Research Of The Single Fiber Optical Tweezers Based On Mode Multiplexing Technology

Optical tweezers,also known as single-beam optical gradient force trap,are devices that use a three dimensional potential well formed by well focused laser beam to provide a non-touch capture and operation.It played unique roles in several fields such as biology chemical and medicine.Since the rapidly development of optical fiber technology can benefit optical tweezers system in degree of freedom of acquisition and flexibility of operation a lot,optical tweezer has become the most assignable tools to explore the micro world.In this article,a new type of single-mode fiber optical tweezers is researched by mode multiplexing technology.By a common optical fiber to achieve a variety of particle capture,this optical tweezers can reduce the difficulty of fabrication of single fiber optical tweezers and enhance single fiber optical tweezers operating functions.The single fiber optical tweezers consists of a special processed standard communication optical fiber.By the LP01 mode beam which transmitted through the optical fiber,non-contact particle capture was achieved.By adjusting the power ratio among LP01 mode and LP11 mode pattern,the particles capture in the fiber axis direction was achieved.By adjusting tension streching interference between LP01 mode and LP11 mode pattern,the particle deflection operation was achieved.And by adjusting the cross-sectional spot of LP11 mode beam pattern,the rotation operation of particle was achieved.To achieve this function of a single fiber optical tweezers,the main work was carried out as follows:First,this thesis designs the overall functionality of single fiber optical tweezers based on mode multiplexing technology.Depending on the application requirements of the optical tweezers,the implement method of overall function is determined and the basic idea of fiber optical tweezers mode multiplexing method is established.To provide theoretical basis and experimental methods for the non-contact control and the particles movement on the axial direction,fully understand the character of LP01 and LP11 mode and the basic principles of higher order mode excitation beam in a standard single-mode fiber.And based on the character of difference between LP01 and LP11 mode,adjusting the power ratio can be achieved by controlling the degree of bending fiber.Based on fiber mode interference between theories,study the character of LP01 and LP11 mode beam interference.By applying a force to axis direction of optical,the fiber length in the axial direction will be stretched and the phase between two modes will be changed.After that,brightness changes of spot in two modes interference occurs that can achieve particle deflection operation.That is a new method to achieve spot rotation by multiplexing mode.Since LP11 beam spot is central asymmetry,rotational operation for particles capture can be achieved by changing stress distribution on the fiber tangential direction.Second,this thesis designes an optical fiber tip with a hemispherical lens.By the method which is combined by electromagnetic momentum conservation law and FDTD,the characteristics of optical trapping force which occurs when LP01 and LP11 were acting on this fiber tip.The distance between optical fiber tip and stability capture point of LP01 mode beam was 10?m which was possible distance for non-contact particle capture.Since two spots on the fiber axis are asymmetric in the plane,space optical trapping force cannot be calculated by two-dimensional simulation.In this paper,space optical trapping force is obtained by sum of multiple two-dimensional calculations.By adjusting power ratio between LP01 and LP11,longitudinal force which the particle suffers can be changed and particle movement therefore can be achieved.And using the combining calculation of optical trapping force of fiber output light field and light torque,the force and torque of particle when deflection or rotation happened in LP11 mode.In simulation,particle will deflect or rotate in light field when the distribution of LP11 mode spots change.Furthermore,a single fiber optical tweezers experimental system is set up.In this system,LP11 mode was excited by dislocation welding between standard single-mode fiber and 980nm fiber.By adjusting the power ratio among LP01 mode and LP11 mode pattern,the particles capture in the fiber axis direction was achieved.By adjusting tension streching Interference between LP01 mode and LP11 mode beam pattern,the particle deflection operation was achieved.And by adjusting the cross-sectional spot of LP11 mode beam pattern,the rotation operation of particle was achieved.In the end,three-dimensional rotation operation and non-contact particle capture of particle had been achieved.A good foundation was layed for mode multiplexing technology in fiber optical tweezers field.