We report a high power all-fiber amplifier with suitable seed power injected by an all-fiber laser. Different seed powers were injected into the all-fiber amplifier during our amplification experiments, and we found the stimulated Raman scattering (SRS) threshold was inversely proportional to the injected seed power. More than 3 kW signal light with good beam quality (M2 = 1.28) has been obtained with a suitable seed power injected, and the slope efficiency of the all-fiber amplifier was about 84.4%.
© 2016 Optical Society of America
High power fiber lasers and amplifiers have a great development in the latest decade . Especially, the applications of key techniques such as large-mode-area (LMA) fiber and double-clad pumping scheme have accelerated the researching of the fiber lasers. The ytterbium (Yb3+)-doped LMA fiber lasers and amplifiers have shown tremendous progress for attaining kilowatt and even multi-kilowatt with near diffraction-limited beam quality [2–5]. Furthermore, the fiber lasers and amplifiers have been applied broadly in scientific and industrial areas by their outstanding characteristics such as high transform efficiency, high brightness and superior reliability etc. Up to now, single-mode fiber-based systems in continuous wave (cw) operation have reached an average power of 10 kW . It shows that the kilowatt-class fiber lasers and amplifiers have a great potential applications and commercial value for researching.
High power signal beam with good beam quality can be obtained by the ways of fiber oscillator and the fiber amplifier. Comparing with the fiber oscillator, the fiber amplifier supplies the simple configuration with fewer fiber components to achieve higher signal power effectively and directly. But with the power increasing of the signal laser, the nonlinear effects such as SRS , thermal effect  etc. will be appeared and limit the output power scaling up.
In this paper, we have demonstrated a highly efficient all-fiber amplifier by bi-directional pumping scheme. In order to obtain high output power and avoid the nonlinear effects, different seed laser powers were injected into the all-fiber amplifier. The SRS threshold powers were checked with the increasing pump power of the amplifier. We found the SRS threshold was inversely proportional to the injected seed power. In the experiment, more than 3 kW output power with good beam quality (M2 = 1.28) was obtained by using a suitable seed power. There were no any nonlinear effects and thermal-optical problems observed during the final experiment. The output power of the all-fiber amplifier was increased linearly well with the increasing pump power, and the corresponding slope efficiency was 84.4%.
2. Experimental setup
The all-fiber amplifier schematic configuration is shown in Fig. 1. The seed laser pigtail fiber was fused with the signal fiber of the 6 + 1x1 fiber coupler, and the signal power was injected to the gain fiber of the amplifier. The gain fiber is a LMA double-clad Yb-doped fiber (YDF) with 25 μm/0.06 NA core and 400 μm/0.46 NA inner cladding. The nominal small-signal pump absorption is 1.8 dB/m at 976 nm. The pump light of the twelve 250 W pump lasers with 976 nm wavelength was coupled into the gain fiber by the 6 + 1x1 fiber couplers of the fiber amplifier. The pump power coupling efficiency of the 6 + 1x1 fiber coupler is about 98%, and the insertion loss of the signal power is less than 0.2 dB. The unabsorbed pump light propagated in the cladding of the passive fiber was stripped out of the fiber by the cladding mode stripper (CMS). The signal beam was obtained from the beam delivery system with collimating lens.
The seed power was supplied by a single-mode CW all-fiber laser with 1080 nm wavelength, and the power could be adjusted up to 1 kW with near diffraction-limited beam quality (M2 = 1.1). The output fiber of the seed laser was 20/400 μm passive fiber. It was fused with the signal fiber of the 6 + 1x1 fiber coupler of the fiber amplifier directly. There was no high power isolator insert between the seed laser and the fiber amplifier in our experiment, because the seed laser had high stabilization and with low output threshold. The backward power of the amplifier was very low because there was no reflection along the direction of the signal light propagating in the fiber amplifier, especially about the output fiber end-cap. Therefore, the backward power from the fiber amplifier had small influence to the seed laser even in high output power level.
In the all-fiber amplifier system, the splice spots of the fiber components were optimized with low fuse-loss as 0.15 dB and coated with low-refractive index polymer. Low fuse-loss of the fiber splicing can help to suppress high-order modes creating in the fiber amplifier. The high-order modes are difficult to be avoided because of the using of the LMA multi-modes fiber in high power fiber amplifier. In order to obtain good beam quality in our experiment, the gain fiber was coiled with a minimum diameter of 15 cm as a mode-filter to suppress the high-order modes .
3. Experimental results and discussion
The pump lasers and the all-fiber amplifier system were cooled by the water chiller in the experiments. The pump light of the fiber amplifier should be kept at the steady wavelength with 976 nm because of the narrow absorbed linewidth of the YDF. In the first amplification experiment,the signal light from the seed laser was injected into the fiber amplifier with different power to find the SRS power threshold in our practical experimental system with the fixed fiber length and fixed mode-field area.
The seed light was propagated through the all-fiber amplifier, and a fraction of light was lost by the absorbing of the active fiber. The remaining seed powers measured from the beam delivery system were 810 W, 580 W, 470 W and 410 W by adjusting the output power of the seed laser respectively. The gain fiber length of the fiber amplifier was about 15 m for increasing the absorption of the pump power. First, a strong signal power with 810 W was injected to saturate the fiber amplifier, and then turned on the pump lasers. With the increasing pump power of the fiber amplifier, the corresponding signal output power was about 1.27 kW (as shown in Fig. 2) when the SRS light was observed from the output beam by an optical spectrum analyzer (OSA) (shown in Fig. 3), and. When the output power exceeded the SRS power threshold, the Raman scattering process transferred energy from the signal wavelength to a longer wavelength region steadily with the pump power increasing. Then we decreased the injected seed power as 580 W and turned on the pump lasers. The signal power about 1.6 kW was measured when the SRS light was observed. In order to improve the output power, 470 W and 410 W seed powers were injected to continue the same amplification experiments respectively. The signal powers were obtained as 1.8 kW and 2 kW with the pump power increasing, and finally the spectrum of the SRS light was also detected by the OSA during the experiments. The output powers of the fiber amplifier with different seed powers were compared in Fig. 2. Before the SRS effect appeared from the fiber amplifier, the output powers were increased linearly. The slope efficiency was measured about 80% as shown in Fig. 2.
According to the classic SRS threshold formula published by R.G. Smith in 1972: Pth = 16 Aeff / gR Leff . The SRS threshold was mainly dependent on the fiber characteristics such as fiber mode-field-area Aeff and the fiber effective length Leff, and gR is the peak Raman gain coefficient (gR = 1 × 10−13 m/W in fused silica at a pump wavelength of 1 μm). About the fiber-amplifier system, it also should be considered the pump power and the signal power in the active fiber of the amplifier, and the SRS threshold should be estimated with a fixed seed power . In our experiment, different signal powers were injected to demonstrate the SRS threshold of the practical fiber amplifier system. It showed that the input signal power of the amplifier performed an important factor to affect the SRS threshold.
In the second amplification experiment, we used the same YDF with 11 m length instead of the amplifier fiber in order to improve the SRS power threshold and the signal output power of the fiber-amplifier system. According to the results of the first amplification experiment and considering the shorter fiber length used, a suitable seed power as 610 W was injected into the fiber amplifier. The output power of the fiber amplifier was increased linearly with the increasing pump power, and it was measured more than 3 kW with high slope efficiency as 84.4% of the all-fiber amplifier shown in Fig. 4. There were no any nonlinear effects during the experiment even in such high power level. The spectrum of the fiber amplifier was analyzed by the OSA shown in Fig. 5. The SRS power threshold was improved effectively, and the Raman scattering light with the wavelength of 1135 nm was not detected at the maximum output power of the amplifier.
At the end of the beam delivery system, the signal beam quality (M2) of the all-fiber amplifier was measured as 1.28 at 3 kW output power shown in Fig. 6. The beam quality of the seed light should be degraded when propagating through the LMA fiber and the fiber components of the all-fiber amplifier. Therefore, it was difficult to obtain so much high brightness signal beam when the seed power was amplified several times. The mode instability effect of signal light of the fiber amplifier could degrade the beam quality obviously [11–13]. The temporal instability of the signal light was analyzed by an InGaAs photo-detector at the end of the beam delivery system of the fiber amplifier with 3 kW output power as shown in Fig. 7(a). The corresponding Fourier spectral distribution was shown in Fig. 7(b). The noise was also recorded and compared with the signal light, and the mode instability phenomenon was not observed from the signal light. The all-fiber amplifier worked steadily without any physical damage during the experiment.
The SRS effect of the all-fiber amplifier is the mainly limit to the power scaling up. Relative shorter fiber length can increase the SRS threshold, but the absorption of pump light needs be insured to improve the efficiency and reliability of the all-fiber amplifier. Enlarging the fiber core is an effective way to increase the SRS threshold by decreasing the power density of the fiber, but it is difficult to remain perfect beam quality of the fiber amplifier. Decreasing the seed power injected to the amplifier is another way to decrease the power density with the fixed fiber type, and the SRS power threshold can be improved obviously. But lower seed power should be decrease the output power of the fiber amplifier under the finite pump power, and the stability of the all-fiber amplifier should also be degraded because of the ASE when the amplifier running in high power level. Therefore, a suitable seed power is necessary for the high-power steady all-fiber amplifier.
We have demonstrated a cladding-pumped Yb-doped all-fiber amplifier by bi-directional pumping scheme. Different seed powers were injected to the fiber amplifier, and we observed the SRS light with different threshold powers. The results of the experiments showed that the SRS threshold power was inversely proportional to the injected seed power. More than 3 kW signal power with good beam quality (M2 = 1.28) of the all-fiber amplifier was obtained by using a suitable seed power injected, and the SRS threshold was increased obviously with lower injected signal power. Therefore, a suitable seed power is necessary for the power scaling up of the high power all-fiber amplifier with the fixed fiber type and length.
This work is supported by the National Key Technology R&D Program under grant No. 2012BAF08B02.
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