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Abstract
Fiber side-pump couplers can enhance the output power of fiber laser due to their dependable and efficient operation and impressive power handling capability. We developed a tellurite fiber side-pump coupler by twisting and fusing a tapered pump fiber onto a target fiber. The effect of twisting parameters on coupling efficiency was comprehensively investigated through theoretical simulations and experiments. Experimental results exhibited an impressive coupling efficiency of 76.5% and a root mean square stability of 0.086% and 0.091% before and after one month, respectively, driven by an incident pump power of up to 4.2 W.
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1. Introduction
Mid-infrared (MIR) fiber lasers have gained increasing prominence in various applications, such as remote sensing, defense security, medical diagnosis, and environmental monitoring, owing to their distinctive absorption properties and atmospheric penetration capabilities [1–4]. The pump coupler is one of the critical optical components in the development of high-power MIR fiber laser systems particularly fiber-based end-pump combiners and side-pump couplers. Typical end-pump combiners, exemplified by tapered fused bundles, although the preparation method is simple and easy to obtain high coupling efficiencies, encounter issues due to fusion splicing and tapering of signal fibers (SFs), including mode-field mismatch, degradation of beam quality, and pump power limitation [5–8]. By contrast, side-pump couplers offer compelling advantages, including uninterrupted signal fiber, uniform multi-point pumping, and effective thermal management, all of which contribute to their potential applications in MIR fiber lasers [9–11].
A myriad of preparation techniques for silica fiber-based side pump couplers are available, including angle polishing, tapered capillary tube, v-groove, GT-wave, and tapered-fused [12–15]. However, applying these methods to soft-glass fibers proves challenging owing to their inherent properties, such as higher refractive index, lower melting temperature, and higher coefficients of thermal expansion compared with silica fibers. Nonetheless, some exciting research advances have been reported. In 2018, Schäfer et al. reported a pioneering fluoride side-pump coupler prepared by angle-polishing, achieving a coupling efficiency of 83% on a high-power fiber laser operating at a wavelength of 2.8 µm [16]. In the following year, their research team implemented these fluoride side-pump couplers to further extend the output power scalability to 30 W [17]. However, angle polishing relies on challenging fabrication steps, such as the angle-polishing and fusion of fluoride fibers. In 2020, Sébastien Magnan-Saucier et al. proposed an innovative fuseless fluoride side-pump coupler, prepared by wrapping a tapered pump fiber (PF) around a SF, achieving a coupling efficiency of 92.8% [18]. Although this preparation method demonstrates simplicity and efficiency, further investigations into its long-term stability are warranted. The specific impact of twisting on coupling efficiency remains inadequately explored. Variations in twisting angles change the propagation path of pump light, and the coupling path of pump light into the SF adjusts accordingly [19,20]. Excessive twisting angles may compromise the total reflection condition, leading to the direct leakage of pump light into the surrounding air and consequently reducing coupling efficiency.
Research on MIR side-pump couplers primarily centers around fluoride fibers [16,18]. Over the past several decades, fluoride fibers have been extensively utilized in MIR lasers due to their advantageous characteristics, including low phonon energy, low refractive index, and exceptional transmission capabilities in the MIR wavelength range [21]. However, their inherent drawbacks such as suboptimal mechanical performance and susceptibility to hydrolysis constrain their application in specific preparation steps and situations. Distinguished by their outstanding chemical durability, thermal stability, and mechanical robustness, tellurite fibers are expected to be an ideal alternative to fluoride fibers [22–24]. Tellurite fiber lasers predominantly employ lens coupling for pumping, as research on side-pump couplers based on tellurite fibers remains relatively limited [25]. In 2023, Xia et al. employed a tapered-fused method to systematically investigate the effects of taper length, waist diameter, and heating time on the coupling efficiency in a tellurite fiber-based coupler, ultimately achieving a coupling efficiency of 67% [26]. Nevertheless, pump power tolerance and coupling efficiency must be further increased to realize high pump power injection and improved pump power utilization. Therefore, the twisted-fused method was applied in tellurite fiber side-couplers to improve the coupling efficiency and output power stability.
In this work, a (1 + 1) × 1 twisting-fused side-pump coupler based on homemade tellurite fiber was investigated through theoretical simulation and experiments. In the simulation, beam propagation method (BPM) was used to analyze the impact of the waist region (including waist length and waist twisting) on the coupling efficiency. The introduction of pitch (P) related to the twisting angle can effectively respond to the effects of twisting on the coupling efficiency, and a suitable pitch range was provided. In the experiments, side-pump couplers with different waist lengths (0, 3, 5, 10, and 15 mm) were prepared, achieving a maximum coupling efficiency of 69%. The maximum coupling efficiency was increased to 76.58% by twisting PF around SF at the rotation angles of 30°. The output power reached 3.2 W at an injected pump of 4.2 W, with root mean square (RMS) stability of 0.086% and 0.091% before and after 1 month, respectively. This study offers a valuable reference for the preparation of MIR side-pump couplers.
2. Theoretical analysis
The diagram of a (1 + 1) × 1 side-pump coupler is shown in Fig. 1. The pump light (980 nm) was coupled through a laser diode silica pigtail (105/125 µm) into a PF (125 µm), which was then coupled into the SF (10/125 µm) from the side surface [27,28]. The refractive index of the silica fiber pigtail core and cladding used in the simulation was 1.445 and 1.428, respectively. The refractive index of the PF was determined to be 2.0281, and the refractive index of the core and cladding of SF were 2.0393 and 2.0281, respectively. The refractive index of tellurite glass fiber was measured by a prism coupler. The inset in Fig. 1 shows the refractive index along x-axis direction and y-axis direction of the taper region cross-section. Fresnel reflection took place at the splice interface between the tellurite PF and silica fiber pigtail due to their difference in refractive index, causing a pump power loss of 16.79% according to the Fresnel formula.
First, the relationship between waist length (Lwaist) and coupling efficiency was investigated by BPM of Rsoft. A taper length (Ltaper) of 40 mm, a waist diameter (Dwaist) of 10 µm, and a fusion depth (FD) of 1 µm were set in the simulation. In our previous work, the maximum coupling efficiency was obtained with these taper pumped fiber parameters [26], where the taper length is the length of the PF taper region and the waist diameter is the diameter of the PF waist region. Figure 2 shows the impact of waist length on pump coupling efficiency without twisting. As Lwaist increases from 0 to 15 mm, the coupling efficiency rapidly increases from 30% to nearly 77%. As the waist length continues to increase, the coupling efficiency eventually reaches a stable value of 87%, indicating that the pump light has been sufficiently coupled into the signal fiber. However, there is still a part of the pump light trapped in the waist, which does not couple into the signal fiber but leaks out at the end of the pump fiber [29]. Therefore, when fabricating side-pump couplers, the length of the tapered PF waist must be increased to allow sufficient coupling of the pump light into the signal fiber.
We presuppose that the PF is wrapped around the SF in a uniform manner and designate the SF axis as the center in the twisting simulation. γ represents the rotation angle of the pumped fiber wrapping path projected onto the cross-section, as shown in Fig. 3(a). The rotation angle of PF twisting one turn is 360°, and the length passed through is equal to 2πP, where P represents the pitch. A rotation angle of 0° indicates that the PF and SF are attached in parallel, and P is considered nonexistent. The waist length is maintained at a constant value, and P decreases with the increase in rotation angle (0°, 90°, 180°, and 360°). Similarly, at a fixed rotation angle (such as 180°), P increases with the waist length, as shown in Fig. 3(b). For the same radius of rotation, P is associated with the waist length and rotation angle. After accounting for the difference in waist length of the tapered PF and diameter of the SF by 1∼2 orders of magnitude in simulation and experiment, the overall length wrapped around the SF is approximately equal to the Lwaist, the calculation formula for P is described as P = (Lwaist*360°)/2πγ.
Fig. 3. (a) Helix situation of the tapered pump fiber under different rotation angles, and (b) different waist lengths.
Ltaper, Dwaist, and FD are fixed at 40 mm, 10 µm, and 1 µm, respectively. Figures 4(a) and (b) show the changes in the coupling efficiency and P with the increase in rotation angle and Lwaist, respectively. In the simulation, the rotation angle is increased from 5° by 5° each time and the Lwaist is increased from 5 mm by 1 mm each time. The coupling efficiency and P increase with the waist length at the same rotation angle. As shown in Fig. 4(a), the coupling efficiency increases rapidly when the rotation angle starts to increase from 5° and then maintains at a relatively high coupling efficiency in the range of 10° to 45°. When the rotation angle exceeds 45°, the coupling efficiency gradually decreases with the increase of rotation angle. The coupling efficiency can even be lower than that at a rotation angle of 0° when the rotation angle approaches 90°. Therefore, the rotation angle is not increased further in the simulation. A waist length of 15 mm and a rotation angle of 40° result in the maximum coupling efficiency of 95% with a P-value of 21 mm, representing a 17% improvement over a rotation angle of 0°. Further increasing the waist length will not lead to a remarkable improvement in maximum coupling efficiency in the range of rotation angles from 10 to 50°. Thus, this paper solely presents findings for a waist length of 15 mm. Nevertheless, the coupling efficiency is comparatively high when the rotation angle increases from 10° to 50°, which corresponds to a P-value ranging from 5 to 85 mm. The decrease in coupling efficiency at the Lwaist of 15 mm and the rotation angle of 25° is due to the fact that the coupling efficiency approaches saturation of about ∼90% at the Lwaist of 10 mm and the rotation angle of 25°. These findings suggest that the coupling efficiency increases with the P-value, exhibiting a relatively high coupling efficiency within a certain range, followed by a decrease. The explanation for this phenomenon is as follows: smaller rotation angle promotes the leakage of pump light out of the pump fiber, including the portion of the pump light that is trapped in the pump fiber, which is beneficial for coupling the pump light into the SF. However, more pump light leaks into the air as the rotation angle increases, resulting in lower coupling efficiency. When preparing twisting side-pump couplers, setting the P-value within an appropriate range is crucial to obtain great coupling efficiency.
Fig. 4. (a) Simulation of the relationship between pump coupling efficiency with the rotation angle and Lwaist, (b) Relationship between P with the rotation angle and Lwaist.
3. Experimental setup and discussion
3.1 Fabrication of the coupler
The bulk glasses of core and cladding were prepared through the melting and quenching technology [23]. The transition temperatures (Tg) and the crystallization onset temperature (Tx) of the cladding glasses of the signal fiber were ∼395 °C and 542 °C, respectively, exhibiting high thermal stability (T = Tx – Tg) of ∼147°C, which could effectively avoid the crystallization caused by high heating temperature during taper drawing and fusing. The homemade tellurite fibers used in the experiment were drawn from preforms prepared by the extrusion method [30]. The fiber propagation loss was measured by the cut-back method to be 1.1 dB/m at 1310 nm, primarily including the electronic absorption, and wavelength-independent scattering caused by impurities, introduced during the glass melting and fiber fabrication process [31]. The PF is a coreless tellurite fiber with a diameter of 125 µm, and the SF shows core and cladding diameters of 10 and 125 µm, respectively. The heating element of the (1 + 1) × 1 side-pump coupler is an Ω-shaped graphite filament that allows for uniform heating around the fiber as shown in Fig. 5(a). The heating power of the graphite filament can be precisely controlled to prepare tapered pumped fibers with specific waist diameter, waist length, and taper length. For the fabrication of twisting-fused side-pump couplers, the PF was tapered near its softening temperature with a specially designed taper length of 40 mm and waist diameter of 10 µm. Thereafter, the coating layer of the signal fiber was removed before it was placed in another groove next to the PF. The PF and SF were then fixed on the left holder, and the fibers were alternately shifted up and down on the right holder to complete twisting. Finally, the graphite filament was used for heating the coupling region. The heating power of the graphite filament was set in the range of 15∼22 W with a preheating time of 3∼10 s. The speeds of graphite filament and motorized stage are set in the ranges of 0.2∼1 mm/s and 1∼10 um/s, respectively. The PF (the part marked in the red dotted circle in Fig. 5(a)) was removed after the fusion. A microscope photograph taken at the twisting regions of the side-pump coupler is depicted in the insert of Fig. 5(a). The coupling efficiency test process is depicted in Fig. 5(b). A 980 nm laser coupled the pump light into the coreless tellurite fiber via the spliced silica fiber pigtail, and subsequently coupled the light into the cladding of the tellurite fiber from the side surface. The splicing loss was measured at 0.7 dB. The coupler was placed on a thermally conductive plate that was cooled by circulating water, and a thermal camera was used to monitor the temperature of the coupler. The output power was monitored by a thermal power meter. η is the defined coupling efficiency of the coupler, η=Pout/Pin, where Pout is the output pump power transmitted through SF, and Pin is the pump power injected into the PF.
Fig. 5. (a) Illustration of the (1 + 1) × 1 side pump coupler fabrication system (inset: partial microscope picture of the side-pump coupler). (b) Experimental setup for coupling efficiency measurement.
3.2 Experimental results and discussion
3.2.1 Effect of PF waist length on coupling efficiency
The relationship between the waist length of the tapered PFs and coupling efficiency was investigated. Five groups of PFs with different waist lengths (0, 3, 5, 10, and 15 mm) were tapered with the same taper length (40 mm) and waist diameter (10 µm), which were then used to fabricate couplers for the investigation. Figure 6(a) demonstrates the scanning curves of a tapered pumped fiber with a linear variation in the tapered region and a fluctuating error of 1∼2 µm in the waist diameter. To observe the variation of the coupling efficiency of the side pump coupler with different waist lengths. Figure 6(b) shows the results of coupling efficiency measured under pump power ranging from 0.1 W to 1 W, which stabilized with the increase in input power. The average value was taken as the coupling efficiency of this coupler. As Lwaist increased from 0 to 15 mm, the coupling efficiency also rapidly increased from 26% to 69.62%, which is in agreement with the simulation results above. However, Fig. 6(b) shows the coupling efficiency increases 5% from 10 to 15 mm, smaller than the 8% increment from 10 to 15 mm. This is due to the fact that as Lwaist increases, it becomes difficult to attach the pump fiber and the signal fiber at a rotation angle of 0°, resulting in errors during preparation. The maximum coupling efficiency in the experiment was lower than the value of 78.2% in the simulation. Excluding pump light leakage, the primary reason is the fiber propagation loss and the inability to precisely control the fusion depth similar to that in the simulation. Increasing the heating power results in a deep fusion, but may also cause deformation of the SF, introducing a non-negligible insertion loss. Conversely, decreasing the heating power results in a low fusion, and consequently a low coupling efficiency [32,33]. Figure 7(a) shows a 1000x cross-section of the waist region for a coupler with a waist length of 15 mm. The fusion portion of the SF and PF was observed at 5000x as shown in Fig. 7(b) (marked in the red dotted box in Fig. 7(a)). As measured through the fused portion of the PF and SF, the fusion depth was roughly 0.7 µm, which is lower than the value in the simulation. Hence, determining an optimal heating power that preserves adequate fusion depth while preventing SF deformation is critical to coupling efficiency.
Fig. 6. (a) Diameter scanning of different waist lengths (0, 3, 5, 10, and 15 mm) at different longitudinal positions for five different tapered pump fibers. (b) Coupling efficiency for different waist lengths at the input power of 0.1-1 W.
Fig. 7. (a) Cross-sectional view of the coupler 5 in the waist region at 1000x (the dotted box shows a portion of the fusion for the PF and SF), (b) Measurement of the fusion depth at 5000x for the PF and SF.
3.2.2 Effect of PF waist twisting on coupling efficiency
The twisting of the PF around the SF during coupler fabrication influences the coupling efficiency. PFs with different waist lengths (5, 10, and 15 mm) were tapered while ensuring that the taper length and waist diameter remained unchanged. The PFs were then twisted around the SF at various rotation angles (0°, 30°, 60°, and 90°) with the same fabrication parameters of graphite filament fusion time and heating power. The PF was twisted by moving it on the grooves of the holder, corresponding to different rotation angles. Figure 8 illustrates distinct sections of the fused-twisting side-pump coupler when observed under a 500x microscope. As displayed in Figs. 8(a) and (b), the PF attached to the SF, maintaining a parallel alignment at the taper region and gradually twisting around the waist region. The end tip of the waist region after removing the other end of the PF is shown in Fig. 8(c).
Fig. 8. Images showing the different sections of a twisting-fused side-pump coupler: (a) the dotted box shows a portion of the PF taper region along the SF; (b) the waist region of the PF twisting along the SF;(c) the end tip of the waist region after removing the pump fiber;
According to the simulation results, the coupling efficiency was relatively high in the range of 10°∼50° of the rotation angle under the same waist length, corresponding to a P-value of 5∼85 mm. The coupling efficiency of the twisting-fused side pump with different Lwaist and rotation angles was measured under pump power ranging from 0.1 to 1 W, and the corresponding average values are shown in Table 1. These results indicate that the coupling efficiency can be improved by appropriately increasing the Lwaist and rotation angles. For rotation angles of 30°, 60°, and 90°, the coupling efficiency can be effectively increased by increasing the waist length. Regardless of the waist length, the coupling efficiency was relatively high at a rotation angle of 30°. As the rotation angle increased, the coupling efficiency subsequently decreased. The coupling efficiency at a rotation angle of 90° was lower than that at a rotation angle of 0°, which is consistent with the simulation results. The maximum coupling efficiency of 76% was achieved at a rotation angle of 30° and a waist length of 15 mm. The coupling efficiency in the experiment was lower than that in the simulation. One primary factor is the propagation loss of the tellurite glass fiber. The total length of the tellurite glass fiber used in the preparation of the side pump coupler was about 50 cm and the pump light loss was about 0.55 dB. Another factor to be considered is the control of fusion depth. Nevertheless, the coupling efficiency can be improved by approximately 8% through twisting.
Table 1. Coupling efficiency of the coupler with different values of Lwaist and rotation angle.
The tellurite fiber coupler demonstrating the maximum coupling efficiency was placed in an environment at room temperature for one month, where its coupling region was exposed to the air without protection, and subsequently tested for output power to evaluate the stability of its output power. Stability testing requires the coupler to be operated at a higher power to observe the operating state. As depicted in Figs. 9(a) and (b), the measured injected pump power to the coupler was 4.2 W. The output power before and after 1 month reached 3.2 W, with RMS stability of 0.086% and 0.091%, respectively. The maximum temperatures recorded during the testing were 57.3 °C and 60 °C, as shown in the inset of Figs. 9(a) and (b). These findings suggest a slight decrease in the coupler’s output power after 1 month potentially due to the UV adhesive used at the junction of the silica fiber pigtail and the coreless tellurite fiber. The elevated temperature of the coupler is caused by the lack of a package, leading to the accumulation of dust on the surface of the coupler. Nevertheless, the RMS stability of the coupler only increased by 0.004%. This finding demonstrated that the tellurite fiber coupler exhibits good output stability. The temperature of the side-pump coupler in the test reached 57°C, this is due to the impurities introduced during the glass melting process. These bubbles and impurities can cause damage to the fiber structure during the fiber drawing process. As a result, heat accumulation occurs when pump light is injected. However, with the improvement of the glass melting process and fiber drawing technology, the side-pump coupler operating at high power is hopeful.
Fig. 9. Output power of the tellurite fiber coupler over time before (a) and after (b) 1 month for 1 hour while pumping with 4.2W at 980 nm.
4. Conclusions
The coupling efficiency of the (1 + 1) × 1 twisting-fused side-pump coupler was analyzed by modeling simulation and experiments. The findings indicate that the coupling efficiency can be enhanced by employing a suitable twisting method, resulting in a maximum improvement of approximately 17% in the simulation compared with that of untwisted couplers. We achieved a maximum coupling efficiency of 76.58% by twisting and appropriately increasing the waist length, enabling an output power of up to 3.2 W with RMS stability of 0.086% for 1 hour. In future works, coupler fabrication techniques and active cooling will be optimized to enhance the injected pump power and stabilize performances. The results have great significance in the development of MIR lasers.
Funding
Natural Science Foundation of Zhejiang Province (No. LR24F050001); Key R&D Program of Ningbo City (2022Z208, 2023Z105); National Natural Science Foundation of China (62090064, 62090065); K. C. Wong Magna Fund in Ningbo University.
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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