Objective Imaging is the most intuitive way to perceive the world. The resolution of traditional imaging method is limited by the diffraction limit caused by the limited aperture of lens system. Ptychography is a non-lens imaging method based on coherent light, which avoids the diffraction limit problem caused by lens system. At present, the mechanical structure is often used to translate the optical probe in Ptychography, which brings errors of the position of the optical probe, resulting in the degradation of the imaging quality. Thus in this paper, we proposed a beam steering chip to avoid the errors of the position of the optical probe. The chip can replace the traditional mechanical optical probe to be used in ptychography. The 100 nanometer processing technology ensures that the position of the optical probe will not have errors, which greatly improves the quality of imaging and the stability of the system. On the other hand, because the size of the chip is only millimeter level, the size of the imaging system is also greatly reduced.
Methods The silicon-based integrated beam steering chip is processed on a 220 nm silicon on insulator (SOI) platform. The main structure of the chip is cascaded optical transmitting antennas with a filter system. The CMOS process with a linewidth of 180 nm is used to ensure that the position error of the optical probe caused by the actual processing process will not exceed one ten thousandth of the distance between adjacent optical transmitting antennas. Therefore, the position error of optical probe can almost be ignored. Light with different wavelengths will be sent to its corresponding optical transmitting antenna. The transverse spacing between adjacent optical transmitting antennas is 120 μm while the longitudinal spacing is 150 μm. After collimated by the lens, the light emitted from each light transmitting antenna can act as a light probe. Because the corresponding wavelength of each optical probe is different, we can adjust the wavelength to decide which optical probe to scan. At the same time, because the position of the optical transmitting antenna on the chip is fixed, there is almost no error in the position of the optical probe in the imaging process. This avoids the influence of optical probe position error on imaging quality.
Results and Discussions When the beam steering chip on the integrated platform is used to replace the mechanical structure for Ptychography, the whole image-forming system can be shown in Fig. 5. Fig.6 shows the diffraction patterns obtained from the CCD, the amplitude and phase information of the sample recovered by the PIE algorithm, and the curve of the error function during the operation of the algorithm. It can be seen that according to the diffraction pattern of CCD obtained in our experiment, the amplitude and phase information of the sample are successfully recovered. Because all 16 diffraction patterns are recorded in one exposure, there is actually overlap between the adjacent diffraction patterns on CCD, and the overlapped part is the high frequency component of the sample, which leads to the crosstalk between the adjacent diffraction patterns. Second, compared with the high-frequency component in the diffraction pattern, the power of the low-frequency component is much stronger. It will lead the center frequency to be overexposed, which also affects the final image quality. According to the introduction, the problem of center frequency overexposure can be solved by using baffle to block the zero order diffraction light. To sum up, we have successfully proved that the integrated beam steering chip can be used to complete space stack diffraction imaging and solve the problem of optical probe position error.
Conclusions In this paper, Ptychography based on integrated beam steering chip is proposed. After collimating, the light emitted from the optical transmitting antenna on the chip becomes the optical probe. Because the filter system is cascaded in the front end of the optical probe, the switch of the optical probe can be controlled by controlling the wavelength of the input light, which increases the diversity of imaging methods. At the same time, due to the fixed position of the optical transmitting antenna on the chip, there is almost no error in the position of the optical probe, which solves the huge impact of the position error of the optical probe on the imaging quality. Compared with the previous research on the error of the position of optical probe, our method solves the problem by device rather than by correction on the algorithm. At the same time, our method not only does not increase the complexity of the imaging system, but also greatly reduces the size of the whole system. The stability and robustness introduced by integrated platform processing will play an important role in the field of laser imaging. Using a small chip instead of a large volume device to realize imaging not only reduces the cost of the system, but also avoids the impact of environmental vibration and noise on the optical path of precision imaging. So using the power of integrated photonics to achieve more complex imaging system will be the focus of our future work.
