive Maximal emitted power:

PMAX ( mW / µm ) = PCOMD PCOMD: internal optical power density at threshold COMD R: mirror reflectivity d: active region thickness (reabsorbing) Γ: overlap of the transverse mode with the active region

Active-Region material P COMD (MW /cm2) InGaAs (0.92 - 0.98µm) InGaAsP (0.81µm) InAlGaAs (0.81µm) GaAs (0.81 - 0.87µm) Al0.07 GaAs (0.81µm) Al0.13GaAs (0.78µm)

18-19 18-19 13-14 10-12 8-9 5

(Botez, SPIE proceedings, vol. 3628, p5, 1999)

1−R d (λ) . 1+ R Γ d : transverse spot size Γ

To increase Pmax: >Structure design: - enlarged guides >Active region materials: - InAlGaAs - InGaAsP (Al-free)

Laser diode with an enlarged active region : advantages Wider transverse near-field (> 0.5 µm) : COMD level increased. Weak internal losses (< 1 cm-1): High external differential efficiency Longer cavity are possible (> 1 mm): thermal and electrical resistivities are decreased Power and efficiency record in CW

QW QW

p Cladding

nid waveguide

1,0

Refractive index, Near field intensity

Refractive index, Near field intensity

Usual cavity (Thershold current minimized)

Enlarged cavity : Γclad => α Γqw => COMD

0,8

QW

0,6

0,4

0,2

0,0

nid Broad Waveguide

-0,2 -4

Distance

n Cladding

p Cladding

n Cladding -2

0

Distance

2

4

Fabrication • Materials

Laser diodes

• Epitaxy • Process Characterization • power and spectrum measurement • far-field and near-field measurement • The M² factor : definition, measurement • Saturation of the power : cause and solution = the large optical cavity Main epitaxial structures : at 808 nm, 940 nm, 980 nm and 732 nm Examples of lasers diodes • Ridge and broad-area lasers at 980 nm • high brigthness lasers near at 980 nm • state of the art • external approach : four-wave mixing • internal approach : the tapered laser diode • Tapered lasers bars at 980 nm coupled into an optical fiber • Laser diodes at 732 nm : broad-area laser results and aging test • Application to photodynamic therapy of laser diodes at 732 nm • Application to surgery and dentistry of laser diodes at 980 nm

Laser structure 732 nm : GaAsP / AlGaAs / AlGaAs

• Tensile strained GaAsP QW TM polarisation

• High Al – content of waveguide and cladding layers

• LOC structure (1µm waveguide)

Fabrication • Materials

Laser diodes

• Epitaxy • Process Characterization • power and spectrum measurement • far-field and near-field measurement • The M² factor : definition, measurement • Saturation of the power : cause and solution = the large optical cavity Main epitaxial structures : at 808 nm, 940 nm, 980 nm and 732 nm Examples of lasers diodes • Ridge and broad-area lasers at 980 nm • high brigthness lasers near at 980 nm • state of the art • external approach : four-wave mixing • internal approach : the tapered laser diode • Tapered lasers bars at 980 nm coupled into an optical fiber • Laser diodes at 732 nm : broad-area laser results and aging test • Application to photodynamic therapy of laser diodes at 732 nm • Application to surgery and denistry of laser diodes at 980 nm

All the results presented here have been obtained in the IST-1999-10356 european project ULTRABRIGHT

Reliable Ultra-Bright Laser Diode Sources For Terabit Telecommunications and Photodynamic Therapy Applications

Results from :

R&T

Thales Research and Technology (TRT) Thales Laser Diodes (TLD) Ceram Optec

CeramOptec GmbH

Ferdinand-Braun-Institut für Höchstfrequenztechnik

Why a high brightness laser diode at 980 nm? Laser with a GaInAs quantum well on GaAs substrate. Losses on the 1.55 µm signal in the optical fibers Erbium doped fiber

1.55 µm signal

pump

optical amplification with the EDFA (Erbium Doped Fiber Amplifier)

Erbium Ion

How can we bring more power in the optical fiber? Efficient coupling of the emitted power in the optical fiber

Pump diode

Coupling optics

Optical fiber

⇒ that’s why we need : high power + high beam spatial quality = high brightness

CW optical power records delivered by single 100µm wide emitters at 25°C

30/06/00 13:35:35

12 11

Max. optical power (W)

10 9 8 (55 µm)

7 6 5 4 3 2

GaAs/AlGaAs : Mitsui Chemicals, Jap. Al-free : Wisconsin U./ Coherent USA AlGaInAs/AlGaAs : Optopower Corp. USA AlGaAs : SDL, USA AlGaAs : Fraunhofer Inst., Ger. GaAsP/AlGaAs : F. Braun Inst., Ger. GaAs/AlGaAs : Thales TLD Al-free : Thales TRT

720 740 760 780 800 820 840 860 880 900 920 940 960 980 1000

Wavelength (nm)

In the perpendicular direction : quasi-gaussian far-field profile (M² close to 1) for all the lasers fabricated with the same epitaxy.

I (u.a.)

1,0

0,5

θ1/e² ≈ 52-65° 0,0 -60

-30

0 Angle (°)

30

60

But in the parallel direction, it depends on the waveguide shape …

In the parallel direction : 2 main kinds of lasers The broad-area laser : high power (∼10W at 0.98µm) but low beam spatial quality 1.0

2 mm 0.8

100 µm

P=1W

I (u.a.)

0.6

M² = 15-20

8,7°

Far field in the // direction

0.4

0.2

0.0 -15

9,8° -10

-5

0

5

10

15

Angle (°)

The ridge laser: high spatial beam quality but a few hundreds of mW 1.0

2 mm

Far field in the // direction

0.8

µm

P =300 mW M² = 1.2

I (u.a.)

0.6

5



0.4 0.2 0.0 -15

14° -10

-5

0

Angle (°)

5

10

15

The broad-area laser 100 µm large * 2mm long : 1,0

The broad-area laser 0.98µm AR/HR 100 µm * 2mm:

8.5W CW, (usually operating at 3-4 W)

0,8

I (u.a.)

Threshold : 340mA (170 A/cm²) Efficiency : 0.95W/A Mondial Record : 10.6W CW (Botez, U Wisc.)

0,6

// Far-field at 1W

0,4

//

0,2

10

2 .5

1 .0 0,0 -15

9 8

6

eff .

1 .0

5 4 3 2

0 .5

0 .6

10

15

0 .4

// Near-field at 1W

0,8

0,6

I (u.a.)

P

1 .5

5

1,0

C

7

0

Angle (°)

Efficiency η

Voltage U /V

V

-5

0 .8 Optical Power P /W

2 .0

-10

//

0,4

0 .2 0,2

1 0 .0

0

2

4

6

8

0 10

0 .0

0,0 0

40

80

120

x (µm)

C u rre n t I /A

At 1W , parallel M² > 15 …

160

200

240

The broad-area laser 100 µm large at 980 nm : reliability PROFIL DE VIEILLISSEMENT TGB 969 P= 1W I=1.4A à 30°C

PUISSANCE OPTIQUE (W)

1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

1000

2000

3000

4000

5000

6000

7000

8000

9000 10000 11000 12000 13000 14000 15000 16000

TEMPS CUMULE (HEURES) 1-4

1-6

1-7

1-8

1-9

2-5

2-7

The broad-area laser 100 µm large at 808 nm : reliability

PROFIL DE VIEILLISSEMENT TGB963D A 1W, 1,4 A, 40°C 1.60 Puissance optique (W)

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000 21000 22000 23000

Temps cumulé (heures) 8-2

8-8

9-1

9-2

9-4

The ridge laser 5 µm large * 2mm long : 1,0

0,8

// Far-field at 300 mW

0,6

I (u.a.)

Laser ridge 0.98µm AR/HR 5µm * 2mm: 300 W CW, (usually operating at 100-300 mW) Threshold : 40mA Efficiency : 0.95W/A Record : 1.2W (with optimized mirrors treatments)

0,4

// 0,2

300 0,0 -25

-20

-15

-10

0

5

10

15

20

25

angle (°)

250 1,0

200

// Near-field at 300 mW

//

150 I (u.a.)

P (mW)

-5

100

0,5

50 0 0

50

100

150

200

250

I (mA)

300

350

400

450

0,0 28

32

36

x (µm)

At 300 mW , parallel M² =1.2

40

44

Others solutions to improve the brightness (1) : internal approach - 1.5W FWHM// 0.26° Ortel IEEEPLT 99

Laser (straight section with a Bragg grating)

- Difficulties:

1 mm

• R2 ≈ 0.01%

R2=0.01%

MOPA I1

I2

Master Oscillator Power Amplifier

• interaction between the 2 sections

Tapered amplifier section

• p-down mounting is difficult

- 0.94 W FWHM// 0.2° SDL CLEO-97

α

- 14 éléments bar, 20 W 75 A, FWHM// 1.1° , SDL CLEO-97

DFB

- weak wall-plug efficiency I

- difficult fabrication

Others solutions to improve the brightness (2) : internal approach The index guided tapered laser bar 30 µm

15 µm

200 µm

- 8W 10A, FWHM// 5° à 4.5W, R2=1%, GEC EL-99

1800 µm

The gain guided tapered laser 100 µm

1900 µm - 2.2 W 4 A FWHM// 0.24° M²=2.2, R2= 0.05%, θ

200 µm Fraunhofer SPIE-98 - 25 lasers bar, 25W 50A, M²=2.6 / emitter,

Ipol Monomode lateral section : ridge

Fraunhofer IEEE-PTL99

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory Optics & Fluid Dynamics Dept.

Improving the Spatial and Temporal Coherence of High-Power Diode Lasers Using Four-Wave Mixing in the Gain Material Paul M. Petersen Optics and Fluid Dynamics Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

P.M. Petersen et al.

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory

Optics & Fluid Dynamics Dept.

New Laser design at Risø National Laboratory

Gain- and refractive index gratings in broad area laser diodes may be used for selective amplification of spatial modes The effect is based on spatial hole burning in the active semiconductor gain material. The third order nonlinear susceptibility and the mode suppressions factor depend on the angle between the interacting beams in the four-wave mixing. A narrow range of angles exist with strong gratings and good mode suppression.

P.M. Petersen et al.

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory

Optics & Fluid Dynamics Dept.

Gain- and refractive index gratings in laser diodes Gain and refractive index gratings 0

Z=0 A1

A3

A2

L Z

Semiconductor amplifier

P.M. Petersen et al.

Electrode

Z=L

The induced gain- and refractive index gratings lead to selective amplification of one spatial mode and suppression of all other modes

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory

Optics & Fluid Dynamics Dept.

Four-wave mixing in diodes using spatial hole burning Gain and refractive index gratings

High reflective coating

0

Z=0

A1

Semiconductor amplifier L

Electrode A3

A2

Z

Antireflection coating

Spatial- and temporal filtering

Output beam M

P.M. Petersen et al.

Z=L The induced gratings eliminate filamentation and lateral lasing in the laser diode.

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory

Intensity [arb. units]

Far-field profiles

Optics & Fluid Dynamics Dept.

1.4 times the diffraction limit

Single- lobe 0

80% of the total power is contained in 0 a diffraction limited 0 output lobe.

Runs freely -5 -4 -3 -2 -1 0

1

2

Radiation angle [deg]

P.M. Petersen et al.

3

4

5

Others solutions to improve the brightness (3) : external approach

Risø National Laboratory

Single-mode operation BW < 0.02 nm

(a) Intensity [arbitrary units]

Singlemode

Optics & Fluid Dynamics Dept.

(b)

(c)

(d)

Wavelength is tunable over 20 nm !

(e) 807

808

809

810

811

Wavelength [nm]

P.M. Petersen et al.

812

813

814

Other solution to improve the brightness at 980 nm developed in Thales Research and Technology (4) : the tapered laser diode Between power (broad-area laser) and spatial beam quality (ridge laser)

Straight section (ridge)

Tapered section Anti-reflection coatings (AR) = 3%

High reflection (HR)

θ output 2-3 mm Ridge L1

Tapered sec. L2

Between the broad-area laser and the ridge: the tapered laser 2300 µm // Far- field at 600 mW

200 µm

1,0

0,8

Laser ridge 0.98 µm AR/HR : 1 W, (usually operating at 500-800 mW) Threshold : 97mA Efficiency : 0.86W/A, max wall-plug efficiency = 51% 1000

Intensité (u.a.)

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