We report a novel center-sunken and cladding-trenched Yb-doped fiber, which was fabricated by a modified chemical vapor deposition process with a solution-doping technique. The simulation results showed that the fiber with a core diameter of 40 µm and a numerical aperture of 0.043 has a 1217 µm2 effective mode area at 1080 nm. It is also disclosed that the leakage loss can be reduced lower than 0.01 dB/m for the LP01 mode, while over 80 dB/m for the LP11 mode by optimizing the bending radius as 14 cm. A 456 W laser output was observed in a MOPA structure. The laser slope efficiency was measured to be 79% and the M2 was less than 1.1, which confirmed the single mode operation of the large mode area center-sunken cladding-trenched Yb-doped fiber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High-power all-fiber lasers have been witnessed tremendous success in scientific research and industrial applications due to their excellent beam quality, better thermal management, higher coupling efficiency and flexibility [1–3]. However, the nonlinear effects and thermal effects caused by high optical intensity in the core, greatly limit the further increase of output power . Subsequently, the large-mode-area (LMA) fibers were developed to overcome these problems .
In conventional all-solid LMA fibers, enlarging the core size is a general method to increase the effective mode area. Ultra-low numerical aperture (NA) and cladding control technology are necessary to ensure the single-mode operation in a large size core. D. Jain et al.  reported an ultra-low NA (as low as 0.038) Yb-doped fiber with a core diameter of 35 μm, which has an effective mode area of 700 μm2 and a Gaussian output beam. Additionally, the gain-guided index antiguided fiber , in which the refractive index of core is lower than inner cladding, also exhibited single transverse mode transmission even with a 100 μm core diameter. Other kinds of fibers utilizing cladding control technologies, such as Leakage Channel Fiber (LCF) [8–11], Multi-trench fiber (MTF) [12–15], Chirally Coupled Core (3C) fibers , Very large mode area (VLMA) fiber  and All Solid Photonic Bandgap Fiber (ASPBF) [18,19], have also been developed to suppress higher order modes (HOM) and efficiently increase the threshold of nonlinear effects. However, LCF, 3C fibers, VLMA fiber and ASPBF have complex structures and hard to be prepare, in addition, the MTF is passive, and only used as power transmission. And most of the structures exhibit large bending loss of fundamental mode (FM) even in a small bending radius.
In this work, we proposed and fabricated a center-sunken and cladding-trenched large mode area Yb-doped fiber (CSCTF). The simulation showed that the center sunken of the refractive index profile would increase the effective mode area, and the multi-trench cladding decreased the FM loss and increased that of the HOM. The Yb-doped CSCTF was prepared using modified chemical vapor deposition process with solution-doping technique. The fiber core has a diameter of 40 μm and a low NA of 0.043. The effective mode area is around 1217 um2 at 14 cm bending radius. The fiber bending radius of the FM bending loss was calculated to be less than 0.01 dB/m and the HOM bending loss was higher than 80 dB/m. A laser slope efficiency of 79% and a maximum output power of 456 W were obtained in the master oscillator power amplifier which was only limited by the available pump sources. The beam quality M2 was measured to be less than 1.1, exhibiting excellent single mode performance in the Yb-doped CSCTF.
2. Fabrication of a center-sunken cladding-trenched Yb-doped fiber
Generally, we can realize uniform profile of core refractive index by controlling the dopants incorporation such as soot porosity, soot oxidation temperature, solution doping condition  and collapse pressure. In order to obtain a center sunken profile, the SF6 was introduced into F300 tube in the process of soot deposition. SF6 can react with SiO2 and generate SiF4, which can decrease the refractive index of core region. A series of fiber preforms were deposited with the process mentioned above. Through optimizing the fabrication process we obtained an Yb-doped CSCT fiber. The refractive index profiles (RIPs) of the preform was measured by PK104 (PHOTO KINETICS). Two cross-sections which were 100 mm and 200 mm away from the end of the preform were selected as the measuring points and the RIPs at the two cross-section were plotted in Fig. 1(a). It is shown that the refractive index distributions of the two interfaces are almost coincidence, implying excellent uniformity along the axis of the preform. Moreover, the center sunken section of the RIP maintains flat along the cross-section. It is shown that the refractive index difference of n1 to n2 was about 0.0004, and the depth of the trench from n3 to n4 was about 0.00035. Correspondingly, the NA of n1 to cladding n3 was determined to be 0.043 and can be defined as the NA of the fiber core. The observed peaks at the center of the profiles are within measurement error and can be neglected.
The preform was firstly processed into a D-shape to facilitate the cladding pumping by breaking the circular symmetry. Then it was drawn into a fiber with core diameter of 40 μm and cladding diameter of 400 μm. The fiber was coated with low index polymer which provides a nominal pump cladding with NA of 0.46 to allow pumping light guided in the cladding. The ytterbium concentration distribution was measured by Electron Probe Micro Analysis (EPMA) which was shown in Fig. 1(b). It indicates that the distribution of ytterbium ions is uniform along the fiber core. The observed peaks at the left of the fiber core profile are merely the measurement artifact.
3. Numerical simulations
A finite element method with Perfectly Matched Layer (PML) was applied to simulate the optical losses of the core modes . A series expension can be established to fit the actual index distribution by following expression:Figure 2(a) shows the calculated loss of the LP01 and LP11 modes at 1080 nm. It is shown that the FM loss rapidly decreases with the increase of bending diameter, while the loss of LP11 mode remains at a high level. It is suggested that the criterion of single mode operation can be set supposing that the loss of HOM is over 10 dB/m and the loss of FM is lower than 0.1 dB/m, which can be achieved at a bending diameter of 24 cm in the CSCT fiber. However in view of the practical application of high power lasers, 0.1 dB/m loss for FM is too large. It will dissipate the laser power and cause other thermal problem. So we select 0.01 dB/m of FM as the criterion loss for single mode transmission. An effective area of 1217 μm2 can be achieved (The step index fiber with the same factor is about 720 um2) at a bending diameter of 28 cm and the corresponding mode loss is 0.007 dB/m for FM and 84.5 dB/m for LP11 mode. The loss ratio of FM to HOM is above 12000, and great suppression of HOMs can efficiently mitigate detrimental modal instability phenomenon . To clarify the influence of the trench to the mode loss, we calculated the loss of LP01 and LP11 mode as a function of different trench number at a bending diameter of 28 cm, which is shown in Fig. 2(b). The FM loss is larger than 10 dB/m when there is no trench, but less than 0.01dB/m with 6 trenches. With the increase of trench number, the FM loss decreased while LP11 mode loss increased and then kept at a higher level. When the trench number reaches to 6, the FM loss was reduced to 0.007 dB/m and the LP11 mode loss went up to 84.5 dB/m. The red point corresponds to the fiber with seven trenches, which shows that the trench number over 6 will impose less influence to LP01 and LP11 mode, and the mode loss will reach a stability.
The above results show that the large effective mode area over 1200 μm2 can be realized in CSCT fiber with a small bending diameter. More specifically, the fiber exhibits excellent mode selection characteristic.
4. Experiment and discussion
The fiber laser slope efficiency was measured in a master oscillator power amplifier system using an 8 m long fiber as shown in Fig. 3(a). Three 200 W fiber-coupled pump diodes modules (supplied by DILAS) operating at wavelength of 976 nm was used as the pump source. The CW seeding laser is working at 1080 nm with a maximum output power of 53 W and the seed structure show in frame. They are both coupled into a pieces of Yb-doped CSCT fiber through a combiner with 20/400 μm pigtail. The fusing point was coated with low index gel. The fiber under test was kept with ~14 cm (+/−0.3 cm) bending radius. Then the Yb-doped fiber was spliced with an end cap to protect the amplifier system. The end cap is a section of ~1 cm coreless fiber was with angle. Figure 3(b) shows the measured slope efficiency with respect to the absorbed pump power. The Yb-doped fiber length was optimized to 8 m with cut-back method. The fiber cladding absorption coefficient at 976 nm was measured as 1.82 dB/m. The laser output with maximum output power of 456 W and 79% slope efficiency were obtained. No power saturation was observed and further power scaling was only limited by the available pump power. The output spectrum with different output power is shown in Fig. 3(c). The acquired 0.96 nm and 1 nm bandwidth corresponds to the 3 dB bandwidth of seed laser and maximum output power.
To explore the single-mode behavior of the CSCT fiber, we used the CCD camera to monitor the output beam from the end cap directly. Figure 3(b) inset shows the obtained output beam at three different pump power levels (1 W, 1.5 W and 2 W). It is shown that only the intensity of the output beam changes and the laser beam maintain Gaussian distribution irrespective of launching conditions.
In order to further evaluate the single-mode behavior of the fiber, a multimode laser beam was injected into the CSCT fiber. Figure 4(a) shows the experiment set-up. Firstly, a 1.5 m long 30/400 Yb-doped fiber (V~5.67) was spliced with the combiner output end and the fiber was placed straight to eliminate the effect of the bending to HOMs. After optimizing the beam path, the M2 of the output beam was measured to be ~2.5 in the x-direction and 2.8 in the y-direction, as shown in Fig. 4(b). The result shows that the HOMs plays the dominate role in the beam. Afterwards, a 1.5m long 40/400 CSCT fiber was spliced with the above 30/400 fiber directly. The 30/400 fiber was still maintained in straight condition. The CSCT fiber was bended with diameter of ~28cm and other experimental conditions were kept the same as above, which was shown in Fig. 4(a). The optimized M2 was measured to be 1.09 in the x-direction and 1.17 in the y-direction, which was shown in Fig. 4(c). This is in good agreement with our simulation result, which indicated a better suppression of the HOMs under bending condition. These measurements verify that an effective single mode operation can be obtained from the center sunken cladding trenched fiber by choosing an appropriate bending diameter to select modes.
Our optimized MCVD process and solution doping technique has shown the versatility to fabricate center-sunken cladding-trenched rare-earth doped fibers. CSCT fibers are all solid structure and are free from any problems caused by air holes or other complex preparation process. It is important to note that the center sunken structure can reach a large effective mode area comparable with PCF but also maintain a uniform rare-earth ions distribution along the profile. The cladding-trenched can reduce FM power tunneling towards the outer cladding and arise resonant coupling between the HOM and cladding-trenched mode, which increase the ratio between the losses of the HOM and the FM [12, 14, 23]. Furthermore, the multi-trench around the core can offer high suppression to the HOMs (80 dB/m) and low loss for the FM mode (0.01 dB/m), which is hard to realize by other kinds of fibers. The mechanism need to be further investigated in the later experiment. Further works will be focused on increasing the laser power and investigating the characteristic of mode instability in our current design. The top-hat beam [24,25] research is also necessary as well.
We have successfully demonstrated an Yb-doped CSCT fiber with a center sunken refractive index core profile and trenched cladding. The fiber was prepared using MCVD technology combined with solution doping method. The fiber has a core diameter of 40 µm and the effective mode area was simulated to be as large as 1217 . The bending radius of 14 cm, the loss of HOMs can be tailored as high as 80 dB/m, and the loss of FM can be controlled as low as 0.01 dB/m, which ensures an effective single-mode operation at 1080 nm. Furthermore, the laser power can reach to 456 W with a 79% slope efficiency and with a beam quality factor M2 less than 1.1. All the results above confirmed that the proposed fiber has a potential application in high power fiber lasers with excellent beam quality.
The National Key Research and Development Program of China (No. 2016YFB0402200); National Natural Science Foundation of China (Grant No. 61735007).
References and links
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