Accuracy design of ultra-low residual reflection coatings for laser optics

• Published in: Chinese physics B Vol. 26; no. 7; pp. 393 - 398
• Main Author: 刘华松 杨霄 王利栓 焦宏飞 季一勤 张锋 刘丹丹 姜承慧 姜玉刚 陈德应
• Format: Journal Article
• Language: English
• Published: 01.06.2017
• Subjects: 不均匀性; 中心波长; 减反射膜; 剩余反射率; 梯度折射率; 涂层材料; 激光光学; 精度设计
• ISSN: 1674-1056; 2058-3834
• DOI: 10.1088/1674-1056/26/7/077801

Refractive index inhomogeneity is one of the important characteristics of optical coating material, which is one of the key factors to produce loss to the ultra-low residual reflection coatings except using the refractive index inhomogeneity to obtain gradient-index coating. In the normal structure of antireflection coatings for center wavelength at 532 nm, the physical thicknesses of layer H and layer L are 22.18 nm and 118.86 nm, respectively. The residual reflectance caused by refractive index inhomogeneity（the degree of inhomogeneous is between -0.2 and 0.2） is about 200 ppm, and the minimum reflectivity wavelength is between 528.2 nm and 535.2 nm. A new numerical method adding the refractive index inhomogeneity to the spectra calculation was proposed to design the laser antireflection coatings, which can achieve the design of antireflection coatings with ppm residual reflection by adjusting physical thickness of the couple layers. When the degree of refractive index inhomogeneity of the layer H and layer L is-0.08 and 0.05 respectively, the residual reflectance increase from zero to 0.0769% at 532 nm. According to the above accuracy numerical method, if layer H physical thickness increases by 1.30 nm and layer L decrease by 4.50 nm, residual reflectance of thin film will achieve to 2.06 ppm. When the degree of refractive index inhomogeneity of the layer H and layer L is 0.08 and -0.05 respectively, the residual reflectance increase from zero to 0.0784% at 532 nm. The residual reflectance of designed thin film can be reduced to 0.8 ppm by decreasing the layer H of 1.55 nm while increasing the layer L of 4.94 nm.