Vogel A, Busch S, Jungnickel K, Birngruber R
H. Wacker Laboratory for Medical Laser Applications, University Eye Hospital Munich, Germany.
Lasers Surg Med. 1994;15(1):32-43. doi: 10.1002/lsm.1900150106.
Nd:YAG laser photodisruption with nanosecond (ns) pulses is an established method for intraocular surgery. In order to assess whether an increased precision can be achieved by the use of picosecond (ps) pulses, the plasma size, the shock wave characteristics, and the cavitation bubble expansion after optical breakdown with ps- and ns-laser pulses were investigated by time-resolved photography and acoustic measurements. Nd:YAG laser pulses with a duration of 30 ps and 6 ns, respectively, were focused into a water-filled glass cuvette. Frequency doubled light from the same laser pulses was optically delayed between 2 ns and 136 ns and used as illumination light source for photography. Since the individual events were well reproducible, the shock wave and bubble wall position could be determined as a function of time. From the slope of these r(t) curves, the shock wave and bubble wall velocities were determined, and the shock wave pressure was calculated from the shock velocity. The plasma size at various laser pulse energies was measured from photographs of the plasma radiation. The breakdown thresholds at 30 ps and 6 ns pulse duration were found to be 15 microJ and 200 microJ, respectively. At threshold, ps-plasmas are shorter than ns-plasmas, but at the same pulse energy they are always approximately 2.5 times longer. The initial shock pressures were 17 kbar after ps-pulses with an energy of 50 microJ, and 21 kbar after 1 mJ ns-pulses. The pressure amplitude decayed much faster after the ps-pulses. The maximum expansion velocity of the cavitation bubble was 350 m/s after a 50 microJ ps-pulse, but 1,600 m/s after a 1 mJ ns-pulse. The side effects of intraocular microsurgery associated with shock wave emission and cavitation bubble expansion can be considerably reduced by the use of ps-pulses, and new applications of photodisruption may become possible.
纳秒（ns）脉冲的Nd:YAG激光光爆破是一种成熟的眼内手术方法。为了评估使用皮秒（ps）脉冲是否能提高精度，通过时间分辨摄影和声测量研究了ps激光脉冲和ns激光脉冲光击穿后的等离子体大小、冲击波特性以及空化泡膨胀情况。分别持续30 ps和6 ns的Nd:YAG激光脉冲被聚焦到一个装满水的玻璃比色皿中。来自相同激光脉冲的倍频光在2 ns到136 ns之间进行光学延迟，并用作摄影的照明光源。由于各个事件具有良好的可重复性，冲击波和泡壁位置可以作为时间的函数来确定。从这些r(t)曲线的斜率可以确定冲击波和泡壁速度，并根据冲击速度计算冲击波压力。通过等离子体辐射照片测量了不同激光脉冲能量下的等离子体大小。发现在30 ps和6 ns脉冲持续时间下的击穿阈值分别为15微焦和200微焦。在阈值时，ps等离子体比ns等离子体短，但在相同脉冲能量下，它们总是大约长2.5倍。50微焦的ps脉冲后初始冲击波压力为17千巴，1毫焦的ns脉冲后为21千巴。ps脉冲后压力幅度衰减得更快。50微焦的ps脉冲后空化泡的最大膨胀速度为350米/秒，但1毫焦的ns脉冲后为1600米/秒。使用ps脉冲可以显著减少与冲击波发射和空化泡膨胀相关的眼内显微手术的副作用，并且可能实现光爆破的新应用。
