This document summarizes the results of experiments scaling the wavelength range of Tm-doped fiber lasers pumped at 790nm. It discusses expanding the typical operating range of 1.95-2.08 microns to shorter wavelengths down to 1.91 microns. To achieve efficient lasing at shorter wavelengths, the fiber design was optimized to reduce reabsorption effects, such as using a large-mode area fiber with a high Tm concentration. Experimental lasers demonstrated over 50% efficiency operating at 1908nm and showed no instability up to 70W of output power. Reliability tests on optimized fiber compositions also showed no significant degradation over 500 hours.

1. Power scaling 790nm-pumped Tm-
doped devices from 1.91 to 2.13µm.
G. Frith, B. Samson, A. Carter, D. Machewirth, J. Farroni and K.
Tankala
22nd January, 2008
www.nufern.com

2. Motivation
• Pumping Tm-doped fibers at 790nm achieves higher overall
optical-to-optical efficiency than cascaded (Er:Yb pumped Tm)
pumping schemes.
– Such systems are typically limited to <30% optical-to-optical
efficiency and 12% electrical-to-optical.
• With high-efficiency, high-brightness pump sources becoming
available, we can now demonstrate E-O efficiencies exceeding
20%.
• Lasers operating at 1.9~2.1µm are of interest for medical,
chemical sensing and direct eye-safe applications as well as
providing an excellent basis for conversion into the mid and far-
IR.
• Tm-doped fibers are much more power scalable than Er:Yb for
eye-safe applications.
2

3. Presentation aims
The aim of this presentation is to answer some
common questions we receive about 790nm-pumped
Tm-doped fibers.
• What are the wavelength limitations?
• What about single polarization?
• What is the fiber reliability?
3

4. Wavelength operating range
λ(µm)
• The broad 3F4 3H6 emission bandwidth of Tm3+ extends from

around 1.5 to 2.2µm.
• Three fundamental factors limit the wavelength range for
efficient operation; reabsorption, gain and background loss.
• In Littrow cavity experiments, 790nm-pumped Tm lasers have
been demonstrated from 1860 to 2188nm. [1,2]
• Efficiencies of these experiments are often limited by external
cavity optics. Here we will compare the performance of
monolithic lasers between 1.91 and 2.13µm
[1] Sacks et al., “Long wavelength operation of double-clad Tm:silica fiber lasers” Proc SPIE 6453-74 (2007)
[2] Clarkson et al., High-power cladding-pumped Tm-doped silica fiber laser with wavelength tuning from 1860
to 2090nm”, Optics Letters, 27 pp. 1989-91 (2002)

5. Wavelength operating range
λ(µm)
Reabsorption increases
rapidly below 1.95µm
Typical absorption profile for
aluminosilicate Tm-doped
fiber

6. Wavelength operating range
λ(µm)
Gain becomes quite low
above ~2.08µm
Typical emission profile for
aluminosilicate Tm-doped
fiber

7. Wavelength operating range
λ(µm)
Background loss
becomes significant
above ~2.15µm
Theoretical background loss
for silica fiber

8. Wavelength operating range
λ(µm)
Normal operating region
Less attention to fibre and device
design required for efficient operation.

9. Wavelength operating range
λ(µm)
OC
HR R~15% nominal
795nm Pump taper
SM-TDF fibre
Experimental setup
• 790nm end-pump cavity based on 130µm fibre.
• Active fibre had 11.1µm MFD @ 2000nm, LP11 cutoff 1.96µm
and ~2dB/m absorption @ 795nm.
9

10. Wavelength operating range
λ(µm)
1.95
• 6m (12dB pump absorption) yielded ~50% efficiency.
• Lasers at 2000 and 2045nm showed similar efficiencies.
10

11. Wavelength operating range
λ(µm)
1.908
• Fibre had to be cut to 3.5m (7dB) to mitigate reabsorption
• Effect of reabsorption evident from efficiency v’s cavity finesse.
11

12. Wavelength operating range
λ(µm)
2.125
• Lower efficiency attributed to cavity finesse
• Onset of ASE seen at ~22W
12

13. Power scaling at shorter wavelengths.
Mitigation of reabsorption:
• The key is to maintain high inversion and limit number of active
ions in cavity. This may be achieved by:
– Core pumping – requires high-brightness pump source.
– Double-passing the pump – impractical for monolithic
cladding-pumped devices.
– Increasing the core-to-clad ratio.
13

14. Power scaling at shorter wavelengths.
• High core/cladding ratios help to mitigate reabsorption effects
however:
– Small claddings place excessive demands on diode
brightness.
– Large cores are not conducive to good mode control and
result in high operating thresholds.
– High core/cladding ratios combined with high active ion
concentrations result in high heat loads.
– High fiber temperatures introduce coating degradation
concerns.
– High core temperatures adversely effect cross-relaxation
efficiency.
– High core/cladding ratios leave little room for stress-rod
insertion for PM operation.
14

15. Power scaling at shorter wavelengths.
To better illustrate the effect of reabsorption:
• Using single-mode fiber with 2dB/m pump absorption, instability
was observed for fiber lengths longer than 3.5m when operating
at 1908nm (at 1950nm we used 6m).
• For a 25/400 fiber, this extrapolates to 1.5m or only 3dB pump
absorption leading to low overall efficiency.
• To obtain better efficiency the core/clad ratio must be increased.
• For 1908nm we developed a large mode area (LMA) fiber with
22µm MFD in 250µm cladding.
• Fiber also incorporated a relatively high Tm-concentration for
optimized cross-relaxation.
• Resultant fiber had ~6dB/m absorption.
15

16. 1908nm MOPA.
• 5W seed at 1908nm (as shown previously).
• 1.7m of LMA fiber counter-pumped with ~130W.
• Fibre mounted on 90mm mandrel with helically cut U-shape
channel for highly effective heat removal.
1.7m length of LMA
MO: 5W @ 1908nm Tm-doped fiber
Mode stripper
795nm
pump
FBGs 2+1:1 combiner
Cladding light
stripper Fiber coupled 792nm
pump modules (2×65W)
16

17. 1908nm MOPA.
• 70W output, pump power limited.
• 53% slope efficiency - artificially low due to diodes shifting off
wavelength (9dB at threshold to 6dB at full power).
• Thermal modeling suggests >100W should be possible before
coating degradation becomes a concern.
17

18. Latest generation LMA Tm-doped fibres
• High Tm concentration cores for high efficiency
• Raised refractive index pedestal to lower the effective core NA
for robust single mode operation.
• Panda stress rods inserted for PM operation.
– Managing 4 different CTE’s requires careful fibre design and
manufacture.
Pedestal
Stress member
Outer
Cladding
Core
18

19. 25/400µm Tm Amplifier
5W @ 2050nm 6+1:1 ~5m length of LMA Tm-
Combiner doped fiber (25/400)
793nm
pump
FBGs Connectorised
endcap assembly
Fiber coupled 795nm
pump modules (6×30W)
19

20. Amplifier performance
• 5W seed @ 2050nm
• 176W coupled pump
• 100.3W output
• Near diffraction limited
beam quality.
FF beam image from PLMA-TDF-25/400 amp
20

21. PLMA-TDF-25/400 performance
• Identical (if not slightly
higher) performance to
regular LMA.
• Birefringence ~2.5×10-4
• PER measurements
pending new polarizers.
21

22. 500 hour test
• New fiber compositions have been designed to maximize cross-
relaxation whilst minimizing energy transfer upconversion.
• 20W laser operating at 1950nm pumped at 792nm
Extrapolated time for 10%
degradation (pump + fibre)
is ~2k hours.
22

23. Conclusions
• Power scaling at wavelengths outside the range of
1.95~2.08µm require specific attention to fiber and device
design to maintain efficient operation.
• We have demonstrated a practical example of how high
efficiency at shorter wavelengths may be achieved.
• 790nm-pumped fibers have to potential to photo-darken
through exposure to visible/UV light generated by energy
transfer upconversion.
• We have shown here that current fibers do not “drop like a
rock”.
• By now applying the lessons we have learnt from improving
photo-degradation in Yb-doped fibers, we believe device
lifetimes should be extendable to tens of thousands of hours.
23