In this paper, two new fabrication methods are developed for highly curved toroidal laser resonator mirrors based on laser micromachining of cylindrical substrates that already have the large lateral curvature RT provided by conventional polishing. These mirrors are produced by a two-step CO2 laser machining process with high resolution ablative laser cutting and then laser polishing, or by vaporization of silica in the CO2 laser smoothing process at higher than usual power. The produced mirrors are with RT = 32 mm and 15 mm radius of curvature and 14 mm lateral length for use with the 200 mum core height Nd:YAG and a 150 mum core Yb:YAG planar waveguide laser respectively

Baker, H. J.

Hall, D. R.

Thomson, I. J.

Wlodarczyk, K. L.

English

Heriot Watt Pure

    Heriot-Watt University Research Gateway                                                          Heriot-Watt UniversityMode-selective toroidal mirrors for unstable resonator planar waveguide and thin slabsolid-state lasersWlodarczyk, Krystian; Thomson, I. J.; Baker, Howard John; Hall, DenisDOI:10.1109/CLEOE-EQEC.2009.5192114Publication date:2009Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Wlodarczyk, K. L., Thomson, I. J., Baker, H. J., & Hall, D. R. (2009). Mode-selective toroidal mirrors for unstableresonator planar waveguide and thin slab solid-state lasers. Poster session presented at CLEO/Europe -European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference,Munich, Germany.10.1109/CLEOE-EQEC.2009.5192114The European Conference on Lasers and Electro-Optics Munich, Germany Mode-Selective Toroidal Mirrors for Unstable Resonator  Planar Waveguide and Thin Slab Solid-State Lasers Krystian L. Wlodarczyk, Ian J. Thomson, Howard J. Baker, Denis R. Hall School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, United Kingdom Summary CO2 laser fabrication of highly curved toroidal resonator mirrors Surface profile and cross-section of a resonator mirror fabricated by laser cutting and laser polishing Surface profile and cross-section of a resonator mirror made by vaporization in laser polishing process Technique  1:  A  two-step  CO2  laser  machining  process  with  high-resolution  ablative  laser cutting (made by PowerPhotonic Ltd.) and then laser polishing. Technique 2:  Vaporization of fused silica in the laser polishing process at higher than usual laser power  Silica  cylindrical  lens  with  240mm  lateral RoC used as a substrate.  Desired shape and transverse curvature of a mirror obtained by precise ablative laser cutting.  Roughness of 300µm wide laser-cut mirror removed by localized laser polishing.  Lateral  RoC  is  decreased  by  surface stress produced by polishing.  2mm thick planar fused silica used as a substrate.   300µm wide trench made by CW raster scan  area  laser  polishing,  but  at  higher than usual laser power.  Depth of desired profile is controlled by laser power used.  Surface roughness  ±20nm Ultra-low surface  roughness ±10nm Testing of fabricated mirrors with Yb:YAG/Sapphire planar waveguide laser [1]  G. J. Markillie, H. J. Baker, F. Villarreal, D. R. Hall, “The effect of vaporization and melt ejection on laser machining of silica glass micro-optical components,” Applied Optics 41, 5660 (2002). [2]   K. M. Nowak, H. J. Baker, D. R. Hall, “Efficient laser polishing of silica micro-optical components,” Applied Optics 45, 162 (2006).  [3]  J. Xu, I. J. Thomson, J. D. R. Valera, H. J. Baker, A. B. Russell, D. R. Hall, “A Planar Waveguide Nd:YAG Laser Using Active  Q-Switching of a Hybrid Unstable Resonator”, IEEE J. Sel. Top. Quantum Electron. 13, 638 (2007). [4]  J. J. Degnan, D. R. Hall, Finite-Aperture Waveguide-Laser Resonators,” IEEE J. Quantum Electron. QE-9, 901 (1973). Case I:  All waveguide modes are efficiently coupled back to the waveguide core – ensuring a multi-mode beam in the transverse axis. Case II:  All the free-space modes are efficiently coupled back to the end-facet – an intercavity aperture is placed in the far field to select the fundamental mode – see for example [3]. We  have  developed  two  new  fabrication  methods  for  mode-selective  highly-curved  toroidal  laser  mirrors based on laser micromachining of cylindrical substrates that already have the small lateral curvature (RL > 1m) provided by conventional polishing. These mirrors are produced by a two-step CO2 laser machining process with high resolution ablative laser cutting [1] and then laser polishing [2], or by vaporization of silica in the CO2 laser smoothing process at higher than usual power. We have produced mirrors with RT = 32mm and 16mm radius of curvature in the transverse direction and 14mm lateral length for use with the 200µm core height Nd:YAG [3] and a 150µm core Yb:YAG planar waveguide laser respectively. Proof of principle of fabricated mirrors was confirmed in the mode-selective configuration (Case III) for the Yb:YAG planar waveguide laser. Low loss configurations for waveguide resonators We can distinguish three configurations (cases) for waveguide resonators with external mirrors [4], where the coupling losses of the fundamental mode between free-space and the waveguide facet are less than 1%. Features of the cylinder mirrors made by vaporization:  Small stress RoC in lateral direction, RL > 2m.  RoC in transverse direction RT from 10 to 45mm.  The mode-selective effect is less pronounced than in the laser-cut mirror but still useful.  Mirror  with  RT  =  16mm  matches  Case  III  coupling  for 150µm core height Yb:YAG planar waveguide laser.  Influence of CO2 laser power on mirror curvature  Features of the ablatively-cut toroidal mirrors:  Toroidal mirrors have 14mm long and 300µm wide optical aperture.  Transverse radii of curvature, RT = 20, 32 and 36mm.  Mirror radius in the lateral direction is well-defined, RL = 216mm.  The  surrounding  rims  of  reverse  curvature  and  side-grooves  designed  to disperse higher order laser modes.   Some low spatial frequency features in the lateral direction that may introduce beam distortion.   The toroidal mirror with RT = 32mm matches Case III coupling for 200µm core height Nd:YAG planar waveguide laser [3].   Mirror suitable for the unstable resonator configuration.  300µm Optical microscope image of a laser-cut resonator mirror Case III: The higher order modes coupled into free-space are returned into the core with decreasing efficiency – natural mode selectivity and improvement of the times diffraction-limit factor (M2) Conclusion Yb:YAG planar waveguide laser  with the pump and cooling system Multi-mode waveguide a Fundamental guided mode M2 = 1.14 in free space Case II z >> zR, R = z  (long radius cavity) z Case III z = zR = (p·a2)/(8·l) R = 2·zR   (confocal) Case I z small, R large (long radius cavity)  Objectives  The  use  of  mode-selective  slit-shaped  toroidal  mirrors  for  the unstable resonator configuration in planar waveguide or thin slab solid-state lasers.  Fabrication of highly-curved (RL/RT > 10) toroidal mirrors using a CO2  laser  for  the  mode-selective  waveguide  resonator configuration (Case III).   Investigation of effectiveness of the fabricated mirrors with the Yb:YAG/sapphire planar waveguide laser.  Beam images for the conventional mirrors 40-400mR40-400mR40-400mR40-400mR40-400mR40-400mR40-400mRRT = 16mm z = 8mm 40-400mR40-400mR40-400mRRT = 19mm z = 8mm Test setup for resonator mirrors Plane 80% Output Coupler Screen 150µm Yb:YAG waveguide section Displacement Test mirror Power meter (temporary)  Camera 250mm  13mm 8 10 12 14 16 18 20 9.40  9.45  9.50  9.55  9.60  9.65  9.70  9.75 Laser power, W Radius of curvature, mm 15.5mm cylinder mirror used in Case III:  95% reflective mirror placed 8mm from waveguide facet.  Output power reduced to 58W because of ~10W leakage from the test mirror.  Suppressed high order modes in the transverse direction.  Unsuitable for unstable resonator configuration – infinite aperture.  1m RoC spherical rear mirror used in Case I:  Mirror placed 0.5mm from waveguide facet.  70 W output power at 35A diode current.  No mode selectivity.  Visible group of at least two transverse modes.  Beam images for the ablatively-cut toroidal mirror (RT = 20mm, RL = 216mm)  Beam images for the cylinder mirrors made by vaporization with different RT  40-400mR40-400mR40-400mR40-400mROutput beam at different distance, z, between  mirror and waveguide facet  z = 8.0mm  z = 9.5mm  Transverse  curvature  slightly  smaller  than required for Case III configuration.  40W output power at 35A diode current.  Mode-selectivity in the transverse direction. References  We  have  developed  CO2  laser-based  fabrication  methods  that  are  capable  of  produce  highly-curved toroidal mirrors with the ratio of RL/RT > 10.  Fabricated mirrors successfully demonstrated mode-selectivity (Case III) in a planar waveguide laser.   Highly-curved slit-shaped toroidal mirrors can be used for the unstable resonator configuration in planar waveguide and thin slab solid-state lasers. 40-400mR40-400mR40-400mRRT = 14mm z = 8mm  Mirror mis-matched Case III coupling.  46W output at 35A.  Visible high order modes.  Mirror matches Case III  56W output at 35A.  Suppressed high order modes in  the transverse direction.  Transverse  RoC  larger  than required.  54W output at 35A.  High order modes appeared.  Appropriate for unstable resonators in planar waveguide or thin slab solid-state lasers.  

Mode-selective toroidal mirrors for unstable resonator planar waveguide and thin slab solid-state lasers

K. L. Wlodarczyk

I. J. Thomson

H. J. Baker

D. R. Hall

Crossref

The European Conference on Lasers and Electro-Optics Munich, Germany Mode-Selective Toroidal Mirrors for Unstable Resonator  Planar Waveguide and Thin Slab Solid-State Lasers Krystian L. Wlodarczyk, Ian J. Thomson, Howard J. Baker, Denis R. Hall School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, United Kingdom Summary CO2 laser fabrication of highly curved toroidal resonator mirrors Surface profile and cross-section of a resonator mirror fabricated by laser cutting and laser polishing Surface profile and cross-section of a resonator mirror made by vaporization in laser polishing process Technique 1: A two-step CO2 laser machining process with high-resolution ablative laser cutting (made by PowerPhotonic Ltd.) and then laser polishing. Technique 2:  Vaporization of fused silica in the laser polishing process at higher than usual laser power  Silica cylindrical lens with 240mm lateral RoC used as a substrate.  Desired shape and transverse curvature of a mirror obtained by precise ablative laser cutting.  Roughness of 300µm wide laser-cut mirror removed by localized laser polishing.  Lateral RoC is decreased by surface stress produced by polishing.  2mm thick planar fused silica used as a substrate.   300µm wide trench made by CW raster scan area laser polishing, but at higher than usual laser power.  Depth of desired profile is controlled by laser power used.  Surface roughness  ±20nm Ultra-low surface  roughness ±10nm Testing of fabricated mirrors with Yb:YAG/Sapphire planar waveguide laser [1] G. J. Markillie, H. J. Baker, F. Villarreal, D. R. Hall, “The effect of vaporization and melt ejection on laser machining of silica glass micro-optical components,” Applied Optics 41, 5660 (2002). [2]  K. M. Nowak, H. J. Baker, D. R. Hall, “Efficient laser polishing of silica micro-optical components,” Applied Optics 45, 162 (2006).  [3] J. Xu, I. J. Thomson, J. D. R. Valera, H. J. Baker, A. B. Russell, D. R. Hall, “A Planar Waveguide Nd:YAG Laser Using Active  Q-Switching of a Hybrid Unstable Resonator”, IEEE J. Sel. Top. Quantum Electron. 13, 638 (2007). [4] J. J. Degnan, D. R. Hall, Finite-Aperture Waveguide-Laser Resonators,” IEEE J. Quantum Electron. QE-9, 901 (1973). Case I: All waveguide modes are efficiently coupled back to the waveguide core – ensuring a multi-mode beam in the transverse axis. Case II: All the free-space modes are efficiently coupled back to the end-facet – an intercavity aperture is placed in the far field to select the fundamental mode – see for example [3]. We have developed two new fabrication methods for mode-selective highly-curved toroidal laser mirrors based on laser micromachining of cylindrical substrates that already have the small lateral curvature (RL > 1m) provided by conventional polishing. These mirrors are produced by a two-step CO2 laser machining process with high resolution ablative laser cutting [1] and then laser polishing [2], or by vaporization of silica in the CO2 laser smoothing process at higher than usual power. We have produced mirrors with RT = 32mm and 16mm radius of curvature in the transverse direction and 14mm lateral length for use with the 200µm core height Nd:YAG [3] and a 150µm core Yb:YAG planar waveguide laser respectively. Proof of principle of fabricated mirrors was confirmed in the mode-selective configuration (Case III) for the Yb:YAG planar waveguide laser. Low loss configurations for waveguide resonators We can distinguish three configurations (cases) for waveguide resonators with external mirrors [4], where the coupling losses of the fundamental mode between free-space and the waveguide facet are less than 1%. Features of the cylinder mirrors made by vaporization:  Small stress RoC in lateral direction, RL > 2m.  RoC in transverse direction RT from 10 to 45mm.  The mode-selective effect is less pronounced than in the laser-cut mirror but still useful.  Mirror with RT = 16mm matches Case III coupling for 150µm core height Yb:YAG planar waveguide laser.  Influence of CO2 laser power on mirror curvature Features of the ablatively-cut toroidal mirrors:  Toroidal mirrors have 14mm long and 300µm wide optical aperture.  Transverse radii of curvature, RT = 20, 32 and 36mm.  Mirror radius in the lateral direction is well-defined, RL = 216mm.  The surrounding rims of reverse curvature and side-grooves designed to disperse higher order laser modes.   Some low spatial frequency features in the lateral direction that may introduce beam distortion.   The toroidal mirror with RT = 32mm matches Case III coupling for 200µm core height Nd:YAG planar waveguide laser [3].   Mirror suitable for the unstable resonator configuration.  300µm Optical microscope image of a laser-cut resonator mirror Case III: The higher order modes coupled into free-space are returned into the core with decreasing efficiency – natural mode selectivity and improvement of the times diffraction-limit factor (M2) Conclusion Yb:YAG planar waveguide laser  with the pump and cooling system Multi-mode waveguide a Fundamental guided mode M2 = 1.14 in free space Case II z >> zR, R = z  (long radius cavity) z Case III z = zR = (p·a2)/(8·l) R = 2·zR   (confocal) Case I z small, R large (long radius cavity) Objectives  The use of mode-selective slit-shaped toroidal mirrors for the unstable resonator configuration in planar waveguide or thin slab solid-state lasers.  Fabrication of highly-curved (RL/RT > 10) toroidal mirrors using a CO2 laser for the mode-selective waveguide resonator configuration (Case III).   Investigation of effectiveness of the fabricated mirrors with the Yb:YAG/sapphire planar waveguide laser.  Beam images for the conventional mirrors 40-400mR40-400mR40-400mRRT = 16mm z = 8mm 40-400mRRT = 19mm z = 8mm Test setup for resonator mirrors Plane 80% Output Coupler Screen 150µm Yb:YAG waveguide section Displacement Test mirror Power meter (temporary) Camera 250mm 13mm 8 10 12 14 16 18 20 9.40 9.45 9.50 9.55 9.60 9.65 9.70 9.75 Laser power, W Radius of curvature, mm 15.5mm cylinder mirror used in Case III:  95% reflective mirror placed 8mm from waveguide facet.  Output power reduced to 58W because of ~10W leakage from the test mirror.  Suppressed high order modes in the transverse direction.  Unsuitable for unstable resonator configuration – infinite aperture.  1m RoC spherical rear mirror used in Case I:  Mirror placed 0.5mm from waveguide facet.  70 W output power at 35A diode current.  No mode selectivity.  Visible group of at least two transverse modes.  Beam images for the ablatively-cut toroidal mirror (RT = 20mm, RL = 216mm)  Beam images for the cylinder mirrors made by vaporization with different RT  40-400mR40-400mROutput beam at different distance, z, between  mirror and waveguide facet  z = 8.0mm z = 9.5mm  Transverse curvature slightly smaller than required for Case III configuration.  40W output power at 35A diode current.  Mode-selectivity in the transverse direction. References  We have developed CO2 laser-based fabrication methods that are capable of produce highly-curved toroidal mirrors with the ratio of RL/RT > 10.  Fabricated mirrors successfully demonstrated mode-selectivity (Case III) in a planar waveguide laser.   Highly-curved slit-shaped toroidal mirrors can be used for the unstable resonator configuration in planar waveguide and thin slab solid-state lasers. 40-400mRRT = 14mm z = 8mm  Mirror mis-matched Case III coupling.  46W output at 35A.  Visible high order modes.  Mirror matches Case III  56W output at 35A.  Suppressed high order modes in  the transverse direction.  Transverse RoC larger than required.  54W output at 35A.  High order modes appeared.  Appropriate for unstable resonators in planar waveguide or thin slab solid-state lasers.  

Mode-selective toroidal mirrors for unstable resonator planar waveguide and thin slab solid-state lasers

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