Particle Capture And Transport Signal Detection Based On Fiber Optic Tweezers Linear Polarization Mode

Optical tweezers are non-contact and non-destructive optical manipulation tools that use focused laser beams to capture,manipulate and transport arranged particles or biological cells.In recent years,optical tweezers technology has spread across a variety of industries,from biomedical applications to physical and material sciences.And fiber optic optical tweezers have attracted great attention in the field of optical trapping due to its flexible operation,compact structure and ease of fabrication.It can capture particles in extremely narrow spaces,and does not require the objective lens to have high numerical aperture conditions,which expands the space for capturing and manipulating particles.In order to realize more functions in single-fiber optical tweezers,a multifunctional optical tweezer structure for non-contact multi-trap trapping and transporting biological cells or particles based on the fiber line polarization mode is proposed.Different shapes of fiber tips were designed,and the multi-trap trapping of Chlorella cells and the transport of polystyrene particles were achieved by exciting high and low order fiber transport modes in a common single-mode fiber with different operating wavelengths,respectively.Firstly,the development status,main structure and application of optical tweezers are briefly described.Secondly,the fundamental of fiber tweezers and several common calculation methods of trapping force for different particles are introduced.The general situation and fundamental of finite element are introduced,and it is used to calculate the trapping force of optical tweezers.Furthermore,the mode theory of optical fiber is discussed in detail,the internal structure and linear polarization mode of optical fiber,and the measuring method and excitation scheme of optical fiber mode are introduced.It provides theoretical guidance and numerical simulation method for analyzing the force of particles under the action of optical force,and lays a theoretical foundation for the application of fiber linear polarization mode in fiber optic tweezers in the following chapters.Second,to address the problems of single location of particle capture based on fiber optical tweezers,we propose a fiber optical tweezer based on LP₁₁mode to achieve multi-directional particle capture.The LP₁₁mode with high purity can be excited in the 980nm single-mode fiber by fusing a common 1550 nm single-mode fiber with a 980 nm single-mode fiber and feeding a 650nm laser source.A special short-tip structure was designed and fabricated using the thermal pulling method to achieve multi-trap trapping of Chlorella cells around the fiber port based on its optical field distribution characteristics.The structure can be used to obtain a high energy ratio LP₁₁mode without mechanical devices,which reduces the loss and fiber damage during beam propagation and enhances the trapping capability of optical tweezers,which is useful for the application of single fiber optical tweezers in bioinformatics engineering and lays the foundation for the subsequent simultaneous trapping of multiple strings of particles and multiple refractive index particles.Finally,in order to solve the problems of difficult fabrication and high power requirement of optical fiber conveyor belt,a finfish shaped optical fiber optical tweezers structure based on optical fiber linear polarization LP₂₁mode which can transmit optical transport function is proposed.The optical field simulation results show that the lateral optical field of the fiber is significantly enhanced after the curved bending structure and gradually extends to the tip,thus increasing the lateral capture force and transport capacity of the particles.The LP₂₁mode with high energy ratio is excited by feeding a 650nm light source into a common single-mode fiber to concentrate the energy around.Experimental results show that at lower optical power,polystyrene particles（5μm）are trapped at the abrupt change in diameter near the arc and transported by scattering forces along the edge of the fiber to the tip,where they are eventually ejected as a particle gun.The superiority of this structure was verified by comparison experiments with a common tapered fiber at different optical powers.In addition,the interferometric signal during particle transport is collected in the experiment,and the displacement information of particle transport can be demodulated and determined.The method is simple in equipment and offers new possibilities for fiber optic manipulation.