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
2θ
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
8°
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.)