We present an abruptly tapered twin-core fiber optical tweezers, which is fabricated by fusing and drawing the twin-core fiber. In the twin-core fiber, the two beams are guided by the tapered fiber. At the end of the fiber tip, a larger converge angle between the two beams are made due to the abrupt tapered shape, which is formed a fast divergent optical field. The microscopic particle trapping performance of this special designed tapered twin-core fiber tip is investigated. The functionality of the proposed novel twin-core fiber optical tweezers is extended since an in-fiber integrated Mach-Zehnder interferometer has been used to control orientation of the trapped particle. The distribution of the optical field emerging from the tapered fiber tip is simulated based on the beam propagation method (BPM). By using this two-beam combination technique, a strong enough gradient forces well is obtained for microscopic particles trapping in three dimensions. The abruptly tapered twin-core fiber optical tweezers is rigid and easy to handle, especially useful for building up a multi-tweezers system for trapping and manipulating micro-scale particles.
©2008 Optical Society of America
Since first demonstrated by Ashkin in 1986[1, 2], optical tweezers (a single-beam gradient force trap) technique has been developed in several directions. Examples are optical micro-machines and micro-components [3–7], the extension of optical tweezers to multiple beam sites to create two-dimensional particle arrays [8,9] as well as stacking of a small number of particles in standing-wave geometries  and by using Bessel light beams  to construct three-dimensional trapped structures . In parallel with this, the optical tweezers technique has been widely used in biology, physics and chemistry. Especially in biology, it has been applied in researches on cells, viruses, bacteria and DNA molecules. Applications now range from the manipulation of the nano-particles to the assembly of the microstructures . Usually a high N.A. microscope objective is necessary to focus the laser beam in the traditional optical tweezers. Despite the relevant results achieved in many fields, the bulky structure of the traditional optical tweezers still limits their utilization in several environments. In order to make the manipulation of the tool easier and more convenient, fiber optical tweezers has been developed since 1993. By using optical fibers to trap a micro-object [15,16] is much easier to handle, and much more suitable for practical use such as in trapping, levitating and rotating of the microscopic particles [17–23]. Usually, etching or drawing method [24–26] was used to fabricate the standard single mode fiber tip for particle trapping in three dimensions. Here we describe a means of implementing optical tweezers by using a twin-core fiber that extends the functionality of the fiber optical tweezers by allowing trapping with orientation, more like the real tweezers (tiny pair of tongs).
2. Twin-core fiber tapered probe
Unlike the polishing and chemical etching manufacture techniques, the twin-core fiber (TCF) optical tweezers was manufactured by heating and drawing the twin-core fiber to form an abruptly taper profile. At the tip of the tapered fiber, a small half spherical high numerical aperture micro-lens is automatically formed due to the surface tension of the fused quartz glass. Figure 1 shows the cross-section of the twin-core fiber and the tapered fiber probe (see Fig. 1(a)) used in our experiment. In the abruptly tapered zone, the two beams coming from each core were guided in the tapered angle, and gradually transferred from the guided mode into the radiation modes due to the core diameter abruptly reduced near the fiber tip, as shown in Fig. 1(b) and (c). The two radiation beams are passing though the tapered twin-core fiber tip and forming a fast divergent-beam away from the fiber tip. By using this two-beam combination technique, a strong enough gradient forces potential well is obtained for micro-particles trapping in three dimensions with the orientation in the twin-core plane.
The configuration of the twin-core fiber optical tweezers is described in Fig. 2. It can be seen that from Fig. 2, an in-fiber integrated twin-core fiber Mach-Zehnder interferometer is build up by using the same twin-core fiber. The tapered twin-core fiber is linked with the Mach-Zehnder interferometer, and the intensities transmitted in the two cores can be modulated by bending the Mach-Zehnder interferometer. Thus, the output two beams from the Mach-Zehnder interferometer is the input two beams of the tapered twin-core fiber. The optical phase of two beams is opposite (represents as green and blue line in Fig. 2) and its intensities can be easily controlled by bending the Mach-Zehnder interferometer, as shown in Fig. 2. In this way, the in-fiber integrated Mach-Zehnder interferometer can be used to control orientation of the trapped particle.
3. Theoretical simulation results
Near the tip of the twin-core tapered fiber end, the intensity distribution of the output optical field is simulated by using the beam propagation method (BPM) . In the simulation, it is assumed that the twin-core fiber tapered probe is immersed in the surrounding medium (say, water) with the refractive index 1.33, and the refractive indices of the twin-core fiber cladding and the fiber core are 1.452 and 1.463, respectively. The 3-dimensional finite difference BPM has been used to solve the Helmholtz equations. This method is exact up to the discretization error. The transverse grid sizes Δx, Δy are 0.03µm and 0.03µm, respectively. The longitudinal step size Δz is 0.5µm. The transparent boundary conditions were applied. And the Gaussian incident field whit opposite optical phase is launched into both fiber cores simultaneously and the wavelength is 0.98µm.
In our calculation, the Gaussian incident optical power is given by
where, the sport size σ=1.85 µ m.
The abruptly tapered fiber profile can be described as
The numerical simulation parameters such as the radius of the twin-core fiber R 0=62.5µm; and the radius of the half spherical micro-lens at the tapered fiber tip is r 0=2.5µm; tapered fiber length l 0=201.5µm, as shown in Fig. 1(a), and the parameter L=288µm, the tapered fiber profile parameter ν=0.018(µm)-1,represents the shape changing characteristics. For the twin-core fiber, the distance between the two cores is 62.5µm, and the diameter of each core is 3.7 µm.
The simulation results of the output far-field intensity distribution are plotted in Fig. 1(b) and (c) and Fig. 3 and 4. From the two-dimensional intensity distribution (Fig. 1(b) and (c)) of the plane in which the two cores lie, it is shown that the optical power is concentrated near the tip of the twin-core fiber. The intensity distributions near the tip at the plane perpendicular to Z direction are given in Fig. 3 and Fig. 4, and the separation distance for each cross-section distribution is 4 µm. It is shown that the two beams are intercrossed inside of the twin-core fiber tip and separated outside of the fiber tip and the divergence angle is over 21 degrees. The difference between Fig. 3 and 4 is the ratio of the intensities transmitted in the two cores, in Fig. 3, it is 1:1 and 1:3 in Fig. 4.
4. Experimental results
The twin-core fiber optical tweezers experimental setup and its working principle are described in Fig. 5. A laser diode (LD) with wavelength at 980nm is used as the light source and its power can be tuned from 0 to 110 mW by adjusting the driving current of the LD. The pigtail fiber of the laser diode is single mode fiber. In order to launch the optical power into the twin-core fiber, firstly, we splice the single mode fiber and the twin-core fiber. Then heating and drawing the fiber at the splicing point, thus the first bi-tapered coupling zone between the single mode single core and twin-core single mode fiber is formed . Finally, we heating and drawing the twin-core fiber at some point between the first bi-tapered coupling zone and the abrupt tapered tip, forming the second bi-tapered coupling zone, in this way, the input power is coupled and shared in the two cores and formed the two beams Mach-Zehnder interferometer. The in-fiber integrated Mach-Zehnder interferometer is used to modulate the intensities of the two beams. Therefore, the transmitted optical power inside in the two cores can be controlled. This leads to the variation of the output optical field distribution at the front of the twin-core fiber tapered end, as shown in Fig. 4.
The microscopic pictures of the twin-core fibers’ cross-section, abrupt tapered fiber probe and a yeast cell has been trapped in water by the twin-core fiber tip are given in Fig. 6. In order to testing the trapping performance of the twin-core fiber abrupt tapered tip, we put the twin-core fiber optical tweezers into the water and the yeast cells have been used as the trapping particles in our calibration experiment. The output optical power can be controlled by adjusting the driving current of the LD, and the output optical power from the fiber tip versus the trapping force in the lateral direction lie in the two cores plain can be approximately calibrated by using the Stokes law,
here, u⃑ represents the moving velocity of the trapped particle and r is the radius of the trapped particle. The diameter (2r) of the yeast cells used in our experiments is about 5.5~6.5µm which are immersed in the water at room temperature 293 K and the dynamic viscosity of water at the room temperature is taken as η=1.002×10-3 (NS/M2). The calibration result is plotted in Fig. 7. Comparing with the case of tapered single mode fiber given in reference , its trapping force is more than 3 times at the same optical power.
In this case, the trap is not isotropic, the axial forces are strongly depend on the moving direction and the shape of the taped fiber tip due to the relative moving speed (along the streamline) at the surface of the trapped particle is different from the moving speed of the fiber itself. In our case, the procedure used for the trapping force calibration is similarly given by [29–30].
The elliptical particle trapping and orientation control experiment has been made. When the output power is adjusted in the order of 5 mW, a yeast cell has been trapped at the twin-core fiber tip. It can be seen from the movie given in Fig. 8, the yeast orientation has been changed and adjusted by bending the in-fiber integrated interferometer, in this way, the coupling ratio between the two cores of the twin-core fiber is varied results in the orientation changing of the elliptical yeast. The experimental process has been record by a real-time movie, as shown in Fig. 8. At the same time, the lowest power against the thermal Brownian motion random force for small particle trapping is also tested. For instance, for the yeast cell with the diameter about 5.5~6.5µm, when the power was reduced in the order of 1 mW, the yeast was released from the optical trap at the fiber tip as shown in the attached movie (Fig. 8).
In summary, we report on the development of twin-core fiber tapered tip, which can be used to trap particles with low powers (<5 mW). The use of the twin-core fiber tapers allows for small probe size, stiff stability and relatively easy to manufacture, and offers potential applications especially in integrated “lab-on-a-chip” devices. The performance of the demonstrated twin-core fiber optical tweezers would lead to functionality extend in three dimensions optical trapping. And it is easy control and conventional operate as tools for biology and micro-assembling by allowing trapping with orientation, more like the real tweezers (tiny pair of tongs).
This research was supported by the National Natural Science Foundations of China, (grant 60577005 and 60707013), and partially supported by the Specialized Research Fund for the Doctoral Program of Higher Education Institute of MOE, China.
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