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Abstract
In this paper, we established a high power tandem pumped fiber amplifier based on tapered ytterbium-doped fiber (TYDF). The TYDF is developed in-house with a core/inner cladding diameter of 30/250 µm at the small-core region and 48/400 µm at the large-core region. The key parameters of the amplifier in a co-pumped and counter-pumped configuration are experimentally investigated, such as slope efficiency, stimulated Raman scattering (SRS) threshold, and beam quality evolution. Up to 10.28 kW laser free of SRS or transverse mode instability is obtained from the counter-pumped amplifier, and the beam quality factor M2 is 2.29, which is significantly improved compared with the 48/400 µm uniform YDF. To the best of our knowledge, this is the highest average output power achieved so far based on the TYDF. This work could provide a solution for balancing the SRS suppression and high order modes control in high power tandem pumped YDF lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High power ytterbium-doped fiber lasers (YDFLs) with the characteristics of compact structure, reliable stability, and high electro-optical efficiency are gradually becoming the mainstream laser sources in industrial manufacture and scientific research [1,2]. Tandem pumping, utilizing 1018 nm fiber lasers to pump YDF, is one of the main technical solutions to obtain >10 kW fiber laser with good beam quality in a single fiber [3], due to its advantages of high pumping brightness and low thermal load. In 2010, IPG Photonics Corporation achieved 10 kW single-mode fiber laser output using tandem pumping technology [4]. Since then, tandem pumping has become a research hotspot in the field of high power fiber lasers [5–7]. However, because the absorption of ytterbium ions at 1018 nm is very weak, ultra-long active fibers have to be used to ensure that the pump laser can be fully absorbed [3], which will lower the threshold for nonlinear effects, such as stimulated Raman scattering (SRS). For instance, Tao et al. [8] constructed a tandem pumping amplifier utilizing YDF with a core/inner cladding diameter of 30/250 µm. The YDF has a length of 32 m for a total pump absorption of 19 dB at 1018 nm, which is several times the length required for pumping at 915 nm. As the output power reaches 3560 W, the intensity of Raman light started to increase nonlinearly, which would hinder further power scaling. To overcome the limitation of SRS, a practical method is to use fibers with larger core diameters to enhance the SRS threshold by reducing the power density in the core [9,10]. For example, a 10 kW tandem pumping fiber amplifier was built employing YDF with a core diameter of 48 µm [9]. Due to the increased mode field area, SRS was effectively suppressed. However, beam quality factor M2 was not revealed in the literature. Notably, the larger core diameter causes a dramatic increase in the number of guided modes in the core and a much smaller effective refractive index difference between the fundamental mode (FM) and the high order modes (HOMs) [11]. This indicates that the commonly used bending filter technique [12] for HOMs control is inefficient. Consequently, it is very challenging to suppress SRS while maintaining excellent beam quality simultaneously if employing ultra-large core fiber.
To solve the problem, researchers have conducted a lot of fiber design work, attempting to change the fiber structure to suppress SRS while maintaining good beam quality [13], such as confined-doped fiber [14,15], photonic crystal fiber [5,16], chirally-coupled-core fiber [17,18], all-solid photonic bandgap fiber [19,20], leakage-channel fiber [21,22], large-pitch fiber [23,24], multi-trench fiber [25,26], and so on. In summary, these special fiber designs aim to change transverse waveguide structures to reduce the overlap between HOMs and the doped region or enhance the loss of HOMs. However, the complex fabrication techniques and poor adaptation to conventional passive fibers limit their application in high power laser systems. In addition to the transverse design, the laser performance can be improved by changing the longitudinal structure of the fiber, such as tapered YDF (TYDF) [27–35]. TYDF generally refers to an active fiber in which the diameters of the fiber core or cladding vary longitudinally. TYDF can be divided into single-tapered fiber [27], spindle-shaped fiber [36], and saddle-shaped fiber [37] according to the longitudinal variation of the geometric dimensions of the fiber. Single-tapered fiber has been widely studied due to its simpler fabrication process compared with spindle-shaped fiber and saddle-shaped fiber. The TYDF mentioned below refers to single-tapered fiber. The TYDF can be divided into three regions according to the fiber diameter distribution along the longitudinal direction: small-core region, tapered region, and large-core region. The small-core region can be designed to control the number of guided modes for better beam quality [32,38], and the tapered region has a length of several meters, which is essentially a mode field adapter (MFA) with good beam-quality preservation [39,40], while the large-core region can increase the effective mode-field area and help to raise the nonlinear effect threshold [33,41]. Compared with conventional uniform fiber, TYDF has a unique advantage in balancing SRS suppression and beam quality improvement. In 2008, Filippov et al. first reported the TYDF and applied it in fiber lasers [27]. Using TYDF with a small core/inner cladding diameter of 5.6/174 µm and a large core/inner cladding diameter of 27/834 µm as the gain medium, a fiber oscillator was built based on bulk optics and 84 W single mode laser (M2 = 1.07) was achieved. Soon in 2009, the output power of TYDF laser reached 600 W [40]. Although the core diameter of the large-core region of the TYDF was as large as 65 µm, single mode laser output with M2 = 1.08 was preserved well. The outstanding performance of TYDF in maintaining good beam quality has attracted much attention and the TYDF have gradually become a selection for high power or high energy fiber lasers [32,42–44]. In 2022, Ye et al. constructed a monolithic amplifier with a homemade TYDF. The core/inner cladding diameter of the small-core region of the TYDF was ∼20/400 µm and that of the large-core region was ∼30/600 µm [34]. And an output power of over 4 kW as well as M2 ∼ 1.46 was achieved. Nevertheless, all previous studies on TYDF lasers were based on LD pumping schemes, and there have been no reports on the tandem pumping schemes. In fact, combining TYDF with the tandem pumping scheme can offer a number of benefits. Firstly, the pump lasers used in the tandem pumping scheme has higher brightness than LDs, which indicates weaker vignetting effect [45] when the pumping light is coupled into TYDF from the large-core end. Secondly, the problem of mode control in tandem pumping amplifiers with large core diameter fiber can be partially addressed by utilizing the beam quality preservation capability of TYDF. Thirdly, the intrinsic mode mixing mechanism of TYDF is beneficial for pump absorption, which can make up for the shortcomings of tandem pumping to some extent [27].
In this paper, for the first time, we build an all-fiberized high power tandem pumping amplifier utilizing the TYDF. The laser performance is experimentally studied in detail for co-pumped and counter-pumped configurations. In addition, we also conduct a comparative study between TYDF and conventional uniform YDF, and the ability of TYDF to improve beam quality is verified.
2. Fiber fabrication and characterization
The fiber preform is fabricated by the conventional modified chemical vapor deposition (MCVD) process combined with a solution doping technique. The TYDF can be fabricated with a variable-speed drawing method [41]. The fabrication process of TYDF mainly includes the following steps. Firstly, the silica rod prepared by the MCVD process is jacketed with a suitable silica tube to form a resulting preform that can fully meet the TYDF design requirement. The diameters of the doping core and the preform are 1.8 mm and 15 mm, respectively. Secondly, the prepared fiber preform is placed on the fiber drawing tower, and the fiber core and cladding diameter are controlled by changing the fiber drawing speed. Finally, the fiber is coated and ultraviolet curing during the drawing process. The refractive index profiles of the TYDF (plotted in Fig. 1) are measured at the large-core region. It is worth noting that there is a refractive index dip at the center of the fiber core. This is owing to the volatilization of the dopant (e.g., Ge or P) from the innermost layers during the collapsing stage [46]. The refractive index difference between core and inner cladding is ∼1.60 × 10−3, and the corresponding core NA is 0.067.
In this work, we fabricate a constant core-cladding ratio TYDF with a length of 60 m. Figure 2 shows the inner cladding diameter distribution of the fiber measured by a fiber dimension monitor during the drawing process. The core/inner cladding diameter of the small-core region is 30/250 µm, while that of the large-core region is 48/400 µm. The taper region has an approximately linearly varying diameter in the longitudinal direction. The lengths of these three regions are about 25 m, 10 m, and 25 m respectively. Notably, the curve in Fig. 2 is fluctuating rather than smooth because the drawing speed is adjusted in real-time during the drawing process. The fluctuations in the diameter of the inner cladding in the small-core region and the large-core region are within ±6 µm and ±8 µm, respectively. Since the core-cladding ratio is 0.12, we can calculate that the fluctuations in the core diameter in the small-core region and the large-core region are within ±0.72 µm and ±0.96 µm, respectively, which are very small. In addition, the absorption coefficient of this fiber is measured to be ∼0.32 dB/m at 1018 nm. For a distinct comparison, a conventional uniform fiber based on the same fiber preform is also fabricated, with a core/inner cladding diameter is 48/400 µm. The length of the uniform fiber is also 60 m.
3. Experimental setup
The laser performance of our homemade TYDF is tested in the amplification stage of a high power co-pumped and counter-pumped amplifier respectively. For the co-pumped amplifier (Fig. 3(a)), the seed laser passes through a chirped tilted fiber Bragg grating (CTFBG) and a forward pump/signal combiner (FPSC) into the small-core end of TYDF. The seed laser is a broadband fiber oscillator with a center wavelength of 1080 nm. The core/inner cladding diameter of its pigtail fiber is 20/400 µm. The power of the seed laser output from the amplifier stage is 160 W. The CTFBG can filter out the Raman noise of the seed laser to enhance the SRS threshold. The core/inner cladding diameter of the CTFBG is 20/400 µm. A (6 + 1) × 1 FPSC is used to couple the seed laser and co-pumped laser into the active fiber. The core/inner cladding diameters of the input and output signal fibers of the FPSC are 20/250 µm and 30/250 µm, respectively. The pump sources are 1018 nm fiber laser modules, which are fusion spliced to the pump ports of the FPSC. The TYDF is coiled in a spiral groove, as shown in Fig. 3(c), with a bending diameter ranging from 28 to 45 cm and a pitch of 1 mm. The TYDF with the small-core end is coiled with a small bending diameter. In addition, signal light propagates from the small-core end to the large-core end. At the large-core end of the active fiber, a cladding power stripper (CPS) is employed to get rid of residual pump light and undesirable cladding signal light. Finally, the amplified laser is output through a quartz block holder (QBH). To match with the large-core region of the TYDF, the pigtail fiber of the CPS2 and the QBH is 48/400 µm. And the total length of the CPS2 and QBH is ∼4 m. All the fiber components are placed on a water-cooled heat sink for efficient thermal management.
Fig. 3. Experimental schematic of the forward (a) and backward (b) tandem pumping amplifier and schematic diagram of the fiber groove (c). (CTFBG: chirped tilted fiber Bragg grating, MFA: Mode Field Adaptor, CPS: cladding power stripper, FPSC: forward pump/signal combiner, BPSC: backward pump/signal combiner, QBH: quartz block head) and measuring system (CO: collimator, HR: high-reflectivity mirror, PM: power meter, OSA: optical spectrum analyzer, PD: photodetector, LQM: laser quality monitor).
In the counter-pumped amplifier, the pump laser is launched into the large-core end of TYDF via a homemade (6 + 1) × 1 backward pump/signal combiner (BPSC), as shown in Fig. 3(b). The core/inner cladding diameters of input and output signal fibers are both 48/400 µm. The total length of passive fiber from the output end of the TYDF to the QBH is 5.5 m. Another stripper (CPS1) is introduced between the CTFBG and the active fiber to get rid of the unabsorbed pump light. The core/inner cladding diameter of the input and output fibers of CPS1 are both 30/250 µm. Notably, CTFBG, FPSC, and BPSC used in the experiment are all made in house.
The output laser from QBH is collimated by a collimator (CO) and then split into two beams by a highly reflective mirror (HR). The reflected beam is collected by a power meter (PM), and its spectrum and temporal trace are monitored by an optical spectrum analyzer (OSA) and a photodetector (PD), respectively. Meanwhile, the transmitted laser enters the laser quality monitor (LQM) for beam quality measurement.
For comparison, we also set up a counter-pumped amplifier with the uniform 48/400 µm YDF mentioned above. Compared with TYDF amplifiers, the change is only in the geometry of the fiber components besides the active fiber. The core/inner cladding diameter of the pigtail fiber of CPS1 is 48/400 µm. In addition, MFA is used between CTFBG and CPS1 to match the mode fields of the seed laser with the active fiber. The core/inner cladding diameters of the input and output fibers of MFA are 20/400 µm and 48/400 µm, respectively.
4. Results and discussion
4.1 Output properties in the co-pumped TYDF amplifier
First of all, the performance of the TYDF amplifier in the co-pumped configuration is studied. As the pump power rises, the output power increases almost linearly with the slope efficiency of ∼82.6%, as shown in Fig. 4(a). The measured spectra at different output powers are shown in Fig. 4(b). The Raman suppression ratio reduces from 50 dB at 3.5 kW to 36 dB at 4.25 kW, which means further power scaling is restricted by SRS. The 3 dB linewidth increases from 1.62 nm of the seed laser to 3.9 nm at 4.25 kW. Self-phase modulation is the main reason for the spectral broadening [47]. Figure 4(c) shows the M2 factors under different output powers. The beam quality of the seed light measured from the QBH is 1.65. Beam quality is maintained well before the power increases to 1 kW. However, there is a slight degradation in beam quality at higher power and the M2 factor reaches 1.86 at 4.25 kW. The reason might be attributed to the thermally induced change in the refractive index of the fiber, which causes mode coupling.
Fig. 4. Output properties in the co-pumped TYDF amplifier. (a) Output power versus pump power. (b) Measured spectra at the different output power. (c) Beam quality under different powers.
4.2 Output properties in the counter-pumped TYDF amplifier
Following this work, the output properties of the TYDF amplifier in counter-pumped configuration are studied at higher power. Benefiting from the advantages of the counter-pumped scheme, the seed power is boosted to 10 kW almost linearly without any sign of SRS. When the maximum pump power is 12.55 kW, the output power is 10.28 kW. Further scaling in the power is only pump-limited. The overall slope efficiency of the TYDF amplifier is ∼80.6%. There are two reasons for the slightly lower efficiency in this case than that of co-pumped. One is the higher insertion loss of BPSC for pump light. The other is due to the extra signal loss introduced by BPSC. The output spectra of the laser are shown in Fig. 5(b). At the maximum output power of 10.28 kW, the signal laser centers at the wavelength of 1080 nm with a 3 dB linewidth of 3.12 nm. Figure 5(c) shows the temporal traces measured at 10.28 kW and corresponding frequency components calculated through Fourier transformation. There is no sign of transverse mode instability (TMI) at the highest output power. The evolution of beam quality with output power is shown in Fig. 5(d). The M2 factor of the seed light measured from the QBH is 1.66, which indicates that the seed light propagating through the TYDF contains HOM components. As the output power increases to 2.23 kW, the M2 increases rapidly to 1.97. This is because the pumped light is primarily distributed in the large-core region for counter-pumped cases, facilitating HOM amplification through mode competition. Then, the beam quality slowly degrades with increasing output power, until reaching a maximum output power of 10.28 kW, where the measured M2 is 2.29, as shown in Fig. 5(e). The slow degradation of beam quality at high power might be related to the continuously rising temperature of the TYDF. Heat not only affects the mode bending loss but also makes the mode coupling coefficients larger [48], which makes the FM coupled to the HOM more easily and leads to the degradation of the output beam quality. In addition, overheating of a BPSC might also deteriorate the beam quality.
Fig. 5. Output properties in the counter-pumped TYDF amplifier. (a) Output power versus pump power. (b) Measured spectra at the different output power. (c) The temporal signal of the PD (inset) and its corresponding Fourier transform results at the maximum power. (d) Beam quality under different powers. (e) The beam profiles of the output laser at 10.28 kW.
4.3 Comparison between the TYDF and uniform YDF amplifier with counter-pumped scheme
For comparison, the characteristics of the uniform 48/400 µm YDF drawn from the same preform are investigated based on the setup depicted in Fig. 3(b). The output power grows with pump power at a slope efficiency of ∼77.7%, as shown in Fig. 6(a). It can be seen that the TYDF amplifier presents higher slope efficiency than that of the uniform fiber amplifier. This is mainly attributed to that the gradually-varying tapered region can enhance the pump absorption [27]. This means that the tapered fiber can shorten the required length. The output spectra of the uniform YDF amplifier are shown in Fig. 6(b). The 3 dB and 10 dB linewidth of the laser measured at 10.07 kW are 3.04 nm and 6.36 nm, respectively. Compared with the TYDF amplifier, the output laser has a narrower linewidth, which is owing to the larger equivalent mode field area of TYDF and the relatively weaker nonlinear effect. It can be observed that there is no Raman scattered light in the wavelength range of ∼1134 nm, and the Raman suppression ratio is greater than 57 dB. Figure 6(c) shows the temporal traces measured at 10.07 kW and its corresponding Fourier transform results, and there is no sign of TMI. The M2 factor of the output laser at different power levels is illustrated in Fig. 6(d). The evolution of beam quality here is similar to the trend depicted in Fig. 5(d). The M2 factor of the seed laser after passing through the amplifier is 1.81 and degrades rapidly to 2.64 at 2.23 kW. Then the beam quality varies slightly at higher power. At the highest output power of 10.07 kW, the M2 factor is measured to be 2.86.
Fig. 6. Laser properties of the amplifier using uniform YDF. (a) Output power versus pump power. (b) Measured spectra at the different output power. (c) The temporal signal of the PD (inset) and its corresponding Fourier transform results at the maximum power. (d) Beam quality under different powers.
4.4 Discussion
Comparing the co-pumped and counter-pumped amplifiers, they have significant differences in beam quality evolution and SRS thresholds. The beam quality of the former is maintained well at lower output power, while that of the latter degrades rapidly. This is because the HOMs have higher gain in the large-core region where the pump power is much higher when counter-pumped. The counter-pumped amplifier has significantly higher SRS thresholds than the co-pumped amplifier. The discrepancy is caused by the diverse distribution of pump light. In co-pumped cases, the signal laser is amplified in the foremost section, where the fiber core diameter is smaller, resulting in a greater power density and lower SRS threshold. On the other hand, for counter-pumped cases, signal laser amplification takes place in the back section, where the fiber core diameter is larger, decreasing the power density and resulting in a higher SRS threshold.
Comparing the uniform YDF amplifier and TYDF amplifier, the beam quality evolution trends are similar for both, but the beam quality of the TYDF amplifier is overall better than that of the former, thanks to the longitudinally varying geometry of the TYDF. The small-core region supports fewer modes, combined with effective mode control methods such as fiber coiling can further suppress HOMs. The tapered region is essentially an MFA with good beam-quality preservation. The variation of core diameter along the longitudinal direction leads to a gradual variation of the difference between the propagation constants of FM and HOMs along the longitudinal direction of the fiber, which can reduce the interference between different transverse modes of the laser transmitted in the fiber and maintain a good beam quality [49]. Indeed, the effective core diameter of TYDF is smaller than that of uniform YDF, which means TYDF has a lower Raman threshold. However, comparing the spectral results of both, there is no evidence of SRS occurring at the highest power, suggesting that both have a high Raman threshold. This is because in the counter-pumped TYDF amplifier, the Raman light is amplified primarily in the back section of TYDF, where the larger diameter can effectively suppress the SRS.
At present, the beam quality obtained from our TYDF is far from single mode. Therefore, future work will focus on the development of a theoretical model for the TYDF amplifier to analyze the mode evolution along the fiber and guide the fiber design. Then, the refractive index distribution and the geometric parameters of the TYDF, such as taper ratio, taper length, longitudinal profile shape, and core/inner cladding diameter, will be optimized to achieve higher output power and improved beam quality.
5. Conclusion
In conclusion, this work presents the first experimental implementation of the tandem pumping scheme combined with tapered fiber. An all-fiberized tapered fiber amplifier with good beam quality was demonstrated by employing homemade TYDF that has a core/inner cladding diameter of 48/400 µm at the large-core region. Compared with the laser performance of uniform 48/400 µm YDF, the TYDF provides slightly higher efficiency and much better beam quality. The maximum power is only 4.25 kW when the TYDF is co-pumped. Further power scaling is limited by SRS. While adopting the counter-pumped scheme, 10.28 kW output power is obtained with a slope efficiency of 80.6%. Further power scaling is only pump-limited. To the best of our knowledge, this is the highest average output power achieved so far based on the TYDF. The M2 at 10.28 kW is 2.29, which is significantly improved compared with that of uniform YDF at the same power level. This work might provide an effective and practical approach to achieve high power and high beam quality fiber lasers by employing TYDF.
Funding
National Natural Science Foundation of China (11974427, 12004431); Science and Technology Program of Hunan Province (2021RC4027); State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR05, SKL2021ZR01).
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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