No abstract

Transverse mode instability (TMI) is one of the main limiting factors in kW-level fiber lasers. Unlike fiber amplifiers, TMI in fiber laser oscillators attracts less attention from researchers. In this work, we construct an all-fiber ytterbium-doped laser oscillator and investigate the performance in co-pumping and bidirectional-pumping configurations, respectively. In the co-pumping scheme, TMI occurs at ~1.6kW and restricts further output power scaling. Different from the characteristic of dynamic TMI in fiber amplifiers, quasi-static TMI is observed in the laser oscillator. Details of the temporal characteristic around the TMI threshold are provided. In the bidirectional-pumping scheme, experimental results validate that the TMI is mitigated notably by employing bidirectional-pumping instead of co-pumping. The output laser power is further scaled to 2.5kW with a slope efficiency of 74.5% and good beam quality (M²~1.3). At the maximum power, the FWHM bandwidth of optical spectra is 5.2nm, and the Raman stokes light is ~20dB below the signal.

Multitone radiation is a promising technique to mitigate stimulated Brillouin scattering effects in narrow-linewidth fiber amplifiers. We demonstrate coherent beam combination of three two-tone fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm. Phase control on the fiber amplifiers are performed by running the SPGD algorithm on a digital signal processor. The contrast of far-field intensity pattern of a coherently combined beam is more than 85%. Experimental results validate that a single-frequency seed laser is not indispensable for coherent beam combination in master oscillator power amplifier configuration.

We demonstrate an experimental study on scaling mode instability (MI) threshold in fiber amplifiers based on fiber coiling. The experimental results show that coiling the active fiber in the cylindrical spiral shape is superior to the coiling in the plane spiral shape. When the polarization maintained Yb-doped fiber (PM YDF: with a core/inner-cladding diameter of 20/400 µm) is coiled on an aluminous plate with a bend diameter of 9-16 cm, the MI threshold is ~1.55 kW. When such a PM YDF is coiled on an aluminous cylinder with diameter of 9 cm, no MI is observed at the output power of 2.43 kW, which is limited by the available pump power. The spectral width and polarization extinction ratio is 0.255 nm and 18.3 dB, respectively, at 2.43 kW. To the best of our knowledge, this is the highest output power from a linear polarized narrow linewidth all-fiberized amplifier. By using a theoretical model, the potential MI-free scaling capability in such an amplifier is estimated to be 3.5 kW.

Bessel beam is the important member of the family of non-diffracting beams and has many novel properties which can be used in many areas. However, the source of Bessel beam generated by the existing methods can be used only in a short distance due to its low power. In this paper, based on the coherent combining technology, we have proposed a method which can be used to generate a high-power Bessel beam. Even more, we give an innovative idea to form vortex phase by using discontinuous piston phase. To confirm the validity of this method, the intensity evolution of the combined beam and the Bessel-Gaussian beam at different propagation distance have been studied and compared. Meanwhile, the experimental realization has been discussed from the existing experimental result related to the coherent combining technology.

We report a high power monolithic ytterbium-doped fiber laser oscillator with an output power of 3.05 kW, which is achieved by simultaneous optimizations of the stimulated Raman scattering (SRS) and transverse mode instability (TMI). The optimizations of the SRS are designed and utilized in the construction of the fiber laser oscillator, while the TMI threshold is optimized with the study of the dependence of TMI threshold on the pump distribution. In the fiber laser oscillator, the TMI threshold is enhanced by ∼30% when the counter-pump scheme is employed instead of the co-pump scheme. By applying bidirectional-pump scheme and appropriately distributing the pump power, the TMI threshold is further enhanced and the monolithic fiber laser oscillator achieves an output power of 3.05 kW with near diffraction limited beam quality.

With an increasing output power, the power scaling of monolithic fiber laser oscillators faces the severe limitations of stimulated Raman scattering (SRS) and the transverse mode instability (TMI) effect. In this work, we report a high power monolithic fiber laser oscillator with a maximum output power of 5.2 kW, which is realized with a trade-off design between the SRS and TMI. The monolithic fiber laser oscillator is constructed with ytterbium-doped fiber with a core/inner cladding diameter of 25/400 µm and corresponding home-made FBG. High-power 915 nm laser diodes are employed as a pump source and are distributed in a bidirectional-pump configuration. By optimizing the bidirectional pump proportion, the monolithic fiber laser oscillator is scaled up to 5.2 kW with a slope efficiency of ~63%. Operating at 5.2 kW, the intensity of the Raman stokes light is ~22 dB below the signal laser and the beam quality (M2-factor) is ~2.2. To the best of our knowledge, this is a record high power for monolithic fiber laser oscillators.

In the power scaling of monolithic fiber laser oscillators, thermally induced transverse mode instability (TMI) is one of the main limiting factors. The onset of TMI deteriorates the output laser beam quality and restricts the maximum achievable laser power. Most studies on TMI focus on fiber amplifiers, while reports on TMI in fiber laser oscillators are much fewer. Here, we report an experimental study on mitigating TMI by detuning the pumping wavelength in a single-end pumped monolithic fiber laser oscillator. The performance of the laser oscillator is investigated with individual 976 nm pumping, with individual 915 nm pumping, and with hybrid pumping. Experimental results show that TMI is mitigated by replacing 976 nm pumping with 915 nm pumping. In the hybrid pumping scheme, the influence of ratios of 976 nm pumping power to 915 nm pumping power on the TMI threshold is studied. Finally, by optimizing pumping power ratio, the output laser power of the monolithic fiber oscillator is enhanced to 2 kW. The M2 factor of the output laser is ∼1.6 and the Raman Stokes light occupies less than 1% of the total power.