1. Single-Crystal YAG Fiber Optics for the Transmission of High Energy Laser Radiation B. Laustsen and J. A. Harrington Department of Material Science & Engineering Rutgers University Piscataway, NJ Web site: irfibers.rutgers.edu B. Laustsen and J. A. Harrington Department of Material Science & Engineering Rutgers University Piscataway, NJ Web site: irfibers.rutgers.edu

2. Single-Crystal (SC) Fiber Optics nCrystalline oxides: sapphire, spinel, YAG,.… nExcellent physical properties High temperature:M p > 2,000 o C Very hard and inert High laser damage threshold Transmission: Visible to about 5 µm nApplications: High power laser delivery Non-linear fibers: LiNbO 3 Active fibers nCrystalline oxides: sapphire, spinel, YAG,.… nExcellent physical properties High temperature:M p > 2,000 o C Very hard and inert High laser damage threshold Transmission: Visible to about 5 µm nApplications: High power laser delivery Non-linear fibers: LiNbO 3 Active fibers

3. SC Sapphire Fiber

4. SC Fiber Materials MaterialSymbolStructure Mp, oCMp, oCMp, oCMp, oC n @ 3 µm Sapphire Al 2 O 3 Rhombohedral20401.709 YAG Y 3 Al 5 O 12 Garnet – cubic 19401.788 GGG Gd 3 Ga 5 O 12 Garnet – cubic 20981.915 Spinel MgAl 2 O 4 Octohedral - cubic 21351.667

5. SC Fiber Optics nRelatively old technology Haggerty, MIT 1972 Bell Labs, Stanford, Univ. South Florida, Rutgers, Shasta, +… nFabrication methods: Laser Heated Pedestal Growth (LHPG) Edge-defined film fed growth (Saphikon, Inc.) nRelatively old technology Haggerty, MIT 1972 Bell Labs, Stanford, Univ. South Florida, Rutgers, Shasta, +… nFabrication methods: Laser Heated Pedestal Growth (LHPG) Edge-defined film fed growth (Saphikon, Inc.) EFGEFGLHPGLHPG CO 2 laser

6. Challenges & Advantages nCrystal pulling ≠ glass drawing nFiber diameter control difficult 300  m ± 1.5  m (0.5%) nCO 2 laser power stability 10 W ± 0.05 W (0.5%) typical nCladding difficult BUT nSmall amount of source material required SC or PC rods Ex: Lu 2 O 3 nSeed: SC fiber or Pt wire nCrystal pulling ≠ glass drawing nFiber diameter control difficult 300  m ± 1.5  m (0.5%) nCO 2 laser power stability 10 W ± 0.05 W (0.5%) typical nCladding difficult BUT nSmall amount of source material required SC or PC rods Ex: Lu 2 O 3 nSeed: SC fiber or Pt wire

7. IR Absorption Edge for Oxide Crystals

8. Bulk Loss at Key Wavelengths Crystal 4 µm absorption dB/m 5 µm absorption dB/m Sapphire – Al 2 O 3 18 410 WOW! YAG – Y 3 Al 5 O 12 5.5* 268 WOW! GGG - Gd 3 Ga 5 O 12 0.5*46 Yttria – Y 2 O 3 0.15*2.3 * Extrapolated from IR absorption edge

9. Laser Heated Pedestal Growth Laser power Source dia. Fiber dia. Growth rate Atmosphere 2 - 25 W 300 - 1000  m 100 - 500  m 1 - 4 mm/min Air, O 2, He Fiber Laser Mic Source CO 2 Laser

10. Rutgers LHPG System

11. LHPG Fiber Fabrication Focusing mirror Fiber Source rod Turning mirror Laser mic

12. Sapphire Fiber Optics Er:YAG

13. SC Sapphire Fiber Lowest loss at 3  m Lowest loss at 3  m n Longest length n Fiber diameter n Slow growth n Polymer coating Lowest loss at 3  m Lowest loss at 3  m n Longest length n Fiber diameter n Slow growth n Polymer coating 0.4 dB/m 0.4 dB/m 5 m 5 m 300 ± 1.5  m 300 ± 1.5  m 1 m in 8 hrs 1 m in 8 hrs FEP Teflon FEP Teflon 0.4 dB/m 0.4 dB/m 5 m 5 m 300 ± 1.5  m 300 ± 1.5  m 1 m in 8 hrs 1 m in 8 hrs FEP Teflon FEP Teflon

14. LHPG Growth – LabVIEW Control

15. Diameter Variations in YAG Fiber nNo diameter control nWith diameter control

16. CharacterizationCharacterization nOptical Loss nFTIR Green Visible (535 nm) Red Visible (635 nm) Nd:YAG (1064 nm) Er:YAG (2.94 μm)

17. Optical Measurement Set-up

18. YAG Source Rod n1 mm × 9 cm long

19. SC YAG Fiber n400 µm × 6 cm long

20. SC YAG Fiber nL = 65 cm, cutback method nFiber non-annealed LaserWavelength Loss, dB/m Green 535 nm 4.4 Red 635 nm 3.6 Nd:YAG 1064 nm 1.9 Er:YAG 3.0 µm 1.1

21. SC YAG Fiber nL = 65 cm, cutback method nFiber annealed LaserWavelength Loss, dB/m Green 535 nm 1.7 Red 635 nm 1.8 Nd:YAG 1064 nm 1.3 Er:YAG 3.0 µm 1.1

22. Effect of Annealing

23. Summary and Future Directions nSC oxide fibers - passive Potentially high laser damage threshold Transmission up to 5 µm Strong, robust, high temperature, inert nActive SC fibers Doped oxide crystal source rods Fiber lasers nDifficulties So far, no proper clad Slow fiber growth Cladding possibilities: nPost clad with glass nCore-clad preform nSC oxide fibers - passive Potentially high laser damage threshold Transmission up to 5 µm Strong, robust, high temperature, inert nActive SC fibers Doped oxide crystal source rods Fiber lasers nDifficulties So far, no proper clad Slow fiber growth Cladding possibilities: nPost clad with glass nCore-clad preform