Mitigating transverse mode instability in a single-end pumped all-fiber laser oscillator with a scaling power of up to 2 kW

Abstract

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.

4.05 kW monolithic fiber laser oscillator based on home-made large mode area fiber Bragg gratings

“…As we can see in Fig 3, Raman-Stokes light is observed at the operation of ∼4.05 kW, and the intensity is around 25 dB below the signal laser. Compared with the reported optical spectra of the fiber laser oscillator employing gain fiber with an ∼20 μm core diameter [14,15,18] , the SRS is remarkably mitigated by employing the LMA fiber with a 25 μm core diameter.…”

“…Figure 2(b) shows the temporal signal at the operation of 4.05 kW and its Fourier spectrum (the inset). The temporal signal is quite stable, and there is no typical modulation frequency of TMI observed in the Fourier spectrum [15,18,28] . From the evidence of temporal characteristics of the output laser, the absence of TMI can be confirmed in the monolithic fiber laser oscillator.…”

“…Theoretically, it is a most effective technique to mitigate the SRS by increasing the mode area of the high-power fiber lasers [16,17] . In consideration of the LMA FBG manufacture techniques, most reported high-power monolithic fiber laser oscillators employed gain fiber with a core/inner-cladding diameter of 20/400 μm and corresponding FBGs, which are commercially available and ensure a near-diffraction-limited beam quality [13][14][15]18,19] . However, as the output power gradually increases, the fiber laser oscillators based on gain fiber with core diameter of ∼20 μm face severe limitation of SRS in the power scaling [14,15] .…”

“…The counter-pumping LDs are combined by a ð6 þ 1Þ × 1 pump/signal combiner, which consists of six pump ports and one signal port with a core diameter of 25 μm and a numerical aperture (NA) of 0.065. In our previous work on the 976-nm-pumped high-power monolithic fiber laser oscillators [15,18] , the occurrence of the transverse mode instability (TMI) usually leads to a rollover output power and limits further power scaling. The 915 nm pumping has a moderate absorption in the YDF compared with the 976 nm pumping we employed in some previous work with observation of the TMI [18,22,23] .…”

“…In our previous work on the 976-nm-pumped high-power monolithic fiber laser oscillators [15,18] , the occurrence of the transverse mode instability (TMI) usually leads to a rollover output power and limits further power scaling. The 915 nm pumping has a moderate absorption in the YDF compared with the 976 nm pumping we employed in some previous work with observation of the TMI [18,22,23] . The bidirectional-pump configuration also distributes the heat load more evenly than the single-end-pump configurations, and thus further enhances the TMI threshold [15,24] .…”