Observation of split evanescent field distributions in tapered multicore fibers for multiline nanoparticle trapping and microsensing

Abstract

The optical attractive force in tapered single-mode fibers (SMFs) is usually uniformly distributed around the tapered section and has been found to be important for trapping and manipulating targeted atoms and nanoparticles. In contrast, a peculiar phenomenon of the evanescent field splitting along the azimuth axis can be experimentally observed by tapering a weakly-coupled MCF into a strongly-coupled MCF to generate supermode interference. Moreover, the supermode interference produces a hexagonally distributed evanescent field and its six vertices give rise to the multiline optical attractive force. For such spectral resonances, the optimum extinction ratio for the transmission dips is given by 47.4 dB, this being determined using an index liquid to cover the tapered MCF. The resonant dips move to a greater extent at longer wavelengths, with the optimum tuning efficiency of 392 nm/RIU for index sensing. The split evanescent fields respectively attract the excited upconversion nanoparticles in the liquid to be linearly aligned and running down the tapered region over the fiber surface, emitting green light with 60° symmetry. The charged nanoparticles were periodically self-organized, with a period of around 1.53 µm. The parallel lines, with 60° rotational symmetry, can be useful for (1) indicating the exact locations of the side-cores or orientations of the tapered MCF; (2) as precision alignment keys for micro-optical manipulation; and (3) enhancing the upconversion light, or for use in lasers, coupling back to the MCF. The split evanescent fields can be promising for developing new evanescent field-based active and passive fiber components with nano-structures.

High Sensitivity Fiber Refractive Index Sensors Based on Asymmetric Supermodes Interference in Tapered Four Core Fiber

“…With continuous tapering, the fourt #4, at the diagonal position with respect to the excitation core gradually enters the e cent fields overlapping region to excite new supermodes, Figure 2h,i, based on th core structure, c#1-#4. Figure 2f-i were the far field mode patterns using whiteligh ning 1250-1650 nm from superluminescent diodes (SLD) under a 1000× CCD micro Partial guiding lights from SLDs near the 1310 nm wavelengths can be observed CCD microscope [23]. From the three-core to four-core structure, the excited super are highly determined by the tapered diameter D of the TFCF and are intrinsically metric due to the vertex-core excitation scheme.…”

“…Figure 2f-i were the far field mode patterns using whitelight spanning 1250-1650 nm from superluminescent diodes (SLD) under a 1000× CCD microscope. Partial guiding lights from SLDs near the 1310 nm wavelengths can be observed by the CCD microscope [23]. From the three-core to four-core structure, the excited supermodes are highly determined by the tapered diameter D of the TFCF and are intrinsically asymmetric due to the vertex-core excitation scheme.…”

“…With this interesting phenomenon, it brings new physical findings and applications for MCF. Except for the excited supermodes, the split evanescent field distribution as well as the evanescent optical trapping force have also been observed in the tapered seven core MCF [23]. In contrast to the large amount of the literature related to the seven core MCF, there have been only a few works reported using FCF for curvature, strain, temperature, pressure, and RI sensing applications [24][25][26][27][28].…”

Multicore Fiber Bending Sensors with High Sensitivity Based on Asymmetric Excitation Scheme

“…MCF has different optical characteristics from ordinary optical fiber. For example, the evanescent wave optical trapping force and split evanescent fields have been found in seven-core optical fibers [ 26 , 27 ]. In addition, attenuated power coupling may occur when the MCF is severely bent [ 28 ].…”

High Sensitivity Strain Sensors Using Four-Core Fibers through a Corner-Core Excitation

“…The supposed mode fields from the individual core generates interferences, which is advantageous to achieve optical sensing with multi-parameters or split optical trapping force [ 27 ]. In fusion splicing, in order to match the core positions between the SMF and MCF for delivering optical power with negligible losses, the 7-core MCFs, with one core sitting at the center position, are more popular than other kinds of MCFs to be used to make various kinds of fiber sensors [ 28 , 29 , 30 ]. However, as a result, the excited supermodes will thus be intrinsically concentric in their fields, and the strain sensors so made are with the strain sensitivity somehow restricted by the limited OPD between supermodes.…”