1. Introduction

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 [4]. Subsequently, the large-mode-area (LMA) fibers were developed to overcome these problems [5].

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. [6] 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 [7], 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 [16], Very large mode area (VLMA) fiber [17] 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 [20] 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.

 

Fig. 1 Index profile of (a) Yb doped fiber preform, (b) Yb concentration of fiber.

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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 [21]. A series expension can be established to fit the actual index distribution by following expression:

 

Fig. 2 (a) The loss of LP₀₁ and LP₁₁ mode with the bending diameter at 1080 nm. Purple and orange line representation the bend diameter of 0.1 dB/m and 0.01 dB/m respectively at LP₀₁, inset is the electric field profile of LP₀₁ and LP₁₁ with bending diameter of 24 cm. (b) Influence of the different trench to the LP₀₁ mode and LP₁₁ mode.

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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 $4°$ 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.

 

Fig. 3 (a) Experimental set-up used for amplifier efficiency measurement, (b) output power as a function of pump power and absorbed power, inset shows the output beam at 1 W, 1.5 W and 2 W pump power, (c) output spectrum with different output power.

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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.

 

Fig. 4 (a)Experimental set-up for single mode verification, a 1.5 m long CSCT fiber coiled with 28 cm bending diameter was used in this experiment, (b) M² measurement of the output mode in 30 µm core fiber, (c) M² measurement of the output mode in CSCT fiber.

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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.

5. Conclusions

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 ${\text{μm}}^{\text{2}}$. 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.