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Featured Application: We propose an SPR sensor with high sensitivity and resolution based on the higher-order modes, which can be applied in the measurement of the refractive index. In this paper, we propose an approach to improve the sensitivity of an optical fiber surface plasmon resonance (SPR) sensor with a pure higher-order mode excited by a designed mode selective coupler (MSC). We calculate the proportion of the power of the higher-order mode in the cladding. Compared to the LP 01 mode, the power proportion of the LP 11 mode ( LP 21 mode) in the cladding theoretically improves by 100% (150%). To generate a relatively pure LP 11 mode or LP 21 mode, a mode selective coupler (MSC, 430–580 nm) is designed. The coupling efficiency of the LP 01 – LP 11 mode coupler is over 80%, and that of the LP 01 – LP 21 mode coupler is over 50%. The simulation results show that the sensitivity of the LP 11   mode and the LP 21 mode increases by approximately 330% and 360%, respectively, using the intensity modulation (n = 1.33–1.38, 430–580 nm); the resolution of the refractive indices of our sensor, using the LP 11 mode ( LP 21 mode), is 2.6 × 10 − 4 RIU ( 2.4 × 10 − 4 RIU). The higher sensitivity and resolution of our presented fiber SPR sensor containing a visible MSC make it a promising candidate for the measurement of refractive indices. [ABSTRACT FROM AUTHOR]

In this paper, we propose an approach to improve the sensitivity of an optical fiber surface plasmon resonance (SPR) sensor with a pure higher-order mode excited by a designed mode selective coupler (MSC). We calculate the proportion of the power of the higher-order mode in the cladding. Compared to the LP01 mode, the power proportion of the LP11 mode (LP21 mode) in the cladding theoretically improves by 100% (150%). To generate a relatively pure LP11 mode or LP21 mode, a mode selective coupler (MSC, 430–580 nm) is designed. The coupling efficiency of the LP01–LP11 mode coupler is over 80%, and that of the LP01–LP21 mode coupler is over 50%. The simulation results show that the sensitivity of the LP11 mode and the LP21 mode increases by approximately 330% and 360%, respectively, using the intensity modulation (n = 1.33–1.38, 430–580 nm); the resolution of the refractive indices of our sensor, using the LP11 mode (LP21 mode), is 2.6×10−4 RIU (2.4×10−4 RIU). The higher sensitivity and resolution of our presented fiber SPR sensor containing a visible MSC make it a promising candidate for the measurement of refractive indices.

Using an all-fiber mode selective coupler (MSC) at the visible band, here we experimentally demonstrate a generating and wavelength multiplexing scheme for the cylindrical vector (CV) and vortex beams (VBs). The proposed MSCs act as efficient mode converters to produce spectrally insensitive high-order modes (HOMs) at the wavelength ranging from 450 to 980 nm, which have broad operation bandwidth (more than 7 nm), high mode conversion efficiency (94%), and purity (98%), and low insert loss (below 0.5 dB). By adjusting the polarization state and the phase shift of linear polarization (LP)11 mode respectively, the donut-shaped CVs and circular-polarization VBs are achieved. The focused intensity distribution of the donut beam on the cross- and axial-sections is monitored by using a confocal system. The all-fiber solution of producing and multiplexing HOMs opens a new route for stimulated emission depletion microscopy applications.

We propose cascaded mode-division-multiplexing and time-division-multiplexing passive optical network (MDM-TDM-PON) based on low mode-crosstalk few-mode fiber (FMF) and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), in which optical network units communicate with the optical line terminal utilizing different time slots and specific optical linearly polarized spatial modes. An MDM optical distribution network (ODN) is cascaded with multiple conventional TDM ODNs to effectively extend a larger scale of current commercial PON systems based on TDM. The upgrade from TDM-PON to the cascaded MDM-TDM-PON is simple and cost-effective. No multiple-input-multiple-output (MIMO) digital signal processing is required to eliminate the mode crosstalk. The all-fiber mode MUX/DEMUXs are composed of mode selective couplers, which simultaneously multiplex or demultiplex multiple modes. We experimentally demonstrate MDM-PON transmission over 10 and 55 km two-mode FMFs and cascaded MDM-TDM-PON transmission over a 10-km two-mode FMF and a 10-km standard single-mode fiber with 10-Gb/s optical on-off keying (OOK) signal and direct detection.

• The new symmetric OAM MSC is with a trade-off between distance and coupling length. • MSC has good performance for three orders OAM modes. • The OAM MSC exhibits wide bandwidth, high mode purity, and short coupling length. • The maximum coupling efficiency is 57.31% and mode purity is in excess of 52.77%. • Over C+L band, mode purity can reach 52%, and coupling efficiency can reach 51%. • The insertion loss is more than −1.73dB and excess loss is less than 0.175dB. We propose orbital angular momentum (OAM) mode generation using an all-fiber coupler based on hollow-core circular photonic crystal fibers (PCF). Using the mode-matching method, we calculated the coupling efficiency of high-order vector modes excited by the OAM coupler. The coupler's structure with optimal parameters was designed to achieve high mode purity and good coupling efficiency for broadband operation. By selecting the proper center-to-center distance and coupling length, the designed mode selective coupler (MSC) generated no more than third-order OAM mode coupling. The MSC's maximum coupling efficiency was more than 51% and its mode purity was in excess of 52% across the entire C + L band. The OAM MSC's design provides a key basis for constructing an all-fiber mode division multiplexing (MDM) communication system. [ABSTRACT FROM AUTHOR]