Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses. II. Transmission, scattering, and reflection

For pt.I see ibid., vol.2, no.4, p.847-60 (1996) We investigated the transmission, scattering, and reflection of plasmas produced in water by Nd:YAG laser pulses of 6-ns and 30-ps duration. The transmission measurements comprise a large energy range at wavelengths of 1064 and 532 nm and various focusing angles between 1.7° and 22°. This parameter range covers the parameters used for intraocular microsurgery, but also allows one to assess the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for picosecond pulses than for nanosecond pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self-focusing, which occurs at focusing angles below 2°, leads to a further increase of transmission. The experimental results were compared with the predictions of the moving breakdown distributed shielding model. Only partial agreement could be achieved, because the model assumes a spatially and temporally constant absorption coefficient within the plasma which is not realistic. The model can, however, be used to determine the average absorption coefficient. Fits of calculated transmission curves to the experimental data at θ=22° yielded 900 cm^-1⩽α⩽1800 cm^-1 nanosecond plasmas and 360 cm^-1⩽α⩽570 cm^-1 picosecond plasmas. The efficacy of plasma-mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the nanosecond pulses nearly 50% of the laser pulse energy is absorbed already at threshold, whereas it is only 8% with the picosecond pulses