Compressed Ultrafast Photography And Its Applications In Ultrafast Optical Field Measurement

It has been a long standing goal to obtain spatial,temporal and spectral information of an optical scene through a single exposure for researchers in the field of optical imaging,which is of great scientific and practical significance for analyzing an optical scene from as many dimensions as possible.The Snapshot Coded-aperture Optical Imaging(SCOI)technology firstly modulates an optical scene with coded apertures,and then reverts the scene measured by image reconstruction algorithms,which makes the optical imaging technology step into a new era of computational optical imaging.According to different imaging dimensions of the target,SCOI technology can realize achievements including but not limited to spatially two-dimensional imaging,spatially three-dimensional imaging,spectral imaging,ultrafast imaging and polarization imaging through a single shot.Among the SCOI technologies with different dimensional capabilities,ultrafast imaging draws a lot of attention because of its outstanding imaging characteristics in detecting unstable or irreversible dynamic scenes.And compressed ultrafast photography,(CUP),as a superb SCOI technology,has obtained tremendous developments in recent years with the imaging speed reaching to 10¹³frames per second(fps),and imaging frames reaching to hundreds of frames.CUP combines compressed sensing theory with space-time transformation imaging technology.Through spatially random encoding,temporal-spatial shearing and spatio-temporal superposition of the dynamic scene,the data is restored to finally realize the reconstruction of the original dynamic scene with compressed sensing algorithm.However,as an emerging ultrafast imaging technology with single exposure and receive-only characteristics,not only CUP's key technical parameters need to be improved,but also its applications in ultrafast optics is awaiting to be explored.Therefore,the work of this paper mainly focuses on improvements of the imaging speed of CUP and its applications in the field of ultrafast optical field measurement.The main work involves two parts of theoreticaly and experimental studies,and the specific achievements are as follows:1.The idea of gas molecule alignment assisted compressed ultrafast photography(MACUP)is proposed.In molecular alignment theory,the change of molecular alignment degree produces the change of refractive index gradient,so the process of using pump light to stimulate and control the gas pool is equivalent to generating a molecular deflector,which replaces the streak camera to realize compressed imaging.Based on the simulation of a CO₂ molecular deflector,combined with the point spread function in imaging,MACUP is able to achieve imaging speeds of more than 180×10¹² fps and sequence depth of approximately 300 frames in a single exposure.To verify the feasibility of MACUP,we simulated the measurement of the spatiotemporal intensity of chirped femtosecond laser pulses,and studied the image reconstruction accuracy during the evolution of intensity and wavelength.These results suggest that MACUP is a potential single-shot ultrafast optical imaging strategy which is expected to reveal the dynamics in ultrafast atomic and molecular optics.2.The picosecond laser fields were measured using the CUP apparatus built by ourselves.In our CUP system,the measurable laser wavelength depends on the spectral response of the streak camera ranging from ultraviolet(200 nm)to near infrared(850nm).Based on the CUP system,we have successfully measured the spatial and temporal intensity evolution of some typical laser pulses,such as 800 nm picosecond laser pulse,800 nm and 400 nm bi-color picosecond laser pulses,and supercontinuum picosecond laser pulse.These experimental results show that CUP technique can well characterize the spatial and temporal intensity evolution of picosecond laser pulse.In addition,the technique has significant advantages in the case of single measurement without reference laser pulse.3.The generation mechanism of ultrafast intensity rotating optical field was studied,and the setup of the rotating optical field was built.The double-lobe ultrafast intensity rotating light field was observed using the CUP apparatus.The modulated laser pulse composed of two vortex beams with different topological charge numbers will generate a rotation process under a certain delay.In theory,we simulated the rotation process of the optical field according to the principle of CUP technology,and verified the feasibility of the experiment.Then,we utilized the CUP system to visually demonstrate the different rotation angular velocity of the two eddy optical fields under the delay of60 fs?120 fs and 150 fs,respectively.The results show that CUP has prominent capability to observe the rotating optical field with a single exposure and break the limit of the number of imaging frames.Research results reveal that CUP equips enormous potential in detecting some complex optical fields.