Optical tweezers were first proposed by Arthur Ashkin in 1986 and were first used in the biological science field in the late 1980s. Since then, optical tweezers have been applied to atomic physics, micromachining, chemistry, biomedicine, and microelectromechanical systems. Meanwhile, on the basis of application development, other new optical tweezer systems such as femtosecond and vacuum laser systems have also been derived. In addition, they have been widely combined with new biomedical technologies in recent years and used for pathological detection of cells, single-cell microsurgery, and designs of biological lasers, cell-based biological photonic waveguides, and biological micro-lenses by using biological entities such as viruses, cells, and tissues. With the rapid development of the microoperation field, functional requirements of fiber optic tweezers are becoming higher and higher, so it is particularly important to improve their utilization efficiency and integration. A variety of methods have been proposed to transport particles, and some of which are made by using double optical fibers to capture cell chains and make a conveyor belt. Some pull the bending port to enhance the evanescent field, while others pull micro-nano fibers and control the power of two lasers to transport them. Most of them transport particles over optical fibers, but optical tweezers with directional emission functions are less involved. Moreover, the use of dual fiber probes requires an extremely precise operation to prevent bending, and it takes time to capture cells when assembling long conveyor belts. Or the bending degree and flame conditions required by nanofibers make it difficult to fabricate fiber ports and thus need to consider repeatability and other issues. To solve the above problems, this paper designs a fin dolphin-shaped fiber optical tweezer structure, which combines with a high-order LP₂₁ mode to realize the function of a fiber conveyor belt. This method is simple and provides a new possibility for optical fiber manipulation.

In this paper, a light source of 650 nm is fed into a high energy ratio LP₂₁ mode in a G.652D fiber with a typical operating wavelength of 1550 nm. The characteristic of the LP₂₁ mode light field is that the outgoing direction is biased towards the central axis and extends symmetrically around, which forms a light field with a unique four-petal center symmetric intensity distribution. Moreover, the high-order four-petal LP₂₁ mode beam has the intensity distribution of high transmission stability in the fiber, and the bending and torsion of the transmission fiber can hardly cause the deformation of the intensity distribution of this mode. In addition, the stiffness of the axial optical trap in the four-beam optical trap is stronger than that in other modes, which not only ensures the stability of the axial capture but also improves the stability of the transverse capture. Therefore, the LP₂₁ mode beam is used in this paper to capture and transport particles. In addition, in order to ensure the stability of particle transport, a fin dolphin-shaped fiber probe is designed, which has a smooth flow arc shape and extends the tip forward, so as to converge the diverging laser on both sides of the fiber. At the same time, this paper makes an ordinary conical fiber for comparative experiments to study the transportation performance of the fin dolphin-shaped fiber and measures the transportation speed of the two kinds of fiber under high and low power. A glass capillary tube is used as the target location for particle transport. The feasibility of the experiments is analyzed by the finite element method.

The experimental and simulation results show that the evanescent field at the sudden change of the diameter of the fin dolphin-shaped fiber is significantly enhanced, and it extends from the arc to the top of the tip, with a strong lateral capture force. Compared with common tapered fibers, the evanescent field near the tip is the strongest. With the increase in the tip distance, the diameter gradually increases, and the light field gradually weakens. Furthermore, the focusing region near the top is shifted to both sides. Therefore, in the particle transportation process, the evanescent field is too weak in the early period, which results in a small light trapping force, and particles are not easy to be captured or escape. In the late period, when particles are near the top, it is easy to deviate from the course and be away from the tip, which brings difficulties for the directional emission of subsequent particles. Several comparative experiments are carried out under different optical power. The results show that the fin dolphin-shaped fiber has more advantages in transportation speed and stability.

In this paper, we design a fin dolphin-shaped fiber optical tweezer structure and enhance the optical field intensity of the side edge of the fiber through a high-order LP₂₁ mode beam to achieve the function of a fiber conveyor belt, and particles are finally transported to the tip for ejection. The finite element method is used to simulate the intensity distribution of the optical field, which shows that the arc structure at the fiber port has significantly enhanced the evanescent field intensity at the side of the fiber. The influence of the side trapping force on the particle transport speed is analyzed by comparing it with conventional conical fiber. The results show that the special structure of the fiber is superior. It expands the direction of the combination of particle transport and particle emission and provides a new possibility for the research on new fiber optic tweezers and the biological cytology field.