Polarimeter—Stress Measurement for Laser Window

The Laser window is used to separate the environment on both sides, such as separating the inside and the outside of the instrument . In this way, the inside and the outside of the instrument are isolated from each other, thereby protecting the internal components. The window is the basic optical element, which is an optical flat plate. It does not change the optical magnification, but only affects the optical path in the optical path. The windows are all coated with anti-reflection coating or without anti-reflection coating. In addition to protecting the laser window, it must also ensure high transmittance and excellent transmitted wavefront. The performance of laser windows is quite different from ordinary optical windows. Its main applications include: laser cutting head protection, optical observation windows, laser protection windows, etc. When choosing a laser window, materials, transmittance, scattering, wavefront distortion, parallelism and the corresponding working environment are all factors that need to be considered.

When birefringence occurs , a Glass Polarimeter can be used to measure optical retardation. Based on experiments , relationship between optical retardation value and the stress value is determined by a special formula. As long as optical retardation value can be measured, the residual internal stress value can be calculated . At present, most Polarimeters are designed to measure optical retardation value directly .

When choosing a window, important parameters should be considered including light transmittance, surface accuracy, thickness, parallelism, substrate material and other properties. Commonly used substrate materials include BK7, UV fused silica, K9 glass ,CaF₂ , Si, Ge, Zn, Se, etc. The corresponding preparation methods include single crystal growth, chemical vapor deposition, hot pressing or isostatic pressing sintering and so on. Since many defects in the preparation process of the subscrates are invisible to the naked eye, this requires us to use some optical equipments for quality control. One of the most widely used instrument is the Polarimeter.

The measurement of a Polarimeter is based on the detection of stress birefringence. For instance , glass is a kind of isotropic object. The refractive index in each direction is the same. If there is stress in glass, the isotropic property will be destroyed, causing a change in refractive index. The refractive index of the two principal stress directions will no longer be the same, thus leading to birefringence .

When birefringence occurs , a Polarimeter can be used to measure optical retardation. Based on experiments , relationship between optical retardation value and the stress value is determined by a special formula. As long as optical retardation value can be measured, the residual internal stress value can be calculated . At present, most Polarimeters are designed to measure optical retardation value directly .