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. High-purity ethanol serving as a saturable absorber operating within its intrinsic absorption darkening region is comprehensively investigated in this work. Ethanol stands out due to its high damage threshold, excellent fluidity, and long-term chemical stability, thereby making it a promising candidate for mid-infrared applications.

Using a custom-designed micrometer-precision liquid cell, the ethanol layer thickness is continuously modulated from 14 μm to 55 μm (±1 μm accuracy), and passive Q -switching can be achieved without the need for an external modulator. The laser system adopts a 248-mm planar resonator, which includes a \varPhi 3 mm × 70 mm Er, Cr:YSGG rod (Cr³⁺ 3% (atomic percentage), Er³⁺ 30% (atomic percentage)), and a flashlamp pumped at 250 μs and 20 Hz. Under these conditions, the output pulse characteristics are governed almost entirely by the ethanol thickness. When the pump energy is fixed at 12.86 J, reducing the layer thickness from 55 μm to 14 μm will shorten the pulse duration from 366.1 ns to 257.9 ns and increase the single-pulse energy from 1.25 mJ to 3.48 mJ. Optimal performance, characterized by 287.6 ns pulses and 11.64 mJ energy, is achieved at a thickness of 45 μm.

While maintaining this optimal thickness, increasing the pump energy from 7.01 J to 10.75 J will further compress the pulses from 629.1 ns to 287.6 ns and increases the output energy from 0.52 mJ to 11.64 mJ, none of which do not cause optical damage, indicating a damage threshold exceeding 10 J/cm². At pump energies exceeding 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, which is an operating regime highly favorable for precision laser ablation.

The beam quality at maximum output is measured to be M_x^2 =7.51 and M_y^2 =7.51 . These results are supported by rate-equation modeling combined with temperature-dependent absorption cross-sections from the HITRAN database, establishing ethanol as an adjustable, high-damage-threshold liquid saturable absorber for compact mid-infrared Q -switched lasers, and emphasizing the broader potential of hydroxyl-containing liquids for next-generation medical and spectroscopic applications.