Advisor: Sheng-Lung Huang Speaker: Sheng-Feng Chen 1

Outline Double clad fibers Fiber Lasers High-power Fiber Lasers Recent developments Related papers 2

Double-clad fibers Def: optical fibers with different waveguide structures for pump and signal light cladding pumping 3

Double-clad Fiber Design propagation modes (inner cladding / core) → incomplete absorption limits gain and power efficiency non-circular shape, fusion splicing fibers problems 4

Polymer outer cladding higher NA of the inner cladding but cannot tolerate very high temperatures higher pump light propagation losses all-glass designs are often preferred for high output powers. 5

Applications cladding-pumped high-power fiber lasers NA)c=0.1,NA)i=0.6 → laser diode efficient coupling high power conversion efficiency (sometimes > 80%) output beam quality can be diffraction-limited 6

Fiber Lasers Def: lasers with a doped fiber as gain medium, or (sometimes) just lasers where most of the laser resonator is made of fibers the gain medium is a fiber doped with (Er 3+ ), (Nd 3+ ), (Yb 3+ ), (Tm 3+ ), or (Pr 3+ ). 7

Special Attractions of Fibers as Laser Gain Media High gain (~0.5dB/mW) compact fiber Bragg gratings and fiber couplers all-fiber setup avoiding free-space optics and any requirement for alignment 8

High-power Fiber Lasers Def: fiber lasers with high output powers of e.g. > 10 W high surface-to-volume ratio(400 → 10 cm 2 /cm 3,50m-long) avoid thermal effect: 1.thermal stress leads to fracture 2.Φ(quantum efficiency ↓ ) 3.Distortion of refraction index 4.Outer cladding(soft fluoro-polymer) chemical instability 9

Launching the Pump Light fiber-coupled pump diodes pump light into passive (undoped) fibers wound around the active fiber several pump fibers and a single active signal fiber are fused together side-pumped fiber coils 10

limitations optical intensities damage the material → increased mode areas / high beam quality. power dissipation per unit length 100 W/m air-cooled fibers → water cooling longer fiber with lower doping concentration → cooling easier / nonlinearities. high concentration/propagation loss ↑ (heat),absorb ↓ 11

Recent developments achieved diffraction-limited beam powers from diode- pumped solid-state lasers. Yb-doped fiber lasers (2001: 100 W → 20 kW) Commercial single-mode lasers have reached 10 kW in CW power. 12

Related papers 13

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Conclusion Diffraction-limited beam quality Power Efficiency Outstanding heat-dissipation capability Compact size and robustness Limited power(limits pulse regime, where peak powers exceed few hundred of kilowatts. 15

Thanks for your listening! Ref: ml 16