ABSORPTION COEFFICIENTS RATIO, 6016, no satu semi Fig.1. Thermal explosion threshold function, (T) for various behaviour of absorption coefficient: I ration, 2-semiconductor-type saturation, 3 conductor-type inclusion plus photoionization Fig.2. Parametric presentation of thermal instability for semiconductor-type inclusion. Fig.4. Explosion induction time versus laser intensity Fig.6. Pulse width dependence of damage threshould intensity for rectangular (I) and Gaussian (2) pulses. 1. Manuscript Received 4-24-89 Experimental Investigation on the Role of Wavelength in the Laser Conditioning Effect J.W. Arenberg and D.W. Mordaunt Hughes Aircraft Company El Segundo, California 90245-0902 This paper reports on an experimental investigation regarding the role of the wavelength in the laser conditioning effect. In this investigation, the threshold enhancement due to conditioning by irradiation at fluence levels below the damage threshold at 1.06 cm is measured at 1.06m and 0.53m. Conditioning of the optic at 1.06m is followed by testing at both 1.06um and 0.53 m. to determine the threshold for the onset of enhancement at each wavelength due to conditioning at 1.06μm. This experiment also allows for some comments to be made regarding the properties for the main actor in the laser conditioning effect. Key words: laser conditioning effect; preconditioning, prepulsing, laser cleaning, water in coatings. Introduction This paper reports on an experiment carried out to collect phenomenological data on the laser conditioning effect. The present effort continues in the vein of earlier experiments.1,2 The laser conditioning effect is the enhancement of the laser damage threshold due to subthreshold irradiation. Previous efforts have been directed at quantifying the magnitude and duration of the threshold enhancement due to conditioning when the conditioning and test wavelength are the same. The experiment reported on herein compares the threshold enhancement at due to conditioning at one wavelength, 1.06m and two different test wavelengths, 1.06m and 0.53pm. The experiment will shed some light on the nature of the major actor in the laser conditioning effect. Water has been hypothesized to be the major actor at longer wavelengths, >1.06m, and the desorption of water has been correlated to threshold enhancement in recent work.3 This paper reports on threshold enhancement due to conditioning at 1.06um at test wavelengths 1.06um and 0.53m for double-V (1.06m/0.53m) antireflection (AR) coated glass. The first item of data collected was the unconditioned and conditioned thresholds at 1.06 m and 0.53um.4 This sets the magnitude of the threshold enhancement at each wavelength. Next, the threshold for the onset for the conditioning effect was estimated. The estimation was made by plotting damage probability versus conditioning fluence. The damage probability as a function of conditioning fluence was generated by irradiating a grid at three subthreshold (unconditioned 1.06um threshold) fluences and then irradiating half of the grid at the 1.06m conditioned threshold and the other half at the 0.53m conditioned threshold and recording the number of sites damaged out of the total irradiated. Finally, the thresholds for the onset of conditioning at 1.06um and 0.53m due to conditioning at 1.06um are compared. The conditioning threshold data is then analyzed to yield some information about the properties of the main actor in the laser conditioning effect. 2. Experimental Conditions A sketch of the test facility is given as figure 1. The test laser is Nd:YAG source for the generation of 1.06pm and a second harmonic generator crystal for the 0.53pm light. The nominal test conditions are given in table 1. As a baseline, the unconditioned and conditioned thresholds were measured for both 1.06um and 0.53μm. The unconditioned threshold is the minimum fluence causing catastrophic failure of the optical surface when each site is exposed to only one test fluence. Damage is denoted by flashes, noise and definite cratering. The test set shown in figure 1 allows for the surface to be observed through a microscope (20X magnification) during irradiation, thus making the association of flashes and the onset of cratering possible. The conditioned threshold was the lowest fluence at which a conditioned site was seen to fail. The irradiance history of the conditioned site was exposure to a slowly increased fluence level, dwelling for 16 shots at each level. The typical conditioned exposure consisted of 5 to 7 fluence levels separated by 4-5 J/cm2. As the conditioned threshold became better defined, smaller fluence steps were taken in the neighborhood of the threshold to attain higher resolution. Following the identification of the 1.06um and 0.53μm conditioned thresholds, a grid was laid out. The grid consisted of 3 rows of 16 spots. Each row was conditioned at a different level. The irradiation levels were 25%, 50% and 75% of the 1.06μm conditioned threshold. Half of the spots in each row were irradiated at the 1.06μm conditioned threshold and the other half at the conditioned threshold at 0.53μm. The frequency of damage was recorded for each test wavelength and conditioning fluence, resulting in a plot of the frequency of damage versus the conditioning fluence at 1.06μm. 4. Results and Discussion The conditioned and unconditioned damage thresholds measured for the coating are given in table 2. From the test results, it is clear that the coating does not condition at 0.53μm due to subthreshold exposure at 0.53pm, while the coating undergoes a noticeable increase, 8+5 J/cm2 when conditioned and tested at 1.06μm. The damage frequency plotted as a function of the 1.06μm conditoning fluence is given in figure 2. In figure 2 there is no sharp onset of conditioning associated with threshold enhancement, as has been noted in other work.2,3 In the case of 1.06um conditioning, there was a continuous decrease in the probability of damage with increasing conditioning fluence. At 0.53Lm |