1. Spectroscopic studies of the influence of the aluminum concentration in heated Yb-doped optical fibers
Christoph Bacher 1
Jonas Scheuner 1
Ali El Sayed 1,2
Sönke Pilz 2
Alex Heidt 1
Manuel Ryser 1
Valerio Romano 1,2
1 IAP, University of Bern, Bern
2 ALPS, Bern University of Applied Sciences, Burgdorf

2. Introduction and motivation
Our group
Our group has two main research fields
Development, fabrication and characterization of optical fibers
Development and design of fiber laser systems

3. Introduction and motivation
High power yellow ( nm) laser
Generation of laser light at a wavelength of 1160 nm is required
Yb-doped fiber lasers at 1160 nm have been demonstrated at Watt level
Yb-doped fibers
+ Commercially available
+ FBG cavity allows narrow banded spectrum
- Not efficient laser emission at 1160 nm
- ASE
- Heating required to suppress ASE (our setup: 155 °C)
It has been reported, that a high aluminum concentration in Yb-doped fibers allow laser emission at 1160 nm at fiber temperatures of 60 °C 1
 Development and fabrication of our own Yb-Al-doped fiber
(Image: Laurent Michaille)
1 Jacquemet, M. et al. (2009)

4. Sol-gel based granulated silica method
Granulated-in-tube method combined with sol-gel process
Sol-gel
grain size ~ nm - μm
every grain
is doped
Granulate-in-tube
granulate-in-tube
preform
cladding drawn
directly from
granulate
laser-based
travelling small-zone
vitrification for
loss reduction
fiber
sintering & milling
removal of OH-groups
grain size ~ 100 μm
Dopants in Ethonal -> mixing
Gelation starts when adding distilled water, evtl. warm up a little bit
Wird zum Pulver getrocknet.
Sintern bis 1550 °C: Pulver verdichten ->Granulat. Mit anschliessendem Mahlen kann Teilchengrösse definiert werden. Für UniBe 150 um – 1 mm.
Vitrifizierung ermöglicht bessere Qualität des Kern. Setzt die Streu- und Propagationsverluste herunter, auf nm

5. Fabrication process of our fiber
Core precursor composition
Vitrification process of core was not performed by laser, but by drawing a drop
Precursor name
Chemical formula
At. %
Ytterbium(III) nitrate pentahydrate
Yb(NO3)3 · 5H2O
0.4
Phosphorus pentoxide
P2O5
2.4
Aluminum nitrate nonahydrate
Al(NO3)3 · 9H2O 4
Vitrifizierungsmaschine kurz erklären, sagen dass Maschine zu diesem Zeitpunkt noch nicht in Bern war

6. Fabrication process of our fiber
Cladding and drawing
40 minutes fiber drawing at 2090 °C  200 m Yb-doped double clad fiber
granulate-in-tube
preform
cladding drawn
directly from
granulate

7. Fiber characterization
Refractive index profile
Index step core/cladding:
5.1317E-3 (err: E-5)
NA core
(err: E-3)

8. Fiber characterization
Lifetime measurement
Lifetime Ƭ = 0.74 ± ms (Commercial: Ƭ = 0.8 ± ms)

9. Fiber characterization
Laser characteristics
1 Measurements taken at University of Bern, Bern
2 Measurements taken at Bern University of Applied Sciences, Burgdorf
Double pass 1
Single pass 2
Pump wavelength
976 nm
940 nm
Fiber length
5 m
2 m
Slope efficiency
4-6 %
0.5 %
Laser threshold
3.5 W
6 W
Temperature
25 °C

10. Fiber characterization
Observations
Heating up the fiber causes a shift in emission as well as absorption spectrum
No lateral scattering was detected with IR viewer
Fiber heats itself up, energy taken from absorbed pump

11. Conclusions
An Yb-doped, high Al-co-dopant double clad fiber was drawn in our in-house fiber drawing tower
Lifetime measurements show similar values to commercial fibers
Could be due to Yb-clustering leading to dipole-dipole interactions
Laser emission was achieved for several pump wavelengths
Heating the fiber shifts emission and absorption spectrum (like commercial fibers do)
Fiber absorbs approx. 60 % of pump power, most of it is converted to heat
No significant increase of spectral emission at wavelengths above 1100 nm was found

12. Outlook
Addition experiments and characterization processes are on progress
Mass spectroscopy
Loss measurements
Sol-gel method allow easily adjustment of dopant concentration
Composition of dopants needs improvement
More efficient lasing
Optimal self heating of the fiber in order to shift the emission spectrum to longer wavelengths

13. Acknowledgments University of Bern Daniel Weber Andres Luder
Prof. Thomas Feurer
BFH
David Kummer
Christian Heger
Dr. Hossein Najafi
Dr. Andreas Burn
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