A Wavelength-Selectable Passively Q-Switched Erbium-Doped Fiber Laser for Addressing Acetylene P-Branch Absorption Lines

A wavelength-selectable passively Q-switched erbium-doped fiber laser (EDFL) is experimentally demonstrated as a pulsed laser source for addressing selected acetylene P-branch absorption lines near 1.53 μm. The cavity combines interchangeable fiber Bragg gratings (FBGs) for coarse wavelength selection with temperature control of the selected FBG for fine wavelength positioning. Target absorption transitions were identified from the HITRAN database; their line positions were converted to the wavelength convention used in the experimental plots, giving P(7), P(9) and P(11) reference wavelengths of 1528.76 nm, 1529.94 nm, and 1531.17 nm, respectively. The laser output was amplified and launched into an acetylene-filled hollow-core fiber (HCF), and the transmitted spectra were monitored during gas filling and FBG thermal tuning. The source exhibited passively Q-switched operation from approximately 7 to 30 kHz over the investigated pump-power range, with pulse widths decreasing from about 12.3 μs to 3.5–4.0 μs. Short-term spectral measurements showed no resolvable central-wavelength drift within the 0.05 nm resolution of the optical spectrum analyzer (OSA) over 30 min. Thermal tuning produced relative transmission minima near the selected P-branch spectral positions, consistent with increased laser–line spectral overlap; these minima are used as wavelength-positioning indicators rather than quantitative absorption measurements. The results establish a compact wavelength-selectable pulsed source architecture for acetylene-filled HCF experiments and related gas–fiber photonic systems.