Research On Doppler Velocimetry Based On Structured Light Field

Since the invention of laser,optical measurement technology,represented by Doppler velocimetry,has developed vigorously,and has now become an indispensable technical means in velocity perception.Previous studies on the Doppler effect mainly focused on the frequency variation of the wave when a linear translational relative motion occurs between the wave source and the observer.In recent years,with the exploration of the physical dimensions of optical fields,it has been discovered that the wave with helical phase distribution can produce a frequency change associated with the lateral rotation of the object,called rotational Doppler effect.Based on this novel effect,the rotational Doppler velocimetry is developed,which opens up a new perspective for the study of angular velocity measurement.However,it still faces some considerable challenges,such as direction discrimination,multi-dimensional measurement,remote measurement,and so on,in both schemes and technical level.It is generally believed that light fields have abundant physically dimensional resources,including intrinsic properties such as energy,momentum,and angular momentum,as well as degrees of freedom in amplitude,phase,polarization,and frequency.In many fields such as optical communications and sensing,the exploration of the physical dimensions of light fields is often a crucial way for researchers to break through the bottleneck of information transmission or acquisition.Therefore,combining the multi-degree of freedom of the structured light with Doppler velocimetry will produce some new sparks,which are expected to make up for the existing technical defects.In this dissertation,the application of various structured light field in Doppler velocimetry is systematically studied.The specific contents are as follows.1)Researches on the direction-discriminated rotational Doppler velocimetry with circularly polarized vortex beam.To overcome the technical limitation of current rotational Doppler velocimetry in obtaining the information of motion direction,we use the circular polarization and orbital angular momentum degree of freedom of light field in the rotational Doppler velocimetry and propose a method for simultaneously measuring the magnitude of the rotational velocity and determining the rotation direction.The feasibility of this method is experimentally demonstrated by detecting a rotating microparticle.In addition,the effectiveness of the different polarization combinations between probe and reference light in such system is compared,and it is concluded that the circular polarization plays a key role for direction discrimination.2)Researches on vectorial Doppler metrology with spatially variant polarized light fields.In early manifestations,the beat frequency detection method was adopted in Doppler metrology using a scalar wave to infer velocity.A consequence of conventional approach is that it is difficult to deduce the motion direction of moving targets directly.We overcome this fundamental limitation using vectorially structured light with spatially variant polarization,allowing the velocity and motion direction of a moving particle to be fully determined by a vectorial Doppler effect.A Doppler polarization signal in vectorial form is observed in the experiment.In addition,the experiments also successfully measure the rotational velocity(magnitude,direction)of a moving particle.The instantaneous position of the moving particle is also tracked under the conditions of knowing its starting position and continuous tracking.Furthermore,we also discuss the moving particle with anisotropy and show its potential to distinguish the rotation and spin of the anisotropic particle and measure its rotational velocity and spin speed(magnitude,direction).3)Researches on structured light interferometry for the extraction of multi-dimensional motion information.Conventional optical interferometry using a Gaussian beam and its modified version using a helically phased beam fail to distinguish the linear and rotational Doppler shift in multi-dimensional motion,which leads to the challenge for the extraction of multi-dimensional motion information.To realize the separation and complete extraction of the information of each component in the overall Doppler effect caused by multidimensional motion,we present a new structured light interferometry,which makes full use of spatial amplitude,phase and polarization degrees of freedom of structured light.Furthermore,the full motion information of the spiral moving particle is measured in the experiment,including translational velocity,rotational velocity,and motion directions simultaneously at one time.4)Researches on remote Doppler velocimetry for rotational motion with fiber-based structured light field.We propose a method to construct a flexible Doppler velocimeter towards practical applications based on structured light field in optical fiber,in which the ring-core fiber supporting vortex mode and the air-core fiber supporting vector mode are respectively employed as the transmission medium in the schemes,so as to realize the remote measurement of rotational motion.In the experiments,the ring-core fiber is first adopted to test the performance of rotational Doppler velocimeter in remote measurement,and then the air-core fiber with kilometer-length scale is adopted for remotely measuring the vectorial information of angular velocity in situ.In addition,it also experimentally substantiates that the measurement system still has commendable accuracy in determining the direction of movement even when the air-core fiber is interfered by the external environment.