The damage of the fused silica surface caused by ultraviolet laser has been the major cause that limits the development of the large aperture high-power laser system [1, 2, 3]. Accurately and effectively detecting the damage threshold is of great significance in assessing damage resistance of optical components. Traditional ways of measuring damage threshold include phase contrast microscope observation [4, 5, 6], plasma flashing , monitoring of the transmission [8, 9], photoacoustic technique  and light Scattering [11, 12], etc. The beam deflection method [13, 14, 15, 16], which is usually applied in measuring the damage threshold of thin film coating, is not often applied in detecting the damage threshold of fused silica. It is the most effective way to repair the damaged fused silica optimal components by applying CO2 laser irradiation thus restricting the growth of the damaged fused silica optimal components. However, CO2 laser treatment [17, 18] is to remove the damaged materials in this way of fusing or evaporating, thus forming a Gaussian smooth pitch , which generates strong optical field modulation and possibly will cause the downstream components [9, 20]. The results show that optical field modulation growth is closely connected with the morphologies of mitigated sites, and it is imperative to detect the morphological properties of mitigated sites quickly and effectively in order to analyze the influence of the mitigated sites on the damage resistance of the downstream optics. This article proposes a quick and effective method of detecting the damage threshold of the fused silica components and the morphological properties of mitigated sites.
2 Experimental set of beam deflection
The experimental set of beam deflection can be seen in Figure 1. The samples are exposed to Nd:YAG pulse laser with 355 nm wavelength and pulse width of 10 ns (1/e). An energy detector (PE25) is used to detect laser energy. The combination of a half-wave plate and polarizer is used to control the energy of each harmonic beam. During the measurements, laser spots are casted to the front of the samples. He-Ne probe beam is casted from a certain angle to the laser region and the transmission light is, via a reflector, reflected into the quadrant detector to measure the laser spot displacement. In Figure 2, the probe beam are casted from three different angles. In mode A, the incident beam is casted to the edge of the laser spot and the emergent beam is far away from the edge of laser spots. In mode B, the incident beam and the emergent beam are all in the laser region. In mode C, the incident beam is far away from the laser spots, while the emergent beam is at the edge of laser spots.
With the increase of irradiation time, the deflections of probe spot in the different modes are shown in Figure 3. In Figure 3(a), the three rows of the picture, from top to bottom, respectively denote the measuring results of mode A, mode B, and mode C. The four columns, from left to right, denote respectively the positions of the deflection beam, before working, 3 seconds later, 6 seconds later, and 9 seconds later. When the probe beam is casted in mode A, the deflection beam moves toward quadrant 1 and quadrant 4. When in mode B, the displacements of the deflection beam can’t be clearly noticed. When in mode C, the deflection beam moves toward quadrant 2 and quadrant 3. In mode A and mode C, with the increase of irradiation time, the displacements enlarge accordingly, and become stable after a period of time. The three displacements of the deflection beam with the increase of irradiation time are shown in Figure 3(b). From Figure 3(b), when probe beams are casted in mode B, the displacements of deflection beam cannot be clearly noticed. When in mode C, the deflection beam moves toward quadrant 2 and quadrant 3. In mode A and mode C, with the increase of irradiation time, the displacements increase accordingly, and become stable after 12 seconds. To improve detection sensibility of the beam deflection, mode A and mode C are adopted to detect the displacements after the irradiation time of 12 seconds.
When laser damage occurs, the deflection displacement of probe beam will increase dramatically, 1-2 order of magnitudes. To verify the accuracy of detecting laser damage threshold with this method, the traditional phase contrast microscope observation is applied to compare the average fluence, respectively, at damage probability of 0%, 50% and 100%, which is shown in Table 1.
From Table 1, the discrepancy between the two methods is within 3%. The beam deflection method developed in this article can be applied to large-aperture, high-power laser systems to quickly detect laser damage threshold in situ. The shift of laser spot and the fluctuation of energy will cause uncertainty of the measurement. The energy detector connected to a computer is used to monitor the average energy of 100 pulses (during 10 seconds). The fluctuation of the measured fluence for each pulse is less than 2.5%. The changes in the microstructure and the refractive index of surrounding medium could occur on the surface of fused silica samples irradiated by the laser pulse with fluence near LIDT, which seriously shortens the lifetime of optical components. The size of these damage sites is too small to be observed by the phase contrast microscopy, used in the experiment, with a resolution of 1.5 μm for these minor sites. Thus, compared to analysis of morphologies, there is less uncertainty using beam deflection method, because this method can detect smaller damage site by adjusting the quadrant detector farther from the sample.
3 Analysis of morphologies of mitigated sites
In the experiment, fused silica samples were 50 × 50 × 5 mm3in size, with the surface roughness (RMS) of less than10 nm. The samples were immersed in HF solution first to remove the redeposition on the surface. Then, irradiated by the 355 nm wavelength and 300 μm diameter laser pulse, a mass of damage sites occur. The range of laser fluence used in the experiment is 5-27 J/cm2 with the fluctuation of fluence ±3%. The damaged fused silica samples were immersed in ultra-high-purity HF solution repeatedly to remove the fragments, and irradiated by 97 W CO2 laser with 7 mmin diameter and the irradiation time for 4 seconds. Subsequently, damage sites became smooth again. The mitigated sites has a Gaussian spatial profile, and their lateral sizes range from 50 μm to 550μm. Lastly, the deflection of the probe beam (He-Ne laser) passing the mitigated sites is detected by the location detector, which is shown in Figure 4.
The characteristics of mitigated sites H(r) is denoted as 
where, H0 is the maximum repair depth, and R0 is the effective half width (at 1/e).
When the probe beam is vertically incident to a mitigated site as shown in Figure 4, the tangent value of incidence angle α is denoted as
From the refraction theorem:
Where β is the refraction angle and n is the refractive index of the medium. According to angle conversion, the sine value of the emergent angle γ can be denoted as
The distance from the samples to the position detector is d, and the deflection displacement can be denoted as
When d[tan(α)]/dr = 0, which means that r = (1/2)1/2 R0, the deflection displacement takes its maximum value. Then, equation (2) can be rewritten as
The data measured by the stylus profilometer indicates that the depth of mitigated sites is much smaller than their width (H0 << R0, that is, tan(α) <<), and the results of equations (3), (4) and (5) can be simplified as
From equation (10), the maximum value of deflection displacements grows with the growing repair points pitch depth. However, the maximum value of deflection displacements decreases with the enlarging repair points width.
Figure 5 indicates the deflection displacements of two different types of mitigated sites at different points. With the probe beam moving toward repairing pitches, the deflection displacements increase first and then decrease. With the beam moving to the bottoms of repairing pitches, the deflection displacements reversely increase first and then decrease. With the beam shifting out of mitigated site, deflection signals return to the equilibrium position.
Figure 6 reveals the changes of maximum deflection with depth-to-width ratios. The experiment values have good agreement with the results from equation (10), which indicates the maximum deflection displacement increases linearly with the increase of depth-to-width ratio. According to the equation (10), the depth-to-width ratios of mitigated sites can be obtained based on the measured maximum deflection displacement. The mitigated sites left from the mitigation process by CO2 laser which will result in light modulation to the downstream optics and the modulation intensity is affected by the morphology of mitigated sites. Thus, the morphology characteristics of mitigated sites obtained by beam deflection method could assess instantaneously damage resistance of the downstream optics in situ.
In this work, a method of beam deflection is developed by adjusting the incident way of the probe beam. The damage threshold of fused silica was quickly detected, with the same result as the one obtained by applying the phase contrast microscope. Then the deflection displacements of the mitigated sites were measured and the morphologies of mitigated sites were analyzed. A method to quickly detect damage threshold of the fused silica components and the morphology characteristics of mitigated sites is proposed, which can provide significant reference as to the online assessment of the damage resistance of the downstream optics and provides the guidance of the repair process.
This work is supported in part by National Natural Science Foundation of China (No. 61505171), a project supported by Scientific Research Fund of Sichuan Provincial Education Department, China (No. 18CZ0014), and was also supported in part by National Nuclear Energy Development Project of China (No. 18zg6103).
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About the article
Published Online: 2018-08-20
Citation Information: Open Physics, Volume 16, Issue 1, Pages 539–543, ISSN (Online) 2391-5471, DOI: https://doi.org/10.1515/phys-2018-0070.
© 2018 J. Zhang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0