Research On Single Optical Fiber Tweezers Technology Based On Graded-index Multi-mode Fiber

The optical tweezers have been gradually applied to biological and medical research fields,such as operations of cell,virus and bacteria,due to its non-invasive properties during microparticles manipulation.Traditional optical tweezers typically require a high numerical aperture microscope to focus the laser beam for micro-particles trapping.However,the traditional optical tweezers have some intrinsic limitations resulting from their bulkiness and lack of flexibility.The realization of optical trapping based on optical fibers allows a miniaturized,versatile and handy tool to be obtained.Optical fiber tweezers can be divided into multi-fiber optical tweezers and single-fiber optical tweezers.Multi-fiber optical tweezers require beam alignment to ensure that beams from different fibers converge at a point.The single fiber optical tweezers may perform the optical trapping without utilizing the beam-alignment,having the advantages of low-cost and easy to manipulate.The single-fiber optical fiber tweezers are divided into single fiber single-functionality optical tweezers and single fiber multifunctionality optical tweezers.The single-functionality optical tweezers based on single-mode fiber(SMF)are easy to prepare and low cost.But it is difficult for single fiber optical tweezers to achieve multiple functionalities based on single core,single-mode fiber unless it combines with other mechanics,such as the thermal effect.On a base of special fibers,the single fiber multi-functionality optical tweezers perform multiple functionalities,such as trapping,rotating and trapping position shifting.However,due to the expensive price and scarce output of the special fiber,the applications of special fiber optical tweezers are narrow.The research work in this paper combines the optical tweezers and graded-index multimode fiber(GIMMF),aiming to fabricate the multi-functionality optical fiber tweezers(optical trapping and controllable trapping position adjustment)by using the special refractive index distribution and large exiting equivalent mode field diameter of GIMMF.Tailoring the trap and axial shift of the microparticle based on the GIMMF provides convenient avenues for fiber optical tweezers applied in practical researches.The main work of this paper is the following two parts:A novel single fiber optical tweezer based on a graded-index multimode fiber(GIMMF)is proposed,which works with a free length GIMMF(>30 cm).We achieve a three-dimensional stable trap of yeast cells by using the GIMMF optical tweezers.Compared with the single-mode fiber optical tweezers,the GIMMF optical tweezers possess large optical trapping forces.Owing to the freedom of the GIMMF length,the fabrication of the GIMMF optical tweezers is simple,repeatable,and highly efficient.However,the optical fiber tweezer mentioned above cannot achieve non-contact capture.A single fiber dual-functionality optical tweezers based on GIMMF is proposed.By using the multi-angle fiber grinding and polishing technology,we fabricate the GIMMF tip to be a special tapered shape,contributing to focus the outgoing beam with a large intensity gradient for the first functionality--three-dimensional contactless trapping of a micro-particle.By adjusting the radial direction offset between the lead-in SMF and the GIMMF,we perform the second functionality--axial shift of the trapped micro-particle with respect to the fiber tip without need of moving the fiber probe itself.