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Status of 100 W Rod System at LZH

Status of 100 W Rod System at LZH. Ralf Wilhelm. Martina Brendel, Carsten Fallnich, Maik Frede, René Gau, Herbert Welling, Ivo Zawischa. Laserzentrum Hannover e. V. Hollerithallee 8 D-30419 Hannover Germany. Modeling. Overview Results. Experimental results. Integrated Diode Units

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Status of 100 W Rod System at LZH

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  1. Status of 100 W Rod System at LZH Ralf Wilhelm Martina Brendel, Carsten Fallnich, Maik Frede, René Gau, Herbert Welling, Ivo Zawischa Laserzentrum Hannover e. V. Hollerithallee 8 D-30419 Hannover Germany

  2. Modeling Overview Results Experimental results Integrated Diode Units Pump Optics Pre-Experiments Outline Resumé/Outlook

  3. pump light distribution • ray tracing • analytical approximation • experimental data Finite Element Method for calculating • temperature distribution • mechanical stress • deformation heat generation gain cooling wave propagation through inhomogenous medium • finite differencing • split step fourier approach calculation of optical properties • thermal lens • stress-induced birefringence k-vector Modeling/Overview

  4. HR 808 coating for double pass reduce thermal gradient in z-direction 100 W Laser Head end-pumped rods

  5. undoped end caps thermal lensing consists of two parts thermal part via reduced by undoped endcaps end effect (bulging of end surface) 100 W Laser Head end-pumped rods

  6. 90° quartz rotator 90° quartz rotator compensates for birefringence 100 W Laser Head end-pumped rods undoped endcaps reduce absolute temperature and thermal lens

  7. Model assumption: cylinder symmetrical pump light distribution

  8. Model assumption: cylinder symmetrical pump light distribution model takes into account wavelength/temperature dependent properties wavelength dependent absorption coefficient

  9. Model assumption: cylinder symmetrical pump light distribution model takes into account wavelength/temperature dependent properties wavelength dependent absorption coefficient temperature dependent heat conducitvity

  10. Model assumption: cylinder symmetrical pump light distribution model takes into account wavelength/temperature dependent properties wavelength dependent absorption coefficient temperature dependent heat conducitvity temperature dependent expansion coefficient

  11. Model assumption: cylinder symmetrical pump light distribution model takes into account wavelength/temperature dependent properties wavelength dependent absorption coefficient temperature dependent heat conducitvity temperature dependent dn/dT temperature dependent expansion coefficient

  12. Model assumption: cylinder symmetrical pump light distribution model takes into account wavelength/temperature dependent properties wavelength dependent absorption coefficient temperature dependent heat conducitvity temperature dependent expansion coefficient temperature dependent dn/dT

  13. 20.1°C 83.7°C Thermal Modeling/Temperature Distribution solution of time independent heat conduction equation by FEM (ANSYS) 1/4 of rod for symmetry reasons

  14. 0.0 MPa 41.3 MPa Mechanical Stress/Von Mises Equivalent Stress fracture limit for YAG 130 thru 260 MPa

  15. Thermal Lens/Abberations

  16. initial distributed E(x,y,z0) (e. g. noise) medium free propagation mirror/aperture free propagation medium free Propagation output power beam quality mirror/aperture free Propagation no yes convergence ? Fox/Li Approach Iterative Solution of Kirchhoff integral equations • inhomogenous distributed gain, refractive index, birefringence concentrated in gain/phase sheets • propagation between gain/phase sheets and in free space described by FFT propagator

  17. First Results mode diameter in rod 1 mm

  18. 100 W 120 W polarisation eigenstates First Results

  19. First Results/Birefringence Compensation

  20. First Results/100 W Head

  21. First Results/100 W Head w/o Abberations

  22. <10 nm Abberations/End Pumped vs. Transversally Pumped

  23. Pump Concepts mode selective pumping w = 1mm

  24. Pump Concepts mode selective pumping w = 2 mm

  25. Homogenization of Pump Light simulation 10 x 800 µm measured 30 x 800 µm

  26. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 5000 m

  27. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 2000 m

  28. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 1500 m

  29. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 1000 m

  30. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 750 m

  31. Thermal Modeling/Temperature Distribution varying with pump spot diameter (pump power kept constant) 500 m

  32. Thermal Modeling/Maximum Temperature

  33. Von Mises Stress varying with pump spot diameter (pump power kept constant) 5000 m

  34. Von Mises Stress varying with pump spot diameter (pump power kept constant) 2000 m

  35. Von Mises Stress varying with pump spot diameter (pump power kept constant) 1500 m

  36. Von Mises Stress varying with pump spot diameter (pump power kept constant) 1000 m

  37. Von Mises Stress varying with pump spot diameter (pump power kept constant) 750 m

  38. Von Mises Stress varying with pump spot diameter (pump power kept constant) 500 m

  39. Mechanical Stress/Von Mises Equivalent Stress varying with pump spot diameter (pump power kept constant)

  40. Experimental/Diode Temperature Control laser diode JENOPTIK 30 W, fiber coupled, NA 0.22; 800 m temperature resolution: 0.01K temperature fluctuations: 2-3 digits temperature stability better than 0.05K

  41. 4 boxes • each 10 X 30 W fiber-coupled diodes  1200 W pump Power Experimental/Diode Box user interface 4 systems (boxes) 40 temperatures laser diode (10) heat sink (2) 4 current controls (1 per box) ADC/DAC upcoming: • 40 diode power measurements  laser power control for each diode overtemp interlocks peltier drivers

  42. 2.5 cm Pump Chamber water flow

  43. Pre-experiments laser rod pump optic TR multimode M2 < 3

  44. Birefringence Compensated Resonator Faraday Rotator thermal lens image Faraday Rotator laser rod pump optic TR

  45. Resumé Modeling • 100 W of output power will be achieveable • abberations will have to be compensated for • abberations are comparable in end pumped and transversally pumped rod Experimental • 4 diode boxes have been set up (1200 W of pump power) • temperature stabilization works • pump light homogenization has been demonstrated • 45 W single mode and 75 W multi mode laser has been demonstrated (single rod, no compensation)

  46. Outlook • optimize overlap of pump light distribution and mode diameter • compare calculated abberations to experiment (Shack-Hartmann sensor, diploma thesis P. Huke) • evaluate conductive cooling (coating of rod‘s shell) -reduce abberations (lower absolute temperature) -avoid contact of cooling fluid with rod • compensate for abberations doped region ? pump

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