We develop coupled-mode formulism for multimode couplings of LP01 core modes to LP1n cladding modes in singlehelix chiral long-period fiber gratings (S-CLPG). Then mode-coupling characteristics of the structure, such as coupling coefficients and relative bandwidths of dips are studied and compared with those of the conventional long-period fiber grating (LPG). Subsequently, transmission spectral characteristics in response to the liquid level are investigated for SCLPG partially immersing in a liquid with refractive index lower than that of the fiber cladding. Simulation results indicate that the dips corresponding to the multimode couplings in the transmission spectrum will shift continuously and monotonically as liquid level changes. Thus S-CLPG could be used for liquid level sensing based on wavelength interrogation, which is beneficial to avoid sensitivity deterioration caused by power instability in power interrogation for most conventional LPG-based liquid level sensors. In addition, we identify that the sensitivity can be enhanced with a thinner fiber cladding or by utilizing higher-order cladding mode resonance. The optimization result indicates that the wavelength shift will be over 1.03 nm per millimeter liquid level change.
For the passive fiber probe, the dependence of the probes' transmission efficiencies on the parameters such as fiber probe shape, taper length, tip diameter and incident wavelength are analyzed with the coupled local-mode theory. It shows that the transmission efficiency of a parabolic fiber probe is about an order of magnitude higher than that of the conical fiber probe. With the increase of the incident wavelength, the transmission efficiency of fiber probes decrease. The images of phase grating by conical fiber probe and by parabolic fiber probe are compared. The Nd:YVO4 frequency-doubling laser with a miniature structure is used as light source in the experiment of SNOM, which provides higher transmission efficiency and make it fascinating in some SNOM's applications. As for active fiber probe, the ASE fiber probe is proposed. It has a wide spectrum of 6nm and may greatly reduce the coherent noise in SNOM image. It may improve the output photon flux several times higher than that of passive fiber probe. Meanwhile, the guide-wave reflection properties in fiber taper are investigated.