Cladding-pumped erbium:ytterbium codoped fiber lasers (EYDFs) are the standard route to high continuous-wave power in the eye-safe 1.5–1.6 μ\mum band, but the ytterbium sensitizer band itself develops net gain because a fraction of Yb ions are effectively isolated from Er ions and the Yb→\toEr energy transfer saturates at high pump power. Jeong et al. (2007) showed that deliberately permitting weak, controlled Yb co-lasing near 1.06 μ\mum clamps the Yb-band gain and prevents catastrophic self-pulsation, enabling a then-record 297 W at 1567 nm, and posed a set of open research needs: systematic experiments mapping how Yb-band feedback quantitatively affects Er slope efficiency, pulsation onset, and the safe operating regime; low-power small-core all-fiber test platforms; EYDF spectroscopy and fabrication that make the Yb→\toEr transfer fast and complete so that Yb-band net gain never appears; coupled numerical modeling of isolated/coupled Yb ions, thermal effects, and multimode propagation; long-term stability validation; and beam-quality fiber designs. The modeling puzzle has since been largely resolved (Dong et al. 2020: a two-population Yb model with a coupled-ion fraction explains continued Er growth past the Yb lasing threshold) and power scaling well beyond 297 W has been achieved by a different strategy (off-resonant and 1018-nm pumping that avoids Yb excitation buildup, reaching 439 W in 2025), while the quantitative experimental mapping of controlled Yb-band feedback and the spectroscopic ideal of isolated-ion-free EYDF remain unaddressed.

Background

Erbium-doped fiber lasers emitting at 1.5–1.6 μ\mum are valued because this wavelength band is relatively eye-safe, transmits well through the atmosphere, and is compatible with mature telecom components. Direct cladding pumping of Er at 980 nm is inefficient, so high-power systems use erbium:ytterbium codoped fiber (EYDF), in which Yb ions act as sensitizers: they absorb high-power 915–980 nm diode light and transfer the excitation nonradiatively to neighboring Er ions (the 2F5/2→2F7/2{}^{2}\mathrm{F}_{5/2}\to{}^{2}\mathrm{F}_{7/2} Yb transition feeding the Er 4I11/2{}^{4}\mathrm{I}_{11/2} level), which then lases on the 4I13/2→4I15/2{}^{4}\mathrm{I}_{13/2}\to{}^{4}\mathrm{I}_{15/2} transition near 1.55 μ\mum. Cladding pumping means the multimode pump light is guided in an inner cladding surrounding the doped core and is absorbed along the fiber length.

Two intrinsic problems limit this scheme at high power. First, the Yb→\toEr transfer is incomplete: time-resolved Yb fluorescence decay in EYDFs shows a fast component (ions coupled to Er, transfer time of order 10 μ\mus) plus a slow tail approaching the ~1.3 ms radiative lifetime, attributable to Yb ions spatially isolated from Er. These isolated ions can be strongly inverted at modest pump power, so the Yb band near 1.06 μ\mum develops net gain. Second, the quantum defect — the energy difference between the ~975 nm pump photon and the 1.56 μ\mum output photon; the pump photon carries about 60% more energy than the output photon, so roughly 38% of the absorbed pump power is deposited as heat — heats the fiber and changes the Yb absorption and emission cross sections. If the Yb-band gain is left uncontrolled, amplified spontaneous emission (ASE, spontaneous emission amplified along the fiber) builds up and can trigger giant self-pulsation — high-energy 1 μ\mum pulses that destroy fiber facets (the polished end faces of the fiber). A point filter at one end is insufficient: without distributed suppression throughout the fiber, Yb-band gain rebuilds along the length. Permitting a weak Yb cavity (co-lasing) instead clamps the Yb inversion at the level set by the cavity loss, draining the excitation stably; this is a gain-clamping or gain-limiting effect.

Jeong et al. (IEEE JSTQE 2007, DOI 10.1109/jstqe.2007.897178) demonstrated a single-stage cladding-pumped EYDF laser (30 μ\mum core, 6 m of phosphosilicate fiber — phosphorus-doped silica, the standard host for Er:Yb codoping — ~975 nm diode-stack pumping) producing 297 W continuous wave at 1567 nm, then a record. The Er slope efficiency fell from 40% to 19% once Yb co-lasing (338 W total at 1.067 μ\mum, threshold near 210 W of launched pump) set in, yet the Er output kept rising without significant slope degradation up to ~450 W of pump and only rolled over above that. Fluorescence decay indicated 2–5% isolated Yb ions — considerably below most EYDFs they had characterized, which they took as the reason for the good scalability. Removing the Yb-band feedback (by angle-cleaving the fiber ends so that they reflect little light back into the cavity) caused unstable Yb emission and fiber-facet failure at Er powers of only 120–150 W, showing the feedback was essential but that its quantitative role was unmeasured. The authors explicitly identified the further studies listed in the abstract. Prior context for the 1.5 μ\mum power record was the 103 W Er:Yb large-core laser of Sahu et al. (2003).

Problem Statement

For high-power cladding-pumped Er:Yb codoped fiber lasers at 1.5–1.6 μ\mum, establish quantitatively how controlled Yb-band feedback — the level of permitted Yb co-lasing near 1.06 μ\mum — affects the Er slope efficiency, the onset of Yb-band self-pulsation, and the safe operating envelope, and determine whether the intrinsic cause, namely Yb ions effectively isolated from Er ions and incomplete Yb→\toEr energy transfer, can be removed at the fiber-fabrication and spectroscopy level so that Yb-band net gain never develops for any pump power of interest. Concretely, per the source paper (Jeong et al., 2007) and the state of the literature: (1) perform the systematic experiments varying Yb co-lasing power/Yb-band feedback that the source called for, and map the resulting Er slope efficiency, pulsation threshold, and damage boundary; (2) develop EYDF compositions and fabrication routes that minimize the isolated-Yb-ion fraction and drive the Yb→\toEr transfer toward completeness, characterizing the transfer by time-resolved fluorescence, and determine the lowest isolated-Yb-ion fraction achieved and the highest launched pump power up to which Yb-band net gain is absent; and (3) where residual Yb-band gain is unavoidable, extend the steady-state two-population models (which split the Yb ions into an Er-coupled fraction and an isolated fraction) to the dynamic regime and validate against experiment. The model-coupling puzzle flagged in the source (explaining continued Er output growth past the Yb co-lasing threshold) has been resolved by the two-population modeling of Dong et al. (2020) and is not part of the open core; the open core is the experimental feedback mapping (1) and the spectroscopic/fabrication goal (2), together with their validation, while the dynamic-regime extension of the two-population models (3) is auxiliary and not required to resolve the open core.

References

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