How To Make A Laser Laser That Can Be Used For A Long Time

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1 High-Power Al-Free Active Region (λ = 852nm) DFB Laser Diodes For Atomic Clocks and Interferometry Applications Vincent Ligeret, François-Julien Vermersch, Shailendra Bansropun, Michel Lecomte, Michel Calligaro, Olivier Parillaud, Michel Krakowski Presented by Michel Krakowski Acknowledgements: Support of the CNES - contract n 0 4/1948/00 DCT094 Acknowledgements: Guido Guliani Measurements of linewidth : University of PAVIA Acknowledgements: L. Teisseire, Y. Robert, C. Dernazaretian, A. Lordereau for excellent technical assistance May 11, 2006 WORKSHOP Laser Diodes in Space - France Thales Research & Technology

2 Our Objective Growing need for diode lasers at 852nm atomic clocks, gyroscopes in positioning systems Satellite, base station, submarine Disadvantages of extended cavity laser configurations for space applications mechanical stability and precise optical alignment in space environment Our goal: a single frequency and single mode reliable laser diode 2

3 852nm Al free active region laser : motivations Better reliability (1) 3 Litterature With a structure in InGaAsP/GaAs at 0.8µm, no failures which could be attributed to a catastrophic growth of dark line defects have been observed to occur in these diodes. D.Garbuzov and all High-Power 0.8µm InGaAsP-GaAs SQW Lasers,IEEE Journal of quantum electronics, vol 27, pp , 1991 Quantum-well, lattice-matched InGaAsP lasers emitting at 0.8 µm are shown to exhibit resistance to 100 dark-line growth. S.L Yellen ans all, Dark-line resistant aluminium free diode laser at 0.8µm, IEEE Photonics Technology Letters, 4(12), pp , December 1992 This document and any data included are the property of THALES. They cannot be reproduced, disclosed or used without THALES' prior written approval.

4 852nm Al free active region laser : motivations Better reliability (2) Our experience Very long lifetest on Al free broad area laser at P=1W, I=1.4A, T j =40 C without any preliminary burn in test : 808nm : 35000hours (4 years) without any degradations 980nm : 28600hours (3 years and 3 months) without any degradations. Easier implementation of epitaxial regrowth on gratings for Distributed Feedback (DFB) laser structure 4

5 852nm laser structure : realisations Broad Area laser Allow to determine the quality of the epitaxial structure (internal efficiency, internal loss, transparency current density) Ridge Fabry-Perot laser diode Single spatial mode emisison Distributed Feedback Laser diode Spectral single mode emission 5

6 Outline I) Al free active region laser structure II) Broad area AR/HR coated 2mm laser diode III) Fabry-Perot Ridge AR/HR coated 2 mm laser diode IV) DFB Ridge 2mm laser diode Emission at 854nm : 150mW optical power (AR/HR) Emission at nm : D 2 line (as cleaved) 6

7 852nm Al free active region laser structure AlGaInP p cladding GaInP optical confinement LOC < 1µm GaInAsP 8nm quantum well GaInP optical confinement AlGaInP n cladding Fast Axis Standard deviation below 1.2nm Large Optical Cavity Structure J 0 = 100 A/cm² α i < 3 cm -1 η = i 94 % 7

8 Device structure ~100µm ~4µm Gold contact Ohmic contact Shottky contact P + Gold contact P Cladding P + Ohmic contact Insulator P Cladding LOC LOC N Cladding QW LOC LOC N Cladding QW N + N + Broad area laser FP Ridge laser 8

9 Device realisation : principal steps initial epitaxy including active region grating definition holography dry etching (Reactive Ion Etching : RIE) (1:1 ratio) wet chemical selective etching (1:3 ratio) epitaxial regrowth of p cladding and p+ contact layers ridge realisation 9

10 10 GaInP GaInP Λ 2500Å GaInAsP GaInP λ B = 2 1 : 1 n eff m Λ Schematic of the Bragg grating structure m = 2 n eff = 3.26 Λ around 2500Å Device realisation : grating GaInP 1: 3 GaInAsP SEM image of grating structure

11 11 Device realisation : ridge Ohmic contact metallisation Ridge definition (photolithography,ion beam etching, Chemical etching) Back surface thinning and polishing, back contact metallisation BCB Polymer planarisation P side contact metallisation

12 12 Outline I) Al free active region laser structure II) Broad area AR/HR coated 2mm laser diode III) Fabry-Perot Ridge AR/HR coated 2 mm laser diode IV) DFB Ridge 2mm laser diode Emission at 854nm : 150mW optical power (AR/HR) Emission at nm : D 2 line (as cleaved)

13 13 Light current characteristics L(I) 5.5W CW 8.5A 15 C 100µm wide area I th = 490 ma J th = 245 A/cm² η = 0.93W/A T 0 = 116K Laser emission up to at least 115 C 1.4W obtained at 100 C and I=3.6A

14 14 Far field in the fast axis Very stable with the output power No beam steering No signs of higher order modes

16 16 L(I) and optical spectrum Up to 250mW kink free at 20 C Optical Po wer (dbm) Wavelen gth (nm) I th = 40mA η = 0.9W/A Max wall-plug Eff. = 0.40 T 0 = 140K P= 50mW, I=120mA, T= 20 C 852nm

17 Determination of T 0 and T 1 0,930 0,920 0,910 0,900 0,890 0,880 0,870 0, Temperature C Temperature C => T 0 = 140K Threshold current (ma) Slope efficiency (W/A) T T T T ( T ) = I ( T ) exp th T η ( T ) = η ( T ) exp 0 T 1 I th => T 1 = 500K

18 18 Near and far field in the slow axis P=230mW, I=280mA, T=20 C

19 M² measurement methods For beams with gaussian intensity profiles : M² = π 4λ θ 1/e² w 01/e² (1) Where θ 1/e² the full divergence of the far-field at 1/e² and W 0 1/e² the full width of the near-field at waist at 1/e² In case of real beam : M 2 calculation with the second moment product of the far-field σ sx and of the near-field σ x0 profiles : = 4πσ x0 σ (2) M² sx Where σ x0 is the second moment of the near-field intensity profile at the waist and σ sx the second moment of the far-field intensity profile (cf A.E.Siegman, «New developments in laser resonators», invited paper, SPIE Vol Optical Resonators 1990 ) This document and any data included are the property of THALES. They cannot be reproduced, disclosed or used without THALES' prior written approval. 19

20 20 M² in the slow axis direction 1,7 1,6 1,5 M² 1/e² M² σσ 1,4 1,3 M² // 1,2 1,1 1, O ptical Power (mw) 230mW monomode (I=280mA) 2 2 M 1.3 = / e ² = M σσ

22 L(I) characteristics Up to 150mW kink free at 20 C 0,40 0, Bragg peak Bragg peak Detuning Detuning +6nm +1.5nm Gain curve Gain curve 0,30 0,25 0,20 0,15 0, T 0 =125K 0,05 0, current (ma) Temperature ( C) I th = 80mA η = 1.05W/A optical power (mw) Wall-plug efficiency T hreshold Current (ma) 50 T 0 = 125K

23 P= 153mW, I=260mA, T= 20 C λ bragg = nm Detuning of 6.2nm nm 40dB 50dB dB ,8 854,0 854,2 854,4 854,6 854, wavelength (nm) 153mW at nm (I= 260mA, 20 C) Optical spectra 0 Optical power (db) optical power (dbm) wavelength (nm) I=200mA, T= 80 C λ bragg = 857.4nm Detuning of -5nm 57dB 54dB wavelength (nm) Optical power (db)

24 24 SMSR as function of current : 20 C Bragg / shoulder Bragg / gain SMSR (db) up to 153mW SMSR around 30dB Current (ma) 260mA SMSR (db) : Bragg peak/gain peak 60 Current (ma) Bragg / Bragg peak base SMSR (db) Bragg peak / Bragg peak base Current (ma)

25 25 SMSR as function of temperature Bragg / shoulder Bragg / Bragg peak base SMSR (db) I=120mA I=150mA I=200mA Temperature ( C) up to 70 C SMSR around 30dB I=200mA, SMSR between Bragg SMSR (db) Bragg peak / Bragg peak base peak and the gain curve around the Bragg peak Temperature ( C)

26 26 Variation of Bragg wavelength Evolution of Bragg relative to gain wavelength nm/ C nm/ C Wavelength (nm) Bragg wavelength Gain wavelength Temperature ( C) Detuning 15 C : 6.7nm Detuning 40 C : 1.5nm

27 27 Variation of Bragg wavelength 857,5 854,4 857,0 854,3 854,2 854,1 854,0 853, nm/mA Bragg wavelength (nm) 856,5 856,0 855,5 855,0 854,5 854, nm/ C 853,5 853, Temperature ( C) Current (ma) Variations of Bragg wavelength as functions of temperature (I = 120mA) Variation of Bragg wavelength as function of current (T= 20 C) Rth d λ b T d ( UI P 0 = Rth = ) UP 37K/W P elec P optical d λ b dt Bragg wavelength (nm)

28 28 1,0 0,8 0,6 0,4 0,2 0,0 Near and far field in the slow axis 1,0 0,8 4µm 8µm 0,6 0,4 0,2 4,2 9,2 0, position µm angle (degre) P=153mW, I=260mA, T=20 C Optical Power (u.a.) Optical Power (u.a.)

29 29 M² in the slow axis direction 1,8 1,7 M² 1/e² M² σσ 1,6 1,5 1,4 1,3 1,2 1,1 1, Current (ma) 153mW monomode (I=260mA) M 2 1 / e ² = 1.3 M 2 σσ = 1.5 M² //

30 30 Outline I) Al free active region laser structure II) Broad area AR/HR coated 2mm laser diode III) Fabry-Perot Ridge AR/HR coated 2mm laser diode IV) DFB Ridge 2mm laser diode Emission at 854nm : 150mW optical power Emission at nm : D 2 line : adjustment of the grating pitch

31 31 L(I) characteristics Up to 85mW kink free at 20 C Up to 75mW kink free at 37 C 100 0, , ,35 0,30 0,25 0,20 0,15 0,10 Optical power (mw) Wall-plug efficiency , 35 0, 30 0, 25 0, 20 0, 15 0, 10 0, 05 0, , , Current (ma) Current (ma) I th = 46mA η = 0.44W/A I th = 52mA η = 0.40W/A Optical power(mw) Wall-plug effiency

32 32 851,20 851,16 851,12 851,08 851,04 851,00 850,96 850,92 Variation of Bragg wavelength at 20 C 852,2 T = 20 C I = 120mA 852,0 851,8 851,6 851,4 851,2 851, Current (ma) Temperature ( C) Bragg wavelength (nm) Bragg wavelength (nm) d λ b = nm ma di / d λ b = 0.06 nm / C dt nm at 37 C and 140mA UP 40K/W DOWN 25K/W

33 Obtention of D2 line λ bragg =852.12nm -10 λ bragg =852.12nm 50dB dB 3 0dB , 6 851,8 852,0 852,2 852,4 852, Wavelength (nm) 40mW at nm (I= 140mA, 36.9 C) Optical power (mw) Optical power (mw) Wavelength (nm)

34 34 Near and far field in the slow axis : nm 1,0 0,9 0,8 0,7 0,6 0,5 0,4 θ // 1/2 = 7.1 P(a.u) 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 w // 1/2 = 3µm 0,3 w // 1/e² = 6µm 0,2 0,1 θ // 1/e² = x (µm) P=40mW, I=140mA, T=36.9 C 0, θ // ( ) 2 2 M 1 / e ² =1.3 =1.3 M σσ =1.5 P(a.u)

35 35 Linewidth measurement 1,6 Laser λ/2 isolator Fibre 852nm 1,4 1,2 1,0 0,8 Experimental data Lorentzien fit IS O IS O 0,6 0,4 0,2 1.34MHz Voltage (V) 0,0-0,2 Collimation lens Coupling lens FPi FSR=150MHz Resolution = 750kHz -0,4-0,6 0,0152 0,0153 0, ,0155 0, ,0157 Time (s) Schematic of the linewidth measurement setup single peak of the interferogram (30mW) low linewidth ν < 2MHz

36 Linewidth : Measurements at Pavia ν min = 0.9MHz at P = 10mW Guido Giuliani : University of Pavia ν min = 0.9MHz at P = 8mW minimum linewidth value ν = 900kHz G.Giuliani, M.Norgia, S.Donati, Laser diode self-mixing technique for sensing applications J.Opt. A, vol.4, n 6, pp. S283-S294, Y.Yu, G.Giuliani, S.Donati, Measurement of the linewidth enhancement factor of semiconductor lasers based on the optical feedback self-mixing effect, IEEE Photonics Technology Letters, vol 16, n 4, pp , 2004 This document and any data included are the property of THALES. They cannot be reproduced, disclosed or used without THALES' prior written approval. 36

37 Broad area laser: Low optical losses (<3 cm -1 ) High internal quantum efficiency (0.95) Low transparency current density (100A/cm²) Conclusion High optical power : 5.5W for AR/HR coated 2mm long broad area laser diode Ridge Fabry-Perot laser: 250mW single spatial mode for AR/HR coated 2mm long ridge diode with M² 1/e² =1.3 Lasing emission at 852nm (P=145mW, T=15 C) Laser emission DFB (AR/HR coated): 854.3nm with a SMSR over 30dB up to 153mW SMSR over 30dB up to 70 C 153mW single spatial mode for AR/HR coated 2mm long DFB diode with M² 1/e² =1.3 and M² σ <1.5 Low value of Bragg wavelength evolutions in current and temperature Low linewidth : ν < 2MHz, minimum linewidth value ν = 900kHz Laser emission DFB uncoated at 852nm nm at 40mW with a SMSR over 30dB This document and any data included are the property of THALES. They cannot be reproduced, disclosed or used without THALES' prior written approval. 37

38 38 Long term ageing for reliability assessment Tests under irradiations Improvement of linewidth measurement Future work