Fundamental Study Of Sub-wavelength Grating Array Wavefront Sensing Technique

Wavefront Sensing technology can measure the phase distribution of optical wavefront. It has been widely used in the Adaptive Optics, Visual Optics, laser quality measurement, aberration measurement of optical components and optical system and so on. The Wavefront Sensers are continuously improving towards high precision, wide dynamic range, high sampling rate, real-time and small size. In recent years, with the progress of micro-structure fabrication technology, such as Electron Beam Lithography, Plasma Etching, make sub-wavelength structures become easy and accuracy. There are many applications of the sub-wavelength structures in the field of optical imaging, filtering, sensing and so on, but it is a novel idea that to apply the sub-wavelength structure to Wavefront Sensing technology.This article proposes a new zonal wavefront sensor, which uses a sub-wavelength grating array to subdivide the measured wavefront. The proposed sensor can provide wide dynamic range, small size, real-time, high general performance and other advantages. The fundamental principles of Sub-wavelength Grating Array wavefront Sensor are described in detail. The parameters of sub-wavelength grating array are optimized. The performances of the proposed sensor are proved by simulations. The content has included as follow:(1) Sub-wavelength Grating Array wavefront Sensing principle research. Analyzed the transmission of the sub-wavelength grating while using different polarization states of light source. The results show that when circularly polarized light is incident on the sub-wavelength grating, transmission will show regular changes with the incident angle of the light. Based on this phenomenon, we propose the basic structure of sub-wavelength grating array wavefront sensor and its signal processing algorithms: the sensor using sub-wavelength grating array to subdivide the measured wavefront. The transmission of each sub-wavelength grating will be modulated by the wavefront slope information. The incident angle of light will be calculated from the imaging received by CCD device. The dynamic range of the sensor depends on the grating period, material, duty and other structure parameters. The sampling rate of the sensor depends on the grating size in array. As a result, the sensor can realize wide dynamic range and high sampling rate simultaneously.(2) Design of Sub-wavelength Grating Array wavefront Sensor. The structure parameters of sub-wavelength grating are optimized by using the rigorous coupled wave method, including the coating material, period, thickness and duty cycle. The diffraction patterns of the sub-wavelength grating array are analyzed by Fresnel Diffraction Theory, and we analyzed the relationships between the parameters of the sub-wavelength grating array and its diffraction patterns by Fresnel Diffraction Theory. The parameters of the distance between the sub-wavelength gratings and the size of the grating are determined. The results show that the sampling rate of the proposed sensor is almost equal to Hartmann Sensor, but the dynamic range is much wider than Hartmann Sensor.(3) Simulations of Sub-wavelength Grating Array wavefront Sensor and error analysis. By combining the scalar and vector diffraction theory, we simulate the diffraction pattern of one sub-unit while the known plane wave is incident. Use Finite Difference Time Domain method to calculate the light propagation in the grating layer. The calculating results are the distribution of complex amplitude in near field. Use Fresnel diffraction theory to calculate the light propagation in the substrate of the grating and air. Finally, we can get the diffraction patterns received by CCD. According to the signal processing algorithm proposed by this article, we can calculate the incident angle of plane wave from the diffraction pattern. The simulation results prove the dynamic range of the proposed sensor is 0 ?? ?15?, which is 3~5 times higher than commercial Hartmann wavefront sensor. The sampling rate is almost equal to Hartmann Sensor. In simulations, the max relative measure error is less than 0.1%, and the average relative measure error is 0.046%.