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The search for suitable saturable absorption materials for the 2.79 μm wavelength range has been a key focus in the development of passive Q-switched laser technology at this wavelength. In this work, we present a comprehensive investigation of high-purity ethanol as a saturable absorber operating within its intrinsic absorption darkening region. Ethanol is distinguished by its high damage threshold, excellent fluidity, and long-term chemical stability—making it a promising candidate for mid-infrared applications.
Using a custom-designed micrometer-precision liquid cell, the ethanol layer thickness was continuously tuned from 14 µm to 55 µm (±1 µm accuracy), enabling passive Q-switching without the need for external modulators. The laser system employed a 248 mm flat–flat resonator incorporating a Ф3 mm×70 mm Er, Cr: YSGG rod (Cr3+ 3 at.%, Er3+ 30 at.%), flashlamp-pumped at 250 µs and 20 Hz. Under these conditions, the output pulse characteristics were governed almost entirely by the ethanol thickness. At a fixed pump energy of 12.86 J, reducing the layer thickness from 55 µm to 14 µm shortened the pulse duration from 366.1 ns to 257.9 ns and increased the single-pulse energy from 1.25 mJ to 3.48 mJ. Optimal performance—287.6 ns pulses with 11.64 mJ energy—was achieved at a thickness of 45 µm.
Maintaining this optimal thickness while increasing the pump energy from 7.01 J to 10.75 J further compressed the pulses from 629.1 ns to 287.6 ns and raised the output energy from 0.52 mJ to 11.64 mJ, all without inducing optical damage, indicating a damage threshold exceeding 10 J/cm2. At pump energies beyond 8.4 J, the ethanol undergoes re-bleaching within its ~20 µs recovery time, resulting in the formation of 2–5 equally spaced nanosecond sub-pulses (6–12 µs spacing, effective repetition ≈ 100 kHz) within a single pump envelope—an operating regime highly favorable for precision laser ablation.
Beam quality at maximum output was measured as Mx2=7.51 and My2=7.48. These results supported by rate-equation modeling incorporating temperature-dependent absorption cross-sections from the HITRAN database, establish ethanol as a tunable, high-damage-threshold liquid saturable absorber for compact mid-infrared Q-switched lasers and underscore the broader potential of hydroxyl-containing liquids for next-generation medical and spectroscopic applications.-
Keywords:
- solid-state lasers /
- Er /
- Cr: YSGG laser /
- passive Q-switching /
- saturable absorber
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