The first laser action in a rare-earth-doped chalcogenide glass fibre — a neodymium-doped gallium lanthanum sulphide (Ga:La:S) fibre oscillating at 1080 nm under continuous-wave 815 nm pumping — was reported by Schweizer et al. in 1997 with two striking limitations: the laser self-pulsed under a continuous-wave pump for reasons stated to be not understood, and its maximum slope efficiency was only 0.7%, leaving the proposed practical devices (efficient 1.3 µm Pr³⁺/Dy³⁺ fibre amplifiers and mid-infrared fibre lasers) unrealized. Later literature has now demonstrated mid-infrared laser oscillation in rare-earth-doped chalcogenide fibres (Tb³⁺-doped selenide glass at 5.38 µm in 2021, continuous operation near 5.25 µm and 150 mW output by 2022-2023, tunable continuous 5.22-5.31 µm lasing in 2025, and an Nd³⁺-doped fibre laser at 5.7 µm in 2026) and has measured mid-infrared signal gain in a Pr³⁺-doped chalcogenide fibre amplifier (4.6 dB at 5.28 µm in 2019). No later work explains the self-pulsing mechanism of the original Nd:Ga:La:S fibre laser, no efficient 1.3 µm Pr³⁺/Dy³⁺ chalcogenide fibre amplifier has been realized, and the output power, slope efficiency, and fibre loss of chalcogenide mid-infrared fibre lasers remain far from practical fibre-laser standards. The open problem addressed here is: identify the physical mechanism of the self-pulsing dynamics of rare-earth-doped chalcogenide fibre lasers, and achieve efficient, stable, practical fibre-laser and fibre-amplifier operation in this glass family.

Background

Chalcogenide glasses are amorphous semiconducting solids built from the chalcogens sulphur, selenium, and tellurium combined with elements such as germanium, arsenic, and gallium. Unlike silica, they are transparent deep into the infrared (sulphide glasses to beyond 6 µm, selenide glasses to beyond 10 µm) and they have very low maximum phonon energies. This matters for rare-earth lasers: an excited rare-earth ion can lose energy non-radiatively by simultaneously emitting several host lattice vibrations (multiphonon relaxation), and the rate of this process falls steeply as each phonon carries less energy, so low-phonon-energy hosts preserve mid-infrared radiative transitions (roughly 3-6 µm) that are quenched in silica. Rare-earth ions such as Pr³⁺, Dy³⁺, Tb³⁺, and Nd³⁺ are trivalent ions whose 4f electronic shells supply sharp transitions used as laser lines; in a fibre laser the doped glass forms the core of an optical waveguide that confines pump and signal light over long interaction lengths, and the geometry also helps dissipate heat. Slope efficiency is the fraction of additionally absorbed pump power converted into laser output above threshold; self-pulsing denotes spontaneous periodic or quasi-periodic pulsing of the output even under a perfectly continuous pump, a behaviour seen in many rare-earth fibre lasers and attributed, depending on the system, to mechanisms such as ion-pair clustering with upconversion, pump-to-signal intensity noise transfer, or saturable reabsorption. Excited-state absorption (ESA) — absorption of pump or signal photons by an already-excited ion — is a common loss channel that raises laser threshold and degrades gain. Ga:La:S (gallium lanthanum sulphide) is a chalcogenide glass family developed at Southampton in the early 1990s precisely because it dissolves rare-earth ions well and transmits to about 8-10 µm; the 1990s motivation was a 1.3 µm optical fibre amplifier for the then-second telecommunications window (Dy³⁺ and Pr³⁺ have 1.3 µm transitions that are quenched in silica, and the practical 1.3 µm amplifiers that did emerge used praseodymium-doped ZBLAN fibre — ZBLAN being a zirconium–barium–lanthanum–aluminium–sodium heavy-metal fluoride glass, a different low-phonon host) and mid-infrared fibre lasers for trace-gas detection, where molecular fingerprint absorption lines require sources beyond 2 µm. Before 1997, laser action in rare-earth-doped chalcogenide glass had only been achieved in bulk samples: a bulk Nd³⁺:Ga:La:S laser at 1080 nm worked continuously but thermally saturated and stopped lasing at higher pump power. In the 1997 Electronics Letters paper, Schweizer, Samson, Moore, Hewak and Payne drew a 22 mm fibre with a 0.05 mol% Nd₂S₃-doped Ga:La:S core (the rod-in-tube technique, in which a doped glass rod is inserted into an undoped glass tube and the assembly is drawn together into a fibre), pumped it with a continuous-wave Ti:sapphire laser at 815 nm, and obtained laser oscillation on the Nd³⁺ ⁴F₃/₂→⁴I₁₁/₂ transition at 1080 nm: thresholds of roughly 100-380 mW depending on output-mirror reflectivity, at most 1.2 mW output, and a maximum slope efficiency of only 0.7%. Unlike the bulk laser, the fibre showed no thermal saturation and ran stably for hours, but its output consisted of regular pulses of about 0.2 µs duration repeating every 2-3.5 µs despite the continuous pump — behaviour the paper left unexplained and flagged for further investigation, while noting that practical devices (efficient 1.3 µm Pr³⁺/Dy³⁺ amplifiers, mid-infrared fibre lasers) were still to be realized on this platform. The question is best understood against the modern context: efficient watt-level mid-infrared fibre lasers up to about 4 µm are now routine using rare-earth-doped fluoride (ZBLAN) glass fibre, while the chalcogenide route — the only glass family that can push rare-earth fibre lasers further into the 4-6 µm range — matured much later and remains at the proof-of-principle stage.

Problem Statement

The 1997 demonstration of Schweizer, Samson, Moore, Hewak and Payne left rare-earth-doped chalcogenide glass fibre lasers with two unexplained and unfulfilled aspects, whose open core survives today: (1) Identify the physical mechanism of the self-pulsing observed in the neodymium-doped Ga:La:S glass fibre laser — regular pulses of roughly 0.2 µs duration repeating every 2-3.5 µs under continuous-wave 815 nm pumping, at 1080 nm on the Nd³⁺ ⁴F₃/₂→⁴I₁₁/₂ transition, in a fibre that otherwise ran stably for hours — and more generally determine what governs the temporal laser dynamics of rare-earth-doped chalcogenide fibre lasers, given that the same glass showed continuous-wave output in bulk form and that the mechanisms established for self-pulsing in silica and fluoride rare-earth fibre lasers (ion-pair upconversion, pump-noise transfer, saturable reabsorption) have never been tested against this low-phonon-energy, short-lifetime (75 µs), lossy host. (2) Achieve efficient, stable, practical rare-earth-doped chalcogenide fibre-laser and fibre-amplifier operation: raise slope efficiency and output power from the 1997 values (0.7% slope efficiency, 1.2 mW) and from the current research-grade mid-infrared results (threshold-level operation in 2021, continuous operation near 5.25 µm and 150 mW beyond 5 µm by 2022-2023, gain of 4.6 dB in a Pr³⁺-doped fibre amplifier) to practical fibre-laser levels — at least 5% slope efficiency and at least 250 mW of stable continuous-wave output power in the 4-6 µm band — overcoming the identified obstacles of impurity-related background loss (hydrogen and oxygen contamination of the melt), excited-state absorption, limited rare-earth solubility, weak thermal properties, and double-crucible fibre-drawing purity (double-crucible drawing melts core and cladding glasses in concentric crucibles and draws them directly into a fibre, so melt purity directly sets fibre loss); and realize the efficient 1.3 µm Pr³⁺- or Dy³⁺-doped chalcogenide fibre amplifier originally proposed, which has still never been demonstrated. Mid-infrared laser oscillation in this fibre platform (beyond 5 µm, with Tb³⁺ and Nd³⁺ in selenide glass) has now been demonstrated and is therefore no longer part of the open core, but it remains subsumed under objective (2) as the principal application target — an efficient mid-infrared chalcogenide fibre laser, for example for trace-gas detection. The open problem has four independent parts: (i) the identified mechanism of the self-pulsing observed in the 1997 Nd:Ga:La:S fibre laser, with experimental and/or modelling evidence that reproduces the observed pulse durations, repetition rates, and their pump-power dependence and excludes the competing candidate mechanisms; (ii) the more general determination of what governs the temporal laser dynamics of rare-earth-doped chalcogenide fibre lasers, taken here as a stated law of the host-class dynamics that predicts whether the output is stable or self-pulsing and is validated against the 1997 Nd:Ga:La:S case together with at least one further rare-earth and host combination; (iii) fibre-laser operation in this glass family at practical efficiency, power, and stability, taken here as at least 5% slope efficiency and at least 250 mW of stable continuous-wave output power in the 4-6 µm band; and (iv) a realized 1.3 µm Pr³⁺- or Dy³⁺-doped chalcogenide fibre amplifier with at least 10 dB of measured small-signal gain.

References

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