Damage to polymer glasses caused by scanning with a cw laser beam

An investigation was made of the damage to "organic glasses" (transparent polymers) by laser radiation applied for duration t_p ≈ 10 ^– 3–1 s. Beginning from t_p ≈ 5 ms, the probability p of damage to polymethylmethacrylate (PMMA) ceased to rise on increase in t_p and even began to fall in the range t_p > 20 ms. Thermal self-defocusing of laser radiation was observed in PMMA, polystyrene, and polycarbonate. Theoretical estimates of the characteristic defocusing time, considered as a function of the parameters of the laser pulses and of the material, were obtained and checked experimentally. Constants which could be used as quantitative criteria of the importance of defocusing were obtained for PMMA and polystyrene. A fall of the dependence p (t_p) observed in the range t_p ≳20 ms was attributed to defocusing, but this explanation did not apply to an inflection at t_p ≈ 5 ms.