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Abstract
We report a new method to fabricate a 4 × 1 signal combiner that comprises an output fiber port and a tapered fused bundle (TFB) with four input fiber ports. The TFB is etched in a solution of hydrofluoric acid and spliced with an output fiber of core diameter 105 μm and cladding diameter 125 μm. Each cladding of the four input optical fiber is etched to approximately 72.5 μm. The etched TFB was fabricated by tapering after forming a bundle of four etched optical fibers. Subsequently, the 4 × 1 signal combiner is fabricated by fusion splicing between the fabricated TFB and output optical fiber with a numerical aperture of 0.15. The efficiency of each port of the fabricated 4 × 1 signal combiner is in the range of 93.3–98.3%. When an optical power of approximately 624.5 W was input to the signal combiner, the maximum output was ~612 W and the efficiency was ~98%. The beam quality factor, M 2is measured to be approximately 14.6, which is calculated as the beam parameter product (BPP) of 5.02 mm·mrad.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber lasers have been developed as optical sources in many fields, such as optical sensors, biophotonics, space industries, and optical communications. In particular, high-power fiber lasers are used in material processing such as welding, marking, and cutting. Their important characteristics often require the stability of the kilowatt band and a short pulse width. However, they have been limited to high output power applications owing to the high thermal effect of the fiber-optic components in conventional fiber lasers [1–11]. To solve this problem, optical combiners (pump combiner or signal combiner) and wavelength division multiplexing (WDM) couplers that can be used for high output power have been developed [12,13]. Various pump combiners have been proposed in literature for end pumping schemes and side coupling schemes with a signal port [1]. In the case of the side coupling schemes, because pump beams are coupled on the side of the output fiber, there are no hot spots that can burn via heat accumulated by the concentrated pump beams, and both free ends of the output fiber can be used to launch a signal beam or to connect coupled devices. In the end pumping schemes, the signal beam is coupled to a core of the output fiber, and the pump beam is focused into the 1st cladding of the same fiber through bulk optics or fiber bundles composed of pump and signal fibers. In this scheme, the structure is simple but it requires careful management in that a hot spot is formed on the end of the output fiber by the pump beams. The fiber type pump combiner used with the end pumping scheme is primarily and commercially manufactured by fabricating the tapered fused bundle (TFB) and splicing it with the output optical fiber [14]. It was developed as an important device for obtaining high output power from the fiber [15–25]. Recently, signal combiners as well as pump combiners for high-power fiber lasers have been developed by many research groups [26–32]. In particular, TFB technology has proven to be excellent for the implementation of optical combiners [20,31,32]. However, one disadvantage of the conventional TFB method is that both the cladding diameter and core diameter of the input optical fiber are simultaneously reduced by the tapering process. The end diameter of the TFB depends on the diameter of the output optical fiber. As the core diameter of the optical fiber decreases, the effective numerical aperture (NAeff), which is defined as the maximum angle with the fiber axis for guided modes, increases [33,34]. When the NAeff of the input optical fiber becomes larger than the numerical aperture (NA) of the output optical fiber by the tapering process, the transmitted light travels into the cladding of the output optical fiber, as shown in Fig. 1. It transfers heat to the coating of the output optical fiber. Consequently, the optical fiber is burnt and cannot serve as a signal combiner.
Meanwhile, when the tapered ratio of the input diameter to the output diameter of the TFB is large, the core diameter of optical fiber becomes small, as shown in Fig. 1. Thus, the multimode light propagating to the core deviates from the critical angle required for total internal reflection and enters the cladding mode. The light propagating to the cladding of the TFB proceeds to the cladding of the output optical fiber, thereby causing optical loss and may even burn the coating of the output fiber. Owing to these problems, selecting appropriate input and output optical fibers is considerably important; therefore, it is a challenging process to manufacture a signal combiner. To achieve a signal combiner with a high stability and a low insertion loss, the cores of the signal fibers that are combined and tapered using the TFB process should not be coupled into the core of the output fiber; instead, all the cores and claddings of the combined signal fibers should be coupled into the core of the output fiber, and the tapered ratio should be minimized to ensure a low NAeff that is less than the NA of the output fiber.
To achieve this experimentally, we herein propose a method of reducing the diameter of the TFB by etching the optical fiber using hydrofluoric acid to minimize the tapered ratio of the TFB. The proposed method can reduce the diameter of only the cladding without affecting the core diameter of the optical fiber. Before the tapering process of the input optical fiber, the cladding portion of the optical fiber is etched by inserting the optical fiber into the hydrofluoric acid. Four input optical fibers etched to 72.5 μm cladding diameters each are fabricated to prepare the etched TFB through the tapering process. During TFB process, the cores of the four closely packed fibers can be thermally expanded according to the temperature of the torch and the processing time. The thermally expanded cores have a low NA due to diffusion of germanium ions and maintain low optical losses for the signal combiner, which will be discussed a little later. The fabricated etched TFB is spliced to an output optical fiber with an NA of 0.15 to fabricate a 4 × 1 signal combiner, and the efficiency and optical characteristics of the signal combiner are analyzed.
2. Theory
The signal combiner is composed of N input optical fibers and one output optical fiber. As shown in Fig. 2, tapering is achieved by pulling both ends of the bundle while heating the bundle of the fibers to a high temperature. Subsequently, the optical fiber bundles fused together by high temperature are tapered by decreasing the diameter of the optical fiber owing to the tension on both ends, and the cores can be thermally expanded to achieve a low NA between the core and cladding.
Figure 3 shows the path of light traveling through a tapered fiber bundle. In the tapered region, the diameter of the core decreases and the reflected angle of the light increases. Therefore, the NAeff of the optical fiber increases in the tapered region, and the value of NAeff can be calculated using the beam parameter product (BPP) relation [35,36]. The BPP relation is a product of the maximum divergence angle of the optical fiber and the diameter of the core, which is a value representing the quality of the laser beam. The product of the diameter and the divergence angle of the tapered optical fiber bundle is constant at the input and output ports of the tapered fiber. The BPP relationship is given as follows:
where Din and Dout are the diameters of the input and output fibers, respectively; θin and θout are the maximum divergence angles of these fibers, respectively [35,36].Figure 4 shows the cross-sectional view of the 4 × 1 signal combiner. The cladding diameter and the maximum divergence angle of the individual input fibers (IFs) of the fiber bundle are denoted by DIF and θIF, respectively. Note that the DIF is not the core diameter, but cladding diameter of the input fibers as all cladding modes caused by the TFB should be coupled to the core of the output fiber. The core diameter and the maximum divergence angle of the output fiber (OF) of the output fiber are denoted by DOF and θOF, respectively. These values can be applied to the BPP relationship as follows.
(2)where k is a constant and is approximately 2.414 for a bundle of four optical fibers. The k value of 2.414 is achieved from adding the diagonal length, DIF between the centers of the four fibers to the fiber diameter of DIF, as shown in Fig. 4.
Because the NAeff is proportional to the maximum divergence angle, Eq. (2) can be written as follows [35,36]:
where the left-hand side of the expression is the value for the input optical fiber, and the right side is the value for the output optical fiber. The NAIF of the beam from the input fiber bundle must be equal to or lower than the NAOF of the output fiber for a low-loss design. Therefore, Eq. (3) is constrained byThe signal combiner designed to satisfy Eq. (4) can theoretically exhibit an efficiency of 100%.
3. Fabrication
Hydrofluoric acid can be used to etch the surface of glass and can thus be used to reduce the cladding diameter of the optical fiber, DIF. As shown in Fig. 5(a), when the optical fiber is immersed in hydrofluoric acid for etching, the diameter of the optical fiber decreases with time. The graph in Fig. 5(b) shows that the diameter of the optical fiber decreases with time. Unlike the tapering process, the core diameter of the optical fiber is not reduced but only the cladding diameter is reduced in the etching process. Therefore, the NA of the optical fiber does not change, and various output optical fibers can be used to fabricate the signal combiner. In addition, the etching process is simple and advantageous for fabricating several etched optical fibers simultaneously. To fabricate a TFB, the optical fiber with a cladding diameter of 130 μm is etched to 72.5 μm. Approximately 1 h and 35 min is required to obtain a diameter of 72.5 μm, as shown in Fig. 5(b). A cladding diameter below 70 μm by the etching process causes handling problems in the bundling and tapering processes, and an extremely small cladding thickness can also induce an additional loss by the evanescent field of the core mode.
Four input optical fibers, which are etched to cladding diameters of 72.5 μm each are aligned as shown in Fig. 6. The maximum diameter of the 4 × 1 bundle is 2.414 × 72.5 μm. It is worth noting that the fiber core diameter is maintained at 20 μm with an NA of 0.08 without regard to the etching process. After bundling the four input optical fibers, the tapering process is performed such that the diameter of the TFB is less than 105 μm, as shown in Fig. 6. From Eq. (4), the value of k DIF NAIF becomes 14.0 from 2.414 × 72.5 × 0.08, and the value of DOF NAOF becomes 15.75 from 105 × 0.15, if we select the output fiber of 105 μm core and 125 μm cladding diameters with 0.15NA. Therefore, Eq. (4) is satisfied if we use this output fiber. The signal combiner is expected to have low loss. Additionally, during the TFB process, thermally induced cores can be formed if we fabricate it with high temperature and long elapsed time. The thermally induced core makes a low NAIF below 0.08, so that the value of k DIF NAIF of Eq. (4) is further reduced; thus, it also makes a low insertion loss of the signal combiner. As the fabricated TFB should be connected to the core of the output optical fiber, the shape of the end of the TFB should preferably be square without air space. Figure 7 is a cross-sectional photograph of the fabricated TFB, showing that the shape is made close to a square. The major and minor axes of the TFB in this study were 104.80 μm and 104.17 μm, respectively.
Figure 8(a) shows the photograph of the alignment before fusion splicing between the TFB and output fiber. In the photograph, the optical fiber on the left side shows the fabricated TFB whose diameter is smaller than the cladding diameter of the output fiber, which is 125 μm. Figure 8(b) shows the photograph for the fusion between the TFB and output fiber. After fusion splicing, the fused portion of the optical fiber exhibits a stepped shape. Therefore, even some light propagating to the cladding of the TFB can be coupled to the core of the output optical fiber. Because high-power lasers transmit the fabricated 4 × 1 combiner, the etched fibers can be easily damaged by the incineration of the dust on the surface of the 4 × 1 combiner due to the leakage of optical power. Therefore, the fabricated signal combiner using the TFB should be protected with proper epoxies.
Figure 9(a) shows the schematic diagram for the protection of the fabricated signal combiner using two types of epoxies, one with a high refractive index (n = 1.565) and another of low refractive index (n = 1.414). When unwanted light from the fusion splice is propagated into the cladding mode of the output fiber, heat may be generated from the coating in the outer region of the cladding. To solve this problem, the cladding mode of the output optical fiber can be eliminated by covering the portion of the optical fiber coated with the high refractive index epoxy, as shown in Fig. 9(a). The low refractive index epoxy covers the fused portion of the TFB and output fiber to protect it from dust and humidity. Figure 9(b) shows a photograph of the fabricated signal combiner using two types of epoxies.
4. Results
Figure 10 shows the experimental setup for measuring the efficiency of the fabricated 4 × 1 signal combiner. Experiments were conducted with four CW lasers that had a wavelength of 1080 nm and output power of approximately 156 W each. The output fibers of each laser had a core and a cladding having diameters of 20 and 400 µm, respectively, with an NA of 0.08; these were spliced into the input fibers of the 4 × 1 signal combiner having a core and cladding of diameter 20 and 130 µm, respectively, with the same NA. The core loss due to fusion splicing can be minimized by proper splicing conditions. The efficiency of each port of the signal combiner was measured with a thermopile power meter.
Figure 11 shows the measured efficiencies of each port of the fabricated signal combiner. From the input signal port 1 of 161.95 W, the combiner output is measured as approximately 152.6 W. The efficiency is approximately 94.2%. Similarly, the efficiency of port 2 is 96.3%, and those of ports 3 and 4 are 98.9% and 96.8%, respectively. The average efficiency is approximately 96.5%. Figure 12 shows the measured efficiencies of the signal combiner when CW laser beams of 156 W are input to the four signal ports simultaneously. When a total optical power of approximately 624.5 W was input to the signal combiner using four CW lasers, the maximum output was approximately 611.9 W, and the total efficiency was approximately 98%. The difference between the average efficiency measured from each port and total efficiency is regarded as an error from the power variation of the thermopile power meter. The fabricated 4 × 1 signal combiner thus exhibits good stability for a combined optical power of 620 W.
Figure 13 shows the results of measuring the M 2 of the signal combiner. When the optical power of 600 W is incident on the signal combiner, the M 2 is measured to be approximately 14.6 and 14.1 for the x and y axes, respectively. Therefore, the BPP of the signal combiner is approximately 5.02 and 4.84 mm·mrad for the x and y axes, respectively, from the relation of BPP = M 2λ/π.
5. Summary
We have successfully demonstrated a 4 × 1 signal combiner using an etched TFB that was fabricated by tapering four input fibers with cladding diameters of ~72.5 μm each. By etching the cladding of the input optical fibers using hydrofluoric acid, the variation in NAeff and core diameter of the optical fiber in the fabricated TFB was minimized. A 4 × 1 signal combiner was fabricated by fusion splicing the fabricated TFB to an output optical fiber with an NA of 0.15. Since all cores and claddings of the tapered input fibers were spliced into the core of output fiber, the 4 × 1 combiner showed a low loss and was also protected against any leaky mode. Two epoxy resins with different refractive indices were used to fabricate protective coverings for the 4 × 1 signal combiner. The some cladding modes of the output optical fiber caused by splicing process, were eliminated by coating the optical fiber with the high refractive index epoxy. Further, the low refractive index epoxy covered the fused portion of the TFB and output fiber and the tapered section. When the optical power of approximately 624.5 W was input to this signal combiner, the maximum output observed was ~612 W, and the efficiency was ~98%. The M 2 was measured to be approximately 14.6, and the BPP of the signal combiner was approximately 5.02 mm·mrad.
Funding
Ministry of Trade, Industry and Energy (10048726).
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