A specially-designed active optical fiber with a constant cladding size and a saddle-shaped profile core is proposed and demonstrated, which called saddle-shaped core (SSC) Yb-doped fiber. The SSC fiber is manufactured through a conventional modified chemical vapor deposition (MCVD) process followed by post-processing of the fiber preforms. The fabricated SSC fiber possesses a large-core region (core/cladding diameter of ∼30/600 μm) at both fiber ends and a small-core region (core/cladding diameter of ∼20.8/600 μm) in the middle length. The output laser characteristics of this SSC fiber are proved through a monolithic high-power continuous-wave MOPA configuration, on the conditions of the co-pumping and counter-pumping manners. It shows that the output lasing powers of co-pumping and counter-pumping regimes are scaled up to ~1.8 kW and ~1.5 kW associated with ~83.3% and ~86.6% slope efficiency, respectively. Before the transverse mode instability (TMI) threshold, a good beam profile quality is measured to M2~1.4, and no evidence of stimulated Raman scattering (SRS) is observed. As far as we know, this is the first demonstration of saddleshaped fiber for kilowatt-level near-single-mode continuous-wave fiber lasers. By further optimizing the fiber design, a higher lasing power connected with near-diffraction-limited beam quality can be achieved in the future.
A high power short-cavity random fiber laser employing the gain mechanism of the Yb-doped fiber and the half-open cavity structure and the temporal optical rogue waves (RWs) behavior are observed and investigated in the paper. The record output power without the stimulated Raman scattering (SRS) is promoted to 26.6 W in the YDRFL with the GDF length of 120 m. The stochastic pulses and temporal optical RWs are observed and demonstrated in the short cavity YDRFL for the first time. It is found that the proportion of RWs depends on the GDF length which can also affect the stability of output lasing. The research results reveal that achieving the relative stable output power requires the greater pump power for the shorter GDF length, although decreasing the GDF length will promote the maximum output power of the YDRFL without the SRS.
In this work, we reported a 2 kW monolithic master oscillator-power amplifier (MOPA) configuration based on a novel constant-cladding tapered-core (CCTC) Yb-doped fiber. The CCTC fiber has a constant cladding diameter of ~400 μm and a varying core diameter along the fiber, with a ~24 μm at both ends and ~31 μm in the middle. This special fiber design can not only effectively suppress the stimulated Raman scattering, but also smooth the thermal load in the fiber. The output performance of this fiber was carefully investigated in a bidirectional pump MOPA configuration with respective co-pump and counter-pump scheme, especially on the aspects of the SRS and TMI. As a result, the TMI threshold is measured at ~870 W and ~1980 W in the co-pump and counter-pump scheme, respectively. The maximum output power of 2023 W is achieved with no sign of SRS in the counter-pump scheme. Before the TMI threshold, the beam quality (M2 factor) remains ~1.4 without any mode distortion, and the M2 factor is measured to be ~1.65 at the maximum output power. These results indicate that the CCTC fiber has great potential to simultaneously mitigate the SRS and TMI effect in the high power operation.
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