Question

A posterior capsulotomy is a non-invasive laser procedure to eliminate the cloudiness that occasionally interferes with a patient's vision after cataract surgery. For this procedure a Nd:YAG laser with pulse duration of 30 ns is focused to 50 – 100 µm in diameter on the posterior capsule to cut a thin membrane which has become cloudy over time. Estimate the pulse energy necessary to achieve optical breakdown during this treatment, because otherwise all laser energy will be absorbed by the retina and other tissues lying underneath.

Answer 1

The intensity required for optical breakdown depends on pulse duration.Generally the intensity is inversely proportional to pulse duration. A standard result is - for 1-ps pulse duration an intensity of ≈ 2 × 10¹³ W/cm² is required for optical breakdown .

So for 30ns pulse duration we need 6010¹⁶ W/cm²

that is 6010¹⁸W/mm²

So pulse energy for an Area with Diameter 50-100 m ( 50-100 10^-3mm) for 30ns is 6010¹⁸3010^-925²10^-6= 0.3510¹³ j (for 50 micron diameter)