Mitigating transverse mode instability in all-fiber laser oscillator and scaling power up to 25 kW employing bidirectional-pump scheme

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.

Development and prospect of high-power doped fibers

“…Optimizing pumping scheme Yang et al constructed a high-power all-fiber laser oscillator to investigate extrinsic mitigation of MI [47] . For co-pumping configuration, max output power is about 1.6 kW, further power scaling is limited by onset of MI, MI threshold is raised to above 2.5 kW by changing co-pump scheme to bidirectional pumping, further power amplification is limited by pump power restriction, which proves bidirectional pump scheme beneficial to MI suppression.…”

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

“…The stimulated Raman scattering (SRS) effect was obviously observed at the operation of 1.5 kW, and further power scaling was limited. In 2016, our group effectively mitigated the SRS by employing a short gain fiber as well as the bidirectional 976 nm pump scheme [15] . A maximum output power of 2.5 kW was achieved in the monolithic fiber laser oscillator with a Raman Stokes intensity of ∼20 dB below the signal.…”

“…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] . In 2017, the researchers in Fujikura Inc. reported a 3 kW monolithic fiber laser oscillator based on their specially designed YDF with a mode area of ∼400 μm 2 and corresponding FBGs [20] .…”

“…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] .…”

“…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] . Besides, according to previous studies [25] , the bidirectional-pump configuration enjoys the advantage of suppressing the SRS.…”