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Selective Excitation ofLP2l Mode

In the following discussion, we will show that LP2imode can be generated by using the most common telecommunication fiber (G.652) and a laser diode with center wavelength of 650 nm. G.652 single-mode fiber is for single-mode transmission at i3i0 and i550nmband for telecommunication; but for 650nm light source, the low- order multi-mode transmission occurs in the fiber, depending on incident angles.

The mode selection theory and experiments have shown that by adjusting the angle of light coupled into the fiber, we can selectively excite an individual low-order mode. By cleaving a fiber carefully to have a flat end surface and allow the beam incident perpendicularly to the end surface, the fundamental mode is mainly excited and the light intensity distribution is a Gaussian. If we adjust the angle between the incident light and fiber end face, different modes can be obtained. Figure3.7 is a schematic diagram of the LP2imode excitation apparatus. The mode selector was fabricated from two ceramic fiber ferrules mounted on a l-axis rotary stage. The incident angles ei and в2 of the LD-coupled fiber, with respect to the testing fiber, can be fine adjusted ranging from 0 to 5 for both pitch and yaw angles, capable of excitation in different few-modes. Separation between LD coupled fiber and receiving fiber also needs tuned to yield a pure LP2i mode. Figure3.8 displays light intensity distribution of LPli mode (b) is a four-lobed spot light intensity distribution diagram actually received; Fig. 3.8a is the actual perfect four-lobed spot light intensity distribution diagram obtained after smoothing.

Although LP2i mode is supposed easy to obtain theoretically according to the mode selection theory, in the actual course of the experiment getting a perfect LPli model is difficult. We often find that we have easy access to a range of approximate

Fig. 3.7 Schematic of an optical fiber mode selector

a Four-lobed LP21 light intensity distribution in 3D obtained in experiment; b Crosssectional intensity distribution

Fig. 3.8 a Four-lobed LP21 light intensity distribution in 3D obtained in experiment; b Crosssectional intensity distribution

a-d Some defected four-lobed intensity distribution

Fig. 3.9 a-d Some defected four-lobed intensity distribution

four-lobed spots, as several conditions shown in Fig. 3.9. These four-lobed spots are not simply a pure LP2i, but coupled multiple modes.

The following properties of LP2i mode are helpful to recognize a relative pure LP2i mode:

  • (1) Spot clarity. In Fig.3.9a, each spot has a circular halo due to coupling of LP2i mode with other modes. There are other modes affecting the LP21 mode characteristics, such as when twisting fiber, if there are other mixing modes, optical fiber twist will make the pattern deformed. In addition other modes will reduce the intensity distribution of four-lobed spot, reducing the optical trapping force.
  • (2) Consistency in size of four spots. By theoretical analysis, four spots of the perfect LP21 mode have the same size. Figure3.9a, b show that a stronger light spot below than that of the above. Adjustment has to be made trying to balance the intensity among four spots.
  • (3) Dark band separating four spots. Observation of Fig. 3.9b leads to that the four- lobed spots are well separated with across-shaped dark band in the middle. Smearing up of light intensity cross the dark band is an indication of mode coupling between LP21 with other modes. It is also this cross-shaped dark band that facilitates rotation of captured cells when we rotate the spot pattern. This is a feature that cannot be neglected in obtaining a pure LP21 mode.
  • (4) When twisting the fiber, the four-lobed spot pattern should not deform. Both theoretical and experimental tests have verified that simply twisting the optical fiber does not cause any deformation of LP21 mode, only give arise to rotation of pattern. In Fig. 3.9c, d, the light intensity distribution changes from four spots into a ring by a process of the energy coupling of the LP21 mode with other modes, ultimately resulting in a change in spot pattern.
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