Study On Plasma Phase Measurement Principle Based On Wide Spectrum Differential Interference

The principle of optical interferometry method utilized to measure laser-induced plasma is summarized in this paper, and more detailed information about extraction method and calculation step of optical field phase modulated by plasma are discussed as well.To calculate electron density of the plasma, this paper gives the optical interference method which can measure the refractive index of plasma while simultaneously studying the corresponding calculation procedure and algorithm. The shearing interferometry using supercontinuum sources is proposed on the basis of theoretical research on refractive index properties of incomplete ionization plasma.Based on plasma excitation mechanism, this paper deduces the relationship between the plasma electrons, ions and neutral atoms and refractive index of incomplete ionization plasma, putting forward the calculation method of plasma electron density by establishing the plasma density equation measured through three kinds of wavelength laser, which can not only improve the measurement precision of free electron density of plasma, but also can calculate ion density and neutral density in plasma, so that this paper also provides a feasible measurement based on the ionization degree of plasma. Because the plasma is usually generated in minuscule spatiotemporal scale, the traditional optical interferometry can hardly detect the information about plasma phase with multiple frequency at the same time. In order to obtain phase information of plasma in the wide spectrum at a certain moment, this paper designs a broadening spectrum detection method by using blazed grating for dispresing the spectrum.This paper establishes the theory model of a complete differential interferometry by using supercontinuum sources, and this method is used to compute coherent parameter and the amount of data. To satisfy the requirements of detecting nanosecond laser induced air plasma, this paper designs a optical path based on the differential interferometry measurement method with the optical path close, resisting disturbance and measuring wave phase at real time. This path is built on the characteristics of plasma phase gradient information, which can be suitable for some higher demands like shock or temperature sensitive condition.This paper also discusses the influence of wavelength, the shearing angle and the detection range on plasma differential interference image information, and calculate the resolution limit of detection components. One-dimensional plasma electron and shock wave phase field is calculated from differential interference image by using stripe tracking technology, and the feasibility of supercontinuum laser differential interference is verified to detect the laser plasma.The results will provide the theory basis for detecting laser induced plasma using wide spectrum differential interferometry, which contributes to the further development and application in the field of plasma diagnosis of optical detection technology.