The Research On Single Photon Imaging Based On Frequency Conversion

With the rapid development of quantum information technology theory and experimental techniques in recent years,many new research areas have emerged.As an indispensable part of quantum information technology,single-photon detection technology can achieve limit sensitivity detection of photons,and has high temporal resolution and low noise count.With the maturity of single-photon detection technology,research on single-photon imaging has received attention,particularly in the emerging field of non-line-of-sight imaging in the past decade.Non-line-of-sight imaging technology is based on the time-of-flight measurement of photons and can achieve 3D spatial resolution imaging of hidden targets outside the line of sight,which has potential application value in rescue,autonomous driving,military and other fields.However,multiple diffuse reflections exist in the non-line-of-sight imaging scene,which brings strong attenuation and complex image reconstruction processes,and high spatial resolution also requires extremely high temporal resolution.In recent years,the development of highsensitivity single-photon detectors with temporal resolution capability has promoted the realization of 3D non-line-of-sight imaging,and the proposed image reconstruction algorithms have paved the way for non-line-of-sight imaging reconstruction.Currently,experimental devices and image reconstruction algorithms based on single-photon detectors can achieve their theoretical spatial resolution limit.However,the spatial resolution is usually limited to the centimeter level due to the temporal resolution capability of single-photon detectors.Further improvement of the spatial resolution requires the temporal resolution capability of single-photon detectors to be improved to the picosecond level or even lower.To meet this demand,we built an up-conversion single-photon detector based on a nonlinear frequency conversion process using pulse pumping in a periodically poled lithium niobate(PPLN)waveguide,which has a temporal resolution capability of 1.4ps in the near-infrared band.By combining frequency domain filtering and time domain filtering,the noise counts from the environment and visible-band single-photon detectors are almost eliminated,and the dark count rate is reduced to 5 cps(counts per second).This high-precision temporal resolved up-conversion single-photon detector has been successfully applied to high-precision 3D non-line-of-sight imaging experiments.It’s usually difficult to realize a coaxial non-line-of-sight imaging system based on conventional single-photon detectors due to the impact of the direct reflected photons from the diffuse wall.Benefiting from separated detection of back-scattered photons from the visible wall and the hidden targets brought by the pulse pumping,we built a highprecision coaxial non-line-of-sight imaging system with the upconvesion single-photon detector,which better adapts to the coaxial hypothesis conditions of many algorithms.Based on this 3D non-line-of-sight imaging system,we verified an axial spatial resolution of 180 μm and a lateral spatial resolution of 2 mm,and achieved 3D imaging of text with a font size of 30 pt outside the line of sight.Further analyzing of the experimental results shows that the wall positioning accuracy and the time bin width in the temporal domain have a significant impact on the imaging precision under high-precision nonline-of-sight imaging scenarios,which provides an exploratory research direction for further exploring higher spatial resolution 3D non-line-of-sight imaging in the future.In addition,our high-precision non-line-of-sight imaging system can provide a good validation platform for more non-line-of-sight image reconstruction algorithms in the future.Many single-photon imaging applications,including non-line-of-sight imaging,require arrayed single-photon detectors to improve imaging speed.However,the current near-infrared avalanche photodiode single-photon arrayed detectors are limited in detection sensitivity due to the high dark count rate.Corresponding solution can be found with nonlinear optics:using nonlinear crystals to up-convert infrared images,followed by detection using visible-band single-photon arrayed detectors.This infrared image upconversion approach has been developed for several decades,but has been hindered by low frequency conversion efficiency,making high-efficiency infrared detection difficult to achieve.Using a high-power,single-frequency pump and an appropriate pump beam waist,we achieved 88%high-efficiency conversion in periodically poled lithium niobate crystals.Together with a visible-band single-photon arrayed detector,a detection efficiency of 10.5%with an average dark count rate of 422 cps/pixel in the communication band is achieved.Furthermore,we conducted single-photon imaging validation,and this arrayed detector is expected to be applied in more complex singlephoton imaging applications in the future.In the research work of this thesis,the frequency conversion technology based on nonlinear optics was applied to single-photon detection,breaking through the temporal resolution capability of traditional single-photon detectors,and was applied in high-precision 3D non-line-of-sight imaging.Compared with the current 3D non-lineof-sight imaging research based on traditional single-photon detectors,the spatial resolution capability of this high-precision non-line-of-sight imaging device has been improved by an order of magnitude.The pulse-pumped up-conversion single-photon detector not only brings high temporal resolution capability,but also solves the problem of ordinary single-photon detectors being unable to achieve precise coaxial non-lineof-sight imaging systems.In the future,this up-conversion single-photon detector can be further improved in terms of temporal resolution and detection efficiency,achieving higher spatial resolution in 3D non-line-of-sight imaging,and expanding its application fields.The near-infrared single-photon arrayed detector developed in our work based on frequency conversion process not only improves the detection sensitivity by an order of magnitude compared to existing semiconductor single-photon arrayed detectors,but its arrayed detection mode also has significant value in improving the imaging speed in single-photon imaging applications.