Intra-cavity spectral modulation observed in high-Q pulsed dye lasers

We investigate a distinct spectral modulation observed in high-Q pulsed dye lasers. The results show that this modulation does not arise from an internal etalon effect, but from the interference between internal reflections within the cavity and the traveling wave in the resonator. Accurate perpendicular alignment of the dye cell relative to the resonator axis is required to enable feedback from these reflected waves. Within the narrow angular range where this feedback occurs (a few hundred microradians), both the modulation pattern and the wavelength spacing between maxima are strongly affected. The optical quality of the components, particularly the dye cell, and precise alignment are critical for reliable modulation control. These findings provide practical insights for improving wavelength tuning and performance stability in such laser resonators.