Laser Damage On Functional Films Of Liquid Crystal Optical Elements

The liquid crystal optical elements have important potential applications on laser beams precision controlling technology (including laser beams shaping, laser polarization state controlling, laser phase modulation, et al.). Laser damage threshold is one of the most important parameters when considering liquid crystal optical elements used in high intensity laser system. Laser damage of the films in liquid crystal optical elements were investigated using CW and pulsed 1064 nm laser in this paper. The damage threshold and damage mechanism of the films were obtained by experiments and simulations.The temperature and stress distributions of indium tin oxide (ITO) films after laser irradiation were simulated. The absorption spectrum and square resistance of ITO were tested. The laser damage on ITO films was studied through pump-probe technology, 50% damage probabilities threshold of ITO were obtained by 1-on-1 testing and square resistance of the films after laser irradiation is measured. The results indicate that the ITO films are damaged due to laser induced thermal stress, and the laser damage threshold decrease with the increase of thickness of ITO films. The damage begins near the interface of ITO film and K9 glass after continuous wave laser irradiation, while the damage begins on the surface of ITO film after pulsed laser irradiation.The coupling and damage mechanisms of near infrared laser interaction with polyimide (PI) films were studied. The laser heats up the films leading to thermal decomposition.100% damage probabilities threshold of PI using pulsed laser (1064 nm,10 ns) is about 25 J/cm2 The thermal-physical mode of laser irradiation on PI was established, considering the exothermic reaction of thermal decomposition. The simulated results of pulsed laser irradiation on PI films basically agree with the experimental data. The temperature distribution of PI films (50μm) after continuous wave laser irradiation is simulated, the result indicates that the laser damage threshold is large.The laser damage on liquid crystal materials (LC) was studied in experiments, 100% damage probabilities threshold of LC materials is about 20 J/cm2. The damage mechanism is that the laser heats up the LC materials, and the electro-optical anisotropy of LC materials turn to electro-optical isotropy when the temperature of LC materials is higher than 65℃, leading to the liquid crystal optical elements fail temporarily. The LC materials decompose and carbonize when the temperature of LC materials is higher than 300℃, leading to the liquid crystal optical elements fail forever. The experimental result indicates that the damage threshold of LC materials is 20 J/cm2 after near infrared laser (1064 nm, 10 ns) irradiation.The calculating model of laser damage on liquid crystal optical elements (including ITO, PI, LC and K9 glass) was established, and the temperature distribution in the liquid crystal optical elements after near infrared laser irradiation was simulated. The large laser absorption coefficients of ITO and PI are the main reason of laser damage.