Manuscript Received 12-14-88 Free vibration pulse laser-induced damage in optical thin films Fan Zhengxiu, Jin Linfa and Luo Miaohong Some experiments on damage in optical coatings induced by Key words: damage; laser; microstructure; optical thin film; overcoat It Free vibration pulse laser can generate a very large energy output. It can be applied into laser processing, laser medicine and be used for research purposes. is of great interest and importance to probe the essence of the interaction between this kind of laser and optical material, and the damage induced by this laser in optical thin films. Some experiments on laser induced damage in optical thin films made in SIOFM are reported here.1 2.1 The pulse structure of the Free vibration pulse laser Experiments were performed on Nd-glass free vibration pulse laser with its maximum output wavelength 1.06 μ and its maximum output energy 1000 J. The pulse structure of this laser is very complex. Figure 1 shows its pulse structure. It is composed of several hundreds of subpulse peaks with each duration several microseconds. The whole pulse duration is about three milliseconds. 2.2 The accumulated effect of laser induced damage in optical thin films The effect of free vibration laser on optical thin films is the same as that of repeatability laser. The induced damage on optical thin films depends either on the peak power of each subpulse peak or on the accumulated effect of the multipulse repetition. The accumulated effect relates not only to the pulse structure of laser but to the material of thin film. Figure 2 and figure 3 give the power damage threshold and energy damage threshold of the Ti02 thin films and ZrO2 thin films vs. laser acting time respectively. To show clearly the accumulated effect of laser induce damage, the laser pulse was cut and divided by a sector which rotated with a very high rate. There was a narrow slit, which could be varied in its width, in this sector to change laser acting time. The wider the slit width, the longer the laser acting time was and also the more frequently the subpulse peak acted on thin films on condition that the rotation rate be definite. Comparing two curves in figure 2, it is evident that the accumulated effect of Ti02 thin film is very different from that of the ZrO2 thin film. With the laser acting time going on, the power damage threshold reduced quickly for the Tio thin film. But for the ZrO2 thin film, the power damage threshold varied slightly as the laser acting time varied from 0.2 ms to 2 ms. The special properties of ZrO2 thin film make it be successfully used in the repeatability laser. 3. 2.3 The effect of thin film structure on laser induced damage The damage resistance of thin film depends greatly on the structure of thin film. Table I is the experimental results of laser induced damage in ZnS thin film which possesses grains of various size. It is shown in the table that the smaller the grain, the higher the damage threshold is. The damage tests of ZrO2 thin film also lead to the same conclusion. Listed in table II are the experimental results of laser induced damage in ZrO2 thin films with various structure. It is shown that the damage threshold of Zro thin film is the highest for cubic crystallinity and the lowest for monoclinic crystallinity. 2.4 The effect of SiO2 overcoating on laser induced damage in ZrO2/S102 2 It is well known that half wavelength S10, overcoating on TiO2/SiO2 multilayers can enhance the damage threshold. We have researched this phenomenon and go a step further. It is found that the level of the damage resistance of the ZrO2/Si02 multilayers with S102 overcoating is related to the thickness of Sio2 overcoating. Table III shows the results of the damage tests on ZrO2/SiO2 reflecting coating with Si02 overcoating of various thickness. is shown that the damage threshold of ZrO2/Si02 multilayers increases with increase of the overcoating thickness and reaches a maximum when the thickness of SiO2 overcoating approximates to twice the wavelength. The damage resistance of ZrO2/Si02 multilayers is related to its microstructure. We can see from figure 4 some electrographies of ZrO2/Si02 multilayers with and without Si02 overcoating, that the surface morphology It becomes finer and more smooth with the increase in thickness of SiO2 overcoating, but roughness when the overcoating thickness exceeds 5/2 the wavelength. Conclusion The damage induced by free vibration pulse laser in optical thin films depends on both the pulse structure of laser and the intrinsic properties of thin films. The accumulated effect of thin film damage depends on the material of thin film. The damage threshold of thin film is related to its structure. In general, the smaller the grains, the higher the damage threshold. The damage threshold of the optical thin film can be enhanced by a SiO2 overcoating on it. The level of resistance to laser radiation is related to the thickness of the overcoating. There exists an optimum thickness of the overcoating for the damage threshold to reach a maximum. 4. References [1] Fan Zhengxiu, Laser Journal, Vol. 9, No. 9, 582; 1982 [2] W. H. Lowdermilk, SPIE, Vol. 541, 124; 1985 Figure 1. The pulse structure of Free vibration laser. Time scale: (a) 0.5ms/measure, Figure 2. The power damage threshold of the optical coating vs. laser acting time. Figure 3. The energy damage threshold of the optical coating vs. laser acting time. |