A singlemode-multimode-singlemode (SMS) fiber structure consists of a short section of multimode fiber fusion-spliced between two SMS fibers. The mechanism underpinning the operation of an SMS fiber structure is multimode interference and associated self-imaging. SMS structures can be used in a variety of optical fiber systems but are most commonly used as sensors for a variety of parameters, ranging from macro-world measurands such as temperature, strain, vibration, flow rate, RI and humidity to the micro-world with measurands such as proteins, pathogens, DNA, and specific molecules. While traditional SMS structures employ a short section of standard multimode fiber, a large number of structures have been investigated and demonstrated over the last decade involving the replacement of the multimode fiber section with alternatives such as a hollow core fiber or a tapered fiber. The objective of replacing the multimode fiber has most often been to allow sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then summarizes and compares the performances of a variety of sensors which have been published under a number of headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their deployment in real world applications and discusses possible future developments of SMS fiber sensors.

Seeking better surface plasmon polariton (SPP) waveguides is of critical importance to construct the frequency-agile terahertz (THz) front-end circuits. We propose and investigate here a new class of semiconductor-based slot plasmonic waveguides for subwavelength THz transport. Optimizations of the key geometrical parameters demonstrate its better guiding properties for simultaneous realization of long propagation lengths (up to several millimeters) and ultra-tight mode confinement (~λ²/530) in the THz spectral range. The feasibility of the waveguide for compact THz components is also studied to lay the foundations for its practical implementations. Importantly, the waveguide is compatible with the current complementary metal-oxide-semiconductor (CMOS) fabrication technique. We believe the proposed waveguide configuration could offer a potential for developing a CMOS plasmonic platform and can be designed into various components for future integrated THz circuits (ITCs).

In this Letter, a novel waveguide based on hybrid surface plasmon polaritons (HSPPs) is proposed and numerically analyzed. This waveguide consists of two dielectric nanowires placed on both sides of a nanowedge-patterned metal film, which can confine light in the ultra-deep-subwavelength region (ranging from ${\lambda }^2/4000$ to ${\lambda }^2/400$) with a long propagation length (ranging from 1200 to 3500 μm). Compared to a previous HSPPs waveguide without the nanowedges, with the same propagation length, our proposed structure has much higher mode confinement with 1 order of magnitude smaller normalized mode area. An investigation of the effect of structural perturbations indicates that our proposed waveguide also has good tolerance of fabrication errors. The proposed waveguide could be an interesting alternative structure to realize nanolasers and optical trapping.

Multiple cladding modes can exist in a small-core optical fiber unaccompanied by core modes, yet this fact has not been sufficiently explored in literature to date. In this article, we study the self-imaging of cladding modes in small core optical fiber interferometers. Our analytical and numerical simulations and experiments show that unlike the self-imaging of core modes, self-imaging of cladding modes only appears at a set of discrete positions along the interferometer axis with an equal spacing corresponding to some discrete values of fiber core radius. This is the first observation of the discrete self-imaging effect in multimode waveguides. More strikingly, the self-imaging period of cladding modes grows exponentially with fiber core radius, unlike the quadratic relationship in the case of core modes. The findings bring new insights to the mode propagation in an optical fiber with a core at micro/nanoscale, which may open new avenues for exploring multimode fiber technologies in both linear and nonlinear optics.