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Abstract
We detail the design and performance of a high efficiency in-band pumped thulium fiber amplifier operating at the 100 W level. Using a novel pumping architecture based on three incoherently combined thulium fiber oscillators at 1904 nm and a seed laser tunable from 1970–1990 nm, efficient amplification is demonstrated in a high dopant concentration 25/65/250 µm thulium fiber. Here we use the 65 µm pedestal surrounding the core as a pump cladding to increase the cladding to core overlap and improve the overall pump absorption. Up to 89% slope efficiency is obtained with ∼100 W output power at 1990 nm. These results indicate that in-band pumping is a viable route to circumvent the thermal limitations associated with 793 nm diode pumping and provide a pathway for development of multi-kW laser sources in the 2 µm spectral window.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Power scaling of single aperture fiber lasers is a persistent area of research focused on increasing the maximum obtainable single mode output power. Over the past several decades, high average power fiber lasers, primarily based on 1 µm Yb-doped fiber lasers (YDFL), have revolutionized a number of scientific, defense and industrial applications [1–3]. Thulium-doped fiber lasers (TDFL) produce the second highest output power after YDFLs and are ideal for various applications such as nonlinear wavelength conversion and material processing where 1 µm based systems fall short [4–7]. However, thulium fiber lasers and amplifiers cladding pumped at 793 nm suffer from large quantum defects and high thermal loads. These drawbacks have historically limited the power scaling potential of TDFLs to the ∼1 kW level. Indeed, the first demonstration of 1 kW output from a TDFL was reported in 2010 [8] with several more recent demonstrations of both ultrafast and CW TDFLs at the 1 kW level [9,10]. Despite this progress, the further growth of output power has been stymied by thermal limitations resulting from the large quantum defect intrinsic to Tm-doped silica pumped at 793 nm [11].
One possible solution which circumvents these thermal limitations is to implement an alternative pumping scheme. While thulium has several energy levels which could possibly be used for laser excitation such as the 3H6→3H5 or 3H6→3F4 transitions, high brightness diode pumps resonant with these transitions and with sufficient output power to enable power scaling are non-existent. Due to the lack of suitable pump diodes, the 3H6→3F4 transition may instead be excited directly with either Er-doped or Tm-doped fiber laser sources owing to its extremely broad bandwidth ranging from ∼1500-1900nm [12]. While Er-doped fiber lasers suffer from their own drawbacks [13,14], pumping with short wavelength TDFLs near ∼1900nm presents a viable route to obtain very high quantum efficiency with correspondingly low heat loads within the active fiber [11]. This approach is described in literature as resonant pumping or in-band pumping [15–18].
Compared with diode pumping at 793 nm which relies on non-radiative decay from the 3H4→3F4 level to populate the upper laser level, in-band pumping instead populates the upper laser level 3F4 directly, where the strong inhomogeneous broadening of the 3F4 manifold leads to the formation of a variety of sublevels which support optical transitions [12]. This removes the need for the 3H4→3F4 non-radiative transition to populate the upper laser level and theoretically enables quantum efficiencies > 90% across a significant part of the Tm emission spectrum. In-band pumping has the added benefit that cross-relaxation no longer plays a role in the laser energetics. Avoiding the need for cross-relaxation removes the dependency of the thulium dopant concentration from the quantum efficiency of the laser system, as is the case in 793 nm cross-relaxation pumping where high Tm3+ concentrations are required to mediate the near field energy transfer process [19], thus providing increased optical efficiency at the expense of high thermal loads. By designing the Tm-doped fiber appropriately for an in-band pumped system, longer fiber lengths may be used compared with 793 nm diode pumping which distributes the thermal load across a much longer fiber and lowers the thermal load per unit length. This significantly lowers the maximum temperature reached within the in-band pumped gain fiber at high power operation, thus allowing for power scaling to higher average powers before thermal degradation becomes an issue.
To achieve the goal of a high slope efficiency Tm:fiber amplifier, in this manuscript we report on the development and performance of a tunable in-band pumped thulium doped fiber amplifier with 100 W output power pumped by three incoherently combined 1.9 µm fiber lasers. Tunability is enabled by use of a custom built tunable seed laser and allows for operation from 1970-1990nm. High slope efficiencies up to 89% are reported.
2. Experimental layout
Figure 1 shows the layout of the in-band pumped TDF amplifier system. The system is free-space pumped and seeded using the pump and seed laser systems described below in Sections 2.1 and 2.2, respectively. Dichroic mirrors are used to spatially overlap the pump and seed beams before launching into a 5.5 m length of 25/65/250 µm TDF (Nufern LMA-TDF-25P/250-M). The dichroic mirrors (HR 1950-2050nm, AR 1850-1930nm) also provide > 30 dB suppression of unwanted radiation entering back into either the 1904nm pump laser or the tunable seed laser, thus preventing free-lasing and potential catastrophic optical damage. Coupling in and out of the TDF is achieved using 25 mm focal length fused silica aspheric lenses AR-coated for 1900-2100 nm. The amplified signal and unabsorbed pump radiation are collimated at the output of the amplifier and separated via a second pair of dichroic mirrors which direct the amplified signal to the diagnostic measurement setup where output power and spectrum are analyzed.
Fig. 1. Schematic of the in-band pumped Tm:fiber amplifier. The system is free-space pumped by up to 144 W from the 50 µm/0.22 NA output of the (3 × 1) laser combiner. DM: dichroic mirror.
Shown in Fig. 2(a), the 25/65/250 µm Tm-doped gain fiber used here was chosen specifically for its high Tm dopant concentration (estimated at 40⋅1025 Tm ions/m3) and correspondingly high pump absorption (11.4 dB/m at 793 nm). The refractive index pedestal around the core is used as the pump cladding in this configuration due to its favorable 65 µm/0.22 NA guidance properties and high core overlap (see Fig. 2(b)). While circular pump claddings typically offer poor pump absorption performance due to skew ray propagation, the high overlap afforded by the 25/65 µm geometry partially circumvents this issue, where data from literature using similar architectures has been previously reported [15]. Although the pump absorption would be significantly enhanced by implementing a non-circular pump cladding, the high core/clad overlap ratio provided by the 65 µm/0.22 NA pedestal still allows for > 1 dB/m pump absorption coefficient at 1904nm when operated at high average power.
Fig. 2. (a) Cross-sectional image of the Nufern LMA-TDF-25P/250-M gain fiber. (b) Measured refractive index profile of the TDF showcasing the 65 µm/0.22 NA pedestal around the core.
Additionally, to avoid any thermal blooming related effects which would prevent efficient free-space pump coupling during high power operation [20], the entire free-space portion of the experiment was placed in a custom built dry box, where the sealed dry box was purged with ultra-dry air prior to laser operation.
2.1 Pump source
Three fiber oscillators were constructed for the purpose of in-band pumping the Tm:fiber amplifier. The schematic of the pump source is displayed in Fig. 3. Each pump laser is a monolithic fiber oscillator formed by a set of low-reflectivity (16-21%) and high-reflectivity (> 99%) fiber Bragg gratings (FBG), and a ∼4 m length of 25/65/250 µm TDF. 793 nm pump light is injected into the oscillators via an intracavity (6 + 1)x1 pump/signal combiner. The FBGs were inscribed in passive 25/250 µm, 0.09 NA fiber via phase mask fs-laser inscription at the Institute of Applied Physics, Friedrich Schiller University Jena [21] and are described in detail in Table 1. During fabrication, the HR-FBG were inscribed via strip and recoat technique, whereas the LR-FBG were written directly through the coating.
Fig. 3. Layout of the three all-fiber 1904nm oscillators combined via the (3 × 1) laser combiner. CLS: cladding light stripper. HR: high-reflective fiber Bragg grating. LR: low-reflective fiber Bragg grating.
Table 1. Description of the LR- and HR-FBGs used to fabricate the three pump units.a
The active fiber used in the oscillators is Nufern LMA-TDF-25P/250-LC, specifically chosen due to its low dopant concentration and favorable thermal management properties. Fibers with low Tm3+ concentration are preferred for lasing near 1900nm due to their higher fractional inversion levels compared with higher concentration fibers when cladding pumped at 793 nm [22]. Because of the significant reabsorption that is present near 1900nm in Tm-doped LMA fibers, high inversion levels are required to achieve efficient lasing without ASE-related or free-lasing issues in the ∼1940-2000nm band. As a result of the low dopant concentration, the pump absorption and thermal load profiles are lower than other commercially available thulium fibers, allowing for efficient thermal management with no long term degradation issues.
After fabrication, the three pump units were spliced onto a (3 × 1) laser combiner fabricated by Neptec Optical Solutions to incoherently combine the three beams into one high power beam. The laser combiner features three 25 µm/0.09 NA input ports and a 50 µm/0.22 NA output delivery fiber with a designed power handling specification of 900 W at 1.9 µm. Due to issues with interactions between ∼2 µm laser light and conventional low index fluoroacrylate polymer claddings [23], care was taken to remove any cladding light before entering the laser combiner by mechanically stripping and chemically etching a 25 cm length of passive fiber to scatter cladding light out of the fiber. The total output power, spectrum of each pump laser and combined output beam profile from the (3 × 1) combiner are shown in Fig. 4(a)-(c). The observed beam profile indicates a highly multimode beam profile as expected from the 50 µm/0.22 NA delivery fiber with a measured beam quality of M2x = 7.43 and M2y = 6.27. At maximum output power, although the laser combiner was angle cleaved to ∼5°, back reflections from the delivery fiber output facet caused free-lasing in two of the pump units. Therefore, the delivery fiber was end-capped with a ∼1 mm length of coreless fiber which was spliced onto the end of the delivery fiber and angle cleaved to further suppress back reflections and effectively suppress free-lasing in the pump units.
Fig. 4. (a) Total combined output power and (b) output spectra recorded at maximum output power of the three ∼1904 nm pump units. Up to 144 W combined output power was obtained from the three pump lasers. (c) Multimode output beam profile from the (3 × 1) laser combiner measured at maximum output power with a measured beam quality of M2x = 7.43 and M2y = 6.27.
Differences in the available 793 nm pump power and slope efficiency of each thulium fiber oscillator lead to varying pump powers from each unit delivered through the (3 × 1) laser combiner. The total output power of each oscillator delivered through the laser combiner is shown in Table 1. Overall, the output of the combiner produced up to 144 W output power at 1904nm (see Fig. 4(a)). Each of the three pump units had excellent spectral purity with < 0.25 nm linewidth at the −3 dB level and high ASE suppression > 50 dB.
2.2 Seed laser
To investigate the dependence of the in-band amplifier slope efficiency and output power on the operating wavelength, a tunable TDFL was constructed. The tunable laser system operates from 1970-1990nm and produces ∼20 W output power across its tuning range. A schematic of the system is shown in Fig. 5.
Fig. 5. Schematic of the tunable TDF seed laser. A wavelength tunable ASE source serves as a spectrally broadband seed. The 3 W output from the ASE source is then injected into the 25/250 µm final amplifier and boosted to the 20 W level.
The seed source is based on a custom developed wavelength tunable ASE source. It can be tuned from 1970–1990nm with a 3 dB spectral bandwidth of approximately 6 nm. The seed source is first pre-amplified to ∼3 W output power. The final high power amplifier utilizes a 3.5 m length of 25/250 µm TDF (Nufern LMA-TDF-25P/250-LC) in a cladding pumped configuration to generate > 20 W average power across the tuning range. The output power and spectra of the seed laser are shown in Fig. 6(a) & (b), respectively.
Fig. 6. (a) Output power and (b) output spectra obtained from the seed source. Over 20 W is achieved across the tuning bandwidth with > 35 dB ASE suppression.
3. Experimental results
Using the experimental setup shown in Fig. 1, power scaling tests of the in-band TDFA were conducted to investigate the high power performance of this pumping architecture. The input seed power was set to 20 W while the input pump power was varied up to 120 W. While up to 144 W of pump power was available to pump the in-band TDFA, losses from various optical elements during coupling limited the total input pump power to 120 W. Coupling of both pump and signal light were ensured by directly imaging the transmitted beam distributions on a Pyrocam III camera. Due to the highly multimode nature of the input pump beam profile, care was taken to minimize the transmitted pump power during operation, therefore ensuring efficient coupling into the pedestal and maximum pump absorption. Further, to prevent thermal degradation of the active fiber and provide higher order mode bend loss, the active fiber was wrapped on a 10 cm diameter water cooled mandrel, where good contact between the fiber and mandrel was enabled by using a high conductivity thermal pad.
Output power curves with respect to the total absorbed pump power are shown in Fig. 7 along with calculated slope efficiencies. Overall, greater than 98 W was obtained across the tuning range with > 100 W produced at 1990nm. The observed slope efficiencies were close to the quantum defect limit and ranged from 88.5-89.1%, as shown in Table 2. It is important to note that as the signal wavelength decreased closer to the pump wavelength, the slope efficiencies increased which agrees with quantum defect scaling.
Fig. 7. Output power vs. absorbed pump power and corresponding slope efficiencies for (a) 1970 nm, (b) 1980 nm, and (c) 1990 nm operation.
Table 2. Output power and slope efficiency at the three central operating wavelengths measured at maximum output power.
Due to the circular 65 µm/0.22 NA pedestal used as a pump cladding in this experiment, absorption of the 1904nm was limited to ∼1.4 dB/m and was primarily hindered by the presence of skew ray modes which had little to no overlap with the core [24]. This was evidenced by an annular shaped pump beam profile of the unabsorbed pump at the output of the Tm-doped gain fiber, as shown in Fig. 8. Despite this fact, 7.8 dB total pump absorption was obtained corresponding to a total absorbed pump power of approximately 100 W at all three operating wavelengths. This can be negated by use of a specialty Tm-doped fiber with a non-circular inner pump cladding.
Fig. 8. Beam profile of the transmitted 1904 nm pump light. Due to propagation of skew rays in the circular pedestal, the unabsorbed pump light shows an annular beam distribution.
The measured output spectra of the in-band TDFA are shown in Fig. 9. The 6 nm FWHM input bandwidth is well preserved after amplification with no measurable spectral broadening. Noticeable ASE is observed at all three operating wavelengths however, indicating that longer wavelength operation is likely preferable in this configuration. The ASE observed here is compounded by the presence of ASE in the input spectrum which also receives gain from the Tm3+ dopant ions.
Fig. 9. Output spectra measured at maximum output power for the three different central operating wavelengths.
To emphasize the low quantum defect heating in this system, the thermal load vs. fiber length was simulated for the in-band amplifier at maximum output power via an in-house developed rate equation model [25]. Simulated results are shown in Fig. 10 and indicate a peak thermal load of 2.9 W/m. Using the cooling mount geometry described above, finite element analysis reveals only a 3.7 °C temperature rise in the low index polymer jacketing based on the simulated thermal load profile. The low thermal load observed here is promising for multi-kW power scaling applications. Even lower polymer temperatures may be obtained with use of a 400 µm outer diameter fiber which provides a larger surface area of the fiber to contact the cooling mount for heat dissipation.
Fig. 10. Simulated thermal load for the in-band amplifier operating at maximum output power. The operating wavelength used for this simulation was set to 1970 nm.
4. Discussion
The system architecture presented within this manuscript was designed specifically to investigate the potential for power scaling to multi-kW average powers in the 2 µm band. This was the primary motivation for our pumping scheme which involved the incoherent combination of multiple lower power TDF lasers to pump a low quantum defect, high power Tm:fiber amplifier. Due to aforementioned thermal issues preventing power scaling to > 1 kW in TDFLs pumped at 793 nm, obtaining multi-kW powers around 1900nm to pump an in-band pumping architecture will necessarily require combination of multiple pump lasers to serve as a high power pump source. Since the total available pump power is limited only by the average power of each pump laser and power handling of the laser combiner, the total pump power can be readily scaled up by fabricating higher power pump lasers and using a laser combiner with more input ports. Based on the high slope efficiencies and low thermal loads demonstrated here and by others, in-band pumping is very likely a viable route to break the ∼1 kW output power barrier which has remained for over a decade and obtain multi-kW output powers in the 1.95-2.1 µm band.
However, several outstanding issues must be addressed in order to realize an in-band pumped TDFA in an all-fiber format. All-fiber systems are preferable due to their environmental stability and lack of free-space optics which are often prone to optical misalignments that can cause optical damage at high power. The two main technologies required for an all-fiber system are the development of Tm-doped fibers with non-circular, all-glass inner claddings and laser combiners with signal input/output ports. While several Yb-doped and Ho-doped triple clad fibers have been reported in literature with octagonal all-glass pump claddings [26–29], this technology has not yet been applied to Tm-doped active fibers with sufficiently high core/cladding overlap ratios to enable efficient in-band pumping. Additionally, the laser combiner geometry used in this experiment was only capable of supporting input radiation from the three pump lasers and did not allow for an input/output signal channel. This is primarily due to the unavailability of a suitable output fiber geometry which can guide both pump and signal light. While the 50 µm/0.22 NA beam delivery fiber on the output of the laser combiner was suitable for pump guidance, a similar fiber which includes a 25 µm/0.09 NA core geometry would also be required to efficiently interface with the 25/65/250 µm TDF used here. Development of such a fiber will therefore be critical to enabling a laser combiner which can support both pump and signal light.
5. Conclusions
In summary, this manuscript presents an in-band pumped thulium fiber amplifier tunable from 1970-1990nm with up to 100 W output power and high slope efficiencies of 88.5-89.1%. By using a high concentration 25 µm core Tm-doped fiber and by injecting 1904nm pump light into the 65 µm pedestal surrounding the core, efficient pump absorption was obtained despite the circularly symmetric pedestal geometry. The pumping scheme implemented here using incoherent combination of three Tm-fiber oscillators represents a scalable approach to producing high average pump powers around ∼1900nm without risk of thermal failure. With further developments of critical enabling components such as laser combiners which include signal input/output ports and more advanced Tm-fiber geometries, these results indicate that in-band pumping provides a direct route to multi-kW output powers in the 2 µm band.
Funding
Air Force Office of Scientific Research (FA9550-19-1-0127); Bundesministerium für Wirtschaft und Energie (BMWi) (ZF4309605DF9).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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