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.