Published on January 28, 2008
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
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
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
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  Sacks et al., “Long wavelength operation of double-clad Tm:silica fiber lasers” Proc SPIE 6453-74 (2007)  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 λ(µm) Reabsorption increases rapidly below 1.95µm Typical absorption profile for aluminosilicate Tm-doped fiber
Wavelength operating range λ(µm) Gain becomes quite low above ~2.08µm Typical emission profile for aluminosilicate Tm-doped fiber
Wavelength operating range λ(µm) Background loss becomes significant above ~2.15µm Theoretical background loss for silica fiber
Wavelength operating range λ(µm) Normal operating region Less attention to fibre and device design required for efficient operation.
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
Wavelength operating range λ(µm) 1.95 • 6m (12dB pump absorption) yielded ~50% efficiency. • Lasers at 2000 and 2045nm showed similar efficiencies. 10
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
Wavelength operating range λ(µm) 2.125 • Lower efficiency attributed to cavity finesse • Onset of ASE seen at ~22W 12
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
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
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
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
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
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
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
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
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
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
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
Power scaling 790nm-pumped Tm-doped ... performance of monolithic lasers between 1.91 and 2.13µm λ(µm ... High-power cladding-pumped Tm-doped silica ...
... (Tm)-doped silica and power scaling studies of fiber lasers ... the Laserline devices to pump ... nm-pumped Tm lasers from 1.91 to 2.13 µm, ...
Developments in thulium-doped fiber lasers ... the power scaling of Tm-doped lasers has rapidly ... compositions shows device lifetimes of several ...
... Fundamentals and Power Scaling. Peter F ... Using a fiber having a 25-µm ... efficiencies of the shorter-wavelength devices. Tm-doped fibers are ...
2 µm Laser Sources and ... In 2006 Kelemen et al. developed and characterised high-power 1.91 µm ... 1989 Spectroscopic and Laser Properties of Tm-doped ...
Nature Photonics Volume ... from multiphonon emission may limit power scaling to ... power cladding-pumped Tm-doped silica fiber laser with ...