Photodarkening-Induced Phase Distortions and Their Effects in Single-Channel and Coherently Combined Yb-Doped Fiber Chirped Pulse Amplification Systems

Abstract

We use numerical simulations to investigate the effects of photodarkening (PD) on pulse compression and coherent combination following chirped-pulse amplification in Yb-doped fiber amplifiers. Even though we increase the pump power to keep the pulse energy constant, PD degrades the pulses due to phase distortions resulting from heating acting already within a pulse as well as to increased self-phase modulation if the fiber length is optimized for the PD-free fiber at beginning of life. By contrast, in a fiber shorter than this, the drop in peak power is a relatively modest 35% even at a PD propagation loss as large as 5 dB, thus providing effective mitigation against PD-induced phase distortions. The improvements extend to the combination efficiency of beam-combined systems.

Single-Shot Phase and Amplitude Fluctuations of Narrow-Line Pulse Bursts in Divided-Pulse Amplifier

“…However, the phase in the trailing edge of the pulses is smaller than that in the leading edge, by an amount of between 0.6 and 1.3 rad at 0.5 ns. This asymmetry may be caused by energy-related phase effects, which build up during the pulse [9]. In contrast to these experiments, the simulations in [9] showed larger phase in the trailing edge than in the leading edge.…”

“…This asymmetry may be caused by energy-related phase effects, which build up during the pulse [9]. In contrast to these experiments, the simulations in [9] showed larger phase in the trailing edge than in the leading edge. This suggests that the refractive index decreases through the combined effect of energy extraction from the Yb-ions and heat deposition.…”

“…This was also seen in simulations in Ref. [9]. The full variations in phase, if dominated by SPM, are expected to be approximately twice as large as the variations over the FWHM duration, since those only correspond to power variations from 50% to 100% of the peak power.…”

“…Higher peak power is more demanding in that a given relative change results in a larger change of the B-integral (which quantifies SPM), and thus in larger stacking-efficiency degradation [7]. Similarly, phase distortions caused by the Kramers-Kronig (K-K) effect [8] and instantaneous heating [9] resulting from the quantum defect or photodarkening increase with higher pulse energy. To understand the potential of CPSA, it is necessary to characterize amplitude and phase distortions within the amplified burst, and estimate their stability, as affected by gain saturation and nonlinearities in parameter regimes of interest.…”

Experimental comparison of Yb/Al/Ce and Yb/Al/P co-doped fibers on the suppression of transverse mode instability

“…By theoretically calculating the thermally-induced mode loss evolution, it showed that the refractive index of the Yb-doped fiber would be changed by the thermo-optic effect, which affected the waveguide structure and altered the bending loss of different modes [24]. Moreover, the phase of the beam could be distorted by PD-induced thermal load [25]. Meanwhile, PD effect would significantly increase the thermal load of the active fiber and aggravate thermally induced refractive index grating, which eventually triggered the occurrence of TMI effect [26].…”