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
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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.
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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?
Wavelength operating range
• 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
λ(µm)
[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)
Wavelength operating range
Reabsorption increases rapidly below 1.95µm
λ(µm)
Typical absorption profile for aluminosilicate Tm-doped fiber
Wavelength operating range
Gain becomes quite low above ~2.08µm
λ(µm)
Typical emission profile for aluminosilicate Tm-doped fiber
Wavelength operating range
Background loss becomes significant above ~2.15µm
λ(µm)
Theoretical background loss for silica fiber
Wavelength operating range
Normal operating region Less attention to fibre and device
design required for efficient operation.
λ(µm)
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SM-TDF fibre
HR 795nm
OC R~15% nominal
Pump taper
Wavelength operating range
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.
λ(µm)
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Wavelength operating range
• 6m (12dB pump absorption) yielded ~50% efficiency. • Lasers at 2000 and 2045nm showed similar efficiencies.
1.95
λ(µm)
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Wavelength operating range
• Fibre had to be cut to 3.5m (7dB) to mitigate reabsorption • Effect of reabsorption evident from efficiency v’s cavity finesse.
1.908
λ(µm)
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Wavelength operating range
• Lower efficiency attributed to cavity finesse • Onset of ASE seen at ~22W
2.125
λ(µm)
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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.
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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.
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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.
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1908nm MOPA.
2+1:1 combiner
1.7m length of LMA Tm-doped fiber
Fiber coupled 792nm pump modules (2×65W)
795nm pump
MO: 5W @ 1908nm
FBGs Cladding light
stripper
Mode stripper
• 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.
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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.
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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
Core
Stress member
Outer
Cladding
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25/400µm Tm Amplifier
Connectorised endcap assembly
6+1:1 Combiner
~5m length of LMA Tm-doped fiber (25/400)
Fiber coupled 795nm pump modules (6×30W)
793nm pump
5W @ 2050nm
FBGs
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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
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PLMA-TDF-25/400 performance • Identical (if not slightly
higher) performance to regular LMA.
• Birefringence ~2.5×10-4
• PER measurements pending new polarizers.
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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.
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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.