Bernd Sumpf
Ferdinand-Braun-Institut
Lichtenwalde, October 18, 2012
Diode lasers for sensor applications
Outline
1. Diode Lasers – Basic Properties
2. Diode Lasers for Sensor Applications
Diode lasers with internal grating
Diode lasers in external cavities
Diode lasers as pump sources for non-linear frequency conversion
Hybrid integrated laser module for ps- and ns-pulses
3. Summary
18/10/2012 2
Diode lasers – Features
18/10/2012 3
Wide spectral range: 0.34 nm ... 33 µm
FBH: 630 nm ... 1.2 µm
High wall-plug efficiency
Easy excitation
Direct Modulation
Small size
Mechanical robustness
Lifetime (> 107 h)
Tuneability
Current
Temperature
External grating
PbxSn1-xSe
PbSxSe1-x
PbxCd1-xS
InPxAs1-x
GaxIn1-xAs
AlxGa1-xAs
GaxIn1-xP
ZnxCd1-xS
AlxGa1-xN
0.5 0.3 1 2 5 3 7 20 10 30
Wavelength / µm
Compound Semiconductors
Laser diodes: High Output Power
18/10/2012 4
Laser diode
P = 20 W
200 µm x 2 µm
Power density
p = 5 MW / cm2
Coal-fired power plant
600 MW
Same power density in a
12 cm cable
Surface of the sun
6 kW / cm2
Laser Diodes: High efficiency
18/10/2012 5
Efficiency 73%
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
80
90
ou
tpu
t p
ow
er
P / W
current I /A
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
vo
lta
ge
U / V
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
C = 73 %
co
nve
rsio
n e
ffic
ien
cy
c
Light bulb:
< 5%
Energy-saving lamp:
< 20%
Laser Diodes: Narrow spectral linewidth
min 10-6 µm
18/10/2012 6
0.9758 0.9760 0.97620.0
0.2
0.4
0.6
0.8
1.0
optical pow
er
P / a
rb. units
wavelength / µm
T = 25°C
P = 400 mW
30 µm
880 km
Paris Berlin
3 cm
Overview: Fabrication Process of a Laser Diodes
Deposition of very thin crystalline layers on a GaAs substrate
Epitaxy
Structuring of devices on the wafer
Processing
Separation of single devices
Cleaving
Mounting of devices on heat sinks
Housing of devices
18/10/2012 7
Schematic view of a high-power DFB laser
18/10/2012 8
Design epitaxial structure
First epitaxy
Manufacturing
of the grating
e.g. using holographic
eposure
Second epitaxy
Process
Facet coating
Mounting
Bragg Grating
Period = 150 … 300 nm
Coupling coefficient
= 1 … 10 cm-1
Ridge waveguide (RW)
WRW = 2 ... 4 µm
neff 3x10-3
7 … 9°
Resonator length
L = 0.75 … 3 mm
HR coating
AR coating
Rf < 10-3
21°
active layer
grating
940 nm DFB lasers for H2O absorption spectroscopy
Transmission measurement
Tuneable Laser
Lambert-Beers-Law
Threshold current Ith = 35 mA
Slope efficiency S = 0.9 W/A
Maximum output power
Pmax 500 mW
Dips in the characteristic due to water absorption
18/10/2012 9
Laser Medium - 0 Det.
LII )(exp)( 0
0 200 400 600 8000
100
200
300
400
500
T = 20°C
Pow
er
P / m
W
Current I / mA
940 nm DFB laser:
Absorptions spectroscopy of water vapour
Spectra calculated based on Lambert-Beers-law
Comparison of calculated spectra to the data
from the HITRAN database
Excellent agreement
Continuous tuning over 5 nm at one temperature
18/10/2012 10
938 940 942 944
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Laser at 50°C
Laser at 20°C
HITRAN
Rela
tive inte
nsity / a
rb. units
Wavelength / nm
Diode lasers in external cavities for Raman spectroscopy
Raman measurement
Fixed frequency laser (e.g. 488 nm, 671 nm, 785 nm)
Spectral emission width: 10 cm-1
Spectral stability 1 cm-1
Well-established contact free method
for material analysis, food safety control, clinical diagnostic.
Excitation in the visual spectral range
Advantages:
Higher Raman signals due to a -4 – dependence
Resonance Raman
Stokes lines in the maximum of the sensitivity of CCDs
Disadvantages:
Possible fluorescence background
Shifted excitation Raman difference spectroscopy
Spectral distance for SERDS SERDS ≈ 10 cm-1
18/10/2012 11
Laser Raman R Det.
Raman
PRaman
671 nm microsystem light source
Gain medium:
Broad area laser
w = 30 µm, 60 µm, 100 µm; L = 2 mm
Output power up to 1.5 W
Resonator
Front facet of the diode laser and
Reflection Bragg Grating
Emission width below 100 pm (10 cm-1)
Active adjustment necessary
18/10/2012 12
RBG
Microoptical bench (13 mm x 4 mm)
Diode Laser
with Rr ≤ 0.1%
and Rf = 1%
CuW-Submount
Microoptics Microoptics
FAC SAC
FAC SAC
BA-Laser
RBG
FAC + SAC
FAC + SAC
IEEE Phot. Tech. Lett. Vol. 20(19), pp. 1627 (2008).
671 nm module – application in Raman-Spectroscopy
Ethanol as analyte (A):
Raman-signals spectrally resolved
Fluorescent interference introduced (B)
Laser dye Cresyl violet
Application of SERDS successful (C)
18/10/2012 13
A B C
Diode lasers in external cavity for interferometry
Absolute distance interferometry (ADI) with 10-6
accuracy requires tunable red emitting diode lasers:
Preferred wavelength ≈ 633 nm
Single-mode operation
Tuning range ≥ 50 pm (40 GHz)
Determines the smallest measurement distance
about 4 mm
Narrow spectral line width ≤ 10 MHz (0.015 pm)
Determines the maximal measurement distance
about 15 m
Output power ≥ 5 mW
Current tunable, no moving parts
Solution:
ECDL with mode spacing larger the spectral width of the RBG RBG 50 pm
n L ≤ 4 mm FP 50 pm - within RBGs spectral width only one mode!
Narrow line width due to high quality resonator (High facet reflectivity)
18/10/2012 14
Scheme of the external cavity laser
Single mode operation
34 pm, i.e. 25 GHz
Emission line width (self-delayed heterodyne)
Between mode hops smaller 10 MHz
coherence length of 30 m
At mode hop increase to about 15 MHz
Side mode suppression ratio: Better than 25 dB
18/10/2012 15
Diode lasers as pump sources
for non-linear frequency conversion, e.g. SHG
Low power application (25 mW) for Raman spectroscopy
Non-linear frequency conversion – Second Harmonic
Generation (SHG)
Pump source
Distributed Feedback (DFB) RW Laser
SHG-crystal
periodically poled MgO:LiNbO3 for 488 nm at 25°C
RW-SHG-Waveguide (3 µm x 5 µm x 11.5 mm)
higher efficiency
18/10/2012 16
Microoptics Diode laser WG-SHG-crystal Microoptics
CuW-submount
Microbench (25 mm x 5 mm)
Diode lasers for the generation of ps- and ns-pulses
Methods:
Gain switching
Current injection
Pulse length: 1 ns – 1 s
Q-switching
Changing the properties of the laser cavity
E.g. implentation of an absorber section
Pulse length: 50 ps – 150 ps
Rep. Rate: up to 0.5 GHz
Mode locking
Coupling of longitudinal modes
Passively (saturable absorber section)
Actively
Pulse length: 1 – 20 ps
Resonator length determines the rep. rate in the
GHz-range
18/10/2012 17
400 - 600µm
120
µm
300 - 3300µm
Ridge waveguide laser
Ridge
0 1 2 3 4
0
1
2
L = 2 mm
W = 6 m
Ith = 50 mA
S = 0.70 W/A
puls
e p
ow
er
P / W
pulse current I / A
pulse
= 10 ns
frep
= 1 MHz
Gain Switching – DFB laser
18/10/2012 18
Geometry: L = 2 mm, W = 6 µm
Pulse length 10 ns … 1 ms; Rep. rate 1 MHz
DFB laser with Popt = 2 W at I = 4 A
MOPA system with up to Popt = 10 W
0
25
50
0
25
50
0
25
50
0 5 10 15 20
0
25
50
75
I = 0.80 A
P = 0.52 W
I = 2.80 A
P = 1.53 W
I = 1.80 A
P = 1.05 W
I = 4.00 A
P = 2.02 W
time / ns
inte
nsity /a.u
.
Q-Switching – Multi-Section RW- and DBR Lasers
e.g. 3 – section DBR laser with gain, absorber, and grating section – length 1.5 … 4.0 mm
Current through gain section varied
Output power can be modulated using the absorber section VSAB = -2.0 V; tmod = 1 ns; frep = 40 MHz
Pulse length 100 ps
Pulse power 300 mW
Amplified power: 20 W
18/10/2012 19
Gewinn-SektionDBR-Sektion
Absorber SektionWiderstand zum Heizen
1500µm
200µm
80 - 300µm
0.00
0.05
0.10
0.00
0.05
0.10
0.00
0.05
0.10
0.00
0.05
0.10
0.0 0.5 1.0 1.5 2.0
0.00
0.05
0.10
72ps
100 mA
82ps
200 mA
72ps300 mA
84ps
400 mA
inte
nsity / a
.u.
118ps500mA
time t / ns
Monolythic devices for mode locking
4 section DBR-Laser
DBR grating determines wavelength
Fast saturable absorber for mode locking
Passive and active mode locking possible
Round trip: ~ 230 ps, rep. rate ~ 4.3 GHz
Pulse length ~ 8 ps, Jitter: < 1 ps
Peak power: 1 W at 8 ps Pulse length
18/10/2012 20
Gewinn-SektionDBR-Sektion Absorber Sektion
10000µm
200µm 8100 -8850 µm 200µm 750 -1500 µm
Kavität
FrontfacetteRückfacette
0 2 40
10
20
time / ns
po
we
r / a
.u.
f active ML = 4.324GHz
0 30 60 90 120 1500.0
0.2
0.4
0.6
0.8
1.0
FWHM; sech
~8.4ps
T = 20.°C
Uabs
= -1.3V
Icav
= 180mA
Igain
= 500mA
inte
nsity / a
.u.
ACF
~13.0ps
time / ps
MOPA system with tapered amplifier
for Q-switched ps-pulses
Master Oscillator: Q-switched DBR
Power Amplifier: Tapered Laser
Amplification of short pulses
Maintaining Beam Quality
Seperate Excitation of RW
and Tapered Section
18/10/2012 21
Generation of current pulses
MO <1 ns, 1 A
PA < 2 ns, 20 A
Electronics also developed at FBH
XXXXXFBH 1
PRE1 TPA1
DC
RW
SAB1
SAB2
RWG1
DBR2
DBR1
DC
RWPuls
0.5 A
ECL
DBR Laser mit GaN Transistor Trapezverstärker mit GaN Transistor
ECL
Puls
16A
t< 2nst 0.5-1ns
50mW...
200mW
100W
GaN Transistor5x8x250
DBR GaN Transistor5x8x500
TPA
RW-Sektion Trapez-Sektion
Results: MOPA system for Q-switched ps-pulses
Pulse length (FWHM) = 73 ps
Pulse power: 20 W
Wavelength defined by MO with emission width (FWHM ~ 0.2 nm)
18/10/2012 22
-90
-60
-30
-90
-60
1020 1040 1060 1080 1100-90
-60
DBR: tpuls,
~ 0.9ns
tperiod
= 25ns
Ig,cw
= 300mA
TPA: tpuls,
~ 1.0ns
tperiod
= 400ns
ITpa,puls
= 16A
IRW,Tra
= 50mA
Pgentec
= 2.13mW
Ppulse
= 12.2W
without seed pulse
with seed pulse
in
tensity / d
B
IRW,Tra
= 100mA
Pgentec
= 2.76mW
Ppulse
= 15.8W
wavelength / nm
IRW,Tra
= 200mA
Pgentec
= 3.53mW
Ppulse
= 20.2W
42dB
0
50
100
150
0
50
100
150
0.0 0.5 1.0 1.5 2.00
50
100
DBR: tpuls,
~ 0.9ns
tperiod
= 25ns
Ig,cw
= 300mA
TPA: tpuls,
~ 1.0ns
tperiod
= 400ns
ITpa,puls
= 16A
IRW,Tra
= 50mA
in
ten
sity /
a.u
.
IRW,Tra
= 100mA
time / ns
IRW,Tra
= 200mA
73ps
Pulse picking using tapered amplifier
Basic principle:
RW-section of TPA as selector
Transparent or absorbing
Controlled with GaN-HF-transistor
Selectable duty cycle
Amplification in tapered section
18/10/2012 23
high frequency pulses (GHz)
HF transistor
low frequency pulses 1kHz - 100MHz
U
ampI
rwI
master oscillator
pulse picker element
DBR
cavity
gain
absorber 1cm
G D S
amplifier section
RW-section for gating
0 2 40
10
20
time / ns
pow
er
/ a.u
.
f active ML = 4.32399GHz
0 5 10 15 200
5
10
15
po
we
r / a
.u.
time / ns
pulse picker f/64 ~ 67MHz
1cm DBR Laser Pulspicker mit HF Transistor Auskoppeloptiken
HF Ansteuerung PulspickerHF Ansteuerung Modenkopplung
Pulse picker – optical micro bench
Integeration of
optical elements
high-frequency electronics
500 mA current pulses
200 ps pulse width
Adjustable rep.rate
1 kHz – 333 MHz
Jitter smaller 25 ps
Small inductivity – short wires
18/10/2012 24
Pulspicker
GaN high
electron mobility
transistor HEMT
Summary and Acknowledgments
Diode lasers:
Compact, reliable, high-power light sources for different applications
Features can be optimized with respect to the application:
Wavelength
Power
Emission width
Beam quality
Pulse parameter
Acknowledgments:
All colleagues at the FBH
Colleague at the Technische Universität Berlin (Agr. Laserspektroscopy)
Financial Support:
Zukunftsfond Berlin
Deutsche Forschungsgemeinschaft
Bundesministerium für Bildung und Forschung
Europäische Gemeinschaft
18/10/2012 25