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Abstract
In this paper, we investigate a (2 + 1)×1 side pump combiner with high power handling capability and high beam quality numerically and experimentally. Through theoretical research, we have found how the combiner’s parameters influence its pump coupling efficiency and maximum load power. According to the numerical analysis, we fabricate a side pump combiner, which consists of two pump fibers (220/242 µm, NA = 0.22) and a signal fiber (20/400 µm, NA = 0.06/0.46). The coupling efficiency of the side pump combiner is 97%, which is tested under 2 kW input pump power. Using two side pump combiners and four high power 976 nm laser diodes, a bi-directionally pumped fiber laser oscillator is constructed and tested. The fiber laser oscillator’s maximum output power is 2150 W when injecting 3070 W pump power, corresponding to a slope efficiency of ∼72%. No stimulated Raman scattering or transverse mode instability was observed during the operation test. Thanks to the advantage of maintaining high beam quality of the home-made side pump combiner, this all-fiber laser oscillator achieves single mode laser output (M2=1.05) even when the output power increases to more than 2 kW. By further improving the fabrication technique and using larger core fibers, output laser power could be greatly increased, which is very important for high-power fiber lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the characteristics such as high optical conversion efficiency, high beam quality, compact system structure and easy heat management, fiber laser sources are favorable in many fields such as industry, remote sensing and medicine [1–3]. To improve the output power of fiber lasers, many fiber components are investigated, such as fiber grating, signal combiner and pump combiner. Among the pump combiner, the most typical type is a fused tapered fiber bundle (TFB) [4,5], which has the advantages of simple structure, high coupling efficiency, and easy packaging. However, suffered from various non-linear effects, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), transverse mode instability (TMI), etc., fiber lasers are limited in power scaling. Counter-pumping or bi-directional pumping scheme is believed to be useful in increasing the threshold of SRS and TMI, and therefore improving the laser’s output power [6,7]. When using as the counter-pumping combiner, there are many disadvantages among the TFB technology, for example, the signal fiber is also tapered with the pump fiber during the fabrication, leading to a degraded beam quality and introducing a non-negligible insertion loss. In addition, it is hard to achieve multi-point cascade pump structure for the end pumping combiner, which will limit the system’s total pumping capacity. In contrast, side pumping technology can solve those problems mentioned above well, because for the side pump combiner, the pump power is coupled through the side of signal fiber, making it easy to achieve multi-point cascade pump structure. In addition, the signal fiber is uninterrupted during the combiner’s fabrication process, which is beneficial to reduce insertion loss and maintain system’s beam quality. Moreover, the excellent backward pump isolation (BPI) of the side pump combiner guarantees it more advantages in counter-pumping or bi-directional pumping scheme [8].
The side pump technology can be divided into two categories, namely non-all-fiber and all-fiber structures. Non-all-fiber structure side pumping is achieved by using facilities such as adhered micro-prism [9], V-groove [10], embedded mirror [11] and diffraction grating [12]. However, suffered from the complex spatial structure, this type of pumping method has a relative low stability, low coupling efficiency and low power handling capability.
All-fiber structure side pump combiners have been recognized as the key components to achieve high power all-fiber laser systems and have therefore attracted more and more attentions in the past few years. In 2012, Theeg et al. developed a (4 + 1)×1 side pump combiner with the coupling efficiency of 90.2% and the maximum handling pump power of more than 440 W [13]. In 2013, the researchers in Tsinghua University fabricated a (2 + 1)×1 side pump combiner with a refractive valley configuration, which can effectively prevent the pump laser from damaging by the backward light [14]. In this experiment, the measured beam quality of M2 factor of the output laser was 1.33 under 244 W output power. Thanks to this special structure, they applied this kind of combiner to 3.89 kW bi-directionally pumped Raman fiber laser, with the measured beam quality of M2 was 1.49 [15]. In 2018, Fanlong Dong et al. constructed a multi-point pumped fiber laser with three side pump combiners. The side multi-pump scheme helps to reduce the temperature of gain fiber to 36.5 °C when injecting 988 W pump power, while in the end pump scheme the temperature is 47.6 °C under the same input pump power [16]. Due to the side pump combiner scheme and the signal fiber’s special structure (triple clad fiber), the measured beam quality of M2 was 1.06 under 645 W output power. In our previous work, we have developed a kind of all-fiber side pump combiners using tapered-fused method [8,17,18], which is based on the fusion of tapered pump fibers to the side surface of double-clad signal fiber. Due to the high coupling efficiency and high power handling capability, the tapered-fused side pump combiners have been used in several high power fiber laser systems [15,17]. The high pump/signal transmission and high-power capability of the side pump combiner based on tapered-fused technique facilitates its wide application in counter-pumping scheme, whose input and output signal fibers usually have the same core size. At present, side pump combiners are usually employed in MOPA (master oscillator power amplifier) structure to achieve the combination of pump light and the signal light in an amplifier, it is rarely reported to be utilized in a fiber oscillator. In 2016, Qirui Tan et al. reported a cascaded combiner structure. They used five (1 + 1)×1 side pump combiners to construct a bi-directionally pumped fiber laser oscillator, achieving 780 W output power when injecting 1096 W pump power [19]. However, the output power of the oscillator was limited by the total pump efficiency and unwanted signal insertion loss of the cascaded structure.
Since the unique advantages of side pump combiner make it possible to be employed in a high-power bi-directionally fiber laser oscillator, in this work we design and fabricate two (2 + 1)×1 side pump combiners and use them to realize an efficient 2 kW single mode all-fiber oscillator in a bi-directionally pumped scheme. Theoretically, we analyze the key parameters that affect the coupling efficiency, backward pump isolation and power handling capability of the side pump combiner, laying the foundation for experimental fabrication. In the experiment, a side pump combiner with high coupling efficiency (∼97%) is developed and applied in a high power oscillator. In the oscillator, two home-made side pump combiners and four 976 nm laser diodes are employed in the bi-directional pumping scheme. The oscillator system generates 2.15 kW laser output at 1080 nm at the pump power of 3.07 kW. Thanks to the good beam quality characteristic of the home-made combiner, the system maintains a single mode laser output (M2=1.05) even when the output power increases to more than 2 kW. In addition, if larger core fibers are used as the signal fiber of the side pump combiner, output laser power could be further scaled by using the side pump combiners, which is very important for high-power fiber lasers.
2. Combiner design and fabrication
2.1 Theories and simulations
Figure 1 shows the structure of (2 + 1)×1 side pump combiner. Two tapered pump fibers (the front side of transition region and the waist) are attached firmly to the signal fiber, and then through the heating and fusion process, the pump fibers are embedded into the signal fiber so that the pump light could be coupled into the signal fiber. The degree of fusion could be expressed by the fusion depth and fusion length, which are described in Fig. 1. However, not all the pump light could couple into the signal fiber, those leaked pump light can be divided three types: leakage power along the transition section (LPT), leakage power at the end of pump fiber (LPE) and leakage power into the coating of signal fiber (LPC) [8]. When the internal total reflection condition is satisfied, those pump light with small propagation angle will leak into the air at the end of pump fiber as LPE, or otherwise they will leak into the air at the surface of signal fiber as LPT. For the pump light coupled into signal fiber successfully, if their propagation angle exceeds the maximum light transmission angle of the signal fiber, they will leak into the polyimide coating as LPC. While LPE and LPT could be handled by the cooling devices easily, making no influence on the combiner’s power handling capability, LPC will cause the temperature the increase of polyimide coating significantly, limiting the further enhancement of the power handling capability of the combiner. In summary, LPC is more critical when compared with LPE and LPT, therefore, we mainly use the value of LPC to measure the combiner’s power handling capability in the theoretical simulation.
The theoretical analysis is based on the beam propagation method (BMP), which is built for solving the Helmholtz equation in the waveguide [20], and some related basic simulations have been given in our previous works [8,17,18]. The core/inner cladding diameter of the double-clad signal fiber is 20 µm/400 µm, and the NA = 0.06/0.46. For pump fiber, the core/cladding diameter is 220 µm/242 µm, and the NA = 0.22. The pump wavelength is set to 976 nm, and since the pump fiber is a multi-mode fiber, the launching pump mode is randomly selected in the simulation (here we choose LP11, LP33 and LP66 as the mode of input pump light). The results of calculation are shown in Fig. 2. It can be observed in Fig. 2(a) that there is an optimal waist diameter of the tapered pump fiber to achieve highest coupling efficiency and this optimal value is larger for higher-order input mode: for LP11 and LP33 mode, the optimal waist diameter is about 20 µm, while for LP66 mode, the optimal value is about 36 µm. From Fig. 2(b), we can find that, for all the input modes, with the increase of fusion depth, the combiner’s coupling efficiency will increase at first and then maintain basically the same value. It can be drawn from Fig. 2(c) that the value of LPC decrease with the increase of fusion length, and the decrease rate decline gradually during the whole process, which means we could improve the combiner’s power handling capability by increase the fusion degree. Moreover, the value of backward pump isolation (BPI) should be taken into consideration when we increase the fusion degree to improve the combiner’s performance of pump coupling efficiency and LPC. The relationship between the BPI (the ratio of amplified signal light and residual pump light leaking to the pump fiber of the backward combiner to the total pump power), with the fusion depth is analyzed and shown in Fig. 2(d): the value of BPI will decrease with the increase of fusion depth, which will bring risks to the pump source when the combiner is used in backward direction. In addition, from all those 4 graphs, we can conclude that the higher-order mode performs worse than the lower-order mode in terms of both coupling efficiency, BPI and LPC, which means the side pump combiner’s characteristics could be improved when injecting high brightness pump light.
Fig. 2. (a),(b) The relationships between coupling efficiency with waist width, fusion depth for different input modes; (c), the relationships between LPC with fusion length for different input modes; (d) the relationships between BPI with fusion depth for different input modes.
2.2 Fabrication and measurements
Figure 3 gives the structure of the side pump combiner fabrication system. The heat source is a hydrogen-oxygen flame and we can control the temperature in tapering and fusion splicing process by changing the flow of hydrogen and oxygen. A pair of fiber holders with 3 special grooves is designed to hold a signal fiber and two pump fibers. The detailed combiner fabrication process can be divided into 3 steps: firstly, two pump fibers are tapered with the pulling speed of 0.15 mm/s and we can scan the diameter of the tapered fiber at different longitudinal positions online by using a CCD. According to the theoretical value, the optimum taper waist is about 20 µm. Secondly, two tapered pump fibers are attached to the surface of signal fiber, in this process, three fibers are fixed horizontally by the grooves in fiber holders: the signal fiber is set at the middle groove and the two tapered pump fibers are fixed at the side grooves. Thirdly, the fusion splicing process of a signal fiber and two tapered pump fibers is carried out by using the torch. A LD (center wavelength: 976 nm) is spliced with one end of the pump fiber in order to achieve online monitor, that is to say, during the heating process we can get the combiner’s real-time coupling efficiency from the power output by the signal fiber. The graph inserted in Fig. 3 is the microscope picture of a fabricated side pump combiner, two tapered pump fibers are 220/242 µm, NA = 0.22, while the double-clad signal fiber in the middle is 20/400 µm, NA = 0.06/0.46.
Fig. 3. The schematic of the combiner fabrication system, insert: the microscope picture of a fabricated side pump combiner.
After the fabrication process, the home-made combiner is set on a glass baseboard and fixed by ultraviolet low refractive index glue. Two laser diodes are spliced with the combiner’s two pump ports for pump coupling efficiency and temperature characteristic testing. The measured pump coupling efficiency is about 97%. In Fig. 4, the highest temperature of the home-made side pump combiner versus the input pump power is illustrated. The highest temperature determines the power handling capability of a side pump combiner. From Fig. 4, we can find that the temperature rise coefficient η is 0.0163 °C/W and there is no problem for this combiner to handle with 2 kW input pump power from laser diode, this value can be increased further if we use pump source with higher brightness. The picture inserted in Fig. 4 gives the thermal image of the combiner when it works with 1700 W pump power injecting. From this picture, we can find that the highest temperature (47.8 °C) locates at the coating edge of signal fiber as described in Fig. 1.
Fig. 4. The relationship between the highest temperature of the combiner with input power, insert: the thermal image of the fabricated side pump combiner when injecting 1720 W pump power.
3. Experimental results and discussion
3.1 Experimental setup
As seen in Fig. 5, an all-fiber bi-directional pumping oscillator is achieved based on the in-house side pump combiners. In the oscillator, the pump sources are four 976 nm laser diodes with the maximum output power of 900 W. The output fibers (220/242 µm, NA = 0.22) of laser diode are spliced with the input ports of the side pump combiners. A 18 m long double-cladding ytterbium-doped fiber (YDF, 20/400 µm, NA = 0.06/0.46) is used as the gain fiber, which is placed in the groove of a special designed water-cooled plate. The absorption coefficient of the gain fiber is about 1.3 dB/m @ 976 nm. Two ends of the YDF are spliced with a high-reflectivity fiber grating (HR, >99.8% reflection, centered at 1079.94 nm, bandwidth of ∼ 3 nm) and an output coupler fiber grating (OC, 8.2% reflection, centered at 1080.05 nm, bandwidth of ∼ 1 nm). A cladding light stripper (CLS) is utilized here to move the residual pump power. Then the output fiber laser passes through an endcap coating with antireflection films to suppress the backward reflection. Finally, the oscillator’s output is delivered to a power meter (PM) and an OSA (optical spectrum analyzer) respectively for measurement.
Fig. 5. Experimental setup of the bi-directional pumping fiber laser oscillator based on two (2+1)×1 side pump combiners, HR, high-reflectivity fiber grating; OC, output coupler fiber grating; CLS, cladding light stripper; PM, power meter; OSA, optical spectrum analyzer.
3.2 Results and discussion
The test results of the system (c.f. Figure 5) are shown in Fig. 6 and Fig. 7. Figure 6(a) depicts the spectrum of the laser at different output power, which is measured by an optical spectrum analyzer (Yokogawa, spectrum covering 600-1700 nm) with the resolution of 0.02 nm. The central wavelength of output laser is 1080 nm and the 3 dB bandwidth rises from 2.4 nm at 537 W to 3 nm at 2100 W. There is no peak at the Raman wavelength, indicating no obvious Raman Effect takes place in the system. The absence of the Raman Effect may due to the bi-directional pumping structure, which means the system has the potential for further power scaling. From Fig. 6(b), we can find that the output power increases linearly with the rise of input pump power. The output power peaks at 2150 W when injecting 3070 W pump power, corresponding to a slope efficiency of 72%. It should be noted that no power saturation is observed in the graph, which means the output power could be further increased with higher pump power available. In addition, the value of BPI is measured to be ∼27 dB by replacing a LD in the backward with power meter. The high value of backward pump isolation of the side pump combiner makes it more profitable in counter-pumping or bi-directional pumping scheme.
Fig. 6. (a) The spectrum of the oscillator with different output power; (b) the output power versus input pump power of the oscillator.
Fig. 7. (a) Experimental setup of beam quality analysis, (b) the results of measured M2 at different output power.
The evolution between the beam quality with the output power is measured by PRIMES LQM system, which is shown in Fig. 7(a). The output laser is split by a highly reflectivity mirror (HR1, reflectivity ∼ 99.95% @ 45°) and the reflected laser is sent to a power meter (PM) for online measurement. A beam splitter mirror, which can transmit the signal light and reflect the pump light, is located behind the HR1 so as to eliminating the influence of the residual pump light, only the signal laser passing through the HR1 and beam splitter mirror is utilized for beam quality measurement. HR2 has the same reflectivity with HR1, which is used here to reflect the signal laser to beam quality analyzer. The measurement results are depicted in Fig. 7(b), the value of laser’s M2 maintains at ∼1.05 when the output power increases from 500 W to 2050 W, suggesting the single mode output of the oscillator system and there is no transverse mode instability in the oscillator at this power scale [21]. The high beam quality characteristics is mainly due to the fact that the signal fiber is uninterrupted during the side pump combiner’s fabrication process, indicating the unique advantages of side pump combiners in achieving high power lasers with high beam quality.
4. Conclusions
In summary, we theoretically design and experimentally fabricate a high coupling efficiency and high beam quality (2 + 1)×1 side pump combiner. In theory, we analyze the key factors affecting the pump coupling efficiency, backward isolation and LPC of the combiner, which laid the foundation for experimental fabrication. In experiment, the side pump combiner is fabricated and tested in a laser oscillator. For the home-made side pump combiner, the pump coupling efficiency is ∼97% and the signal light insertion loss is less than 3%. The maximum output power of the laser oscillator is 2150 W when injecting 3070 W pump power, corresponding to a slope efficiency of ∼72%. Thanks to the high beam quality characteristic of the home-made side pump combiner, the oscillator maintains a single mode output (M2=1.05) even when the output power reaches more than 2 kW, suggesting the unique advantage of the side pump combiner in achieving high power and high beam quality laser system.
Funding
Natural Science Foundation of Hunan Province (2019JJ20023); National Key Research and Development Program of China (2017YFF0104600); National Natural Science Foundation of China (11974427).
Disclosures
The authors declare no conflicts of interest.
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