1. 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

2. 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

3. 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

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

5. 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

6. 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

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

8. 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

9. 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

10. 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

11. 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

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

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

14. 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

15. Beam twister coupler Multi bar to fiber
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Solid-State Laser Crystal and Device Laboratory

16. 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

17. One M-block module Multi-mini bar to fiber
6 mini-arrays of 4 emitters each
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Solid-State Laser Crystal and Device Laboratory

18. Stack M-block modules Multi-mini bar to fiber
10 M-blocks total 60 mini array
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Solid-State Laser Crystal and Device Laboratory

19. Stack M-block modules to fiber Multi-mini bar to fiber
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Solid-State Laser Crystal and Device Laboratory

20. 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

21. 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

22. 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

23. 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

24. 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

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