Topic report 11/09/29 Brightness conversion efficiency from LD to fibers Speaker: Chieh-Wei Huang Advisor: Sheng-Lung Huang Solid-State Laser Crystal and Device Laboratory

What is “Brightness” In the context of laser technology, the brightness of a laser source (in a quantitative sense) is generally understood as being equivalent to its radiance, which is the total power divided by the product of the mode area in the focus and the solid angle in the far-field, the units are then usually: W‧cm−2‧sr−1 For a diffraction-limited beam with moderate divergence (so that the paraxial approximation is valid), the brightness can be expressed as: 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Why we need the high brightness source? High brightness becomes more and more important in diode laser applications for fiber laser pumping and materials processing. For OEM customers fiber coupled devices have great advantages over direct beam modules: the fiber exit is a standardized interface, beam guiding is easy with nearly unlimited flexibility. In addition to the transport function the fiber serves as homogenizer: the beam profile of the laser radiation emitted from a fiber is symmetrical with highly repeatable beam quality and pointing stability. However, efficient fiber coupling requires an adaption of the slow-axis beam quality to the fiber requirements. Diode laser systems based on standard 10mm bars usually employ beam transformation systems to rearrange the highly asymmetrical beam of the laser bar or laser stack. These beam transformation systems (prism arrays, lens arrays, fiber bundles etc.) are expensive and become inefficient with increasing complexity. This is especially true for high power devices with small fiber diameters. On the other hand, systems based on single emitters are claimed to have good potential in cost reduction. Brightness of the inevitable fiber bundles, though, is limited due to inherent fill-factor losses. 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Two basic arrangements are used for most commercial fiber laser designs 2018/5/27 Solid-State Laser Crystal and Device Laboratory

General survey LD (Single emitter) to fiber LDA to fiber Multi single emitter end pump configuration Multi-bar or Multi-mini bar end pump configuration LD pigtail fiber+ tapered fiber bundle coupler+ Clad-adhesion side pump Coupling power N/A Brightness Brightness conversion efficiency 2018/5/27 Solid-State Laser Crystal and Device Laboratory

JDSU module LD to fiber Wavelength: 9XX nm Fiber core: 105 mm Fiber NA: 0.22 Output power: 2 W Coupling efficiency: 50% LD brightness: 9.29 MW /cm2-steradian Coupled fiber brightness: 0.15 MW /cm2-steradian Brightness conversion efficiency: 1.6% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Typical configuration of laser diode array LDA to fiber 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Prism beam shaping mechanism LDA to fiber Wavelength: 9XX nm Fiber clad: 400 mm Fiber NA: 0.22 Output power: 40 W Coupling efficiency: 70% LDA brightness: 5.5 MW /cm2-steradian Coupled fiber brightness: 0.206 MW /cm2-steradian Brightness conversion efficiency: 3.7% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

nLIGHT module (1) Multi-single emitter to fiber Wavelength: 940 nm Fiber clad: 200 mm Fiber NA: 0.2 Total emitters: 72 Output power: 700 W Coupling efficiency: 50% LD brightness (for each): 20.9 MW /cm2-steradian Coupled fiber brightness: 17.7 MW /cm2-steradian Brightness conversion efficiency: 1.2% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

nLIGHT module (2) Multi-single emitter to fiber Wavelength: 979 nm Fiber clad: 105 mm Fiber NA: 0.12 Total emitters: 10 Output power: 80 W Coupling efficiency: 50% LD brightness (for each): 20.9 MW /cm2-steradian Coupled fiber brightness: 20.3 MW /cm2-steradian Brightness conversion efficiency: 9.7% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

nLIGHT module (3) Multi-single emitter to fiber Wavelength: 9XX nm Fiber clad: 105 mm Fiber NA: 0.15 Total emitters: 14 Output power: 100 W Coupling efficiency: 40% LD brightness (for each): 20.9 MW /cm2-steradian Coupled fiber brightness: 20 MW /cm2-steradian Brightness conversion efficiency: 6.8% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Step-mirrors coupler Multi-bar to fiber Coupling efficiency 67% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Spatial multiplexing (wavelength multiplexing) Multi-bar to fiber 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Spatial multiplexing Multi-bar to fiber Wavelength: 9XX nm Fiber clad: 400 mm Fiber NA: 0.22 Cascade for 12 stack Output power: 480 W Coupling efficiency: 65% LDA brightness (single): 5.5 MW /cm2-steradian Coupled fiber brightness: 2.5 MW /cm2-steradian Brightness conversion efficiency: 3.8% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Beam twister coupler Multi bar to fiber 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Results of beam twister coupler Multi bar to fiber Coupling efficiency is 65% into a 200 mm fiber 2018/5/27 Solid-State Laser Crystal and Device Laboratory

One M-block module Multi-mini bar to fiber 6 mini-arrays of 4 emitters each 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Stack M-block modules Multi-mini bar to fiber 10 M-blocks total 60 mini array 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Stack M-block modules to fiber Multi-mini bar to fiber 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Coupling power and coupling efficiency measurement Multi-mini bar to fiber Wavelength: 9XX nm Fiber clad: 400 mm Fiber NA: 0.22 Stack for 10 M-block 60 mini array 240 emitters Output power: 1000 W Coupling efficiency: 50% LD brightness (single emitter): 10.9 MW /cm2-steradian Coupled fiber brightness: 5.15 MW /cm2-steradian Brightness conversion efficiency: 0.2% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Coupling efficiency: 58.5% LD brightness (single emitter): LD pigtail fiber + tapered fiber bundle coupler + Clad-adhesion side pump scheme Wavelength: 9XX nm Fiber clad: 220 mm Fiber NA: 0.22 7 LD single emitters Output power: 100 W Coupling efficiency: 58.5% LD brightness (single emitter): 20.9 MW /cm2-steradian Coupled fiber brightness: 1.7 MW /cm2-steradian Brightness conversion efficiency: 1.16% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

General survey LD (Single emitter) to fiber LDA to fiber Multi single emitter end pump configuration Multi-mini bar end pump configuration LD pigtail fiber+ tapered fiber bundle coupler+ Clad-adhesion side pump Coupling power 2W 40W 700W 1000W 100W Brightness 0.15 MW/cm2-steradian 0.206 MW/cm2-steradian 17.7 5.15 1.7 Brightness conversion efficiency 1.6% 3.7% 1.2% 0.2% 1.16% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Grating coupler Wavelength: 976 nm Fiber clad: 400 mm / 105 mm Fiber NA: 0.22 1 LDA with 19 emitters Output power: 20 W, 20 W Coupling efficiency: 50%, 50% LDA brightness (for each emitter): 3.7 MW /cm2-steradian Coupled fiber brightness: 0.103 MW /cm2-steradian 5.11 MW /cm2-steradian Brightness conversion efficiency: 2.78 % 7.2% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Compare with others The factor will influence the brightness is “coupling efficiency” after stack multi-stage LD (Single emitter) to fiber LDA to fiber Multi single emitter end pump configuration Multi-mini bar end pump configuration LD pigtail fiber+ tapered fiber bundle coupler+ Clad-adhesion side pump Grating coupler Coupling power 2W 40W 700W / 100W 1000W 100W 20W/ 20W (105mm) / 200W Brightness 0.15 MW/cm2-steradian 0.206 MW/cm2-steradian 17.7 / 20 5.15 1.7 0.103/5.11/10.3 Brightness conversion efficiency 1.6% 3.7% 1.2% / 6.8% 0.2% 1.16% 2.78 % 7.2% 2018/5/27 Solid-State Laser Crystal and Device Laboratory

Thanks for your attention! 2018/5/27 Solid-State Laser Crystal and Device Laboratory