Optical Tweezer Using Focused LP2i Mode
A tapered fiber structure fabricated by mechanical polishing has been very mature to produce an optical fiber axicon on a fiber end, which can produce a Bessel beam with a high convergence. This section describes the manipulation of bio-particles by using fiber axicon to focus LP21 mode.
A Gaussian beam emitted from an optical fiber is generally difficult to be focused effectively to a small beam spot with a long focal depth, to solve this problem Bessel beams are frequently used in fiber-optic cell trapping/manipulation systems. Since J. Durlin proposed zeroth-order Bessel beam without diffraction effect in 1987, people started to pay attention to this energy-trapped beam with a long depth of focus. Meanwhile, J. Durlin and J.H. Eberly proposed the experimental apparatus which was capable of producing a zero-order Bessel light beam-cone lens structure [18, 19], which has greatly accelerated the Bessel light application in various fields. There are several methods to fabricate tapered structure in an optical fiber. Compared with the other methods of fabrication such as chemical etching or focused ion beam milling, mechanical polishing provides a precise taper angle with high quality surface
finishing. As such we limit our discussion on grinding/polishing method as the major means for fabrication of fiber tapered structure.
Prior to the preparation and processing optical fiber tapered structure, we need to analyze what cone angle we need. Bessel beams can be produced by micro-cone structures, while the most important parameter that impacts beam spatial characteristics is cone angle (apex angle). Many literature and experiments showed that the larger the cone angle, the longer the depth of focus can be produced, and the greater the FWHM (Full width at half maximum) of the Bessel beam; on the contrary, the smaller the taper angle, the shorter the depth of focus, and the smaller FWHM of Bessel beams. Obviously, the ideal condition is to get a long depth of focus and small FWHM Bessel beam simultaneously, however, these two parameters are mutually exclusive, and an optimal configuration needs to be found through analysis and tests.
In order to find the most suitable cone angle to implement particle manipulation, we first use Rsoft® waveguide simulation software to simulate light intensity output of tapered optical fiber for a range from 60° to 150°. At wavelength of 680nm in a conventional single-mode fiber, simulations are conducted by Rsoft for apex angles from 60° to 150° to obtain beam intensity distributions from these axicons, as shown in Fig. 3.12.
Based on this simulation result, a fiber cone angle of 120° is chosen for a tapered fiber . The selection of 120° cone angle for application in bio-particle manipulation and rotation results from that a trade-off point is reached between the energy confinement capability and the depth of focus. According to size of cells, we need to have a focal spot diameter of about 1 ^m and tens of micrometers depth of focus to facilitate cell manipulation.
In grinding and polishing process, as shown in Fig. 3.13, an optical fiber sleeved in a ceramic ferrule holder is rotated by a motor at an angular speed of 100-200 rpm; an abrasive discaf fixed to another motor is rotated at a 2000 rpm. The fiber axis must be completely collinear aligned with that of the rotating chuck, forming a certain angle with the disc for formation of apex angle. On the other hand, the optical fiber is controlling by a high-precision nano-translation stage so that feeding of the fiber tip in contact with the disc is controlled stably. Usually the grinding process only takes a few minutes with the help of real-time monitoring by a microscope with long working distance. After forming of cone structure, abrasive disc needs to be changed with low roughness in series, till a polishing disc is used to obtain a smooth cone surface finishing.
With the above setup, we can get a cone angle of the tapered fiber at any designed angle. Figure3.14a shows a microscopic image of a tapered fiber with 120° cone angle, and Fig.3.14b, c demonstrate the simulation of output light and measured cross-sectional intensity distribution of Bessel spot . We found that by polishing method for processing 120° tapered fiber surface is very smooth, to ensure the integrity of the outgoing beam Bethel. In Fig. 3.14c the rings of Bessel beam are very clear and well-separated, with spatial distribution of multiple orders agreed with simulated result shown in Fig.3.14b.
Fig. 3.12 Light intensity distribution from fiber axicons with different apex angles at the wavelength of 680 nm, simulated by Rsoft software
Fig. 3.13 Schematic of tooling for fiber axicon fabrication
Fig. 3.14 a Microscope image of an axicon fabricated on end surface of a single mode optical fiber. b Numerical simulation of the Bessel beam exiting the axicon along the propagation path. c Experimental measurement of a cross-section of the Bessel beam from the fabricated axicon
By exciting LP21 mode in an optical fiber, a focused LP21 spot is obtained, as shown in Fig. 3.15, where Fig. 3.15a shows the experimentally measured averaged pre-focused intensity distribution of the four lobes in the LP21 beam. Figure 3.15b displays microscopic images of the focused beam spots both on focal plane and 90 ^m in front of the focal plane, with their beam waists being 6.2 ^m and 1.4 ц,ш, respectively. Two yellow lines indicate beam spot shrinkage through beam focusing.
The setup for cell capture and rotation is shown in Fig. 3.16. A laser diode (Mitsubishi ML101U29) with a wavelength of 650nm is coupled into a G.652 singlemode fiber using a 10x microscope objective (N.A. = 0.25). A beam in LP21 mode was generated with a mode selector by adjusting the coupling incident angle through minute angle tuning, with a maximum LP21 mode power of 15-20mW. A mechanical fiber rotator is incorporated in the beam train, capable of twisting a segment
Fig. 3.15 a Measured intensity distribution of LP21 beam from the fiber end (before being focused by axicon fiber lens); b Microscopic image of the focused beam intensity distribution 90 in front of focal plane, and the beam profile at focal plane
Fig. 3.16 Schematic of a setup for cell manipulation
of fiber to control the rotation of the LP21 beam spot . The terminal fiber end controlled by manipulator was shaped into a conical tip (axicon lens, apex angle 120°), which converts the LP21 fiber mode into a Bessel-like beam with minimum FWHM = 1.5 m . Yeast cells are placed on a cover glass of a microscope slide mounted on a 3D piezoelectric nano-stage with a movement resolution of 20nm (Thorlabs NanoMax, Max311D). Images were recorded by a microscope system with a CCD(SUNTIME300E), and a high-pass filter (650 nm high-pass filter, Thor- labsFEL0650) to block the scattered laser light. The intensity was adjusted by an N.D. filter. A LED white light source was used to illuminate the samples for image recording.
The mode selector is a tilt adjustable stand with three-dimensional adjustment. Mode excitation in an optical fiber is directly related to the angle of incidence, so a fine adjustment of the angle is very important. In addition to a stable LP21 mode, we also need to improve the coupling efficiency. This is because that the largest loss throughout the system is the mode excitation and coupling part. In achieving a high efficiency of mode generation, an appropriate distance between input fiber end face and the fiber connecting to laser diode needs to fine tuned too.
Cell samples are flow inside a simple micro fluidic channel fabricated on microscopic cover glass as platform, which was affixed on a three-dimensional precision piezoelectric nano-stage with 20nm resolution (Nanostage, Thorlabs NanoMax, Max311D). Yeast cells (diameter 3-6 ц-m) are used as the dielectric particles, the end face of the optical fiber located on the substrate is immersed in a diluted solution of yeast cells. In imaging system a beam splitter constitutes dual observation paths, allowing human eye and CCD (SUNTIME300E) record the microscopic images/videos simultaneously. A high-pass filter (650 nm high-pass filter, Thorlab- sFEL0650) is added in the light path to filter 650nm laser light, preventing eyes from laser light damage, but also to prevent the CCD saturation by light source. In direct observation the CCD will be saturated by LP21 four spots, thus an adjustable neutral density attenuator (ND) is placed before the CCD, with an attenuation range fromND2 to ND400, minimizing the influence of laser source in recording process.
Lighting subsystem is of critical importance in imaging process. A proper lighting provides not only suitable illumination intensity and angle, but also fully utilize numerical aperture to maximize spatial resolution of microscopic imaging system. Since the optical fiber is placed against the objective, direct illumination is not available, yet yeast cell is partial transparency, difficult to observe directly by point source lighting system. After a trial and error, it is found that a LED spatial array can serve as a light source well, capable of achieving good lighting effects.