Mitigating of modal instabilities in linearly-polarized fiber amplifiers by shifting pump wavelength

Abstract

We investigated the effect of pump wavelength on the modal instabilities (MI) in high-power linearly polarized Yb-doped fiber amplifiers. We built a novel semi-analytical model to determine the frequency coupling characteristics and power threshold of MI, which indicates promising MI suppression through pumping at an appropriate wavelength. By pumping at 915 nm, the threshold can be enhanced by a factor of 2.1 as compared to that pumped at 976 nm. Based on a high-power linearly polarized fiber amplifier platform, we studied the influence of pump wavelength experimentally. A maximal enhancement factor of 1.9 has been achieved when pumped at 915 nm, which agrees with the theoretical calculation and verified our theoretical model. Furthermore, we show that MI suppression by detuning the pump wavelength is weakened for fiber with a large core-to-cladding ratio.

Study of Wavelength Dependence of Mode Instability Based on a Semi-Analytical Model

“…In [17], the measured change in threshold by shifting the pump wavelength from 977nm to 970nm was ~78%, which is larger than the calculated 40%. This may be due to that the fiber calculated in our paper has a larger core-clad ratio than that measured in [17], which results in weaken of gain saturation [25] and weaken the suppression capability of MI by shifting the pump wavelength [26]. It can be seen from Fig.…”

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier

“…However, when the output power exceeds 2995 W, the optical-to-optical efficiency falls dramatically to 28.8%. One can see that the increase trend of backward power indicates no stimulated Brillouin scattering (SBS) effect, so the power rolling over is the sign of the onset of mode instability 3,6 . Then the beam quality at different output powers is illustrated in Fig.…”

“…The main amplifier is made up of a counter (6 + 1) × 1 signal/pump combiner, a 18 m long Nufern 25/400 large mode area Yb-doped fiber with pump absorption factor of 0.6 dB/m at 915 nm, two cladding power strippers (CPSs) and an output quartz block holder (QBH). Wavelength-locked 915 nm laser diodes (LDs) are employed to pump the gain fiber and achieve higher TMI threshold 6 , and the CPSs are utilized to remove unwanted cladding light. To obtain good beam quality, the active fiber is coiled with a minimum diameter of 10 cm, and water-cooled on a heat sink.…”

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

“…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] .…”