High-brightness and high-power fiber lasers operating near 980 nm have important applications because they can not only be applied for pumping the ultrafast solid-state and fiber lasers but also generate the emission around 490 nm by frequency-doubling. However, it is difficult to achieve high-power near-diffraction-limited operation for the fiber laser operating near 980 nm. The main reason is that the large core-to-cladding area ratio (CCAR) of the active fiber is needed to suppress the amplified spontaneous emission (ASE) around 1030 nm. Although some micro-structured fibers have been designed for suppressing the ASE around 1030 nm and maintaining good beam quality, their difficult manufacturing and drawing processes limit the popularization of pertinent studies. Therefore, more studies have been carried out by using double-cladding Yb-doped fibers (DCYFs). However, the beam quality achieved by the DCYF is generally poor. Moreover, the strong in-band ASE is observed, but its effect on the beam quality is not clear. Therefore, in this paper, an experimental study is carried out to explore the effect of the in-band ASE on beam quality with the help of a 50-W-level master oscillator power amplifier (MOPA) fiber laser operating near 980 nm. It is also demonstrated that the suppression of in-band ASE can be improved by shortening the length of DCYF. We hope that our study can be helpful in designing the high-power near-diffraction-limited fiber laser near 980 nm with DCYF.

As shown in Fig. 1, the MOPA fiber laser consists of a seed oscillator and an all-fiber amplifier. Firstly, the seed oscillator was fabricated, and the output properties of seed light were measured. The effects of the mode field adapter (MFA) and filter on the beam quality of seed light were investigated. Then, the all-fiber amplifier was fabricated and tested with the seed light. The active fiber length was determined to be 0.56 m, so as to suppress the ASE around 1030 nm, and the output power, spectrum, and beam quality of the amplifier were measured. The suppression of the in-band ASE could be revealed with the output spectrum. After that, the active fiber length was shortened to 0.47 m for improving the in-band ASE. Then, the output power, spectrum, and beam quality of the amplifier were measured, and the suppression of the in-band ASE was obtained with the output spectrum. By comparing these results with the case of an active fiber with a length of 0.56 m, the improvement of in-band ASE suppression and beam quality could be revealed.

The seed light is firstly investigated in the experiment. We measure the output properties of seed light at points A and B successively (Fig. 1) to reveal the effect of MFA and filter on the seed light. Experiment results imply that the effect of MFA on the beam quality of seed light should be negligible. However, the filter makes the output beam quality (M² factor) worsened from 1.11 to 1.57. As a result, the seed light with the output power of 7.04 W and M² factor of 1.57 can be provided. Then, based on the seed source, the MOPA fiber laser is studied in the experiment. A 0.56-m-long active fiber is firstly used in the amplifier, and 51.2-W-level output power is achieved with a slope efficiency of 17.2%. The ASE around 1030 nm is well suppressed, and its suppression is more than 30 dB. However, the suppression of the in-band ASE is not so well that the parasitic laser oscillator is present with 16.6-dB peak-to-peak suppression at the maximum output power. As a result, the output beam quality (M² factor) is about 1.79. Then, the length of active fiber in the amplifier is shortened to 0.47 m, and the peak-to-peak suppression of the in-band ASE is improved to 33.2 dB at the maximum output power, with no parasitic lasing oscillation observed. As a result, the output beam quality is improved to 1.48, which suggests that the in-band ASE should be well suppressed to improve the beam quality of the Yb-doped fiber laser operating near 980 nm. With the 0.47-m-long active fiber, the 51.3-W-level output power is obtained with a slope efficiency of about 14.0%. The peak-to-peak suppression of the ASE around 1030 nm is 46.5 dB.

In this paper, based on a step-index DCYF, a 50-W-level near-diffraction-limited fiber laser around 980 nm is demonstrated, and the effect of the in-band ASE on the beam quality is revealed. Experimental results imply that the strong in-band ASE should do harm to the beam quality. As a result, only improving the beam quality of seed power cannot effectively improve the beam quality of fiber laser, and in-band ASE should be significantly suppressed. In our experiment, the suppression of the in-band ASE is improved by shortening the length of an active fiber. Then, with the 50-W-level output power, the peak-to-peak suppression of the in-band ASE increases from 16.6 dB to 33.2 dB, and the output beam quality (M² factor) is improved from 1.79 to 1.48.