This paper first proposes and verifies two simplified mode diversity receivers (MDRs) for free-space optical (FSO) communication systems under moderate-to-strong turbulence. The MDRs are designed and implemented for coherent detection of the FSO communication system based on few-mode-fiber (FMF) coupling. The first MDR replaces the coherent receiver with a Kramers–Kronig (KK) receiver, which reduces the number of photodetectors (PDs) and high-speed analog-to-digital converters by 3/4. The second MDR realizes few-mode KK detection by utilizing a few-mode local oscillator and only one PD. This method simplifies the system of a MDR to the extreme. Experimental results show that the average received power of a FSO communication system based on six-mode fiber has approximately 6 dB gain compared with the one based on single-mode fiber (SMF) under moderate-to-strong turbulence. The two proposed MDRs have approximate performance, improving by 5 dB to 6 dB compared to SMF-based heterodyne detection under moderate-to-strong turbulence. In addition, the simplified schemes reduce the volume and the cost of the optical receiver, and it has some practical significance for FSO communication.

We report on the fusion splice loss characteristics of each mode in few-mode fiber (FMF). The fusion fault loss characteristics of all modes of the six-mode step-index and graded-index fibers are measured, respectively. The high-order modes with high fault detection sensitivity are presented. The fault events with different loss amplitudes can be accurately characterized by high-order modes. On this basis, a fault detection and location method for FMF based on backscattered light of high-order modes is proposed and experimentally demonstrated. Rayleigh backscattering light of high-order modes is utilized to detect the faults of 7.2 km six-mode step-index fiber with three fusion splice points with different fusion quality, the detection results of each mode are compared. The high-order modes ${\mathrm{LP}}_{21\mathrm{a}}$, ${\mathrm{LP}}_{21\mathrm{b}}$, and ${\mathrm{LP}}_{02}$ with high fault detection sensitivity are shown, three fault points located around 1.03, 3.1, and 3.2 km of the FMF are successfully detected. While the ${\mathrm{LP}}_{01}$, ${\mathrm{LP}}_{11\mathrm{a}}$, and ${\mathrm{LP}}_{11\mathrm{b}}$ modes can detect only one fault point, which is located around 3.19 km of the FMF.

A Si₃N₄ slot waveguide has the ability to confine light tightly in the slot, shows weak absorption of 980 nm pump light, and has lower transmission loss compared to a Si slot. Hence, the optical waveguide amplifier based on Er³⁺ and Yb³⁺codoped was proposed to be integrated with a Si₃N₄ slot to increase the gain. The core-shell NaYF₄/NaLuF₄: 20%Yb³⁺, 2%Er³⁺ nanocrystals-polymeric methyl methacrylate covalent linking nanocomposites were synthesized and filled into the slot as gain medium. The concentrations of Er³⁺ and Yb³⁺ were increased compared with traditional physical doping methods. High-efficiency emission at 1.53 μm was achieved under 980 nm laser excitation. The slot waveguide was accurately designed using the semivector finite difference method in combination with the maximum confinement factors and the minimum effective mode area. The optimum width of the slot was 200 nm, and the optimum height and width of the silicon strip waveguide were 400 nm and 400 nm, respectively. The six-level spectroscopic model was presented, and the gain characteristics of the slot waveguide amplifier were numerically simulated. A net gain of 8.2 dB was achieved, which provided new ideas and directions for waveguide amplifiers.

In this paper, we propose and experimentally demonstrate a nondestructive method for simultaneously measuring the mode-dependent loss (MDL) and mode coupling (MC) in few-mode fibers (FMFs). The method is based on analyzing the Rayleigh backscattering amplitudes obtained with an optical time domain reflectometer (OTDR). The experimental results show that, in about 4 km six-mode FMF, the MDL and MC values are 0.132 dB and $-23.13\mathrm{dB}/\mathrm{km}$ between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{11a}$, 0.176 dB and $-23.73\mathrm{dB}/\mathrm{km}$ between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{11b}$, 0.272 dB and $-26.17\mathrm{dB}/\mathrm{km}$ between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{21a}$, 0.284 dB and $-26.70\mathrm{dB}/\mathrm{km}$ between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{21b}$, and 0.380 dB and $-20.21\mathrm{dB}/\mathrm{km}$ between ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{02}$. And it also demonstrated that the proposed scheme can be scalable to measure MDL and MC between higher-order modes. The values of the MC and MDL obtained by the proposed method agree well with that by the conventional transmission method. However, the proposed method has the merits of simultaneous and single-end measurement of MDL and MC in the FMF and could be a good solution to the characterization of FMFs used in large-capacity mode-division-multiplexing transmission systems.

Waveguide amplifiers based on slot waveguide have enormous capacity due to their ability to confine light strongly to a narrow slot waveguide. NaYF₄:Er³⁺ nanoparticles-polymeric methyl methacrylate covalently linked nanocomposites were synthesized and filled into the slot. The stability and the Er³⁺ concentration doped in this novel material were improved. The slot waveguide was designed accurately. The rigorous numerical method, full-vector finite difference method, was used to analyze the modal characteristics and optimize the slot combined with the maximum power confinement in the slot and the minimum effective mode area of the slot. A four-level spectroscopic model pumped at 1480 nm was presented. The rate equations and propagation equations were solved and the gain characteristics of the slot waveguide amplifier were numerically simulated. The primary parameters were optimized. A net gain of 5.78 dB was achieved when the signal power was 0.001 mW at 1530 nm, pump power was 20 mW, Er³⁺ concentration was 1.3 × 10²⁷ m⁻³, and the waveguide length was 1.5 cm.