The high absorption confined-doped ytterbium fiber with 40/250 μm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 μm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber lasers/amplifiers have developed rapidly and gained widespread attention in the last several decades owing to their compact structure, high conversion efficiency, high-power capability, and good beam quality, etc [1–3]. Hitherto, the output powers from fiber lasers/amplifiers have already exceeded the 10-kW level [4,5], which enabled a wide variety of applications, especially in industrial manufacturing [6,7]. However, further power scaling of fiber lasers/amplifiers is challenged by the emergence of nonlinear effects resulted from the high power density within the core, such as stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS) [8–10]. The most straightforward solution is to increase the effective mode area by employing large mode area (LMA) fibers, but the cost is a degraded beam quality owing to the increment of supported modes, and very possibly a significantly reduced transverse mode instability (TMI) threshold [2,11]. To increase the output power while maintaining good beam quality in the LMA fiber lasers/amplifiers, it is essential to reduce the number of supported transverse modes, and in fact, lots of efforts have been devoted to the LMA fiber designs.
There are several fiber design strategies to reduce the number of supported modes and even realize effective single-mode operation in an LMA fiber. One way is to decrease the numerical aperture (NA) of the LMA fiber [12–15]. However, the low-NA LMA fiber is sensitive to bending due to the weak mode confinement, making it more demanding in practical use. Another practical issue is that the core NA of the optical fiber could not be arbitrarily reduced, mainly restricted by the technical and physical limitations , therefore, there is limited potential for the core size scaling by lowering the NA. Another approach is designing specialty optical fiber with microstructures or special refractive index profiles, such as chirally-coupled core fiber , hollow-core/hole-assisted photonic crystal fiber [18–20], all-solid photonic bandgap fiber [21,22], large pitch fiber [23,24], tapered fiber , and single or multiple trenched fiber [26,27], etc. These fiber designs could help realize effective single-mode operation in an LMA fiber, but they either require a complicated fabrication process or are difficult for post handling (cleaving and splicing) . Moreover, some of them are difficult to be fabricated into active fibers, making them less attractive in fiber oscillators/amplifiers . Therefore, an easy-to-fabricate and easy-to-use fiber design with good compatibility is in great demand.
A very promising solution is the confined-doped fiber design, in which only part of the core is selectively doped. In a conventional active fiber, the core is fully doped, and all the supported modes that propagate in the core could experience the gain. While in the confined-doped fiber, transverse mode discrimination, also known as the ‘gain filtering effect’, is established by separating the waveguiding function and the gain through selective doping, and only the modes that occupy the doped regions of the core could extract the gain. It has been proven both theoretically  and experimentally  that confining the doping area to the central part of the core can facilitate fundamental mode laser output. In recent years, high-power fiber lasers/amplifiers employing the confined-doped LMA ytterbium-doped fiber (YDF) have also been reported. In 2016, Mashiko et al. demonstrated a 2-kW fiber oscillator operating at 1080 nm with the beam quality factor M2=1.2 based on the confined-doped fiber , and the output power was further scaled to 3 kW with M2=1.3 in the following year by increasing the pump power . The effective mode area of this confined-doped fiber is ∼400 μm2, however, one of the key parameters, i.e., the relative doping ratio of the core, was not mentioned. In 2018, Liao et al. fabricated a confined-doped YDF using the modified chemical vapor deposition (MCVD), the core and inner cladding diameters of which are 35 and 400 μm, respectively . Around 51% of the central core area was doped with Yb-ions. Finally, the beam quality factor was improved from 2.8 in a fully-doped fiber with a similar core/inner-cladding diameter to 1.5 by using this confined-doped fiber in a fiber oscillator. And a similar beam quality factor was obtained in a ∼450-W fiber amplifier. In the same year, Seah et al. reported a 4.1 kW tandem-pumped 1060 nm fiber amplifier by using the confined-doped fiber . The core/inner-cladding diameter of this confined-doped fiber is 42/250 μm and around ∼75% of its core diameter is doped with Yb-ions. The beam quality factor M2 was 1.59 under the maximum output power, which was significantly improved compared with the beam quality factor of M2=2.77 from the fully-doped fiber amplifier. In 2020, Zhang et al. fabricated a confined-doped Yb/Ce co-doped aluminosilicate fiber with 33/400 μm core/inner-cladding diameter, where ∼70% across the core diameter was doped with Yb/Ce-ions . By using this confined-doped fiber in a fiber amplifier, the TMI threshold increased by 74% (to ∼1.25 kW) comparing to its fully-doped counterpart, and the beam quality factor reached 1.43 under the output power of 1.2 kW. Subsequently, they further reported a 3 kW fiber oscillator using this confined-doped fiber, however, the beam quality was not characterized . In the same year, Wang et al. fabricated a confined-doped YDF with a 30/400 μm core/inner-cladding diameter, and the diameter of the doped region was 20 μm . Based on this fiber, 2.42 kW output power at 1080 nm was obtained with the beam quality factor of M2=1.32 in a fiber amplifier. All these results indicate the confined-doped fiber design is a viable solution to realize high-output power while maintaining good beam quality in an LMA fiber.
In this contribution, we theoretically analyze the mode discrimination property of the confined-doped LMA YDF with the core/cladding diameter of 40/250 μm and core NA of 0.08, according to which, the high absorption confined-doped LMA YDF with a 0.75 relative doping ratio is successively designed, fabricated and applied in a 1018 nm tandem-pumped fiber amplifier. Finally, an efficient 6.2 kW fiber amplifier is obtained with good beam quality and high SRS effect suppression. Comparison work is also carried out by utilizing a piece of 40/250 μm fully-doped ytterbium fiber in a fiber amplifier, where the TMI threshold and the beam quality factor evolution are studied and compared with the confined-doped fiber amplifier results, further proving the intrinsic advantages of the confined-doped fiber in high-order mode suppression and TMI mitigation.
2. Fiber design strategy
We consider using an LMA fiber with the core/cladding diameter of 40/250 μm and NA of 0.08 for tandem pumping application. To find the optimized parameters, simulations of a fiber amplifier employing the confined-doped YDF are carried out based on the rate equations incorporating mode distributions and transverse spatial-hole burning . In the proposed simulation, the population inversion in the steady state can be described as:
The gain of each mode is given by:
The propagation equation is given by:
In the simulation, the fourth-order Runge-Kutta method is used to solve the interaction between each transverse mode and the ytterbium-doped gain medium along the propagation direction with the iteration step size of 0.005 m. The four lowest-order modes, i.e., LP01, LP02, LP11, LP21, are considered in this model, the mode distributions of which are acquired through the finite element method. The image resolution of the transverse mode distribution is 0.25 μm×0.25 μm. The main parameters used in the simulation are listed in Table 1.
In this simulation, the proportion of the fundamental mode (defined as ‘mode purity’) of the seed laser varies from 0.7 to 0.99, and the rest power is shared by the other three high-order modes equally for simplification. The relative doping ratio, which is defined as the diameter of the doped region to that of the core diameter, of the active fiber varies from 0.4 to 1. First of all, the impact of the pump directions on the mode purity of the output laser is simulated by injecting the seed laser with a mode purity of 0.9. The signal power is 100 W, and the pump power is 5000 W in total, which is equally distributed in both pump directions in the bidirectional pump scheme. The simulation results are shown in Fig. 1(a). When the fiber is fully doped, the mode purity decreases after amplification in all pump schemes since all the high-order modes can extract the gain. In comparison, the co-pumping scheme outperforms in terms of the mode purity of the output laser, while the counter-pumping scheme is the worst arising from higher high-order mode gain. As the relative doping ratio decreases, the difference between these three pumping configurations becomes negligible owing to the gradually establishing mode discrimination of the confined-doped fiber. Therefore, co-pumping is a preferred choice for obtaining output laser with better fundamental mode purity regardless of the relative doping ratio. Subsequently, the mode purity of the output laser from the confined-doped fiber amplifier is simulated under different injected seed purities and different relative doping ratios of the active fiber in a co-pumping scheme, and the simulation results are shown in Fig. 1(b). The gain filtering effect could be observed only when the relative doping ratio decreases to a certain level, and as the mode purity of the seed laser increases, it requires a smaller relative doping ratio to realize the gain filtering effect. For instance, the confined-doped fiber with a relative doping ratio smaller than 0.83 could manifest the gain filtering effect when the injected mode purity is 0.7 while it would require a relative doping ratio smaller than 0.78 when the injected mode purity is 0.95. The mode purity of the output laser maximizes around the relative doping ratio of 0.5, and the mode purity would decrease when the doping ratio goes smaller, arising from the growing proportion of the LP02 mode.
According to the simulation results, the relative doping ratio of 0.5 seems to be the best choice for gain filtering. However, there is a contradiction between the relative doping ratio and the length of the YDF. A smaller relative doping ratio means weaker absorption and, therefore, would require longer YDF for sufficient pump absorption. For example, it requires 15 m fully-doped YDF for sufficient pump absorption at 1018 nm while it would increase to 60 m YDF with the relative doping ratio of 0.5 for equivalent pump absorption. The long fiber length would lead to a significantly decreased SRS threshold, limiting the power scalability. Therefore, compromise should be made between the gain-filtering effect (i.e., the relative doping ratio) and the required length of the active fiber. According to the simulation results, when the relative doping ratio is 0.75, even seed laser with very high purity (i.e., 0.99) could be slightly improved. Therefore, a confined-doped fiber with a 0.75 relative doping ratio is chosen.
The designed confined-doped fiber is subsequently fabricated using the MCVD in conjunction with the chelate gas deposition technique and drawn into fiber with the core/inner-cladding diameter of 40/250 μm. The picture of the fiber cross-section as well as the refractive index profile along the diameter of the fabricated confined-doped fiber are presented in Fig. 2. The inner cladding is processed into an octagonal shape with a diameter (flat-to-flat) of 250 μm and the core diameter is measured to be ∼40 μm, as indicated in Fig. 2(a). As shown in Fig. 2(b), the refractive index of the doped region is slightly higher than the undoped region, which occupies around 75% of the core, indicating good adherence to our design parameters. The absorption coefficient of this fiber is measured to be ∼0.8 dB/m at 1018 nm in the low power regime.
3. Experimental setup
The fabricated confined-doped fiber is applied in a master oscillator fiber amplifier (MOPA) platform similar to our previous experiments [39,40], which consists of a seed laser and a one-stage fiber amplifier as depicted in Fig. 3. The seed laser is a 100 W-level single-mode fiber laser operating at 1080 nm, which is injected into the fiber amplifier through a (6 + 1)×1 pump and signal combiner (PSC). The core/inner-cladding diameters of the PSC’s input and output signal port are 10/125 μm and 40/250 μm, respectively. A piece of 25-meter-long the as-fabricated confined-doped YDF is used to provide the active gain. The pump sources are three 3 kW level 1018 nm fiber laser modules, which are fusion spliced to the pump ports of the PSC. The redundant pump ports of the PSC are angle cleaved to 8°. A cladding mode striper (CMS) is spliced after the confined-doped YDF to remove the residual cladding power. Finally, the amplified laser is output through a quartz block holder (QBH). The core/inner-cladding diameter of the CMS as well as the QBH’s pigtail fiber is 40/250 μm. The confined-doped fiber is water-cooled on a heat sink.
The output laser from the QBH is first collimated by a collimator and then split into two beams by a highly reflective mirror (HRM). The reflected beam is measured by a power meter. Since the HRM could only reflect majority of the 1080 nm laser, the 1018 nm pump laser can pass without significant loss. Although the amount of 1018 nm pump laser is tiny, it could still affect the beam quality measurement. In order to get the beam quality factor accurately, the transmitted laser first passes through a dichroic mirror to eliminate the residual 1018 nm pump laser and then goes into the laser quality monitor (LQM) for beam quality measurement.
4. Results and discussion
According to previous study , fiber bending could shift the fundamental mode off the core center and reduce its overlap with the doped region, making the confined-doped fiber less effective in gain filtering and resulting in a degraded beam quality. Therefore, the effect of bending radius of the confined-doped fiber on the beam quality is first studied in a kW-level fiber amplifier. The confined-doped fiber is successively coiled to the bending radius of 10 cm, 15 cm and 20 cm in the inner circle, as shown in Fig. 3. When the pump is not applied, the above-mentioned fiber bending radii do not affect the mode content of the propagating laser, and the beam quality factor of the seed laser after passing through the fiber amplifier remains almost unchanged under these coiling radii, being ∼1.70. While when the pump laser is applied, the beam quality factor of the fiber amplifier with the bending radius of 10 cm continuously degrades to 1.98 at ∼1 kW output power while that of the fiber amplifier with 15 cm and 20 cm coiling radius is mostly preserved, being 1.73 and 1.72 respectively at a similar power level. Although the fiber amplifier with the coiling radius of 20 cm has slight beam quality improvement compared with that of 15 cm coiling radius, considering our available heat sink for efficient heat dissipation for multi-kilowatt fiber amplifier, the bending radius of 15 cm is finally chosen for further power scaling.
Then, the output power of the 1080 nm seed laser is set to 110 W. By continuously increasing the 1018 nm pump power to 7419 W, the output power from the fiber amplifier increases steadily from 100 W to 6200 W with an overall optical-to-optical efficiency of ∼82.22%. No power rollover is observed and the slope efficiency remains stable during the power scaling process, as shown in Fig. 4(a). The output spectra under different output powers are presented in Fig. 4(b), which are recorded by an optical spectrum analyzer with a 0.2 nm resolution. The full width at half maxima (FWHM) linewidth of the output spectrum increases from ∼1.14 nm of the seed laser to ∼4.46 nm under the maximum output power. Benefiting from the large mode area, the signal-to-noise ratio, defined as the intensity difference between the signal laser at 1080 nm and the Raman components at ∼1135 nm, is about 40 dB, indicating very good SRS suppression. Moreover, the proportion of the residual 1018 nm pump power in the total output power is calculated through spectral integration, which is ∼ 0.4‰. Therefore, the residual pump power only occupies a negligible proportion of the total output power.
The evolution of the beam quality factor (M2) is also measured under different output powers. The seed laser is a single-mode laser based on double clad fiber with 10 μm core size and core NA value of ∼0.08, however, when the pump laser is not applied, the beam quality factor of the seed laser becomes 1.43 after passing through the PSC, which is possibly resulted from the imperfect fabrication of the PSC. Then, the beam quality factor further degrades to 1.70 after passing through the confined-doped fiber owing to the refractive index profile mismatch between the 40/250 μm signal fiber of the PSC and the 40/250 μm YDF, as the refractive index profile of the confined-doped fiber is not uniform across the core (shown in Fig. 2(b)). The beam quality factor M2 maintains relatively well during the power scaling process, being 1.72 at the output power of 4.42 kW, as presented in Fig. 5. Further increase of the output power to 4.74 kW leads to the onset of TMI, where the beam quality starts to degrade, reaching 1.89 at the output power of 5.07 kW. As the output power continuously increases, the beam quality factor degrades rapidly to 2.05 at 5.37 kW and 2.3 at 6.20 kW owing to the severer TMI effect.
The temporal traces and the corresponding radio frequency spectra of this fiber amplifier are also recorded under different output powers. As can be distinguished in the radio frequency spectrum in Fig. 6(a), there is a small bump around 2 kHz when the output power reaches 4.74 kW, which is a typical characteristic of the onset of TMI. As the output power continuously increases, the small bump evolves into distinct peaks around 2.5 kHz and the circularity of the beam profile also becomes distorted, as shown in Fig. 6(b-c), which is an implication of the stronger TMI phenomenon. The occurrence of the TMI is also reflected in the standard deviations (STDs) of the temporal traces, as indicated in Fig. 6(d). The STD remains relatively stable before the emergence of TMI at 4.74 kW, and then grows rapidly from 0.0075 to 0.0351 as the output power increases to 5.37 kW.
In this experiment, the beam quality is basically maintained rather than improved in this designated confined-doped fiber. This could be attributed to several reasons. First, the actual mode distribution in the as-fabricated fiber is different from the calculated result. In the proposed simulation, the refractive index is regarded as uniform across the core, however, it is slightly higher in the doped region than the undoped area, making the mode shrink to the center of the fiber core. To be specific, the calculated effective mode area of the fundamental mode in the simulation is ∼740 μm2, however, the effective mode area decreases to ∼ 530 μm2 in the fabricated fiber, according to the measured refractive index profile depicted in Fig. 2(b). The reduced mode area could make the gain filtering less effective. Second, the fiber is bent with a radius around 15 cm in the amplifier, although the bend loss is negligible, the fundamental mode is slightly shifted away from the center of the fiber owing to the bending, which could also affect the gain filtering effect. Therefore, better beam quality could be expected by spooling the fiber with a larger coiling radius, even though only slight beam quality improvement was observed in a kW-level fiber amplifier with 20 cm coiling radius. Besides, in order to further improve the beam quality, one can either flatten the refractive index of the core by compensating the refractive index of the undoped region, thus enabling a more spread mode distribution, and at the same time adopt a large coiling radius to alleviate bend distortion, or incorporate the graded-index fiber design to make the fiber resistant to bend distortion .
Furthermore, a piece of fully-doped YDF with core/cladding diameter of 40/250 μm is adopted to study the beam quality evolution and the TMI threshold. This fully-doped YDF is coiled to an equivalent bending radius to that of the confined-doped fiber. As the output power increases, the beam quality factor of the proposed fiber amplifier gradually degrades from 1.52 of the seed laser to 2.28 at 1.76 kW, and then to 2.56 at the output power of 2.45 kW, as indicated in Fig. 7. The TMI threshold is around 1.76 kW, which is nearly 3 kW lower than our confined-doped fiber amplifier. The beam quality factor of the seed (M2=1.52) after passing through the fully-doped fiber amplifier is slightly better than that of passing through the confined-doped fiber amplifier (M2=1.7), which could result from a better mode matching between the output signal port of PSC and the fully-doped YDF. However, the beam quality factor of this fully-doped fiber amplifier constantly degrades even before the TMI appears. These results further prove the advantages of the confined-doped fiber for high-order mode suppression and TMI mitigation.
In summary, the gain property of the fundamental and high-order modes in the confined-doped fiber with 40/250 μm core/inner-cladding diameter was theoretically analyzed by simultaneously considering the transverse mode distribution and transverse hole burning effect. Subsequently, the confined-doped fiber with a 0.75 doping ratio was designed and fabricated after a comprehensive evaluation of the gain filtering effect and the fiber length. Based on the fabricated confined-doped fiber, 6.2 kW output power at 1080 nm was realized in a fiber amplifier through the forward tandem pumping scheme. Notably, the beam quality was well maintained during the power scaling process: the beam quality factor of the seed laser was 1.7 and only slightly increased to 1.72 at 4.42 kW, and then to 1.89 at 5.07 kW owing to the onset of TMI at ∼4.74 kW. Moreover, the SRS was also suppressed to about -40 dB lower than the signal laser at the output power of 6.2 kW thanks to the large mode area design. Further power scaling and better beam quality could be expected by mitigating the TMI effect. In stark contrast, the beam quality factor of the fiber amplifier based on fully-doped 40/250 μm YDF degraded to 2.56 at the output power of 2.45 kW and the TMI threshold was around 1.76 kW. Therefore, compared with the fully-doped fiber amplifier, not only the beam quality was improved, but also the TMI threshold was increased by ∼170% in the confined-doped fiber amplifier. This work reveals the intrinsic advantages of the confined-doped fiber for high-order mode suppression as well as TMI mitigation and proves the feasibility of employing the confined-doped fiber for realizing high-power fiber laser with good beam quality. Single-mode operation in even higher power levels could be expected by systematically optimizing the fiber parameters, coiling radii, and pumping schemes, etc.
Innovative Research Groups of Hunan Province (2019JJ10005); Hunan Provincial Innovation Construct Project (2019RS3018); National Natural Science Foundation of China (62035015).
The authors would like to thank Dr. Zilun Chen’s group for fabricating the cladding mode striper and the pump and signal combiner, and Liang Xiao, Jiawei He, Jiaxin Song as well as Cong Zhou for their kind help in the experiment.
The authors declare no conflicts of interest.
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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