Institut d'optique http://www.iota.u-psud.fr/~roosen/ Self-organizing laser diode cavities with photorefractive nonlinear crystals Nicolas Dubreuil, Gilles Pauliat, Gérald Roosen Nicolas Huot, Laurent Meilhac, Antoine Godard, Sébastien Maerten, Vincent Reboud 30-07-05 Tutorial / www.bright.eu 1
Self organizing laser cavities Laser cavity containing a dynamic holographic material, holog >> laser At switching on : Modal structure imposed by the cavity with no hologram Amplifier = competition for gain Modification of modes Dynamic hologram = spectral, spatial filter Desired steady-state = Single transverse and longitudinal mode oscillation 30-07-05 Tutorial / www.bright.eu 2
Self organizing laser cavities No adjustment : Self-adaptation to the operating point, to the thermal or mechanical change, to ageing... Dynamic holographic media Thermal holograms Carmacho et al., Opt. Lett., 24 (1999) Gain holograms Sillard et al., JOSA B, 14 (1997) Absorption holograms Horowitz et al., Opt. Lett., 21 (1996) Photorefractive materials Whitten and Ramsey, Opt. Lett., 12 (1987) 30-07-05 Tutorial / www.bright.eu 3
Self organizing laser cavities Extended cavity laser diodes A.R. 15 mm BaTiO3:Co Diode r u R=57 % t= 0 s : no hologram Temporal evolution of the spectrum 30-07-05 Tutorial / www.bright.eu 4
Self-organizing laser cavities 1. Introduction 2. Photorefractivity and Self adaptive intracavity Fabry-Perot filter 3. Self-organization on spectral modes o Extended cavity laser diodes Laser diode made single mode Stabilisation of a tunable laser diode 4. Self-organization on spatial modes: high power laser diodes o Principle o Extended cavity broad area laser diode 5. Self-organizing extended cavity with tapered laser diode (WP3.2) 6. Conclusion, perspectives 30-07-05 Tutorial / www.bright.eu 5
Photorefractivity and adaptive filter A +1 Photorefractive crystal A 1 Photoexcitation Electric field ± n Electric charges - - - - - - - - - Bragg grating photoconductor electro-optic photorefractive 30-07-05 Tutorial / www.bright.eu 6 I n
Photorefractivity and adaptive filter A +1 Photorefractive crystal A 1 n 1 n 2 n 1,sat = n max A +1 A 1 * A +1 2 + A 1 2 A +2 Photoexcitation Electric field ± n Electric charges - - - - - - - - - Bragg grating A 2 n 1 Adaptation speed : (Intensity) -1 ==> Stationary state independent of intensity dependent on modulation Gratings share n max in proportion to their respective modulations Dynamic stationary state n 1,sat n 2,sat n 1,sat n 2,sat = A +1 A 1 * A +2 A 2 * 30-07-05 Tutorial / www.bright.eu 7 t
Photorefractivity and adaptive filter A +1 Photorefractive crystal A 1 A + - n 1,sat = n max A +1 A 1 * A +1 2 + A 1 2 Photoexcitation A +diffracted + A A diffractée Electric field ± n Electric charges A +diffracted A + n A + A * + A 2 A + + A n 2 max +/- A Same phase and amplitudes structures for diffracted and oscillating waves (A +diff + A - ) remains the same cavity mode ==> Preservation of the modal structure Crystal orientation -> sign of interferences Self-pumped Phase Conjugated Mirror 30-07-05 Tutorial / www.bright.eu 8
Photorefractivity and adaptive filter Self-adaptive Fabry-Perot filter L r u l exc d Constructive interferences Steady state reflectivity in the single mode regime r m Amplitude reflectivity r m L. Meilhac, N. Dubreuil, G. Pauliat, G. Roosen, Optical Materials, Vol. 18, n 1, pp. 37-40, 2001. Mode # Frequency 30-07-05 Tutorial / www.bright.eu 9
Self-organizing laser cavities 1. Introduction 2. Photorefractivity and Self adaptive intracavity Fabry-Perot filter 3. Self-organization on spectral modes o Extended cavity laser diodes Laser diode made single mode Stabilisation of a tunable laser diode 4. Self-organization on spatial modes: high power laser diodes o Principle o Extended cavity broad area laser diode 5. Self-organizing extended cavity with tapered laser diode (WP3.2) 6. Conclusion, perspectives 30-07-05 Tutorial / www.bright.eu 10
Extended cavity laser diodes A.R. Diode 15 mm BaTiO3:Co r u R=57 % SDL 5411, 100 mw, 810 nm diffraction limited multimode BaTi0 3 Co doped (D. Rytz, FEE) 45 cut ordinary polarization thickness = 2 mm S. Maerten, N. Dubreuil, G. Pauliat, G. Roosen, D. Rytz, T. Salva. Opt. Commun, Vol. 208, n 1-3, pp. 183-189, 2002. 30-07-05 Tutorial / www.bright.eu 12
Tunable extended-cavity semiconductor laser Continuous tuning over 100nm around 1.55 m Self adapted intracavity Fabry- Perot: mode stabilization loss without photorefractive filter with photorefractive filter CdTe crystal 4.5 mm, =0.8 cm -1 N. 1/2 = L 1 Factory constraints relaxed with with no no mode hops Single mode operation maintained for for higher output power N-3N-2 N-1 N N+1N+2 N+3 A. Godard, G. Pauliat, G. Roosen, P. Graindorge, P. Martin, 30-07-05 ( 1 ) Eur. Phys. J. A. Phys., vol. 20, n 3, pp. 191-196, 2002 Tutorial / www.bright.eu 13
Self-organizing laser cavities 1. Introduction 2. Photorefractivity and Self adaptive intracavity Fabry-Perot filter 3. Self-organization on spectral modes o Extended cavity laser diodes Laser diode made single mode Stabilisation of a tunable laser diode 4. Self-organization on spatial modes: high power laser diodes o Principle o Extended cavity broad area laser diode 5. Self-organizing extended cavity with tapered laser diode (WP3.2) 6. Conclusion, perspectives 30-07-05 Tutorial / www.bright.eu 14
Broad area laser diodes Power up to 4 W but several transverse and longitudinal modes => low brightness Intensity x, Far field spectrum x, Goal : increase of brightness; => 1 single transverse/longitudinal mode. Wavelength 30-07-05 Tutorial / www.bright.eu 15
Principle of spatial self organization Assumption mode 1 the strongest For mode 1 Broad area diode Photorefractive crystal mirror Diffracted profile A + diffractée A + A + * A A + 2 + A 2 n max A Photorefractif effect = diffracted profile identical to the incident profile => Decrease of losses 30-07-05 Tutorial / www.bright.eu 16
Principle of spatial self organization Assumption mode 1 the strongest For another mode << mode 1 Diffracted profile A + diffractée A + A + * A A + 2 + A 2 n max A Photorefractive effect = diffracted profile differs from the incident profile Increase of losses for the less intense modes Favors the strongest mode Reboud, V., Dubreuil, N., Pauliat, G., and Roosen, G. in:osa Trends in Optics and Photonics, 87, pp. 535-540, 2003 30-07-05 Tutorial / www.bright.eu 17
Extended cavity broad area laser diode A 30-07-05 Tutorial / www.bright.eu 18
Extended cavity broad area laser diode Single mode oscillation Cavity geometry with no spatial filter Power (mw) Without PR crystal With PR crystal Modeling V. Reboud, N. Dubreuil, G. Pauliat, and G. Roosen, Photonics Europe, Strasbourg, 2004, SPIE, Vol 5452 pp. 244-249 (2004) L 2 =f 2 L 2 (mm) 30-07-05 Tutorial / www.bright.eu 19
Extended cavity broad area laser diode Objectives Cavity optimization Modeling Determination of : lowest-loss lateral mode characteristics (shape, M 2 factor), output power versus injected current Fox and Li method; steady state with single frequency; BPM inside the amplifier (charge carrier distribution, antiguiding, ); Two-wave mixing for PR crystal, diffraction (FFT in the extended cavity) No mode coupling effect Good agreement between numerical simulation and experimental results : -> beam shape, M 2 value, power 30-07-05 Tutorial / www.bright.eu 20
Self-organizing laser cavities 1. Introduction 2. Photorefractivity and Self adaptive intracavity Fabry-Perot filter 3. Self-organization on spectral modes o Extended cavity laser diodes Laser diode made single mode Stabilisation of a tunable laser diode 4. Self-organization on spatial modes: high power laser diodes o Principle o Extended cavity broad area laser diode 5. Self-organizing extended cavity with tapered laser diode (WP3.2) 6. Conclusion, perspectives 30-07-05 Tutorial / www.bright.eu 21
Extended cavity tapered laser diode (WP3.2) In the extended cavity, the beam already has a single transverse structure Make it single longitudinal mode with a self-organizing cavity mirror Photorefractive crystal Collimating lens AR 1% Tapered diode Self-adapted Fabry-Perot Quarterly report or Periodic Progress report n 2 February 2005 Meeting in Regesburg, Feb 2005 Tapered lasers provided by IAF (976 nm) and FBH (808 nm) To minimize mode coupling --> design of a short extended cavity 30-07-05 Tutorial / www.bright.eu 22
Extended cavity tapered laser diode (WP3.2) High power single mode operation around 970 nm (Laser IAF) Signal (db) -20-30 -40-50 -60 967 968 '1.5 A' '1.8 A' '2.0 A' '2.2 A' '2.4 A' '2.6 A' 969 970 971 972 Signal (a.u.) Spectrum 2.5 A <->2.6 A Wavelength (nm)) Output power : 740 mw at 2.6 A; (few s adaptation time); Single longitudinal mode; Side mode rejection ratio : > 30 db; Coherence length : > 1 m FSR Frequency Quarterly report or Periodic Progress report n 2 February 2005 - Meeting in Regesburg, Feb 2005 30-07-05 Tutorial / www.bright.eu 24
Conclusion, perspectives Self-organizing laser cavities Self-learning and self-adaptive process to make laser single mode spatial and spectral Physics + Devices Photorefractive materials : - from blue to > 1.6 m - «perfects» for some applications Demonstrations in a large variety of lasers : Dye, Ti:Sapphire (pulsed), Nd:YVO 4, diffraction limited and Broad Area Laser diode N. Huot, J.M. Jonathan, G. Pauliat, P. Georges, A. Brun, G. Roosen, Appl. Phys. B 69, n 2, pp. 155-157, 1999. Modeling of dynamics New geometries (2 and 4 wave mixing) Coupling of cavities Industrial transfers 30-07-05 Tutorial / www.bright.eu 25
Conclusion, perspectives Current results: Diffraction limited laser diodes made single mode o Wavelengths: 635 nm, 660 nm, 810 nm, 1.55 m o Linewidth: 250 khz (coherence length > 600 m) o Output power: limited by laser chip (> 1000 mw ) o Rejection of secondary modes: > 1000 o No stabilization, slight frequency drift: < 200 MHz (> 1day) Potentialities o Wavelengths: any from blue to > 1.6 m, if diode availaible o Frequency locking possible if required o Frequency tunability possible if required 30-07-05 Tutorial / www.bright.eu 26
References N. Huot, J.M. Jonathan, G. Pauliat, P. Georges, A. Brun, G. Roosen, Appl. Phys. B 69, n 2, pp. 155-157, 1999. Laser mode manipulation by intracavity dynamic holography : application to mode selection. L. Meilhac, N. Dubreuil, G. Pauliat, G. Roosen, Optical Materials, Vol. 18, n 1, pp. 37-40, 2001. Modeling of laser mode self adapted filtering by photorefractive Fabry-Perot interferometer. S. Maerten, N. Dubreuil, G. Pauliat, G. Roosen, D. Rytz, T. Salva. Opt. Commun, Vol. 208, n 1-3, pp. 183-189, 2002. "Laser diode made single-mode by a self adaptive photorefractive filter". N. Dubreuil, A. Godard, S. Maerten, L. Meilhac, G. Pauliat, J.M. Jonathan, G. Roosen, J. Phys IV France, Vol 12, Pr5, pp. 99-106, 2002. Cavités laser auto-organisables. A. Godard, G. Pauliat, G. Roosen, P. Graindorge, P. Martin, Eur. Phys. J. A. Phys., vol. 20, n 3, pp. 191-196, 2002. Stabilization of a 1.55 m extended-cavity semiconductor laser by intracavity dynamic holography, G. Roosen, A. Godard, S. Maerten, V. Reboud, N. Dubreuil, G. Pauliat, Opt. Materials, Vol. 23, n 1-2, pp. 289-293, 2003. "Self organization of laser cavities using dynamic holograms". A. Godard, G. Pauliat, G. Roosen, E. Ducloux, Appl. Optics, Vol. 43, n 17, pp. 3543-3547, 2004. "Relaxation of the single-mode emission conditions in ex tended-cavity semiconductor lasers with a selforganizing photorefractive filter". A. Godard, G. Pauliat, G. Roosen, E. Ducloux, IEEE, Journ. Quantum Elect., Vol. 40, n 8, pp. 970-981, 2004 Modal Competition via Four-Wave Mixing in Single-Mode Extended-Cavity Semiconductor Lasers 30-07-05 Tutorial / www.bright.eu 27