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Abstract:
A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers (CLSs) has been demonstrated by employing bidirectional-pump scheme. 2.009 kW signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%. Raman Stokes light is less than -47 dB at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 mm/400 mm. The beam quality M2≤1.2 and the spectral FWHM bandwidth is 4.34 nm. There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.
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Key words:
- fiber laser /
- high power /
- stimulated Raman scattering /
- power stability
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A high power single-mode fiber laser is an attractive laser source which has various application fields such asmaterials processing and long-distance laser energy transmission. In the last decades, the output powers of monolithicfiber lasers have increased remarkably. However, the power scaling of the single-mode fiber lasers is limited by the traditional fiber nonlinear effects and transverse mode instability (TMI). Among fiber nonlinear effects, the stimulated Raman scattering (SRS) is the main limiting factor for continuous-wave fiber lasers. SRS results in the output power instability of the fiber laser. Backward SRS is also damagingly raised by the reflected light from the optics or the work piece ina practical laser system. Therefore, a laser with high SRS suppression is desired for stable operation of a laser system.Besides, photodarkening is also found in Yb-doped high power fiber laser, which can cause the decrease of output power,long term stability and operation life of the fiber laser. Koponen has observed a seventh-order dependence of the PD rateon the excited-state Yb concentration for two different fibers. This result implies that PD of an Yb-doped fiber sourcefabricated using a particular fiber will be strongly dependent on the Yb inversion rate and lower inversion rate can beobtained by using 976 nm pump light as compared to 915 nm. A single-mode fiber laser employing bidirectional-976 nmpump scheme and high SRS suppression and high power stability are demonstrated without any TMI. We investigate thesignal power vs. pump power (S-P) performances, beam distribution and quality, output spectrum and output powertemporal characteristic of the fiber laser oscillator with bidirectional-pumping configuration. The forward-pumping isfirstly utilized and then bidirectional-pumping is utilized to further scale the output laser. Besides, the performance ofthe homemade CLS used in the 2 kW system is described. By employing bidirectional-pumping, the TMI is remarkablymitigated and when the output power is further scaled to 2 kW with a slope efficiency of 76.6%, the Raman stokes lightis ~47 dB below the signal power even with a 10-m delivery fiber with core/inner cladding diameter of 20 μm/400 μm.Nearly diffraction-limited beam quality is also confirmed with the measured M2 below 1.2 and no residual pump andcladding light are observed. Beside, remarkable power stability is also demonstrated because of more uniform temperature distribution on Yb-doped gain fiber by a special thermal management. From the experimental results and theoretical evaluation, an output power of 3 kW is believed to be achieved by further increasing pump power.
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Figure 7. (a) The picture of the encapsulated CLS. (b) The picture of the surface of CLS after abrading.
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