Objective In 2015, Liu et al. used a narrow linewidth super-fluorescent fiber source as a seed to be amplified, and then achieved a 1.5 kW high-power laser output with a spectral linewidth of 0.8 nm at the highest power. It has been successfully applied in a high brightness spectral beam combining system, which lays a foundation for the application of a narrow linewidth and high power super-fluorescent source. When the narrow-linewidth laser is used as the sub source of the spectral combining system, the brightness of the spectral combining system can be effectively improved by narrowing the spectral linewidth of the laser. Therefore, it is of great significance to study a high power fiber super fluorescent source with a narrower linewidth.
Methods An all-fiber super-fluorescent source is established. Laser diode, (2+1)×1 pump-signal combiner, ytterbium doped fiber, optical isolator, optical circulator are fused as the schematic illustration shown in Fig.1. By setting the current of the laser diode lower than the stimulated radiation threshold, the laser works under a super-fluorescent state. The backward output of the broadband source is filtered by a fiber Bragg grating (FBG), and a narrow linewidth laser signal is obtained at the port 3 of the circulator. The full width at half maximum (FWHM) of this FBG is 0.16 nm, and the reflectivity is greater than 99% at the central wavelength of 1064.1 nm. In order to further improve the spectral signal-to-noise ratio of laser signal and get a narrower spectral linewidth, the laser signal passes through another filter after proper power amplification, and the FWHM reaches 0.08 nm. After two stages of pre-amplification, the power of the laser signal is increased to 15 W, which is injected into the main power amplifier stage and then amplified to 1.08 kW. The spectra for different output powers in the main power amplifier stage are shown in Fig.2 (b). A 4 km G652D passive fiber is fused between the second stage pre-amplifier (Amp2) and the third stage pre-amplifier (Amp3), and then the spectrum at 1.08 kW is narrowed as shown in Fig.3 (a). The comparison of spectral linewidth versus output power is shown in Fig.3 (b) with and without the 4 km passive fiber.
Results and Discussions By comparing the results of these two experiments, it can be found that the spectral FWHM is narrowed by 0.03 nm and the RMS linewidth is narrowed by 0.06 nm at a 1.08 kW output power by fusing a 4 km passive fiber between Amp2 and Amp3. Limited by the available pump power, the change of spectral width at a higher power has not been further verified, but according to the linear broadening law of laser spectra, this method via fusing an additional long-distance energy transfer fiber to compress the laser output spectrum will play a more obvious role in the laser at higher power. Some previse researches have shown that the time-domain characteristics of laser seed source will affect the speed of spectral broadening, but the comparison of the time-domain probability density functions (PDFs) of the laser signals with and without the 4 km fiber, shown in Fig 4(b), implies that the improvement of time stability is not very significant. In order to explore the reasons for the change of spectral broadening speed, the length of the passive fiber is reduced to 3 km, the spectral FWHM is broadened from 0.18 nm at 15 W to 0.23 nm at 1.08 kW, and the broadening slope is 0.05 pm/W. By further shorting the passive fiber to 2 km, the spectrum is broadened from 0.16 nm to 0.25 nm with a 0.09 pm/W slope. By lengthening the passive fiber to 5 km, the change of FWHM versus power is same as that for 4 km fiber. The difference is that the spectral side-mode is enhanced. One possible explanation is that the dispersion of a long-distance fiber can change the phase matching condition between the sub-wavelengths in the laser signal and weaken the intensity of the FWM effect, and then the spectral broadening speed in the process of power amplification is restrained.
Conclusions A narrowband all-fiber super-fluorescent source is established with a power of 1.08 kW after multi-stage power amplifiers. The full width at half maximum is 0.23 nm under the maximum output power. The narrowband source is optimized by dispersion control, and thus the FWHM is compressed to 0.20 nm under the maximum power with spectral-broadening-free property when power rising. It can be used in a spectral beam combining system to improve the diffraction beam quality.
