Diode lasers for sensor applications Bernd Sumpf Ferdinand-Braun-Institut Lichtenwalde, October 18, 2012
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 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 (> 10 7 h) Tuneability Current Temperature External grating Al x Ga 1-x As Ga x In 1-x P Compound Semiconductors Zn x Cd 1-x S Al x Ga 1-x N InP x As 1-x Ga x In 1-x As PbS x Se 1-x Pb x Cd 1-x S Pb x Sn 1-x Se 0.3 0.5 1 2 3 5 7 10 20 30 Wavelength / µm 18/10/2012 3
Laser diodes: High Output Power Laser diode Coal-fired power plant Surface of the sun P = 20 W 200 µm x 2 µm Power density p = 5 MW / cm 2 600 MW Same power density in a 12 cm cable 6 kw / cm 2 18/10/2012 4
voltage U / V output power P / W Laser Diodes: High efficiency Efficiency 73% Light bulb: < 5% Energy-saving lamp: < 20% 1.8 90 0.9 1.6 C = 73 % 80 0.8 1.4 1.2 1.0 0.8 0.6 0.4 70 60 50 40 30 20 0.7 0.6 0.5 0.4 0.3 0.2 conversion efficiency c 0.2 10 0.1 0.0 0 10 20 30 40 50 60 70 0 current I /A 0.0 18/10/2012 5
optical power P / arb. units Laser Diodes: Narrow spectral linewidth min 10-6 µm 1.0 T = 25 C P = 400 mw 0.8 0.6 0.4 30 µm 0.2 0.0 0.9758 0.9760 0.9762 wavelength / µm 3 cm Paris 880 km Berlin 18/10/2012 6
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 HR coating Design epitaxial structure First epitaxy Manufacturing of the grating e.g. using holographic eposure Second epitaxy Process Facet coating Mounting Ridge waveguide (RW) W RW = 2... 4 µm n eff 3x10-3 AR coating R f < 10-3 Resonator length L = 0.75 3 mm 21 7 9 Bragg Grating Period = 150 300 nm Coupling coefficient = 1 10 cm -1 grating active layer 18/10/2012 8
Power P / mw 940 nm DFB lasers for H 2 O absorption spectroscopy Transmission measurement Laser Medium - 0 Det. Tuneable Laser Lambert-Beers-Law I ( ) I0 exp ( ) L 500 T = 20 C 400 300 Threshold current I th = 35 ma Slope efficiency S = 0.9 W/A Maximum output power P max 500 mw Dips in the characteristic due to water absorption 200 100 0 0 200 400 600 800 Current I / ma 18/10/2012 9
Relative intensity / arb. units 940 nm DFB laser: Absorptions spectroscopy of water vapour 0.6 Spectra calculated based on Lambert-Beers-law 0.5 Laser at 50 C 0.4 Comparison of calculated spectra to the data from the HITRAN database 0.3 Laser at 20 C Excellent agreement 0.2 Continuous tuning over 5 nm at one temperature 0.1 HITRAN 0.0 938 940 942 944 Wavelength / nm 18/10/2012 10
Diode lasers in external cavities for Raman spectroscopy Raman measurement Laser Raman R Det. 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 P Raman Raman 18/10/2012 11
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 RBG BA-Laser FAC + SAC FAC + SAC RBG Microoptics SAC FAC Diode Laser with R r 0.1% and R f = 1% FAC Microoptics SAC Microoptical bench (13 mm x 4 mm) IEEE Phot. Tech. Lett. Vol. 20(19), pp. 1627 (2008). CuW-Submount 18/10/2012 12
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) A B C 18/10/2012 13
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:LiNbO 3 for 488 nm at 25 C RW-SHG-Waveguide (3 µm x 5 µm x 11.5 mm) higher efficiency Diode laser Microoptics WG-SHG-crystal Microoptics CuW-submount Microbench (25 mm x 5 mm) 18/10/2012 16
120µm Diode lasers for the generation of ps- and ns-pulses Ridge waveguide laser Methods: Ridge 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 400-600µm 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 300-3300µm 18/10/2012 17
pulse power P / W intensity /a.u. Gain Switching DFB laser Geometry: L = 2 mm, W = 6 µm Pulse length 10 ns 1 ms; Rep. rate 1 MHz DFB laser with P opt = 2 W at I = 4 A MOPA system with up to P opt = 10 W 2 1 pulse = 10 ns f rep = 1 MHz L = 2 mm W = 6 m I th = 50 ma S = 0.70 W/A 0 0 1 2 3 4 pulse current I / A 50 25 0 50 25 0 50 25 0 75 50 25 0 0 5 10 15 20 time / ns I = 0.80 A P = 0.52 W I = 1.80 A P = 1.05 W I = 2.80 A P = 1.53 W I = 4.00 A P = 2.02 W 18/10/2012 18
intensity / 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 V SAB = -2.0 V; t mod = 1 ns; f rep = 40 MHz Pulse length Pulse power Amplified power: Widerstand zum Heizen DBR-Sektion 200µm 100 ps 300 mw 20 W 80-300µm Absorber Sektion 1500µm Gewinn-Sektion 0.10 0.05 0.00 0.10 0.05 0.00 0.10 0.05 0.00 0.10 0.05 0.00 0.10 0.05 100 ma 200 ma 300 ma 400 ma 500mA 72ps 82ps 72ps 84ps 118ps 0.00 0.0 0.5 1.0 1.5 2.0 time t / ns 18/10/2012 19
power / a.u. intensity / a.u. 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 DBR-Sektion Kavität Gewinn-Sektion Absorber Sektion 750-200µm 8100-8850 µm 1500 µm 200µm Round trip: Pulse length Peak power: ~ 230 ps, rep. rate ~ 4.3 GHz ~ 8 ps, Jitter: < 1 ps 1 W at 8 ps Pulse length Rückfacette 10000µm Frontfacette 20 f active ML = 4.324GHz 1.0 0.8 0.6 ACF ~13.0ps T = 20. C = -1.3V U abs I cav I gain = 180mA = 500mA 10 0.4 FWHM; sech ~8.4ps 0.2 0 0 2 4 time / ns 0.0 0 30 60 90 120 150 time / ps 18/10/2012 20
MOPA system with tapered amplifier for Q-switched ps-pulses Master Oscillator: Q-switched DBR DBR Laser mit GaN Transistor Trapezverstärker mit GaN Transistor Power Amplifier: Tapered Laser Amplification of short pulses Maintaining Beam Quality Seperate Excitation of RW and Tapered Section DBR GaN Transistor 5x8x250 DBR2 SAB2 50mW... FBH XXXXX 1 100W GaN Transistor 5x8x500 TPA DBR1 SAB1 RWG1 200mW PRE1 TPA1 t 0.5-1ns Puls 0.5 A DC RW DC RW t< 2ns Puls 16A ECL ECL RW-Sektion Trapez-Sektion Generation of current pulses MO <1 ns, 1 A PA < 2 ns, 20 A Electronics also developed at FBH 18/10/2012 21
intensity / db intensity / a.u. 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) -30-60 -90-60 DBR: t puls, ~ 0.9ns t period = 25ns I g,cw = 300mA TPA: t puls, ~ 1.0ns t period = 400ns I Tpa,puls = 16A without seed pulse with seed pulse I RW,Tra = 50mA P gentec = 2.13mW P pulse = 12.2W I RW,Tra = 100mA P gentec = 2.76mW P pulse = 15.8W 150 100 50 150 100 50 I RW,Tra = 50mA I RW,Tra = 100mA DBR: t puls, ~ 0.9ns t period = 25ns I g,cw = 300mA TPA: t puls, ~ 1.0ns t period = 400ns I Tpa,puls = 16A -90-60 42dB I RW,Tra = 200mA P gentec = 3.53mW P pulse = 20.2W 0 100 50 I RW,Tra = 200mA 73ps -90 1020 1040 1060 1080 1100 wavelength / nm 0 0.0 0.5 1.0 1.5 2.0 time / ns 18/10/2012 22
power / a.u. power / a.u. 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 master oscillator DBR cavity gain absorber 1cm 20 10 f active ML = 4.32399GHz 0 0 2 4 time / ns pulse picker f/64 ~ 67MHz U HF transistor G D I rw S high frequency pulses (GHz) RW-section for gating 15 10 Iamp low frequency pulses 1kHz - 100MHz amplifier section pulse picker element 5 0 0 5 10 15 20 time / ns 18/10/2012 23
Pulse picker optical micro bench GaN high electron mobility transistor HEMT HF Ansteuerung Modenkopplung HF Ansteuerung Pulspicker Pulspicker Integeration of optical elements high-frequency electronics 500 ma current pulses 200 ps pulse width Adjustable rep.rate 1cm DBR Laser 1 khz 333 MHz Jitter smaller 25 ps Small inductivity short wires Pulspicker mit HF Transistor Auskoppeloptiken 18/10/2012 24
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