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We have demonstrated ultra-high efficiency amplification in Tm-doped fiber with both core- and cladding-pumped configurations using a resonant tandem-pumping approach. These Tm-doped fiber amplifiers are pumped in-band with a 1908 nm Tm-doped fiber laser and operate at 1993 nm with >90% slope efficiency. In a core-pumped configuration, we have achieved 92.1% slope efficiency and 88.4% optical efficiency at 41 W output power. In a cladding-pumped configuration, we have achieved 123.1 W of output power with 90.4% optical efficiency and a 91.6% slope efficiency. We believe these are the highest optical efficiencies achieved in a Tm-doped fiber amplifier operating in the 2-micron spectral region.
© 2014 Optical Society of America
1. Introduction
The efficiencies achieved by fiber amplifiers have enabled significant power scaling. Yb-doped fiber amplifiers have achieved 1.5 kW of output power with 85-90% slope efficiency and spectrally narrow (<10 GHz) output by pumping with 975nm diodes [1]. Alternatively, 10 kW of single-mode output has been achieved with broader linewidth using tandem pumping at 1018 nm [2]. Yb-free Er-doped fibers using resonant pumping at 1470 nm and 1530 nm and emitting at 1560-1590 nm have shown ~85% efficiency using core pumping and have also been scaled to 88 W using cladding-pumping with 69% efficiency [3,4]. For >2-micron output, Tm doped fibers have reached the 1 kW level [5], and Ho-doped fibers are currently at the 407 W level [6]. In all cases, further power scaling is ultimately limited by thermal effects due to heating of the fiber acrylate coating, thermally-induced mode coupling [7,8], nonlinear effects [9,10], and pump brightness.
Power scaling of diode-pumped Yb-doped fiber amplifiers is currently limited by the availability of high-brightness pump sources. With the current state-of-the-art diode technology, Nufern has demonstrated 1.5 kW with narrow linewidth and high slope efficiency [1]. IPG has shown that by tandem pumping with Yb-doped fiber lasers operating at 1018 nm, 10 kW can be achieved, albeit with spectrally broad output [2]. This is the current high power mark for single-mode fiber lasers. Further power scaling will most likely need to address pump absorption issues at high power levels as well as thermal and nonlinear limitations.
Yb-free Er fiber lasers are attractive due to their ability to generate wavelengths in the 1550-1600 nm region. The lack of Yb as a sensitizer is advantageous to eliminate 1-micron ASE and enables resonant pumping [3,4]. However, this requires the use of InP diodes operating at 1470 nm or 1530 nm. Unfortunately, the overall output power and brightness of these diodes are significantly lower than typical 976 nm diodes. This is one reason why further power scaling has not been realized.
Recently, Ho-doped fibers gained significant attention, as they enable operation in an excellent atmospheric window around 2.1-microns. Demonstrations of resonantly-pumped Ho-doped fibers lasers and amplifiers exploit tandem pumping using 1950 nm Tm-doped fiber lasers. This tandem pumping approach eliminates diode brightness issues when power scaling, as diffraction-limited fiber lasers are orders of magnitude brighter than even the brightest multimode pump diode. However, efficiency is currently limited to ~60% due to silica and OH losses [6]. This unexpectedly low efficiency can limit significant power scaling due to thermal heating of the fiber acrylate coating (as opposed to pump-source brightness).
For applications requiring high power in the 2-micron band, thulium-doped fibers offer several advantages, including: low propagation losses due to the short wavelength of operation in comparison to holmium; windows of good atmospheric propagation [11]; and a wide range of pumping approaches. Although these benefits allow for a significant amount of output power with moderate efficiency, power scaling is ultimately limited by thermal effects.
Other than ytterbium, thulium is the only other ion in silica-doped fiber which has reached the 1 kW power level [5]. However, this amplifier was only ~50% efficient [5], resulting in ~1 kW of waste heat generation in the fiber. Improving this efficiency will be essential for the scaling of Tm-doped fiber output power.
For Tm-doped fiber to approach the power levels of Yb-doped fibers, two issues must be addressed: pump brightness and efficiency. 793 nm diodes are not as bright or efficient as 976 nm diodes used for Yb. In addition, because of the large quantum defect from 793 nm to >2000 nm, the efficiency of a diode-pumped approach is fundamentally limited. Granted, efficiencies higher than the quantum defect can be achieved as a result of the 2-for-1 cross-relaxation process, but these are still limited to ~60% at high powers [11–13]. In order to scale to significantly higher output power, brighter pump sources must be used, and the quantum defect must be reduced. Both of these can be address by tandem pumping with a fiber laser.
We recently reported the first demonstration of a resonantly tandem-pumped Tm-doped fiber laser, showing >88% slope efficiency using two different fibers with different doping concentrations and geometries [14]. This novel pumping approach yields two distinct benefits over conventional 793 nm diode pumping: (1) ultra low quantum defect resonant pumping (1908 nm to 2000 nm has a quantum defect of only 4.6%) and (2) use of a high brightness single-mode fiber laser pump source. The initial experiments in [14] showed the feasibility of resonant tandem pumping of a Tm-doped fiber with another Tm-doped fiber laser. In this paper, we use the tandem-pumping approach in an amplifier configuration and with significantly higher power. We compare core- and cladding-pumping and show that this is a viable and practical way to scale Tm-doped fiber output power while minimizing excess heat generation.
The thulium fiber emission and absorption cross-sections are shown in Fig. 1. The common pumping regions are shown along with the absorption cross-sections at those wavelengths. For high power operation, 793 nm pumping has been the method of choice due to the cross-relaxation process and availability of mature high power diodes [5,11–13,15]. Alternatively, tandem pumping at 1560 nm has achieved >400 W of output power when using an Er:Yb-doped fiber laser as a pump source, with high efficiency [16]; however, the overall efficiency is still about the same as 793 nm pumping due to the 2-for-1 cross-relaxation process [11–13,15]. Regardless of these approaches, both are fundamentally limited in efficiency, and thus power scaling, due to the large quantum defect. 1908 nm pumping, as demonstrated in [14] and discussed in this paper, is a very attractive pumping region due to the low quantum defect. In addition, high power 1908 nm light is easily generated in Tm-doped fiber lasers.
Fig. 1 Absorption and emission cross-sections of Tm-doped fiber based on measured data and [11–15, 17–19]. Dashed lines indicate common pumping regions for Tm-doped fibers.
The advantages of low quantum defect pumping at 1908 nm are obtained at the expense of pump absorption. As seen in Fig. 1, the absorption cross-section is ~37 times lower at 1908 nm than at 793 nm [11–15,17–19], but the low quantum defect allows for very high optical efficiencies. Since 1908 nm diodes are immature, inefficient, low power, and expensive, Tm-doped fiber lasers become an obvious choice for generating high power 1908 nm pump light.
Other proposed tandem pumping architectures recommend pumping near the peak of the thulium absorption using either Er:Yb fiber lasers or cladding-pumped Raman lasers operating in the 1560-1700nm region [16,20]. However, we use the 1908 nm region because of the direct transition in Tm-doped fiber, allowing for high power output with good efficiency. The trade-off with our pumping approach is the reduced pump absorption. The advantages of high-brightness and low quantum defect resonant pumping outweigh the compromise in reduced pump absorption, as discussed in this paper.
2. Fibers tested
In these experiments, we used two different Tm-doped fibers from Nufern with identical geometries, but with different doping concentrations. Both fibers have a nominal 25 µm core diameter, a 45 µm pedestal diameter, and a 250 µm cladding diameter. These fibers were designed for 793 nm pumping. To achieve the high efficiency with 793nm pumping, the core of the fiber must be doped with alumina in order to achieve the 2-for-1 cross-relaxation effect [21]. As a result, the index of refraction of the fiber core increases significantly. To achieve single-mode operation given the high alumina doping, Nufern places an intermediate doped region (or pedestal) around the core. The pedestal is doped to have a slightly lower index than the core, allowing for the core to have a numerical aperture of 0.1, which is lower than if the pedestal was not present. The pedestal is essentially an intermediate cladding defining the waveguiding properties of the core and can be exploited as a multi-mode waveguide. Cross-sectional views of these fibers are shown in Fig. 2. For the remainder of this paper, we will refer to the low concentration fiber as TDF-LC and the high concentration fiber as TDF-HC.
Fig. 2 Cross-sectional views of both fibers used in this testing. (a) Low concentration fiber (TDF-LC); (b) high concentration fiber (TDF-HC).
For our core-pumped experiments, we coupled light into the 25 µm core only. Traditional cladding-pumped experiments could not be performed using the 250 µm diameter cladding, as the low-index acrylate coating would burn if exposed to high power 1908 nm light. To simulate cladding pumping, we coupled the pump light into the pedestal region. This pedestal pumping allowed the use of commercially available fibers for this testing. In this case, consider the pedestal to be an inner pump cladding, and the 250 µm cladding to be an outer cladding, similar to a pumping technique used in [22].
During our initial testing of the resonantly tandem-pumped thulium oscillators, we observed a significant amount of pump bleaching, resulting in lower overall pump absorption compared to the ground-state absorption [14]. This required the use of longer fiber lengths to achieve efficient pump absorption. Prior to this testing, we repeated the pump transmission measurements using the fibers shown in Fig. 2. The experimental setup is shown in Fig. 3.
In the setup, we focus the 1908 nm pump laser into the core of an undoped, pedestal-free 25/250 fiber. The undoped fiber was stripped of cladding modes to remove any 1908 nm light which is inadvertently launched into the cladding. This ensures that only core light is present. This undoped fiber was then fusion spliced onto a short length of each Tm-doped fiber so we could record the pump transmission as a function of pump power/intensity. Both ends of the fibers were angle cleaved at 8 degrees to prevent parasitic lasing. The pump transmission results for both the TDF-LC and TDF-HC are shown in Fig. 4.
Fig. 4 Pump transmission versus pump intensity at 1908 nm. Blue dots are measured values for TDF-LC; blue dashed line is a fit to the data. Red squares are measured values for TDF-HC; red dotted line is a fit to the data.
As expected, the TDF-HC fiber does not bleach as much as the TDF-LC fiber for the same pump intensities due to the higher doping concentration. By fitting these as we did in [14], we are able to estimate the lifetime, doping concentration, and pump absorption cross-section for each fiber. This allows us to calculate the ground-state pump absorption in the Tm-doped core in the absence of bleaching. These are summarized in Table 1 and are consistent with previously reported values [11–13,17,18].
Table 1. Calculated Tm-doped Fiber Parameters Based on Pump Transmission Measurements
3. Core pumping
Our initial demonstration of a tandem-pumped oscillator was an all-fiber configuration which utilized core pumping [14]. This technique allowed us to characterize the performance using commercially available parts. In similar fashion, we decided to begin these amplifier experiments with core-pumping as it allows us to optimize the test setup, measure performance, and easily swap fibers without changing our seed or pump coupling efficiency.
Due to the lack of available pump combining components in the 2-micron spectral region, we setup a free-space configuration in order to combine the pump and signal light. This is shown in Fig. 5. In this setup, the seed laser is a 1993 nm Tm-doped fiber laser, built in-house. It is a conventional 793 nm pumped fiber laser generating several Watts of output power. The output from the 1993 nm seed laser is spliced directly to an optical isolator. This prevents feedback from the amplifier from destabilizing the 1993 nm oscillator cavity. The output from the oscillator is collimated using an infrasil aspheric lens and passed through a narrow bandpass filter. This filter has a narrow (~2 nm) bandpass and transmits >90% of the 1993 nm seed light while reflecting >99.8% of the 1908 nm pump light. The 1908 nm pump laser is similar to the 1993 nm seed laser and is configured according to [23]. The output from the 1908 nm laser is collimated using an infrasil aspheric lens, reflected off the bandpass filter, and is coupled, along with the 1993 nm seed light, into the core of a 25/250 undoped fiber by using another infrasil aspheric lens. All lenses used in the setup have the same focal length. The undoped fiber is a standard Nufern fiber with a 25 µm core diameter and a 250 µm cladding diameter designed to match the waveguiding characteristics of the Tm-doped fiber. Note that this undoped fiber does not have a pedestal, so any light not coupled into the core will propagate in the cladding. As in the pump transmission experiments, a cladding stripper is used to remove any 1908 nm light which is inadvertently launched into the cladding. This ensures that only core light is present. This cladding stripper is essential to ensure core-pumped operation. The output from this passive fiber is spliced directly to a length of the TDF-LC or TDF-HC fiber. At the output of the amplifier, we have another infrasil aspheric lens which collimates the beam so it can be directed to our diagnostic and equipment.
The Tm-doped fiber was loosely coiled on an optical table for fiber management purposes only. We did not attempt to cool the fiber during the experiments. Each amplifier was seeded with 5.2 W of 1993 nm power. For the TDF-LC, we used an 8.5 m length, and for the TDF-HC, we used a 2.5 m length. Due to the limited fiber we had in-house for these experiments, we did not perform a detailed cut-back. A coarse cut-back was performed, but there was no attempt to optimize the fiber lengths for the core-pumped setup, as this was intended as a baseline test and proof-of-concept only. The results of the core-pumped experiments are shown in Fig. 6.
Fig. 6 Core-pumped fiber amplifier results. (a) Amplifier extracted power versus launched 1908 nm pump power for both the TDF-LC and TDF-HC fibers; (b) amplifier efficiency versus launched pump intensity. Data points represent measured data. Dashed lines are fits to the data.
Both amplifiers exhibit very similar slope efficiencies for comparable absorption lengths (in the presence of bleaching), showing insensitivity to doping concentration. The TDF-HC fiber has a slightly higher slope efficiency of 92.12%, compared to the 91.54% slope of the TDF-LC. We believe this is due to the shorter fiber length (2.5 m versus 8.5 m), resulting in slightly lower overall propagation losses. The optical efficiency of the TDF-HC was 88.4%, while the TDF-LC fiber has an optical efficiency of 86%. Output power was measured directly at the fiber output as well as through a narrow bandpass filter to eliminate the residual pump light and confirm performance at 1993 nm. A typical unfiltered output spectrum is shown in Fig. 7.
Both amplifiers exhibited similar spectral characteristics. The output spectrum was measured using a Yokogawa AQ6375 optical spectrum analyzer (OSA). The measurement was set to a 0.05 nm resolution over a 200 nm scan window. The amplified 1993 nm signal has a full-width as half maximum (FWHM) of 0.1 nm, which is identical to the input spectrum. This corresponds to a FWHM spectral width of ~8 GHz. There is still some residual 1908 nm pump power visible at the output of the amplifier, but it is more than 27 dB down from the peak of the 1993 nm. This amount of pump absorption indicates that the fiber length in this case may be too long, resulting in excess pump absorption. Amplifier efficiency may be improved by shortening the fiber length.
4. Cladding pumping
Conventional high power fiber lasers use cladding pumping rather than core pumping. Obviously, this allows for more pump power to be launched into the fiber. However, in the 2-micron region, the arcylate coatings which act as an outer cladding and provide waveguiding properties to the pump are absorptive. If too much 1908 nm pump power propagates in the 250 µm cladding, the fiber acrylate coating will burn. This is not an issue for Yb-doped systems, but Ho-doped fibers have been designed to account for this acrylate absorption through the use of a glass-clad fiber [6]. To simulate cladding pumping without designing a custom glass-clad fiber and while not damaging the outer acrylate coating, we decided to pump the pedestal of the doped fiber. The test setup is shown in Fig. 8.
Fig. 8 Schematic of the cladding-pumped experimental setup. This is an identical configuration to the core-pumped configuration, except the passive fiber and cladding stripper at the input of the amplifier are removed.
This setup is nearly identical to the setup used for core-pumping (Fig. 5), except the undoped fiber input and cladding stripper are removed. We focus the 1993 nm seed in the 25 µm core of the Tm-doped fiber and slightly defocus the 1908 nm pump such that it couples into the 45 µm pedestal. In this case, the pedestal is our pump cladding, and this allows us to simulate cladding pumping without damaging the fiber. Through careful alignment and optimization, we were able to increase the core coupling efficiency and 12.2 W of 1993 nm seed power was used for these experiments.
Again, we tested both the TDF-LC and TDF-HC fiber, as we did in the core-pumped experiments. With the TDF-HC, 4.5 m of fiber was used. With the TDF-LC, 8.5 m of fiber was used. To achieve these lengths, we performed a coarse cut-back experiment. The fiber amplifier results are shown in Fig. 9. Note that the slopes shown in Fig. 9 are for extracted amplifier power. The 12.2 W of seed input is subtracted from the total measured output power to show the explicit amplifier performance. The slopes are very similar between the two fibers. The TDF-LC and TDF-HC fibers have nearly identical slope efficiencies of 91.46% and 91.57% at ~87 W total output power, respectively. Both amplifiers reach transparency at nearly the same pump levels and show extremely similar performance. The TDF-LC fiber had 88% optical efficiency (extracted power to launched pump power), while the TDF-HC amplifier had 89.1% optical efficiency (extracted power to launched pump power). Spectral output is shown in Fig. 10.
Fig. 9 Cladding-pumped amplifier results. (a) Amplifier extracted power versus launched 1908 nm pump power for both the TDF-LC and TDF-HC fibers; (b) amplifier efficiency versus launched pump intensity. Data points represent measured data. Dashed lines are fits to the data.
Fig. 10 Filtered output spectrum from a cladding-pumped amplifier over a broad spectral range showing 1993 nm output with no residual pump. Inset: Amplifier output spectrum over a narrow spectral window.
To measure the output power and spectrum, we used another narrow bandpass filter at the output of the amplifier to reflect the residual 1908 nm pump and transmit the amplified 1993 nm power. The FWHM of the 1993 nm output signal was 0.1 nm, which was the same as the seed input linewidth, indicating that there is no significant spectral broadening. As shown in Fig. 10, there is also no measurable 1908 nm pump leakage through the filter, verifying that all measured power was at 1993 nm.
With the TDF-LC fiber, we performed a cut-back experiment so we could look at the effects of fiber length on output power and slope efficiency. Those results are shown in Fig. 11. We did not perform a detailed cut-back with the TDF-HC fiber because we did not have a long enough length of fiber for the measurement. One interesting point to note in Fig. 11 is that slope efficiency is not highly dependent on fiber length. If fiber is long enough to absorb the pump, there is little excess loss in un-pumped lengths of Tm-doped fiber at the output wavelength of 1993 nm. At this wavelength, the fiber is quasi-four-level and does not suffer from significant ground-state absorption. This can also be seen in the absorption cross-sections of Fig. 1.
Fig. 11 Plot of slope efficiency versus Tm-doped fiber length for a 65 W amplifier. Both plots are the same but with different lower bounds on the Y-axis. (a) Full scale showing relatively flat performance; (b) zoomed-in scale on Y-axis to show detail in the slope efficiency changes versus length. The TDF-LC was used for this measurement.
4.1 Power scaling
After achieving more than 87 W of output power in both fibers, we decided to power scale an amplifier to the >100 W level. To do this, we simply increased the output power of the 1908 nm pump laser. This was done by attaching a higher power 793 nm diode to the 1908 nm pump laser. We only used the TDF-HC fiber for this experiment, as most of the TDF-LC fiber was consumed in the cut-back experiment. As in the previous experiments, the Tm-doped fiber remained uncooled and was loosely coiled on an optical table. The results are shown in Fig. 12.
Fig. 12 Power scaled cladding-pumped amplifier results. (a) Efficiency and total output power versus launched pump power using 4.5 m of the TDF-HC fiber. Data points represent measured data. Dashed lines are linear fits to the data. (b) Amplifier temporal characteristics over 15 seconds of operation measured with an extended InGaAs detector.
The cladding (pedestal) pumped amplifier has a slope efficiency of 91.6%. For 123.07 W of 1908nm pump power, 111.32 W are extracted in the amplifier. That is an optical conversion efficiency of 90.4%. When seeded with 11.8 W of 1993 nm light, the amplifier has a total output power of 123.13 W, resulting in only 11.75 W of unconverted pump power. By pumping harder, the slope efficiency remained nearly identical to the slope in Fig. 9; however, as the pump intensity increased, the optical efficiency of the amplifier improved and is approaching the slope efficiency. No roll-over is observed. Temporally, the output is fairly well behaved. There is no spiking at turn-on; however, it does take the laser approximately 7.5 seconds to stabilize in output power. This is mostly due to the stability of the 1908 nm pump laser, which does have some degree of thermally-induced power fluctuation at turn-on. The wide voltage response Fig. 12 (b) is due to the use of a biased extended InGaAs detector, as can be seen at the low-level when the pump laser is off.
5. Discussion
Although pumping a Tm-doped fiber with another Tm-doped fiber may seem counterproductive due to the efficiency penalty imposed by the intermediate conversion stage necessary to generate high power at 1908 nm, it is required to scale above the 1 kW power level. Resonant pumping enables ultra-high optical efficiency (90.4% in the case of our 123 W amplifier), and thus low thermal loading, which is essential for significant power scaling. By using this pumping approach to scale above the 1 kW level, the waste heat dissipated in the Tm-doped fiber amplifier would decrease by an order of magnitude from 1 kW [5] to <100 W.
The apparent efficiency penalty of our approach is less than one would expect. Consider the electrical-to-optical (E-O) efficiency for the three methods of 2-micron generation: Er:Yb fiber laser pumped, 793 nm diode pumped, and 1908 nm tandem pumped. A summary of these methods are summarized in Table 2.
Table 2. Summary of High Power 2-micron Generation in Silica Fibers
For Er:Yb tandem pumping, assuming E-O efficiency of fiber-coupled 980 nm diodes of 55%, a 45% efficient Er:Yb fiber pump laser operating at 1567 nm, and a Tm-doped fiber conversion stage with 60% efficiency [16], an overall E-O efficiency of 14% is achieved. For 793 nm diode pumping, with an E-O efficiency of 38% for the pump diodes and 52% optical efficiency for the Tm conversion stage, a total E-O efficiency of 20% is achieved [5]. For a 1908 nm tandem pumped approach with 45% diode efficiency, a 45% efficient 1908 nm Tm-doped fiber laser, and a 90% efficient 1993 nm tandem-pumped Tm fiber amplifier, an overall E-O efficiency of 18% is realized. This tandem pumping approach is actually comparable in efficiency to a direct 793 nm diode-pumped Tm-doped fiber laser, but without the thermal and pump brightness limitations. This could potentially allow for significantly higher power scaling in the 2-micron spectral region. Ho-doped fiber results are also included in Table 2 for reference. Although Tm-doped fiber pump lasers operating at 1950 nm (for holmium pumping) are more efficient than at 1908 nm due to lower ground-state absorption losses [23,24], the decreased optical efficiency in Ho-doped fiber makes the overall electrical-to-optical efficiencies for these two tandem pumped approaches comparable.
To further compare feasibility of our approach for power scaling beyond 1 kW we must address pump absorption and bleaching. It is interesting to look at the pump absorption in comparison to Beer’s Law given this pumping approach. This is shown in Fig. 13 for both the TDF-LC and TDF-HC fibers.
Fig. 13 Pump power absorption as a function of fiber length for the TDF-LC and TDF-HC fibers when cladding/pedestal-pumping (in the absence of extraction). Solid lines show the absorption with the effect of bleaching. Dashed lines show the Beer’s Law absorption.
Since bleaching is a function of pump intensity, we cannot use a constant absorption coefficient when looking at pump absorption. The absorption in the gain fiber is dependent on the pump intensity and must be considered when designing for higher pump levels. To estimate the effects of bleaching, we calculate the amount of pump power transmitted through a differential length of fiber according to [14]. We then adjust the pump intensity accordingly, and iterate across the entire length. As shown in Fig. 13, the pump absorption as a function of fiber length in the presence of bleaching (and in the absence of extraction) is significantly different in comparison to the Beer’s Law absorption. This may be advantageous for high power scaling, as it helps distribute the heat load more evenly along the length of the fiber. With the TDF-LC fiber at the 82 W pump level, pump absorption is nearly linear until ~4 m, when the absorption tails off as the pump intensity is decreasing, and the fiber is less bleached. The use of a higher doping concentration does help mitigate the bleaching effect to some extent, as expected due to the presence of more thulium ions. The bleached absorption shown in Fig. 13 neglects the extraction that occurs in an amplifier. This is a simplified case to show the effects of bleaching for purposes of discussion.
Due to bleaching, power scaling is not trivial. Increasing pump power results in a decrease in pump absorption, for a fixed fiber geometry. This means that the fiber will bleach faster if we pump harder, which will require a longer length for pump absorption. As pump intensity increases and bleaching dominates the absorption, longer fiber is needed to maintain overall absorbed pump power. However, increasing fiber length lowers the thresholds of degrading nonlinear processes and places practical restrictions on building a device [9,10]. As such, designing an amplifier for power scaling becomes a balance between pump power, fiber geometry (core/clad ratio), doping concentration, fiber length, and nonlinear effects.
6. Conclusion
We have demonstrated highly efficient, uncooled, Tm-doped fiber amplification using a resonant tandem pumping approach. The novelty of this approach is the use of a Tm-doped fiber laser (operating at 1908 nm) to pump a Tm-doped fiber amplifier in the tail of the absorption with very low quantum defect. The quantum efficiency from 1908 nm to 1993 nm is 95.7%. With this architecture, we demonstrated more than 123 W of output power in a cladding-pumped amplifier with 91.6% slope efficiency, which is 95.7% of the quantum efficiency. In comparison to Yb-doped systems, pumping at 975 nm and lasing at 1064 nm yields a 91.6% quantum efficiency. High power Yb-doped fiber amplifiers have been operated with 90% slope efficiency [1], which is 98.3% of the quantum efficiency. Similarly, tandem pumping of Yb has yielded 95-96% slope efficiency for a 98% quantum efficiency, resulting in 97-98% of the quantum efficiency [25], so our results coincide with the highest efficiencies reported for Yb-doped fibers. The results achieved in this paper demonstrate the effectiveness of this pumping approach for improving amplifier optical efficiency, and thus reducing thermal loading, showing a path toward higher power scaling in the 2-micron spectral region via in-band tandem pumping.
References and links
1. S. Christensen, “Narrow linewidth fiber amplifiers,” in Advanced Photonics Congress, OSA Technical Digest (Optical Society of America, 2012), paper SW3F.3. [CrossRef]
2. V. Gapontsev, V. Fomin, A. Ferin, and M. Abramov, “Diffraction limited ultra-high-power fiber lasers,” in Lasers, Sources and Related Photonic Devices, OSA Technical Digest (Optical Society of America, 2010), paper AWA1.
3. M. Dubinskii, J. Zhang, and V. Ter-Mikirtychev, “Record-efficient, resonantly-pumped, Er-doped singlemode fiber amplifier,” Electron. Lett. 45(8), 400–401 (2009). [CrossRef]
4. J. Zhang, V. Fromzel, and M. Dubinskii, “Resonantly cladding-pumped Yb-free Er-doped LMA fiber laser with record high power and efficiency,” Opt. Express 19(6), 5574–5578 (2011). [CrossRef] [PubMed]
5. T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, G. Rines, and P. F. Moulton, “1kW, all-glass Tm:fiber laser,” presented at SPIE Photonics West, Fiber Lasers VII: Technology, Systems, and Applications, San Francisco, CA, 23–28 January 2010.
6. A. Hemming, N. Simakov, A. Davidson, S. Bennetts, M. Hughes, N. Carmody, P. Davies, L. Corena, D. Stepanov, J. Haub, R. Swain, and A. Carter, “A monolithic cladding pumped holmium-doped fibre laser,” in Conference on Lasers and Electro-Optics 2013, OSA Technical Digest (Optical Society of America, 2013), paper CW1M.1. [CrossRef]
7. A. V. Smith and J. J. Smith, “Mode instability in high power fiber amplifiers,” Opt. Express 19(11), 10180–10192 (2011). [CrossRef] [PubMed]
8. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, “Theoretical analysis of mode instability in high-power fiber amplifiers,” Opt. Express 21(2), 1944–1971 (2013). [CrossRef] [PubMed]
9. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. J. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240–13266 (2008). [CrossRef] [PubMed]
10. J. W. Dawson, M. J. Messerly, J. E. Heebner, P. H. Pax, A. K. Sridharan, A. L. Bullington, R. J. Beach, C. W. Siders, C. P. J. Barty, and M. Dubinskii, “Power scaling analysis of fiber lasers and amplifiers based on non-silica materials,” Proc. SPIE 7686, 768611 (2010). [CrossRef]
11. P. F. Moulton, “Power scaling of high-efficiency Tm-doped fiber lasers,” Proc. SPIE 6873, 687315 (2008).
12. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: Fundamentals and power scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]
13. P. Moulton, “High power Tm:silica fiber lasers: current status, prospects and challenges,” in CLEO/Europe and EQEC 2011 Conference, OSA Technical Digest (Optical Society of America, 2011), paper TF2_3.
14. D. Creeden, B. R. Johnson, S. D. Setzler, and E. P. Chicklis, “Resonantly pumped Tm-doped fiber laser with >90% slope efficiency,” Opt. Lett. 39(3), 470–473 (2014). [CrossRef] [PubMed]
15. G. D. Goodno, L. D. Book, J. E. Rothenberg, M. E. Weber, and S. B. Weiss, “Narrow linewidth power scaling and phase stabilization of 2-μm thulium fiber lasers,” Opt. Eng. 50(11), 111608 (2011). [CrossRef]
16. M. Meleshkevich, N. Platonov, D. Gapontsev, A. Drozhzhin, V. Sergeev, and V. Gapontsev, “415W Single-Mode CW Thulium Fiber Laser in all-fiber format,” in CLEO/Europe and IQEC 2007 Conference, OSA Technical Digest (Optical Society of America, 2007), paper CP2_3.
17. G. Turri, V. Sudesh, M. Richardson, M. Bass, A. Toncelli, and M. Tonelli, “Temperature-dependent spectroscopic properties of Tm3+ in germinate, silica, and phosphate glasses: a comparative study,” J. Appl. Phys. 103(9), 093104 (2008). [CrossRef]
18. S. D. Jackson and T. A. King, “Theoretical modeling of Tm-doped silica fiber lasers,” J. Lightwave Technol. 17(5), 948–956 (1999). [CrossRef]
19. B. M. Walsh and N. P. Barnes, “Comparison of Tm:ZBLAN and Tm:silica fiber lasers; spectroscopy and tunable pulsed laser operation around 1.9µm,” Appl. Phys. B 78(3-4), 325–333 (2004). [CrossRef]
20. J. Ji, S. Yoo, P. Shum, and J. Nilsson, “Minimize quantum-defect heating in thulium-doped silica fiber amplifiers by tandem-pumping,” in Proceedings of IEEE Photonics Global Conference (2012).
21. S. D. Jackson and S. Mossman, “Efficiency dependence on the Tm3+ and Al3+ concentrations for Tm3+-doped silica double-clad fiber lasers,” Appl. Opt. 42(15), 2702–2707 (2003). [CrossRef] [PubMed]
22. C. A. Codemard, J. Nilsson, and J. K. Sahu, “Tandem Pumping of large-core double-clad ytterbium-doped fiber for control of excess gain,” in Advanced Solid-State Photonics, 2010 OSA Technical Digest (Optical Society of America, 2010), paper AWA3.
23. A. Carter, A. Hemming, and N. Simakov, “An efficient, high power, monolithic, single mode thulium fibre laser,” in Workshop on Specialty Optical Fibers and their Applications, OSA Technical Digest, (Optical Society of America, 2013), paper T2.4. [CrossRef]
24. D. J. Creeden, B. R. Johnson, and S. D. Setzler, “High efficiency 1908nm Tm-doped fiber laser oscillator,” in Advanced Photonics Congress, OSA Technical Digest (Optical Society of America, 2012), paper SW2F.4. [CrossRef]
25. T. Yao, J. Ji, J. K. Sahu, A. Webb, and J. Nilsson, “Tandem-pumped Ytterbium-doped aluminosilicate fiber amplifier with low quantum defect,” in Conference on Lasers and Electro-Optics 2012, OSA Technical Digest (Optical Society of America, 2012), paper CM4N.7. [CrossRef]
