The optical manipulation of particles is important in biomedicine, physics, and optics. Given the rapid development of the field of micromanipulation, there is considerable demand for improved functionality of optical tweezers. Currently, conventional optical tweezers and fiber optic optical tweezers can achieve limited particle transport without moving the probe. This limitation may have certain operational and analytical implications in the measurement of the angular frequency of cells for malaria diagnosis using optical capture techniques. Thus, there is a requirement to perform research on controlled and stable particle capture using fiber optic optical tweezers without moving the fiber and reciprocal axial transport between different capture sites. Certain researchers proposed multiple cases of reciprocal particle transport using fiber optic optical tweezers such as mode multiplexed tweezers and dual fiber optical tweezers. Most mode multiplexing tweezers use misaligned fusion to generateLP01 and LP11 modes in single-mode fibers, and these modes can realize the reciprocal motion of particles without moving the fiber. These tweezers have special requirements for shaping the fiber tip and cannot achieve reciprocal motion over long distances owing to the formation of two focal positions. Dual fiber optical tweezers are used to change the force balance of particles by adjusting the magnitude of the optical power emitted from both fiber ends so as to achieve particle capture and reciprocal motion. However, this method is a complex experimental setup, involves cumbersome operation, and has a limited particle transport distance owing to the two fiber ends. Therefore, in this study, a new fiber optic optical tweezer device is proposed that enables stable particle capture and the controlled manipulation of motion distance and velocity without moving the fiber optic probe.

A new optical tweezer is proposed using the balance of solution evaporation force and optical force in which an appropriate amount of blue butyl glue is glued to each of the four corners of the slide, and then a coverslip is placed on the blue butyl glue to form the sample chamber. Here, the height of the port where the fiber optic probe is placed should be higher than the height of the port on the opposite side while the remaining two sides should be in parallel. Such an arrangement increases the contact area between the solution and air at the port where the fiber optic probe is placed, thus increasing the evaporation force of the port. Then, the configured sample suspension is injected into the sample chamber using a syringe. The liquid does not flow out of the sample chamber when the solution fills it owing to the tension between the surface molecules of the solution and sample chamber. The solution will drive the particles to the fiber side owing to the effect of evaporation force, thus providing a force opposite to the optical force. When the optical force and evaporation force of the solution reach equilibrium, a stable capture of particles can be achieved. Moreover, the output power of the fiber probe can be periodically changed by modulating the driving current of the laser. When the optical force is more than the solution evaporation force, the particles will move far away from the fiber tip, but when the optical force is less than the solution evaporation force, the particles will be pulled to the fiber tip. The distance and speed of particle movement can be controlled by adjusting the amplitude and period of the modulation signal. Finite element analysis was used to analyze the optical field distribution at the tip of the fiber and the magnitude of the force on these particles.

The experimental and simulation results demonstrate that the proposed method can achieve stable capture of polystyrene spheres and repeatable axial reciprocal transport without moving the fiber. When the driving current of the laser increases, the peak of the particle motion curve increases as the modulation current of the laser gradually increases, moreover, the motion distance of blob increases. The slope of the particle motion curve increases as the modulation current increasing, indicating that the particle motion is faster. The motion distance of the particle is linear with the modulation current. Moreover, the laser light source with 980 nm used in the experiment can effectively reduce the photothermal effect during particle manipulation and avoid the damage caused during particle transport. As a controllable all-fiber integrated device, the method proposed in this study enhances the functionality and flexibility of the optical manipulation method, providing a potential technical support for its application in the fields of micromanipulation and biomedical research.

In this study, a new single-fiber optical tweezer based on current modulation is proposed to achieve stable particle capture by adjusting the magnitude of the evaporation force of the solution. Moreover, the driving current of the laser is modulated to change the output power periodically and change the force of the particle so as to achieve the controlled manipulation of particle transport distance and transport speed. In this study, a simulation model is built to analyze the force situation of the particles during the motion, and the mechanism of the periodic reciprocating motion of the particles is provided. The experimental results demonstrate that stable particle capture and reproducible axial reciprocal transport are achieved with polystyrene microspheres and yeast cells as target particles without moving the optical fiber. Moreover, the correspondence between the modulation current of the laser and the particle motion distance are analyzed, the particle motion curves under different driving currents are plotted, and the particle motion distance is demonstrated to be linearly related to its modulation current as presented in the fitting equation. As a controllable all-fiber integrated device, the proposed method extends the application possibilities of single-fiber optical tweezers.